Economic Geology Vol. 91, 1996, pp 1239-1262 Hydrothermal Characterization of theWest Gore Sb-Au Deposit, Meguma Terrane, Nova Scotia, Canada DANIEL J. KONTAK, RICItARD J. HORNE, ANDPAUL K. SMITH Nova Scotia Department of Natural Resources, P.O.Box 698, Halifax, Nova Scotia, Canada BSJ 2T9 Abstract The West Gore Sb-Au deposit is anomalous inthe Megurea terrane ofNova Scotia because ofits enrichment in Sb, a metal that isessentially absent from other Megurea gold deposits. Thedeposit ishosted bygraphitic and sulfide-bearing slates of the lower Paleozoic Halifax Formation that were deformed intoa northeast- trending, upright, closed syncline and metamorphosed to thegreenschist facies during theregional Acadian orogeny (ca. 400Ma).Mineralized veins at thedeposit define a single structure trending 110 øthat probably formed the dextral component ofaconjugate shear system as part ofregional, northwest-directed compression. Theveins crosscut a penetrative regional schistosity (S•)in thehost slates and vein formation is constrained by (1) thepresence of cleaved wall-rock slates in theveins, (2)vein-related sulfides overgrowing theStfabric in xvall-rock fragments, and (3) a 370Ma 4øAr/•9Ar plateau age forhydrothermal muscovite. Mineralization occurs asstibnite, native antimony, aurostibnite, Au-Sb alloys, andAu-Sb-O phases in veinquartz with associated Fe,As, Pb, Zn,Cusulfides and chlorite-carbonate gangue; wall-rock alteration isvariably developed as narrmv zones peripheral toveins enriched in sericite, calcite, sulfides, tourmaline, and chlorite. Early stages of mineralization areconstrained to maximum temperatures of 495øC from arsenopyrite geothermometry, but lower temperatures are recorded by chlorite geothermometry (350ø-390øC). Textures of veinquartz include comb and plumose varieties with a bimodal grain size andalso a coarser, anhedral quartz. Fluid inclusion studies indicate that thefluid xvas a mixed H•O-NaCI-CaC12-CO2-CH4 •type and that fluid urnnixing occurred, albeit in minor amounts. Thermometric measurements reveal maximum Thof 375øC and a range of salinities (0.4-28.1 wt % NaC1 equiv). Thevolumetric properties of fluid inclusions reflect cycling of fluid pressures with supralithostatic pressures (to4-5 kbars P,,•d) recorded bythe presence of H20-CO2 inclusions (to20 mole % COs) xvhich are interpreted to have resulted from fluid-rock interaction. Subsequent reduction in fluid pressures (toca. 2 kbars) resulted in decreasing amounts of CO2 (<1-2 mole %) and fluid unmixing thatgenerated higher and lower salinity trends observed in Th-salinity plots. Vein-forming fluids are characterized by 613C•c = -19.5 to-22.6 per rail, 6•sOn•o = 9to15 per mil, 6D = -42 to -64 per rail, and 634S•_•s = 9.7to 10.3 per rail based onanalysis ofcalcite, quartz fluid inclusion extracts in quartz and stibnite. Vein tourmaline associated with stibnite has SVSr/S%r .......... d of 0.71707 to 0.72284. These data areconsistent witha metamorphic origin forthefluid withlocal inheritance of C and S isotope signatures from interaction of the orefluid withgraphite and sulfide wall-rock slates of the Halifax Formation. The West Gore deposit originated from infiltration of metamorphic-derived fluids generated during the waning stages ofthe Acadian orogeny contemporaneous with generation and eraplacement offelsic and mafic magmas. The mineralizing fluids werefocused to higher crustal levels (Pfiuld : ca.2 kbars) where brittle- ductile conditions prevailed and veins werelocalized to the dextral component of a conjugate shear system related to movement along a major dextral strike-slip fault orshear zone (Cobequid-Chedabucto fault system). Deposition of veinconstituents resulted froma decrease in temperature andchanges in fluidchemistry (decrease in fs• andfo•) induced byfluid unmixing and interaction with thewall rock. Lithogeochemistry of the local stratigraphy does not indicate any regional enrichment in Sb, Au,or other metals. Theoccurrence of thestibnite may, therefore, reflect either telescoping of metals in a Au-W-Sb province or enrichment of Sb in thesource area or fluid conduit relative to other Meguma gold deposits. Introduction EX•MIN^TION of the metallogenetic mapof the Megurea terrane ofNova Scotia (Chatterjee, 1983) indicates theanom- alous presence of the West GoreSb-Au district among the numerous gold deposits of this metallogenetic province. This Au-Sb association is unusual in Megurea gold deposits since Sbmineralization is rarely recorded even asa trace constit- uent ofmineralized veins. However, the metallogenic associa- tion of Au-Sbis common elsewhere in metasedimentary- dominant terranes, asin New Zealand (Henley et al., 1976; Paterson, 1986), Ireland (Steed and Morris, 1986), Turkey (Aral, 1989; Gokce andSpiro, 1994), Australia (O'Shea and Pertzel, 1988), British Columbia (Madu et al., 1990), Bolivia (Ahlfeld and Schneider-Scherbina, 1964),andthe Appala- chians (Mossman et al., 1991).Colvine et al. (1984) and Nes- bitt and Muehlenbachs(1989) have noted the association of Sbwithmesothermal lode gold deposits of the Superior province and Canadian Cordillera, respectively, although Hodgson andTroop(1988)do not attach anysignificance to an Au-Sb association for Archcan deposits of the Abitibi greenstone belts. Additional areas of Sb mineralization in the Canadian Appalachians include the Moreton's Harbour area of Newfoundland where stibnite ishosted bymafic and felsic volcanic rocks (Kayand Strong, 1983), the past-producing Lake George Sb deposit of NewBrunswick that ishosted by Silurian metaturbidite rocks and a 412 Ma monzogranite 0361-0128/96/1868/1239-2455.00 1239