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An in-depth analysis of the mineralogy and textures of the Greens Creek Massive Sulfide Deposit in Alaska. It includes descriptions and photographs of various ore types, such as massive sulfide, white ore, and pyrite, as well as their textures, including brecciated, tectonically banded, nodular diagenetic, and colloform. The document also discusses the presence of gold, silver, mercury, and other minerals in the ore. Students of geology and mining may find this document useful for studying the mineralogical and textural features of massive sulfide deposits.
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Contents Abstract ....................................................................................................................................................... 187 Introduction................................................................................................................................................. 187 Mine Location, General Host Rocks, Metamorphic History, Deposit Morphology, and Structural History ................................................................................................................. 189 Previous Mineralogical and Mineral Geochemical Studies ............................................................... 191 Ore Types..................................................................................................................................................... 194 Primary Ore Mineralogy and Textures ................................................................................................... 196 Recrystallized and Remobilized Ore Mineralogy and Textures.......................................................... 196 Free Gold—Mineralogy and Textures .................................................................................................... 199 Paragenetic Relationships—Physical and Chemical Modification of Ores .................................... 200 LA–ICP–MS Mineral Chemistry Studies ................................................................................................ 204 Methods.............................................................................................................................................. 204 Results ................................................................................................................................................ 209 Discussion ................................................................................................................................................... 230 Conclusions................................................................................................................................................. 234 References Cited........................................................................................................................................ 234 Figures
The Greens Creek deposit is a 24.2-million-ton poly- metallic massive sulfide with a diverse base- and precious- metal-rich mineralogy, which has been subjected to regional lower greenschist facies metamorphism. Roughly 30 percent of the ores retain primary mineralogy and mineral textures as well as gross original ore stratigraphy. Ore lithologies fall into two groups: massive sulfide ores (greater than 50 percent sulfides) and semimassive or disseminated sulfide gangue-rich “white” ores (less than 50 percent sulfides). There are two types of massive ore: massive pyritic and massive base-metal- rich ore. The white ores are of three types: white carbonate and white siliceous ores (common), and white baritic ore. All the ore types can be further subdivided on the basis of modi- fication by veins, breccias, and gouge or rubble zones pro- duced during faulting or folding. Veining due to metamorphic remobilization and recrystallization can result in enrichment of free gold and a variety of silver-sulfosalt minerals. The ore stratigraphy at Greens Creek is characterized by proximal copper-arsenic-gold-enriched massive pyritic ores centered over white siliceous ores and silicified footwall. Laterally, the white siliceous ores grade outward into white carbonate and barite ores. White ores are overlain by massive pyritic ores that change upward and outward toward lower copper-gold grades. Proximal ores transition to increasingly higher grade zinc- lead-silver-(gold)-rich, massive, fine-grained, base-metal-rich ores toward the argillite hanging wall and the margins of the deposit. Distal ore commonly is characterized by carbonate- and barite-rich white ores against footwall phyllites, which grade into massive, fine-grained, base-metal-rich ores toward the hanging wall. This progression of ore types is the most common throughout the mine. Primary mineral textures are characterized by framboidal, colloform, dendritic, and “spongy” pyrite intergrown with base-metal sulfides and sulfosalts. Primary assemblages also include sphalerite, galena, tetrahedrite, chalcopyrite, free gold, and a variety of lead-antimony-arsenic (-mercury-thallium) sulfosalts. The abundance of polyframboidal, colloform, and nodular pyrite textures coupled with their δ^34 S-depleted isotopic signature provides strong evidence that the main stage of massive sulfide mineralization at Greens Creek occurred primarily during early diagenesis, synchronous with accumu- lation of the hanging-wall sediments. The early development of framboid-derived, atoll-shaped textures in the ores may indicate that a zone-refinement process occurred in the pres- ence of colloidal base-metal-sulfide gels. Metamorphic recrys- tallization produced advanced atoll-shaped structures in the ores and resulted in much coarser textures and the formation and(or) remobilization of secondary, precious-metal-enriched minerals. Secondary minerals are present as matrix to pyrite euhedra and in late fractures and veinlets. Secondary mineral- ogy includes chalcopyrite, sphalerite (low iron), galena, free gold, electrum, tetrahedrite (antimony-rich), pyrargyrite, and many other sulfosalt minerals. Metamorphism resulted in vis- ible cleaning and coarsening of the ore mineralogy and caused local trace-element redistribution and upgrading of the ores.
