IX Congreso Peruano de Geologia. Resumenes Extendidos.

Sociedad Geol6gica del Peru, Vol. Esp. 1(1997), Lima, p. 121-127

HYPOGENE OXIDATION AND LATE DEPOSITION OF PRECIOUSMETALS IN THE PIERINA HIGH-SULFIDATION DEPOSIT AND OTHER

VOLCANIC AND SEDIMENTARY ROCK-HOSTED GOLD SYSTEMS

DONALD C. NOBLE1, BRIAN PARK-LJI, WILLIA.\f B. HENDERSON2 & CESAR E. VmAL3

Mackay School of Mines. University of Nevada, Reno. Reno. Nevada 89557. U.S.A.U.S.Mineral Laboratories. 6723F 32nd Street. North Highlands. California 95660. U.S.A.Compania de Minas Buenaventaura S.A.. Carlos Wllardn 790. Lima 13. Perno

INTRODUCTION

Hypogene oxidation was an important process during the formation of high-sulfidation golddeposits at Pierina, Yanacocha, Tantahuatay and other districts, and may be important in certainsedimentary rock-hosted deposits, including some of the sandstone-hosted gold deposits of northernPerf. Mineralization at Pierina involved a complex sequence of events that included a stage ofpervasive hypogene oxidation. Gold and, to a greater extent silver, was mostly introduced by latehydrothermal solutions that destroyed pyrite, enargite and native sulfur and deposited barite. Variousminor elements were introduced at one or more times as hydrothermal activity proceeded, and a numberof elements, including Fe, Ti, Te, Se, TI, Zn and Cd were hydrothermally leached by oxidizing solutions.The results suggest that concentrations of iron, particularly specular hematite, devoid of precious metals,but containing elevated Hg, As, 'n, Te, Se, Bi and Sn in high-level argillically/advanced argillicallyaltered rock can be used to infer the presence of subjacent mineralization.

P ARAGENETIC SEQUENCE AND GEOCHEMICAL BUDGET AT PIERINA

Pierina is a newly-discovered world-class high-sulfidation type Au-Ag deposit of middle Miocene(ca. 14.5 Ma) age situated in the Cordillera Negra of northern Perf a short distance northwest of the cityof Huaraz (Lowell, 1997). High-grade ore is hosted by intensely altered porous welded tuff (Huanquiand Calle, 1996) of early Miocene age. Based on reconnaissance surface studies by Noble and Park-Liprior to drilling (Noble, 1996a), hydrothermal solutions may have ascended, at least in part, alongsteeply-dipping, generally NW-SE trending structures. The high-grade ore zone is overlain, and perhapsintruded and truncated by, breccias that were provisionally interpreted as filling a diatreme or brecciapipe and/or breccia dikes. Data subsequently obtained by American Barrick through intensive drillingand trenching will amplify, and perhaps contradict, some of these early observations and interpretations.

ParagenesisA remarkable feature, first observed by Patrick Hillard, Freddy Huanqui and other Acuarios

personnel, is the presence in the ore zone of irregular residual masses (cores) several cm to 0.5+ m indiameter of rock containing small amounts of fine-grained pyrite and variable, but commonly large,amounts of native sulfur in porous, completely silicified welded tuff. The sulfide-native sulfur bearingmasses are surrounded by dark selvages (rims) several mm thick containing abundant covellite. Therims in turn are surrounded by rock containing iron oxides/hydroxides. In addition, there is rock withfew or no visible iron oxides that contains considerable amounts of coarse barite. Some rocks containgrains and aggregates of a black mineral that we identified in polished section as acanthite, in partintergrown with barite. During field observations in May, 1996, of material taken from two explorationtunnels, we observed that in some rocks barite also occurs as irregular, mm to cm wide veins that cut notonly oxidized rocks but also the pyrite plus native sulfur-bearing residual cores, with the veins beingbordered by selvages of oxidized material. Our observations led to the following paragenetic sequence,shortened and reworded from Noble (1996b):

