Structurally controlled vein barite mineralization in the Maritimes Basin of eastern Canada: Geologic setting, stable isotopes, and fluid inclusions
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| Authors: | Kontak, DJ; Kyser, K; Gize, A; Marshall, D |
| Year: | 2006 |
| Journal: | Econ. Geol. 101: 407-430 Article Link (DOI) |
| Title: | Structurally controlled vein barite mineralization in the Maritimes Basin of eastern Canada: Geologic setting, stable isotopes, and fluid inclusions |
| Abstract: | The Brookfield barite deposit occurs in the Carboniferous Maritimes basin of eastern Canada, an area known for ca. 300 Ma Mississippi Valley-type (MVT) Zn-Pb-Ba mineralization, including the past-producing 'Walton barite and Gays River Zn-Pb deposits. In contrast to most of MVT mineralization in the Maritimes Basin that is hosted by Visean carbonate rocks of the Windsor Group, the Brookfield deposit is hosted by red terrestrial clastics of the underlying Tournaisian Horton Group. The barite mineralization occurs as <= 25-m-wide fault fill with both coarse-textured, pristine barite and mylonitic-textured barite situated close to the Cobequid-Chedabucto fault system, separating the Meguma and Avalon terranes. The breccia vein mineralization has a simple mineralogy and consists of barite with up to 1.3 wt percent Sr, minor siderite, and trace amounts of late-stage quartz and calcite. The host rocks close to the barite mineralization are intensely altered, with quartz and kaolinite replacing muscovite and complete removal of diagenetic hematite. A Grant-type isocon plot indicates mass loss (C-A = 0.82 degrees C-O) during wall-rock alteration with BEE profiles maintained but relative chondrite-normalized abundances reduced. Although an Fe-Mn zone occurs a few 100 m along strike, no such mineralization occurs in the barite deposit. Fluid inclusion studies indicate that vein fluids were saline brines (20-30 wt % NaCl + CaCl2 equiv) that coexisted with an immiscible N-2-CO2-CH4 gas. Laser Raman analysis of the most common gas-rich inclusions indicates a uniform fluid chemistry with 66 mol percent N-2 and 34 mol percent CO2 and is consistent with compositions inferred from thermometric measurements. Homogenization temperatures for primary, aqueous fluid inclusion asemblages in quartz intergrown with barite indicate minimum trapping temperatures of 210 degrees C. Conditions at the time of entrapment, based on intersection of aqueous and gaseous isochores, are estimated at similar to 775 bars and 250 degrees C. Stable isotope data for vein minerals (barite, siderite, quartz) are uniform in nature and indicate delta O-18(fluid) = +12 per mil at 250 degrees C for siderite and +9.8 per mil for quartz, delta C-13(fluid) = -4 to -6 per mil from siderite, and delta S-34(fluid) = +12 per mil from barite. Fluid inclusion extracts indicate delta D values of -47 to -71 per mil. Collectively, these isotopic compositions are consistent with a basinal-type fluid derived from modified meteoric water with sulfur derived from Carboniferous evaporites and carbon of mainly marine limestone origin (i.e., Carboniferous Windsor Group) with a minor biogenic component from the Horton Group. Mineralization at Brookfield resulted from focusing of heated, overpressured brines of modified basinal origin into an active fault zone environment. The association of the barite mineralization with intensely altered wall rock represents a rare example of such alteration in the MVT metallogenic domain of the Maritimes Basin. However, similar alteration associated with Ba-Fe-Mn mineralization along the Cobequid-Chedabueto fault system raises the possibility that the Brookfield deposit may instead be part of another mineralizing event that was distinct from the 300 Ma MVT deposits. |
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