DEPARTMENT OF THE INTERIOR U.S. G E O L O G I C A L S U R V E Y

FOLIO OF THE MOUNT HAYES QUADRANGLE TO ACCOMPANY MAP MF-1996-A

METALLfFEROUS MXNERAL RESOURCB ASSESSMENT OF T H E MOUNT HAYES QUADRANGLE,

EASTERN ALASKA RANGE. ALASKA

Waren J. NoUebag,Ian H. Lange, Donald A. Singer. Gary C. C w t h .

Richard 8. Mpp, D avld I.. Campbell, and W w e n Y eend

SUMMARY OF HETALLIPEROUS H I M E R A t RESOURCE ASSESSHEN?

This rep& measea t he m etalltferoua mineral resources of the Mount Hayes quadrangle based on extensive geologic, geochemical. and geophysical investigations, and on investigations of mineral deposits, prospects, and occmrencea. The assessment consisb of the following steps: (1) in tepat ing geological, geoche mical, and geophystcal data t o idenWy favorawe gealogic environments fcr 1 4 types of mineral deposits tha t occur. or may occw in the quadrangle; (2) developing mod& for t he type3 of mineral de-ts; (3) defining recognition crlterla for the types of mineral depoedta; and (4) assigning potential, for exam ple, the likelihood for undiscovered mineral deposits, for each type of depodt by area, based on the num bsr and the quality of recognition criteria.

A major result of the asssssment Is that specific types of mineral deposlts are rsstrlcted t o specific geologic units, such as tectonostratiwaphic terranes, or younger granitic plutons. In particular, t he following areas exhibit high

,potential for undLscovered mineral deposits: (1) The metavdcanic rock unit of the Jarvis Creek Glacier subterrane of the Yukon-Tanana terrane has high potential fo r Kuroko m asslve sulfide depoaits (areas A and D , sheet 2); (2 ) The Slana River aubterrane of the WrangeVia t m a n e exhibits hlgh potential for porphyry Cu-Au-Ag deposits in the soutkcentra l and southeatern parts of the quadrangle (areas G 2 and I, sheet 2); (3) The S a n a River subterrane exhibits high potential fcr porphyry C u-M o deposits in s m a l l granitic plUtOnS (areas K and M , sheet 4) in the south-central part of the quadrangle; in addition, the Maclaren Glacier metamwphtc belt of t he Maclaren terrane exhibits high potential for porphyry CwMo deposits in the southwestern part of the quadrangle; and (4) The Tangle subtewane of the WrangeUra terrane in the southwest part of the quadrangle exhibits high potential for W-Mo and C u-Zn-Pb skarn depodts adjacent t o a s m a n granitic pluton (area L2, sheet 4). Areas with moderate or low potential for various undiscovered types of mineral deposits are described in tables 1 t o 14 and sheets 1 t o 4.

INTRODUCTION

This report and the accompanying maps assess the m etalJiferous mineral resource potential of the Mount Hayes quadrangle, eastern Alaska Range, Alaska. The assessment is the mdor result of a five-yew study done under the Alaskan Mineral Resource Assessment Program (AM R AP). Field work for the assessment was done during the summers of 1978 through 1982. Laboratory investigations and office synthesis of data started in 1979. This report la one part of a falio on the quadrangle. In adjacent quadrangles, mineral resource assessments have been completed for the Big Delta quadrangle (Menzie and Fmter. 19781, t h e Nabesna

quadrangle (Richter and others, 1975), the Talkeetna Mountains quadrangle (Singer and others, 19781, and the Tanacross quadran@ (singer and others, 1976).

Nonmetallic com m odities in the quadrangle consist of sand and gave l , marble, and quartzite. Although much of this material is suitahle for conswuction wes, i t s remoteness from markets would result in hlgh, uncompetitive development and transportation costs. Local granite and decomposed granite are obtdned from one or two pits along the Alaska Highway in the northeast part of the quadrangle. The geologlc environment in the quadranqe is unfavorable for oil, gas, or geothermal energy. Coal has been sporadically mined from the Jarvis Creek coalfield which has been studied and assessed by Wahrhaftig and Hickox (1 955). This mineral resource assessment is based on the following new geologic, geochemical, and geophysical data:

(1) Geologtc mapping of the quadrangle a t scales of 1 :63,360 and 1:250,000;

(2) Petrogaphic study of 3,332 thin sections of rock sa m ples;

(3) Geolo@c mapping and sampling of known mineral depodts, prmpects, and occurrences (Nokleberg and others, in pessl, including study of 128 polished thin sections of sulfide and odde minerals from mineral deposits, Fospects, and occurrences;

(4) Semiquantitative e m i d o n spectrogfaphic analysis for 31 elements and interTetation of 1,976 rock, mineral deposit, prospect, and occurrence samples for 31 elements (zehner and others, 1985):

(5) Semiquantitative emission spectrographic analysis for 31 elements and interpfetation of 976 stream-sediment, g lac ia l -dew, and heavymineral-concentrate samples for elements (O'Leary and others, 1981, 1982; C urtin and others, in press);

(6) Identification and interpretation of the distfibution of heavy minerals in heavy- mineral-concentrate samples;

(7) Examination of 52 panned samples of gravel and sand from, or in the vicinity of known or suspected placer mines or deposits (Y eend, 1980, 1981 a , b);

And (8) analysis and interpetation of the aerom agnetic map for the quadrangle (State of Alaska, 1974);

This assessment is for undiscovered mineral resources tha t might be expected t o Occur in the quadrangle as of the time of publication. Subsequent new techniques may be developed or new mineral resources may be defined tha t are not envisioned in this ass@ssment. The term "mineral resourcen is defined as a natural concentration of elements in such form that economic extraction is currently or potentially feasible.

ACKNOWLEDGMENTS

We greatly appreciate the excellent published geologic studies for the quadrangle. The geologic, mineral deposit. and deposit compilation studles of Rme (1 965, 1966a, b,

1967), Rose and Saunders (19651, Cobb (1972, 1979), and MacKevett and Holloway (1 977) were particularly useful. Valuable t o w s of lode mineral deposits, prospects, and occurrencm, were provided by Michael Ross, C,R, Nauman, and S.R. Newldrk, and of placer mines by William Beerman, Douglas Culp, Lewis E l m e r , John Johnson, and Richard Osborne. Valuable discussions of geology and mineral deposits were held with H.C. Berg, Richard Cavalero, E,R. Chipp, Donald Grybeck, D.L. Jones, E.M. MacKevett, Jr., C . R . Nauman, S.R. Newkirk, D.H. Richter, Michael R o s s , N.J. Silberling, and T.E, Smith. We are indebted t o H . C . Berg, Donald Grybeck, T.P. MUer, A.T. Ovenshine, and G,R. Winkler who encowaged and supported this mineral resource assessm ent. This assessm ent benefited from the constructive reviews of Donald Grybeck and J.E. Case.

G E O L O G I C S U M M A R Y

The Mount Hayes quadrangle is in the eastern Alaska Range, which forms a r e a t , glacially sculptured, arcuate mountain wall extending appoxtmately 1,000 km from the Canadian border on the east t o the Aleutian Range on the west and southwest. The eastern Alaska range is characterized by high p e a k ranging t o over 4,180 m in elevation and spectacular valley glaciers as long as 65 km. The range is bisected by the Denali fault, which i s a major gealoglc and geowaphic boundary between the Yukon River basin in interior Alaska t o the north, and the Copper River basin of southern Alaska t o t he south.

The bedrock geology is subdivided into variow tectonostratigraphlc tem+ane~ (Mg. 1). The term lltectonostratigraphic ten*anel' (hereafter referred t o as terrane) is deflned as a fault-bounded gealoglc entity wlth a distinct geologic history, stratigraphy, structure, and (or) types of mineral deposits, all Ufe r ing markedly From those of aqoinlng neighbors (Jones and SflberYng, 1979). In the northern part of the quadrangle, north of t he D enall fault;, the bedrock geology i s dominated by the Devonian and Mississippian or older Yukon-Tanana terrane, a complex of multiply deformed and rn etam orphosed sedimentary, volcanic, and plutonic rocks (fig. 1; Jones and othera, 1987; Aleinikoff and Nokleberg, 1985a, b; Nokleberg and Aleinikoff, 1985). In t he southern part of the quadrangla, the bedrock geology is dominated by the Mesozoic Madmen and Paleozoic and Mesozoic Wrangellia terranw (fig. 1; Jones and othem, 1987; Nokleberg and others, 1982, 1985). A moderate number of gani t ic and lesser gabbroic plutons, chiefly of Mesozoic age, occur both north and south of the D enall fault.

In the las t two decades, the Mount Hayes quadrangle has been the focus of many bedrock geologic studies. Bedrock gealogic maps have been published by Halmes (1 9651, Rose (1965; 1966a, b; 19671, Rose and Saunders (19651, Holm es and Foster (1 968), M atteson (1 9731, Bond (1 9761, Stout (19761, Richter and others (1977), and Nokleberg and others (1982). Isotopic studies have been published by Smith and Turner (19731, Twner and Smith (1974), Wilson and Turner (1 9751, Aleinikoff and others (1 981, 19831, Aleinikoff (1 9841, Aleinikoff and Nokleberg (1984a, b, 1985a, b), LeHurray and others (1 9851, and Nokleberg and others (1 986). Stratigraphic and structural studlee have been pubLished by Bond (1973, 1976), Richter and D utro (1 9751, Stout (1 9761, N okleberg and others (1 981 a, b, c: 1983; 1985): and N okleberg and Aleinikoff (1985). Summary studies of mineral deposits and m etallogenesis have been published by N okleberg and others (1984) and Nokleberg and Lange (1985), A geologtc bibliogaphy of the Mount Hayes quadrangle was puhllshed by Zehner and others (1 980). Geophysical studies have been published by the State of Alaska (19741, Barnes (19771, HiUhouse and Grom me (1 9 8 4 , and CampbW and Nokleberg (1 985).

B E D R O C K GEOLOGY N O R T H O F DENALIFAULT Y ukon-Tanana terrane

The most extensive bedrock unit north of the D enaU fault is the Yukon-Tanana terrane (Jones and others, 19871, which in this report is subdivided from north t o south, into the Lake George, Macomb, Jarvis Creek Glacier, and Hayes

Glacier subterranev wig. 1 ; Aleinikoff and N okleberg, 1985a; Nokleberg and Aleinikoff, 1 985; NoUeberg and others, 1986). These subterranes are in terpeted as various levels of a complex and highly metamorphosed Devonian and Mississippian continental margin igneous arc (Nokleberg and Aleinikoff, 1985). Because of reeonal tilting toward the south near the Denali fault, the deeper gani t ic r o c b of the arc occur t o the north, and the shallower volcanic rock of the arc npcur t o the south. The Lake George, Macom b, Jarvia Creek Glacier, and Hayes Glacier units were initially defined as separate terranes (Nokleberg and Aleinikoff, 1985); however, these units a re now defined as subtemanes in wder t o emphasize their genetic relations as variow structural levels of t he Y ukon-Tanana terrane.

