GRANITES HOSTING SHKOLNOYE GOLD DEPOSIT (NORTHEASTERN RUSSIA):

PETROLOGY AND RELATIONS WITH MINERALIZATION

Alshevsky A.V., Akinin V.V., Gelman M.L.

Northeastern Interdisciplinary Research Institute, Magadan, Russia, gelman@neisri.ru

The medium-sized Shkolnoye gold deposit hosted in the granite Burgagy (BG), is exploing by «Nelkobazoloto» Co. It is situated in the Yana-Kolyma belt (where a. 3000 tonnes Au are obtained from abundant placer deposits) closely to its boundary with the Okhotsk-Chukotka volcanic belt (OCVB), containing Au-Ag epithermal deposits. The Yana-Kolyma belt is extended along the MZ₂ Kolymian granitic belt. The latter is composed by (1) J₃ batholiths of Ga-Crd adamellite which dimensions are of hundreds and thousands km², (2) J₃ batholiths less than 200 km² and minor intrusions, (3) K_1-2 plutons of alkaline and midalkaline granites. Plutons (1) were originated due to ascent of «dry» magma and (2) formed as a result of its following enrichment in water. Plutons (3) are regardless of both (1) and (2) and are related to a general change of synmagmatic fluid. J₃ plutons (2) host Au quartz veins and cause a zonality of Au mineralization in country rocks. Some Sn deposits are related to K_1-2 plutons. A knowledge about connection of the Yana-Kolyma belt and the Kolymian batholithic belt had to conduce already in the 1930-s to reveal the world importance of that metallogenic belt. But in individual Au placers districts, there minors are only common not batholiths. Granite hosted Au occurrences belong to the Au-Bi ore type whereas placers are due to the Au quartz ores, which are hosted by metamorphosed sedimentary rocks (C₃-J₃) and MZ₂ minors. The Shkolnoye is unique among exploited Au deposits in the Yana-Kolyma belt which is hosted in granite. And this phenomenon is worthy of attention.

BG which occupies 5 km², is intruded into P₂ deposits underwent regional greenshist metamorphism. It is a zoned pluton composed by gabbro (only big xenoliths); gabbrodiorite, camptonite, diorite-porphyrite, diorite (pregranitic dykes and early intrusive rocks); younger quartz diorite close to tonalite and sodic quartz syenite (endocontact rocks); adamellite and granite which are perhaps in intrusive succession (central part of pluton); aplite, pegmatite and silexite, porphyritic granite, lamprophyres, dacite and rhiolyte (vein series). The ages are determined by traditional and modern methods. U-Pb age of zircon from adamellite is 149,7±2 Ma (a new data, SHRIMP-RG), Ar-Ar age of biotite too frome adamellite is 148±2 Ma (but there is age 96,3±1 Ma too). The K-Ar and Ar-Ar ages of endocontact rocks and hornfelses belong to J₃ too. Some K-Ar ages of adamellite are close to 150 Ma. But according to this method, rocks of central part of BG look young than J3 (till 137-122 Ma). And the vein series is more younger, their ages are 123 to 109 Ma. BG corresponds to adamellites of the kolymian batholiths due to the age a. 150 Ma as well as belonging to the ilmenite series. BG is a zoned pluton, and that together with its small size as well as presence of pregranitic more basic rocks, occurrence of Hb (besides Bi) in adamellite are comparable to J₃ small batholiths, hosting Au occurrences. But unlike the typical granitogene ores, the ore bodies position in BG does not depend on its own structure. These bodies, like the rocks of vein series, follow along the fault. And we suppose that the Major ore body at the depth about 800 m will come «under the pluton», i.e., the source of mineralization is deep in character. Petrological analogy of namely kolymian adamellites with the rocks of BG is not very strong. In the first rocks, Hb is not replaced by Bi, but is formed instead of it in that place where the water has come. The crystallization sequence in BG is as follows: Cpx-Hb-Bi like in K_1-2 granitoids of the OCVB. In BG, rocks of the major phase are more ancient, but they formed, probably, by the same way as the OCVB ones under the influence of the deep water fluid (but non-oxidazed fluid, like in the Kolyma belt). Opx that is characteristic for BG, is a mineral of magmatic rocks of the outer zone of OCVB. Some differences of BG with kolymian rocks are due to the general lateral petrographic zonation on the Siberian North-East: a small decreased K-Na ratio and the presence of gabbro (not only gabbrodiorite) apply to that. Even more peculiar character is in rocks of the vein series of BG. Their numerous K-Ar ages are close to ages of K_1-2 rocks in the Yana-Kolyma belt, and to formation of the OCVB. It is an interesting fact that BG is located near the Butygychag U-Sn deposit. Schlieren pegmatites, a phenomenon of voluminous greisenization of BG rocks, an increase content of Sn in them (up to 100 g/t) mention the magmatic rocks hosting tin deposits. Dacite and rhyolite dykes we could consider as the subvolcanic facies of ignimbrites of the OCVB. Argillization in metasomatic halos of ore bodies, propylitic epidotization of contact zones of BG are also subvolcanic phenomena. There are some features that adamellites have undergone the superimposed adularization and biotitization like in deep zones of propylitic facies.

Gold ores are of two types. 1 - Au-Bi: dispersed sheelite-molybdenite mineralization, zones of sulfide impregnation, quartz veins. 2 - gold-quartz, high-grade ores, which are found only in the zone of faults. The 1 type is confined to vein series, to greisenizated adamellites. Partly, it is earlier that the 2 type. We have determined: in ores of the 1 type arsenopyrite (+loellingite), Sb-bismuthinite, A joseite, tetradymite, ingodite, bismuth, gold 858 (in small grains). The Sb admixture in bismuthinite is very high (up to 8,2%), that determines an antimony style of the Au-Bi-Te mineralization of the type 1 and its geochemical character close with the antimonybearing industrial ores of type 2. Ores of 2 type are sulfide-quartz veinlets, and quartz and hydro-micas-quartz stockworks, which are pivotal in the zones of beresites and argillizites, poor in sulfides (1-2%) mainly represent with pyrite and arsenopyrite as in type 1. The following minerals are specific features of type 2 ore: Ag-tetrahedrite, boulangerite, jemesonite, gold 700-800 (large grains), sulfides of Pb, Zn, Cu as well as Sb-sulfides - antimonite, berthierite, bournonite, famatinite, including rare Ag-containing minerals such as diaphorite, owyheeite (teremkovite). Tetrahedrite is very reach in Ag (15 to 19%); and Au:Ag values in the ores of 2 type are high (1:1) due to that. In whole, this is sulfoantimonite type of gold-quartz ores poor in sulfides.

The BG features are characteristic for MZ₂ granite-metamorphic domes adjacent to the OCVB from the side of mesozoids. In the Priokhotsk part of the OCVB, the such domes are poorly eroded. But on the neighbor with BG Degdekan gold ore area, there are known lamprophyre dykes, containing numerous xenoliths of diaphthorites and anatectites delivered probably from a dome core. The ages of some zircons of these rocks are 142±3 and 131±5,6 Ma (the same data that in BG!). In BG zircons with age 149 Ma, we discovered inclusions of Ilm and Ap, melted inclusions – «drops» with djerfisherite (?), and Mss (Ni-Fe-S) and Iss (Cu-Fe-S). These inclusions show the peculiarities of the deep petro- and metallogenesis. The Shkolnoye deposit is similar to «above dome» Maiskoye deposit (Chukotka). Its projects could be widen by the search of impregnated porphyry ores similar to ore of the Maiskoye deposit, in P₂ country rocks, where they have underwent metasomatic changes.