The Greens Creek deposit is a mineralogically diverse, polymetallic (zinc-lead-silver-gold-copper) massive sulfide of Late Triassic age and unusually high silver content. The global resource is currently 24.2 million tons at an average grade of 13.9 percent zinc, 5.1 percent lead, 0.15 troy ounces per ton gold, and 19.2 troy ounces per ton silver. Twenty-nine ore sul- fide minerals have been reported, 18 of which are silver bear- ing (table 1). In addition to the metals of economic interest, the ores are enriched in trace elements such as As, Sb, Hg, Tl, Ni, Co, Cr, and Mo. These metals and minerals are contained in a suite of five ore types, which define an ore stratigraphy that progressed from early white ores to main-stage massive sulfide ores during the growth of the system. This progression (see chap. 15 for a more thorough description) is thought to have started at low temperature, at or very near the sediment/ water interface in a basin containing oxygenated seawater, and then evolved to higher temperature within a growing sulfide blanket that formed in and beneath a progressively thickening layer of black shales. As deposition progressed, pore fluids in the shales and probably the overlying water column gradu- ally became anoxic. These early and main-stage ore-forming
Mine Location, General Host Rocks, Metamorphic History, Deposit Morphology, and Structural History 189 processes produced a trace-element-rich orebody with a diverse silver- and base-metal-rich mineralogy, primarily by replacement of shale, early sulfides, and silica-barite gangue. Remnants of primary mineral textures are represented by fine- to very fine grained aggregates of framboidal, dendritic, and colloform or botryoidal pyrite intimately intergrown and banded with base-metal sulfides and the diverse suite of trace- element and silver-bearing sulfides. Following the cessation of mineral deposition in lat- est Triassic time, the deposit was buried by continued shale sedimentation, which effectively prevented sea-floor oxi- dation of the newly formed sulfides. During the next 215 million years the deposit went through at least three phases of deformation (see chap. 7) and a regional subgreenschist to lower greenschist facies metamorphic event (see chap. 2) that reached temperatures of 275–350oC (see chap. 12) and buried host rocks in the Greens Creek area to a depth of about 17–18 kilometers (Himmelberg and others, 1995). Although the metamorphism and multiple deformation of the orebody resulted in pronounced structural dismemberment, folding, recrystallization, and remobilization of the ores, large areas of the deposit preserve primary mineralogy, textures, and chemistry. A range of preservation states exists, from pristine to completely modified. This fortuitous situation presents an opportunity to study the effects of deformation and metamor- phism upon massive sulfide mineral textures and to understand the details of mineral residence of specific trace elements and their redistribution as a result of textural modification. Most of the initial mineral identifications, descriptions of ore textures, and preliminary determinations of mineral chem- istry are contained in unpublished mining company reports (see “Previous Mineralogical and Mineral Geochemical Studies” section). In particular, electron microprobe mineral-chemistry studies were conducted to improve the beneficiation of various mill products, and metal zonation studies of tetrahedrites were conducted in an attempt to obtain geochemical vectors within ore assemblages. We summarize these unpublished reports, compile their mineral identifications and textural observations, and significantly extend the petrographic descriptions and geo- chemistry of the various ores at Greens Creek. Here we describe the five principal ore types in detail and the mineralogy and mineral textures present in these ores. We then discuss the paragenesis of the ores and the progressive modification of the mineralogy and mineral textures as a result of deformation and metamorphism. We follow the descriptive portion of the chapter with the presentation of preliminary Laser Ablation–Inductively Coupled Plasma–Mass Spectroscopy (LA–ICP–MS) mineral geochemical studies of the ores. The LA–ICP–MS studies provide mineral-specific geochemical data in support of the petrographic observations. These data demon- strate that the specific mineral residence and concentrations of trace elements in the ores change as a result of recrystallization and remobilization, in addition to the resulting mineralogical and