Origin of dissolution vugs, caverns, and breccias in the Middle Devonian Presqu'ile barrier, host of Pine Point mississippi valley-type deposits

Abstract

Mississippi Valley-type lead-zinc ore deposits at Pine Point occur in dissolution vugs, caverns, and breccias, which previously were interpreted to be related to meteoric karstification during Middle Devonian sub-Watt Mountain exposure. Studies of open pits at Pine Point and core samples from 34 wells from the subsurface Presqu9ile barrier revealed that dissolution related to the sub-Watt Mountain exposure was relatively minor and differed from the large-scale hydrothermal dissolution that occurred prior to or was contemporaneous with the emplacement of Mississippi Valley-type deposits. Dissolution vugs, caverns, and breccias related to the sub-Watt Mountain exposure are (1) commonly filled in with green shale, and locally, with pendant calcite cements, (2) preserved both in limestones and dolomites, (3) restricted to the strata immediately beneath the unconformity, and (4) formed prior to stylolitization, dolomitization, and mineralization. Diagnostic meteoric karst features, such as rundkarren, dolina, calcrete, terra rossa, and speleothems are not observed in the Presqu9ile barrier. The limited evidence of meteoric water during sub-Watt Mountain exposure is possibly due to an arid climate and relatively short periods of exposure.In contrast, hydrothermal dissolution vugs, caverns, and breccias that host Mississippi Valley-type ore deposits at Pine Point are filled with laminated dolomites, saddle dolomites, sulfide minerals, late-stage calcite, and pyrobitumen that are associated with hydrothermal fluids; occur both above and below the Watt Mountain unconformity; follow the regional conduit system that parallels the barrier; postdate stylolites, suggesting that dissolution must have occurred during burial; and occur only in dolomites interpreted to have been formed from burial fluids. Fluid inclusion measurements indicate that the saddle dolomites precipitated from fluids between 92 degrees and 178 degrees C having salinities between 11 and 29 wt percent NaCl equiv. Thus the majority of dissolution vugs, caverns, and breccias that host Mississippi Valley-type ore deposits at Pine Point appear to be related to hydrothermal brines, not meteoric fluids, and formed before and/or during late-stage dolomitization and Pb-Zn mineralization. Several mechanisms have been proposed to account for the hydrothermal dissolution, including acid basinal fluids generated by thermochemical sulfate reduction, diagenesis of organic matter, and/or cooling of CO 2 -rich brines during their updip movement, all of which may have been active at Pine Point.