Interpretation of mineralization styles, alteration, mineralogy, paragenesis, and geochemistry at Gold Quarry, Carlin trend, Nevada: Implications for Northern Nevada rift-related, Miocene, intermediate-sulfidation, base-metal mineralization
AuthorSherlock, Wesley Kingston
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AbstractNewmont’s Gold Quarry deposit is a world class Carlin-type deposit located on the southern portion of the Carlin trend. The deposit is hosted primarily in the Rodeo Creek unit, which is extensively faulted and locally exhibits multiple types and intensities of alteration and mineralization. The majority of the remaining ore is sulfidic-refractory consisting of Eocene Carlin-type mineralization where the gold is hosted as a solid solution within the structural lattice of arsenian pyrite. Extensive Miocene base-metal mineralization locally overprints Eocene Carlin mineralization and contains rare local arsenian pyrite and widespread small, generally less than 1 to 10 µm, late free gold grains. Miocene gold may also be sequestered locally in other sulfides such as arsenopyrite. Silver is constrained to Miocene base-metal mineralization and is not represented above trace element levels in Eocene Carlin mineralization. Miocene base-metal sulfides and sulfosalts are common throughout the Gold Quarry deposit. Massive sulfide veins ranging in size from less than 1 cm to over half a meter in width cut earlier Eocene Carlin-type mineralization including cutting up through reactivated mineralized Eocene structures. Carlin-type mineralization immediately bounding the base-metal veins is generally destroyed by Miocene vein-associated alteration. Base-metal mineralization occurs in four primary styles at Gold Quarry: fracture/fault controlled massive sulfide veins; replacement base-metal mineralization; hydrothermal breccia base-metal sulfide matrix mineralization; and weak disseminated base-metal mineralization. Miocene massive sulfide base-metal mineralization is divided into three phases separated by two quartz events. The three mineralization events are preceded by an argillization event that locally ranges from weak to intense. Argillization locally overprints or destroys Eocene Carlin mineralization and is itself cut or replaced by Miocene base-metal mineralization. Eocene Carlin gold, however, does not appear to be remobilized by the overprinting argillization. Base-metal Phase I mineralization is iron sulfide-dominant comprised primarily of pyrite and marcasite with minor As and Ni iron sulfides and late sphalerite. Phase II mineralization is base-metal sulfide-and sulfosalt-dominant with a clear metallogenic progression from early copper dominant minerals to silver-rich minerals and terminates with late sphalerite and greenockite. Phase I sphalerite has a low Fe content and commonly exhibits chalcopyrite disease, while Phase II sphalerite in contrast has a high Fe content and no accompanying chalcopyrite disease. The primary Ag-bearing minerals include tetrahedrite, proustite-pyrargyrite, galena-associated Pb-Sb sulfosalts, and aramayoite. Phase III represents system shutdown and is gangue mineral (quartz, barite, and carbonate) dominant with only minor local pyrite and sphalerite. Phase III also contains the majority of the free gold complement with rare encapsulated free gold also associated with the end of the Phase II quartz event. Local supergene alteration primarily consisting of oxidation is generally in close proximity to structures, especially at depth. Base-metal sulfide/sulfosalt paragenesis is consistent throughout the deposit. The full paragenetic sequence is represented in all of the mineralization styles except for hydrothermal breccia base-metal sulfide matrix mineralization zones, which have been found to be primarily restricted to Phase I mineralization. Select sample 50-element geochemistry was used to derive the unique geochemical signatures for the various mineralogy and alteration styles. This allows for the interpretation of the mineralogy of structures or zones that have been altered or that were decomposed. It also provides a template for the interpretation of the 20-foot interval composite geochemical data. The geochemical correlations consistently support the mineralogical associations and paragenesis. The select sample massive sulfide vein correlations indicate that most of the veins experienced mineralization episodically throughout the Miocene mineralization event. However, several vein samples showed tight correlation even with large separation distances. Massive sulfide veins of only one inch in width exerted sufficient influence on the geochemical signature of a 20-foot interval to be recognized and tentatively identified mineralogically.Select sample geochemical analysis shows strong correlation of Se with base-metal mineralization. Te shows a strong correlation with Bi. Maximum Se (674 ppm) and Te (366 ppm) values for select samples and 590 and 61.4 ppm for composite-intervals respectively, suggests Miocene association. Miocene massive sulfide veins display a wide range of Au values from essentially barren to approximately 0.5 oz/t, while Ag values locally exceed 105 oz/t, indicating locally significant Au, and the majority of the Ag component at Gold Quarry, is of Miocene origin. Multiple extensive base-metal intercepts including 120 feet of Zn at 3.39% and 40-feet of Cu at 0.0206% are represented in the interval composite geochemistry. A 20-ft interval also displays high Au (28.3 ppm) and Zn (16,700 ppm) suggesting Miocene reactivation and mineralization of earlier Eocene mineralized feeder structures. Miocene base-metal mineralization style, sulfide assemblage and paragenesis are indicative of a relatively deep seated intermediate sulfidation source. Moderate to intense argillization is locally extensive at Gold Quarry. High to intermediate sulfidation systems are commonly initiated by an early magmatic vapor phase resulting in barren argillized zones (Sillitoe, 1995). Locally, the argillized zones are not barren due to residual Eocene Carlin Au. The gradational transition of the Miocene base-metal sulfide and sulfosalt assemblage from a higher-sulfidation state to a lower-sulfidation state is represented in the paragenetic sequence. Two distinct sphalerite mineralization styles, low-iron Phase I and high iron Phase II, along with intervening chalcopyrite-tennantite-tetrahedrite support this interpretation. The high Se values, alunite date and intermediate sulfidation state of the Miocene base-metal mineralization suggest a direct relationship with the northern Nevada rift. This is also supported by the spatial association of the Carlin trend with the northern Nevada rift. Several samples containing zoned tabular euhedral alunite have been identified. The alunite generally occurs replacing Phase III barite and in direct physical contact with and paragenetically prior to crystalline dickite, and is interpreted as representing system shutdown of the Miocene base metal mineralization event. Both of these minerals are interpreted to have formed under hypogene processes as dickite forms at approximately 175 to 250ºC (Zotov et al., 1998). One particularly clean alunite sample with zoned tabular crystals has been analyzed during this study by 40Ar/39Ar and rendered a date of 16.03 ± 0.10 Ma. Estimates of the temperature of formation of supergene alunite, recovered from much higher in the deposit, at Gold Quarry by previous workers is 80 to 100ºC (Heitt, 1992; Arehart et al., 1993a) with corresponding K/Ar dates of 30 ± 1.2 Ma, 27.9 Ma, and 25.9 ± 0.6 Ma. This suggests that the supergene alunite is the result of the alteration of Eocene Carlin mineralization.