Iron Oxide-rich Mineralization and Related Alteration in the Yerington District, Lyon County, Nevada

摘要

This study documents two magnetite deposits that formed in similar host rocks during the emplacement of the Jurassic Yerington batholith: the Minnesota (Standard Slag) mine and the Pumpkin Hollow deposit. Copper-sulfide skarn deposits in the district, hosted in the same sedimentary units (e.g., the Ludwig and Mason Valley mines), have been well-documented by previous studies. Basin and Range normal faulting in the district has extended and rotated exposed rocks to create Jurassic cross-sectional views through these hydrothermal systems, allowing for documentation of alteration in lateral and vertical extents. Recent work in the Yerington district has documented alteration associations, mineral assemblages, and timing relationships that allow for comparison between various Fe oxide-rich deposits, such as the Cu-poor Minnesota mine and the Cu-rich Pumpkin Hollow deposit, as well as comparisons between these Fe oxide-rich deposits with the Fe oxide-poor copper skarn deposits. Mineralization in the Minnesota mine and Pumpkin Hollow deposits is hosted in Mesozoic carbonate, fine-grained clastic, and Jurassic intrusive rocks and is linked to voluminous Ca(-Na) alteration of associated intrusive units, multiple episodes of brec-ciation, and replacement dominated by magnetite with subordinate calc-silicates. At the Minnesota mine, Ca-Na alteration of the McLeod Hill quartz monzodiorite is dominated by plagioclase-actinolite-epidote-titanite (sphene) associations. Magnetite mineralization at the Minnesota mine within the Mason Valley Limestone appears to zone from central magnetite replacement with minor pyrite-calcite to magnetite clasts supported by tremolite-calcite-quartz-chlorite ± serpentine matrix at the contact with relict marble and hydrothermal dolomite. The McLeod Hill quartz monzodiorite at Pumpkin Hollow is host to a variety of alteration associations, ranging from plagioclase-actin-olite-epidote-titanite to plagioclase-garnet-diopside-calcite. Magnetite mineralization at Pumpkin Hollow is complex, but much of the Mason Valley Limestone replacement is comprised of magnetite with lesser diopside (often replaced by actinolite) ± pyrite-chalcopyrite-pyrrhotite. Crosscutting relationships between igneous rocks and hydrothermal alteration indicate that magnetite-dominated mineralization at both deposits occurred relatively early during emplacement of the batholith, and mainly predates granite porphyry dikes that elsewhere (Ludwig, Mason Valley) are linked to porphyry-related skarn mineralization. Fe oxide-poor systems are dominated by Si-addition (high temperature, anhydrous, calc-silicate mineralization), low volumes of Fe-oxide, and late Cu-mineralization generally linked to granite porphyry dikes. These relationships suggest that Fe oxide-rich systems are distinctly different from Fe oxide-poor systems in the Yerington district. The Fe oxide-rich systems are similar to iron oxide-copper-gold (IOCG) systems documented elsewhere around the world that are linked to processes dominated by external fluids, whereas the Fe oxide-poor systems are magmatic-hydrothermal deposits dominated by magmatic fluid sources.