Tungsten–molybdenum skarn and stockwork mineralization, Mount Reed – Mount Haskin district,...

Tungsten-molybdenum skarn and stockwork mineralization, Mount Reed - Mount Haskin district, northern British Columbia, Canada

SUSAN J . GOWER,' ALAN H. CLARK, AND C. J A Y HODGSON Department of Geological Sciences, Queen's University, Kingston, Ont., Canada K7L 3N6

Received June l I, 1984 Revision accepted November 8, 1984

In the Mount Reed - Mount Haskin area, intrusive- and calc-hornfels-hosted stockwork Mo(- W) mineralization, oxidized W-Mo and Zn-Pb-Cu skarn mineralization, and Ag-Zn-Pb vein mineralization are spatially associated with multiphase, epizonal, magnetite-bearing granite stocks of Eocene age, emplaced into the Lower Cambrian Atan Group of the Cordilleran miogeocline.

Mo(>W)-stockwork mineralization partly predated the main exposed intrusions and was related to bodies of aplitic syenogranite, vestigially preserved. Two extensive, mineralogically and texturally distinctive, but probably coeval, zoned exoskam systems constituting the main stage (stage I) are recognised at Mount Reed. The B- and F-rich inner system was controlled by fracture permeability in dolomitic marble. It is zoned from massive magnetite skarn, at the intrusive contact, through concentrically banded ("wriggliteW-textured), diopside - phlogopite -magnetite - chondrodite -vesuvianite - (Mo-rich) scheelite skam, to stockwork veinlets with diopside cores and serpentine-chondrodite-humite-ludwigite-magnetite marginal zones. The calcic outer system was controlled mainly by bedding-plane permeability at a gradational marble - metapelitic homfels contact and consists of banded skarn zoned from garnet-, through ferrosalite-, to wollastonite-dominant assemblages, with fluorite as a minor but persistent constituent.

Superimposed on both systems are dykelike bodies of vesuvianite-garnet skam (stage 11) and amphibole-biotite-sulphide skarn (stage 111) associated with quartz veins. Scheelite is markedly enriched in the latter, retrograde, skarn facies, which is, however, of only limited development. Because the main stage skarns appear to be truncated by the Mount Reed monzogranite, it is tentatively proposed that they formed adjacent to the early aplitic syenogranite and may thus have been coeval with the Mo-stockwork system.

The unusually varied exoskarn assemblages at Mount Reed well illustrate the influence of protolith composition and structure and of fluid-rock interaction on skarn development.

Dans la rkgion du mont Reed - mont Haskin, un intrusif- et des cornkennes calciques servent de roche-magasin a un stockwerk de Mo(-W)-, a un skarn oxydC contenant W-Mo et Zn-Pb-Cu et a une minCralisation en veine d'Ag-Zn-Pb, et ils sont associCs a distance avec des stocks de granite a magnetite, multiphasCs, Cpizonaux, d'gge Cocene, et mis en place dans le groupe Atan du miogCoclinal cordillCrien.

La minCralisation du stockwerk de Mo(>W) est partiellement antkrieure a la mise en place des intrusions exposCes, et elle est apparentke a des corps de syknogranite aplitique dont certaines parties sont pr6servCes. Deux systemes d'exoskam zonCs de grande supeficie, caracttrisCs par des textures et minkralogies distinctes, mais probablement contemporains, representent la phase dominante (phase I) au mont Reed. Le systeme interne enrichi en F et en B Ctait contrBlC par la permCabilitC de fracture dans le marbre dolomitique. La zonation passe d'un skarn a magnCtite massif au contact de I'intrusif, a un skam a diopside - phlogopite - magnttite -chondrodite-vtsuvianite - scheelite (riche en Mo) concentriquement ruban6 (texture de "wrigglite"), a un stockwerk de petites veines dont le centre est occupC par du diopside et dont les zones de bordure cornportent de la serpentine-chondrodite-ludwigite-magnCtite. Le systeme exteme calcique Ctait contr61C principalement par la perme- abilitC des plans de stratification au contact graduel de comtennes mCtapClitiques a des cornCennes a marbre, et il est constituC d'un skam rubane avec une zonation d'assemblages ou domine successivement le grenat, la ferrosalite et la wollastonite, et la fluorite est toujours prCsente comme constituant mineur.

Des corps en forme de dykes de skarn a ~Csuvianite-grenat (phase II), et un skarn a amphibole-biotite-sulfures (phase 111) associC avec des veines de quartz sont superposCs aux deux systemes. La scheelite est remarquablement plus abondante dans ce dernier facies de skam rhograde, lequel cependant est peu dCvelopp6. Vu que la phase principale des skams semble &tre tronquCe par le monzogranite du mont Reed, nous proposons que les skams ont dQ se former en bordure du syenogranite aplitique plus ancien, et ont pu Ctre possiblement contemporains avec le stockwerk a Mo.

Les assemblages tres diffkrents et exceptionnels des skarns du mont Reed illustrent bien le rBle important de la composition du protolithe et de la structure, et de l'interaction des fluides avec la roche, dans le dCveloppement des skams.

[Traduit par le journal]

Can. J. Earth Sci. 22. 728-747 (1985)

Introduction The skarn and associated stockwork and vein mineralization

in the Mount Reed - Mount Haskin area of northern British Columbia (ca. lat. 59'19'N; long. 12Y27'30"W; Fig. 1) illustrate with unusual clarity some of the characteristic geological relationships of W - Mo- base-metal hydrothermal systems associated with epizonal granitoid intrusions. Such relationships include (1) zoning in the mineralogy and texture of skarn min-

'NCe Bamhill. Present address: 34 Esdaile Ave., Dartmouth, N.S., Canada B2Y 3N6.

eralization, and in the forms of the mineralized bodies, both areally and temporally, and on both the deposit and district scales; (2) the occurrence of multiple mineralizing and intrusive events that partly overlap; and (3) variability in the type of structural and rock permeability control on mineralization.

The first mineral discovery in the immediate area, in 1937, was a Pb-Zn-Ag vein, by prospector J . Reed, who still lives on the mountain that bears his name. During the boom period of porphyry-deposit exploration in the decade 1965 - 1975, stockwork-type molybdenite mineralization was discovered and drilled in the granitic stock and intruded rocks at Mount

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COWER ET AL. 729

Rc. I. Location map, Mount Reed - Mount Haskin district (star), northern British Columbia.

H a ~ k i n ; ~ work by several companies resulted in the delimitation of a "Moly zone" containing 11 Mt grading 0.15% MoS,.

The latest target of mineral exploration has been the W-Mo skarns at Mount Reed. Initially drilled in 1969, the property on which the skarns occur was optioned to Canadian Superior Exploration from 1978 to 1982. During the summer of 198 1 , the senior author was employed by this company to carry out mapping and core logging. This paper is based on that work and on subsequent laboratory studies (Barnhill 1982). It focuses on the documentation of the mineralogy and form of the mineralization in the district, with emphasis on the scheelite-bearing exoskarn bodies at Mount Reed.

The mineralized area at Mount Reed (lat. 59'18'N; long. 129O25'W) is located adjacent to Highway 37, 24 km east of the town of Cassiar. Elevations range locally from 1050 to 1970 m above sea level (asl). Exposure is generally good above treeline but is sparse at lower levels. Mount Haskin (59"201N; 129"30rW) is located ca. 4.5 km northwest of Mount Reed. No bedrock mineral production has been recorded from the district.

District geology Sedimentary rocks

A simplified geological map of the area, lying within the northern extension of the Omineca lithotectonic domain, is presented as Fig. 2. Mounts Reed and Haskin are underlain by a northwest-trending belt of Lower Cambrian miogeoclinal sediments assigned to the Atan Group (Gabrielse 1963). Three members can be distinguished within the group in the Mount Reed - Mount Haskin area: a lower siltstone-quartzite member, a middle carbonate member, and an upper argillite-

siltstone-quartzite member. The middle carbonate member is ca. 360 m thick and comprises two main lithofacies, an argillaceous limestone to interbedded limestone-pelite facies and a massive, partially dolomitic limestone facies (Barnhill 1982).

