Tectonic control and fluid evolution in the Quartz Hill, California, lode gold deposits

Humboldt State Univeristy, Department of Geology, Arcata, CA, United States

At Quartz Hill, in the Klamath Mountains of northern California, gold-bearing quartz-carbonate-pyrite-chlorite-sericite veins occupy reactivated moderately southeast-dipping faults that originally formed synchronously with the transcrustal Soap Creek Ridge fault. Geophysical evidence suggests that the Soap Creek Ridge fault dips shallowly eastward to a depth of approximately 14 km, where it intersects a major crustal suture zone that joins layered accreted oceanic crust to crystalline continental crust. Gold mineralization occurs in the footwall along the linear trace of this fault. CO ₂ -rich fluids reacted with wall rock to form alteration zones containing pyrite-siderite, sericite-ankerite, and chlorite-calcite. Microthermometric studies reveal coexisting high-salinity (11-16 wt % salt) aqueous and CO ₂ -rich (13-51 mole % CO ₂ ) inclusions that homogenize at temperatures greater than 285 degrees C. These inclusions probably represent the unmixing of a parent fluid that was metamorphic in origin. Low-salinity (0-6 wt % salt) inclusions with variable CO ₂ concentrations (0-9 mole % CO ₂ ) homogenize at temperatures from 170 degrees to 270 degrees C. These inclusions are likely to have formed from the mixing of hot, saline, CO ₂ -rich solutions with cooler, less saline fluids of meteoric origin.K-Ar analyses of two size fractions of hydrothermal sericite from a Quartz Hill mine yielded concordant dates of 145.8 + or - 3.0 and 147.7 + or - 2.8 Ma and probably represent the age of mineralization. Similarities in tectonic setting, local structural control, host rocks, fluid inclusion properties, and alteration and vein mineralogy suggest that the Early Cretaceous gold mineralization in the Sierra Nevada foothills was equivalent to the ore-forming episode at Quartz Hill and throughout the Klamath Mountains.A tectonic model for ore genesis consistent with thermochemical, geophysical, structural, and geochronological data is that hydrothermal solutions were derived from subducting oceanic plates by devolatilization reactions associated with prograde metamorphism. These CO ₂ -rich fluids migrated upward through a crustal suture zone and were channeled along a transcrustal fault. Deposition occurred in response to pressure fluctuations within brittle fault zones. Important changes in Farallon-North American convergent plate motion occurred at 150 and 133 Ma. The synchronism of these plate motion changes with regional tectonic activity and gold mineralization suggests a genetic relationship. These plate motion changes produced crustal-scale tectonic responses, which generated fluids and created pathways for ore fluid transport.

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