PROBABLE MVT DEPOSITS IN INDIANA: PRELIMINARY FIELD, PETROGRAPHIC AND FLUID INCLUSION DATA FROM DIAGENETIC MINERALS IN SILURIAN AND DEVONIAN CARBONATES

Late Silurian and Middle Devonian limestones and dolostones in northern Indiana contain a suite of diagenetic minerals similar to Mississippi Valley-type deposits found elsewhere in North America. Through preliminary field and petrographic studies, we have identified fluorite, calcite, quartz, pyrite, sphalerite, and chalcopyrite cement crystals, some associated with petroleum residues and/or localized replacement chert. The minerals occur in vugs, fractures, and stylolites in the Late Silurian Pleasant Mills and Wabash Formations and the Middle Devonian Detroit River and Traverse Formations. Preliminary fluid inclusion analyses of the fluorite crystals suggest that the diagenetic fluid from which they crystallized was a warm, very saline, Na-Ca-Cl-rich brine. Forty-three primary fluid inclusions from three fluorite crystals have yielded homogenization temperatures of 105 - 125°C, and five primary fluid inclusions in two fluorite crystals analyzed for freezing/melting behavior have yielded eutectic temperatures of -61 - -52°C and final melting temperatures of -21.1 - -21.8°C. These fluid inclusion data are similar to those of several MVT deposits found elsewhere in the U.S., suggesting that Paleozoic carbonates in northern Indiana experienced an MVT event similar to those at classic MVT locales. Further field, petrographic and fluid inclusion studies of these rocks should better define the characteristics and timing of this probable MVT event.