The clastic and carbonate host rocks of mineralization in the Mount Reed area display a wide range of oxidation state: some units contain graphite, and others hematite.

Igneous rocks Two small granitoid stocks, located 5 km apart on the flanks,

respectively, of Mount Reed and Mount Haskin, have intruded the Atan Group sediments.

The Mount Reed stock (Figs. 3, 4) is elongate parallel to the strike of the sediments and measures approximately 1000 m long by 225 m wide. The Mount Haskin stock (Fig. 5) is circular in plan, with a diameter of ca. 1200 m. The two intrusions are similar in composition and texture and have yielded essentially identical, Early to Middle Eocene, K- Ar biotite ages of 48.8-51.5 Ma (Christopher et al. 1972: ages recalculated using the revised decay contacts of Steiger and Jager (1977)). Each stock is composed of three map units: a coarse granite, a fine granite, and an aplitic facies (Figs. 3, 5).

Coarse-grained quartz and alkali-feldspar megacrystic monzogranite (classification after Streckeisen 1976) makes up much of the Mount Reed and Mount Haskin stocks; this rock typically contains 30% quartz, 1-3% biotite, and approximately equal proportions of K-feldspar (predominantly as 6- 12 mm megacrysts) and plagioclase (An,,-,,) (Fig. 6). A/CNK values approach unity (Table l ) , indicating a metal- uminous character according to Shand's (1927) classification. Mesocratic xenoliths of plagioclase-rich monzogranitic composition (Fig. 6) are locally abundant in the Mount Reed monzogranite; these are tentatively regarded as representing an early, less differentiated facies of the pluton.

Fine-grained granite (monzogranite-syenogranite) is found in a narrow zone ranging from 2 to 30 m wide at the borders of the intrusions. Within the fine-grained units, layered and pegmatoid phases are locally present (Fig. 4); these contain no scheelite or molybdenite. The fine- and coarse-grained granites have gradational contacts, suggesting that they formed from a single pulse of magma and that the finer textures resulted from chilling at the contacts of the plutons.

The areally restricted aplitic facies occurs as a cap at the apex of the Mount Reed intrusion and as a lens at the northwest contact of the Mount Haskin stock. The aplite is a quartz-rich syenogranite (Fig. 6). Much of the aplite is cut by an intense stockwork of apparently weakly mineralized molybdenite- quartz veins, which are not developed in the contiguous granite. Furthermore, xenoliths of aplite, cut by quartz veins, are found within the main granites (Figs. 4, 7a) , and coarse granitic dykes cut the aplite facies (Fig. 7b), truncating both molybdenite-quartz veinlets and fractures that offset the veinlets. These cross-cutting relationships demonstrate that the aplitic rocks were emplaced prior to the coarse and fine granites and that some mineralization (apparently Mo rich) was associated with the earlier igneous event.

The granitoid rocks of the area contain magnetite and may thus be assigned to the "magnetite series" of Ishihara (1977), as well as to Chappell and White's (1974) I-type clan.

Contact metamorphism 'Christopher J . Hodgson. 1977. Unpublished property examination A poorly defined contact-metamorphic aureole extends at

report, Mt. Haskin - Mt. Reed properties. AMAX, Vancouver, least 0.5 km around the granitic intrusions at Mount Reed and B.C., 19 p. Mount Haskin; it is superimposed on greenschist-facies re-

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CAN. I. EARTH SCI. VOL. 22, 1985

Cretaceous I Eocene intrusions

0 Mississippian ultramafics

0 Devono - Miss. Sylvester Gp.

Devonian McDame Gp.

Cambrian I Ordov. Kechika Gp.

Cambrian Atan Gp. Ists.

0 Cambrian Atan Gp. Clastics

[ml Proterozoic lngenika Gp.

0 Ordov. I Silurian Sandpile Gp. Lithophile metal occurrences

( W, Mo, Be 1

+I+ Syncline, anticline

FIG. 2. Geological map of area surrounding the Mount Reed - Mount Haskin district (simplified after Gabrielse 1963).

gional metamorphic assemblages that, in pelitic rocks, include the assemblage Fe-rich chlorite + muscovite. Biotite has developed at the expense of chlorite in the outer zone of the contact aureole. Within about 100 m of the intrusions, andalusite and cordierite porphyroblasts appear. Andalusite + K- feldspar and corundum + K-feldspar are found in pelitic rocks at the contact of the Mount Reed intrusion and are presumed to reflect prograde breakdown of muscovite.

Metamorphic minerals developed in carbonate units include tremolite, diopside, and, in the innermost aureole, periclase, which shows partial retrograde alteration to brucite.

The mineral assemblages in the carbonate and pelitic rocks demonstrate that metamorphic conditions attained the pyroxene hornfels facies adjacent to the Mount Reed stock. The formation of periclase through the reaction,

Do1 = Per + Cc + CO,

requires temperatures in excess of 610°C (Harker and Tuttle 1955) and very low Xco, values (<0.04). Similarly, the assemblage K-feldspar + corundum requires temperatures of at least ca. 600°C (Evans 1965), and the presence of andalusite indicates that confining pressures did not exceed ca. 1.8 kbar

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COWER ET AL.

. . . . . . . . . . . u . . . . . . . .

( CLASTIC )

/ Geological contact

FIG. 3. Geological map of Mount Reed area. Note facies of the granitic stock and contact-parallel foliation in Atan Group marbles around the southeastern lobe of the intrusion.

(180 MPa) (Holdaway 197 1 ; Evans 1965). In the thermal aureoles at both Mount Reed and Mount

Haskin, biotite hornfels in contact with carbonate-rich units has been converted to calc-silicate hornfels. These fine-grained rocks contain diopsidic clinopyroxene, quartz, K-feldspar, garnet, plagioclase, and sphene. Little or no scheelite or base-metal sulphide mineralization is associated with the calc- silicate hornfelses, which are tentatively considered to have generally formed through bimetasomatic reactions between carbonate and pelitic lithologies during contact metamorphism. However, pale green zones composed of diopside, plagioclase, and quartz that cross-cut sedimentary laminations in the Mount Haskin "Moly zone" and very locally at Mount Reed indicate that fracture-controlled Ca-metasomatism of metapelitic hornfels also occurred (Dick 1980).

Structural relationships The Paleozoic strata in the area trend northwest (Fig. 2) and

dip 45-60" to the southwest. Faults follow two dominant trends. Early northwest-trending faults parallel the stratigraphy and the long axis of the Mount Reed intrusion and are thought to be responsible for the repetition of Atan Group strata at Mount Haskin. The second series of faults trends northeast and cuts the northwesterly trending faults. Faults exert a local control on the extent of skarn development and control the locations of Ag-, Pb-, and Zn-bearing veins.

Much of the volume occupied by the two epizonal stocks was generated by structural deformation of the Atan Group sediments, which were warped such that the regional dip is steep- ened near the stocks (metasedimentary xenoliths are extremely

rare in the intrusions). Small-scale, disharmonic, parallel folds, attenuation features, and a contact-parallel schistosity (defined by alternating spar and calcarenite layers) are developed in the limestone of the Atan Group in the inner aureole around the southeastern part of the Mount Reed intrusion (Fig. 3).

Mineralization Four types of mineralization are associated with the in-

trusions at Mount Reed and Mount Haskin (Fig. 8): molybdenite-quartz stockworks in the plutons and in metasomatized metapelitic hornfels; W(-Mo) exoskarn bodies in marble; massive base-metal sulphide skarn lenses along carbonate- hornfels contacts; and Ag-Pb-Zn veins in fault zones.

Exoskarn development at Mount Reed Several distinct exoskarn facies and three stages of meta-

somatism have been recognized in the carbonate rocks near the Mount Reed stock (Table 2). The areal distribution of the skarn facies at surface is outlined in Fig. 9, and their inferred disposition at depth is indicated in Fig. 10 a and b. The successive stages of skarn formation are discussed below and are tentatively correlated with associated alteration and mineralization in the intrusion and metapelitic hornfels.