Lake George Subterrane of Y ukon-Tanana terrane

The Lake George subterrane (fig. 1; Aleinikoff and i

Nokleberg, 1985a, b: Nokleberg and Aleinikoff, 1985) occurs in the northeastern part of the quadrangle, and is composed of: (1 ) polydeform ed, coarse-grained pelitic muscovite- quartz-biotitegarnet schigt derived from quartz-rich t o clay- * rich shale of Devonian or older age; and (2) relatively younger Devonian and Mjsaiasippian medium-pained, gneissose panodlorite and diorite, and coarse-pained augen gneiss derived from r a n i t e and panodior-ite. The pelitic schist and m etam orphosed plutonic rocks are ductily deiorm ed and regionally m etam orphosed a t middle or upper am phibalite facies into m ylonitlc schist and m ylonitic gneiss, and exhibit local retrogression t o t he lower greenschlst facies (Nokleberg and others, 1986). The pelitic schist and metamorphosed plutonic r o c k are intruded by mid-Cretaceous t o early Tertiary diorite, granodiorite, and granite. Small areas of some plutom are extensively hyd-othermally altered. These plutonic rocks are locally slightly t o moderate schistose and are weakly metamorphosed a t lower weenschht facies, The Lake George subterrane is bounded t o the south by the Tanana River fault.

M acom b Subterrane of Y ukon-Tanana terrane

The Macomb subterrane (fig. 1; Nokleberg and Aleinikoff, 1985) occurs south of the Lake George subterrane in the northeastern part of the quadrangle. The Macomb subterrane is com posed of: (1) older, polydeformed, rn edium- grained pelitic schist, calc-schist, and quartz-feldsparcbiotite schist derived from shale, marl, and sandstone of Devonian or alder age; and (2) a s a t e of relatively younger, shallow-level, fine- t o medium-gained gneissme gani te , granodiorite, quartz diorite, and diorite of Devonian age. The rnetasedim entary rocks and the m etam orphosed plutonic rocks are ductily deformed and regionally metamorpho9ed a t epidote-am phibolite t o upper greenschist facies into m ylonitic gneiss and schist (Noklebag and others, 1986). The rn etasedt m entary and metamorphosed plutonic rocks are lntruded by younger and less deformed and rn etam orphosed mid-Cretaceous and possihly younger granitic rocks ranging in corn position From quartz diorite t o granite. These younger plutonic rocks are locaLly weaUy t o moderately deformed and m etamorphoaed. The Macom b subterrane Is bounded t o the south by the Elting Creek fault,

Jarvis Creek Glacier subterrane of Y ukon-Tanana terrane

The Jards Creek Glacier subterrane (fig. 1; NoCcleberg and Aleinikoff, 1985) occurs a c r m the northern part of the quadrangle, south of the Macomb subterrane. The Jarvis Creek Glacier subterrane consists of fine-gained, polydeform ed schist derived from D evonian or older sedimentary and volcanic rock. T M unit is subdivided into two major units: a metasedimentary rock unlt rich in fine- gained m etasedirn entary rocks with minor m etavolcanic r o c h , and a metavolcanic rock unit rich in finegrained rn etavolcanic roch with moderate amounts of fine-grained m etasedimentary rocks. The m etasedim entary and m etavolcanic rock3 are alm ast totally recrystallized (Nokleberg and others, 1986). rhe rn etasedim entary rocks

cases, the type of deposit could not be defined because the avanahle data are not sufflclent for classification.

SUMMARY OF LODE MINERAL DEPOSITS, PROSPECTS, A N D OCCURRENCES NORTH OF DENALIFAULT

Lake George and M acom b subterranes of Y ukon-Tanana terrane

A minor lode mineral occurrence in the Lake George subter'rane is on the south shore of Lake George, where a r a b sample of silicified, iron-stained pyritequartz-actinolite schist contains 30 ppm Sn. Minor lode mineral occwences in the M aco q b subterrane occur: (1 ) on the north side of Elting Creek, where a grab sample of pyrite-bearing irori-stained quartz-biotite schist contains 3.2 ppm Au; and (2) on the northwest side of t he West Fwk of the Robertson R iva , where a p a b sample of pyroxene cumulate contains @+eater than 5,000 ppm Cr. Except for t he lat ter . these occurrences are asocia ted with quartz veins, or occur in metasedlmentary schist near pSanltlc plutom. These occmences are p'obahly either gold quartz vein or epither m a1 p e c i o m and b a s e m eta1 deposit4 tha t formed dwing regtonal metamorphlsm and (or) during intrusion of Cretaceolls gani t ic plutom.

Jar* Creek Glacier subterrane of Y ukon-Tanana terrane

The Jarvls Creek Glacier subterrane locally contalm substantial lode mineral deposits, p-ospects, and occwences . The major lode mineral deposits and occmences are in the metavalcanic rock unit and consist of 15 s m a l l - t o moderatesize Kwoko massive sulfide deposits. These deposib, pospects, and occwences form two maor belts; a western belt. west of the Delta River, between the Hayes and M cGinnis Glaciers, and an eastern belt in the area southeast of the West Fork of the Robertson River.

Five fraspects and occwences com prlse the western belt, which is about 32 km long. The vestern belt prospects and occlnrences generally contain disseminated t o massive chalcopyrite, galena, sphalerite, pyrite, pyrrhotite. Selected samples contain as much as 9,200 ppm Cu, 2,500 ppm Pb, 23,000 ppm Zn, 5,000 ppm A s , 50 ppm Ag, 0.20 ppm Au, and 100 pprn Sn. Ten deposits and occlnrences comprise the eastern belt, which is about 26 km long. The eastern belt of deposits and occmences contain t he same sulfide minerals as the western belt. Selected samples contaln as m uch as 110,000 ppm Cu, 110,000 pprn Zn, 15,000 ppm Pb, 10,000 ppm As, 300 pprn Ag, 1,9 ppm Au, 300 ppm Sn, and 2.000 ppm Sb. In both belts, the massive sulfide deposits occur discontinuously as irregularly shaped, generally fault-bounded pods, lenses. and stringers. The deposlts and occurrences are hosted in mainly m eta-andesite, m etadacite, metamorphosed quartz keratophyre flows and tuffs, m etam orphased volcanic grayw acke and siltstone, and quartz schist, part of which may be derived from quartz exhalite. Preliminary studies of these deposits have been published by Nauman and others (1 9801, Andenon ( I 982), Culp (1 9821, Lange and Nokleberg (1 984), and Nokleberg and Lange (1 985). Field, petrographic, geochemical, and isotopic data indicate these deposits formed in a Devonian submarine Island-arc environm ent and were sutaequenuy deformed, m etam orphceed, and remobllized in the mid-Cretaceous (Lange and Noklekrg, 1984; Nokleberg and Lange, 1985).

0 ther lode mineral occurrences in the m etasedimentary rock unit of the Jarvis Creek Glacier subterrane are: (1 ) r a b samples of quartz veins in iron-stained schist, usually with disseminated pyrite, and containing as much as 450 ppm Pb, 30 ppm Sn, and 7 ppm Ag; (2) w a b samples of iron-stained schist, usually with disseminated pyrite, and containing as much as 350 ppm Pb, 300 ppm Sn, 30 ppm Mo: and (3) one grab sample of chalcopyrite and malachite i n meta$abb*o containing 55,000 ppm Cu, 7 ppm Ag, and 0.10 ppm Au. Most occurrences are in quartz veins or in metasedimentary schist and are either robably gold quartz vein or epithermal p rec iow and base-metal deposits t ha t Formed during regional m etam orphism and (or) intrusion of Cretaceous wanitic

plutons.

Hayes Glacier subterrane of Y ukon-Tanana terrane, and Windy and Aurora Peak terranes

Mineral occurrences in the Hayes Glacier subterrane condsist: of three areas of grab samples of altered q u a r t r mica-g~apkdte schist containing diseminated pyrite and quartz Vein% Selected samples contain as much as 720 ppm Zn, 30 ppm Mo, and 3 pprn Ag. Two occurrences are on the northwest side of the Trident Glacier in the western par% Of the quadrande; the other occurrence is in the southeastern part of the quadrangle. A minor lode mineral occurrence in the Windy terrane, in the southeastern part of the quadrangle, consists of a grab sample of metamorphosed quartrwhite mica raywacke tha t contains 5,000 ppm A s . M o s t of these occwences are in quartz-rich schist w phylllte. These occurrences are pobahly either gold quartz vein or epithermal p-ecious and base-metal deposits t ha t formed during regional metamorphlsm and (or) intrusion of Cretaceous granitic plutom.

Cretaceous wanitic plutons north of the DenaLi fault

Sparse lode mineral prmpecta and occurrences in, Or near Cretaceous and early Tertiary granitic plutons occur north of t he Denall fault in the Macomb and Jarvis Creek Glacier subtemanes. In the M acom b sub tmane , the mineral occwences conslst of: (1) altered quartz monmnite co~~ta in ing 0.25 ppm Au; (2) a s m a l l altered aplite dike containing 2.8 ppm Au and 70 ppm Sn; and (3) two areas of altered pyrite-bearing aplite or quartz monzonite with values of as much as 7 ppm Ag and 110 ppm Pb. In the Jarvis Creek Glacier subterrane, the mineral occurrences consist of three areas west of M olybdenu m Ridge, in the western part of the quadrangle, where g a b samples of granodiorite with molybdenite contain a.3 much as 0.1 ppm Au, and 5 ppm Ag, and 70 ppm Mo. In both subterranes, these occurrences are interweted as porphyry CwMo or porphyry Cu-Au-Ag deposits.

SUMMARY OF LODE M I N E R A L DEPOSITS, PROSPECTS, A N D OCCURRENCESSOUTH O F DENALIFAULT

Slana River subterrane of W rangellia t w a n e

The Slana River subterrane contains abundant lode mineral p-ospects and occurrences. Most of the p-aspects and occurrences are related t o igneous activity during l a t e Paleozoic island-arc volcanism (Nokleberg and others, 1984). About 19 s m a l l - t o moderatesize porphyry Cu-Au-Ag prospects and occurrences are located in the central-southern and eastern-southern parts of the quadrangle and consist of disseminated t o local s m all masses of chalcopyrite, bornite, malachite, and pyrlte in or near m etamorphosed and altered dacite porphyry. Selected samples contain as much as 100,000 ppm Cu, 5,000 ppm Pb, 530 pprn Zn, 70 ppm A$, 2.0 pprn Au, 1,500 ppm As, 50 pprn Mo, and 30 ppm Sn.

About nine am al l skarn p-aspects and occurrences occur in the south-central and southeastern parts of the quadrangle. The skarns are hosted in marble interlayered with l a t e Paleozoic metavolcanic rocks that are intruded by gabbro, diabase, or dacite. These skarn p-ospects and occurrences consist of disseminated t o local small masses of chalcopyrite and pyrite. Selected samples contain as much as 56,000 ppm Cu, 720 ppm Zn, 300 ppm Ag, 1.2 ppm Au, and 2,000 ppm Co. These skarn p-ospects and occurrences are COm monly associated with porphyry C u-Au-Ag pscepects and occurrences, and robahly formed during la te Paleozoic island-arc volcanism and associated igneous activity.

