Porphyry-copper mineralisation in the central Srednogorie zone, Bulgaria

Abstract

The porphyry-copper systems in the central part of the Srednogorie zone (Bulgaria) are represented by three major deposits (Elatsite, Medet and Assarel) and several smaller deposits and occurrences, all of them within the Panagyurishte ore district. The hydrothermal systems are related to Late Cretaceous calc-alkaline igneous complexes. Ore mineralisation is developed predominantly in the apical parts of subvolcanic and intrusive bodies as well as within the volcanic and basement metamorphic rocks. Several of the porphyry systems are spatially associated with shallow-level intermediate and high-sulphidation volcanic-hosted epithermal deposits of economic importance, such as the major gold-copper mine at Chelopech located 10 km from the Elatsite porphyry-copper deposit. Mineralisation processes in the porphyry deposits start with intensive hydrothermal alteration of the wall rocks. K-silicate alteration is characteristic for pre-ore hydrothermal activity in all of them, and it is located mostly in their central parts. Propylitic alteration is prominent in the Medet and Assarel deposits. The Assarel deposit is located in the central part of a palaeovolcanic structure and shows a large spectrum of pre-ore alterations, including propylitic, sericitic, and advanced argillic assemblages. The initial stages of the hydrothermal systems are characterised by high temperatures (>550–500 °C) and highly saline (50–20 wt% NaCl equiv.) and vapour-rich fluids of likely magmatic origin. The composition of the fluids gradually changes from H2O–NaCl±FeCl2 to H2O–NaCl–KCl and H2O–NaCl-dominated as the fluids cool, react with wall rocks, and may become diluted with meteoric water. Fe-Ti-oxide mineral associations were formed early in all deposits, later followed, in the Elatsite deposit, by an assemblage of bornite, chalcopyrite, platinum group element (PGE) phases, Co-Ni thiospinels, Ag- and Bi-tellurides, and selenides. The main ore stage in all deposits is dominated by chalcopyrite ± pyrite, associated with propylitic alteration at Medet, K-silcate-sericite alteration at Elatsite, and chlorite-sericite alteration at Assarel. Specific geochemical features of the ore systems are marked by precipitation of numerous fine-grained mineral inclusions in chalcopyrite and pyrite (palladoarsenide, Pd-rammelsbergite, Cu-Ni sulpharsenides in Elatsite; Co-and Ni-pyrite, carrolite, vaesite, sulvanite and colusite in Medet; enargite, As-sulvanite, colusite, goldfieldite, wittichenite, calaverite and aikinite in Assarel). Native gold and electrum are observed as small grains, except in Elatsite where larger aggregates up to several millimetres are found. Molybdenite is present as small flakes within chalcopyrite, or forms felted aggregates within thin quartz veinlets in the Elatsite and Medet deposits, and very rarely at Assarel. Thin veinlets of quartz-galena-sphalerite are found at the upper parts of the deposits. Supergene alteration of the primary sulphide minerals is developed in the Assarel deposit where a thick blanket of secondary copper mineralisation (chalcocite, covellite) has formed. In the other two deposits, secondary mineral associations (Cu and Fe oxides and hydroxides) are limited to near the surface. Differences in the deposits studied suggest that Elatsite and Medet are formed probably at deeper levels of the porphyry systems, whereas Assarel is developed at rather shallower and lower-temperature conditions typical of the apical part of a subvolcanic body intruding its volcanic superstructure.