Mineralogic, fluid inclusion, and stable isotope studies of several gold mines in the Mother Lode, Tuolumne and Mariposa counties, California

Pa. State Univ., Dep. Geosci., University Park, PA, United States

Gold-bearing quartz veins in the southern part of the Mother Lode, California, formed from fluids which contained Si, K, CO ₂ , Ca, S, and metals and which were focused into permeable fault zones. In the Coulterville district of the Mother Lode, these fluids altered serpentinite to a talc-chlorite-magnesite-quartz assemblage which is crosscut by veins containing quartz, dolomite, sericite, sulfides, tellurides, and gold. Textural relationships among the vein-filling minerals reveal that galena, tellurides (Pb, Ni, Ag, Au-Ag), and native gold precipitated last among the vein minerals.Observations from the Oro Rico and McAlpine mines, Coulterville district, indicate that these deposits were subjected to deformation during and after their formation. Fluid inclusions in the milky vein quartz exhibit only a narrow range in T (sub m _ice ) but a large range in T _h which probably reflects posttrapping deformation. Microthermometry of fluid inclusions from vug-filling quartz in the Alleghany district, in the northern portion of the Mother Lode, reveals low-salinity (<2 equiv wt % NaCl), CO ₂ -H ₂ O fluids with T (sub mco ₂ ) = -56.7 degrees C, T (sub m _clathrate ) = 9.5 degrees C, and T (sub h _total ) = 150 degrees to 200 degrees C. Visual estimates of CO ₂ content indicate a maximum of 10 mole percent CO ₂ . In contrast, inclusions from vug-filling quartz in the Coulterville district are filled with low-salinity, low T _h (110 degrees -140 degrees C), H ₂ O-rich fluids which were probably later than, and unrelated to, the gold-mineralizing fluids.Oxygen isotope values of vein quartz from the southern Mother Lode indicate formation by ¹⁸ O-enriched fluids in the range 6 to 11 per mil (vs SMOW). These values are similar to estimates of 8 to 15 per mil from vein quartz of the Alleghany district (Taylor, 1981; Boehlke and Kistler, 1986). The average delta ¹³ C values for magnesite and dolomite of -4.5 + or - 0.6 per mil (vs PDB) from the southern Mother Lode are identical to results from the Alleghany district 200 km to the north (Rosenbaum and Taylor, 1984). These data suggest either a single or well-mixed source of oxidized carbon for the fluids. Oxygen isotope compositions of magnesite, calculated using an experimentally derived "phosphoric acid fractionation factor" at 50 degrees C of 1.0096, are enriched in ¹⁸ O relative to coexisting dolomite as predicted for isotopic equilibrium.The chemistry of the Mother Lode ore-forming fluids is characterized by several approaches. With an X (sub CO ₂ ) estimate of 0.1 from fluid inclusions, quartz-magnesite assemblages in the Coulterville serpentinite alteration zones limit the temperature of formation to below 325 degrees C for P < 2 kb. Hexagonal pyrrhotite inclusions in pyrite yield a lower T limit of 250 degrees C. This temperature range (250 degrees -325 degrees C) is compatible with the 235 degrees to 386 degrees C range obtained from quartz-mica isotope geothermometry in veins from the Alleghany district (Boehlke and Kistler, 1986). Microthermometric evidence of one-phase trapping of the CO ₂ -H ₂ O fluid, coupled with the assumption of trapping temperatures of 300 degrees C, suggests minimum trapping pressures of 1,000 to 2,000 bars. Assuming equilibrium, the assemblage of daughter minerals described by Coveney (1981) indicates neutral pH conditions of 5.5 to 6.0 at the prevailing P-T conditions. Sulfide mineral stabilities imply that H ₂ S was the dominant sulfur species and that a (sub O ₂ ) of the mineralizing fluids was in the range 10 (super -32) to 10 (super -35) .Our preferred model for the formation of the Mother Lode vein system involves magmatic activity associated with a subducting slab. In this model, generation of ore-forming fluids is possible by a variety of mechanisms including metamorphic devolatilization reactions in rocks adjacent to the plutons, direct magmatic input, and deep circulation of meteoric water. Any of these mechanisms is possible given the available data on the Mother Lode deposits.

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