J.V. Tingley | Search USGS

USGS2005

Base- and precious-metal deposits in the Basin and Range of Southern California and Southern Nevada - Metallogenic implications of lead isotope studies

Southern California and southern Nevada contain abundant lead-zinc deposits with strikingly different characteristics. On the west, the Darwin Terrane contains abundant Jurassic and Cretaceous intrusions surrounded by lead-zinc skarn and replacement deposits rich in pyrite and manganese minerals. The Tecopa Terrane is east of the Darwin Terrane and contains some lead deposits that are hosted by the Proterozoic Noonday Dolomite. These lead deposits have no consistent relation to igneous rocks; they contain mainly galena, and are devoid of pyrite and manganese minerals. Other skarn and vein deposits in the Ivanpah and Tecopa districts are more closely associated with igneous rocks. Mississippi Valley type lead-zinc deposits are present still farther to the east in the Goodsprings Terrane in Nevada. These deposits are hosted by breccias formed below the Mississippian-Pennsylvanian unconformity and are unrelated to igneous rocks. Deposits in the Darwin Terrane have lead isotopic signatures that lie along a mantle-sediment mixing line indicating that they formed in a continental arc setting analogous to that for the plutons in the Sierra Nevada batholith [Chen, J.H. and Tilton, G.R., 1991. Application of lead and strontium isotopic relationships to the petrogenesis of granitoid rocks, central Sierra Nevada batholith, California. Geological Society of America Bulletin 103, 439-447]. Encroachment of this continental arc on the North American continent in the eastern part of the Darwin Terrane resulted in a lead isotopic signature that is like that of the strongly contaminated plutons [Chen, J.H. and Tilton, G.R., 1991. Application of lead and strontium isotopic relationships to the petrogenesis of granitoid rocks, central Sierra Nevada batholith, California. Geological Society of America Bulletin 103, 439-447]. Many deposits from the Inyo Mountains on east side of the Owens Valley have lead isotopic signatures that reflect this encroachment. To the east in the Tecopa Terrane, encroachment of the continental arc on the Mojave crust resulted in partial melting of 1.7 Ga amphibolite and granulite facies rocks to produce the plutons and mineral deposits associated with plutons in this terrane. Lead from deposits in this terrane hosted in the Proterozoic Noonday Dolomite and associated rocks have a lead isotope signature that reflects hydrothermal circulation of fluids in the Mojave supracrustal rocks. The boundary between the Darwin Terrane and the Tecopa Terrane lies just west of the Ash Valley-Panamint Range Fault and is bounded by the Towne Fault on the north and the Garlock Fault on the south. Lead isotopic data from the Goodsprings district in southwestern Nevada, east of the Tecopa Terrane, form a 1.45 Ga linear array that is indicative of Mississippi Valley type mineralization. Although we have no independent evidence of the timing of the tectonic events that formed these deposits, it is likely that all formed as a result of the Mesozoic collision of the Panthalassen crust with the North American continent. ?? 2005 Elsevier B.V. All rights reserved.

Earth-Science Reviews↗

USGS2003

Structural localization and origin of compartmentalized fluid flow, Comstock lode, Virginia City, Nevada

Bonanza-grade orebodies in epithermal-style mineral deposits characteristically occur as discrete zones within spatially more extensive fault and/or fracture systems. Empirically, the segregation of such systems into compartments of higher and lower permeability appears to be a key process necessary for high-grade ore formation and, most commonly, it is such concentrations of metals that make an epithermal vein district world class. In the world-class silver- and gold-producing Comstock mining district, Nevada, several lines of evidence lead to the conclusion that the Comstock lode is localized in an extensional stepover between right-lateral fault zones. This evidence includes fault geometries, kinematic indicators of slip, the hydraulic connectivity of faults as demonstrated by veins and dikes along faults, and the opening of a normal-fault-bounded, asymmetric basin between two parallel and overlapping northwest-striking, lateral- to lateral-oblique-slip fault zones. During basin opening, thick, generally subeconomic, banded quartz-adularia veins were deposited in the normal fault zone, the Comstock fault, and along one of the bounding lateral fault zones, the Silver City fault. As deformation continued, the intrusion of dikes and small plugs into the hanging wall of the Comstock fault zone may have impeded the ability of the stepover to accommodate displacement on the bounding strike-slip faults through extension within the stepover. A transient period of transpressional deformation of the Comstock fault zone ensued, and the early-stage veins were deformed through boudinaging and hydraulic fragmentation, fault-motion inversion, and high- and low-angle axial rotations of segments of the fault planes and some fault-bounded wedges. This deformation led to the formation of spatially restricted compartments of high vertical permeability and hydraulic connectivity and low lateral hydraulic connectivity. Bonanza orebodies were formed in the compartmentalized zones of high permeability and hydraulic connectivity. As heat flow and related hydrothermal activitv waned along the Comstock fault zone, extension was reactivated in the stepover along the Occidental zone of normal faults east of the Comstock fault zone. Volcanic and related intrusive activity in this part of the stepover led to a new episode of hydrothermal activity and formation of the Occidental lodes.

Economic Geology↗