A GENETIC MODEL FOR MINERALIZATION OF LOWER WINDSOR (VISEAN) CARBONATE ROCKS OF NOVA-SCOTIA, CANADA

This paper summarizes and integrates some of the results of a three-ye ar research project on the source, path, and timing of fluid movement within and between sub-basins in Nova Scotia, lower Carboniferous (Vis ean) carbonates, and the relationship of these fluids to base metal mi neralization. The Kennetcook, Shubenacadie, and Musquodoboit sub-basin s in central mainland Nova Scotia and the River Denys sub-basin on Cap e Breton Island are excellent examples of platformal onlapping sedimen tation of the larger Maritimes basin. Within each of these sub-basins one base metal deposit was selected for detailed research. Prior to mi neralization, potential host rocks in the four sub-basins contained su bstantial zones of porosity as a result of various processes. For exam ple, at Walton, synsedimentary breccias provided abundant open space i n the otherwise fine-grained, tight Macumber Formation carbonates. The lower grade mineralization at Gays River was deposited in primary por osity within reefal facies of the Gays River Formation. Both overpress uring and gravity-driven flow have been proposed and tectonic conditio ns prevalent during the time of mineralization provide permissive supp ort for either process, especially in the Walton and Gays River areas. Tectonic breccias, developed in Macumber Formation carbonates during movements associated with the Ainslie detachment, are the main host of the Jubilee deposit. In all three deposits, however, open-space filli ng was accompanied by widespread replacement of the host carbonate. Wi th increasing burial, Windsor Group limestones were selectively sideri tized at Walton, totally dolomitized at Gays River, and simply lithifi ed at Jubilee. Preore burial temperatures reached 70 degrees C at Walt on, 135 degrees C at Gays River, and 65 degrees C at Jubilee. Timing o f mineralization has been determined by a variety of methods indicatin g that ore fluids entered the Gays River area at about 320 to 300 Ma, the Walton area at 333 to 307 Ma, and the Jubilee area at 330 to 310 M a. Given the uncertainty in the dating methods used, these data are in terpreted to indicate overlapping mineralization among the three areas . The distribution of deposits and occurrences in the Kennetcook and S hubenacadie-Musquodoboit sub-basins suggest a northerly source of flui ds for deposits in these sub-basins; the direction of fluid flow in th e River Denys sub-basin remains speculative. Evaporated seawater was t he most likely original source of ore fluids, but these were modified during burial by halite dissolution and by reaction with underlying si liciclastic aquifers. Temperatures of ore-stage deposition decrease fr om Walton (median similar to 200 degrees C, range 100 degrees-300 degr ees C), to Gays River (median similar to 140 degrees C, range 80 degre es-220 degrees C),and to Jubilee (median similar to 80 degrees C, rang e 80 degrees-220 degrees C). The high-temperature, high-salinity ore-f orming fluid is interpreted to have entered the host rocks and mixed w ith, and was cooled by an ambient low-temperature, high-salinity fluid . Several possible sources of die extraordinary heat in the primary fl uid are proposed and include an underlying basalt plate, high heat-pro duction granites, an increase in geothermal gradient caused by coeval extension, or a combination of these. Ore fluids appear to have been p rimarily complex NaCl-CaCl2-KCl-MgCl2-FeCl2 aqueous solutions with les ser amounts of light gases such as CO2 and CH4. Hydrocarbons were pres ent in ore-stage fluids at both Walton and Jubilee and more than 99.9 percent of the hydrocarbon occurs as liquid petroleum. Given the high temperatures of these deposits, the presence of liquid petroleum indic ates a short-lived mineralizing system, probably on the order of a few hundred thousand years. A clear relationship with underlying basement lithologies is indicated by the sub-basin-specific character of Pb an d Sr isotopes in ore-stage minerals, suggesting that ore fluids leache d metals directly from basement rocks, or more likely, from sediments derived from it. Isotopic data indicate that sulfur at Walton, Gays Ri ver, and Jubilee was derived from the same source, probably deeply cir culating sulfate-rich brines. The data also suggest that sulfate reduc tion at Gays River must have taken place away from the deposit area wh ereas data from the Walton and Jubilec deposits suggest in situ thermo chemical reduction of sulfate by hydrocarbons. Compared with other Car boniferous, carbonate-hosted deposits in Newfoundland and Ireland, tho se in Nova Scotia are considerably younger, relative to their host roc ks; are more inhomogeneous, in terms of Pb-206/Pb-204 ratio, than thos e in Newfoundland but compare well with those in Ireland; possess Pb i sotope compositions which plot above the Stacey-Kramer growth curve wh ereas those in both Newfoundland and Ireland plot below it; and exhibi t fluid inclusion temperatures and salinities comparable with those of Newfoundland deposits but many are hotter and most are more saline th an those of deposits in Ireland. Collectively, the following common pa rameters are suggested for deposits in all three localities: (1) forma tion of most, if not all, by replacement of lithified carbonate rocks; (2) derivation of metals from directly underlying basement rocks, res ulting in the control of ore-lead isotope compositions according to ag e and composition of basement lithologies; and (3) ore-stage mineraliz ation at temperatures higher than those typical of Mississippi Valley type deposits but lower than those typical of sedimentary exhaltive de posits.