Mississippi Valley-type (MVT) ore deposits arc epigenetic carbonate-hosted Pb-Zn deposits that contain galena, sphalerite, fluorite, bar&e, dolomite, calcite, and quartz. Although they are thought to form from basinal brines, their exact origins are still unclear, partly because of the scarcity of reliable geochronological data. Rb-Sr dating of sphalerites has recently been shown to be a promising technique for the direct dating of ore minerals in MVT deposits. This paper reports the results of a reconnaissance study of sphalerites, their fluid inclusions, and associated minerals from MVT deposits of North America. Sphalerites from Immel mine, Mascot-Jefferson City district, east Tennessee, define a Rb-Sr age of 347 f 20 Ma consistent with a Rb-Sr age of 377 +-29 Ma for sphalerites from Coy mine in the same district, but inconsistent with models that ascribe their genesis to the effects of the late Paleozoic Alleghenian orogeny. Rb-Sr isotopic analyses of K-feldspar from Immel mine preclude the possibility that the Rb-Sr data reflect feldspar inclusions. Sphalerites from the main ore zone of Daniel's Harbour mine, Newfoundland, do not form a linear isochron and open behavior of the Rb-Sr system is suspected. Sphalerites from the Pine Point district, Northwest Territories, Canada, define a Rb-Sr age of 36 1 + 13 Ma, indicating that the mineralization took place shortly after the deposition of the middle Devonian host carbonate rocks. These results are not compatible with mineralization models based on regional fluid migration related to early Tertiary Cordilleran deformation. Sphalerites from northern Arkansas have very low Rb and Sr concentrations (less than 0.1 ppm). The Rb-Sr data do not form isochrons and the sphalerites have higher '%r/%r ratios than expected, given their Rb/Sr ratios and reasonable constraints on their ages. The sphalerites are suspected to contain clay inclusions; and it is likely that the Sr isotopic compositions of these sphalerites, which have very low Sr concentrations, were affected by small amounts of inherited inclusions. Except for sphalerite from northern Arkansas, SEM studies and isotope dilution trace element measurements have so far failed to identify any suitable phases other than sphalerite that might be a host for the Sr.

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Analyses of fluid inclusion gases from Mississippi Valley-type districts in east Tennessee reveal the presence of several distinct aqueous solutions and vapors that were part of the mineralizing process. Inclusion contents were released by crushing 5 to 25 mg mineral samples and by decrepitating individual inclusions; all analyses were obtained by quadrupole mass spectrometry. Most analyzed inclusion fluids consist of Hz0 with significant amounts of CH4 (0.3 to 2.9 mol%), COz (0.1 to 4.7 mol%), and smaller amounts of C2H6, C,Hs, H& SO*, N2, and Ar. In general, inclusion gas abundances are greatest for sphalerite from the Mascot-Jefferson City district, lower for the Sweetwater district, and lowest for the Copper Ridge district. Compositional similarities in the inclusion fluids from the three districts imply that mineralization probably formed from fluids that permeated the entire region, rather than from completely separate fluids at each site. Saturation pressures calculated for these fluid compositions range from 300 to 2200 bars. Burial depths for the host unit have been estimated to be about 2 to 3 km in the east Tennessee area during Devonian time, the age of mineralization indicated by recent isotopic ages. Pressures at these depths, whether hydrostatic or lithostatic, would not have been adequate to prevent phase separation. Thus, our gas analyses represent either a mixture of vapor-rich and liquid-rich inclusions, or liquid-rich inclusions that trapped excess vapor. A lack of visible vapor-rich inclusions, high gas contents in individual fluid inclusion gas analyses obtained by decrepitation, and a positive correlation between decrepitation temperature and gas content for individual inclusions strongly suggest that the samples contain liquid-rich inclusions that trapped varying amounts of excess vapor. This excess gas probably accounts for the anomalously high homogenization temperatures and the wide range of homogenization temperatures observed in fluid inclusions in these ores. The vapor phase could have formed either by phase separation resulting from over-pressured aqueous fluids migrating into a region of hydrostatic pressure, or by incorporation of a pre-existing gas cap at the sites of deposition into the invading aqueous fluid. Exsolution of a vapor phase from the mineralizing brines should cause precipitation of carbonate and sulfide minerals, but reaction path modelling indicates that the resulting sparry dolomite:sphalerite ratios would be too high to form an ore-grade deposit. On the other hand, if the vapor phase was from a preexisting sour gas cap that was intercepted by a Zn-rich brine, large amounts of sphalerite would precipitate in a fairly small region. Preliminary mass balance calculations suggest that a gas cap of dimensions similar to the individual districts in east Tennessee could have contained enough H2S to account for the total amount of sphalerite precipitated.

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