Gold and Base Metal Mineralization, Hydrothermal Alteration and Vein Paragenesis in the Spring Valley Deposit, Pershing County, Nevada
AuthorCrosby, Betsy Leigh
AdvisorThompson, Tommy B.
Geological Sciences and Engineering
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Spring Valley is a gold-base metal deposit located on the eastern flank of the Humboldt Range in Pershing County, Nevada. Mineralization is primarily hosted within the Feldspar Porphyry, Agglomerate, Welded Tuff, Volcaniclastic Siltstone and Quartz-eye Rhyolite units of the Rochester Formation and in lithic tuffs and greenstones of the Limerick Formation. It occurs in a series of crosscutting quartz-carbonate ± pyrite ± tourmaline ± sphalerite ± galena veins of clear mesozonal origin associated with weakly to strongly pervasive quartz-sericite-pyrite-carbonate ± tourmaline alteration. Initial classification as a porphyry gold system is thus disproved, and alternate orogenic and reduced intrusion-related gold interpretations are presented. Vein-related alteration is relatively consistent throughout the Quartz-eye Rhyolite, Welded Tuff, Agglomerate and Feldspar Porphyry members of the Rochester Formation, though it is better developed in the latter. In the Feldspar Porphyry, proximal alteration consists of a light green to greenish-gray, weakly to strongly pervasive quartz-sericite-pyrite-carbonate ± tourmaline assemblage. Tourmaline is typically only found in selvages adjacent to tourmaline-bearing veins. Proximal quartz-sericite-pyrite-carbonate-tourmaline alteration grades outward into an intermediate to distal quartz-hematite alteration assemblage, characterized by the presence of very fine-grained hematite "dust" that imparts a distinct red-brown coloration to groundmass and feldspar phenocrysts. Quartz-hematite alteration may grade further outward into a distal quartz-specularite assemblage, in which very-fine grained, platey specularite lends a dark gray coloration to groundmass. Alteration in the overlying Agglomerate unit of the Rochester Formation is best observed in reactive greenstone clasts. It consists of a light green quartz-sericite-pyrite-carbonate ± fuchsite ± tourmaline assemblage, where similarly to proximal alteration zones in the Feldspar Porphyry, tourmaline occurs consistently outboard of tourmaline-bearing veins. Proximal alteration grades outward into a dark gray quartz-specularite ± hematite assemblage. Hydrothermal alteration of Welded Tuff and Feldspar Porphyry clasts is similar to that of their source units. Alteration effects are generally not present in groundmass or unreactive quartzite and carbonate clasts. Proximal alteration in the remainder of the Rochester Formation and in the Limerick Formation is consistent with that observed in the Feldspar Porphyry and Agglomerate units, though intermediate to distal alteration varies considerably. In the Welded Tuff, proximal alteration consists of a light green to gray quartz-sericite-pyrite-carbonate ± tourmaline assemblage that occurs interstitial to silicified fiamme. At depth, proximal alteration may grade outward into a dark gray to purplish quartz-specularite ± hematite assemblage, though at shallow levels, it generally grades outward into relatively unaltered wallrock. In the Quartz-eye Rhyolite, proximal light green quartz-sericite-pyrite-carbonate alteration grades outward into a greenschist-facies sericite-carbonate assemblage. In the Limerick Formation, proximal light green quartz-sericite-pyrite-carbonate ± fuchsite ± tourmaline alteration crosscuts a distinct, greenschist-facies chlorite-carbonate-hematite ± epidote assemblage. Proximal alteration may grade outward into a hematite-rich zone, but most commonly does not. Following characterization of alteration assemblages and zoning, major element compositions of representative samples from alteration zones in the Feldspar Porphyry were determined. Al2O3 generally decreases from unaltered to proximally altered samples. It is broadly paralleled by an increase in SiO2 and is thus likely due to increasing secondary quartz in quartz-sericite-pyrite-carbonate alteration zones. Decreasing CaO from unaltered to proximally altered zones is likely due to an increasing presence of iron in carbonate. Increasing K2O, at first counterintuitive due to the certain loss of K2O during sericitization of K-feldspar, is likely due to the relatively high volumes of sericite introduced in anastomosing veinlets. BaO trends mimic those of K2O and are likely related to the substitution of Ba for K in sericite. Quartz vein textures at Spring Valley are indicative of formation at mesozonal depth under high pressure and are thus also inconsistent with a porphyry interpretation. Pre-ore ataxial and ore-stage crack-seal and non-directional quartz veins were identified. Ataxial veins are composed of distinctive fibrous quartz and are associated with moderately to strongly pervasive silicification. Many