1 Alteration of tuff to a mixture of quartz+aluniteOhematite or pyrite.

2 Formation of vuggy silica rock by alunite-destructive solutions.

3. Deposition of pyrite accompanied by Cu, As and perhaps Au in the vuggy silica rock.

4. Deposition of native sulfur in at least some parts of the pyritic vuggy silica rock

5. Ingress of relatively oxidizing and/or sulfur-poor solutions causing destruction of pyrite anddissolution of Cu-bearing minerals. The released Fe formed hypogene hematite and perhapsother Fe oxide/hydroxide phases; the Cu in part migrated to zones with native sulfur, wherecovellite, chalcocite, enargite?, etc., were precipitated, in the local high aS2 environment toform Cu-rich rims around the native sulfur-bearing masses. Gold was either introduced at thistime (perhaps during the latter part of this stage?), or(and?) remained where it had beendeposited in stage three.

6. Deposition of acanthite plus barite (in part intimately intergrown with acanthite) in portionsof the mineralized rock. The (chloride-rich?) solutions that brought in Ag and Ba appear tohave removed Fe and Cu.

Alunite rock is porous and little or no silica was introduced; alteration may have taken place in amagmatic steam environment. Gold-bearing friable silica rock was later recognized as a distinct rocktype. Reflected-light and electron-beam microscopy show small amounts of enargite with pyrite in theresidual cores and small amounts of chalcocite as well as covellite in the rims, and confirm theoccurrence of acanthite.

The paragenetic sequence is considerably more complicated than that recognized in deposits suchas Julcani (Noble and Silberman, 1984) and other high-sulfidation type gold deposits (e.g., Arribas,1995). Nevertheless, certain stages observed at other deposits, such as the deposition of late-stagealuniteOquartz, have not been seen.

TIMING OF ADDITION AND REMOVAL OF AU, AG AND OTHER ELEMENTS

Multielement geochemical analyses provide additional information on the timing of introductionof Au, Ag and other elements, and demonstrate that a number of elements were hydrothermallyremobilizied and leached during one or more of the paragenetic stages. Based on the field, petrographicand geochemical studies, the following stages are recognized:

1. Early alunite

2. Vuggy silica

3. Pyrite-enargite

Native sulfur

5. Hypogene oxidation

6. Barite-precious metaVfriable silica

Barite-precious metal ore consists of rock that contain visible acanthite (barite-ananthite on Fig.1) and specimens with abundant go.1d but no visible acanthite (barite-gold). Later stage(s) may bepresent: The breccias overlying and perhaps cutting high-grade ores have Agl Au ratios 100+ timeshigher than the ores, and local breaking of brittle high-grade ores into decimeter- to meter-sized blocksmay have been caused by late hydrothermal explosions.

Analytical data on matched pairs of pyrite-native sulfur and hypogene oxidized materials fromfive hand specimens demonstrate that Au and particularly Ag were mostly added during the hypogeneoxidation and barite-precious metal stages (F. 1). Relatively small amounts of Au, and perhaps barite,probably were introduced during the pyrite-enargite stage, but virtually no silver was introduced prior tohypogene oxidation. Analyses of the covellite-rich rims show high concentrations of Ag as well as Cu,indicating introduction of significant amounts of silver during stage 5. Two samples of early alunite rockcontain extremely low Au and Au, consistent with previous observations at many deposits and prospectsthat early quartz+alunite+pyritelhematite altered rock is almost devoid of precious metals.

A preliminary summary of the geochemical budget of hydrothermal alteratiol) is given below,with certain of the relations summarized in Figure 1. Because of the complicated evolutionary history,

122

Block Shawing residual pyrite.enargiter later native sulfur assemblage within vuggy silica rock surrounded by gold-rich hypogeneoxidized ore. Note darlc silver-rich band containing abundant hypogene covel lite separating sulfide + native sulfur-bearing and barite-rich oxidizid rock@.

