ISSN:
1432-1866
Source:
Springer Online Journal Archives 1860-2000
Topics:
Geosciences
Notes:
Abstract The Polaris deposit is one of the largest Mississippi Valley-type deposits in the world, with 22 million tonnes of ore at 14% Zn and 4% Pb contained in a single, compact orebody surrounded by dolomitized host rocks. Using detailed sampling of carbonates in the orebody and the dolostone halo, this paper aims to characterize the temporal and spatial evolution of the mineralizing system, and to understand the mechanisms that controlled the accumulation of this large, compact Zn–Pb deposit. Five types of dolomite have been distinguished, including three replacement (RD) and two pore-filling dolomites (PD). The paragenetic order is RD1, RD2, RD3, PD1, and PD2. Pore-filling calcite (PC) postdates all other minerals. In most cases, sulfides and dolomite did not co-precipitate, but sphalerite and galena largely overlap with RD3 and PD1. Various dolomites are dissolved or replaced by sulfide-precipitating fluids; sulfides in turn can be overgrown by dolomites. Colloform texture in sphalerite is widespread. Fluid inclusions were studied in RD3, PD1, PD2, sphalerite, and PC. The overall ranges of homogenization temperatures (T h) and last ice-melting temperatures (T m-ice) for fluid inclusions in dolomites and sphalerite are from 67 to 141 °C and from −46.7 to −27.0 °C, respectively, consistent with warm basinal brines with high salinities and Ca/Na ratios. Gas chromatographic analysis of these fluid inclusions indicates low concentrations of hydrocarbons (〈0.06 mol%). C, O, and Sr isotopes were analyzed for all dolomites and PC, as well as for the fine-grained host limestone and early diagenetic calcite (SC–RC). The isotopic values of RD2, RD3, PD1, and PD2 cluster tightly and form largely overlapping domains. With respect to the host limestone, they are depleted in 18O, similar in δ13C, and slightly enriched in 87Sr. There are no regular spatial variations for fluid inclusion and isotope data, indicating an overall geochemical homogeneity in the hydrothermal system. However, certain samples close to the fracture zones in the orebody with slightly elevated T h and 87Sr/86Sr values and depleted δ18O values suggest that the fracture zone was the conduit for the hot brines. Based on the geological and geochemical characteristics of the deposit, we propose that sulfide precipitation at Polaris was caused by mixing of a reduced, metal-rich, sulfur-poor fluid with a reduced, metal-poor, sulfur-rich fluid at the site of mineralization. The metal-carrying fluid ascended along fractures from below the deposit and was hotter than the host rocks, whereas the reduced sulfur-carrying fluid was delivered to the site of mineralization laterally and was in thermal equilibrium with the host rocks. This model can readily explain the dissolution of dolomite during sulfide precipitation and the abundance of colloform sphalerite, as well as the low concentrations of hydrocarbons in fluid inclusions.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/s001260050257
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