Locally abundant occlnrences of podiform chromite occw in mafic or ultramafic dikes and ,sills in the Upper Triassic Nikolai Greenstone, or i n m afic and ultram afic r o c b that are pobahly com agm atic with the basalt potoli th for the Nikolai Greenstone. These occurrences consist of disseminated t o small lenses and stringers of podiform chromite in cumulate ultramafic rock and are located in the central and western parts of the subterrane, These

mineralized areas and t o aid in defining the types of the mineral occrnrences within theae wem. The studies included the collection of stream-sediment samples a t 795 site on tri utary streams with drainage baeins ranging Prom 1 to 5 m i 4 in area (O'Leary and others, ,981. 1982; Curtin and others, in p-em). In addition, com poiite samples of glacial d e w were collected a t 116 sites on tributary glaciers. These samples were subsequently concentrated t o yield a minlls-80-mesh fraction and a nonmagnetic heavpmineral concentrate fraction with a specific r a v i t y r e a t e r than 2.85. For the purposes of this study, analytical data from glac ia l -dew sam ples were com bined with t hme of stream- sediment samples because statistical analysis of the analytical data showed tha t these two media are chemically s i m i l a r .

In general, the analytical results from both the heavy- mineral concentrate samples and the minus-80-m esh Paction of the stream-sedimenb samples are useful in identifying and outlining areas of known or inferred mineral occwences . The data Porn the heavy- mineral concentrate swvey are especially lrseful Iw delineating the distribution and abundance of are minerals, became the dilution effect of low-dedty , barren minerals has been removed, The analytical results of minll4-80- m esh sedi m ent s a m ples reflect the metal content of we-related minerals, barrenlow-derdty minerals, and metals that have been scavenged pirnarily by am crphous-*on and m anganeseoldde coatin@ on sedlm ent raim. In addition t o analysis of the explwation geochemical Sam ples, the mineralogy of the heavy- mineral-concentrate samples was miwascopically determined t o identify ore minerals.

The geochemical data indicate tha t the individual terranes have distinctive geochemical. characteristics. ConsequentLy, t he terranes are treated aa separate populattons in determining the distribution and abundance of the elements in the quadrangle, and separate data se ts were pepared f r r each major terrane. The major areas of interest a re described Mow.

Summary of m u l t 4

A few notable associations of high-m eta1 concentrations and areas of known Kwoko massive s M d e deposits occur in the Jarvis Creek subterrane (NoUebet-g and others, in w-1. ~ i g h values of Ag, Cu, Pb, and Zn occur in heavy-mineral concentrates and in stream sediments in the metavalcanic rock unit of the Jarvis Creek Glacier subterrane. In addition, heavy-mineral-concentrate samples contain pyrite, galena. sphalerite, chalcopyrite, arsenopyrite, and scheellte. Arsenopyrite, chalcopyrite, pyrlte, galena, and sphalerite also occur in the metasedimentary rock unit. These data Outline known mineral occurrences and undiscovered Kwoko m ass lve sulfide and epithermal p*eciom and base-metal mineral occurences in t he q etavolcanic rock unit.

Three areas north of the Denali fault underlain by pani t lc r o c k are characterized by high concentrations of Sn, W and, and Sb in heavpmineral concentrates. In the Macom b sub tmane , especially in the Berry Creek drainage, high values of Sn together with those of Cu, Pb, Zn, and Mo outline an area tha t has a moderate potential for undiscovered porphyry Sn, porphyry CwMo, and skarn mineral deposib. Similar high concentrations of Sn and W in the Macom b Plateau area also suggest local areas that are favorable for undiscovered porphyry Sn and W-skarn mineral deposits, Mineralogical examination of the heavy mineral- concentrate samples confirmed the pasence of cassiterite, scheelite, and fluorite, aU co m m on minerals in porphyry Sn deposits, together with gold, chalcopyrite, and galena.

The gan i t e of Granite Mountain in the central part of the quadranae is charactedzed by high concentrations of Sn, W , and Sb and high concentrations of Ag, Cu , Pb, and Mo. Chalcopyrite, cassiterite and arsenopyrite, monazite, thorite, malybdenite, and pow e U t e occur in the heavy- mineral concentrates in tNs area. These relations indicate a t least a low potential f w undiscovered porphyry Cu-Mo and porphyry Sn depodts. The pan i t e of Molybdenum Ridge is also characterized by high values of Sn, W and Sb in heavy-mineral concentrates, Monazite, thorite, m alybdenite and pow e U t e

aLw occur in the heavy-mineral concentrates from Granite Mountain and Malybdenum Ridge. These data indicate a t least a low potential for undiscovered porphyry Sn deposits.

South of the Denali fault, in the Wrangellia terrane in the southwest part of the quadrangle, another notable area exhibits high concentrations of Ag, Cu, Pb, A s , Mo, and local Sn and W in heavy-mineral concentrates and in stream- sediment samples, except for Sn and W . Gold, cinnabar, scheelite, chalcopyrite and menopyrite occur in heavy mineral-concentrate samples *om several d t e s in this area. These high concentrations occw in an area of h o w n C W A g quartz vein occwences (Nokleberg and others, in p'ess), and the exploration geochemical data outline areas with mininum potential for Cu-Ag quartz vein deposits.

To the northwest in the East Susitna batholith of the M aclaren tesrane. abundant m alybdenite, pow mte and scheelite occur in heavy-mineral concentratw. These occwences correlate with moderately high concentrations of W , Cu, and Sn in heavy-mineral concentrates and may indicate local small areas with a minimum potential for porphyry M o or skarn mineral deposits.

Gold and cinnabar were identified in a number of heavy- mineral concentrates from the Wrangellia terrane. The source of the gold may be epithermal p e c i o w and b a s e metal deposits associated with Mesozoic ganiUc r o c b intruding the Wrangellia terrane or C u-Ag quartz vein depaaits in mafic and intermediate igneoua rocks of the WrangeYia t m a n e . The source of cinnabar may be Tertiary volcantc r o c k that occur within the Wrangellia terrane in the central and eastern parta of the quadrangle.

S U U M A R Y OF GEOPHYSICALINVESTIGATIONS Methods

Geophysical investigations consisted of collecting and analyzing gav l ty and aeromagnetic data and swveying a short, very low-Prequency, electromagnetic p50flle over the Miyaoka m&ve sulfide occlesence in the northwestern part of the quadrangle. GravIty measurements were made a t 80 new stations in the quadrangle and augment 300 gavi ty measurements made by Barnes (1977). These g-avity measurements g e a t l y aid the tectonic interwetations of the reglon in the quadrangle. However, the density of the gravity survey over most of the quadrangle is too sparse t o define s m a l l anomalies that might indicate known or undiscovered mineral de posh .

Aeromagnetic fields were analyzed by Campbell and Nokleberg (1984, 1985) on the aeromagnettc map of the quadrangle publbhed by the State of Alaska (1974) to: (1) separate magnetically distinct terranes; (2) delineate app'oxim a t e boundaries of magnetic plutons and tabular bodies; and (3) identify lineaments t ha t may rep-esent faults. Probahle magnetic source roc& were identified by: (1) comparing detailed aeromagnetic and geologic maps a t 1:63,360 scale; (2) visiting o u t w o p and measwing magnetic suseptibilities; and (3) performing new aeromagnetic surveys using a helicopter-borne magnetometer.

Subsurface models were also calculated of magnetic structures along six profiles, five of which were constructed From the aeromagnetic map of the quadrangle, and one of which was surveyed by the h e l i c o p t ~ b o r n e m agnetom eter. These models show that the major magnetic anomalies of the Tangle subterrane of the WrangeUia terrane are probably due t o thick tabular bodies of cumulate W a m a f i c rock. The Nikolai Greenstone, which occurs a t the surface, is less magnetic and results in lesser anomalies. The models atso indicate: (1) tha t deep-rooted gani t ic plutons probahly c a w e most of the anomalies of the Slana River subterrane of the Wrangellia terrane; (2) the major pluton in the Lake George subterrane is steep slded and deep seated; and (3) t ha t in the central part of the quadrangle, a large gani t ic batholith trim med by the Denali fault is exceptionally deep rooted (Campbell and Nokleberg, 1985).

Geophysical indications of porphyy deposits

Analysis and interpretation of an aerom agnetic features map reveals plutons and tabular magnetic bodies tha t may be

related t o specific types of mineral deposits. In particular, several U-shaped anomalies were identified, for exam ple, strong, local equidimensional aerom agnetic highs with reentrant or central lows. MulUstage intruaiom, such as porphyry CL- Mo deposits, a re sometimes characterized by this aeromagnetic signatures. The reenwant aeromagnetic low of the U-shaped anomaly may occw over the zone of m o s t intense alteration, and hence, can be wed to define exploration targets (Cunningham and others, I 984). However, U-shaped anom d i e s may arise from several other causes, and fwther m w e , not all porphyry systems have associated anomalles of this shape. For instance, the system must be eroded t o an a p r o p i a t e level for the anomaly t o occw.

Three U-shaped anomalies are of interest for delineating porphyry deposits in the quadrangle. (1) An area in the eastern part of the quadrangle, northwest of the headwaters of Rumble Creek in the Jarvis Creek Glacier subterrane; (2) an area in the southwattern part of t he quadrangle in the Maclaren terrane; and (3) an area in the south-central part of the quadrangle in the Slana River subterrane of the WrangelJla terrane. More detailed geophysical s u r v e p are needed in these areas to determine whether porphyry deposits are fresent.

Certain anomalles are p-obahly not due to pwphyry bodies. The granite of Granite Mountain intrudtng the Jarvis Creek Glacier sub tmane and the gani t ic plutons intruding the Lake George and Macomb subtesranes have complex aeromagnetic signature, which can be explained by variations in susceptibility of and depth t o the sowce rocks without requiring pervasive alteration of magnetic minerals. A strong U-shaped anomaly occurs in the Mount Hajdukovich area in the central part of the quadranglee. Field examination indicates this anomaly is due t o an exhumed radially zoned Fanit io pluton whose core exhibits no alteration.

Geophysical indications d skarn deposlta

The aeromagnetic map (State of Alaska, 1974) also shows several local highs in areas of sedimentary or m etasedim entary rocks. Such geopaphically s m a l l highs, of low t o moderate amplitude, occur in low-magnetic fields. This type of anomaly is often associated with skarn deposits. This type of anomaly can reflect relatively magnetic plutons, t he sowce of the magnetic bighs, t ha t intrude calcareous rocks which are non-magnetic, Com monly, the pluton may not be particdarly magnetic, but the skarn zone may contain strongly magnetic minerals. Generally, skarn deposits are s m a l l , and as a result, aeromagnetic su rvep should be flown a t closer than the one mile spacing, which were used for this assessment, if such deposits are t o be defined by this method. Clearly, not all such anomalies are due t o skarns nor do all skarns exhibit such anomalles. A s a result, this criterion merely indicates areas where plutonic r o c k may intrude and alter cartonate r o c k , The areas with this type of ano m aly are: (1) an area In the southeastern corner of the quadrangle bounded t o the north by the Denali fault and t o the west by a north-trending magnetic Uneam ent in the Slana River subterrane; and (2) an area of known volcanogenic massive sulfide deposits in the central-eastern part of the quadrangle In the Jarvis Creek Glacier subterrane,

M E T H O D O L O G Y A N D C R n E R I A F O R M I N E R A L R E S O U R C E A S S E S S M E N T

Methodology

The method used in thi3 mineral resource assessment ls based on a report of a resowce app-aisal workshop held i n Golden, Colorado, in December 1981, and on the subsequent wwk of Pratt (1981) and colleagues in the Rolla, Missouri quadrangle. This form of assessment was first applied by Richter and others (1 975) in the Nabesna quadrangle. The method consists of the following steps ( 1 ) Compilation of geolodc, geochemical, and geophysical. maps of the quadrangle t o identify the known and inferred geologic environm ents favorable t o mineral deposlts; (2) Determination of types of mineral deposits that could be expected to occur in the quadrangle on the basis of known world- wide associations of' certain mineral-deposit types with

geologic environments and on known mineral types of deposits; (3) Derivation of descriptive modelsfor t he types of mineral deposit; (4) Derivation of recognition criteria each type Of mineral-deposit; ( 5 ) Systematic examination of the available data for the existence of the recognition criteria; (6) Evaluation of the geographic distribution and relative importance of various recognition criteria t o apwaise a low, moderate, os high potential fos undiscovered deposits in specific areas, or t o indicate areas where data are insufficient for a knowledgeable assessment. And, (7) description of grade-tonnage models for well-defined types of d e H B to define the podb le dzes and gadea of undiscovered deposits. Further descriptions of the gade- tonnage models are described by Singer and Mosier (1 983a, b) and Cox and Singer (1 986).

