Polymetallic, sulphide ore deposits and associated volcanic rocks from, the harsit river area, N. E. Turkey

Abstract

The Pontid magmatic arc developed during the late Cretaceous and early Tertiary as a result of the northward subduction of Tethyan ocean- floor beneath "Pontian Land", due to the relative northwards movement of Anatolia. Two volcanic cycles, both basalt-andesite-dacite-rhyodacite sequences, can be distinguished in the northern Harsit river area. Basic members of the Upper Cretaceous Lower Volcanic Cycle include tholeiitic basalts and andesitic lavas. They are overlain by dacite lavas. Only the waning stage of this cycle, the rhyolites, tuffs and breccias, contain abundant pyroclastics. This stage is closely related to the mineralisation and constitutes the host-rock horizon. The host-rock and its associated mineralisation show spatial association with the regional fault pattern. The early Tertiary, Upper Volcanic Cycle shows evidence of explosive vulcanicity in the basalts of the Upper Basic Series. Dacites and rhyodacites are locally developed and again show spatial association with the faulting. Both the major and trace element chemistries of the two volcanic cycles Udemonstrate the clear separation into a lower tholeiitic and an upper calc-alkaline cycle. The rocks show similar chemistry to volcanics from island arcs in other areas. The origin of the tholeiitic magma is ascribed to melting of "dry" amphibolite formed by metamorphism of Tethyan oceanic crust during early subduction. Fractional crystallisation of this magma has led to the development of the Lower Volcanic Cycle. The calc-alkaline magma is thought to have formed during a later stage in the subduction process when melting of amphibolite was joined by melting of biotite or phlogopite. This produced a relatively "wet" magma which suppressed plagioclase fractionation until a late stage and prevented enrichment of iron in the residual melts. The volatiles produced in this process may have promoted some melting of Iherzolite overlying the subducted slab. A "high-level" fractional crystallisation of the calc-alkaline magma is thought to have yielded the Upper Volcanic Cycle. Massive, polymetallic mineralisation is associated with the final phase of the Lower Volcanic Cycle. The mineralisation is characterised by a sequence in which a quartz-pyrite and/or sphalerite and chalcopyrite stockwork ore is overlain by massive ore containing galena, sphalerite, chalcopyrite, barite and sulphosalts. This is, in turn, overlain by a horizon in which barite is dominant, succeeded by hematitic and/or manganiferous tuffs and sediments. The ore deposits, in their mode of occurrence, in their mineralogy and morphology bear close resemblance to the Kuroko deposits found in the Miocene, tholeiitic, felsic volcanic rocks of Japan. The lack of evidence for the derivation of ore fluids from igneous activity during the Tertiary era, and the unmineralised nature of the early differentiates of the Lower Volcanic Cycle restrict the mineralising episode to a short period of the felsic, Upper Cretaceous volcanism. The ore fluids responsible for the ore-bodies are thought to be the metal-rich fluids that separated during the final stage of fractionation and solidification of the tholeiitic silicate magma. The ascending magmatic ore solutions interacted with.-sea water. This process resulted in the exchange of Co, Ni, Se, Mg, Ca, Na and K, in a decrease in temperature and salinity, and in an increase in the oxidation state and pH relative to the initial composition of the magmatic fluids. Lithogeochemical analysis and wall-rock alteration patterns in the host-rock and hanging wall may be used in conjunction with detailed stratigraphical and structural analysis to provide directional vectors to determine the proximity of ore bodies.