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Geochemistry of a high-T hydrothermal dolostone from the Emirli (Ödemi, western Turkey) Sb-Au deposit

M. Akçay^1,*, H. M. Özkan, B. Spiro², R. Wilson³ and P. W. O. Hoskin⁴

¹ Karadeniz Teknik Üniversitesi, Jeoloji Müh. Böl. 61080 Trabzon, Turkey
² NERC Isotope Geosciences Laboratory, Keyworth, Nottingham NG12 5G, UK
³ Department of Geology, University of Leicester, Leicester LE1 7RH, UK
⁴ Institut für Mineralogie, Petrologie und Geochemie, Albert-Ludwigs-Universität Freiburg, Albertstrasse 23 B, D-79104 Freiburg im Breisgau, Germany

^* E-mail: akcay{at}ktu.edu.tr

A dolostone layer is found in spatial association with the Emirli epithermal Sb-Au deposit in western Turkey. It occurs within an argillic alteration zone adjacent to the major Emirli fault zone, which is the controlling structure for the deposit, and is composed of large closely packed subhedral to anhedral planar and non-planar dolomites. Pyrite is the only accessory mineral in the layer and occurs as disseminations and veinlets up to 100 µm wide. Dolomite crystals are petrographically homogeneous and have consistent deep-red cathodoluminescence (CL) colour with no zoning which implies single-stage dolomitization of a calcitic precursor which is partly preserved as remnant patches of orange-coloured CL zones. Some crystal boundaries have dark (or no) CL colour. Electron microprobe line-scan analyses across these regions indicate intense enrichment of Fe and Mg depletion, revealing late-stage Fe-metasomatism (ankeritization) especially prevalent near pyrite veinlets and disseminations.

Dolomite crystals are composed of 52.4–55.0 mol.% CaCO₃, 29.9–41.2 mol.% MgCO3, 1.8–14.4 mol.% FeCO₃ and 0.75–3.2 mol.% MnCO₃ indicating ferroan dolomite. The relationship between Ca and Mg is not stoichiometric due to substitution of Mg²⁺ by Fe²⁺ after dolomitization, as demonstrated by a strong negative correlation between Fe and Mg. Whole-rock Fe contents of the dolostone layer increases toward the Emirli fault zone.

The ¹³C_(PDB) compositions of the Emirli dolomite, calcitic marbles, and graphite-schists are in the ranges of –1.6 to 0.8, 1.5 to 1.5 and –6.6 to –23.5, respectively, indicating that dolomite was formed due to interaction of light-carbon-enriched fluids with calcitic marbles; light-carbon may have been derived from decarboxylation of the graphitic schist layers. ¹⁸O_(PDB) values of dolomite and marble range –15.2 to –11.2 and –2.4 to –3.5, respectively. This large isotopic difference between dolomite and marble was probably inherited from oxygen isotope exchange between the dolomitizing fluid and the precursor calcites, as well as other minerals enriched in light-oxygen.

Fluid inclusions in dolomite are two-phase, and homogenize into liquid within the temperature interval 242–362°C with a mode of 290°C, and have salinities of 1–3 wt.% NaCl equiv. Using this modal temperature, the average ¹⁸O_(PDB) composition of water in isotopic equilibrium with the Emirli dolostone was estimated to be –19.4±2.1, which is interpreted as an indication of modified surface-waters; this interpretation is also supported by low fluid salinity and Na and Sr contents. These fluids migrated along graben-related faults, penetrating deeper levels where they were transformed into hydrothermal fluids due to the high heat-flow of the Küçük Menderes graben system, and flowed-up mainly through the Emirli and Haliköy faults that control mineralization in local deposits of Sb-Au and Hg, respectively. Due to interaction with chlorite-bearing graphite-schists, the fluid may have dissolved Mg²⁺ from chlorite and been enriched in isotopically-light carbon due to decarboxylation of graphite. Dolomitization occurred as a result of the interaction of these fluids with a calcitic marble band adjacent to the Emirli fault zone. Subsequent introduction of Fe²⁺ caused ankeritization along dolomite crystal boundaries during first-stage Sb-Au mineralization.

KEYWORDS: hydrothermal dolomite, dolostone, stibnite, Sb-Au deposit, Emirli, Turkey