textural modifications. Based on the petrographic observa- tions and mineral chemistry presented, we propose a sequence of events that explain the formation and modification of the mineralogy, textures, metal endowments, and zonation of the ores throughout the deposit. The chapter concludes with a discussion of the mobility of metals in deformed massive sulfide deposits and a comparison of Greens Creek mineralogy and chemistry to modern and ancient analogs. Mine Location, General Host Rocks, Metamorphic History, Deposit Morphology, and Structural History The Greens Creek mine is 29 km south of Juneau, Alaska, near the northern end of Admiralty Island and within Admiralty Island National Monument (fig. 1). The deposit is located at the contact between a footwall sequence of predom- inantly phyllitic mafic volcanic rocks and mafic-ultramafic hypabyssal sills and intrusions, and a hanging wall of black argillites. Geochronologic studies (chap. 11) have established that the hanging-wall argillites have an age of 220 Ma and are thus part of the sedimentary portion of the regionally extensive Hyd Group of latest Triassic (Norian) age. Less precise age controls suggest strongly that the mafic-ultramafic intrusive suite in the footwall is also of Late Triassic age and are the parent rocks to the Hyd Group basalts that cap the argillites. Uncertainty remains as to the age and identity of the footwall phyllites. They are most likely either of middle Permian or of Late Triassic age. Constraints on the metamorphic history of the host rocks and ore at Greens Creek are few. The Late Triassic and older rocks of most of Admiralty Island are within the Admiralty Island Metamorphic Belt of Brew and others (1992), which refers to most of the island west of the Glass Peninsula. Within this region, mineral assemblage data are limited, no isograds have been mapped, and no thermobarometric data are avail- able. No clear trends in the metamorphic pattern are discern- ible, and rocks from subgreenschist through amphibolite facies are present, produced during post-Triassic metamorphism and deformation (Brew and others, 1992). Loney (1964) suggested there is a westward decrease in metamorphic grade present in correlative rocks on the southern portion of Admiralty Island. Regional mapping of metamorphic units (Dusel-Bacon, 1994) places the Admiralty Island Metamorphic Belt in the green- schist facies; descriptions of Late Triassic and younger rock units within this belt in the area around the mine (Lathram and others, 1965; S.M. Karl, written commun., 2003) suggest subgreenschist to lower greenschist facies metamorphic grade. Limited petrographic and electron microprobe work conducted during this study on Hyd Group basalt samples from the immediate vicinity of the mine suggest a dominantly chlorite- albite-epidote-quartz-sericite-calcite assemblage with occa- sional relict clinopyroxene and(or) hornblende phenocrysts. Tentative petrographic and electron microprobe identification of minor pumpellyite may indicate that the Late Triassic and younger rocks of the mine area are near the prehnite-pumpel- lyite to lower greenschist facies transition.
190 Geology, Geochemistry, and Genesis of the Greens Creek Massive Sulfide Deposit, Admiralty Island, Alaska SOUTHEA ST ERN (^) AL A SKA Mansfield Peninsula Admiralty Island Juneau f. CascadeCreek
-. . Killer Creek Bruin (^) Creek Cub Creek Gallagher Creek Big Sore Creek Cliff Creek Gre ens (^) C r eek CREW FERRY TERMINAL HAWK INLET FACILITY "A"R oad "B"R oad Tributary Creek Zinc Creek Mammoth Claim EXPERIMENTAL FOREST Big Sore Big Boil
YOUNG BAY HAWK INLET Ma^ riposite Ridge GREENS CREEK MINE PORTAL TONGASS NATIONAL FOREST Gallagher Ridge Lil Sore East Lil Sore National Monument Boundary Zinc Creek roadcut Antenna Mountain High Sore Zinc Creek Pass (^0) 4,000 8,000 FEET 0 1,000 2,000 METERS N Area of figure 1 ALASKA National Monument Wilderness Boundary Lakes District
192 Geology, Geochemistry, and Genesis of the Greens Creek Massive Sulfide Deposit, Admiralty Island, Alaska LS LS LS LS XS5000 XS4000 XS3000 XS2000 XS1000 XS Southwest orebody 5250 orebody 9A orebody West orebody Northwest orebody Shop orebodies 224 West Bench 200 West Bench 200 South orebody Upper Northwest orebody MAKI FAULT ZONE MAKI FAULT ZONE
LS4500 LS5500 LS 224 West Bench Southwest orebody West orebody East orebody 9A orebody Cross section 2400 view looking grid North 1,000 feet elevation 0 feet elevation (sea level) –1,000 feet elevation CROSS-SECTIONAL VIEW OF THE GREENS CREEK DEPOSIT. PLAN VIEW OF THE GREENS CREEK DEPOSIT.