Recognition criteria

Recognition criteria, a s defined by Pratt (1 981 ), are those geologic parameters that affect the favorability for the pesence of' an undiscovered mineral deposit and may be either diagnostic, secondary, or negative. The term "secondary" is used in place of the term "permissive" wed by Pratt (1 981) because b t h diagnostic and secondary criteria can be regarded as permisdve.

Diagnostic criterla are pesent in nearly a l l known deposib and are generally conaidered to be requlred for the wesence of a mineral deposit. Conversely, the known absence of such criteria may either severely l i m i t or definitively rule out the possibility of the p-esence of a deposit. Diagnostic criteria are a favaall le indication tha t a deposit may be wesent, but do not guarantee that a deposit is pesent. For example, Kuroko m asdve sulfide depodts are characterized by rhyolite or dacite being much more abundant than basalt. Thus, the pesence of r h y d t e or dacite in much greater amounts than basalt is a diagnosttc criterion, without which, the existence of such deposits can be ruled out.

General examples of diagnostic cri twia include: (1) a speclfic favorable geologic environment; (2) a known mine, deposit, prospect, or occurrence; (3) a specific geologic relation including stratigraphy and (or) age, petrology, structure, or erosional stage; (4) a specific rock type; (5) a specific geochemical association such a s anomalous concentrations of associated elements in rocks; (6) occurrence of an associated mineral suite; and (7) a specific alteration pattern. The m o s t important recognition criterion is a specffic favorable gealoglc environment. This criterion is used t o initially define an area on a geolodc map with a t Least low potenttal for a particular type of mineral deposit. A l l other recognition criteria, whether diagnostic or secondary, are built upon the exlstance of a specific favoraue geologic environment.

Secondary criteria are those tha t are ~ e s e n t in enough known deposits Chat they may be considered t o favor the presence of a deposit, although they a re not required. Their existence enhances the possibility of a mineral deposit, but their absence does not lessen the posdbjlity. Exam ples of secondary criteria are: (1) a specific geologic relation; (2) a general geoche mica1 association; (3) specific an0 m alous concentrations in rock samples: (4) specific anomalous concentrations in stream-sedi m ent or heavy- mineral- concentrate samples; (5) a specific geophysical anomaly; and (6) pathfinder accessory elem e n h in rock or strea m-sedi m ent 3am ples.

R ecognitton criteria were developed from the descriptions of types of mineral deposits and are described in the fallowing sections and listed in tables 1 through 14 on map sheets 1 through 4. Recognition criteria were developed only for edsting data; for example, criteria for data not obtained are not listed. For some types of deposits, recognition criteria could not be separated into diagnostic and secondary criteria.

The following mineral abb-eviations are used in tables 1-1 4.

a r arseno pyrite gn galena ca cassiterite m o m olybdenite cin cinnabar PY p ~ r i t e

cp chalcopyrite pyrr pyrrhotite e p epidote sh scheelite fl fluorite s p sphalerite

1. GOLD PLACER D E P O m S

(References: Lindren, 1933: Yeend, 1980, 1981a, b: W m e n Y eend in Cox and Singer, 1986)

General description

Gold placer deposits consist of ele m ental gold in gains and rwely nuggets in wavel, sand, snt, and clay and their consalldated equivalents, in alluvial, beach, aeolin, and rarely g ladal de@ts. The m & com mon h a t rocla are alluvial r a v e l and conglomerate with white quartz clasts and heavy minerals tha t are indicative of low-pad8 m e t a m a p h i c r o c k oontaining quartz veins or of quartz veins in the upper-level exposwm of wardtic plutons. Sand and sandstone are of secondary importance. The depoaIts occw in a high-energy alluvial d e m t i o n a l environment where r ad i en t s decrease and river velocities are lower. The m ajcs deposit minerals are gold, sometimss with attached qu&z, magnetite, and (a) i l m enite.

Recognition criteria

1. Geologically favorahle envlronment consisting of stream ~ a v a l s a conglomerates in a reglon containing gold- bearing lode depodts.

2. Known mine, deposit, ~ o s i E d , or occwence. 3. Occurence of gold and cinnabar in heavy- minwal-

concentrate sam ples. 4. Anomalous values of Au in heavy-mineral-

concentrate samples.

AS5EiSment (tahle 1, map sheet 1 )

Areas 1 and 2 in the northern part of the quadrangle, and area 3 in the southwestern part of the quadrangle (sheet 1) are geologically favorahle for undiscovered gold placer depoait4 because they contain stream ravels , conglo m erates, or glaciofluvlal deposits occwring do w natrea m or do w nglacier from areas with anom alow values of Au in bedrock and gold in heavy-mineral-concentrate samples. Areas 4, 6, 7, 9, and 11 (sheet 1) in the south-central and southeastern part of the quackangle are also geological favorable because of containing stream Favels, conglomerates, w glaciofluvial deposits that occw downstream a- downglacier from Tertiary sandstone and conglom erate in the WrangelJla terrane tha t exhibit gold in panned sam ples.

Areas 1 through 4, 9, and 11 are assessed t o have a moderate potential for undiscorwed gold placer deposits, because of containing mines or occurrences, gold in h e a v r mineral-concentrate Sam plea, and (or) anomalous values of Au in heavy-mineral-concentrate samples (criteria 2 tkough 4). Areas 6 and 7 are assessed t o have a low potential because of containing only occurrences of gold and cinnabar in heavy- mineral-concentrate Sam ples (criterion 3). A gradetonnage model suggests that one-half of gold placer deposits contain 1.1 million tonnes or more and gold wades g e a t e r than 0.2 g / t for one-half of the deposits (G.J. Orris and J.D. Bliss i n Cox and Singer, 1986).

2. PLATIUUM P L A C E R DEPOSITS (References: t i ndgen , 1933; Stum pfl and Tarklan. 1976:

W m e n Y eend and N . J Page in Cox and Singer. 1986)

General description

Platinum placer deposits consist of elemental platinum and Pt-woup minerals in wains and rarely nuggets in alluvial, beach, aeolin, and rarely glacial deposits. The most com m on host r o c k are alluvial ~ a v e l and conglomerate with clasts of cum ulate m afic or ultram S i c rock or alpinetype peridotite and heavy minerals indicative of mafic or ultramafic temane. The deposits occur in a high-energy alluvial

depositional environment where gradients flatten and river velocities lessen. The m ajar deposit minerah are Pt-Woup alloys, 0s-It alloys, magnetite, chromite, and (or) ilmenite.

R ecognition trite-

1. Geologically favorahle envlronm ent consisting of stream gave l s or conglomerates in a r e a n containing cum ulate m S i c or fitram afic r o c k OF alpine peridotites.

2. Known deposit, wwpect , or occwence .

Assess m ent (table 2, map sheet 1)

Areas 5. 8, and 10 (sheet 1) are geolo@caUy favorahle for undiscovered platinum placer deposits because of containing stream wavels, congio m eratea, or giaciofluvlal depadts tha t occw downstream or downglacier from cumulate mafic or ultramafic roc& or alplne peridotites, Theae areas are assessed t o have a very low potential f w undiscovered platinum placer deposits because of not containing any known depodts (criterion 2). A wade-tonnage model suggests that onehalf of platinum placer dewsits contain 1.1 m u o n tonne3 or more and at grades of 2.5 g/t or more in onehal f of the deposits (D . A. Singer and N .J Page in Cox and Sing*. 1986).

3- H O T S P R I N G A u DEPOSITS (Reference: B. R. Berger in Cox and Singer, 1 986)

General description

Hot-spring Au deposits consist of finely disseminated gold in subaerial, intermediate volcanic, or volcaniclastic roclcs tha t are extensively silicified and txecciated. The host r o c b are generally dacite and andedte with lesser rhyodacite, rhyolite, or valcaniclastic sedimentary rocla. F in~gra ined silica, particularly chalcedony, and quartz veins occur in the silicified Weccia with gold, pyrite, and S F and As-sulfides. Extensive alteration occurs with formation of siliceous sinter, stoc kw o r b , veins, and cemented treccia usually controlled by a pervaslve fracture system, The ore- depositional environ m ent consists of hot springs in a volcanic pile of an Andean-type arc or in a continental-rift setting. T h i s type of mineral deposit grades downward into epithermal precious- and base- m eta1 deposits.

~ i agnos t i c criteria

1. Geologically favorable environment of dacite and andesite with lesser rhyodacite and rhyolite formed a t or near the surface.

2. Known deposit, prospect, or occurrence. 3. Large amount of felsic shallow-intrusive and

extrusive rock. 4. Extensive areas of strong silification. 5. Brecciated volcanic rock, 6. Disseminated pyrite or rellct disseminated pyrite, 7. H ot-spring deposits.

Secondary criteria

1. Stockw orks formed by abundant quartz veins. 2. Local areas of argiLLc t o advanced arglllIc alteration. 3. Anomalous values of A s , Sb, Ag, or Au in rock

sam ples. 4, Anomalous values of As, Sb, Ag, or Au in stream-

aedim ent Sam ples. 5. Anomalous values of A s , Sb, Ag, or Au in heavy-

mineral-concentrate s a m ples. 6. Occurrence o l pyrite, gold, or cinnabar in heavy-

mineral-concentrate samples.

Assess m ent (table 3, mapsheet 2)

Areas J through N (sheet 2) a re underlain by Tertiary sedimentary and volcanic r o c b and are geologically favorable

fOP undiscovered hot-sp-ing Au deposits. Area J ia assemed t o have a moderate potential for undFscovered deposits because of exhibiting all other diagnostic criteria (3 t kough 71, anomalous values of A s , Sb, Ag, or Au in rock samples (secondary criterion 31, and pyrite, gold, or cinnabar in heavy- mineral-concentrate samplas becondary criterion 6; table 3). Areas K through N are assessed t o have a low potential became of exhibiting only local areas of strong dtification (dlagnosttc criterion 4) and (or) because of exhibiting, only locally, pyrite, gold, a cinnabar in heavy-mineral- concentrate samples (secondary criterion 6; table 3).

A grade-tonnage model for hot-spllng Au depodts b not available. The few w ell-studied deposits contain m ore than 2 million tonnes and a m uch as 90 m u o n tonnes. Gold wades Fobably range fYom 1 t o 6 g/t. Saver may be pesent in Fades higher than gold wades.

