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 Viséan 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 iso-con plot indicates mass loss (CA = 0.82*CO) during wall-rock alteration with REE 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 co-existed with an immiscible N2-CO2-CH4 gas. Laser Raman analysis of the most common gas-rich inclusions indicates a uniform fluid chemistry with 66 mol percent N2 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°C. Conditions at the time of entrapment, based on intersection of aqueous and gaseous isochores, are estimated at ~775 bars and 250°C.

Stable isotope data for vein minerals (barite, siderite, quartz) are uniform in nature and indicate δ18Ofluid = +12 per mil at 250°C for siderite and +9.8 per mil for quartz, δ13Cfluid = −4 to −6 per mil from siderite, and δ34Sfluid = +12 per mil from barite. Fluid inclusion extracts indicate δD 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-Chedabucto 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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