Chemical evolution of brines during mississippi valley-type mineralization; evidence from East Tennessee and Pine Point

Abstract

The major element chemical composition of inclusion fluids from Mississippi Valley-type deposits has been evaluated semiquantitatively using scanning electron microscopy and energy dispersive analysis of individual inclusion decrepitates in ore and gangue minerals. The volcano-shaped decrepitates are produced by heating polished plates above the decrepitation temperature of the inclusions, forcing the enclosed fluids to the surface where the nonvolatile components precipitate. Energy dispersive analyses of these precipitates, here called decrepitates, are expressed in cation and anion weight ratios and carry errors of + or - 10 percent.Application of this technique to fluid inclusions in sphalerite and dolomite from the Mascot-Jefferson City zinc district of East Tennessee and the Pine Point lead-zinc district of the Northwest Territories reveals that fluids attending sulfide deposition in both districts are dominantly Na-Ca chloride brines. Subordinate but significant amounts of K, Mg, and locally Sr and Fe are also present. CaCl 2 /(CaCl 2 + NaCl) weight ratios vary greatly among the samples from both deposits but are consistently lower (0.1-0.3) in gangue dolomites not associated with sulfides than in sphalerite and ore-stage dolomite (0.2-0.6 at East Tennessee; 0.4-0.8 at Pine Point). Although Cl is the only significant anion detected in the East Tennessee analyses, a significant proportion of decrepitates from sphalerite and ore-associated dolomite at Pine Point contain appreciable S, occasionally in amounts equal to or exceeding Cl.Mass balance calculations show that the increase in the CaCl 2 /(CaCl 2 + NaCl) ratio in decrepitates associated with ore from both districts cannot result simply from limestone dissolution at the site of brecciation and mineralization. Instead, they appear to require either fluid mixing or a chemical evolution of the fluids entering the carbonate host rocks. The presence of significant Ca and S together in the Pine Point fluid inclusions and the absence of daughter anhydrite requires that most S was reduced, which lends support to mixing models involving metal-rich brines and sulfur-rich waters in ore formation in that district. Further, the presence of excess S in ore-stage decrepitates from Pine Point suggests that sulfide de-position there was limited by the amount of metal-rich brine entering the H 2 S-rich barrier reef complex. In contrast, the lack of detectable sulfur in the decrepitates from Mascot-Jefferson City ores suggests that mineralization there may have been limited by the availability of S, but does not permit selection between mixing and fluid evoluton as a cause of ore deposition.