Abstract

Fifteen North American Mississippi Valley-type districts, distributed from the Arctic archipelago to Arkansas and in host rocks ranging in age from Late Cambrian to early Carboniferous, were selected for abroad test survey of Mississippi Valley-type district thermal histories. For each district in which data were collected, histograms of homogenization temperatures for the ore minerals were compared with the temperature range of the corresponding host rocks, as indicated by color alteration index (CAI) determinations on conodonts in the host-rock carbonates. Although a special effort was made to collect conodonts from the Upper Cambrian Bonneterre Formation, host rock to the Southeast Missouri district, all samples were devoid of conodonts. Thus only 14 districts were used in the final analysis.Comparison of fluid inclusion homogenization temperatures in main-stage ore minerals with color alteration index-determined host-rock temperatures reveals that a majority of Mississippi Valley-type districts (Pine Point, Newfoundland Zinc, Mascot-Jefferson City, Copper Ridge, Sweetwater, Central Missouri, Northern Arkansas, and Tri-State) are representative of group 1 (i.e., they are in thermal equilibrium with their host rocks insofar as could be determined 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, define a positive thermal anomaly. Host rocks to the Austinville-Ivanhoe, northern Newfoundland, and Robb Lake deposits (group 3) revealed the highest color alteration index values of North American Mississippi Valley-type districts, with host-rock temperatures greatly exceeding ore mineral fluid inclusion temperatures (negative thermal anomaly).With respect to major tectonic elements of North America, group 3 deposits (cool ore in hot rocks) are positioned within orogenic belts but on the continental side. Some group 1 deposits (ore temperature = host-rock temperature) are well within the continental interior but others are adjacent to orogenic belts on the continental side. Group 2 deposits (hot ore in cool rocks) are all within the continental interior.Current hypotheses regarding the formation of Mississippi Valley-type deposits and simultaneous heating of their host rocks invoke the migration of hot brines set into motion craton-ward by the hydraulic gradient produced by orogenic uplift at the craton edge. Once the ore-bearing fluids entered the foreland carbonate platforms, they traveled through sedimentary aquifers of the continental interior raising the temperature of the thinly covered carbonate rocks. The fluids left, as evidence of their passage, elevated color alteration index values and group 1 Mississippi Valley-type deposits distributed with decreasing homogenization temperatures away from the orogenic front. The correspondence between host-rock and ore temperatures suggests that group 1 deposits were deposited essentially within their own aquifers.Group 2 deposits were formed when the same, or genetically similar, fluids rose from the regional aquifer(s) into structurally controlled conduits in the continental interior, possibly developed as a result of synorogenic doming and arching. This upward channeling of the fluids resulted in rapid and localized fluid flow which precluded or at least inhibited wide-spread heating of the host rocks by ore fluids; consequently, color alteration index values in group 2 districts record only maximum burial depths of the host rocks and bear little relation to temperatures of the ore fluids. Although the cause of the unusually high color alteration index values (4-5) of group 3 districts is not fully understood, it is noted that these districts occur near the extreme outward edge of foreland platforms, near or within the leading margins of orogenic belts. Whether these group 3 host rocks were heated by simple tectonic burial or by passage of heated fluids under shallower conditions is not known.

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