The petrology and geochemistry of volcanic rocks and associated sulphide deposits of the S.E. Anatolian Ophiolite belt, near Malatya, Turkey.

Abstract

The Fe-Cu-Zn sulphide mineralisation of the Polusagi and Canakci areas are two of many cupriferous deposits which occur along the S.E. Anatolian ophiolite belt. The Polusagi stratiform mineralisation is within the lavas of an Upper Cretaceous-Eocene submarine volcanogenic sequence which is thrust over the crystalline Palaeozoic basement rocks. The Canakci mineral isation occurs within the volcanogenic sediments of probable Upper Cretaceous- Eocene age. The host and associated rocks of the mineralisation at Polusagi are studied in detail. Extensive submarine alteration has changed the igneous mineralogy and rock chemistry. Element mobility during the alteration is determined and a chemical classification scheme is proposed on the basis of the immobile elements Zr, Y, Nb, La, Ce and Ti. The igneous rocks belong to a tholeiitic suite and were formed during the early stages of an island- arc development on a Tethyan marginal basin floor. The Polusagi magma originated from a high degree partial melting of an upper mantle source rock and the various rock types were produced by a process of fractional crystallisation involving olivine, clinopyroxene, magnetite, plagioclase and apatite. Mineralogy and mineral chemistry of the Polusagi rocks are studied in detail. The rocks were metamorphosed to prehnite-pvunpellyite facies at higher stratigraphic levels and to greenschist facies at lower, under pressures < 3kb and probably around 2kb, Metamorphism was basically a result of sea-water-rock interaction i.e. "ocean-floor hydrothermal metamorphism". Ore mineral chemistry and mineralisation at both areas are examined in detail. The Polusagi massive sulphides are fine grained and composed mainly of pyrite, chalcopyrite and sphalerite with minor fahlore, galena, bornite and covellite and traces of magnetite, hematite and idaite. They show strong mineralogical banding and exhibit features ascribable to precipitation from solutions in an unrestricted environment. The massive sulphides are underlain by pyritic stockwork mineralisation and are overlain by and associated with Fe-Mn-oxide rich siliceous sedimentary horizons. The mineralisation at Canakci is in two forms : massive and disseminated. In many aspects, the former is similar to the massive banded ore and the latter to the stockwork mineralisation of the Polusagi deposit. The Canakci massive sulphide is composed mainly of pyrite, bornite, fahlore, sphalerite and galena with minor chalcopyrite, digenite, idaite, covellite and traces of mawsonite, hexastannite, colusite and native gold, A model is presented to account for the genesis of both deposits. In the submarine geothermal system set up following the volcanism, the sea water percolated through the volcanic pile, became heated and reacted with and leached metals from the rocks. The hydrothermal metal bearing solutions were then discharged onto the sea-floor through channels now represented by the stockwork mineralisation. Upon reaching the sea-water rock interface with temperatures up to 400ºC, the solutions mixed with the sea-water and precipitated massive sulphides as a result of a sudden drop in temperature and increases in pH and oxygen fugacity. With further dilution, Fe-Mn oxide rich siliceous sediments precipitated from cool and now oxidised solutions.