Mineralogy of low temperature argillic alteration in the porphyry-epithermal Çöpler gold deposit, Erzincan, East Central Turkey

The Çöpler porphyry-epithermal gold deposit, is associated with middle Eocene dioritic and granodioritic intrusive rocks. It is a porphyry Cu-Au system characterized by a potassically altered (biotite-K-feldspar-magnetite) core overprinted by extensive phyllic (quartz-sericite) and argillic (quartz – montmorillonite – kaolinite - cristobalite) alteration zones. The low temperature (argillic) alteration zone was investigated by optical microscopy and X-ray diffraction (XRD) methods. Bulk mineralogical compositions of the altered samples are quartz + feldspar + clay. Hydrothermal alteration-related clay mineral compositions of the argillic zone samples are quartz + smectite (montmorillonite) + kaolinite in the outer parts, and quartz + mixed-layer illitesmectite (I-S) and pure cristobalite in the inner parts (toward phyllic zone). Dioctahedral smectites are mainly Ca-montmorillonite, while samples close to inner parts include minor amounts of Na-smectite. Clays were neoformed within the pores of groundmass, and as replacements of plagioclase crystals. I-S has an R3 ordering type (85% illite, 15% smectite). Mineralogical associations indicate lowest grade of alteration (argillic zone), at low temperatures (< 200°C). In general pure smectite and I-S minerals represent potential minerals to aid in elucidating the origin of late stage hydrothermal fluids and the duration of hydrothermal activity which can be obtained from their stable and radiogenic isotope data.