麻豆传媒

Mineralizing fluids for lead-zinc and barium deposits located at the margin of the Minas Sub-basin of the major Fundy/Magdelen Basin were hot (150-250oC) saline brines with hydrogen and oxygen isotopic compositions in the overlapping fields of basinal brine, and metamorphic and juvenile fluids. Strontium isotopic analyses of calcite and barite and of lead isotopes of galena indicate that there is a genetic link between these mineral deposits. The Horton Group rocks which are direct clastic derivatives of the basement are a likely source for some of the calcium and barium as well as the lead in the mineralizing fluids.

Sulfur, probably derived from the voluminous Windsor evaporites, was likely carried as sulfate (in a fluid with neutral pH) to the Pb-Zn depositional sites, where complete (non-biogeneic?) reduction (at >200oC) occurred. The carbon budget of the ore-forming fluids was dominated by oxidized carbon from a carbonate source locally influenced by low 13C organic carbon.

The likely age for this mineralizing event is ~300 Ma. Rb/Sr and ⁸⁷Sr/⁸⁶Sr analyses of a suite of clays separated from Horton Group sandstone near a brecciated horizon indicate a strontium-homogenizing event at 300+Ma. Fission track dating of zircon in host rock gives mineralization ages of 313+44 (2 ) Ma at the Gays River Zn-Pb deposit and 250+34 (2 ) Ma at the Smithfield Pb-Zn deposit. An ⁴⁰Ar/³⁹Ar age spectrum of a ~610 Ma muscovite from Meguma wallrock at Gays River indicates a partial resetting at <330 Ma. These ages coincide with a pulse of regional tectonism recorded in the stratigraphic succession.

A basinal brine expulsion model is favoured. The hot mineralizing fluid originated at >5km depth under the Windsor evaporites in the deep southern Magdalen Basin. Horton Group clastic rocks and the Pembroke breccia are likely aquifers. Mathematical modelling using the TWODEPEP finite element program indicates that, during periods of rapid sediment or tectonic loading, fluid pressures under the evaporite seal could have approached the lithostatic load. At least one sudden massive hydrofracturing, fluid expulsion event is likely to have occurred by 299 Ma and possible as early as late Windsor time (336 Ma).

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