Geologic, mineralogic, geochemical and genetic studies of Midi (Karamustafa/Gümüşhane) Zn-Pb deposit, Northeast Turkey

SUMMARY Geologic, Mineralogla, Geochemicai and Genetic Studies of Midi (Karam ustafa- Gfîmüşhane) Zn- Pb Deposit, NE Turkey The Midi deposit is composed of two different ore bodies namely the Köstürelik and Maden Dere. Köstürelik orebody trending NW-SE and dipping NE is hosted by Kntıllık Breccia and Maden Dere orebody trending E-W and dipping N is located at the bottom part of the Zimonköy formation. Both are confined to fault zones. Maden Dere orebody occurs as lenses, whereas the Köstürelik as an irregularly outlined conical form. The orebodies have been formed by both replacement of the host rocks and Millings of fractures. Main ore minerals are sphalerite, pyrite, galena; lesser chalcopyrite, pyrrhotite, tetrahedrite, bornite and gold and tellurides in traces. Alteration developed along the fault zones extends about 1 to 10 metres into wall rock but displays indistinct spatial zonation. Quartz, sericite, chlorite, epidote and carbonate minerals are the dominant alteration minerals. The mass change calculations based on the Zr, Hf, Nb determined to be immobile indicate a 24.26 % volume increase in the ore zones, mainly due to addition of Ca(25 g/lOOg), Fe(12 g/lOOg), Al(8 g/100g), Sİ(3 g/lOQg) and ore-forming elements. Mineral precipitation took place in a temperature interval between 250 and 388 °C. The mean homogenization temperature obtained from fluid inclusions (306.6±25 °C) is consistent with those obtained from S isotope thermometry (301±22 °C), arsenopyrite thermometry (331 °C), and Na- K alkali thermometry (298 °C). Ore forming fluids evolved in the NaCl±KCl-H20 system with low to medium salinity values (6.1±1.4 NaCl % equiv.) and a mean denisty of 0.76±0.05 g/cm3 are slightly acidic to alkaline in composition and are enriched in Ca, Na, K, Mg and Fe but relatively depleted in Si. Thermodynamic parameters calculated at 325 °C and 275 °C respectively revealed a /S2 of -30.4/-26.64, /02 of -12.7/-7.3 and pH of 3.38/5.35 for the ore forming fluids. In this system Pb is transported as chloride complexes at high temperatures, but as sulfure complexes at low temperatures, whereas Zn is transported mainly as chloride complexes. Variation in pH values played much more important role in precipitation of both galena and sphalerite than those in temperature. S^S values (+2 - +6.7 %o) of pyrite, galena and sphalerite indicate a magmatic source for the sulfure. Carbon and oxygen isotopic composition of the fluid in equilibrium with calcite and ankerite point out that oxygen is magmatic in origin, but carbon is derived from another source. C and O isotope modelling studies indicate that water-rock interaction (10-400 w/lr) played the main role in deposition of carbonate minerals and isotope fractionation. Key Words: Midi, Gümüşhane, Epi-mesothermal, Zn-Pb deposit, Mass transfer, Geothermometry, Stable isotope geochemistry, Isotope modelling IX