Perspectives on biogeochemistry

Abstract

Carbonate rocks from the Superior and Slave Provinces of Canada, Kaapvaal Craton of South Africa and the Pilbara Block of Australia, considered of hydrothermal origin from field criteria, have been characterized mineralogically, isotopically (Sr, O, C) and chemically (major and trace elements). In agreement with previous studies, the bulk chemical composition suggests that the carbonate rocks originated by massive carbonatization, silicification and K (±Na) metasomatism of intermediate to ultramafic silicate precursors. The carbonate component itself defines two partially overlapping populations. The ferroan dolomite + breunnerite assemblage is confined mostly to shear zones, conduits and stockworks, while the calcite + ferroan dolomite (ankerite) ± siderite mineral assemblage represents a wider regional halo. This dichotomy may result from a multistory—not necessarily coeval—plumbing system of the volcanosedimentary piles, with the “regional halo” population being a product of shallow circulation cells, while the “conduit” assemblage probably formed from deeper focussed flows. Trace element chemistry of carbonates is consistent with their precipitation from relatively low salinity solutions (⩽2× sea water) and with derivation of solutes from the contemporaneous volcanosedimentary piles (87Sr86Sr 0.7020 ± 0.0008). The hydrothermal waters were derived either from metamorphic dewatering at the base of the piles, or from a magmatic source with δ18O of about +6 ± 7%. SMOW. In contrast, carbon dioxide in the “regional” and the “conduit” assemblages had dissimilar principal sources. In the former it was derived from exogenic sources (decarbonation of marine carbonates and thermal decomposition and oxidation of organic matter), while in the latter the CO2 was of “mantle” origin (δ13C of +2 to −10 and −4 ± 2%. PDB, respectively).