4. EPYTHERHAL PRECIOUS- A M D BASE-METAL D E P O m S (References: Lindgen, 1 933; SaUtoe. 1 977; D.P. Cox. 0 .t.

Moaier, Takeo Sato, N.J Page, D.A. Singer, and B.R. Berger in Cox and Singer. 1986)

General deswiption

Epitherm al r e t i o w and base-m etal deposits consist of gold or d v e r in vuggy quartz veins and disseminated in w a l l rock and are associated with abundant pyrite, arsenopyrite, tetrahedvite, and locally sphalerlte, chalcopyrite, enargite. and galena. The hmt roclcs consist of andeslte t o rhyolite flows, ash flows, CufTs, and associated volcaniclastic rocks, sometimes overlying older volcanic sequences or igneous intrusions. Thh deposit type consists of two subtypes, quartz adularia and quartz alunite. Both subtypes may wade upward into hot-sping Au deposits. The quartz-adularia subtype is further divided into t b e e subtypes on the baais of rock type beneath the deposits. Here, all of these types a r e com bined. The ore depositional enwonment is valcanic centers such a s calderas generally with a though-going fracture or fault system in an ~ndean-type arc or subaerial continental-rift setting.

D iagnostic criteria

1. Geologically favoraue environment of a large and thick volcanic field of andesite t o rhyolite flows, ash flows, tuffs, and volcaniclastic rocks locally with interbedded fluvlal or lacustrine sedimentary rocks.

2. Known deposit, pospect, or occurrence. 3. Quartz veins, stockworlcs, or Weccia pipes. 4. Open-space filling in veins and altered areas with

banded veins, vuggy. finegrained crystals, or possibly large zoned crystals.

5. Quartz, adularia, chalcedony, carbonate, barite, and (or) fluorite fillings of veihs and open spaces.

6. Conspicuous wall-rock alteration consisting of extensive replacement by ~ o p y U t i c , sericitic, and argWtic asse m blages and replace m ent by silica, sericite, or alunite, withtn or adjacent t o veins.

7. Disseminated pyrite.

secondary criteria

1. Anomalous values of Cu. Pb, Zn, A s , Sb, Ag, or Au in rock sam pies.

2. Anomalous values of Cu. Pb, Zn, A s , Sb, Ag, or Au in s t rea m-sedim ent Sam plea.

3. Anomalous values of Cu, Pb, Zn, As, Sb, Ag,or Auin heavy- mineral-concentrate Sam plas.

4. 0 ccurrence of gold, chalcopyrite, sphalerite, galena, cinnabar, menopyrite, tetrahedrite or fluorite in heavy- mineral-concentrate sam pies.

Assessm ent and g-ade-tonnage models (table 4, map sheet 2)

The geological favorable areas for undiscovered epithermal precious- and base-metal deposits are the Tertiary sedimentary and volcanic roclcs in the Slana River subterrane

of the Wrangellia terrane (areas J through N , sheet 2, table 4). Area J Is assessed to have a low potential for undiscovered deposits because of exhibiting quartz veins, stockworb, or lreccia pipes (diagnostic criterion 3). C O ~ S ~ ~ C ~ O U S w all-rock alteration and dtsse m inated pyrite (diagnostic criteria 6 and 7), and cinnabar and chalcopyrite in heavy- mineral-concentrate sam ples (secondary criterion 4; table 4). Areas K th'ough N are assessed to have a very low potential because of exhibiting no secondary critwia, or only locally anomalous values of C u In rock Sam ples (secondary criterion 1) and or cinnabar, epidote, sphalerite, and galena in heavy- mineral-concentrate s a m ples (secondary criterion 4; table 4).

Grade-tonnage m odds for the quartradularia and quartz-alunite subtypes of epithermal p e c i o u s and b a s e metal deposrts are published by D.L. M c e i e r and W .D. Menzie in Singer and M osier (1 983a) and Cox and Singer (1 986). The data suggest tha t if quartz-adularia type deposits exlst in the quadrangle. then one-half would contain 690.000 tonnes or more. whereas the quartz-alunite type would contain 460.000 tonnes or m me.

For the quartradularia deposits, gold grades range Worn 4.3 g/t or more for the richest half of the deposits to 19 g/t or more in the richest tenth of the deposits. Sflver wades vary fPom 130 e/t or more in t he richest half, while ten percent of the deposits contain 600 g l t a* m we. Copper and zlnc Fades are low. Silver wades tend t o be higher in the quartradularia type. Lead and zinc a r e wesent in some quartradularla deposits. Copper is p-esent in less than o n e half of the deposits. For the quartpalunite deposits, gold grades range from 5.6 g/t or more for the richest half of the deposits t o 12 g/t or more in the richest tenth of the deposlts. Silver grades vary Porn 13 ut or more in the richest half, while ten percent of the deposits contain 62 g/t or more.

5. G O L D Q U A R T Z V E I N DEPOSITS (References: Clark, 1969; Boyle, 1961; B.R. Berger in Cox

and Singer, 1986)

G eneral description

Gold quartz vein deposits consist of gold in veins of massive quartz, sometimes with minor pyrite and arsenopyrite. Gold quartz vein deposlts, termed low-sulfide Au quartz vein deposits by Cox and Singer (19861, a r e generally hasted in reens tone beLtp-regionally m etam orphosed and penetratively deformed oceanic strata, including graywacke, shale, and chert tha t are intruded by pani t ic plutons. Grade of metamorphism is usually greenschist facies. The ore depositional environment consists of a mobile belt of accreted terranes along a continental margin, sometimes associated with an Andean-type volcanic arc and associated batholith.

D iagnostic criteria

1. Geologically favorable environment of regionally m etamwphosed and penetratively deformed grayw acke, shale, or chert intruded by gani t ic plutons.

2. Known deposit, prospect, or occurrence. 3. Greenschist facies regional m etam 0rphism. 4. Quartz veins, with or without with F e c a r b n a t e ,

pyrite, arsenopyrite, and base- m etal sulfides.

Secondary criteria

1. Intrusion of calc-alkaline plutons during or just after regional meta q orphlsm and penetrative deform ation.

2. Quartz-vein emplacement along major faults, shear zones, axial planes, and fold axes.

3. Anomalous values of A s , Sb, Cu, Mo. W , Au, Ag, or H g in rock samples.

4. Anomalous values of A s , Sb, Cu, Mo, W , Au, Ag, or Hg in stream-sediment Sam ples.

5, Anomalous values of As, Sb, Cu, Ma, W , Au, Ag, or H g in heavy mineral-concentrate s a m plea.

6. Occurrence of gold, pyrite, or arsenopyrite i n heavy-

Secondary crlteola

1 . A m ydgules in m etabasalt with chlorite, chalcedony, quartz, epidote, zeolites, calcite, C u-sulfides, and rare native copper.

2. Anomalous values of Cu, Pb, Zn, or Ag in rock sam ples.

3. Anomalous values of Cu, Pb, or Ag in stream- sedim ent sam ples.

4. Anomalous values of Cu, Pb, Zn, or Ag in heavy- mineral-concentrate samples.

5. Occurrence of chalcoclte, bornite, CoveUte, galena, sphalerlte, cr pyrlte in heavy-mineral-concentrate samples.

Assess m ent (table 7, map sheet 3)

The geologically favorahle areas for undiscovered Kennecott Cu-Ag d e p m t s are parts of the S a n a River and Tanae subterranes of the WrangWla terrane where the Nikolai Greemtone is overlain by Triassic limestone. The only area where this relation occurs is in the eastern part of t he Slana River subterrane (area F, sheet 3, table 7). Farther west in the Tangle subterrane, Trlassic limestone is faulted against other bedrock units. Area F is assessed t o have a moderate potential for undiscovered deposits becatise of exhibiting lower greenschist-facies m etam wphism and quarteCksulFlde veins in metabasalt (diagnostic criteria 3 and 5; table 7). In addition area F exhibib amydgules in m etabasalt with various sLUcate and C wsulfide m i n e r a , anomalous values of Zn and Ag in heavymineral-concentrate samples, and chalcopyrite and pyrite in heavy-mineral- concentrate samples (secondary criteria 1, 4. and 5; table 7). A lack of abundant deposits yecludes CO~SWuCtiOn of a pade-tonnage model.

8. K U R O K O M A S N R S U L P I D E D E P O m S (R efePence.9: Lam bert and Sato, 1974; Scott, 1980; Franklin

and others, 1981 ; D. A. Singer in Cox and Singer, 1986).

General description

Kuroko massive sulfide deposits consist of C w, Pb-, and Ztr-sulfides tha t occur in submarine volcanic rocks of interm ediate t o f &c com position containing lesser m afic volcanic rocks and locally abundant sedimentary rocks. The volcanic rocks occur as flows, ash flows, tuffs, breccia , and in some cases in feLsic domes. The ore depositional. environment is mainly hot s p i n @ related t o marine volcanism in island-arcs or extensional rifting regimes. The deposit minerals include pyrite, chalcopyrite, sphalerite, and Lesser galena, tetrahedrite, tennantite, and magnetite. Local zeolite, clay, sericite, chlwite, and silica alteration may occur.

D iagnostic Criteria

1. Geologically favorable environment of subm arlne volcanic rock of intermediate t o felslc, generally c a l c alkaline cam position, and associated tuffs, keccias, and sedimentary rocks.

2. Known deposit, prospect, or occurrence. 3. F elsic pyroclastic deposits. 4. SlLiceous chemical sedimentary rocks. 5. H ydrotherm ally altered volcanic rocks.

Secondary criteria

I. Primary barlte or gypaum in volcanic or sedim entary rocks.

2. H ydrother m a1 alteration along a narrow stratigaphlc interval.

3, Anomalous values of Cu, Pb, Zn, As, Ag, Au, Sn, or Sb in rock Sam ples.

4. Anomalous values of Cu, Pb, Zn, Ag, Au, Sn, or Sb in stream-sediment sam ples.

5. Anom alow values of C u, Pb, Zn, A g, Au, Sn, or Sb in heavy- mineral. concentrate samples.

6. Occurrence of chalcopyrite, sphalerlte, galena, arsenopyrite, tetrahedrite, a- pyrite, in heavymineral- concentrate s a m ples.

A s s e s s m ent and gfadrtonnage m ode1 (table 8, map sheet 2)

North of the Denali fault, the geologically favorahle areas fo r Kwoko m &ve sulFide deposits are the submarine metavalcanlc r o c b of the Jarvis Creek Glacier and Hayes Glacier subterranes (areas A through F, sheet 2, table 8) of the Yukon-Tanana terrane. South of the D e n d fault, the gealogically favorahle areas are t he Tetelna Volcanics and g a n a Spur Formation of the Slana River s u b t m a n e of the WrangeYJia twrane (areas G through I, sheet 2, tahle 8).

Areas A and D a re asaessed t o have a high potential for unttlscovered deposits because of exhibiting moderately abundant known deposits, prospects, or occurrences, felsic pyroclastic deposits, and siliceous chemical sedimentary rock (diagnmtlc criteria 2, 3 and 4; table 8). In addition, these areas exhibit rellct hydrothermal alteration, anomalous values of appopsiate dements in rock, stream-sedi m ent. and heavy-mineral-concentrate samples, and basem etal sulfides in heavy-mineral-concentrate samples kemndary ctiterla 2 tkough 6; table 8).

