Abstract

The Lone Tree deposit is located in the northern Battle Mountain mining district, Nevada. Prior to mine closure in 2006, Santa Fe Pacific Gold and Newmont produced 4.2 Moz of gold at an average grade of 2.06 g/t at Lone Tree, primarily from the N-S– to NNW-SSE–striking Wayne zone. The ore is located between the Roberts Mountain and Golconda thrusts in siliciclastic rocks of the Ordovician Valmy Formation and in the Pennsylvanian-Permian Battle Mountain and Edna Mountain Formations, and above the Golconda thrust in siliciclastic and carbonate rocks of the Mississippian to Permian Havallah sequence. Ore is also hosted by rhyolitic dikes that were emplaced at 40.95 ± 0.06 Ma based on zircon U-Pb chemical abrasion-thermal ionization mass spectrometry.

The gold is associated with sericitic and argillic alteration of the siliciclastic rocks and dikes and with decarbonatization and Fe carbonate alteration of the carbonate-bearing units, as well as in Fe-As sulfide and finegrained quartz alteration of all rock types. Oxidation affects 30 to 45% of the deposit, penetrating into the stratigraphy along numerous steeply dipping north-south, east-west, and north-northeast–south-southwest structures. Gold is positively correlated with Ag, As, Hg, and Sb. The highest Au grades occur in quartz-sulfide ore hosted in siliciclastic and carbonate sedimentary rocks and rhyolitic intrusions. In this ore style, fine-grained quartz and sericite are intergrown with disseminated sulfide minerals (quartz-sericite-pyrite alteration), constituting cores of weakly mineralized pyrite or marcasite, which are surrounded by fuzzy arsenopyrite rims that contain up to ~2,000 ppm Au. Low gold grades occur in late-stage banded pyrite breccias consisting of a finely zoned Au-poor pyrite matrix surrounding jigsaw-fit clasts of quartz-, illite-, barite-, and adularia-altered siliciclastic rock. The timing of main-stage mineralization is bracketed between the emplacement of the dikes and an adularia 40Ar/39Ar age of 40.14 ± 0.74 Ma.

Sericite intergrown with arsenopyrite-rimmed pyrite in phenocrysts of the rhyolite dikes gave δ18O values of 1.6 to 9.5‰ and δD values of –105 to –145‰. For temperatures of 300 ± 100°C, the calculated fluid isotopic compositions are consistent with felsic magmatic water and minor modifications by mixing with meteoric water and exchange with wall rocks. In the silica-sulfide ore, in situ isotopic laser ablation-multicollector-inductively coupled plasma-mass spectrometry (LA-MC-ICP-MS) analyses of pyrite cores yielded δ34S values ranging from 3.4 to 7.7‰, with average values of 5.6‰ in the felsic dikes, 4.5‰ in the siliciclastic rocks, and 5.3‰ in the carbonate rocks. These values match conventional pyrite δ34S data reported for Eocene porphyry systems elsewhere in the district. Nanoscale secondary ion mass spectrometry analyses show that gold and associated trace elements occur in submicron-scale zones within arsenopyrite rims on pyrite. The average δ34S values of the arsenopyrite rims are 5.3 to 6.5‰ heavier than the pyrite cores, indicating cooling and an increasing H2S/SO2 ratio. The highest grades resulted from episodic pulses of a gold-rich fluid that was partly derived from, or exchanged with, the sedimentary host rocks. In situ LA-MC-ICP-MS δ34S values for the late-stage banded pyrite breccia become progressively lighter from veinlet margin to center, reaching a low of –32‰. These veinlets indicate a shift from main-stage quartz-sericite-pyrite and intermediate argillic alteration to more neutral pH and oxidizing conditions during late-stage mineralization, indicating either increasing interaction between the fluid and sedimentary sulfur sources in the host-rock package or bacterial sulfate reduction and supergene sulfide precipitation.

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