THERMAL COMPARISON OF MISSISSIPPI VALLEY-TYPE LEAD-ZINC DEPOSITS AND THEIR HOST ROCKS USING FLUID INCLUSION AND CONODONT COLOR ALTERATION INDEX DATA

Fifteen North American Mississippi Valley-type districts, distributed from the Arctic archipelago to Arkansas and in host rocks ranging in a ge from Late Cambrian to early Carboniferous, were selected for a broa d test survey of Mississippi Valley-type district thermal histories. F or each district in which data were collected, histograms of homogeniz ation temperatures for the ore minerals were compared with the tempera ture range of the corresponding host rocks, as indicated by color alte ration index (CAI) determinations on conodonts in the hostrock carbona tes. Although a special effort was made to collect conodonts from the Upper Cambrian Bonneterre Formation, host rock to the Southeast Missou ri district, all samples were devoid of conodonts. Thus only 14 distri cts were used in the final analysis. Comparison of fluid inclusion hom ogenization temperatures in main-stage ore minerals with color alterat ion index-determined host-rock temperatures reveals that a majority of Mississippi Valley-type districts (Pine Point, Newfoundland Zinc, Mas cot-Jefferson City, Copper Ridge, Sweetwater, Central Missouri, Northe rn Arkansas, and Tri-State) are representative of group 1 (i.e., they are in thermal equilibrium with their host rocks insofar as could be d etermined by the color alteration index method). Ore minerals in group 2 districts (Upper Mississippi Valley, Polaris, and central Tennessee ) are significantly hotter than surrounding host rocks and, as such, d efine a positive thermal anomaly. Host rocks to the Austinville-Ivanho e, northern Newfoundland, and Robb Lake deposits (group 3) revealed th e highest color alteration index values of North American Mississippi Valley-type districts, with host-rock temperatures greatly exceeding o re mineral fluid inclusion temperatures (negative thermal anomaly). Wi th respect to major tectonic elements of North America, group 3 deposi ts (cool ore in hot rocks) are positioned within orogenic belts but on the continental side. Some group 1 deposits (ore temperature = host-r ock temperature) are well within the continental interior but others a re adjacent to orogenic belts on the continental side. Group 2 deposit s (hot ore in cool rocks) are all within the continental interior. Cur rent hypotheses regarding the formation of Mississippi Valley-type dep osits and simultaneous heating of their host rocks invoke the migratio n of hot brines set into motion cratonward by the hydraulic gradient p roduced by orogenic uplift at the craton edge. Once the ore-bearing fl uids entered the foreland carbonate platforms, they traveled through s edimentary aquifers of the continental interior raising the temperatur e of the thinly covered carbonate rocks. The fluids left, as evidence of their passage, elevated color alteration index values and group 1 M ississippi Valley-type deposits distributed with decreasing homogeniza tion temperatures away from the orogenic front. The correspondence bet ween host-rock and ore temperatures suggests that group 1 deposits wer e deposited essentially within their own aquifers. Group 2 deposits we re formed when the same, or genetically similar, fluids rose from the regional aquifer(s) into structurally controlled conduits in the conti nental interior, possibly developed as a result of synorogenic doming and arching. This upward channeling of the fluids resulted in rapid an d localized fluid flow which precluded or at least inhibited widesprea d heating of the host rocks by ore fluids; consequently, color alterat ion index values in group 2 districts record only maximum burial depth s of the host rocks and bear little relation to temperatures of the or e fluids.