Mineralogy, paragenetic sequence, geochemistry and genesis of the gold and silver bearing Upper Cretaceous mineral deposits, northeastern Turkey

The Upper Cretaceous mineralizations, located in northeastern Turkey, are the largest sources of base metals in the country. Vein-type and the volcanogenic massive sulfide-type deposits (VMS) are present and both are closely associated with felsic volcanism of Upper Cretaceous age.; The formation of the volcanogenic massive sulfide deposits is strictly associated with the evolution of a very complex volcanic arc structure formed during Kimmeridgian-Alpine epoch, called “the Eastern Pontid tectonic belt”. The VMS deposits of the region present a specific genetic type, and it is highly similar with well-known Kuroko-type mineralizations of Japan. This volcanic arc contains about 62 massive sulfide deposits along its axial zone.; The volcanogenic massive sulfide mineralizations can be subdivided into three types: black-ore dominated deposits, yellow-ore dominated deposits, and deposits containing black (or semi-black) and yellow ore. The deposits are relatively small in reserves compared to the larger porphyry-type Cu-Mo deposits, but they have much higher base metal contents. The orebodies are mostly lens-shaped and flat lying or slightly tilted. Replacement textures dominate in all of the deposits investigated. Almost all the major occurrences are hosted by felsic rocks. In most of the occurrences, the mineralized hanging wall rocks are overlain by either barren volcanic rocks or by volcano-sedimentary sequence.; Sulfur isotope data from the sulfide minerals revealed a very narrow distribution implying a homogeneous source and formation temperature, possibly from the underlying volcanic rocks. Oxygen isotope data showed that the ore-forming fluids could be seawater or mixed with magmatic waters in origin. Lead isotope data also indicated a homogeneous metal lead source for the VMS deposits and showed a less radiogenic character. Lead isotopes also indicated that the VMS deposits were of 89 my in age.; The VMS deposits contain submicroscopic gold in the form of electrum and petzite. Gold occurrences were found to be restricted to the yellow ore, and gold grains occur mainly within chalcopyrite. Silver occurs in solid solution, especially in tedrahedrite-tennantite and bornite.; The vein-type mineralizations are relatively small in size, but they are rich in base metal contents and they also contain gold and silver. They are structurally controlled and mesothermal in character. Types of mineralizations include open space, replacement, vug-filling, and veins. According to the proposed generalized paragenetic sequence, pyrite and sphalerite with barite and quartz are present in most of the mineralizations. The vein-type deposits of the region contain gold in the form mainly of native gold and electrum. Silver occurs as freibergite and in solid solution in tetrahedrite, pyrite, galena, and zinkenite.; Sulfur isotope compositions for the vein-type deposits also have shown a narrow distribution that can be attributed to a homogeneous source and a narrow range of formation temperatures. The vein-type deposits have more radiogenic lead that may indicate an older or sedimentary source for the metal lead. The geological setting suggests that the Eocene magmatism in the surrounding vicinity could have been a source for the ore-forming fluids and for the extensive faulting. (Abstract shortened by UMI.)...