Geology and geochemistry of the Cortez gold deposit, Nevada

Abstract

Gold has been discovered recently at Cortez, Nevada, about 45 miles southwest of Carlin in carbonate rocks in a window of the Roberts Mountains thrust. The host rock consists of laminated to thin-bedded dark-to light-gray, silty dolomitic limestone and calcareous dolomitic siltstone in the upper part of the Silurian Roberts Mountains Limestone. These rocks contain sparse pyrite cubes and aggregates and some organic carbon. The rocks have been faulted and folded repeatedly during their complex geologic history. The gold is disseminated in a large zone where the rocks have been fractured and bleached and the pyrite oxidized. During oxidation the iron was redistributed, giving the rock a color ranging from light gray to dark red. The alteration zone envelops a 34-m.y.-old intrusive body of biotite-quartz-sanidine porphyry, which is also altered. No genetic relationship between the mineralization and the intrusive body is known. Silicification, iron-oxide staining, decalcification and, in extreme cases, dedolomitization generally accompanied the gold metallization, although any one of these phases of hydrothermal alteration may have been well developed without introduction of significant amounts of gold. Some clay alteration occurred in the igneous rock but none in the ore body. The gold is in micron-sized particles of native gold. Gold is taostly with silica between original silt grains and to a lesser extent in quartz-filled microfractures and hematite-goethite pseudomorphs after pyrite. The gold was discovered during the examination of an arsenic-antimony-tungstenmercury geochemical anomaly known in the area. Other gold deposits in north-central Nevada are associated with such anomalies. Contents

Stratigraphy, petrology, and some fossil data of the Roberts Mountains Formation, north-central Nevada

“…These features have been studied at the Cortez deposit by Wells, Stoiser, and Elliott (1969), Wells and Elliott (1971), Wells and Mullens (1973), and Erickson and others (1966). The Gold Acres deposit was briefly described by Ketner (in Gilluly and Gates, 1965, p. 134) and in more detail by Wrucke and Armbrustmacher (1975).…”

Magmatism, ash-flow tuffs, and calderas of the ignimbrite flareup in the western Nevada volcanic field, Great Basin, USA

“…The most important mineral deposits in the western Nevada volcanic fi eld are the 35.7 Ma Cortez and Cortez Hills Carlin-type gold deposits and the 26.1 Ma Round Mountain epithermal gold deposit. The Cortez and Cortez Hills gold deposits are spatially and temporally associated with rhyolite porphyry dikes, sills, and fl ow domes (the Cortez dikes; Wells et al, 1969;Muntean et al, 2011). The Round Mountain deposit is not associated with exposed intrusive rocks, but it formed along the structural margin of the Round Mountain caldera within 0.5 Ma of caldera formation (Henry et al, 1996(Henry et al, , 1997.…”

Igneous Geology of the Carlin Trend, Nevada: Development of the Eocene Plutonic Complex and Significance for Carlin-Type Gold Deposits

“…At Cortez, 35 Ma, hydrothermally altered, porphyritic rhyolite dikes are variably interpreted as pre-or postmineral (Wells et al, 1969;McCormack and Hays, 1996;Mortensen et al, 2000). Eocene volcanic or intrusive rocks coeval with the time of mineralization are known in two of the three districts.…”