FLUID COMPOSITION AND THERMAL REGIME DURING ZN-PB MINERALIZATION IN THE LOWER WINDSOR GROUP, NOVA-SCOTIA, CANADA

The Zn-Pb-Ba-Cu-Ag-F deposits hosted by Lower Windsor Group (early Car boniferous) carbonates in Nova Scotia are located along the southern m argin of the Maritimes basin. Various geochemical studies indicate tha t the deposits were formed by fluids expelled from the basin, but it r emains unclear whether the ore-forming fluids for different deposits w ere derived from a common source region in the central part of the bas in, as proposed previously, or from separate source regions. It is als o unknown how the ore fluids were conveyed from the source regions to the sites of mineralization and whether or not the mineralizing system s of different deposits were hydrologically related. This paper addres ses these problems through a compilation of regional fluid composition and temperature data and thermal comparisons between the ore-forming fluids and their host rocks. The compositions and temperatures of the fluids were evaluated from fluid inclusion data, whereas host-rock tem peratures were estimated from vitrinite reflectance measurements. The ore-forming fluids are characterized by: (1) variable, but generally h igh salinities (mainly between 15 and 30 wt % NaCl equiv), (2) variabl e, but generally detectable amounts of bivalent cations (mainly Ca2+), (3) significant concentrations of hydrocarbon species, and (4) high t emperatures (mainly between 150 degrees and 250 degrees C). NaCl/(NaCl + CaCl2) wt ratios in fluid inclusions show a broad district-scale va riation: mainly < 0.6 for the Jubilee deposit and several showings on Cape Breton Island, and mainly > 0.4 for the Gays River and Walton dep osits in mainland Nova Scotia. CH4/higher hydrocarbons ratios of the g as components are greater for the Gays River deposit (3.8-9.7) than th e Jubilee deposit (0.7-1.3). Fluid inclusion homogenization temperatur es do not show a negative correlation with the distance between the de posits and the central part of the Maritimes basin, as might be expect ed if the fluids for different deposits were derived from the same sou rce region. The background temperatures prior to mineralization were e stimated from mean vitrinite reflectance (R-o) in host rocks which wer e relatively far from the deposit, and were probably least affected by the mineralizing events, and from homogenization temperatures of flui d inclusions in preore minerals. These temperatures are in the range o f 70 degrees to 155 degrees C, which are significantly lower than thos e of the ore-forming fluids. The paleogeothermal gradients estimated f rom the background temperatures andbunal depths are ca. 65 degrees C/k m. Vitrinite reflectance values in host rocks adjacent to the deposits indicate that these rocks were heated by the mineralizing fluids to v ariable degrees, but thermal equilibrium between the fluids and rocks was not attained in most cases. This thermal relationship suggests foc used and short-lived, rather than pervasive and long-lived, fluid flow . The regional variation of fluid composition and thermal patterns sup ports a model in which the ore-forming fluids of individual deposits w ere derived from separate sources; fluid flow was focused along confin ed conduits and individual mineralizing systems were unconnected hydro logically. Under paleogeothermal gradients of ca. 65 degrees C/km, flu ids with temperatures of ca. 250 degrees C could have been generated i n sub-basins proximal to the deposits. Sudden release of overpressured fluids from the basal part of the proximal sub-basins, characterized by high flow rate and short duration, is proposed as the mechanism of ore-forming fluid flow.