Areas B, E and G are assessed t o have a moderate potential because of exhibiting few, if any known d e p d t s , pcepects, or occurrences, siliceous che mica1 sedim entary rock (diagnostic criteria 2 or 4; table 81, and few anomalous values of apwowiate elements in rock, stream-sediment and heavy- mineral-concentrate sam ples, and few base-m etal sulfldes in heavy- mineral-concentrate sam ples (secondary criteria 3 through 6; table 8).

Areas F, H, and I are assessed t o have a low potential because of exhibiting only a few anomalous values of a p p o p l a t e elements in rock, stream-sediment, and heavp mineral-concentrate sample, and a few basem etal sulfides in heavy- mineral-concentrate Sam ples (secondary criteria 3 through 6; tame 8).

Area C is assessed t o have a very low potential because of a few ano m alous values of appopriate elem ents in stream- sediment samples.

A gradetonnage m o d d was prepared by D . A . Singer and D.L. Moaier in Cox and Singer (1 986). The plotted grades and tonnages of the pototype deposits demonstrate tha t if K uroko m asdve $ a i d e deposits exist in the quadrangle, then onehalf of the deposits should contain 1.6 million tonnes or more and the largest tenth of the deposits contain 1 9 m u o n tonnes or more. Fifty percent of the deposiiz have average copper grades of 1.3 percent or more and the rlchest tenth have a t leas t 3.5 percent copper. Average zinc wades of 2 percent or more occur in onehalf or more of the deposits. The richest tenth contain a t leas t 1.9 percent lead. Precious metals are reported in over half the deposits with the tichest tenth having a t leas t 2.3 g/t gold; the median silver ~ a d e is 11 B/t whereas 10 percent of the deposits contain 98 g/t or m ore of sl2ver.

9. P O R P H Y R Y Cu(AU-Ag) DEPOSITS ( R eferences: Titley, 1 975; Sutherland Brown, 1 976; C olley

and Creenbaum , 1980; D.P. Cox i n Cox and Singer, 1986).

General description

Porphyry Cu(AkAg) deposits conaist of chalcopyrite, bornlte, or pyrite, and minor m oly bdenite, sphalerite, galena, or arsenopyrite in stockw ork veinlets in hydrother m ally altered, shallowly em placed porphyry and adjacent country rock. The granitic host rocks include quartz diorite t o quartz monzonlte, syenite, and small, hypabyssal andeslte to rhyodacite, and trachyte s t o c k , dikes, and sills. Local disseminated and massive sulfide minerals may occur in coeval volcanic rocks, along with quartz veins, and dikes with sulfide minerals. The ore depositional environment con-ts

of epizonal intrwive roc& with abundant dikes, breccia pipes, and cupolas of batholiths tha t a re intruded t o shallow levels in either an island-arc or Andean-type arc setting. In this study, porphyry C U ( A W A ~ ) deposits include associated poly m etallic vein deposits.

Diagnostic criteria

1. Geological favorable environment of calc-alkallc and alkalic porphyritic granitic plutons and stocks and (or) stocks, dikes, or sills of andesite t o rhyodacite or trachyte.

2. Known deposit, praspect, or occurrence. 3. Coeval coeval granitic, hypabyssal, and (ot) volcanic

roc b. 4. Hydrothermal alteration in and adjacent t o i n t r d v e

rocks.

Secondary criteria

1. M asslve sulfide minerals in volcanic rocks or in skarns fcrm ad in carbonate layers in volcanic pLle.

2. Anomalous values of Cu, Pb, Zn, As. Mo. Ag, or Auin rock samples.

3. Anomalous values of Cu, Pb, Zn, As, Mo, Ag, or Au in stream-sedim ent Sam ples.

4. Anomalous values of Cu. Pb, Zn, A s , Mo, Ag, or Auin heavy- mineral-concentrate s a m ples.

5. Occurrence of chalcopyrite, bornlte, pyrite, m alybdenite, sphalerite, galena, arsenopyrite, rn agnetite, m onozlte, and thoti te in heavy mineral concentrate Sam ples.

6. Local equidimenslonal aeromagneuc highs, particularly with reentrant ot central lows.

Assessment and gradetonnage model (tahle 9, map sheet 2)

The geologically favwahle areas for unundiscovered porphyry Cu(Au-Ag) deposits are the Tetelna Volcanics and S a n a Spur Formation in the Slana River subterrane of the Wrangellia t a r a n e (areas G-1 through G 3, H , I, sheet 2, table 9). These units a re t he only ones in the quadrangle tha t are intruded by hypabyssal granitic plutons in this case, shallow- level. andeslte t o dacite stocks. dikes and sills.

Areas G 2 and I are assessed t o have a high potential for undigcovered deposits because of exhibiting known deposits, Fmpecta, or occurrences, coeval granitic r an i t i c , hypabyssal, and volcanic rocks, and hydrothermally altered intrwfve rocks (diagnostic criteria 2 through 4; table 9). In addition, these areas exhibit massivesulfide layers in volcanic rocks, anomalous values of appropriate elements in rock, stream-sedim ent and heavy- mineral-concentrate samples, b a s e m etal sulfides in heavy- mineral-concentrate Sam ples, and a local, equidim endonal aerom agnetic high (secondary criteria 1 through 6; table 9).

Areas G I and H are assessed to have a moderate potential because of exhibiting m a s t of the same diagnostic criteria as above ( 2 through 41, but fewer anomalous values of appropriate elements in rock, stream-sediment, and heavy- mineral-concentrate samples, and few er b a s e m e t a sulfides in heavy- mineral-concentrate s a m ples, and local equidimensional aeromagnetic highs (secondary criteria 1 through 6; table 9).

Area G 3 i s assessed t o have low potential because of exhibiting only coeval granitic, hypabyssal, or volcanic rocks and hydrotherm ally altered igneous roclcs (diagnostic criteria 3 and 4; table 9), and locally chdcopyrdte in heavy-mineral- concentrate sarn ples and an appropriate aerom agnetic high (secondary criteria 5 and 6; table 9).

A grade-tonnage rn odel was prepared by D . A . Singer and D.P. Cox in Cox and Singer (1986) for porphyry Cu-Au deposits. The plotted gades and tonnages of the pototype depodts demonstrate tha t if porphyry C u-Au deposits exist in the quadrangle, then onehalf of the deposits should contain 150 m u o n tonnes or more, and the largest tenth of the deposits contain 91 0 million tonnes w more. F i f t y percent of the deposits have average copper wades of 0.50 percent or more, and the richest tenth have a t least 0.71 percent copper. Average molybdenum gl-ades of 0.0036 percent O r

m ore occur in onehalf or m ore of the deposits. Gold grades of 0.31 g/t occur in onehalf or more of the deposits, and the richest tenth have a t leas t 0.64 g/t.

10. P O R P H Y R Y Cu-Mo DEPOSITS (References: ~ o w m and Guilbert, 1970; Sutherland Brown,

1976; White and others, 1981 ; D.P. Cox in Cox and Singer, 1986)

G enerat description

Porphyry C w M o depodts consist of pyrlte with lesser chalcopyrite and molybdenite, and minor sphalerite or galena. The sulfides occur in stockwcsk veinlets in porphyritic gani t ic rocks or hypabyssal intrusive rocks or in wall rocks adjacent t o t he igneous rocks. The intrustve rocks include quartz diorite t o granite plutons or andesite t o rhyolite stocks. Local replacement sulfide bodies may occur in coeval volcanic rock or in older w a l l rocks, some t ime assodated with quartz veins or dikes tha t also contain sulfide minerals. Associated alteration consists of sodic. potas ic , phyllic, argilllc, and p-opylitic types. The m e depositional environment consists of shallowly emplaced granitic plutons in either an island arc, Andeartype arc, or a rifted continental setting. The areas of favorable environment are either surface outcrops of gani t ic rocks, or areas adjacent to granitic rocks where geophysical data indicates favorable areas in the subsurface. In this study, porphyry CLI-Mo deposits include associated palym etallic vein deposits.

Diagnostic criteria

1. Geologically favorahle environment of plutons of generally porphyritic quartz diorlte t o quartz monzonite or hypabyssal stocks of andesite t o rhyolite,

2. Known deposit, praspect, or occurrence. 3, Coeval shallow-granitic, hypabyssal, or volcanic

rocks. 4. N um erous faults and iyecciated country rock: 5, Intrusion of igneous rocks controlled by regional-

scale faulting. 6. H ydrother m a1 alteration.

Secondary criteria

1. El ultiple intrusive phases, some porphyritic. 2. Volcanic or inbusive Ereccias, locally with

disseminated or m a v e sulfides, 3. Dikes, quartz veins, or stockwork veinlets with

sulfide minerals. 4. Replacement massive sulfide minerals or skarns In

country rock. 5. Breccia pipes locally with sulfides. 6. Anomalous values of Cu, Mo, Pb, Zn, As, A u , OP Sb in

rock sa m ples. 7. Anomalous values of Cu, Mo, Pb, Zn, Ag, Au, or Sbin

strea m-sedi m ent sam ples, 8. Anomalous values of Cu, Mo, Pb, Zn, Ag, Au, or s b in

heavy- mineral-concentrate sarn ples. 9. Occurrence of chalcopyrite, malybdenite, pyrite,

sphalerite, galena, scheeLitepoweVLte, and (or) fluorite in heavy- mineral-concentrate samples.

10, U-shaped aeromagnetic anomaly patterns, for exam ple, strong, local equidim ensional aerom agnetic highs with reentrant or central lows.

Assessm ent and grade-tonnage m odel (table 10, map sheet 4)

The geologically favorable areas for undiscovered porphyry Cu-Mo depodts w e granitic plutom throughout the quadrangle (areas A through F , G 1 through C4, H I through H4, 1 through K , L 1 t hough L4, and M through R , sheet 4, table 10). North of the Denall fault, abundant granitic plutons occw in the Lake George, M acom b, and Jarvis Creek Glacier subterranes of the Yukon-Tanana terrane and the Aurora Peak terrane (areas A , C , E , G, H , sheet 41, and south of the Denali fault, in the Maclaren terrane and the Slana

River subterrane of tkie WrangellIa terrane (areas J, N , 0, sheet 4). For a detailed assessment, areas G , H , and L are divlded into subareas (sheet 4, table 1 O),

Areas K and M , s m a l l isolated granitic plutons in the Wrangellia terrane, a re assessed t o have a high potential for undiscovered deposits because of exhibiting known deposits, pospect;s, or occurrences; coeval gani t ic , hypabymal, or volcanic r o c b , numerous faults and kecciated counwy r o c k or hydrothermal alteration (diagnostic criteria 2 through 4 and 6: table 9). In addition, areas K and M exhibit anomalous values of app'opiate ele m en- in rock, speam-sedi m ent, and heavy- mineral-concentrate sam ples, and base-m etal sulfides in heavy mineral-concentrate sam ples (secondary criteria 6 through 9; table lo).

R dat ive t o areas K and M with high potential, areas E , G4, H2, I. L1, L4, and N are ase&.ed to have a moderate potential because of exhibiting fewer diagnostic criteria. mainly coevai granitic, h y p a b p a l , c? volcanic rocks; numerous faults and keccia ted country rock; and hydrothermal alteration (diagnmttc criteria 3, 4, and 6). In addition, these areas relative t o areas K and M , exhibit fewer secondary witem, for example, a moderate number of anomalous values of a p p o m a t e elements in rock, stream- sedim ent, and heavy- mineral-concentrate Sam ples, and a m oderate am aunt of b a s e m eta1 sulfides in heavy- mineral conceritrate samples (secondary criteria 6 through 9) (table 10).

