Fluid inclusion chemistry in the exploration for Mississippi Valley-type deposits: an example from East Tennessee, U.S.A.

Abstract

Scanning electron microscopy and energy dispersive analysis of fluid inclusion decrepitates in sparry dolomite cements associated with sphalerite mineralization in the Mascot-Jefferson City Mississippi Valley-type district of East Tennessee show a markedly lower Na/Ca ratio than those in dolomites unassociated with ore. The exploration potential of this variation in fluid inclusion chemistry has been evaluated by analyzing inclusion decrepitates from white sparry dolomite along a 150 m traverse in the New Market West mine area. The traverse extends from the central portion of a large domal collapsebreccia orebody with greater than 3% Zn, into the dolomite-veined, but unmineralized Knox Group limestones. Ratios of Na/Ca as low as 0.8-1.2 were common in dolomite-hosted decrepitates from the high-grade collapse ores. This anomalous decrepitate chemistry was not observed in sparry dolomites less than 12 m from the margins of the breccia bodies where Na/Ca ratios ranged from 1.8 to 3.6. Dolomite cements in low grade (<1.0% Zn) parts of the collapse bodies showed the greatest variation in Na/Ca ratio, with values ranging from 0.9 to 3.5. No systematic trends were observed in K/Na and S/Cl ratios from the same samples.