Stage I : main stage The most extensive metasomatism of metacarbonate rocks in

the Mount Reed aureole occurred during stage I. At this time, two essentially distinct, zoned, skarn systems were simultaneously generated. Of these, the inner skarn system was developed in the central portion of the middle carbonate member

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732 CAN. J. EARTH SCI. VOL. 22. 1985

SECTION 13-50 NW

. . , . . . . . - . .

inclusions of aplite '.:.:-:-:.: iiii: * with Mo veining . . . . .::. . . . .::.

-, ;:;:;::. % banded fine granite :::;:%,. . . .

.I..'...'. U" .l'.'.l.* ..... . . . . . , . . .

b ! I

9 +00 Tot00 1 1 +DO l 2 + O U SW SW SW SW

SECTION 14-50 N W 7-50

--I50

0:: rnesocratic xenoliths

re-entrant in the southeastern contact of the stock, the texture of the inner skarn shows an outward transition from massive (magnetite-rich), through concentrically banded, to stockwork (Fig. 9).

Massive magnetite skarn is limited in extent (Fig. 9). Minor amounts of and diopside and rare molybdenum-rich scheelite occur with the magnetite.

The main minerals constituting the concentrically banded skarn (Figs. 11, 12) are, in order of abundance, clinopyroxene, phlogopite, magnetite, chondrodite, and vesuvianite. Minor components are forsterite, garnet, chlorite, serpentine, sericite, talc, molybdenum-rich scheelite, and molybdenite. Electron microprobe analyses of several of these minerals are presented in Table 3.

The pale green clinopyroxene (Table 3: analyses 1-3) coexisting with magnetite closely approaches diopside in composition. Mg-rich phlogopite (analysis 4) and muscovite (analysis 5), locally with talc and chondrodite (analyses 6, 7), apparently constitute an equilibrium assemblage. The vesuvianite (analyses 8- 10) is A1 deficient and Mg and Fe rich relative to that in later vesuvianite-garnet skarn (see below and Table 5). Two analyses of the garnet (1 1, 12) reveal mod- erately to extremely andradite-rich compositions. In this skarn, serpentine and chlorite are clearly of retrograde origin.

The characteristic concentric layering in this skarn type (Figs. 11, 12) is caused by changes in grain size, mineral assemblage, and (or) mineral proportions. Blackish layers alternate with light green, white, or brown layers; the former are rich in magnetite and the latter in diopside and chondrodite. Individual layers are less than 1 mm in width and are irregularly sinuous. They are disposed in crudely cylindrical and spher- oidal configurations, ranging in diameter from 1 to 50 cm. In

lying adjacent to the stock at surface (Fig. 9); the protolith consisted of massive, finely crystalline limestone with pods and beds of dolomitic limestone, grading to coarsely crystalline dolostone. In contrast, the outer skarn system formed predominantly farther from the intrusion at the upper and lower contacts of the same marble unit, in dolomite-poor grey limestone with argillaceous intercalations. The two systems have

0

sharp contacts with one another or, alternatively, are separated by unreplaced marble. At depth, on the southeastern margin of the intrusion, the skarns of the inner system pinch out, and the

some rocks the layering is chaotic or has a parallel, sheeted disposition, commonly paralleling fractures.

Similar contorted layering in fluorite- and magnetite-rich skarns has been well described by Triistedt (1907), Knopf (1908), and, recently, by Kwak and Askins (198 1 a,b). Kwak and Askins have reviewed the literature on this skarn texture, originally termed "wrigglite" by Askins (1976). It is generally accepted that such mineralogical layering can be best explained

outer skarn system locally occurs in contact with the granite (Fig. 10a).

Skarns in the inner skarn system are fracture controlled and, even in dolomite-poor host-rock domains, are characterized by magnesium-rich silicate minerals and abundant magnetite. In a

/ / /

r0 DDH in section by a Liesegang-type process in which metasomatic constituents metres involved in the replacement process diffuse outward from and

4 D D H cutting section inward toward fractures. In contrast to other wrigglite skarns, that at Mount Reed contains no fluorite; its role as a fluorine-

~ c . 4. ( a ) cross section A (Fig. 3) and ( b ) cross section B bearing mineral is fulfilled by chondrodite in this more mag- (Fig. 3) through the southern part of the Mount Reed stock. Geological nesia,, environment, and neither tin nor beryllium minerals contacts, in part schematically shown, based on surface exposures and have been confirmed. Assays do, however, record anomalous drill-core intersections.

Sn values (<0.05%) in this skarn facies. Stockwork skarn consists of randomly oriented, polystage

veinlets of skarn in brucite(-periclase) marble. The average interveinlet spacing in this facies decreases from ca. 5- 10 cm farthest from the granite to less than 1 cm at the generally abrupt contact with wrigglite skarn. Narrow, 1 mm wide, black veinlets consist of ludwigite and magnetite, both of which also occur as euhedral crystals disseminated within the adjacent marble. Identification of the ludwigite, which forms acicular euhedra with rhombic cross sections, was confirmed by X-ray powder diffraction techniques and by electron-probe micro- analysis: partial analysis yielded MgO 34.76 and FeO 47.29 wt.%, indicative of an intermediate member of the ludwigite- vonsenite series. The mineral is opaque in thin section, and in reflected light it displays optical properties matching those reported by Leonard et al. (1962) and Picot and Johan (1977).

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COWER ET AL.

UPPER ( CLASTIC MEMBER, ,

. . . . . . . . . . . . . . . . . . . . . I . . . . . . .

Massive sulphide akarn

FIG. 5. Geological map of Mount Haskin area (modified after Christopher J . Hodgson).' Ornaments as in Fig. 3, except where indicated.

In some zones, the ludwigite is extensively replaced by Fe-rich chlorite.

Cutting the thin black veins are wider ( 1-5 mm) pale green- ish yellow, or locally red, green, or blue veins of magnesium silicate minerals. Serpentine is the most abundant mineral in these veins and is associated with lesser amounts of humite (Table 4: analyses 1 , 2), chondrodite (analysis 3), diopside (analysis 41, forsterite, and hematite. Ludwigite-magnetite selvages are generally developed around the veins (Fig. 13a).

The wider veins ( 1 -20 cm) in more intense stockwork skam show bilaterally symmetrical c~ lou r banding, with green diopsidic cores and yellow to bIack marginal zones composed of alternating, thin bands of serpentine, chondrodite, humite, and ludwigite + magnetite, resembling crude wrigglite. The cores

of wider veins comprise garnet, vesuvianite, Mo-rich scheelite, and molybdenite. Although there is local evidence for replacement of chondrodite (Fig. 13b) and olivine by serpentine, much of the latter mineral appears to represent a prograde constituent of the later veins in the stockwork.

The various mineralogical faciex of the inner skarn system are thought to have formed as a result of a progressive, but essentially single-stage. alteration process. The stockwork skarn represents the least. the wrigglite skarn the intermediate, and the massive magnetite skarn the most intense alteration of the predominantly dolomitic marble protolith.

