Relation between chemistry and ore content of ultrabasites in the Kalna massif (Western Sayan)

The Kalna ultrabasite massif is situated in the Western Sayan ophiolitic belt, at the northeastern termination of the Kurtushubin-sky fault branch. It pertains to the Idzhim complex. The ultramafite body is isometric and deformed by sublatitudinal and NW-trending tectonic faults. The body includes the northern and eastern blocks. The northern block is 35 km , and its northern termination is a direct target of research. Within the massif there are dunites, harzburgites, chrysotile-antigorite and antigorite-lizardite-chrysotile serpentinites with highly subordinate quantity of large-crystalline diopsiditites in form of lenses up to 20 m thick. In the northern part of the body there prevail dunites, and to the southwest there is a bended dunite-harzburgite complex. Contacts with the host rock are of tectonic type with serpentinite melange. The area under study is situated in the Upper Amyl potentially chromite-bearing region with the nearby Ergak ultrabasite massif. Within the Kalna massif there are seven ore-bearing zones with an extent of 900-3000 m and a thickness of 70-500 m. Chromite ores are rare, medium, and dense inclusions of chromite varieties, among which there are 1 cm thick veinlets. Chromespinelides are high-chromium, up to 65.33 % of Cr ₂O ₃, average alumina content is 9.41 %. The authors used the method of evaluation of chromite-bearing potential of ultrabasite massifs based on their chemical composition to estimate the potential of the studied area. The method has been derived from data on reference targets in the Urals by linear discriminant functions based on more than 300 silicate analyses. Two complex petrochemical factors were derived, with their positive values defining that ultrabasite composition is favorable for localizing chromite mineralization, and their negative values indicating that it is not. The method is entirely empirical and is not directly connected with other criteria of chromite ore prospecting. Thirty-five analyses of silicates made by different authors were processed. A sum of two discriminators (Z ₃) was used, which gives the most contrast anomalies. The derived distribution pattern of the factor resembles that in the ore-bearing serpentinites in the Urals, and its isolines at the northern termination of the massif show that all existing ore-bearing zones are within the areas with positive values. Prolongation of positive anomalous zones of the discriminator resembles extension of ore bodies. Moreover, the most promising sites to localize mineralization (Z ₃ >10) are situated to the south of the known ore-bearing areas that have not been subjected to detailed exploration, which suggests their strike continuation. The findings support relation between chemistry features of ultramafites and occurrence of chromite mineralization in them. This allows us to recommend using the method in prospecting.

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