East orebody Central zone South zone
MAKI FAULT KLAUS 5250 FAULT orebody North zone Figure 2. Plan and cross-sectional views of the Greens Creek deposit showing features described in the text.
Previous Mineralogical and Mineral Geochemical Studies 193 show a mix of primary and intensely granulated textures but no evidence for ductile deformation, and (3) the white ore sample contains abundant cymrite (hydrous barium alumino- silicate) and probably barian muscovite. Importantly, Alorno (written commun., 1987) also noted that the ore sulfides, particularly tetrahedrite, were confined to veinlets in the white ore and were not part of the “early assemblage.” Both poly- crystalline quartz and rotated dolomite porphyroblasts were observed in the phyllites, establishing a predeformation age of silica-carbonate alteration, similar to the findings of this study (V.M. Anderson and C.D. Taylor, unpub. data, 2000), and compositional layering in the phyllite occasionally was truncated against pyrite bands. If so, this observation suggests the presence of a premineralization fabric in the phyllites that is probably equivalent to the S 1 foliation of current usage (see chap. 7). Alorno’s report documents the occurrence of all of the currently known major sulfide minerals as well as gangue graphite, rutile, ilmenite, “green mica,” and the postulated barian muscovite. Although cymrite may be present, it has not been documented in any of the later studies. He notes the presence of delicate botryoidal features in pyrite-tetrahedrite- sphalerite bands, describes either cataclasis or the lack of deformation features in pyrite, and “granoblastic,” “annealed,” and “flame” textures in galena and sphalerite. Parageneti- cally, Alorno (George Alorno, written commun., 1987) notes the remobilization of tetrahedrite and galena into fractures in, and as matrix to, pyrite and refers to “pseudoframboidal” pyrite features produced by replacement of base-metal sulfides by pyrite. Additionally, he makes the important observation that early primary textured pyrite is commonly overgrown by euhedral pyrite. The next and most significant effort at understanding the mineralogy and mineral chemistry of the ores at Greens Creek is contained in a series of internal mining company reports. These reports detail the results of three major phases of work. The first phase consists of three reports that describe the mineralogy and mineral chemistry, as determined by electron microprobe analysis, on a variety of mill products drawn from East orebody ore: mill head; lead (Pb), zinc (Zn), and bulk concentrates; tails; and final tails (E.U. Petersen, written commun., 1991, 1992a, b). These early studies recognized the complexity of the silver (Ag) mineralogy at Greens Creek (see table 1) and placed some preliminary constraints on the mineral chemistry of a few of the major phases. East orebody sphalerite was shown to have fairly homogeneous low iron (Fe) content (less than 1.0 percent) and measurable cadmium (Cd), mercury (Hg), copper (Cu), and manganese (Mn) (less than 1.0 percent combined). Analyses of tetrahedrite showed them to be the zinc-antimony-rich, iron-arsenic-poor end member, with measurable cadmium. Silver in tetrahedrite ranged from about 0.5 to 6.5 percent with several exception- ally silver-rich analyses up to 15.9 percent. E.U. Petersen (written commun., 1992) also noted the common occurrence of an unknown silver-rich mineral (up to 44 percent silver), previously identified as covellite, which plots along the covel- lite (CuS) – acanthite (Ag 2 S) join. Results of the second phase of mineralogical research at Greens Creek are contained in a series of memorandums from Chris J. Carter to the Greens Creek mine staff (C.J. Carter, writ- ten commun., 1994a, b, c, d). In these memorandums, Carter presents a reevaluation of Petersen’s concentrate microprobe data and a preliminary evaluation of microprobe data from a set of 12 samples collected from the Southwest orebody. A report containing detailed descriptions and assay data provides con- text