little covariance can be seen in multielement geochemical data obtained on systematically taken core orchannel samples. Careful paragenetically controlled selective sampling was necessary to revealgeochemical patterns. Aluminum, the alkali and alkaline earth elements, Fe, Mn and P were largely re-moved during the formation of vuggy silica rock. A number of elements introduced with pyrite andenargite were leached during the hypogene oxidation and barite-precious metal/friable silica stages.Most obvious was the removal of iron. The elements Cu, Zn and Cd were almost completed leached, andTe, Se, and Tl were largely removed. Aluminum, almost entirely leached during the formation of vuggysilica rock, was further depleted. Highly variable 1i contents indicate hydrothermal leaching, transportand local deposition of this generally immobile element, as has been observed at the Paradise Peak (Ne-vada) high-sulfidation Au-Ag-Hg deposit (John et al., 1991). In contrast, significant amounts of leadwere added during the last two stages. Appreciable amounts of Bi and As were introduced during thepyrite-enargite stage, and were both variously added to and removed from rock masses of hand-specimen size during hydrothermal oxidation and the following barite-precious metal/friable silicastage. Highly variable Sn contents in the range 1 to 35 ppm suggest than Sn was variably introducedand leached. Although Sb contents likewise vary considerably in rocks of the hypogene oxidation andlater stages, the generally higher Sb contents suggest that significant amounts were introduced duringthe late stages of mineralization. Mercury was introduced during the barite-precious metal/friable silicastage and perhaps during the preceding hypogene oxidation stage. Manganese is 040 ppm in all rocks.

Rocks of the barite-precious metal stage have highly variable Agl Au ratios, suggesting theexistence of two or more substages. Although friable silica rock generally is very poor in iron, onespecimen cut by hematite veinlets contains elevated Au, Fe, Pb, Bi and Te and low Ba, Sr and Ag,indicating the presence of a compositionally distinct mineralizing fluid that deposited Au and Fe.Although we suspect that friable silica rock formed before barite-acanthite rock, these stages orsubstages my overlap or reflect multiple hydrothermal pulses. Only detailed field paragenetic studies inmine workings will provide definitive answers. The fate of the large amounts of native sulfur isuncertain, although a possibility, supported by the barite-acanthite intergrowths, is that some is presentin the barite, having acted to precipitate Ba from late reduced, sulfate-poor and precious metal-bearing,hydrothermal solutions.

123

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The evolutionary history of hydrothermal solutions seen at Pierina is in all probability not unique.Vuggy silica rock containing pyrite and native sulfur separated from oxidized rock by covellite-richselvages has been observed at the Tantahuatay district, and late barite-gold paragenetic stages have beendescribed or observed at a number of districts. Pierina has a number of geological, mineralogical andchemical similarities to, and some differences from, the Paradise Peak high-level high-sulfidationdeposit (John et al., 1991).

Constraints on Temperature and Pressure of MineralizationPierina appears to have been formed at unusually low temperatures, probably well below 200~ C.

Geological relations suggest a moderately, but not extremely, shallow depth of mineralization.Distribution of native sulfur within cavities and the local movement of native sulfur outside the remnantpyrite+enargite zones indicates temperatures above the melting temperature (113~ C). Consistently lowtemperatures of hydrothermal activity are supported by the virtual absence of secondary fluid inclusionsof any type in quartz phenocrysts within the host dacite tuff. The intense, relatively high-temperatureearly quartz-alunite-pyrite alteration by relatively saline fluids recognized at such districts at Julcani,Goldfield, Sumrnitville and Ccarhuaraso (Bruha and Noble, 1983) is conspicuously absent.

Evidence For H.,'pogene Oxidation At Pierina, Yanacocha And Other DistrictsThe breakdown and removal of sulfide and sulfosalt minerals by hydrothermal solutions in vein

and related deposits is a long and well established geological process (e.g., Lacy and Hosmer, 1956). AtPierina, introduction and profound depletion of various elements concurrent with and later than thebreakdown of pyrite, enargite and native sulfur strongly argues that their destruction was caused byhypogene solutions. It would have been very difficult, if not impossible, to have added the amounts ofAu, Ag, Ba, Sr and Pb present in both the oxide-bearing and iron-leached silicaObarite rock, and toremove one or more weight percent of iron, by supergene processes, particularly in the absence ofoverlying rocks that were originally rich in precious metals and sulfides. Moreover, the irregularveinlets filled with barite and hematite indicate a late stage of fracturing best explained by hydrothermalprocesses.

