A STUDY OF FLUID INCLUSIONS IN SULFIDE AND NONSULFIDE MINERAL PHASES FROM A CARBONATE-HOSTED ZN-PB DEPOSIT, GAYS RIVER, NOVA-SCOTIA, CANADA

This paper reports on a detailed fluid inclusion study of synore (spha lerite, calcite) and postore (calcite, fluorite, quartz, barite) miner al phases at the Gays River Zn:Pb deposit (2.4 Mt, 8.6% Zn, 6.3% Pb), Nova Scotia, Canada. The deposit is hosted by dolomitized Visean carbo nate rocks, in the basal part of the Windsor Group, that formed during a marine transgression on terrestrial clastics of the Tournaisian Hor ton Group. Mineralization is dominantly of replacement style, with les ser porosity infilling, and mainly occurs in the fore reef of a carbon ate bank, the shallow water equivalent of the laterally extensive, lam inated algal and bituminous Macumber Formation. Paragenetically, const ant volume dolomitization of the host rock is followed by euhe dral, m anganiferous dolomite cement, then sphalerite and galena. Syn- to post ore calcite with trace amounts of barite, fluorite, and quartz, occlud e the remaining porosity. Fluid inclusions types include: (1) aqueous, L rich +/- solids, including halite, (2) methane, (3) H2O-CO2, (4) mo nophase L and V types, and (5) liquid petroleum +/- aqueous phase. Typ e 1 inclusions are most abundant, types 2 and 3 are rare, and type 5 a re postore. Type 4 inclusions are mostly related to postentrapment cha ng es (e.g., necking). Thermometric data indicate that the mineralizin g fluids were high-salinity, NaCl-CaCl2-H2O brines with 20 to 28 wt pe rcent NaCl equiv. Hydrohalite ice-melting relationships indicate consi derable variation in the NaCl/(NaCl + CaCl2) ratio of the fluids (0.2- 1). Lower salinity fluid inclusions (0-16 wt % NaCl equiv) are restric ted to postore calcite and barite. Combined SEM EDS analyses of decrep itate mounds identify Na, Ca, and Mg as the major solute components. H omogenization temperatures (Th) for all mineral phases range from 70 d egrees to 250 degrees C, but ranges of less than or equal to 10 degree s to 20 degrees C occur within isolated groups of inclusions. Homogeni zation data on liquid petroleum inclusions and associated aqueous incl usions in fluorite are 120 degrees to 150 degrees C, whereas three pet roleum inclusions in syn- or postore calcite have TI, values of 53 deg rees, 53 degrees, and 167 degrees C. Bulk crush gas chromatography ind icates that inclusion fluids contain up to 1.4 mole percent combined C OB and CH4, with the most abundant condensable gases in sphalerite and galena. Since the high pressures (less than or equal to 2,000 bars) r equired to retain this gas as dissolved species in the fluid are incom patible with the local stratigraphy, it is inferred that the gas was e ither picked up along the fluid path and transported as an immiscible phase or was produced locally in the carbonate bank by thermal degrada tion of organic material. The data are interpreted to indicate that a high-temperature (less than or equal to 250 degrees C), saline (ca. 25 wt % NaCl) metalliferous fluid migrated into the carbonate bank where it reacted with reduced sulfur generated by thermochemical sulfate re duction and mixed with an equally saline but lower temperature fluid. The variability of the NaCl/ (NaCl + CaCl2) ratio of the fluids sugges ts contamination by dissolution of the host dolostone; this dissolutio n may also have provided some of the COP component of the gas. The pre ssure of the mineralizing environment is constrained at ca. 400 bars b y the presence of liquid petroleum, aqueous and methane inclusions, an d their respective isochoric projections. The high homogenization temp eratures for the mineralizing fluids may reflect the regional setting of the deposit; That is, it is located on the margins of a large stabl e craton in an area that was structurally active during the Carbonifer ous and within which widespread thermal disturbances are known. The hi gh temperatures also suggest affinities with the Irish carbonate-hoste d base metal deposits.