THE ORIGIN OF A MISSISSIPPI VALLEY-TYPE DEPOSIT IN THE VIBURNUM TREND, SOUTHEAST MISSOURI.

摘要

Basse-metal sulfide minerals in the Buick mine, southeast Missouri, formed by precipitation from hot, saline, aqueous solutions, which transported both the base metals and reduced sulfur to the site of deposition. The precipitation of the sulfides probably occurred because the brines dissolved the dolostone host-rock--the upper Cambrian Bonneterre Formation. Cooling or dilution of the brines could also have been important causes of sulfide precipitation. In the earliest stage of mineralization, fine-grained pyrite and marcasite replaced dolomite, fine-grained iron oxides and organic carbon. The iron disulfides were later replaced during the main stage of sphalerite mineralization ((delta)('34)S 16.5 to 19('0)/00) while the ore-forming solutions dissolved the dolostone host-rock ((delta)('13)C -1.0 to +.6, (delta)('18)O 21.5 to 24.9('0)/00). Following the sphalerite precipitation, two main stages of galena were precipitated. An early cuboctahedral stage of galena precipitation ((delta)('34)S 10.0 to 21.3('0)/00) during which galena replaced dolostone and grew in vugs in the dolostone or in early sphalerite, was followed by precipitation of cubic galena ((delta)('34)S 0.9 to 10.5('0)/00) and minor amounts of pale sphalerite ((delta)('34)S 14.0 to 16.5('0)/00). The two stages of galena precipitation were separated by a period during which the early dark sphalerite and cuboctahedral galena were partially dissolved and marcasite ((delta)('34)S 2.3 to 4.1('0)/00) was precipitated. The dissolution was probably caused by decrease in pH during marcasite precipitation from a solution supersaturated with respect to pyrite. Sparry and white dolomite rhombs ((delta)('13)C -2.9 to -0.9 and (delta)('18)O 20.3 to 21.9('0)/00) were precipitated at the same time as some of the early sphalerite, the late cubic galena and pale sphalerite, possibly because of increased carbonate alkalinity when organic matter in the dolostone was oxidized.;Throughout the paragenesis, there is an overall trend towards isotopically lighter sulfur, correlated with isotopicallyl more radiogenic lead. This trend suggests two principal sources of the sulfur and the lead in the deposit. According to one possible model, brines containing isotopically heavy sulfur and radiogenic lead were progressively flushed out of the system by brines containing isotopically very light sulfur and extremely radiogenic lead. However, at least some of the early radiogenic lead was probably leached from the underlying Lamotte Sandstone, which was an important aquifer for the brines. The isotopically heavy sulfur in the early cuboctahedral galena was probably derived from early Paleozoic evaporites in a nearby sedimentary basin. The isotopically light sulfur is too light to be derived directly from an evaporitic source, however, it could have been derived from diagenetic pyrite, petroliferous materials or crystalline igneous basement rocks.