“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, which has been called the “Pontid- 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. This region contains more VMS deposits than any other district in the world. The Japanese volcanic arc contains 56 massive sulfide deposits, and the Noranda district in Canada, a Precambrian equivalent of this type, contains about 50 massive sulfide deposits.
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 (from a few hundreds to 40 million tons) compared, for example, 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 (up to 5° to 10°). Replacement textures dominate in all of the deposits investigated. The mineral paragenesis for the yellow-ore dominated deposits was found to be pyrite-chalcopyrite (I)-sphalerite-galena-tetrahedrite-chalcopyrite(II)-bornite, and for the black-ore dominated deposits, pyrite-chalcopyrite-sphalerite-galena-tetrahedrite. Formation temperatures, measured as part of this dissertation research, fall in the range of 160°C- 320°C, concentrating between 240°C-300°C. Almost all the major occurrences are hosted by felsic rocks that contain rhyolite, rhyodacide, dacite, their lava and pyroclastics for which “felsic volcanic complex” as a term is proposed here. In most of the occurrences, the mineralized hanging wall rocks are overlain by either barren volcanic rocks or by volcano-sedimentary sequence. Alteration is limited to the host rocks, and the hanging wall rocks are not altered to any significant degree. Electron microprobe analysis (EPMA) study during this dissertation research on the major sulfide minerals for trace elements revealed a strong similarity between the VMS deposits of the region implying coeval formation through a similar genetic mechanism. Thus, they are considered “sister deposits”.
Sulfur, oxygen, and lead isotopic analyses were determined as part of this dissertation research. 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 (Au, Ag) and petzite (Ag3AuTe2). Gold occurrences were found to be restricted to the yellow ore, and gold grains occur mainly within chalcopyrite (CuFeS2). Gold grades for the ores vary between 0.24 ppm and 7.55 ppm. Silver occurs in solid solution, especially in tedrahedrite-tennantite (Cu12SbS13- CU12AS4S13) and bornite (Cu5FeS4.
Unlike the Green Tuff district that host the Kuroko-type VMS deposits, the Pontid region does not have a basinal structure. The ore deposits are oriented mostly along a line corresponding to the axis of a complex volcanic arc. Based on the region’s geology, dacitic-rhyodacitic-rhyolitic domes appear to be genetically more related to VMS deposits, although it is postulated that the magnitude of the domes is not sufficient for adequate heat production to produce large scale convection cells to form ore deposits, and therefore larger heat sources beneath the domes are required. This implies a magmatic source that is a more plausible model for the region due to the domes and nearby granitoid intrusions.
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 (160°C-300°C by the fluid inclusion measurements presented here). 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 grading from 0.60 ppm to 35 ppm, 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 (δ34S = -0.5 - +4.2) 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, pages iii-v.
Hagni, Richard D.
Wronkiewicz, David J.
Hogan, John Patrick
Gregg, Jay M.
Geosciences and Geological and Petroleum Engineering
Ph. D. in Geology and Geophysics
University of Missouri--Rolla
xxvii, 251 pages
© 2000 Emin Ciftci, All rights reserved.
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Ciftci, Emin, "Mineralogy, paragenetic sequence, geochemistry and genesis of the gold and silver bearing Upper Cretaceous mineral deposits, northeastern Turkey" (2000). Doctoral Dissertations. 1340.
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