Summary:
Both the Tulkubash and the Kyzyltash mineralisation are classified as orogenic gold deposits. The Tulkubash mineralisation exhibits characteristics of shallow epithermal mineralisation, and is further classified an epizonal orogenic deposit. The Kyzyltash mineralisation formed in a much deeper environment and is classified as mesothermal orogenic gold deposits.
The Chaarat Property, located within the Middle Tien Shan Province, locates within the Tien Shan Metallogenic Belt, a Hercynian fold and thrust belt that crosses Central Asia.
The Sandalash River valley down cuts a northeast-trending sequence of Cambro-Ordovician siliciclastic sediments which comprise the Chaarat Formation. This in turn is overthrust by a sequence of younger Devonian-age quartzites which make up the Tulkubash Formation. The sedimentary rocks hosting mineralisation strike north-easterly and exhibit dips between 40° and 75° to the northwest. Younger, Permo-Triassic-age granodiorite and diorite phases intrude the sediments and are closely associated with the gold mineralisation and, in some areas, are themselves mineralised.
Mineralisation and associated hydrothermal alteration at Chaarat are genetically associated with igneous intrusive rocks along a system of regional-scale, sinistral, oblique-slip faults. Within this setting, there are two distinct types of mineralisation: the Tulkubash-type and the Kyzyltash-type. However, the proximity of the two types of mineralisation and the common structural controls suggest that both were the result of a common hydrothermal event.
The Chaarat Formation is made up of three members which exhibit a sequential package of alternating, moderately- to well-bedded, dark coloured, siltstones, shales, quartzites, and greywackes, with minor limestone interbeds. The lower member is up to 170 m thick, consisting of grey siliceous siltstone interbedded with minor dark siltstone and shale. The middle member is approximately 300 m thick. It consists of interbedded fine- and mediumgrained sandstones, greywackes and siltstones, with a basal zone consisting of lenticular beds of polymictic gravely conglomerates and sandstones. The upper member is dominated by shales and rhythmically interbedded siltstones and finegrained sandstones which commonly exhibit graded bedding. The member is 70 m to 90 m thick whereas the thickness of individual beds ranges between 1 m and 2 m.
The Chaarat Property lies within the Sandalash Fault Zone (SFZ), a zone defined by a series of subparallel brittle shear zones that are the result of the local, predominantly sinistral strike-slip, displacement of the SFZ. The gold mineralisation occurs in various extensional structures, related to pressure relief during faulting (Kramer 2009; Jakubiak 2017). The SFZ comprises three mineralised fault zones, namely the Tulkubash Structural Zone, the Contact Fault, and the Main Zone Fault as well as one unmineralised zone called the Irisay Fault.
Gold mineralisation within the Chaarat Property is divided into two styles of mineralisation:
- The Kyzyltash mineralisation, which is divided into the Main and Contact zones. This mineralisation is sulphide-rich and refractory; and
- The Tulkubash mineralisation, which is oxidised and can be processed through conventional heap leach methods.
The Tulkubash mineralisation is the primary subject of this feasibility study; however, the Kyzyltash mineralisation is briefly described for completeness.
The Tulkubash zone is a mineralised structural zone that trends northeastsouthwest and dips steeply 55° to 75° to the northwest. The Tulkubash zone is interpreted to be a brittle shear zone that developed as the result of predominately sinistral strike-slip motion within the SFZ. Gold mineralisation within the Tulkubash zone occurs within zones of intense silicification and quartz flooding, which form individual gold-bearing lodes that can range from 5 m to 45 m in true thickness. Where multiple lodes are present, the Tulkubash zone can have a width of up to 250 m with the individual lodes separated by unmineralised country rock. Development drilling of the Tulkubash deposit indicates that the zone is remarkably continuous, however its thickness does vary along strike.
A distinctive feature present in areas of strong mineralisation are ovoid shaped hydrothermal breccias which are interpreted as fossilised steam vents. They form resistant spires up to 10 m high and 5 m to 10 m in cross section. The breccias are clast-supported with less than 5% carbonate cement and are easily identified in outcrop by the distinctive preferential growth of lichens on the carbonate cement. The breccias are typically barren but occur within areas of strong gold mineralisation. Goldbearing lodes are characterised by red and red-brown hematitic iron oxides, with minor yellowbrown limonite, and rarely occurring jarosite and stibiconite. The Tulkubash zone is largely oxidised with low oxidation material occurring at the base and more strongly oxidised material at the top. The contact between unoxidised sulphide ore and oxidised ore can be gradational but is often observed with a sharp contact, suggesting at least some of the oxidation is hypogene.
Using Tulkubash composites, gold particles are identified in heavy liquid separates (Kirchner and Coetzee 2011). The gold occurs as electrum containing a low silver content, typically ranging between 4% and 8%, with a few grains at 16% silver. Silver was also observed as silver-rich tetrahedrite and within a silver-rich lead-antimony-sulfosalt. The widespread silicification and deep oxidation is in distinct contrast to the Kyzyltash zone, where minor quartz occurs in thin veinlets with no significant oxidation.
The Kyzyltash zone is a series of sulphide-bearing ore bodies made up of the Main zone and Contact zone mineralisation and locates to the East and northeast of the Tulkubash Zone. The mineralised zones occur within two subparallel northeast-trending structural zones that have been traced for 10 km along strike. The ore consists of goldarsenopyrite-stibnite- tetrahedrite mineralisation occurring in sheared and altered wall rock. The ore exhibits strong sericitic alteration, with lesser amounts of quartz, quartz vein stockwork, ankerite, and calcite gangue. In some areas, antimony and silver are significant constituents of mineralisation, the latter particularly in the Contact zone and in the M7000 ore body (about 21 g/t silver average). Antimony, in stibnite and various sulfosalts, can locally reach values of 10% or more over 1 m to 2 m thick zones. Trace amounts of copper and molybdenum are also present in some of the ore.
Petrographic work completed by Chaarat on more than 50 thin sections showed that free gold is present in the ore and occurs as inclusions in quartz and arsenopyrite. The gold mineralisation is, to some extent, correlated with arsenic, which mostly occurs as arsenopyrite. In some localised zones, there are very high silver values (greater than 400 g/t silver). The distribution of silver values is not fully understood, and transitions from silver-rich areas to silver-deficient areas can occur over distances of less than 20 m along strike.