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Appendix E Comparison to Other Deposits The Statement of Task requests that the committee include a discussion of current gold mining operations at sites with comparable geologic, mineralogical, hydrologic, and climatic characteristics to those found in the Com- monwealth (see Box 1-3). Below, we summarize those features that are common to all (or most) deposits in Virginia and discuss a few individual non-Virginia deposits that show many features that are similar to Virginia deposits. All of the gold deposits in Virginiaâs gold-pyrite belt and in the Virgilina district are classified as orogenic deposits (Goldfarb et al., 2005). This class of gold deposit is associated with mountain building, metamorphism, deformation, and regional shearing. Orogenic gold deposits are further subdivided based on the intensity, or tem- perature and pressure, of the metamorphic and deformation events, and include deposits hosted in subgreenschist to lower greenschist (~<1â1.5 kbar; <~5 km) rocks, greenschist grade (~1.5â3 kbar; <~5â10 km) rocks, and amphibolite grade (~3â5 kbar; <~10â18 km) rocks. Deposits in the gold-pyrite belt and the Virgilina district are associated with greenschist grade metamorphic rocks, although some approach amphibolite grade and some in the Virgilina district are lower greenschist grade. The tectonic, structural, and alteration characteristics of gold- pyrite belt deposits match the greenschist class of orogenic deposits well, but differ from many greenschist-hosted orogenic deposits in that a larger proportion of gold in this class occurs as âfreeâ gold, whereas most gold in the gold-pyrite belt is encapsulated in pyrite. The gold-pyrite belt deposits are also lower in arsenic than the typical greenschist grade orogenic deposit, and veins/lenses in gold-pyrite belt deposits tend to be smaller than the aver- age reported for this class of deposit (Goldfarb et al., 2005). COMPARISON WITH OTHER NON-VIRGINIA DEPOSITS Below, we summarize the characteristics from non-Virginia deposits that have comparable geologic, miner- alogical, hydrologic, or climatic characteristics to Virginia gold deposits. Haile Gold Mine, South Carolina The Haile Gold Mine is comparable to gold deposits in Virginia in that it is found in the Piedmont physio- graphic province, which has a humid, subtropical climate, and aquifers hosted in fractured bedrock (SCDHEC, 2022). Similar to Virginia gold mines, Haile Gold Mine is an orogenic deposit that occurs in greenschist facies rocks (although of lower metamorphic grade than in Virginia) that has quartz-sericite (carbonate) alteration and low pyritic ores that are associated with shearing and occur along a linear trend. However, Haile differs from 205
206 THE POTENTIAL IMPACTS OF GOLD MINING IN VIRGINIA Virginia deposits in that much of the mineralization is disseminated in a siltstone at Haile rather than occurring in distinct quartz veins, the mineralized zones are much wider with barren or low-grade rock between mineralized zones (which requires open pit mining), and the ores are more arsenic rich and contain much fewer base metals (Foley and Ayuso, 2012; Foley et al., 2001; Mobley et al., 2014; SRK Consulting, 2020). As a result, even though the original formation of gold deposits in Virginia may have occurred in similar tectonic environments as gold deposits in South Carolina, the gold deposits in these two states have experienced different postformation histories that have led to different styles of mineralization. Another important difference between the Haile mine and gold deposits in Virginia is the scale of the deposits. The Haile mine extracts much larger volumes of rock compared to mining that would occur in Virginia, generating much more waste and tailings, and requiring a much larger surface footprint to accommodate the infrastructure and waste and tailings storage. The potential environmental impact of a tailings dam failure or release of water from the site at Haile is commensurately larger because of these various factors. Dahlonega District, Georgia The gold deposits of the Dahlonega district, Georgia, are very comparable to the Virginia deposits. Similar to Virginia, the Dahlonega district deposits are associated with shear zones in greenschist wallrock, have gold hosted in pyrite that is found in steeply dipping veins, and have relatively low arsenic content. Graton and