Abstract

Vein stockworks and lacustrine developments of cryptocrystalline magnesium carbonates of Neogene and Quaternary age occur within the partially serpentinized, discontinuous ultramafic belts of southwestern Turkey. They are comparable to the Neogene cryptocrystalline magnesite bodies elsewhere in the Alpine orogen to the northwest and southeast. Our previous work (Fallick et al., 1991) suggested that cool (<100°C) modified meteoric water was the mineralizer, that ultramafic rock was the source of the magnesium, but that there were three separate sources of the (bi)carbonate. These sources were distinguishable by their stable isotope composition as follows: (1) low-temperature carbonate with δ18O(SMOW) values of ~36 per mil and δ13C(PDB) values of ~4 per mil, derived from atmospheric CO2; (2) moderate-temperature carbonate with δ18O(SMOW) values of +28 per mil and δ13C(PDB) values of –15 per mil, derived by decarboxylation of organic-rich sediments; and (3) higher temperature carbonate with δ18O(SMOW) values of ~19 per mil and δ13C(PDB) values of ~3 per mil, assumed to have been generated by thermal contact metamorphism of Paleozoic marine limestone at depth. In general these magnesite deposits were found to fall into two groups, comprising carbonate generated on two mixing lines. The first group spanned the putative mixing line from the "atmospheric" source (1) to "organically derived" source of CO2 (2). The second group extended between atmospheric source (1) and the "thermal" source (3), although there were concentrations either around the atmospheric end, or precisely at the contact metamorphic end of the line.

In the present study we found that large stockwork deposits at Helvacibaba and Koyakci Tepe have δ13C(PDB) and δ18O(SMOW) values averaging ~–12 and ~+27 per mil, respectively, indicating a derivation mainly by oxidation of organic-rich metasediments perhaps underthrust at depth (end-member 2), with some involvement of atmospheric carbon dioxide as bicarbonate in the circulating, hot, and modified meteoric water (end-member 1). Calcite veinlets in a meta-argillite of the Cambro-Ordovician Seydisehir Formation, most likely to have been underthrust beneath the stockworks, yielded δ13C(PDB) values of –20 per mil, consistent with, though not proving, oxidized organic carbon being one of the sources of carbonate. The δ18O(SMOW) values of these same veinlet carbonates are also rather low (22‰), indicating precipitation from heated ground water, though their age is unknown.

The major stratiform magnesite deposit at Hirsizdere in the center of the Menderes graben has δ13C(PDB) and δ18O(SMOW) values averaging ~3 and ~25 per mil, respectively, and thus appears to be an example of the hydrothermal-sedimentary (i.e., exhalative) type (Ilich, 1968). In contrast, the hydromagnesite stromatolites presently growing in Salda Gölü (Lake Salda) are apparently developing at cool ground-water seepages. The gross morphology of the Salda Gölü stromatolites and the hydromagnesite sediments derived therefrom is reminiscent of that revealed in the Bela Stena magnesite pit in Serbia. These lacustrine deposits have mean δ13C(PDB) values of ~4 and ~2 per mil and mean δ18O(SMOW) values of ~36 and ~33 per mil, respectively, i.e., they both plot broadly over the atmospheric CO2-meteoric water field (end-member 1), consistent with microbially mediated precipitation at cool ground-water seepages in enclosed evaporating lakes.

You do not currently have access to this article.