Geology, mineralogy and geochemistry of the gold-bearing stibnite and cinnabar deposits in the Emirli and Halıköy areas (Ödemiş, İzmir, West Turkey)

Abstract

Deposits of stibnite and cinnabar show a strong spatial association with metamorphic rocks and occur mainly within orogenic belts. Our study addresses the Emirli stibnite and Halıköy cinnabar deposits within the Menderes Massif in western Turkey. Stibnite mineralisation occurs as stratabound lenses in graphite- and mica-schist, and as structurally controlled veins. Both types have a common mineral paragenesis containing pyrite, arsenopyrite, stibnite, gold, base metal sulphides, marcasite, realgar, orpiment and cinnabar. Realgar, orpiment and cinnabar are not present in the stratabound lenses. Quartz is the principle gangue mineral in all the deposits. The initial fluids were dilute, carbonic and rather hot (with Th > 300 °C). Deposition of pyrite and arsenopyrite is probably due to cooling of the fluids and to sulphidation and/or decreasing fO 2, caused by interaction with syn-sedimentary sulphides in the country rocks. Arsenopyrite geothermometry indicates an average depositional temperature of 330 °C in the initial quartz vein. Vein formation increased the fluid pressure and caused effervescence, which resulted in a sudden pressure drop from 1.4 kbar during the initial quartz formation to ∼440 bars, and stibnite precipitated at 232 to 307 °C. Boiling and concomitant sulphur loss during stibnite precipitation may have also initiated gold deposition. Further cooling below 190 °C under a pressure of ≤ 600 bars caused cinnabar deposition, mainly in the Halıköy deposit. Salinity of the fluids during stibnite and cinnabar precipitation is 0.8 to 7.1 wt.% NaCl eq. and suggests a meteoric fluid. The structural framework induced by the initiation of extensional tectonics served as conduits for the penetration of surficial fluids to the environs of a possible magmatic heat source evidenced by thermal waters in the region. Stibnite from the stratabound (graphite-schist hosted) lenses and the crosscutting veins in the Emirli deposit have sulphur isotope compositions in a range of − 5.8‰ to − 8.9‰. δ 34S values of pyrite from the graphite-schist zones and mica-schists are in the ranges of − 8.9‰ to − 7.7‰ and − 10.2‰ to − 11.7‰, respectively. It is therefore concluded that the sulphur in stibnite originated from sulphide minerals in the country rocks, which in turn may have a source related to bacterial reduction of sulphate within the pre-metamorphic sediments. Cinnabar has heavier δ 34S contents (− 6.5‰ to − 6.8‰), suggesting a minor magmatic sulphur contribution towards the final stages of mineralisation.