2011

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Ore deposits in the Ledinsk cluster of alkali-ultrabasic rocks and carbonatites (Sette-Daban, eastern Yakutia)

G.S.Anisimova^*, L.A.Kondratieva^* and K.K.Struchkov^**

^*Diamondand Precious Metal Geology Institute, Siberian Branch, Russian Academy of Sciences, Yakutsk, Russia; ^**Northeast Federal University, Yakutsk, Russia

g.s.anisimova@diamond.ysn.ru

Tectonically, the Ledinsk ore cluster forms part of the Belorechensk zone of the Sette-Daban paleorift (Sagir et al., 2001), while metallogenically it belongs to the Sakhara ore district within the Sette-Daban metallogenic zone. Three massifs of alkali-ultrabasic rocks and carbonatites are known from there. These are the Povorotnyy, Gok and Voin massifs confined to the Burkhala deep fault. Description of the massifs is taken from Kolodeznikov et al. (1996). The Povorotnyy massif has a lens-like form in plan view, is 3x1 km in size, and occurs 70 km north of the Gornoye Ozero massif. It extends northeasterly (azimuth 20^o) along the strike of the deep fault zone, and is hosted in the Lower and Middle Cambrian shales and limestones. South of the massif, along its long axis, occurs the Labaz-Voy zone of hydrothermally altered rocks consisting of hyperbasite and syenite dikes and carbonatite veins. Most abundant are amphibolized pyroxenites. Nepheline syenites and ijolites occur as relict blocks within carbonatites. Thin dikes of nepheline syenite are seen to intersect carbonatites locally. Absolute age determinations (Zaitsev et al., 1997) yielded 484 Ma for amphibolized pyroxenites, 382 Ma for carbonatites, and 379 Ma for nepheline syenites.

The Gek massif is a lenticular body steeply dipping to the south and extending northeasterly along the strike of the Ledinsk cluster. It is hosted in the Middle Cambrian terrigenous-carbonate rocks, and measures 0.4 x 2.3 km at the day surface. The massive contains pyroxenite, ijolite, and alkali and nepheline syenite. Carbonatites that make up a significant part of the massif are compositionally similar to those from the Gornoye Ozero and Povorotnyy massifs.

The Voin massif has a stock-like form slightly elongated in a northwesterly direction. It is 0.15 x 0.4 km in size at the day surface. Magmatic rocks are hosted in the Lower Ordovician limestones and, as indicated by the available geological data, dip steeply to the southwest. The massif is largely made of nepheline and alkaline syenites, with the central part consisting of pyroxenites intersected by syenite dikes.

The described massifs and carbonatites of the Ledinsk cluster have associated rare metal, rare earth and As-Ni-Cu-sulfide mineralization, the latter being discovered there for the first time.

Rare metal mineralization. The Povorotnyy Ta-N mineralization is associated with the massif under the same name, which is made of pyroxenites, ijolites, nepheline syenites, and carbonatites. Rare metal mineralization occurs in calcite carbonatites (Entin et al., 1991).

Rare earth mineralization has a wide distribution in the Ledinsk cluster area, and is associated with the ankerite facies of carbonatites. Ce-group minerals are represented by bastnaesite and parasite. Accessory and ore minerals include magnetite, titanomagnetite, ilmenite, rutile, sphene, anatase, apatite, zircon, orthite and perovskite (Sagir et al., 2001).

It was found that pyroxenites of the Povorotnyy massif contain a rare earth mineral of the monazite-group chemically corresponding to Ce-monazite (Kondratieva et al., 2010). Also present are titanite, carbonate-F-apatite, an undefined mineral of complex composition (Ca-Ti-Fe-Mn-O). Accessory minerals include ilmenite, rutile and zircon.

As-Ni-Cu-sulfide mineralization. Within the Ledinsk area there is the Meteor stratiform copper occurrence localized in carbonate rocks.

The authors found vein-disseminated sulfide mineralization both within the ultrabasic massifs and in carbonate rocks (€₂-O₁) away form them. As-Ni-Cu mineralization is localized in quartz and carbonate-quartz cross veins and in micaceous metasomatites.

According to mineragraphic and microprobe data, the ores have a uniform mineral content. The principal vein mineral is quartz. Less abundant are calcite, dolomite, and ankerite. The dominant ore mineral is chalcopyrite. First found there are tennantite and gersdorffite (sulfarsenide of Ni).

Chalcopyrite is the main ore mineral. It occurs as small irregular grains and crystals in quartz and carbonate. Microscopic studies revealed the presence of chalcopyrite relics in hydrous ferric oxides. Chalcopyrite, as an indicator of copper mineralization, contains no trace elements and its chemical composition is close to theoretical.

