Age and source of the Küre/Ödemiş arsenopyrite-gold mineralization (Menderes Massif, Turkey) determined by Re-Os-He isotopes

Abstract

The Menderes Massif (MM) is considered to be one of the principal regions hosting vein-type quartz-arsenopyrite orogenic gold deposits and occurrences in Turkey. Gold mineralizations in the MM can be grouped into: (i) schistosity-conformable E-trending veins and (ii) shear-controlled NW-trending veins. Arsenopyrite is one of the major gold-bearing minerals in these deposits or occurrences. Rhenium–Osmium isotopic dating of two types of arsenopyrite from the schistosity-conformable and vein-type Küre (Ödemiş) gold deposit has yielded two groups of model ages (maximum ages) at 557–574 Ma and 246–250 Ma, respectively. The data suggested that the quartz-arsenopyrite gold deposit formed in the Neoproterozoic and Early Triassic, with the former and the latter corresponding to Pan-African compressional orogenesis and Palotethys rifting, respectively. Fluid inclusions in Küre arsenopyrite have 3He/4He ratios of ~0.08–0.09 Ra (Ra = 1.39x10-6, the 3He/4He ratio of air) being within the range of middle to upper crustal values. A minor helium contribution of mantle fluids to the Küre gold deposit is possible because the 4He concentrations in arsenopyrite are enriched 170 to 9520 times relative to argon and typical atmospheric values, indicating that contribution of atmospheric He to the mineralizing fluids is negligible, and 0.08–0.09 Ra is slightly above typical crustal 3He/4He ratios. Arsenopyrite yields relatively non-radiogenic initial Os isotopic compositions, also indicating possiblecontribution from mantle fluids. Carbonic fluid inclusion type-2 (H2O-CO2-NaCl ± CH4) in quartz of NW-trending veins at Küre are gas–liquid-hydrate-crystal-rich and their homogenization temperatures (Th) range from 244 to 387 °C. Salinity values range from 0.2 to 5.7 wt% NaCl equiv. The homogenization temperatures (Th) of aqueous fluid inclusions type 1 (H2O-NaCl) in quartz from NW-trending veins at Küre vary between 237 and 358 °C whereas salinity values range from 1.7 to 7.7 wt% NaCl equiv. Quantitative EPMA spot analyses on arsenopyrite minerals have shown that As concentrations in EW-trending and NW-trending veins range from 32.5 to 33.8% and 32.3–33.8%, respectively. Based on a phase diagram for the Fe–As–S system along with the arsenopyrite geothermometer, the ranges of temperature and corresponding logf S2, at which arsenopyrite is in equilibrium with pyrite are between 380 and 480 °C, and −7.4 and −4.3 for Apy-1, respectively. These values are between 390 and 475 °C, and −7.0 and −4.5 for Apy-2, respectively.