GEOLOGY AND GEOCHEMISTRY OF THE CHIMNEY CREEK GOLD DEPOSIT, HUMBOLDT COUNTY, NEVADA

Abstract

Chimney Creek is a sediment-hosted disseminated precious metal deposit located in the Osgood Mountains, Humboldt County, Nevada. It is the first large deposit of this type discovered in rocks affected by the Antler, Sonoma, and Nevadan orogenies and by Tertiary extension. The orebody is in alkalic basalts of the Mississippian Goughs Canyon Formation and the Etchart Limestone, a mixed siliciclastic-carbonate shallow-water sequence of Pennsylvanian- Permian age. These strata are bounded by the Devonian-Mississippian Roberts Mountain Thrust and the Permo-Triassic Golconda Thrust and are dissected by Nevadan and Basin and Range block faulting. The mid-Cretaceous Osgood Mountains Granodiorite intruded these rocks and a complex hydrothermal system evolved in which the Chimney Creek deposit and the other ore deposits of the Osgood Mountains developed. A magmatic-metasomatic auriferous CO₂-H₂O-CH₄ fluid of pH = 4 to 5 flowed upward along fractures in the Goughs Canyon Formation and spread out laterally along permeable beds in the lower member of the Etchart Limestone under a minimum pressure of 800 bars. Pathways in the basalt were armored with wide phyllic selvages and the carbonates were dissolved. Resulting space in the Etchart Limestone was partially filled by stratabound and stratiform silicification when the CO₂-H₂O-CH₄ fluid mixed with a dilute aqueous meteoric fluid and Au was deposited. On average, high enthalpy silica undersaturated solutions removed quartz from the deposit core and low enthalpy silica saturated solutions deposited quartz at the margin, so an outer annulus of bedded jasperoid surrounds a sanded core. This acid-sulfate alteration converted detrital feldspars and illite to phyllosilicates and sulfates in an area at least one kilometer in diameter coincident with conodont CAIs of 4 to 5 and fracture densities greater than 0.2 cm⁻¹. Chimney Creek is a mesothermal fluid mixing acid sulfate sediment-hostel precious metal deposit that formed along with skarn and hot spring type deposits as the Osgood Mountains Granodiorite cooled.