for the 12 hand samples (Debora Apel, written commun., 1993). Carter’s evaluation of Petersen’s data focuses on the variation of silver content in tetrahedrite and the possible utility of using silver as a vector for exploration. He suggests that the broad range of silver/copper values determined for tetrahedrites from East orebody concentrates indicates that original metal zoning features have probably been preserved and thus might provide a useful exploration criterion. Petrographic and geochemical studies of mineralogy in the Southwest orebody samples noted a broad range of sulfide textures and grain sizes. Four distinct textural varieties of pyrite were observed: euhedral, spongy, colloform, and framboi- dal. Minor to moderate amounts of arsenopyrite also were observed, and microprobe data on pyrite, arsenian pyrite, and arsenopyrite indicated measurable gold and nickel (Ni). Galena was noted to be weakly argentiferous and thus not a signifi- cant contributor to the silver budget. The antimony content of tetrahedrite was shown to vary greatly from about 13 percent (expressed as the molar ratio Sb/[Sb+As]) to about 81 percent, demonstrating that a series of tetrahedrite-tennantite miner- als are present in the Southwest orebody. These tetrahedrites also have three times the average silver content (expressed as the molar ratio Ag/[Ag+Cu]) than those in the East orebody (17.6 versus 5.2 percent), lower zinc content, and higher iron and cadmium content. They also report silver-rich minerals belonging to the proustite-pyrargyrite series, jalpaite, and an unidentified mineral with 86 weight percent silver (C.J. Carter, written commun., 1994). Ag/Ag+Cu versus Sb/Sb+As plots of tetrahedrite analyses were interpreted as having three distinct trends, indicating three distinct starting fluid compositions may have contributed to the ore-forming process (C.J. Carter, writ- ten commun., 1994). Sphalerite iron content was observed to be as homogeneous as in East orebody sphalerites with perhaps a slightly higher average value (0.9 weight percent). The potential use of iron in sphalerite and Ag-Cu versus Sb-As trends in tetrahedrites as exploration tools led to a third phase of mineralogical research. E.U. Petersen and D. Chirban (written commun., 1994) conducted a study aimed at charac- terizing the mineralogical and chemical zoning of the Greens Creek deposit based on tetrahedrite chemistry. Tetrahedrites in 44 samples primarily from drill holes on the 2400 cross sec- tion of the Lower Southwest orebody, representing four major ore types, were analyzed by electron microprobe (Denise Chirban, written commun., 1995; Denise Chirban and E.U. Petersen, written commun., 1996). The concentration of silver in tetrahedrite was found to vary from 2.45 to 20.95 weight percent and was accompanied by large variations in all three solid solutions as expressed by
196 Geology, Geochemistry, and Genesis of the Greens Creek Massive Sulfide Deposit, Admiralty Island, Alaska Primary Ore Mineralogy and Textures Roughly 30 percent of the Greens Creek ores retain primary mineralogy and textures. With one notable exception, primary features are present in massive, homogeneous, fine-grained MFPs that usually contain a high percentage of silica gangue. Apparently, large, cohesive blocks of pyritic ore have acted as undeformed nuclei around which the more ductile MFBs have flowed during deformation. Brecciation of the MFPs is a com- mon feature within these cohesive blocks, resulting in angular to subangular, tectonically milled clasts of primary-textured MFP in a sheared sulfide matrix consisting mostly of more ductile and recrystallized base-metal sulfides (fig. 4 A ). Clear evidence for primary depositional layering of sulfides is lacking. However, mineralogically banded massive ores are common and are com- posed of entirely recrystallized sulfides (fig. 4 B ). Within primary-textured MFPs, framboidal, colloform, and dendritic-textured pyrite is common and is intimately intergrown with a variety of base-metal sulfides and sulfosalts. Framboids