PETROLOGICAL, MINERALOGICAL, FLUID INCLUSION, AND STABLE-ISOTOPE STUDIES OF THE GIES GOLD-SILVER TELLURIDE DEPOSIT, JUDITH MOUNTAINS, MONTANA

The Gies gold-silver telluricle deposit, Judith Mountains, Montana, occurs in steeply dipping quartz veins near the contact of Late Cretaceous to Paleocene alkaline porphyritic intrusions (quartz monzonite and syenite stocks, and tinguaite dikes), and sedimentary rocks (shale, sandstone, conglomerate, limestone, and dolomite) of Middle Cambrian to late Paleocene age. Trace element geochemistry suggests that these intrusions were possibly derived from the shallowly dipping subducting Farallon plate lithosphere or the overlying asthenospheric mantle. Vein-related alteration is variable and does not extend more than 100 m away from veins. Potassic alteration and silicification are most closely associated with telluride mineralization. The vein mineralogy is complex and consists of at least 36 minerals including two unknown possibly new minerals and six tellurides. Four stages of vein mineralization are recognized in the Gies deposit, the dominant mineral assemblages are (I) quartz-pyrite-roscoelite-carbonates, (II) quartz-pyrite-base metal sulfides-carbonates-minor tellurides-roscoelite, (III) quartz-tellurides-pyrite-roscoelite-base metal sulfides-sulfosalts, and (IV) sulfides-sulfosalts-native gold. Fluid inclusion studies of quartz from stages I, II, and III yield homogenization temperatures of 270-degrees to 319-degrees-C (mean = 293-degrees +/- 10-degrees-C), 222-degrees to 282-degrees-C (mean = 256-degrees +/- 9-degrees-C), and 195-degrees to 250-degrees-C (mean = 220-degrees +/-13-degrees-C), respectively. Assuming hydrostatic pressure, a 5-degrees-C correction due to pressure yields mean trapping temperatures of approximately 300-degrees, 260-degrees, and 225-degrees-C, respectively, for stages I, II, and HI. Salinities of primary fluid inclusions in all three quartz stages range from 5.7 to 8.0 wt percent NaCl equiv. Isotopic studies show that deltaS-34 values between -1.0 to 3.1 per mil for vein sulfides are most consistent with a magmatic origin. Oxygen isotope values of quartz in stages I, II, and III range from 13.1 to 18.1 per mil, whereas values for feldspar in unaltered alkaline intrusions range from 7.1 to 9.8 per mil, feldspar in altered alkaline intrusions from 7.3 to 9.8 per mil, and a single sample of stage III kaolinite is 7.9 per mil. Values of deltaO-18. computed from quartz and fluid inclusion trapping temperatures for stage I, II, and III fluids are 5.9 to 9.4 per mil, 6.4 to 8.7 per mil, and 3.6 to 6.2 per mil, respectively. A deltaO-18 value of 3.3 per mit for fluid in equilibrium with stage III kaolinite agrees well with values derived from fluids in equilibrium with stage III quartz. Hydrogen isotope values of stage II roscoelite, stage III kaolinite, and fluid inclusions in stage I, II, and III quartz range from -115 to -78 per mil, whereas fluids in equilibrium with these minerals range from -115 to -88 per mil. Oxygen and hydrogen isotope evidence suggests that gold telluride mineralization precipitated by conductive cooling in response to increased mixing of evolved meteoric water with magmatic fluids. Concomitant with a decrease in temperature from stage I through stage IV are decreases in pH, f(S2), and f(O2) and increases in f(Te2) and vanadium content of the ore-fluid. Geologic and mass balance calculations are consistent with a low water/rock ratio (< 0.1) and indicate that magmatic fluids and many of the ore-forming components (e.g., Au, Ag, Te, V, S) have possibly been derived from a buried intrusion (e.g., tinguaite or carbonatite) at approximately 59 Ma.