Geology, base-precious metal concentration and genesis of the silica-carbonate alteration (listwaenites) from late Cretaceous ophiolitic melanges at central east Turkey
The late Cretaceous (pre Maastrichtian) alteration of serpentinite bodies in the Divrigi and Kuluncak ophiolitic melanges has formed two distinct types of listwaenite. The earliest is silica-carbonate listwaenite (type I), which is dominated by silica + calcite + dolomite + ankerite +/- magnesite. Later, carbonate listwaenite (type II) comprise calcite + dolomite + ankerite + magnesite, and lack any significant introduced silica. Accessory ore minerals including pyrite, hematite, limonite, gersdorffite, marcasite, carrollite, langisite, and chromite are less abundant in carbonate listwaenite than they are in silica-carbonate listwaenite. Both types of listwaenite have been distinguished in the Guvenc and Karakuz areas, but in Curek only silica-carbonate has been recognized. The two listwaenite types are dissimilar in their major oxide and base-precious metal contents. Silica-carbonate (type I) listwaenite has, in order of relative abundance, SiO2, CaO, Fe2O3 and MgO, whereas carbonate (type II) listwaenite is dominated by CaO, Fe2O3 and MgO. The contents of Al, Ti, Mn, Na, K and P oxides are negligibly low in both listwaenite types in all study areas. Concentrations of base and precious metals are much higher in silica-carbonate listwaenites than in carbonate listwaenites but Au and Ag are present only in very low concentrations in both listwaenite types. Concentrations of Co, Ni, Pb, As, Sb and Ag are higher in listwaenite than in associated serpentinite, and these elements have probably been leached in part from adjacent serpentinite by hydrothermal fluids. Silica-carbonate (type I) and carbonate (type II) listwaenite in the Karakuz and silica-carbonate listwaenite in Guvenc, are formed along thrust fault zones. However, the majority of silica-carbonate in Curek and carbonate in Guvenc are not thrust fault controlled. In clearly fault-related listwaenites, thrust fault(s) acted as pathway for mineralizing and altering fluids. In non fault-related listwaenites, hydrothermal fluids moved along highly serpentinized microfractured, stock-worked and porous ultramafic rocks. The hydrothermal fluids involved in the formation of carbonate (type II) listwaenite, differed from those that formed silica-carbonate (type I) listwaenite, which were enriched in SiO2, as well as CO2, Ca and H2O, whereas those that formed carbonate listwaenite were SiO2 deficient, and enriched only in COP, Ca and H2O.