Magmatic sulphides in high-K calc-alkaline to shoshonitic and alkaline rocks

Abstract

Abstract. We investigate in both mineralised and barren systems the occurrence and chemistry of magmatic sulphides and their chalcophile metal cargo behaviour during evolution of compositionally different magmas in diverse geodynamic settings. The investigated areas are: (a) the Miocene Konya magmatic province (hosting the Doganbey Cu-Mo and Inlice Au-epithermal deposits) (Post-Subduction) and (b) the Miocene Usak basin (Elmadag, Itecektepe and Beydagi volcanoes, the latter associated with the Kisladag Au porphyry) in Western Turkey (Post-Subduction). For comparison we also investigate (c) the barren Plio-Quaternary Kula volcanic field, west of Usak (Intraplate) and finally we discuss and compare all the above areas with the already studied (d) Quaternary Ecuadorian volcanic arc (host to the Miocene Llurimagua Cu-Mo and Cascabel Cu-Au porphyry deposits) (Subduction). The volcanism of the studied areas displays a wide range of SiO2 spanning from basalts to andesites/dacites and from high K-calc-alkaline to shoshonitic series. Multiphase magmatic sulphides occur in different amounts in all investigated areas and based on textural and compositional differences, they can be classified in different types, which crystallised at different times (early versus late saturation). A decrease in the sulphide Ni/Cu (proxy for mss-monosulphide solid solution/iss-intermediate solid solution) ratio is noted with magmatic evolution. Starting with an early stage, saturating Ni-richer/Cu-poorer sulphides hosted by early crystallising minerals e.g. olivine/pyroxene, leading up to a later stage, producing Cu-richer sulphides hosted by magnetite. The most common sulphide type resulting from an early saturating stage is composed of a Cu-poor/Ni-rich (pyrrhotite/mss) and one/two Cu-rich (cubanite, chalcopyrite/iss) phases making up 84 and 16 area % of the sulphide, respectively. Our results suggest that independently of the magma composition, geodynamic setting and whether or not the system has generated an ore deposit on the surface, sulphide saturation occurred in variable degrees in all studied areas and magmatic systems and is characterised by a similar initial metal content of the magmas. However not all studied areas present all sulphide types and the sulphide composition is dependent on the nature of the host mineral. In particular sulphides, resulting from the late stage, consisting of Cu-rich phases (chalcopyrite ,bornite, digenite/iss) are hosted exclusively by magnetite and are found only in magmatic provinces associated with porphyry Cu (Konya and Ecuador) and porphyry Au (Beydagi) deposits.

Gold endowments of porphyry deposits controlled by precipitation efficiency

“…Since there is some scatter in the SiO 2 -Cu relationship, all possible values within the upper and lower boundaries of this scatter have been considered and implemented in the Monte Carlo modelling. Georgatou and Chiaradia 46 have shown that also high-K calc-alkaline, shoshonitic and other alkaline rocks follow a similar trend in the SiO 2 -Cu plot. Therefore, the Cu contents of melts in the model were taken to be the same for calc-alkaline and variably alkaline rocks.…”

“…s) Time Random between 0 and 5 Ma Pressure (calc-alkaline systems) Random between 0.15 and 0.9 GPa Pressure (alkaline systems) Random between 0.15 and 0.6 GPa H 2 O in parent magma Random between 2 and 4 wt% 49 H 2 O in crustal rocks Random between 0.2 and 1 wt% 2 Fluid-melt partition coefficient of copper Random between 2 and 100 2 Fluid-melt partition coefficient of gold Random between 10 and 100 27 Gold content in calc-alkaline magmas Random between 6 and 9 ppb 47 Gold content in alkaline magmas Random between 10 and 32 ppb 25 Copper content in calc-alkaline and alkaline magmasConstrained through SiO 2 -Cu relationship of alkaline and calc-alkaline magmas45,46 Cu precipitation efficiency in Cu-rich calc-alkaline systems 50%(Fig. 3a-c, Fig.…”