Geochemistry and origin of the Asimotrypes carbonate hosted mesothermal gold deposit, Pangeon mountain, N. Greece

Abstract

The Asimotrypes mesothermal gold deposit located in the Pangeon Mt (E. Macedonia, N. Greece), is part of the Western Rhodope Massif (WRM), which represents the metamorphic core complex of an alpine collision orogen. Alpine metamorphism, of U. Cretaceous - M. Eocene age, reached upper greenschist to lower amphibolite conditions. A low-pressure greenshist fades retrograde overprint during uplift marked the end of Alpine metamorphism in the Miocene. Post-metamorphic Miocene uplift has been recognised in the Pangeon Mt and it was ascribed to Late Cainozoic extensional tectonics.

The Asimotrypes ore, of replacement and shear-zone style consists mainly of arsenopyrite, pyrite and gold, with subordinate sphalerite, galena, chalcopyrite, pyrrhotite, tetrahedrite-tennantite, marcasite, covellite and malachite. Gold is either refractory occurring mainly in arsenopyrite and to a lesser extent in Aspyrite, or free in the oxide minerals.

Three types of fluid inclusions (with subtypes) were recognised based on constituent phases at room temperature and microthermometric behaviour: (i) H₂O-CO₂ 3-phase inclusions: L₁ (H₂O)+L₂ (CO₂)+V (CO₂); (ii) Aqueous 2-phase inclusions: L+V; and (iii) Naturally decrepitated and/or leaked inclusions: V or L+V. The fluids have low salinity (<5 wt % NaCl equiv.), but variable CO₂/H₂O ratios. Microthermometric studies in gangue quartz indicate early ore deposition at P-T conditions of 275º-310ºC and 2.7-3.1 kb during unmixing of the mineralising fluids, followed by deposition at temperatures down to 130ºC and low near surface pressures.
δ³⁴S values of primary sulphide minerals suggest a magmatic source for the sulphur. δ¹³C values in marble calcite are indicative of a marine environment of deposition, also supported by a plot of δ¹³C versus δ¹⁸O for the same samples. Calculated isotopic composition of ore fluids in quartz, sericite and whole rock at 275ºC and 340ºC are consistent with values of metamorphic fluids. Whole rock hydrogen isotopic composition of -117±7.5 (lσ) indicates that the mineralising fluid was of meteoric origin. Sr isotope data implies seawater origin for strontium. Lead isotope data in ores from the Rhodope showed that Pb is derived from crustal rock types. 
Sulphide species such as Au (HS)^-₂, were probably the most effective complexing agents for gold in the Asimotrypes fluids, which were typically low in salinity. A decrease of sulphur species activity and cooling are suggested to be the favoured depositional mechanism in a reducing environment. Combined with geological evidence, the fluid inclusion and stable data of the Asimotrypes gold ore, are consistent with genesis from deeply convecting meteoric waters driven by regional uplift through rocks undergoing retrogressive greenschist facies metamorphism.

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