The geochemical record of hydrothermal mineralization and tectonism inboard of the Appalachian Orogen: The Ottawa Embayment

Document Type

Article

Publication Date

6-15-2003

Department

A. E. Seaman Mineral Museum; Department of Geological and Mining Engineering and Sciences

Abstract

The history of mineralization of the Ottawa Embayment includes migration of basin brines within the confines of an Early Paleozoic basin, and cross-stratal fracture flow linked to structural reactivation of an underlying Neoproterozoic failed-rift system. Paleoporosity consists of paleokarst (PAL) in Lower Ordovician dolostone (Beekmantown Group), and fractures (now veins) that cross-cut both Lower Paleozoic (PL) and Precambrian (PC) strata. A series of contrasting mineral assemblages, which define changing fluid compositions, fill paleokarst cavities and rare interconnected paleofractures (now veins). First, there is an early-PAL succession of quartz, saddle dolomite, then calcite, with accessory hydrocarbon as well as sulphide and sulphate minerals. The quartz defines migration of saline (> 20 wt.%) silica brines and hydrocarbon that spans a temperature increase (defined by fluid inclusions) from < 100 to < 200 °C. This gradient is registered also by a change in state, from liquid to methane, of hydrocarbon trapped within the quartz crystals. The succession of saddle dolomite, then calcite associated with liquid hydrocarbon, marks an overall cooling to < 110 °C. δ13C of the dolomite identifies strong sedimentary rock-water interaction, but a depleted 13C composition (∼-14‰) for the calcite illustrates that, with cooling, paleokarst cavities were sites of bacterially mediated sulphate reduction. Remaining paleokarst porosity is filled with a late-PAL mineral assemblage that contains low-temperature (mostly < 60 °C) calcite with a varied assemblage of sulphide and sulphate minerals. However, a similar mineral assemblage occurs in regionally abundant PL calcite veins that, locally, visibly intersect paleokarst void space. Furthermore, PL veins are also connected to PC veins. Thus, the late-PAL period of mineralization is linked to regional fracture-controlled cross-stratal fluid migration, which locally intersected paleokarst. Isotope (S, O, C) geochemistry, mineral assemblages, and fluid inclusions from vein and late-PAL minerals identify mixing among four endmember solutions: evolved hydrothermal ( < 130 °C) basement-derived brines; cool ( < 60 °C) meteoric-derived fluids; dissolved Middle Ordovician evaporite; and residual basin brines. The age of mineralization awaits future analysis, but patterns and compositions of fluids in this interior basin are consistent with being far-field effects of Paleozoic and Mesozoic tectonism in the Appalachian orogen.

Publication Title

Chemical Geology

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