EVOLUTION OF EPITHERMAL GOLD-SILVER VEINS USING AL AND TI IN HYDROTHERMAL QUARTZ OF THE ORO FINO MINING DISTRICT, DEER LODGE COUNTY, MONTANA

The Oro Fino veins are hosted by the Butte granite and occupy east-west trending shears. The veins are dominated by multiple generations of quartz, silicified-brecciated granite and minor electrum with a variety of silver sulfide and sulfosalt minerals. Quartz samples selected from the veins were examined by scanning electron microscope-cathodoluminescence imaging to identify quartz generations and analyzed by electron probe microanalysis (EPMA) for Al and Ti. Aluminum saturation in hydrothermal fluids is known to be pH dependent. High Al concentrations >2000 ppm in quartz can be an indication of crystallization from an acidic fluid. Titanium concentrations > 21 ppm in quartz is temperature dependent and can indicate crystallization temperatures above 600^o C. All but the earliest quartz generations had Ti below its detection limit 21 ppm; most generations had Al above its detection limit 13 ppm. Brecciated quartz included in the veins from the host granite had Ti above the detection limit. Early barren quartz veins filling the fractures in the included quartz grains also had detectable Ti. Al and Ti concentrations in early quartz veins, suggest the first mineralizing fluids had an acid pH and a temperature exceeding 600^o C. These acidic high temperature fluids reacted with the included granite and wall rock causing silicic alteration. With dropping temperature, kaolinite and montmorillonite alteration followed and the pH became basic, resulting in the crystallization of electrum and sulfide minerals in association with the altered granite. Quartz crystals that are co-genetic with the sulfide minerals show a depletion in Al in the crystal cores (< 2000 ppm) when compared to the outer growth zones (>2,000 ppm) suggesting a pH change from basic to acidic and Ti below detection indicating crystallization temperatures were below 600^o C. Fluid inclusion homogenization temperatures (150^o – 300^o C) support low temperature as well. A final quartz generation is represented by chalcedony filling open spaces. EPMA analysis indicates Al and Ti was not detected suggesting this late-stage quartz crystallized from a low temperature hydrothermal fluid of neutral or basic pH.