However, the time-space relationships of mineral development in the system remain unclear. Thus, magnetite (with ludwigite) formed early in the stockwork facies but was essen-

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734 CAN. J. EARTH SCI. VOL. 22, 1985

TABLE 1. Whole-rock analyses and normative mineralogies of igneous rocks at Mounts Reed and Haskin

(1) (2) (3) (4) (5) (6) (7) (8) (9) (10)

MAJOR OXIDES ( ~ t . %) SiOZ 72.9 74.6 74.3 74.2 74.0 72.7 74.4 75.0 68.7 65.1 Alz03 13.0 13.0 13.0 12.7 12.2 13.7 13.4 12.9 15.6 15.8 CaO 1.3 0.8 1.09 0.80 1.07 1.56 0.90 0.68 2.27 2.98 MgO 0.28 0.1 0.20 0.02 0.31 0.01 0.01 0.00 0.64 1.04 NazO 3.90 4.21 3.98 3.69 2.88 4.54 4.04 3.66 4.46 4.22 K20 5.01 5.18 4.65 5.55 5.16 5.31 4.56 5.07 4.19 4.37 Fe203 1.71 0.72 1.07 0.76 1.32 0.36 0.87 0.58 2.86 3.65 MnO 0.07 0.03 0.04 0.02 0.04 0.08 0.07 0.03 0.11 0.10 Ti02 0.22 0.11 0.17 0.06 0.13 0.05 0.11 0.10 0.34 0.59 pzo5 0.06 0.01 0.04 0.00 0.02 0.00 0.02 0.00 0.09 0.19 LO1 0.39 0.47 0.54 0.93 1.17 0.62 0.23 0.23 0.85 0.70 Total 98.8 99.2 99.1 98.8 98.9 99.0 98.6 98.3 100.1 98.7

MINOR ELEMENTS (ppm) Cr 109 89 130 130 117 109 89 82 116 89 Zr 110 70 80 20 50 20 60 70 370 280 Sr 250 10 140 20 120 130 70 20 520 720 Rb 300 260 280 320 430 610 430 450 280 280

NORMATIVE COMPOSITIONS

Q 29.61 29.96 31.68 31.49 35.84 22.72 32.08 32.86 21.42 16.98 Or 30.19 31.13 27.91 33.68 31.39 31.51 27.39 29.94 24.94 26.28 Ab 33.63 36.21 34.19 32.04 25.08 38.56 34.73 30.94 38.00 36.32 An 3.23 1.30 3.94 1.75 5.28 1.33 4.40 3.36 10.25 11.47 Co 0 0 0 0 0 0 0.23 0.16 0 0

A/CNK (mol AI2O3/(Ca0 + Na20 +K20)) 0.91 0.92 0.95 0.93 0.99 0.85 1.01 1.01 0.97 0.92

D.I. (norm. Q + Or + Ab + L C + Ne + Ka) 93.44 97.31 93.78 97.22 92.31 92.79 94.21 93.7 84.37 79.59

SAMPLE DESCRIPTIONS AND LOCATIONS: (1) Coarse granite, DDH 81 -6, 448 ft, Mt. Reed; (2) fine granite, 10+50 SW, 13 +25 NW, Mt. Reed; (3) aplite, DDH 81-6,55 ft, Mt. Reed; (4) fine granite (minor phyllic alteration), DDH 81-1 1,703 ft, Mt. Reed; (5) coarse granite (intense argillic alteration), DDH 81-18, 491 ft, Mt. Reed; (6) dyke, 1 m wide, cutting skarn (endoskam locally developed adjacent to fractures), 11+75 NW, 8+50 SW, Mt. Reed; (7) coarse granite, Mt. Haskin; (8) fine granite, mineralized (Mo) and altered drill-core from "Moly zone," Mt. Haskin; (9) mesocratic xenolith, 15+50 NW, 9+25 SW, Mt. Reed; (10) mesocratic xenolith, DDH 81-15, 268 ft, Mt. Reed.

quartz 0 Coarse Granite

Fine Granite MT. REED

O MT. H A S K I N

x Aplite

alkali feldspar plagioclase

FIG. 6. Modal compositions of granitoid rock types. Samples from Mount Reed stock, except where indicated.

tially coeval with the Ca-Mg silicate minerals in the wrigglite. We tentatively propose that an early hydrothermal phase dominated by magnetite formation generated the proximal massive oxide skarn and, peripherally, the magnetite-ludwigite vein-

TABLE 2. Exoskarn stages and facies, Mount Reed

Proximal Distal

MAIN STAGE (I) Inner skarn system Outer skarn system

Massive magnetite skam i. Wollastonite skarn + Concentrically banded, Andradite- ferrosalite-

diopside -phlogopite- fluorite skarn magnetite-chondrodite Ferrosalite - andradite - skarn + fluorite skarn +

Stockwork, magnetite- (low-grade W >> Mo) ludwigite-diopside- serpentine - humitel chondrodite skarn (low-grade; W > Mo)

STAGE 11 ......................... Vesuvianite-prnet skam .........................

(low-grade W-Mo)

NOTE: Extent of skam facies development at present exposure interval: I >> I1 > 111.

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Frc. 7. ( a ) Xenolith of aplite in coarse granite of Mount Reed stock. The aplite alone is cut by a series of subparallel quartz veinlets containing minor molybdenite; fractures that cut the veinlets are themselves truncated by the enclosing granite. (6) Veinlets of coarse granite cutting aplite and quartz vein stockwork. (Location 9+00 SW; 13+00 NW.)

lets and that this gave way to a phase (possibly with higher as,/ap,) that formed the wrigglite, the magnetite-bordered Ca-Mg silicate veinlets, and the magnetite-free skarn veinlets.

The outer skarn system of the main stage is characterized by a banded texture and by Fe- and Ca-rich and Mg-poor skarn minerals (Table 5). These distal skarns are zoned in- wards, away from the contacts of the marble with the upper and lower clastic units, from pyroxene-rich to garnet-rich garnet-

I pyroxene(-fluorite) skarn to wollastonite skarn. Southwest of the intrusion, garnet-pyroxene(-fluorite)

I skarn forms a band of outcrops, 20 m broad and 500 m in length, along the marble - biotite hornfels contact (Fig. 9).

I Northeast of the granite, banded garnet-pyroxene(-fluorite) skarn is developed at the lower marble-homfels contact, whereas massive skarn is observed in outcrops closer to the intrusion contact.

Garnet-pyroxene(-fluorite) skarn generally displays ca.

2 cm wide mineralogical banding, perhaps reflecting laminated distribution of argillaceous impurities, in which dark green ferrosalite (average composition ca. Hd61J~hllDi28; Table 5) layers alternate with brown-red garnet (An&-,,) layers. Fluorite, not easily identified in hand specimen, is ubiquitous in thin section. Wollastonite, vesuvianite, calcite, and scheelite are minor constituents, the last-named occurring as fine dis- seminations and in garnet - fluorite veinlets. Clinopyroxene- rich garnet-pyroxene skarn is interbedded with calcareous hornfels towards the clastic members.

The high manganese contents of the clinopyroxene and, particularly, the wollastonite in this skarn facies are noteworthy. The markedly manganoan and ferroan composition of the wollastonite is matched by a small proportion of the wollastonites described by Matsueda (1973, 1974); the Mount Reed mineral is classifiable as a manganoan iron-wollastonite, or perhaps as a calcian bustamite. Matsueda (1974) argued for the existence

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736 CAN. J . EARTH SCI. VOL. 22, 1985

TABLE 3. Electron-probe microanalyses of minerals in magnetite-rich "wrigglite" skarn, Mount Reed

Clinopyroxene Chondrodite Vesuvianite Garnet Phlogopite Muscovite

I 2 3 4 5 6 7 8 9 10 11 12

SiO, A1203 Ti02 Fe203 FeO MgO MnO CaO Na,O K2O Cr203 Hz0 Total

(Total cationic charge = 12; all cations occupy 3 sites)

(Total cationic charge = 22)

33.53 33.65 0.30 0.46 0.00 0.15 0.00 0.00 4.05 3.02

51.73 52.68 2.78 2.83 0.20 0.25 0.19 0.14 0.00 0.00 0.00 0.00 5.02 5.07

97.80 98.25

PROPORTIONS (Total cationic

charge = 18; all cations occupy

7 sites)

Si ~ 1 ' ~ AIV' Ti Fe3+ Fez' Mg Mn Ca Na K Cr 0 OH. F

(Total cationic charge = 72, cations occupy 25 sites)

(Total cationic charge = 12, all cations except Si occupy 5 sites)

8.821 2.993 3.028 0.179 0.000 0.000 3.032 0.377 0.073 0.442 0.015 0.006 0.504 1.582 1.927 0.626 0.054 0.072 1.725 0.000 0.015 0.138 0.051 0.105 9.234 2.910 2.798 0.270 0.000 0.000 0.000 0.000 0.013 0.029 0.011 0.000

34.000 12.000 12.000 4.000 0.000 0.000

END-MEMBER COMPOSITIONS

Almandine 1.8 2.4 Pyrope 0.0 0.5

Spessartine 1.7 3.5 Grossular 17.2 3.4 Andradite 78.7 96.9 Uvarovite 0.5 0.0

NOTE: These and the following electron probe microanalyses were carried out on an A.R.L.-AMX instrument, employing the energy-dispersive mode, set up and supervised by P. L. Roeder. The analyses were converted to structural formulae through use of computer programmes written by D. M. Carmichael, Queen's University, Kingston, Ont.

of a miscibility gap between iron-wollastonite and wollastonite, but our analytical coverage is insufficient to indicate whether both phases are represented in the Mount Reed assemblage. We prefer to ascribe the high Mn and the Fe contents of the wollastonite to the composition of the hydrothermal fluid rather than of the protolith.

