Fluid-mineral reaction in the Lake George granodiorite, New Brunswick, Canada: Implications for Au-W-Mo-Sb mineralization

The Early Devonian Lake George granodiorite stock, New Brunswick, Canada, is spatially and temporally associated with early W-Mo-Au and later Au-Sbmineralization. The former mine was the largest antimony producer in North America until the mid-1990;. On the basis of fluid-inclusion analyses, multiple generations of fluids evidently interacted with the granodiorite, with the dominant fluids approximated by the system H2O-NaCl-CO2-CH4. Additional components of the fluid, S-Ca-Fe-K-Mg-Mn, reflect fluid-rock reaction and partial re-equilibration among the rock-forming minerals (i.e., plagioclase, orthoclase, biotite, and hornblende) in the granodiorite. The CO2-bearing inclusions (Type 1) have a relatively low salinity, ranging from 0 to 15 wt.% equiv. NaCl (ave. 4 wt.%), and most homogenize to the CO2 phase at temperatures of 200 to 470degreesC. Inclusions that homogenize to the aqueous phase do so over a narrower range of temperatures (250-350degreesC). Fluid pressures were probably 740 to 2210 bars for Type-1 inclusions. Aqueous liquid + vapor (Type 2) and aqueous liquid + vapor + halite or sylvite (Type 3) inclusions are recorded as well. Type-2 inclusions show a broader range of temperatures of homogenization (mostly to the aqueous phase), from 150 to 580 C, with a salinity between 2 and 19 wt.% equiv. NaCl. The stable isotope compositions of the rock-forming minerals in the granodiorite exhibit a disequilibrium relationship, e.g., either delta(18)O(K-feldspar) > delta(18)O(quartz). delta(18)O(plagioclase) > delta(18)O(quartz), or delta(18)O(plagioclase) > delta(18)O(K-feldspar). The whole-rock delta(18)O values range from 9.4 to 10.7parts per thousand, consistent with derivation by contamination by I80-rich sources. Values of deltaD(biotite) range from-75 to-64parts per thousand. The bulk fluids entrapped in quartz phenocrysts have deltaD values of -40.2 +/- 5.5parts per thousand, consistent with those of the fluids in equilibrium with biotite at temperatures between 350 and 480 degreesC. The similarities among the fluid compositions in the granodiorite, Au-W-Mo quartz-carbonate veinlets and stockworks, and the large Sb veins, suggest that they are genetically related. These fluids are interpreted to be mainly derived from progressive exsolution of volatiles from the cooling magmas at depth, although the CO2-CH4-bearing fluids were derived from the intrusion by wall-rock hybridization and skarn-forming reactions and later interaction with groundwater. These CO2-CH4-bearing fluids may have reacted with the granodioritic magma to lower its redox conditions, as reflected by a low magnetic susceptibility (ave. 6 X 10(-5) SI). Together with the petrochemical features, fluid composition and T-P characteristics in the Lake George granodiorite and associated W-Mo-Au-Sb deposit share many similarities with other reduced I-type granitic-intrusion-related gold systems.