Pyrite is less abundant than chalcopyrite. It is present in all structural-morphological types of rocks, and occurs in veinlets and as disseminations in quartz, carbonate and the host rocks. Its chemical composition is close to stoichiometric.

Sphalerite is a rare mineral in quartz and carbonate-quartz veins. It occurs as irregular phenocrysts and monocrystals, has a light brown coloring, and is associated with chalcopyrite and hydrous ferric oxides. Sphalerite is Fe-free, while Cu and Pb are constantly present in trace quantities.

Tennantite is first found in quartz veins localized in carbonatized rocks in the Kishra-Kim interfluve and along Labaz creek (right tributary of the Leda r.). The mineral occurs as xenomorphic phenocrysts and granular aggregates. Associated minerals are chalcopyrite, pyrite, and gersdorffite.

With regard to chemical composition, the mineral is an end arsenic member of the tetrahedrite-tennantite series. Its Ag content is low, rarely attaining 3.19%. It contains less Zn (5.04%) and more Fe (3.33%) than tennantite from the neighboring Shorokinsk ore cluster (7.36% Zn and 0.90% Fe) (Anisimova et al. 2010). Some of the tennantite grains show prevalence of Fe (6.49%) over Zn (2.20 %).

Gersdorffite is present in quartz veinlets in association with tennantite. This sulfarsenide of Ni forms dendritic aggregates and symplectic intergrowths in chalcopyrite grains in paragenesis with tennantite. Another associated mineral is pyrite. Gersdorffite contains trace amounts of Fe (up to 9.58%) and Cu (up to 11.01%), which is likely due to its close paragenesis with chalcopyrite. As indicated by data from the literature, the mineral characteristically contains no copper.

Trace elements of ores and rocks. Atomic-absorption analysis of the ores and rocks at the Laboratory of Physical and Chemical Methods, Diamond and Precious Metal Geology Institute, Siberian Branch, Russian Academy of Sciences, revealed their low gold and silver content. The bulk of the samples contain 0.00n g/t Au, with the maximum value (0.8 g/t) observed in the ores at Kishra, where gold content of sulfidized rocks is also relatively high (0.0n g/t). The same trend is evident at Labaz. Ag content is by an order of magnitude higher that of Au.

Other trace elements in the ores and rocks were determined by the method of semiquantitative spectral analysis. Cu, Ni and Co are present constantly. Pb and Zn occur rarely. The Gek and Povorotnyy magmatic rock massifs are characterized by high Ni, V, Sr, Ba, Yb and Y contents.

Conclusions. Alkali-ultrabasic rocks and carbonatites of the Ledinsk cluster host rare metal, rare earth, copper and As-Ni-Cu-sulfide deposits. The latter were first found by the authors. Unlike stratiform Cu deposits of the Sette-Daban metallogenic zone they are localized in cross-cutting ore bodies, representing a later metamorphogenetic-hydrothermal mineralization. Also first found in the Ledinsk ores are tennantite and gersdorffite. The find of the end arsenic homologue confirms a general trend in the composition of fahlerz – a decrease in Sb content from center to periphery and from west to east.

A wide distribution of ultrabasites, picrites, lamproites and the finds of diamonds in the Belorechensk paleorift zone, of which the Ledinsk ore cluster forms part, indicate its high diamond potential (Levashov et al., 1970, Sagir et al., 2001).

This stady was completed under the Integration Project of SB RAN, N37.

References:

Anisimova, G.S., Kondratieva, L.A. and Popova, S.K., 2010. Composition of fahlore as reflection of mineral zonation// Modern mineralogy: from theory to practice. Materials of the International Conference “XI Congress of the Russian Mineralogical Society”, St. Petersburg. p. 139-141 (in Russian).

Entin, A.R, Zaitsev, A.I., Lazebnik, K.A. et al., 1991. Carbonatites of Yakutia: (Mineral content, mineralogy). Yakutsk:YaNTs SB RAS, 240p. (in Russian).

Kolodeznikov, I.I., Levashov, K.K.,Marshintsev, V.K. et al., 1996. Geology and diamond potential of the southeast margin of the Siberian platform and Sette-Daban. Moscow: Nedra, 160 p. (in Russian).

Kondratieva, L.A., Anisimova, G.S., and Struchkov, K.K., 2010. Ore and diamond content of the Belorechensk zone of the Sette-Daban paleorift//Otechestvennaya Geologiya, N6, p. 3-11 (in Russian).

Levashov, K.K., 1070. Problems of diamond tenor in Sette-Daban// Dokl. AN SSSR, V. 195, N2, p. 426-428 (in Russian).

Sagir, A.V., Dorofeeva, R.N., Krukovskiy, P.Yu. and Filatova, Yu.B., 2001. Structure, magmatism and metallogeny of the Belorechensk zone of the Sette-Daban paleorift//Otechestvennaya Geologiya, N1, p. 34-43 (in Russian).