are commonly in the 1–5 micrometer (mm) size range and are as large as 30 mm (figs. 5 A – C ). Colloform-textured areas can be millimeters in size (fig. 5 D ), with individual mineral bands ranging from less than 1 to 10 mm in width (fig. 5 E ). Mineral bands are most commonly composed of pyrite interbanded with sphalerite, galena, tetrahedrite, and a lead-antimony sulfosalt mineral (fig. 5 F ). Rarely, chalcopyrite replaces pyrite or infills bands between pyrite layers in the more copper-rich orebodies (fig. 5 G ). Dendritic-textured areas of pyrite are less common, only occur in MFP, and can have individual dendrites in the 100–200 mm size range (fig. 5 H ). Common accessory mineral- ogy in the primary textured ores includes sphalerite, galena, tetrahedrite, chalcopyrite, free gold, and a variety of Pb-Sb-As (-Hg-Tl) sulfosalts. Primary-textured sulfides are also present in WCAs. Rounded grains of pyrite up to a centimeter in diameter with radial growth textures occur in a matrix of massive, coarse crystalline dolostone. These features produce a “pudding stone” texture and are reminiscent of diagenetic pyrite nodules (fig. 4 C ). Similar nodular pyrite textures, fragments of nodules, pyrite rinds, and concentrations of framboidal and colloform pyrite intergrown with base-metal sulfides also occur in associa- tion with concentrations of white, hydrothermal dolomite that typically cuts more massive, coarse crystalline dolostone. These textures are clearly linked to hydrothermal veining and in some cases may be produced by solution brecciation of footwall dolostones and WCAs followed by precipitation of dolomite and quartz cements in the resulting cavities (fig. 4 D ). Recrystallized and Remobilized Ore Mineralogy and Textures Recrystallization of the primary textures in individual hand samples ranges from 20 percent to 100 percent and results in a range of textures. Incipient recrystallization results in the formation of polyframboidal aggregates (rogenpyrite; fig. 6 A ), “spongy” textured pyrite, and atoll-shaped pyrite. Spongy texture is most common in the massive ores in millimeter- to centimeter-scale areas of massive and aggregated pyrite. These areas often show relict framboidal and colloform textures. The pyrite itself commonly appears quite “clean” or monomineralic. However, the spongy areas are full of micrometer-scale inclu- sions of other sulfides, sulfosalts, and gangue minerals (fig. 6 B ). The centers of euhedral pyrite crystals larger than about 100 mm also tend to have spongy texture. As recrystallization pro- gresses, atoll-shaped pyrite forms at the expense of framboids and colloform pyrite (fig. 6 C ). The larger aggregates of poly- framboids and spongy textures coalesce further into large areas of inclusion-rich anhedral to subhedral pyrite (fig. 6 D ). The major effects of more advanced recrystallization is much coarser grain sizes and the formation and(or) remobi- lization of secondary, precious-metal-enriched minerals (fig. 6 E ). Pyrite recrystallization is characterized by development of euhedral crystals and polygonal textured masses that appear to be unzoned and free of mineral inclusions (fig. 6 F ). Recrys- tallization of pyrite tends to roughly double the grain size; for example, pyrite euhedra in areas of 1–5-mm framboids are 5–10 mm in size. In recrystallized aggregates or massive areas of pyrite with development of polygonal texture, individual grains are commonly in the 50–100-mm range. In MFPs that have been entirely recrystallized, pyrite euhedra are commonly in the 0.1- to 1-mm size range or larger (fig. 6 G ). Secondary minerals occur as discrete rounded inclusions within pyrite grains (figs. 6 H and I ), as growth zones within, and as mar- gins to, pyrite (figs. 6 J and K ), and as matrix to pyrite euhe- dra. Subhedral to euhedral sphalerite occurs in recrystallized MFB, usually in a matrix of anhedral galena (figs. 6 L and M ). Secondary mineralogy includes chalcopyrite, sphalerite (low iron), galena, free gold, electrum, tetrahedrite (antimony-rich), bornite, covellite, pyrargyrite, and