Relative t o the areas with moderate potential, areas A- D , F, GI-G3, HI, H 3 , H4, J, L2. L3, and 0-R are a s s w e d t o have only a low potential because of exhibiting fewer and sparger diagnostic crlteria, mainly s m a l l and sparse areas with coeval granitic, hypabyssal, or volcanic rocka, and numerous fault4 and becciated country rock (diagnostic criteria 3 and 41, and fewer and sparser secondary criteria, mainly a few anomalous values of appo!Aate elements in rock, stream-sediment, and heavpmineral-concenwate samples, and a few basemeta l sulfldes in heavpmineral- concentrate samples becondwy criteria 6 through 9; table 10).

A pade-tonnage model for porphyry Cu-Mo deposits was pepared by D . A . Singer, D.L. Mosier, and D.P. Cox in Cox and Singer (1986). The plotted pades and tonnages of the prototype deposits demonstrate tha t if porphyry Cu-Mo deposits exlst i n the quadrangle, then onehalf of the deposits would contain 140 million tonnes or more, and the largest tenth of the deposits would contain 1,100 million tonnes or more. F f t y percent of the deposits have average copper grades of 0.5 percent w more, and the richest tenth have a t least 1.0 percent copper'. The richest tenth contain a t least 0.03 percent molybdenum and having a t leas t 0.4 g/t gold; 10 percent of the deposits contain 2.5 g/t or more of silver. For porphyry Mo deposits (W.D. Menzie and T.G. Theodorein Cox and Singer, 1986), one-half of the deposits would contain 93 million tonnes or more, and the largest tenth would contain 630 million tonnes or more. Fifty percent of porphyry M O deposits contain 0.084 percent or more molybdenum. The richest tenth contain 0.1 3 percent molybdenum.

11. U-MO A N D CwZn-PbSKARN DEPOSITS (References: Einaudi and others, 1981 ; D.P. Cox and T.G.

Theodore in Cox and Singer, 1986).

General description

W-Mo and CpZn-Pb skarn deposits consist of various com binations of schedl tepow ellite, m alybdenite. chalcopyrite, bornite, sphalerite, galena, pyrite, pyrrhotite, and (or) magnetite with accessory menopyrite, tetrahedrite, gold, or other ore minerals that occur in contact m etasom atized calcareous rocks or in nearby m etasom atized pant t ic r o c k . The contact metasomatic r o c k or skarns are generally adjacent to granitic plutons ranging in corn position from quartz diorite t o granite. The extent of replacement of calcareous roc& varies from a few meters t o a few hundred meters away from the r a n i t i c rocks. The extent of replacement ki highly variable and often is controlled by fractures, faults, and folds. Skarns corn monly exhibit a

complex mineralogic zonation. Replace m ent minerals and CexCWeS are Often extremely varied, with the most common minerals being andraditegossulari te garnet, diopside- hedenbergite clinopyroxene, w ollastonite, epidote, idocrase, hornblende, quartz, fluorite, white mica, and chlorite. The ore depositional environment conststs of granitic plutons tha t intrude either continental shelf sedimentary r o c b in an Andean-type arc setting a platform or oceanic sedimentary rocks in an island-arc setting.

Diagnostic cri twia

1. G ealogically favorahle environrn ent of calc-alkame plutonic rocks intruding calcareous or impure calcareous sedim entary r o a b

2. Known deposit, prmpect, or occwence. 3. Replacement of calcareous wall rocks by irregular

q asses of contact m etasom atic minerals, including andradite- gossularite, d iopaidehedenber~te , hornblende, w ollastonite, epidote, actinollte, idowase, and quartz.

4. Bleaching of calcareous w a l l rocla. for example, disappearance of gaphi te and local stllcification.

Secondary criteria

1. Abundant fractures, folds, or faults in calcareous sedimentary rocks.

2. Replacement of gan i t l c rocks adjacent t o calcareous sedimentary r o c b by andradite-grossularite, diopslde hedenbagite, epidote, hornblende or actinollte, chlorite, calcite, or quartz.

3. H ydrother m a1 alteration of plutonic roc&. 4. Anomalous values of W , Mo, Cu, Pb. Zn. Ag. Au, or

Sn in rock samples. 5. Anomalous values of W , Mo. Cu, Pb. Zn, Ag, Au, or

Sn in stream-sediment samples. 6. Anomalous values of W , Mo, Cu, Pb, Zn, Ag, or ~ u i n

heavy- mineral-concentrate samples. 7. Occurrence of scheellte-powelUte, malybdenite,

chalcopyrite, bornite, sphalerite, galena, pyrite, arsenopyrlte, gold, or fluorite in heavy-mineral concentrate samples,

8. Local aero m agnetic h igh, particularly geographically s m a l l highs of low t o moderate amplitude in regions of otherwbe low-m agnetic Pields.

Assesam ent and grade-tonnage m ode1 (table 1 1, map sheet 4)

The geologlcay favorable ateas f w undiscovered W - Mo and C u-Zn-P b skarn deposits are carbonate rocks intruded by gani t lc plutons in the Macomb and Jarvls Creek Glacier subterranes of the Yukon-Tanana terrane and the Aurora Peak terrane north of the Den& fault (areas A , C , HI t bough H4, sheet 4, table 1 I), the East Susltna batholith of the M adaren terrane, and the Wrangellla terrane south of the Dena.li fault (areas J, L1 th~ough L4, P-R, sheet 4, table 11). For a detailed assessment, areas H and L are divided into subareas (sheet 4, table 11 1.

Area L 2 is assessed t o have a high potential for undiscovered deposits because of exhibiting known deposits, pOspects, or occurrences, skarn masses, and bleaching of calcareous wall r o c k (diagnwtic criteria 2 through 4; table 11). In addition, area L 2 exhibits abundant structures in calcareous sedi m entary rocks, anomalous valuas of appropiate elements in rock and stream-sedim ent sam @as, and oddes and base-m etal sulfides in heavy-mineral- concentrate samples, and an app-opiate aero m agnetlc signature (secondary criteria 1, 4, 5. 7, 8; table 11).

Relative t o area L2, areas A , C , H2, H3, L1, L3, L4, and P-R are assessed t o have a moderate potential because of exhibiting fewer and very sparse diagnostic criteria, mainly two sites of sillcate skarn minerals, and one si te of bleaching of calcareous walt rocks (diagnostic criteria 3 and 4; table 11). In additton, these areas exhibit, relative to area L2, fewer secondary crlteria, mainly sparse anom alow values of appopriate elements in rock, strea m-sedim ent, and heavy- mineral-concentrate s a m ples, sparse oxides and base- m etal sulfides in heavy-mineral-concentrate samples, and in some

areas an app.op-iate aeromagnetic signature becondary criteria 4 through 8; tahle 1 1 ).

Relative to the above areas with moderate potential. areas HI. H4, and J are assezaed to have a low or very low potential became of exhibiting only a geologically favoralUe area (diagnostic criterla 1). In addition, these areas exhibit very few and s p m e secondary criteria, for example, rare ano q alom valuas of app-oprSate elem ents in rock, stream- sediment, and heavy mineral-concentrate samples, sparse oddes and base m eta1 sulfides in heavy- mineral-concentrate samplas, and an app-oflate aeromagnetlc signatwe (secondary criterla 4 tbough 8; tahle 1 1 1.

Grade-tonnage models were pepared for Cu and W * akarn deposits by G.M. Jonm and W .D. M enzle in Cax and

Singer (1986). The plotted grades and tonnages of the pototype deposits demonstrate that if Cu skarn deposits exist in the quadranae, then onehalf would contain 0.6 m u o n tonnes or more, and the largest tenth of the deposlts contain 9.6 million tonne$ or more. Fifty percent of the deposlts have average copper gadea of 0.7 percent or more. and the richest tenth have at least 4.0 percent copper. The. richest tenth contain a t least 2.2 g/t gold and 34 B/t snvw. For W-skarn deposits, Onehalf of the deposits would contain 1.1 million tonnm w mae. and the largest tenth should contain 22 million tonnes or mae. Fifty percent of the deposits have average tunmen wades of 0.67 percent W 0 or more, and the richest have at least 1.4 percent W 0 3.

12. P O R P H Y R Y Sn D E P O m S (References: M ulligan, 1974; Smith and Turek, 1976; B.L.

Reed in Cox and Singer, 1986).

General descsi@ion

Porphyry Sn deposits consist of ctisseminated cassiterite and accessory tow maline, topaz. and w hite mica in the upper, highly altered parts of leucocratic quartz monzonlte CP rani te . The hhst granitic rocla are generally intensely hydrothermally altwed to various corn binations of K-feldspar. albite, sssidte, chlorite, quartz. topaz. towmaline, and fluorite. The ore dewdtional environment consist4 of intrusion of SUiceolls granitic rocks into a continental fold belt of thick platform rock$ with minor volcanic rocks. This deposit type may be assodated with Sn-gmisen deposits. However, no weisen occmences were observed in the field, either because of poor exposwes i n geologically favorable areas or because of a lack of occurence.

Diagnostic crlteria

1. Geolodcally favorable envbonment of granite lntruded into continental plaWorm sedimentary rocks.

2. Known deposlt, pospect, or occurrence. 3* Continental fold belt of thick platfmm sedimentary

r o c k and minor volcanic rocks, 4. Epizonal m ultlphase stocks of granitic roclcs.

Secondary crlterla

1. U ppeplevel cupolas and roof zones of plutons. 2. Locally exbenaive alteration in granitic rocks

consisting of replacement K-feldspar, albite, sericite, chlorite, fluorite, or arsenopyrite.

3. Postorogenlc intrusion of eranltdc rocks. 4. Associated tin weisen. 5. Associated tin placer deposits. 6. Anomalous values of Sn, Mo, As, or W i n rock

sa m ples. 7. Anomalous values of Sn, Mo, A s , or W in stream-

sedlm ent sam ples. 8. Anomalous values of Sn, Mo, As, or W in heavy-

minwal-concentrate samples. 9. Occurrence of cassiterite, fluorite, molybdenite, or

arsenopyrlte in heavy- mineral-concentrate sam ples.

Assessm ent (tahle 12, map sheet 4)

The geologically favorable areas for undiscovered pcrphyry Sn deposits are wanitlc plutons intruding falded, continental platform sedimentary rocks in the Macomb and Jmvis Creek Glacier subterranes of the Yukon-Tanana terrane and the Aurora Peak terrane north of the Den& fault (areas A. D, and F-H, sheet 4 , table 12). For a detailed assessment, areas G and H are divided into subareas lsheet 4, table 12). A gradwtonnage model is not available.

Areas A , D , F, and H2-~4 are assessed to have a moderate potential for unrncovered depoaits because they exhibit folded continental-platform sedimentary rocks or local epizonal ar mul'dphase granitic rocks (diagnostic witeria 3 and 4; table 12). In addition, most of these areas exhibit locally extensively altered granitic r o c h (secondary d t e r i a 2 ) (areas A, D , and F). Although all areas show anomalous values of appropriate elem enta i n rock, stream- sedl m ent and hgavy- mineral-concentrate sam flles, and oxldes and base- m eta1 sulfldes in heavy- mineral-concentrate Sam plm (secondary criteria 6 through 9; table 12).