Wollastonite skarn contains greater than 20% fibrous wollastonite and has an irregular mineralogical banding. Other com- mon minerals in this facies are andraditic garnet, pale, probably diopsidic pyroxene, vesuvianite, fluorite, spinel, calcite, and scheelite.

The mineralogy and zonal pattern in the outer skarn system are typical of many "infiltration-type'' skarns. Dick (1980) and Dick and Hodgson (1982) suggested that the mineralogical zonation wollastonite + garnet + pyroxene is the result of progressive metasomatism, where the ratio of the activity of

Ca, which is progressively being removed from the rocks, to the product of the activities of constituents, such as Fe and Al, which are being introduced by the hydrothermal fluids, decreases in the fluid towards the fluid source. The areal distribution of distal main stage skarn zones at Mount Reed suggests that the marble-metapelite contact acted as the major channelway for fluids, because this interface is associated with the skarn minerals most depleted in calcium.

Stage 11: vesuvianite-garnet skarn Both the outer and inner skarn systems are cross-cut by

tabular bodies of massive, coarse-grained vesuvianite-garnet skarn associated with thin, discontinuous quartz veins. The bodies range from thin veins (Fig. 14) to dykelike systems up to 15-20 m wide. A fracture control is widely evident, but much of the skarn formed through replacement of either marble

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TABLE 4. Electron-probe microanalyses of selected minerals in magnetite-rich stockwork skarn, Mount Reed

Humite Chondrodite Clinopyroxene

I 2 3 4

SiOZ 36.64 37.86 35.26 56.91 A1203 0.00 0.00 0.00 0.00 TiO, 0.00 0.07 0.10 0.00 FeZ03 0.76 0.00 0.00 0.00 FeO 0.71 0.47 1.30 1.43 MgO 56.29 58.27 57.19 17.69 MnO 1.00 0.44 0.98 0.65 CaO 0.31 0.20 0.00 26.68 NaZO 0.00 0.00 0.00 0.00 KzO 0.06 0.03 0.05 0.02 Crz03 0.00 0.00 0.00 0.01 Hz0 3.68 3.77 5.25 0.00 Total 99.45 101.11 100.13 103.39

IONIC PROPORTIONS (Total

cationic I / / / / Mo Stockwork (Total cationic charge = 18; (Total cationic . ... .:..... W (Mo) Skarn charge = 26; all all cations charge = 12; * - Massive Sulphide 0 1 2 cations occupy occupy 7 all cations - 10 sites) sites) occupy 4 sites)

Pb - Ag -Zn Veins km

FIG. 8. Areal distribution of mineralization types, Mount Reed - Mount Haskin district (modified after Canadian Superior Ltd., unpublished report, 1978).

or main stage skarn. This skarn type also occurs as narrow zones between fine granite or alaskite dykes and other skarn facies.

Vesuvianite-garnet skarn is composed of large (up to 8 cm, but averaging 1 cm), grass-green clusters of radiating vesuvianite crystals and variable proportions of large (0.5-2 cm), well zoned garnets. Minor amounts of quartz, diopside, calcite, chlorite, and coarse-grained, Mo-poor scheelite and molybdenite occur interstitially. Microprobe analyses (Table 6) show that the vesuvianite is A1 rich and that the garnet is zoned from grossular-rich cores to margins enriched in andradite and, to a lesser degree, in spessartine and almandine.

Stage I l l : amphibole-biotite skarn Amphibole-biotite - Fe sulphide skarn occurs locally as

selvages adjacent to quartz veins or as patchy replacement zones or pods in the skarn types discussed above. Thin sections of amphibole-rich skarn clearly indicate that pargasitic amphibole pseudomorphs pyroxene; thus, this skarn facies may be regarded as retrograde, in the sense of Einaudi et al. (1 98 1 ). Other minerals that occur in amphibole-rich skarn include garnet, quartz, hematite, fluorite, blue- and yellow-fluorescing scheelite, calcite, chlorite, pyrite, and pyrrhotite. Relatively high tungsten assay values were obtained from restricted drill sections of amphibole-rich skarn.

Si AI" ~ 1 ~ ' Ti Fe3+ Fez' Mi? Mn Ca Na K Cr 0 OH, F

growth of pink, isotropic garnet, clinopyroxene, fluorite, and anhedral scheelite. Garnet is concentrated along fractures, whereas the other endoskarn minerals are disseminated in a mosaic of sutured plagioclase and quartz grains. The endoskarn is locally traversed by quartz-fluorite-magnetite veins (Fig. 15).

Ca metasomatism of granite dykes varies widely in intensity. Initial alteration is represented by veins of fibrous wollastonite and (or) equant garnet, bordered by plagioclase-enriched envelopes. Endoskarn development in the dykes culminates in massive plagioclase (An-5,) rocks with accessory garnet and minor diopside, chlorite, sphene, and scheelite.

The marginal zone of the aplitic cap of the Mount Reed stock contains small pods of almost monomineralic clinopyroxene endoskarn.

Skarn-type alteration of hornfels Skarn development in granitic and metapelitic rocks As noted above, calcification of metapelitic hornfelses at

Endoskarn both Mount Reed and Mount Haskin has occurred adjacent to Limited development of calcic endoskarn in the Mount Reed carbonate horizons or along calcite veins and fractures, gener-

stock is evident in the coarse and fine granite facies, in granitic ating calc-silicate (diopside-plagioclase-quartz) hornfelses. In dykes, and, to a greater extent, in the aplite unit (Fig. 9). addition, fracture-controlled Ca metasomatism locally affected

The margin of the stock adjacent to exoskarn bodies is con- biotite hornfels at Mount Reed and, particularly, Mount verted, over a thickness of ca. 1 m, to a fine-grained inter- Haskin, giving rise to hornblende-rich rocks (Dick 1980). In

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1 COWER ET AL. 739

FIG. 10. Inferred distribution of skarn facies in vicinity of southwest contact of Mount Reed stock. (a) Section A-B and ( b ) section C-D (Fig. 9), both sections looking southeast.

the northern part of the "Moly zone" at Mount Haskin (Fig. 5), a stockwork of molybdenite- and scheelite-bearing quartz veins (Mo/W approximately 2 : 1) displays hornblende-rich selvages, passing outwards into envelopes of disseminated hornblende, sphene, and pyrite. The veins also contain locally abundant fluorite and minor K-feldspar, chlorite, beryl, pyrite, and pyrrhotite. This mineralization facies was thus clearly associated with Ca metasomatism of the hornfelses, perhaps associated with skarn development in nearby unexposed marble.

At Mount Reed, several drill-core intersections of scheelite- bearing quartz veinlets, with intensely biotitized envelopes, were encountered in metapelitic hornfels.

Vein mineralization in granitic rocks Mo stockworks and phyllic alteration

A weak development of molybdenite-pyrite-quartz veins characterizes the southeastern part of the Mount Reed intrusion. There, the fine granite is cross-cut by an open stockwork of veinlets ranging from 1 mm to 2 cm wide and with an average intervein spacing of ca. 20 cm. The veinlets have phyllically altered envelopes, in which muscovite forms coarse main clusters. and biotite is chloritized. These veinlets are

in the Mount Haskin stock, where the southern part of the "Moly zone" stockwork is developed in both fine and coarse granite facies (Fig. 5). The Mount Haskin aplitic unit, situated in the centre of the "Moly zone," has (apparently) disseminated molybdenite throughout and also displays molybdenite-coated fractures. These are truncated by dykes of coarse granite, thus demonstrating, as noted above, the occurrence of two episodes of molybdenite mineralization in this area.

