The geology and mineralogy of copper lead, Zinc sulphide veins from Bulancak, Turkey

Abstract

The Bulancak Cu-Pb-Zn sulphide veins are the best representatives of their type among the varied deposits of the Eastern Pontus Ore Province. Kuroko, vein, stockvjork, prophyry-type copper, skarn-type iron and manganese deposits with subduction related volcanicity have been described from this province by various authors. Petrographic study of the Country rocks, despite their hydrothermal alteration, shows that they are mainly calc-alkaline. Volcanicity took place from Upper (possibly Lower) Cretaceous to Miocene as indicated by intrusives dated at 25 million years. Ore microscopy studies have proved the first occurrence (in Turkey) of Betekhtinite (Cu, Fe)(_21)Pb(_2)S(_15) and Aikinite (PbCuBiS(_3))-Bismithinitre(Bi(_2)S(_3) solid solution minerals. Idaite is reported for the first time from the study area. Field and Laboratory work have shown the structural control of Mn, Fe and Cd distribution in sphalerites. Mn follows a NW-SE trending zone from Kusdere to Kasyatak whereas Fc-Cd distribution follows a SW-NE trending zone between Kovalik Sr. and Kuloglu along Kűçűkgűre Dere (stream), reflecting the dependance on the two major fracture systems in the area. Data based on a study of more than 2500 primary and secondary fluid inclusions in transparent ore and ganque minerals is given, including measurements on the temperature, pressure, density and salinity. The inclusions were moderately saline, salinity varying within the range 13.2 equivalent wf.% NaCl to almost fresh water indicated by a 3 density change from 0.74gm/cm(^3) in the early vein stage to 0.98 at the end of the base metal sulphide stage. These variations way reflect the mixture of deeply circulating meteoric waters with rising, saline, hydrothermal fluids. Homogenization temperatures ranged from ca. 340 C to 80 C. Inclusions in neighbouring early formed quart2 and sphalerite minerals from the same vein show a temperature rise from ca. 230 to 330 C in the early vein stage, and eventually boiling conditions were reached. At a later stage the fluids cooled from 330 C to ca. 80 C. in the base metal stage. The highest temperatures were obtained from the Tekmezar Group Veins, in comparison to the DarikBy Group Veins, indicating a horizontal temperature zoning in which the Bi-content of the aikinitebismuthinite solid solution minerals increases from West (Saridiken Dere) to east (Kornali Dere). Data obtained from isotherms indicates that the ore-bearing fluids rose along NW-SE trending faults. This explains the concentration of veins around the Selmanoglu, Gonurca Dere, and Uzllinltik Dere Faults. Simple cooling of the solutions, pressure fluctuations, density and replacement processes played major roles in the formation of these veins. Fluid inclusion studies combined with geological data, are considered to be useful parameters in mineral exploration to pinpoint the richest part of an orebody. Metal transport and the thermal history of vein mineralisation are discussed in the light of fluid inclusion data.