Genesis of Vein Stockwork and Sedimentary Magnesite and Hydromagnesite Deposits in the Ultramafic Terranes of Southwestern Turkey: A Stable Isotope Study

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 (less than or equal to 100 degrees 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 delta(18)O((SMOW)) values of similar to 36 per mil and delta(13)C((PDB)) values of similar to 4 per mil, from CO2; (2) moderate-temperature carbonate with delta(18)O((SMOW)) values of +28 per mil and delta(13)C((PDB)) values of -15 per mil, by decarboxylation of organic-rich sediments; and (3) higher temperature carbonate with delta(18)O((SMOW)) values of similar to 19 per mil and delta(13)C((PDB)) values of similar to 3 per mil, assumed to have been generated by 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 source (1) and the thermal source (3), although there were concentrations either around the 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 delta(13)C((PDB)) and delta(18)O((SMOW)) values averaging similar to-12 and similar to+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 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 delta(13)C((PDB)) values of -20 per mil, consistent with, though not proving, oxidized organic carbon being one of the sources of carbonate. The delta(18)O((SMOW)) values of these same veinlet carbonates are also rather low (22 parts per thousand), 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 delta(13)C((PDB)) and delta(18)O((SMOW)), values averaging similar to 3 and similar to 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 Golu (Lake Salda) are apparently developing at cool ground-water seepages. The gross morphology of the Salda Golu stromatolites and the hydromagnesite sediments therefrom is reminiscent of that revealed in the Bela Stena magnesite pit in Serbia. These lacustrine deposits have mean delta(13)C((PDB)) values of similar to 4 and similar to 2 per mil and mean delta(18)O((SMOW)) values of similar to 36 and similar to 33 per mil, respectively, i.e., they both plot broadly over the CO2-meteoric water field (end- member 1), consistent with microbially mediated precipitation at cool ground-water seepages in enclosed evaporating lakes.