a host of other sulfosalt minerals. Rare realgar has been identified (by X-ray diffrac- tion) in the WBAs of the 200 South orebody. Base-metal sulfides in recrystallized ores most commonly occur as anhe- dral intercrystalline masses of secondary sphalerite, galena, chalcopyrite, tetrahedrite, and pyrargyrite. Individual anhedral crystals are commonly 50–200 mm in size (fig. 6 N ). Second- ary sulfides also commonly occur in late fractures and veinlets millimeters wide and centimeters long (figs. 6 O and P ). An interesting and potentially genetically important modification that affects MFPs in proximal, copper-arsenic- gold-rich areas of the deposit is the occurrence of subhedral to euhedral, light creamy yellow to white, arsenian pyrite (FeS 2 with variable substitution of arsenic for iron) and arse- nopyrite (FeAsS, 46 percent arsenic). These phases appear to replace primary pyrite (figs. 7 A – C ) or form as overgrowths on primary-textured pyrite masses (figs. 7 D and E ). They are present in trace amounts to as much as 50 percent of a sample. Powder X-ray diffraction analyses of two MFP ores, each with approximately 50 percent of this sulfide visible in polished thin section, confirmed the presence of arsenopyrite, but only in trace to minor amounts. In both cases the major
198 Geology, Geochemistry, and Genesis of the Greens Creek Massive Sulfide Deposit, Admiralty Island, Alaska Figure 5 (above and facing page). Photomicrographs of polished thin sections showing primary mineral textures: ( A ) framboidal pyrite (rib sample 96GC–14, 31’, 344 crosscut, Lower Southwest orebody); ( B ) pyrite (py) framboids and galena (gn) spheres in spongy-textured to massive pyrite (drill-core sample GC1643–04, 75’ 200 South orebody); ( C ) aggregated pyrite framboids in a matrix of sphalerite (sph) (drill-core sample GC1643–03, 54’, 200 South orebody); ( D ) colloform-textured aggregate of pyrite with a spongy-textured interior and growth zones composed of sphalerite, galena, and tetrahedrite (rib sample 96GC–12, 21.5’, 344 crosscut, Lower Southwest orebody);
Free Gold—Mineralogy and Textures 199 in chemically remobilized veins and is particularly abundant in association with late quartz-carbonate-pyrargyrite veins. Concentrations of these precious metal enriched veins usually occur in or within close proximity to areas of primary-textured ore and account for the highest grade ores in the mine, with total metal values reaching $10,000/ton. Free Gold—Mineralogy and Textures Free gold is a relatively common feature at Greens Creek and has been found in a wide variety of both ore and host-rock lithologies. Two types are visually distinguish- able, and a third was identified during LA–ICP–MS analysis. Bright yellow gold of high fineness occurs in thin section as rounded blebs and inclusions from 5 to 100 mm in size, although the most common size is about 5 to 20 mm (fig. 8 A ). Most of the grains are in cracks in pyrite and in the matrix to pyrite grains in association with secondary chalcopyrite, sphalerite, galena, tetrahedrite, and pyrargyrite (figs. 8 B and C ). Most gold grains are in contact with either pyrite or ar- senian pyrite (fig. 8 D ); however, a few grains were observed included within coarse-grained subhedral to euhedral pyrite (fig. 8 E ). Yellow gold is most commonly seen in the large blocks of MFP ore but also has been observed in all of the ore types at the mine and in both hanging-wall and footwall rocks, usually in association with quartz-carbonate veins. The second type of free gold is silvery white and occurs predominantly as a thin foil coating late fractures and shear planes in underground exposures and hand samples (figs. 8 F and G ). In thin section, white gold appears as thin veinlets and spaced sheets that occupy fractures up to 0.5 millimeter wide. These fractures are paragenetically late and appear to be tension gashes and piercement structures that occur at high angles to S 2 foliation (fig. 8 H ). Geochemical data sug- gest this is a gold-silver electrum. Free