Argillic alteration Both the Mount Reed and Mount Haskin intrusions display

zones of intense argillic alteration. This alteration is particularly well shown by the topographically lower, western part of the Mount Reed pluton, where the monzogranite has been converted to an assemblage of kaolinite, sericite, and quartz; less altered rocks display argillized plagioclase but essentially fresh K-feldspar. Weak argillic alteration is evident in most exposures and drill-core intersections at Mount Reed. Argillized granite is locally cut by thin molybdenite-quartz- calcite veins and contains rare disseminated molybdenite and chalcopyrite.

Base-metal- silver mineralization .2

apparently similar in all salient aspects to those associated with Massive sulphide skarns the early, aplitic episode of intrusion. Whereas little tungsten-bearing exoskarn is developed near

Molybdenite-quartz-muscovite veining is more extensive the Mount Haskin stock, several bodies of massive, pyrrhotite-

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CAN. J. EARTH SCI. VOL. 22, 1985

FIG. 11. Megascopic textures in concentrically banded, wrigglite-textured skam, Mount Reed. Dark zones are dominated by magnetite, and the paler by diopside and chondrodite. ( a ) Cross section, perpendicular to axis, through large columnar structure. (Location, 10+00 SW, 14+00 NW.) ( 6 ) Section through group of narrower pipes, with axes at ca. 30' to field of view. (Location, diamond-drill hole (DDH) 80-3; 61.5 ft.)

rich, garnet skarn have been delineated along carbonate- hornfels contacts (Fig. 8). The skarns contain abundant sphalerite and lesser amounts of chalcopyrite, galena, and arsenopyrite.

The most extensive bodies of base-metal skarn are the "A zone" (a Zn-Pb lens adjacent to the "Moly zone") and, apparently farther from the stock, the larger "B zone" (a Zn - Cu - Pb body), with a lower base-metal content than the A zone but showing an enrichment in Ag and Bi.' The Snow Zn-Pb zone, located near the summit of Mount Haskin. is restricted to a thin

scribed "Joe Reed's Vein," located 1.5 km southeast of the summit of Mount Reed. Ag-rich sphalerite-galena-pyrite mineralization there occurs patchily along some 80 m of a 3-6 m wide north-trending shear zone, which juxtaposes clastic and carbonate rocks of the Atan Group.

Several other sphalerite-bearing veins, with variable proportions of galena and chalcopyrite, have been observed in Atan Group and McDame Group limestones at the southern limits of the Mount Reed area.

horizon of siltstone at the base of an isolated synclinal outlier Discussion of limestone. Alone among the skarns of the Mount Haskin

The areally and temporally variable patterns of mineral- area, it contains significant Sn (ca. 0 . ization and alteration at Mounts Reed and Haskin are clearly

Peripheral vein mineralization related to the emplacement, in two separate pulses, of granitoid Ag - Pb - Zn-bearing veins occur peripherally to the other magmas and to the complex hydrothermal systems that

economic mineral prospects in the area, normally at the greatest developed near these intrusions. The temporal and areal inter- distances from the stocks (Fig. 8). Gabrielse (1963) has de- relationships of the successive skarn-forming events that

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,& , . - .?'" "a; r@k * - *A . - > ,.

FIG. 12. Photomicrograph of wrigglite textures, Mount Reed. Opaque magnetite bands alternate with zones of diopside, chondrodite, and vesuvianite. (Transmitted, plane-polarized light; location same as for Fig. I 1 b. )

affected the aureole of the Mount Reed stock are schematically assemblage of the inner skarn system and the calcic mineral depicted in Fig. 16, together with the inferred correlations with assemblage of the outer skarn system are interpreted to reflect mineralizing events in the intrusions and metapelitic horn- the initial difference in the overall Mg and Ca contents of the felses. It should be emphasized that the exact age relationships heterogeneous carbonate protoliths of the two skarn bodies: between the episodes of intrusion and the development of the dolomitic limestone in the inner system and limestone in the exoskarns are unclear. The absence of scheelite in the early outer system. The abundance of ludwigite-vonsenite in the molybdenite-quartz stockwork veinlets associated with the inner skarns and the apparent absence of tourmaline in the aplite, the presence of scheelite-bearing endoskarn in the partially Al-rich outer skarn environment suggest that boron coarse-grained granite, the broad parallelism of the skarn was extracted from the hydrothermal fluids shortly after their bodies and the intrusive contact, and the textural- exit from the intrusion. In this context, the "truncation" of the mineralogical zonation of the magnetite-rich inner skarn B-rich inner skarns by the stock (Fig. 10a) may indicate that system (Fig. 9) all suggest that the tungsten-rich hydrothermal monzogranite intrusion followed this metasomatic event. In fluid pulse was related to the main exposed stock in this area. contrast, all stage I skarns are relatively rich in fluorine, which However, the intersection, at depth, of the outer skarn system was precipitated in chondrodite-humite near the stock and in by the stock (Fig. IOU) implies that emplacement of the fluorite in the distal alteration facies. monzogranite postdated main stage skarn development. The absence of scheelite in the aplite-hosted veinlets may reflect a

et al . 1968). From the mineralogical zonation and cross-cutting relation-

ships described above, it is possible to place certain constraints on the physical and chemical nature of the mineralizing fluids and how they must have changed in time and space as the mineralized bodies formed.

Process and conditions of mineralization The mineralogical zonation in the stage I skarns at Mount

Reed indicates that the inner and outer skarn systems developed separately, but sensibly simultaneously, as a result of the progressive reaction of broadly similar fluids moving through different conduit systems and reacting with different wall rocks (Figs. 3, 9). A system of stockwork fractures in the dolomitic marbles, localized at the granite-marble contact, focused fluids in the inner skarn system. Fluids that formed the outer system followed a zone of bedding-parallel permeability centred in the interlaminated, nonmagnesian marble-metapelite zone at the contacts of the carbonate member with the

That the fluids responsible for formation of the two systems differed in their fo,/fs, ratios is indicated by the sulphide- oxide mineral assemblages: the magnetite and Mo-rich scheelite in the inner system indicate a higher fo,/fs, than do the pyrite, pyrrhotite, molybdenite, and scheelite in the outer system. This difference may be due to the reducing effect of carbonaceous material in the pelitic rocks, which have reacted with fluids in the outer but not in the inner system. It should be emphasized, however, that the presence of grossular- andradite garnet rather than grossular-almandine garnet in the outer system indicates it to be an oxidized type ofskarn in the sense of Einaudi et al. (1981), and as such it is quite different from the large, reduced-W skarns of the CanTung-MacTung type (Dick and Hodgson 1982; Mathieson and Clark 1984).

Superimposed on the main stage skarn assemblages are the more restricted stage 11 vesuvianite-garnet skarns. The presence of Mo-poor scheelite and molybdenite in these indicates an fo,/fs, ratio in the fluid similar to that that formed the main stage outer skarn system. The zonation of garnet grains from grossular rich to andradite rich indicates that a,3+/ac,z+ in fluids passing a reference point in the system increased with time, but there is no indication of variation with time in fo2/fs2.

The origin of the stage I1 skarn is problematic. The occur- metapelite member of the Atan Group. The magnesian mineral rence of the dykelike bodies of vesuvianite-garnet facies im-

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742 CAN. I. EARTH SCI. VOL. 22. 1985

FIG. 13. ( a ) Photomicrograph of Mg silicate-rich skarn veinlet in dolomitic marble, Mount Reed. Marginal zone of acicular ludwigite crystals, with minor magnetite and retrograde Fe chlorite, flanks a core zone dominated by diopside, chondrodite, and humite. (Transmitted, plane- polarized light; location, DDH 81-9; 310 ft.) ( b ) Axial zone of magnesian skam veinlet, Mount Reed. Chondrodite, in part displaying lamellar twinning, partially replaced by serpentine. (Transmitted light, crossed nicols.)

mediately adjacent to the contacts of the Mount Reed stock implies a genetic relationship. We tentatively consider the time-space relationships to be consistent with a pre- monzogranite origin for the main stage skarns; the stage I1 bodies represent a comparatively restricted hydrothermal event coinciding with the consolidation of that body.