Lindgren (1906) note that the mode of origin was similar to that of the Californian and Australian deposits, which are classified as oro- genic gold deposits today. The main differences between the Dahlonega deposits and those in Virginia are that there are no massive sulfide deposits spatially associated with the Dahlonega deposits, the gangue mineral assemblage at Dahlonega contains less carbonate (and no reported ankerite) as is common in the Virginia deposits, and pyrrhotite appears to be more common in the Dahlonega deposits. Most other characteristics of the Dahlonega deposits are in good agreement with the Virginia gold deposits (Graton and Lindgren, 1906; Jones, 1909; Yeates et al., 1896). Most of the mining in the Dahlonega district occurred in the saprolite using hydraulic mining methods. In some cases, such as at the Findley Mine, the quartz veins in saprolite are identifiable and competent and mined using conventional methods (not hydraulic mining) (Jones, 1909). The Kensington Deposit, Alaska The Kensington deposit is an orogenic gold deposit that consists of the Kensington, Raven, Eureka, Jualin, and Elmira deposits/ore bodies. Vein mineralization in the Raven ore body is characterized by gold and goldâsilver telluride minerals, where most of the gold is contained in calaverite (AuTe2) that occurs in association in and interstitial to pyrite grains and in microfractures in pyrite. In the other ore bodies, mineralization occurs primarily as disseminated pyrite or auriferous pyrite seams and blebs. The Kensington deposits are comparable to deposits in Virginia in terms of genetic classification as orogenic depositsâboth are associated with shearing, have steeply dip- ping veins, show similar alteration assemblages, are low in arsenic, and have similar gold grades. The Kensington deposits differ from gold-pyrite belt deposits in that schists are not associated with the Kensington deposits, and mineralization occurs completely within one rock type (Jualin diorite). Additionally, there are no associated mas- sive sulfide deposits at the Kensington deposits (Cox and Bagbey, 1992; Pascoe et al., 2022; USGS, 2017c). The scale of the mining operation and the processing methods used at Kensington are similar to what might occur in Virginia. Kensington has a total of 983,000 indicated and measured ounces of gold (Pascoe et al., 2022), which is approximately one to two orders of magnitude larger than deposits in Virginia. The mining operations use underground mining, crushing, and flotation to produce a concentrate that is then shipped off-site for further processing (Pascoe et al., 2022). Bousquet Group, Canada The Mic Mac, Mooshla A, Mooshla B, and parts of the Mouska deposit at the Bousquet Group of orogenic gold deposits in Quebec, Canada, are comparable to the Virginia deposits in that there is a clear genetic association
APPENDIX E 207 between their shear zoneâhosted gold mineralization and nearby volcanogenic massive sulfide deposits. In other respects, the Bousquet deposits are not comparable to Virginia gold deposits. For example, their upper greenschist to amphibolite metamorphic grade is distinct from the lower to mid greenschist metamorphic grade in Virginia. Additionally, Bousquet has a much higher pyrite content than Virginia, different alteration assemblages, and gold associated with chalcopyrite, bornite, and pyrrhotite, instead of pyrite (Groves et al., 2003; Mercier-Langevin etÂ al., 2007; Tourigny et al., 1993). Mikado Deposit, Alaska The Mikado deposit is an orogenic deposit that is most comparable to the Virginia deposits in terms of size, geometry, and the scale of the mining operation required. In other ways, the Mikado deposit is not similar to Virginia deposits. For example, the Mikado deposit is much higher in arsenopyrite than Virginia ores, veins that represent open space filling rather than replacement, wall rocks that are highly graphitic, and more free gold than gold encapsulated in sulfides (Ashworth, 1983). Eagle Shawmut Mine, Mother Lode District, California The Eagle Shawmut Mine is an orogenic deposit in the Mother Lode of California is comparable to Virginia deposits in many respects. For example, the Eagle Shawmut deposit is of a similar size and is associated with steeply dipping veins in greenschist-hosted shear zones. However, the deposit differs from Virginia in that it has higher arsenic and contains more free gold (Knopf, 1929).