Hypogene conversion of pyrite and other sulfide/sulfosalt minerals to hematite (oxidation) can bebrought about by some combination of: 1) increase in the f02 and 2) decrease in total dissolved sulfurcontent of a solution initially in equilibrium with pyrite. The presence of highly oxidized/low totalsulfur hypogene fluids at the Yanacocha district is shown by the local presence of alternating bands ofquartz and coarse (as much as 2-3 mm) specularite crystals as open-space fillings within high-grade orezones of the Carachugo deposit. Additional evidence for hypogene oxidation is provided by the greatdepths of oxidation observed in several districts. Perhaps the best example is provided by the middleMiocene Yanacocha Sur deposit, in which oxidation extends to a depth of at least 350 m below the sur-face. Although supergene oxidation is present to depths of greater of 400 m in pipes of polymetallic oreof limestone replacement origin at the even younger (ca. 7.5 Ma) Yauricocha district of central Perf(Lacy, 1949; Alvarez et al., 1989), the district is in a region of extreme topography, which would havefacilitated downward movement of oxygenated groundwater.

Although oxidized sedimentary rock-hosted gold ores at the Carlin mine, Nevada, have beeninterpreted to have formed by hypogene processes (Radtke et al., 1980), other workers (e.g., Kuehn andRose, 1992) have argued strongly for a supergene origin. There has been similar controversy at theParadise Peak deposit (John et al., 1991; Sillitoe and Lorson, 1994). However, other partly oxidizedsedimentary rock-hosted deposits in Nevada, such as Rain (Williams, 1992), possess geometric,nuneralogical and textural features suggestive of hypogene oxidation. Relict blocks of sulfide-bearingrock are locally present in sandstone-hosted gold ores exposed in the Tentadora open pit of COMARSA inthe Angasmarca district, northern Perf (Montoya et al., 1995; D. C. Noble, unpub. data). At Tentadora,oxidation is deep and hematite is common, and a hypogene origin for the oxidation is a definite possibility.

Leyenda pag. 124

Figura 1. Diagrams summarizing certain minor-element relations of in ores from Pierina. All values have beenrecalculated free of S, Ba. Sr and associated oxygen, with the exception of the Ba-Au plot, for which the data have beenrecalculated free of sulfur. A = alunite rock; closed circle = pyrite+enargite+native sulfur rock; open diamond =covel lite-rich rim; open square = oxidized rock associated with pyrite+enargite+native sulfur cores; cross = oxidizedfriable silica rock with minor barite; inverted triangle = barite-gold rock; triangle = barite-rich rock with visible acanthite.Dashed lines connect samples of different ore types from same specimen. Note that logarithmic scale.s are used for bothaxes of aU the plots.

125

TIMING OF PRECIOUS-METAL DEPOSITION

At Pierina, copper was transported and deposited before, and by markedly different hydrothermalsolutions than, most of the gold. A similar timing appears to hold for most, and perhaps the greatmajority of high-sulfidation deposits in which gold is the main economic metal. In some cases, forexample EI Indio, deposition of enargite accompanied by modest amounts of gold was followed by agold bonanza stage (Jannas et al., 1990). In the Angasmarca district, gold is late in a multistagehydrothermal history, being associated with Fe-oxides that fill fractures. In many deposits, late depo-sition of gold can be inferred from the rapid rates and high percentages of gold recovery in cyanidecolumn tests and heap-leach operations. The situation is very different from the strong coherencebetween Cu and Au observed in porphyry Au-Cu deposits. The very late deposition of gold and thecomplete or partial separation in time of late Au, Ag, Ba, Hg, Sb, etc. from earlier deposited Cu, Zn, Te,TI, Bi, As, etc., have important implications for the timing and mechanisms of extraction and transportof these elements in high-level high-sulfidation mineral systems.

