Summary:
In the Kapan mining district, mineralisation dominantly occurs as two styles – as chalcopyritepyrite veins and stockworks and poly-metallic vein systems hosted by a volcano- sedimentary rock sequence.
The Shahumyan deposit exhibits numerous syn-volcanic dykes and sills of andesitic to gabbroic composition. Such intrusions are pre-mineralisation. Pre-mineralisation dykes often have mineralised contacts and are always subject to propylitic alteration or higher grade (typically within contacts of veins or major structures).
At Shahumyan mineralisation is hosted by a suite of sub-parallel veins, the current interpretation of the deposit includes in excess of a hundred veins. In many cases the continuity of these veins is yet to be established and future drilling will be important to validate the current model. Regardless, development along and across the vein system has confirmed the prevalence of approximate east-west strikes and steep dips of the veins.
Hydrothermal-altered breccia units are also notable within the deposit makeup. Such breccias are generally found on the eastern flank of the deposit and contain polymictic clasts (andesite, basalt, dacite) within a sulphide rich matrix. Mineralised clasts can be found which are thought to have occurred due to precipitation of sulphides within vesicular lithologies.
Mineralisation within the Shahumyan deposit is characterized by narrow veins (0.2 – 2.0 m), steeply dipping and striking approximately east-west that host a polymetallic (Cu-Zn-Pb-AuAg) mineral assemblage. Veins are often found in networks and “zones” that may be shadowed by a wide zone of argillic alteration and disseminated mineralisation. Such zones can be as wide as 25 m (for example vein 5) but typically are 10-15 m wide and appear most common in higher elevations within the deposit.
Mineralisation is primarily semi-massive to massive sphalerite, pyrite, chalcopyrite, galena and bornite and occasionally, found with the latter mineral assemblage of tennantite, tetrahedrite, bornite, enargite, chalcocite, covellite, Pb, Au, Ag, Bi tellurides and native gold and silver (recorded during petrographic analysis). Gangue minerals present are typically quartz, calcite, gypsum, anhydrite and rhodochrosite. Veins exhibit a variety of textures which relate to the numerous stages of mineralisation and tectonism that have occurred.
According to Tate (2012) the mineralogy, scale and distribution of veins and alteration are consistent with the upper parts of a large porphyry style hydrothermal system with the Shahumyan vein system representing a basement carbonate zone within the system. The Centralni stockworks represent the lower transitional parts of a high sulphidation epithermal zone within the porphyry hydrothermal system.