AQUEOUS AND LIQUID PETROLEUM INCLUSIONS IN BARITE FROM THE WALTON DEPOSIT, NOVA-SCOTIA, CANADA - A CARBONIFEROUS, CARBONATE-HOSTED BA-PB-ZN-CU-AG DEPOSIT

The Walton Ba-Pb-Zn-Cu-Ag deposit, with a historical (1941-1978) produ ction of 4.4 Mt of barite and 0.36 Mt of massive sulfide ore, is hoste d by sideritized, Visean carbonates of the Macumber Formation which fo rms the basal member of the marine Windsor Group in southern Nova Scot ia. Strata-bound mineralization occurs as massive barite which gives w ay to barite and sulfide mineralization at depth; locally, mineralizat ion transects bedding and may occur within the underlying terrestrial clastics of the Tournaisian Horton Group. Barite occurs in a variety o f forms (massive to bladed, fine to coarse grained) and colors (white, red, orange, and shades of these) and overlaps the deposition of sulf ide minerals. All barite types are characterized by the presence of ab undant aqueous and liquid petroleum fluid inclusions that are of domin antly equant shape, of primary and secondary origin, and less than or equal to 100- to 150-mu m size. Inclusions are classified as (1) L- an d V-rich aqueous with rare solid phases, (2) mixed CO2 and CH4 (rare), and (3) liquid petroleum with rare bitumen. It is not possible to est ablish a chronology for the different fluids and all types may, in fac t, be closely related temporally and spatially. The occurrence of thre e-phase aqueous-petroleum inclusions, indicative of heterogeneous trap ping, indicates contemporaneity of the aqueous and petroleum inclusion s. The aqueous inclusions are dominated by high-salinity types (20-28 wt % NaCl equiv), but intermediate- and low-salinity types also occur. Observations of first melting (eutectics at -55 degrees and -35 degre es C) and final melting (ice and hydrohalite) indicate that fluid comp ositions varied in terms of NaCl/ (NaCl + CaCl2) (less than or equal t o 0.1-1.0) and NaCl/(NaCl + MgCl2) (0.2-0.84) ratios. Preliminary SEM- EDS analysis of decrepitate mounds confirms the presence of Na, Ca, an d Mg and also indicates K, Fe, and Mn in solution. The lack of clathra te phases suggests that, if present, condensed gases (e.g., CH4, CO2) occur in only minor to trace amounts in aqueous inclusions. Homogeniza tion temperatures cover a broad range with maximum temperatures of ca. 300 degrees C. Liquid petroleum inclusions, with a blue-white color u nder ultraviolet light, occur in isolated populations with uniform L/V ratios characterizing a particular group. Homogenization temperatures range from ca. 100 degrees to 300 degrees C, similar to the range for aqueous inclusions but within a group a much narrower range is record ed. The high homogenization temperatures and rare occurrence of bitume n in the inclusions predicates rapid, postentrapment cooling of the ar ea (i.e., within a few kiloyears), otherwise degradation of the petrol eum would have occurred. The delta(34)S signatures of sulfides at Walt on suggest that mineralization occurred in the presence of liquid petr oleum with the petroleum causing reduction of aqueous sulfate to H2S. Collectively the fluid inclusion data indicate that mineralization occ urred at ambient conditions of 250 degrees to 300 degrees C and pressu res of ca. 400 bars, as constrained for different inclusion types. The liquid petroleum is considered to have been generated by thermogenic processes via interaction of the heated, highly saline mineralizing fl uid with organic-rich beds within the Horton Group (Horton Bluff Forma tion) and was subsequently entrained as an immiscible phase within the aqueous fluid. Generation of the hydrothermal petroleum at Walton is analogous to the present-day occurrence of liquid petroleum vented fro m black smokers on the ocean floor (e.g., Guaymas basin).