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Paragenetic Evolution of the Robertson Deposit: Eocene Intrusion Related Gold Deposit in the Northern Shoshone Range, Nevada
AdvisorMuntean, John L
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The Robertson deposit, formerly known as Tenabo, is an Eocene intrusion-related gold deposit 5 km north of Pipeline, which is a 20 Moz+ Carlin-type gold deposit in the northern Shoshone Range, Nevada. Carlin-type gold deposits have been the focus of many studies as they represent the second largest accumulation of gold in the world, and a growing body of scientific literature indicates that the source for this gold may be related to Eocene magmatism. The presence of an Eocene intrusion-related gold deposit within 5 km of a Carlin-type deposit begs the question of whether the two deposits are genetically related, however detailed characterized of Robertson deposit is needed before this link can be examined. Most of the previously defined gold mineralization at Robertson occurred near the contacts of intrusions and hosted by hornfels in the overlying lithologies. Ongoing exploration has discovered significant free visible gold both within the stock and in distal settings within metasedimentary host rocks. Here, gold mineralization is spatially and temporally related to an intrusive complex referred to as the Tenabo stock, and based on petrographic and geochemical analysis, is composed of successive intrusive phases including an early ilmenite-bearing diorite, andesite dikes, hypabyssal dacitic porphyry dikes, composite ilmenite-bearing granodiorite, granitic porphyry dikes (formerly called feldspar porphyry dikes), pebble dikes, and subvolcanic rhyolitic dikes. The purpose of this study is to characterize gold mineralization as it relates to intrusive phases, alteration styles, and structural controls by combining fieldwork, petrography, geochemistry and geochronology. This study is the first in depth examination of Robertson focusing on gold mineralization within intrusive phases and in distal settings, presenting six new U/Pb LA-ICP-MS ages of intrusive phases, one molybdenite Re/Os age, and one hydrothermal orthoclase 40Ar/39Ar age, conservatively constraining gold mineralization to 39.65-35.77 Ma. Emplacement of the Tenabo stock resulted in contact metamorphism of the host siliciclastic lithologies yielding quartz, biotite, and calc-silicate hornfels, which formed an aureole up to 1 km from the stock. Subsequent hydrothermal alteration followed, including endo/exoskarn, potassic alteration, sericitic alteration, and chloritic alteration. Petrographic observations of veins at Robertson indicate the succession of three main stages referred as 1) pre-gold, 2) syn-gold, and 3) post-gold. The pre-gold stage includes barren quartz veins, potassic alteration veins comprised of biotite veinlets and quartz-K-feldspar, quartz – arsenopyrite, quartz – chalcopyrite, and quartz – molybdenite veins. The syn-gold stage is composed of quartz – gold ± Bi-Te-Se-Pb-Ag-S sulfosalt veins, consisting of native gold seen as single grains in quartz gangue and/or on the surface of pyrrhotite, arsenopyrite, chalcopyrite, and loellingite grains. The post-gold veins encompass pyrite ± quartz, quartz – calcite – polymetallic sulfide veins with Pb-Zn-Sb-Ag-Sn base metal sulfides, bull quartz veins, and calcite veins. Cross-cutting relationships along with geochronological data indicate that gold mineralization resulted from the emplacement of upper Eocene (40.99-37.38 Ma) reduced ilmenite-bearing intrusive phases that intruded into Ordovician-Devonian siliciclastic sedimentary lithologies, forming a high-tonnage, low-grade gold deposit characterized by a Au-As-Bi-Te-Cu metal association, similar to reduced intrusion related gold deposits and gold skarns described globally. Based on petrographic and geochemical evidence, it is concluded that mineralizing conditions for gold precipitation were under relatively low sulfur fugacity and oxidation state, leading to preferential formation of minerals such as native Bi, arsenopyrite, pyrrhotite, and loellingite, typical of low-sulfidation assemblages. Under such conditions it is possible to transport gold to lower temperatures <300°C, where native gold is commonly associated with Bi-bearing minerals or to even lower temperatures (~150-200°C) where it remains within the lattice of sulfides as invisible gold, typifying Carlin-type deposits.