A study of fluid inclusions in sulfide and nonsulfide mineral phases from a carbonate-hosted Zn-Pb deposit, Gays River, Nova Scotia, Canada

Abstract

This paper reports on a detailed fluid inclusion study of synore (sphalerite, calcite) and postore (calcite, fluorite, quartz, barite) mineral 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 carbonate rocks, in the basal part of the Windsor Group, that formed during a marine transgression on terrestrial clastics of the Tournaisian Horton Group. Mineralization is dominantly of replacement style, with lesser porosity infilling, and mainly occurs in the fore reef of a carbonate bank, the shallow water equivalent of the laterally extensive, laminated algal and bituminous Macumber Formation. Paragenetically, constant volume dolomitization of the host rock is followed by euhedral, manganiferous dolomite cement, then sphalerite and galena. Syn- to postore calcite with trace amounts of barite, fluorite, and quartz, occlude the remaining porosity.Fluid inclusions types include: (1) aqueous, L rich + or - solids, including halite, (2) methane, (3) H 2 O-CO 2 , (4) monophase L and V types, and (5) liquid petroleum + or - aqueous phase. Type 1 inclusions are most abundant, types 2 and 3 are rare, and type 5 are postore. Type 4 inclusions are mostly related to postentrapment changes (e.g., necking). Thermometric data indicate that the mineralizing fluids were high-salinity, NaCl-CaCl 2 -H 2 O brines with 20 to 28 wt percent NaCl equiv. Hydrohalite ice-melting relationships indicate considerable variation in the NaCl/(NaCl + CaCl 2 ) ratio of the fluids (0.2-1). Lower salinity fluid inclusions (0-16 wt % NaCl equiv) are restricted to postore calcite and barite. Combined SEM-EDS analyses of decrepitate mounds identify Na, Ca, and Mg as the major solute components. Homogenization temperatures (T h ) for all mineral phases range from 70 degrees to 250 degrees C, but ranges of < or = 10 degrees to 20 degrees C occur within isolated groups of inclusions. Homogenization data on liquid petroleum inclusions and associated aqueous inclusions in fluorite are 120 degrees to 150 degrees C, whereas three petroleum inclusions in syn- or postore calcite have T h values of 53 degrees , 53 degrees , and 167 degrees C.Bulk crush gas chromatography indicates that inclusion fluids contain up to 1.4 mole percent combined CO 2 and CH 4 , with the most abundant condensable gases in sphalerite and galena. Since the high pressures (< or = 2,000 bars) required to retain this gas as dissolved species in the fluid are incompatible with the local stratigraphy, it is inferred that the gas was either picked up along the fluid path and transported as an immiscible phase or was produced locally in the carbonate bank by thermal degradation of organic material.The data are interpreted to indicate that a high-temperature (< or = 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 reduction and mixed with an equally saline but lower temperature fluid. The variability of the NaCl/(NaCl + CaCl 2 ) ratio of the fluids suggests contamination by dissolution of the host dolostone; this dissolution may also have provided some of the CO 2 component of the gas. The pressure of the mineralizing environment is constrained at ca. 400 bars by the presence of liquid petroleum, aqueous and methane inclusions, and their respective isochoric projections.The high homogenization temperatures for the mineralizing fluids may reflect the regional setting of the deposit. That is, it is located on the margins of a large stable craton in an area that was structurally active during the Carboniferous and within which widespread thermal disturbances are known. The high temperatures also suggest affinities with the Irish carbonate-hosted base metal deposits.