gold occurs in ores that exhibit a high degree of recrystallization and remobilization of the more ductile sulfides and are usually in close proximity to areas of primary- textured ores. Exceptions include yellow gold that occurs in quartz-carbonate veins in the host rock and rare instances of white gold foil in highly sheared and fractured argillites in the immediate hanging wall. These electrum-bearing quartz-car- bonate veins typically are within 1–2 m of high-grade primary ores. Yellow gold has also been observed in association with chalcopyrite and tetrahedrite in hairline fractures cutting late diabase dikes in the 200 South portion of the Lower Southwest orebody. This relationship thus requires remobilization of gold at some time after emplacement of the dikes. The two types of gold, as well as native silver, are easily distinguished based upon LA–ICP–MS geochemical studies of free gold separates recovered from the mill (figs. 9 A – I ). Laser traverses across 0.5- to 1.0-mm-sized grains recovered from heavy concentrates, represented as time versus elemental intensity plots, clearly indicate the presence of nearly pure gold grains with a high ratio of gold to silver (figs. 9 A , D – F ). Other grains that appear silvery-white on polished surfaces show lower gold-to-silver ratios and consistently elevated mercury, demonstrating that the silvery white gold is actually a ( E ) colloform banded spheres in pyrite with growth bands filled with galena or perhaps lead-antimony sulfosalt minerals (rib sample 96GC-13, 25.5’, 344 crosscut, Lower Southwest orebody); ( F ) aggregate of colloform shapes in pyrite (rib sample 96GC–13, 25.5’, 344 crosscut, Lower Southwest orebody); ( G ) chalcopyrite (cpy) infilling or replacing bands between pyrite layers in colloform-textured MFP (drill-core sample GC1527–09, 402.5’, Northwest orebody); and ( H ) dendritic-textured pyrite fronds in massive pyrite (rib sample 96GC–16, 43.5’, 344 crosscut, Lower Southwest orebody).
Paragenetic Relationships—Physical and Chemical Modification of Ores 201 Figure 6 (pages 15–17). Photomicrographs of polished thin sections showing recrystallized ore textures: ( A ) growth of polyframboidal clusters in a matrix of sphalerite (sph) (rib sample 96GC–12, 21.5’, 344 crosscut, Lower Southwest orebody); ( B ) spongy-textured pyrite (py) with relict framboidal and colloform shapes, and numerous inclusions of sulfides, sulfosalts, and gangue (rib sample 96GC–18, 51’, 344 crosscut, Lower Southwest orebody); ( C ) atoll structure of clean recrystallized pyrite forming around a grain of spongy- textured pyrite (rib sample 96GC–13, 25.5’, 344 crosscut, Lower Southwest orebody); ( D ) clean anhedral pyrite with numerous rounded inclusions of tetrahedrite (tetr) and galena (gn) (drill-core sample GC1643–06, 119.5’, 200 South orebody); ( E ) veinlets of chalcopyrite (cpy), tetrahedrite (tetr), galena (gn), and yellow gold (Au) cutting recrystallized pyrite (drill-core sample GC1643–05, 100.5’, 200 South orebody); ( F ) clean, completely recrystallized, polygonal-textured pyrite (rib sample 96GC–14, 31’, 344 crosscut, Lower Southwest
202 Geology, Geochemistry, and Genesis of the Greens Creek Massive Sulfide Deposit, Admiralty Island, Alaska orebody); ( G ) recrystallized pyrite euhedra in a matrix of chalcopyrite, galena, tetrahedrite, and sphalerite (drill-core sample GC1643–08, 147.5’, 200 South orebody); ( H ) clean, recrystallized anhedral pyrite with rounded inclusions of sphalerite (drill-core sample GC1643–08, 147.5’, 200 South orebody); ( I ) partially recrystallized spongy pyrite with rounded inclusions of chalcopyrite and galena (drill-core sample GC1643–07, 143.5’, 200 South orebody); ( J ) clean, recrystallized pyrite with discrete, atoll-shaped growth zones of chalcopyrite and galena GC1527–08, 397.5’, Northwest orebody); ( K ) recrystallized pyrite with rims of galena (drill-core sample GC1643–11, 438’, 200 South orebody); ( L ) recrystallized euhedral pyrite and sphalerite in a matrix of anhedral tetrahedrite and galena (drill-core sample
