GENESIS AND ORE POTENTIAL OF RARE-METAL LEUCOGRANITES OF KHARALGA

VOLCANO-PLUTONIC COMPLEX (KHAPCHERANGA ORE ZONE)

Zubkov V.S.

Institute of Geochemistry SB RAS, Irkutsk, Russia, zub@igc.irk.ru

Available geochemical data suggests that sulfide-cassiterite deposits are associated with Jurassic plumasitic rare-metal leucogranites (PRMG) of the Khapcheranga volcano-plutonic complex located in Central Trans-Baikal area (Tauson, 1977; Kozlov, Svadkovskaya, 1977; Troshin, 1978; Gundobin, 1980; etc.). In this complex, formation of magmatic rocks is due to intra-continental rifting processes proceeding along the Onon-Turinsky deep-seated fault and elsewhere. The author recognized rhythmically alternated effusive rocks of profoundly differentiated latite series (from shoshonites to rhyolites) and acid ignimbrites and tuffs of the Kharalga complex lying on the sides of the depressions tracing this fault. In the Onon depression on the southern termination of the deep fault, these are latites and Kharalga acid volcanics that are dominant, while on the northern termination, trachytes originated in the Tyrgetuy-Zhimbirinsky depression. Every depression is characterized by a specific sequence of exploding volcanics of various composition. The bimodal distribution of effusive rocks does not allow to refer the Kharalga volcanics to the differentiated rocks of latite series. The effusives of this series are enriched in volatiles (F, B), ore (Sn, Be, etc.) and some incompatible elements (Ba, Sr, LREE); whereas contents of volatile and ore elements diminish in the Kharalga acid volcanics that is explained by the loss of exploding melts during degassing (Zubkov, 1979; Zubkov, Legeido, 1982; Zubkov, Petrov, 1984). The low ratio of strontium (⁸⁷Sr/⁸⁶Sr = 0.7050) isotopes and their relation to deep-seated faults indicate the mantle genesis of latite series rocks. Close spatial and temporal associations of the Kharalga acid volcanics and latites, their joint occurrence in the zones of deep faults, presence of the high concentrations of F, Sn, Be and light REE in cesium volcanic glasses of the Kharalga complex as well as in latites indicate the possibility for PRMG and volcanic analogs to be derived under the action of fluid and heat flows supplied from shoshonite-latite melts. Thus, the data obtained point to the paragenetic relationship of rare-metal leucogranites and rocks of latite series, its presence in Trans-Baikal area being substantiated (Tauson et al., 1984).

The regional ore-bearing potential of granite massifs may be assessed, using the geochemical data. Thus, the rare-metal index (F/1000 х (Li+Rb)/(Sr+Ba)) of the phases of additional intrusions of the Kyrinsky complex, which formed earlier than the Kharalga one (J_1-2), is only 0.2, and that of PRMG is 3.4 (Tauson et al., 1984). The required condition of potential ore-bearing of granites is a high concentration of volatile components ensuring the emanative concentration of ore and rare elements in residual melts (Tauson, 1977). The study also provides the geochemical ranking and assessment of potential ore-bearing by the formula, where the numerator contains an association of microelements, and denominator contains the reflectance ratio (the ratio of element contents in granite of the massif to its clark value) (Kozlov, Svadkovskaya, 1977).

Let us consider assessment of the local potential ore-bearing of PRMG on example of the Khapcheranga intrusion (KI) (20х5-8 km), which embedded along the sublatitudinal Tarbaldzhey fault in the Middle-Upper Jurassic. The subsurface KI contour is defined to depth 1 km from data of gravitational survey. Four domes are located on its surface, three occurring at depth from 0.2 to 0.6 km, and one Khapcheranga stock (2 km²) is exposed by erosion processes. Location of domes is defined by the degree hornfelsation of sandstones and schists (Gundobin, Zubkov, 1973) and zonal structure of emanation halo (Tauson, 1984; Troshin, 1978; Gundobin, Zubkov, 1973). The Khapcheranga stock hosts the Sn-W- greisen deposit Granitnoye, and Khapcheranga tin-polymetallic deposit occurs to the south of this stock. The spatial-temporary association, zonal structure, as well as composition of primary aureoles point to the genetic affinity of the Khapcheranga deposit to the stock with the same name. The middle Jurassic dykes of spessartites, being similar, by petrochemical composition, to dykes and latite flows (Zubkov, 1979), preceded the KI intrusion. It appears that this intrusion PRMG was formed in melting crustal rocks under the action of latite magma and intratelluric fluid flows. The local potential ore-bearing of KI domes is affected by the degree of concentration of volatile and ore elements in residual melts, which may be evaluated with the coefficients of their distribution between mineral phenocrysts and granite-porphyry bulk mass (Tauson et al., 1981). According to calculations provided in (Troshin et al., 1972), the amount of ore and volatile elements in the KI emanation aureole is much higher than that in hydrothermal veins of the Khapcheranga ore knot deposits. The authors proposed the methods how to calculate the potential ore productivity of blind domes, based on tin reserves in Granitny and Khapcheranga deposits and the area of their aureoles, as well as considering the deficit of metal in aureoles versus the elements distributed similarly to tin (Cs, Li). The study also offers a series of formulae for calculating the element contents in the emanation aureole of KI and its domes (Zubkov, 1981).

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