ECONOMIC CONSIDERATIONS

The existence of hypogene oxidation processes has implications for mining and exploration.Hypogene oxidation can be expected to extend to considerably greater depths than supergene oxidation,increasing the tonnage potential for more economically treatable oxide (non-refractory) ores. Indeed, insome cases the intensity and pervasivness of hypogene oxidation processes may increase with depth.Because hypogene oxidation and leaching can take place at depth beneath rock that has been relativelylittle affected by oxidation and/or other types of alteration, care must be taken in discarding prospectsthat have weak surface geochemical and alteration signatures. Moreover, the demonstrated mobilizationof certain elements during the deposition of Au and Ag suggests that concentrations of hematite withelevated contents of Hg, As, n, Se, Te. Bi and Sn, may serve, in the absence of a significant precious-metal signature, as a geochemical guide to buried gold mineralization.

SUMMARY OF CONCLUSIONS

The results at Pierina, Yanacocha, Tantahuatay and other Peruvian systems confirm and extendknowledge of high-sulfidation Au-Ag deposits. Significant observations and conclusions include the fol-

lowing:

The evolution of high-sulfidation gold systems can be appreciably more complicated thancommonly stated, involving multiple stages of addition, mobilization and/or removal of metalsand other elements; this conclusion is consistent with observations at Pierina and ParadisePeak.

.

. Hypogene oxidation may take place in high-sulfidation systems, and hematite is a commonmineral in ore of high-sulfidation Au-Ag deposits.

. Iron and also Ti, Zn, Cu, Se, Sn, Te, and Tl, may be leached during and after oxidation,particularly during the formation of high-grade ores.

. Major differences in the concentrations of various major and minor elements in unoxidized(refractory) and spatially associated oxidized rock provide strong evidence for hypogeneoxidation at Pierina, and has the potential of confirming or ruling out hypogene oxidationprocesses in other volcanic and sedimentary-rock hosted deposits.

. Iron, transported upward by vapors and deposited as hematite at higher levels, provides auseful exploration guide for buried high-sulfidation gold deposits, and particularly whenaccompanied by elevated concentrations of Hg, As, n, Te, Se, Bi and Sn.

. Prospects with weak and/or high-level alteration features and precious-metal anomalies shouldnot be discarded without a careful evaluation of the full geochemical signature.

. Ore-grade concentrations of gold are formed very late, although gold some may have beenintroduced earlier during the formation of pyrite+enargite.

. Vuggy silica rock, although a much better sampling media than early-stage alunitized rock,commonly contains little gold, even if pyrite and enargite are present in cavities; oxidizedvuggy silica rock is preferable.

1'/\

. The highest Au and Ag contents commonly are associated with abundant bariteAg association is very important.

the Ba-Au-

. Solutions containing high concentrations of precious metals develop very late in the evolutionof high-sulfidation systems, and gold mobilization and deposition is largely separated in timefrom that of copper.

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37 - 1-38.ARRmAS, A., Jr., 1995, Characteristics of high-sulfidation epithermal deposits, and their relation to magmatic fluid, in

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Volume 23, p. 419-452.BRUHA, D. J., and NOBLE, D. C., 1983, Hypogene quartz-aluniteOpyrite alteration formed by moderately saline,

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271, p. 147-156.LOWELL, J. D., 1997, Discovery of the Pierina, Peru, gold deposit by Arequipa Resources: 1997 SME Annual Meeting

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MONTOYA, D. E., NOBLE, D. C., EYZAGUIRRE, V. R., and DESROSIERS, D. F., 1995, Sandstone-hosted golddeposits: A new exploration target is recognized in Per£: Engineering and Mining Journal, v. 196, no. 6,

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and silver at Pierina: Lima, Unpublished memorandum to Brian Park-Li, P. Hillard and D. Lowell. 3 p.

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RADTKE, A. S., RYE, R. 0., and DICKSON, F. W., 1980, Geology and stable isotope studies of the Carlin gold deposit,

Nevada: Economic Geology, v. 75, p. 641-672.SILUTOE, R. H., and LORSON, R. C., 1994, Epithermal gold-silver-mercury deposits at Paradise Peak, Nevada: Ore

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1228-1248.WILLIAMS, C. L., 1992, Breccia bodies in the Carlin trend, Nevada; Classification, interpretation and role in ore

formation: Unpublished M.S. thesis, Colorado State University, Fort Collins, 213 p.

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