Eocene Igneous Geology and Relation to Mineralization: Railroad District, Southern Carlin Trend, Nevada

摘要

The Railroad District (Railroad), in the northern Pi?on Range, at the southern end of the Carlin trend, Nevada, underwent intense and complex magmatism that domed the intruded Paleozoic rocks and generated several episodes of late Eocene hydrothermal activity. Mineralization types present at Railroad are porphyry Cu-Mo, skarn Ag-Cu-Pb-Zn-Mo, skarn overprinted by Carlin-type Au, and Carlin-type Au (Jackson et al., this volume; Koehler et al., this volume). Field study, chemical analyses, and ~(40)Ar/~(39)Ar and U-Pb dating of igneous rocks reveal that igneous activity occurred between about 38.9 and 37.4 Ma and that Railroad was a major intrusive center coeval with the adjacent Indian Well Formation volcanic field. Intrusive rocks at Railroad were emplaced during at least five separate episodes that produced at least ten distinct rocks types: early quartz-absent, quartz-poor, and quartz-phyric dacite sills in the North Bullion deposit (~38.4-38.9 Ma); Bullion granodiorite stock and related coarsely porphyritic rhyolites (-38.2-38.4 Ma); Pine Mountain rhyolite dikes (undated but at least 38.0 Ma); Indian Well Formation ash-flow tuffs, rhyolite lava dome, and quartz porphyry dikes east of the Bullion fault (-37.8-37.9 Ma); and late rhyolites (~37.4-37.6 Ma). Igneous rocks generally became more silicic through time, beginning with dacites with 65-70% SiO_2 and progressing to granite and high-SiO_2 rhyolite with 75-77% SiO_2. Most rocks are compositionally similar to other calc-alkaline rocks of late Eocene magmatism in northern Nevada. The Pine Mountain rhyolites are distinctive in having major and trace element compositions indicative of extreme differentiation. Regional study of the Indian Well Formation volcanic field, east and southeast of Railroad, shows that this volcanic field was active over the same time span as intrusive activity at Railroad and probably underwent a broadly similar compositional evolution. For example, quartz-phyric dacite ash-flow tuff at the base of the section in the southern Pi?on Range south of Railroad is 38.47±0.15 Ma, and rhyolite ash-flow tuff at the top of the section and a porphyritic rhyolite intrusion are 37.70±0.06 and 37.75±0.19 Ma, respectively. ~(40)Ar/~(39)Ar ages of four alteration minerals at Railroad (coarse muscovite in altered granodiorite, biotite in hornfels, sericite in porphyritic rhyolite, and adularia in quartz-phyric dacite) range from 38.15±0.11 to 37.83±0.03 Ma. Dateable hydro-thermal alteration was contemporaneous with at least two of the igneous episodes-the Bullion granodiorite stock and related coarsely porphyritic rhyolites at 38.2 Ma and the rhyolitic episode at 37.8-37.9 Ma. Carlin-type Au mineralization at North Bullion is no older than~38.4 Ma, the age of dacite dikes and sills that are weakly mineralized, but currently cannot be dated more precisely. Skarn and porphyry mineralization are best developed in and adjacent to the late rhyolites but are partly in and around, and dated contemporaneously with, the Bullion stock. Porphyry Cu-Mo mineralization may have been a cumulative process that occurred over the lifetime of the Railroad igneous center. The presence of porphyry, skarn, and Carlin-type mineralization at Railroad provides a unique opportunity to assess the physical and temporal relationship and deposit-type zoning of these deposit types relative to intrusive activity and the contribution of magma to economically significant metals. Other parts of the regional Railroad-Indian Well volcanic field should also be considered prospective for these deposit types. Railroad may be particularly favorable because it was an intense, long-lived intrusive system with relatively little volcanic eruption that might have dispersed volatiles and metals.