Relative to the above areas with moderate potential, areas G 1 twough G4 and H 1 are assessed to have a low potential because of exhibiting fewer and sparser diagnostic criteria, m alnly folded continental-platform seam entary rock , and in a few areas, epizonal panitic rocks (diagnostic criteria 3 and 4: table 12). In addition, these areas exhibit iewer and spamer secondary criteria, mainly, local areas of a few anom alow values of app.op-iate elements in rock, stream-sedim ent and heavy-mineral-concentrate sam ptes, and a very Few oxides and sulfides in heavy mineral-concentrate samples (secondary criteria 6 tbough 9; table 12).

13. G A B B R O I C Ni-CU DEPOSITS (References: Naldrett, 1981; Ross and Travis, 1981; N.J

Page i n Cox and Singer, 1986)

General description

Gabtroic N i - C u deposits (adapted from synorogenic- synvolcanic Ni-Cu deposit of Cox and Singer, 1986) consist of pmhotite, pentlandite, chalcopyrite, platinum-roup minerals and accessory pyrite that occur mainly as &eminations and 1- m Mvesu l f ide lenses in large sills of cum ulate m afic and ultram afic r o c k and i n am alzer dikes, sills, and masses of gablrcns and norites. The mafic and ultramafic rocks generally intrude greenstone belts and are locally intemely deformed and m etam orphceed. The host rocks consist of various combinations of olivine-pyroxene cum ulates, plagloclase-pyroxene cum flat=, cv olivine- plagioclase aumulates, gabbro, and norite. The ore depositional environm ent consists of moderate to large bodies Of cum d a t e mafic and ultram afic rocks, and gabbro or norite dtkes and qKI3 intruded into geenstone b e l b , possibly associated with rWng, followed by a period of accretion, deform ation, and regional-grade m etam orphlsm .

Diagnostic criteria

1. C ealoglcally favorable envlronm ent of cum ulate m afic or ultram &ic rock and gabbro or norite dikes and sills intruding ar associated with greemtone belt.

2. Known deposit, prcepect, or occurrence.

Secondary criteria

1. Anomalous values of Cu, Ni, or Co in rock sarn ples. 2. Anomalous values of Cu, Ni, or Co in stream-

sediment a m plefa. 3. Ano m alow values of C u, N i , or C 0 in heavy- mineral-

ooncentrate Sam plm. 4. Strong aeromagnetic gadient or high.

Assessm ent and gadetonnage m ode1 (table 1 3, map sheet 1)

The geologically favorable areas for undiscovered gabtroic Ni-Cu deposits are intrusive gabtrce, diabases, and cum ulate m afic and ultram afic rocks in the Slana River and Tangle subterranea of the Wrangaa t m a n e (areas A-E,

sheet 1, table 13). These rocks are interweted as being comagm +'fc with the magma8 that alBa farmed tho #ubmarlna and subae, -1 baaalrs of the Uppsv 'TPlsaaia Nlk~lrl Greenstone.

Areaa A artb D are &damned t13 have 4 modr~ i r t r potential Por undlnooversd depoelta bsaauao af wrhiUting R known p-oapeot nr oooirrenQe (dik4~noati0 oriterlon 21 tabla 13) and beomuse of rxhlblWng anamdaue vPus8 o f CU, N1, or C o i n rock, atreem-nedi m ent, or hoavy- mlnersll-oan~11ntr61L61 8ampLea and a strong aerom~gnet lo psdient ar Mgh beoondary oriteria 1 through 41 tahlc 13).

Areas 8, C , and E are PmIWad t o have a low pobentlal beoauae or exhibiting only n ravorabla gsulog l~ Invironmsnt (diagnostlo ordtrrlon 1 I tahle 13), end beasuae oF rrxhlbiUnp rew and rparoe 8no m aloun valuon d C u, Ni, op C B i n rook, atream-aedim ant, and heavy- miner l l -oonoant~~ts san flee and a strong reromagnrtlo gradient or high beoondrry criteria 1 through 4; table 131, Ths awomagnrtio aurvmy (Stat@ a? Aleska, 1974) indioeter that the oum ulate ultram nElo r o o k i n weaa 0 and C are pmmt at ohallow d r p t h and Farm u oontlnuoua U-shaped band open t o the wwt. A r r w paripherol to areas 0 and C may alm have low potantid far grbffolo N1- C u depoelts.

A wade-tonnage model w a n wepared by D , A , Slngar, N.J Page, and W . D . Msnzia i n Cox and 8ingw (1986). The plotted wades and tonnaps of the pototypcr dspedb demonatrate tha t If gabbroic N1-C u bynvaloant~nynorogonla Ni-Cu) depoelta e d a t i n the quadrangle, thsn one-hrlf of the depadta should oontaln 2.1 mllllon tonnw or rime, and the largest tenth should contain 17 m u o n tonnm or more. F i f t y pwoent of the depoalFs have avwagr coppw grade4 of 0.47 peroent cr awe , and the richest tenth have st 1rMt 1.3 percent copper. F l f t y peroent o f the depolrlta have avoraga nickel graded of 0.77 percent a* more, and the riohsat t r n t h have a t least 1.6 percent n l c k l . Cabalt, gold, and the pLatinum-roup elements are r e a s n t i n some aP throe deposits.

14. P O D I F O R H C H R O l l K T E DEPOSlT (Refwencm: NaLdrett and Cabrl, 19761 J.P. ALberr i n Cox

and Slnger, 1986)

General desalpt lon

Podlfwm chromlte dapnclita c o m b t o f chramitx ilf14 accemwy p la t i num-~oup mlnwals that we occw I n ~dlik m a m m l n ultramafic r a c k t ha t I n evms oasw we Nghly d e l f f m sd and rn &am o r ~ o a a d , The hoat r o c k indude dunlte and hazbu@ta, assocLatcJd maClc i g n w w rocla, and cumulate e afic and ultram dlc rocks, spa etimes sxtsnaivaly eerpentlnlised. The w e dapmdtlonal environment a a W t s of t ea tmized ultramxic roak formed I n the basal p&7 of ophiolite~ ar cum W e ignecua rock formed in the uppar parts o f ophiolltes w along rifts,

1. GaoLoglcaUy favoratb environment af rnebawphlc- textwed mfllc m Utram&lc r o c k , aaaoWed w a f k i n t r d v e rocks, a cumulate malic w ultralealic rmb.

2. Known deposlt, prospect, a occurrence. 3. redode emplacement.

5ewndary Criteria

1. Anamalotm values of Cr, It, crr C o L n r O C k f M m ~ . 2. Anomalous value3 of Cr, W1, ar Co IH $ b a a -

a e Q m ent sam *. 3. Anam a law values vf Cr , Ni. or Co In M a ~ y - m i n W d -

mncentrate sa m am. I. strong awom apetic padlent or hi@.

Asamsent and gpadgtonnage nod4 (tabln ll, map shR& 1)

The gepLogicaUy favwat8s areais IW wdlswvwed poWwm chromlte &@ts are the terrane V f U t m m Mi0 an4 wwxiated roc@ In .the sautheastfffl part M the quab.an@

(areas F-H, sheet 1, table 14) and the cumulate ultramafic @no m 8fio roalcs In the 3lanr Rivar end Tslngle auhLarr3#nsr of tha WrmlilsUa twrsna (ape*# A-n, ahrtet 1, CabLe Ill),

Arsae A - D rand I! we maawed t o have R mrad@rl*tcr pahential xlw rndtroaverad dapuait.a beowma of sxhibi#n@ known p~wperat~ 811 aoawrcnrwe or taotonio rlmplflndtmrnt (dlagna#Me BPILRP~II 2 and 31, I n addMan, thfsss *Paw exhlMt rnam4lileua vuluaa s f CP, Ni, or Cn i n ~ a o k , utresrn-flscLim@nt, and hlavy-tntn~r&~on~anC~#ts Ramplss, and R ntrang rwornrpnnLicl v w a o n t ap Tdph (aaoandwy flrltbtris I t W o ~ # h 41 ts l* 14).

Arwm F and 4 w@ mnaassd La have 8 low pnt;smLlsll beaeuel d IxhiMtinR n a y testenlo am plraoam snC (di&#nmWo er i tw ia 3) and b @ o # ~ a s ol e x l ~ W t i n ~ npapsa and Paw slnamaloup vauw af CP, N i , or Ca in oweam-waldimmt w r hoavpminwa-a-sanssntrat~ wa w @as and a swan@ aerern~gnatio Fntilprnt w high bsaondary rtt4Lepla 2 tkou8h 4 I tablo 1 4),

A grade-tannwga modal wrrrr p'trparad by D. A. dingar and N,J Prgs In Cox and dinam (1 986) baaed an psdusrm depoidh From Ci%Warnla nnd O~sgon, C tvsmlts ptlds Is na&aLlvsly amdated with Mnnea. The potted padss and tsnn*unr cif the p'ototypr dsgesllt8 tlsm anntret~r Lhat iP podifwrn ohPemtts drpadts rat i n tha quadran@@, thsn o n ~ h a l f ehsuld aantan 130 tonne* w mare, and the lar+#sst tenth aP bhe d s p r i t r aontdn 2,000 tonno# or murs. F l f ty peroant aP ths drrpncllki hrvs 8vl)rLgo ~hroml tc i wades o l 44.0 p s r ~ s n t or more C r 0 and the r iahmt tenth hsve a t l emL 50 pws rn t C r 2 Q . cO%& and the platinum-psup dam antn Ws ~ s ~ e n C I n nom 1 a l theor do podh,

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EXPLANATION TERRANE O F ULTRAMAFIC AND ASSOCIATED ROCKS

1 CENOZOIC

C e n o z o i c sedimentary and v o l c o n l c rocks

U n c o n f o r m 1 t y

MACLAREN TERRANE

TERT I ARY

G r o n ~ t i c p l u t o n s EARLY TERTIARY AND OLDER

E a s t S u a i k n o b a t h a l i t h F n u l t e d ~ n t r u s ~ v e c o n t a c t CRETACEOUS AND

JURASS I C

LATE JURASSIC OR OLDER

1

Gobbro t o g r a n o d l o r l t e p l u t o n a f I n t r us i ve c o n t a c t

M a c l a r e n G l a c l e r metamorphic b e l t

CLEARWATER TERRAE YUKON-TANANA TERRANE

t LATE TRIASSIC MISSISSIPPIAN AND OLDER

YTL

YTL, L a k e George s u b t e r r a n e YTM, Mucomb suhterrane

YTJ, J a r v l s C r e e k G l o c l e r s u b t e r r a n e YTH, Hayes G l a c i e r s u b t e r r a n e

S p l a y o f B r o x s o n G u l c h thrust

WRANGELLIA TERRANE

YTM

r JURASSIC TO LATE PALEOZOIC

Nenano G l o c i e r f a u l t

AURORA PEAK TERRANE

YTJ

WRS. S l a n a R i v e r s u h t e r r o n e WRT. T a n g l e s u b t e r r a n e

YTH

CRETACEOUS TO SILURIAN

C o n t a c t -.. F a u l t - - d o t t e d where c o n c e a l e d S p l a y o f Den011 f a u l t

WINDY TERRAE

DEVONIAN AND SILURIAN

S p l a y o f D e n a l l f a u l t

Figure 1. Tectonclstratigraphic terrane map of the Mount Hayes quadrangle, eastern Alaska Range, Alaska-Continued