The stage 111 skarns, characterized by amphibole, biotite, sulphides, quartz, calcite, and abundant fluorite, are superimposed on stage 11 and appear to record a marked increase in the activities of the volatile components H20, CO,, and probably H2S, possibly as a result of a decrease in temperature late in the history of the hydrothermal system. The occurrence of amphibole envelopes on quartz-molybdenite veinlets in the "Moly zone" suggests that this "retrograde" stage in the skarns and perhaps also the molybdenite-pyrite-quartz veinlets with

phyllically altered envelopes in the granites at Mounts Reed and Haskin were contemporaneous.

The temporal relationship of base-metal skarn and vein mineralization to the skarns is unknown, but, by analogy with other areas, they probably formed late and (or) peripherally in the hydrothermal systems of the area.

Regional analogues Although apparently of small size, the skarn and stockwork

deposits in the Mount Reed - Mount Haskin district constitute a remarkable variety of mineralization types. In some aspects, these bear similarities to other skarn-stockwork centres in the northeastern part of the Canadian Cordillera (Dick 1980), but in total the deposits under discussion display several unusual features.

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TABLE 5. Electron-probe microanalyses of minerals in garnet-pyroxene skam, Mount Reed

Wollastonite Garnet Clinopyroxene (manganoan,

ferroan) I 2 3 4 5 6 7 8 9 10 11

SiOz A1203 TiO, Fez03 FeO MgO MnO CaO Na20 KzO Cr203 Hz0 Total

Ti Fe3+ Fez+ Mg Mn Ca Na K Cr 0 OH, F

Almandine Pyrope Spessartine Grossular Andradite Uvarovite

35.91 36.50 50.31 3.47 6.49 0.16 0.00 0.04 0.00

26.01 21.64 1.79 1.53 1.76 16.88 0.00 0.18 5.42 1.17 2.02 2.97

31.14 30.52 23.10 0.00 0.00 0.18 0.00 0.00 0.06 0.12 0.11 0.23 0.00 0.00 0.00

99.35 99.27 101.10

IONIC PROPORTIONS

(Total cationic charge = 12; all cations except Si occupy 5 sites)

3.027 3.001 2.985 3.030 3.023 3.031 0.000 0.00 0.000 0.000 0.000 0.000 0.771 0.570 0.481 0.253 0.344 0.635 0.001 0.022 0.002 0.003 0.000 0.002 1.207 1.374 1.503 1.739 1.648 1.353 0.144 0.128 0.196 0.121 0.018 0.123 0.009 0.005 0.015 0.024 0.000 0.022 0.166 0.115 0.106 0.092 0.083 0.142 2.693 2.774 2.865 2.764 2.809 2.716 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.013 0.012 0.000 0.000 0.000 0.000 0.013 0.012 0.003 0.008 0.007

12.000 12.000 12.000 12.000 12.000 12.000 0.000 0.000 0.000 0.000 0.000 0.000


4.8 4.2 6.5 4.0 3.6 4.1 0.3 0.2 0.5 0.8 0.0 0.7 5.5 3.8 3.5 3.1 2.8 4.7

29.3 23.0 13.7 5.1 10.8 22.5 60.1 68.2 75.1 86.9 82.4 67.6 0.0 0.6 0.6 0.1 0.4 0.4

(Total cationic charge = 12; all cations occupy 4 sites)

1.975 1.996 1.987 1.982 0.007 0.004 0.013 0.000 0.000 0.005 0.005 0.000 0.053 0.000 0.008 0.002 0.053 0.008 0.013 0.038 0.554 0.601 0.629 0.635 0.371 0.298 0.225 0.268 0.099 0.112 0.133 0.104 0.971 0.968 0.937 0.957 0.014 0.008 0.014 0.010 6.003 0.000 0.006 0.000 0.007 0.000 0.000 0.004 6.000 6.000 6.000 6.000 0.000 0.000 0.000 0.000

(Total cationic charge = 6; all cations occupy

2 sites)

As pointed out by Dick (1980), the Mount Haskin mineralized area may be classified as a Mo-W vein-stockwork system with an outer (distal) zone of Zn-Pb(-Ag) skarns and veins, broadly comparable to the much larger Logtung W - Mo deposit, situated 75 km to the northwest (Noble et al. 1984). Both deposits show a significant enrichment in scheelite relative to molybdenite in passing from the stock into the hornfelsic country rocks.

The study area lies on a southeasterly extrapolation of a line of W(-Mo) skarn deposits, including the Stormy and Mid/ Nite groups in Yukon Tenitory (Dick 1980). These small and, in general, low-grade scheelite-molybdenite skarns are broadly comparable to the outer calcic, exoskarn W(-Mo) bodies associated with the Mount Reed intrusion but differ radically from the large and rich W-Cu(-Zn) skarns of the easterly belt overlapping areally with the Yukon - Northwest Territories border. The oxidized outer skam system at Mount Reed is dominated by grossular-andradite garnet and ferro-

salitic pyroxene, contains accessory wollastonite and vesuvianite, and is poor in sulphide minerals, whereas the large W-Cu(-Zn) skams are dominated by grossular-almandine garnet and hedenbergitic pyroxene and are rich in sulphide minerals. The relatively high pyroxene/garnet bulk ratio of the Mount Reed skarns in comparison with that in the similar bodies in the Stormy and Mid/Nite deposits (Dick 1980) may reflect the mixed marble - metapelitic hornfels protolith. The high fluorite content of the outer skarn system is not displayed by the Yukon deposits described by Dick.

Oxidized Mo-rich tungsten skarns of this type in the northeastern Canadian (and western United States) cordillera apparently contain negligible scheelite when compared with the reduced, pyrrhotitic, W-Cu(-Zn) skarns (e.g., Dick and Hodgson 1985; Newbeny 1983) but are elsewhere (e.g., King Island, Tasmania; Sangdong, South Korea) major tungsten re- positories. Dick and Hodgson (1985) suggested that the reduced and S-rich composition of the large W-Cu(-Zn) skams

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CAN. 1. EARTH SCI. VOL. 22, 1985

TABLE 6. Electron-probe microanalyses of minerals in vesuvianite-garnet (stage 11) skarn

Garnet (single grain)

I 5 Vesuvianite (core) - 7 3 4 (rim) 6

SiOz A1203 TiOz Fez03 FeO MgO MnO CaO NazO KzO Crz03 Hz0

IONIC PROPORTIONS

(Total cationic charge = 12; all cations except Si occupy 5 sites)

(Total cationic charge = 72; all cations occupy

25 sites)

Si AI'" AIV' Ti Fe3' Fez+ Mg Mn Ca Na K Cr 0 OH. F

2.987 2.952 2.969 0.000 0.000 0.000 1.805 1.741 1.430 0.002 0.005 0.001 0.184 0.247 0.552 0.076 0.110 0.122 0.000 0.012 0.000 0.153 0.197 0.119 2.774 2.686 2.681 0.000 0.000 0.000 0.000 0.000 0.000 0.007 0.002 0.016

12.000 12.000 12.000 0.000 0.000 0.000


Almandine 1.8 2.5 3.7 4.1 7.4 PY rope 0.1 0.0 0.4 0.0 0.6 Spessartine 5.1 5.1 6;6 10.5 6.8 Grossular 79.8 82.8 76.9 61.0 42.3 Andradite 1 3.1 9.2 12.3 27.6 39.1 Uvarovite 0.1 0.4 0.1 0.8 0.1

may be a reflection of the graphitic and pyritic nature of the metapelitic country rocks. It is also possible, however, that the compositions of the ore-forming fluids reflected those of the parent granitoid magmas (Mathieson and Clark 1984) and that the great tungsten enrichment in the W-Cu(-Zn) skarns relative to the W(-Mo) skarns in the region may have been ultimately controlled by specific deep-crustal protoliths that were involved in magma genesis: metasedimentary, alumi- nous, and reduced in the former case, and meta-igneous and oxidized in the latter.

