Geology and Mineralisation in the Chaarat Licence Area
The Chaarat Project is located in a mountainous area along the Sandalash River valley, on the western border of Kyrgyzstan. The valley marks the north-easterly trending hinge zone of an anticline, the north-western limb of which consists of a sequence of Upper Proterozoic and Cambrian-Ordovician siliciclastic rocks - the Chaarat formation, which dips at around 400 northwest. The formation comprises greywacke, sandstone with siltstone, shale, rhythmically bedded siltstone and black shale with limestone lenses and an upper tillite.
Figure 2: Project GeologyThis stratigraphy is disrupted by several NE-SW trending oblique-slip faults and by a major thrust dislocation which brings the Devonian age Tulkubash Formation quartzites and Carboniferous age carbonate rocks into contact with the Chaarat formation, some 500 to 600 metres above the Sandalash River. The siltstones show an irregular axial planar fabric that locally becomes intense.
Permian-Triassic age syenite and diorite have intruded along many of the faults. Other intrusive rocks include small cross cutting granites bodies and post tectonic mafic dykes. The diorite dykes seem to be closely related to mineralization and in some areas their contacts are mineralised. Skarn mineralogy, pyroxene and garnet, is locally present along the diorite contacts.
The mineralised zones exhibit pervasive sericitic alteration with lesser amounts of quartz, ankerite and calcite gangue. There is limited oxidation near surface and arsenopyrite and stibnite are locally visible in surface outcrops. In some areas, antimony (Sb) and silver are significant constituents of mineralisation. Silver becomes quite important in the Contact Zone (about 21 g/t Ag average) and in M7000 (Karator), where it locally obtains values of a few hundred g/t. Antimony, in stibnite and various sulfosalts, can locally reach values of 10% or more over one to two meters thick zones. Copper and Molybdenum are rarely significant constituents of the mineralisation.
Three sub-parallel mineralised zones have been identified at Chaarat: the Main Zone, the Contact Zone and the Tulkubash Zone.
The Main Zone is a north-east, south-west trending, system of anastomosing and braided oblique slip faults steeply dipping to the north-west. Abundant conjugate structures and riedel shear intersections provide favourable areas for mineralization. The Main Zone structure is developed within the siltstones of the Chaarat formation on the lower slopes of the Sandalash River valley and include several discrete mineralised bodies along the strike. As of the 9th March 2010 seven such mineralised bodies have been identified and have been drilled. The Main Zone has been further subdivided into six project areas, referred according to their position from the base line, including, M2400, M3000, M3400, M3900, M4400 and M5000 and M6000.
The Contact Zone
The Contact Zone is a shear system developed between the Chaarat and Tulkubash formations and mineralisation in this zone has been discovered intermittently over a length of ten kilometres. The Contact Zone structure strikes north-east, south-west and dips at 450 to 600 to the north-west. Three separate project areas occurring along strike have been drilled: C5300, C4600 and C4000 with extensive underground development at C5300. Gold mineralization is hosted in sheared siltstones which display sericitic alteration with minor quartz veins and calcite-ankerite, and contains some 10 to 15% pyrite, stibnite, tetrahedrite and arsenopyrite, associated with the gold mineralisation.
The Tulkubash Zone is hosted in quartzites and the mineralised zones exhibit pervasive silicification and pyritization (quartz-pyrite) with local argillic alteration. Stibnite is usually present in mineralised zones, both as disseminations and in thin veinlets. The Tulkubash Zone strikes parallel to the Contact and Main Zones along most of its strike, but to the south, in the M0700 area, mineralisation also occurs along east-south-east trends where it occurs as steeply dipping en-echelon vein sets and sometimes brecciated zones between north-east, south-west oblique-slip structures, dipping north-west at 400.
The mineralized system
Mineralisation in Chaarat is defined as “deep epithermal”. The “epithermal” designation is due to the “epithermal” element suite – Au, Ag, Sb and As. Depth at time of formation, based on stratigraphic reconstruction, was in excess of 1000 metres. Mineralisation was controlled by the chemical reaction of hydrothermal solutions with wall rocks, as opposed to physical processes of cooling and boiling, dominating “shallow epithermal” systems. The pervasive sericitization, disseminated sulfides and ankeritization within mineralised lodes, and the relative paucity of quartz veinlets (usually less than 5% volume) are taken as indications of the lesser importance of cooling and the importance of reaction of solutions with feldspar rich wall rocks in controlling mineralisation.
Deep epithermal systems are formed in nearly isothermal conditions and are less controlled by near surface sharp temperature gradients which dominate in “shallow epithermal systems” and as a result can extend to great depths. Mineralization in Chaarat has been demonstrated over vertical range of close to 800 meters, between elevations of 2800 metres to 2000 metres ASL, and is still open at depth.
The best analogues to the Chaarat ore bodies are the world class Natalka, Maisk and Olimpiada gold deposits in Russia which extend over a vertical range of a few thousand meters. ( A.V. Volkov, 2007, “Model of multilevel structure of ore-columns and conditions of formation of large and superlarge Au-As-Sb disseminated deposits of invisible, refractory gold” Proc. 9th biennial meeting of the Soc. Geol. applied to Mineral deposits, Dublin, pp.573-576). We believe Chaarat has a similar depth potential.
Also noteworthy is the connection between porphyrytic diorite intrusives and mineralisation, manifested locally in mineralisation along intrusive contacts, and in the spatial association between areas with abundant intrusives and mineralization, evident in the Contact Zone (but not in the Main or Tulkubash zones).
The presence of prograde metamorphic, high temperature calcsilicates (pyroxene-garnet) along the diorite contacts suggests gold is related to late retrograde sericitic alteration, both along intrusive contacts and mostly along high angle fault zones.