trend northwest, suggesting formation during NW/SE-directed crustal shortening. Crack-seal and non-directional quartz veins are associated with previously described quartz-sericite-carbonate-pyrite ± tourmaline alteration and may host sphalerite-galena-gold ± chalcopyrite ± tetrahedrite mineralization. Crack-seal veins are generally earlier and are composed of elongate-blocky quartz ± pyrite, with late carbonate and tourmaline that formed previous to the crack-seal process. Non-directional quartz veins are composed of granular quartz-carbonate ± pyrite ± tourmaline ± sphalerite ± galena ± chalcopyrite ± tetrahedrite ± gold and record the first introduction of Pb, Zn and Au mineralization. Ore stage veins are primarily brittle features, with internal textures indicative of formation during extension, including relatively planar, matching vein walls, open-space filling quartz textures, growth of elongated grains perpendicular to vein walls, and evidence of vein-opening direction, such as inclusion bands in crack-seal veins. Eight vein stages were recognized at Spring Valley: 1. Pre-ore ataxial veins associated with moderately to strongly pervasive silicification; 2. Early gray granular quartz-tourmaline ± pyrite veins; 3. Early ore-stage gray elongate-blocky to granular quartz-carbonate ± pyrite veins; 4. White to gray elongate-blocky quartz ± carbonate-pyrite veins; 5. Gray granular quartz-carbonate ± pyrite veins; 6. Gray granular quartz-tourmaline ± carbonate-pyrite veins; 7. White granular quartz-tourmaline ± carbonate-pyrite; 8. White granular quartz-carbonate-pyrite ± sphalerite ± galena ± chalcopyrite ± tetrahedrite ± gold. The cyclical transition between granular and crack-seal veins likely reflects fluctuations in pressure and changes in fluid chemistry throughout the evolution of the deposit. Fluctuations in temperature were not likely a factor. Analyses of 129 LH2O-VCO2 and LH2O-LCO2- VCO2 inclusions identified in 6 of 8 ore-stage quartz veins indicate that the minimum temperature of formation was consistent throughout vein formation and ranged from 210°C to 340°C. An applied pressure correction 120°C to 180°C brings the range of trapping temperatures to approximately 330°C to 520°C. Gold mineralization is also inconsistent with classification as a porphyry system. It may occur in all ore-stage vein stages, particularly where proximal to later Au-bearing veins. It is primarily associated with pyrite, though it may occur without pyrite and/or with later sphalerite and galena. In contrast to porphyry systems, sulfide content is generally less than 2%, and only trace amounts of copper sulfides--primarily chalcopyrite and tetrahedrite--are present. Ore mineralogy is much simpler than that of nearby deposits, with virtually no silver and volumetrically minor sulfide mineralization. Ore mineralogy, alteration mineralogy and zoning, and vein textures are inconsistent with classification as a porphyry gold system. Two alternative interpretations are proposed: orogenic and reduced intrusion-related gold. Spring Valley shares many similarities with orogenic gold deposits, including the mineralogy and consistency of alteration assemblages and zoning, the mineralogy and simplicity of gold and associated sulfides, and crack-seal and non-directional vein textures indicative of formation under mesothermal conditions. Similarly to most orogenic gold deposits, mineralization occurs in greenschist-facies host rocks where it is clearly retrograde in origin. However, the geometry of extensional crack-seal and non-directions veins is dissimilar to that of extensional veins in established orogenic gold deposits, and a definitive structural setting cannot be established. Reduced intrusion-related gold (RIRG) deposits share many of the above characteristics with orogenic gold deposits and the gold occurrence at Spring Valley. Several additional lines of evidence suggest Spring Valley is a reduced intrusion-related gold deposit: 1. Mineralization is spatially and possibly temporally associated with the Late Cretaceous Rocky Canyon granodiorite; 2. Mineralization is low grade and irregularly distributed; 3. Quartz vein geometries are more in line with those of proximal RIRG deposits; and 4. Metal zoning in the Humboldt Range is strikingly similar to metal zoning in established RIRG districts. However, RIRG deposits are typically associated with more complex ore and alteration mineralogy, and no evidence of a genetic relationship between varying deposits in the Humboldt Range has been established. In addition, a broad, NS-trending zone of Au-bearing metamorphic quartz veins similar in geometry, morphology and orientation identified throughout northwestern Nevada suggests that vein formation occurred independently of Late Cretaceous magmatism (Cheong, 1999). In the absence of an age of mineralization, the style of gold mineralization at Spring Valley cannot be definitively established