The inner skarn system at Mount Reed is unusual both in the northeastern cordilleran context and with respect to mineralized skams in general. The largely dolomitic protolith has given rise to highly magnesian mineral assemblages (diopside, phlogopite, humite-chondrodite, Mg-rich vesuvianite, forsterite, and serpentine), and the abundance of magnetite and andraditic garnet records a high fo,, probably a function of the composition of the ore-forming fluids rather than of the protolith.

The extensive review of Einaudi et al. (1 98 1) demonstrates that magnesian W(-Mo) skarn deposits are very rare; the Mg-rich skarns of Costabonne, France (Guy 1979) bear some re- semblances to the Mount Reed exoskarn in their mineral species but display significantly different time-space relationships of mineral development. Moreover, the high content of boron in the hydrothermal fluids implied by the occurrence of ludwigite-vonsenite is a feature not matched by tungsten skarns in general. Indeed, closer analogies are evident with the magnesian tin skarns (Einaudi et al. 1981), although only traces of tin occur in the Mount Reed system. When compared with other documented magnesian lithophile - metal skams, however, those at Mount Reed differ in the apparently simulta- neous formation of clinopyroxene, forsterite, magnetite, chondrodite, phlogopite, and vesuvianite in the "wrigglitic" facies. Further, ludwigite appears to have formed, together with magnetite, in the initial stage of stockwork skarn development, whereas in other deposits documented by Einaudi et al. (1981)

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RG. 14. Veinlet of vesuvianite-garnet (stage 11) skam cutting magnetite (bottom) - diopside-rich skam of the inner system. (Lo- cation, Mount Reed, 10-50 SW; 12+50 NW.)

these minerals crystallized at a later, retrograde stage. The well defined wrigglite structures in the Mount Reed

inner skarn system are perhaps the first to be recorded from W-rich rather than Sn-rich skarns (cf. Kwak and Askins 1981a,b); although restricted wrigglite bodies occur in the Fujigatani and Kuga scheelite skarns, Honshu, Japan (A. H. Clark, unpublished data, 1978), such structures have not been observed in the sulphide-rich W-Cu(-Zn) skarns of Yukon Territory and the Northwest Territories. The Mount Reed wrig-

RG. 15. Garnet-clinopyroxene-scheelite endoskam, cut by magnetite-fluorite veinlets. (Location, Mount Reed stock, 9+00 SW; 16+00 NW.)

glite differs from most, if not all, other occurrences in the absence of fluorite, but the abundance of chondrodite suggests that high fluorine activities may indeed be essential for the development of this texture.

Conclusions The following are the main conclusions of the study. (1) A wide variety of mineralization types occurs in the

Mount Reed and Mount Haskin area: both intrusive- and calc- hornfels-hosted stockwork Mo(-W); oxidized W-Mo and

calc - i l l r ~ i f l ~ : lhornhlenue - r tcn I rnetasnme~lsrn ana METAPELIT'C rnorv. schecllle - s t r , u e i n r , HORNFELS ficheellle - 012 V B l n s wilh LEGEND

I ( c a l c i c , argil laceous)

MARBLE

( magnesian )

I

Calc - silicate hornfels I outer skarn Garnet - pyroxene - fluorite

System

0 Wollastonite

Massive magnetite

inner skarn @ Stockwork

system Concentrically banded

~ ' w r i g g l i t e ~ l /

(=irl Vesuvianite -garnet ( Stage II)

@ Marble

I I moly - q t r . p h ~ l l i c m t t s r a ~ l o n

? s t o c k w o r k + m o l y - q t z . valns ? I GRANITE -- endoskarn ( minor scheelite ) - -

TIME

I FIG. 16. Schematic diagram illustrating the inferred temporal and spatial evolution of W-Mo exoskarn and associated mineralization types

at Mount Reed. It has been assumed that the main stage skams were associated with the aplitic (syenogranite) intrusion and were coeval with the molybdenite-quartz stockwork.

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746 CAN. J . EARTH SCI. VOL. 22, 1985

Zn-Pb-Cu skam; and vein-type Ag-Zn-Pb. All are spatially related to two Eocene, magnetite-bearing, polyphase, epizonal (P < 1.8 kbar (1 80 MPa) from contact assemblages) monzogranite-syenogranite stocks emplaced into the Lower Cambrian Atan Group miogeoclinal metasediments.

(2) Mo-stockwork mineralization developed in two episodes after each of the two intrusive phases, i.e., the first aplitic phase and the second coarser grained granite. Although individual quartz veinlets in the Mount Reed aplitic inclusions appear weakly mineralized, the very close spacing of the observed veinlets suggests, tantalizingly, that an unusually high-grade stockwork deposit may have been stoped out by the intrusion of the Mount Reed stock. In the younger granitoid rocks, Mo-quartz veinlets have phyllically altered envelopes, whereas Mo-W stockworks in the hornfels at Mount Haskin have hornblende envelopes.

(3) Skam W-Mo mineralization at Mount Reed occurs mainly in two bodies (stage I), both with well developed mineralogical and textural zonation caused by one main stage of progressive fluid-rock interaction but formed within different rock types. In the outer skam system, zoned calcic skam (edge to core: wollastonite + garnet-pyroxene + pyroxene- garnet), controlled mainly by bedding-plane permeability, formed in the interbedded marble and i elite at the contact of a dominantly calcareous member with a dominantly argillitic member of the Atan Group. In the inner skarn system, a texturally and mineralogically zoned, ferroan-magnesian skarn (edge to core: ludwigite-magnetite-diopside vein-stockwork + magnetite-rich, concentrically banded skarn + massive magnetite skarn), controlled mainly by fracture permeability, formed in dolomitic marble towards the contact of the Mount Reed intrusion with the marble. There are two, weakly developed, superimposed stages (I1 and 111) of skarn mineralization, both of which contain tungsten mineralization; they record a change in fluid composition and (or) in conditions in the depositional zone late in the evolution of the system. It is possible that the main stage exoskams were associated with the earlier, aplitic, intrusive episode and that the second episode of prograde, anhydrous skam (stage I I ) was generated by the coarse-grained monzogranite (Fig. 16).

(4) The nature of the mineralization indicates that major amounts of Fe, Al, Si, B, and F, as well as Mo, W, and base metals, were camed into the depositional system by hydrothermal fluids. The deposit is similar to other Cretaceous- Tertiary W-Mo deposits in northeastern British Columbia and southeastern Yukon, such as Logtung (Noble et al. 1984), although it is much richer in B- and bearin in^ minerals. In its oxidized nature, it contrasts sharply with the large reduced W-Cu(-Zn) skarns of the Mackenzie Mountains, Yukon - Northwest Temtories border area, such as CanTung and MacTung (Dick and Hodgson 1982; Mathieson and Clark 1984).

(5) The restricted development of the scheelite-rich stage 111 amphibole skarns at the present erosional level is a critical constraint on the economic potential of the Mount Reed centre.

Acknowledgments The authors thank Canadian Superior Exploration, Ltd., for

access to the Mount Reed property, for providing company- owned analytical data and samples, and for contributing to the costs of rock analysis. Special thanks are due to John Watkins and Marilyn Atkinson for their generous assistance in the field. Peter Roeder and Dugald Carmichael, at Queen's University,

gave instruction in microprobe analysis and interpretation. The assistance of Chris Peck in drafting and photography and of Linda Harris in typing is gratefully acknowledged.

Analytical and travel expenses were covered partly by Natu- ral Sciences and Engineering Research Council of Canada (NSERC) operating grants to A.H.C. and C.I.H. The senior author was supported by NSERC awards while engaged in this research.

We greatly appreciate the detailed and constructive criticism of an anonymous reviewer of our initial text.

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Tungsten–molybdenum skarn and stockwork mineralization, Mount Reed – Mount Haskin district,...

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