tHE PECULIARITIES OF MOLYBDENITE-QUARTZ VEINLET STRUCTURAL AND MINERAL COMPOSITION IN GRANITES OF THE PERVOMAISKY MASSIF

(DZHIDA ORE FIELD, SOUTH-WEST TRANSBAIKALIA)

Damdinova L.B.

Geological Institute SB RAS, Ulan-Ude, Russia, damdinova@geo.buryatia.ru

The structure and mineral composition of host rocks and ore-bearing molybdenite-quartz veinlets have been studied within the stockwork Mo deposit (Dzhida ore field). The rocks that host the mineralization are represented by the porphyry-like granites of the Pervomaisky massif that have relatively persistent composition (Table 1). Fine-grained matrix of these rocks is composed by potassium feldspar (to 30-40%), quartz (30-35%), plagioclase (20-25%), as well as biotite and muscovite, their quantities ranging from 5 to 10%. In addition, rare impregnations of (1-2%) pyrite, fluorite and apatite (< 1%) have been noted. The impregnations regularly distributed in the matrix reach 5 mm in size and are represented by quartz, potassium feldspar and rare plagioclase.

Table1. Average contents of main (mass.%) and some admixture (ppm) elements in granites of the

Pervomaisky massif (upper lines), and standard deviation (lower lines) on four samples.

Note: silicate analysis of granites has been carried out in laboratory of GI SB RAS, analysts Ivanova V.A., Guseva N.L., Borzhonova I.V., Tsyrenova A.A. The admixtures were analyzed in Irkutsk Institute of Geochemistry SB RAS by ICP-MS technique, analyst Smirnova E.V.

Molybdenite-quartz veinlets are composed by (80–90%) isometric grains of quartz; columnar grains that grow perpendicularly to walls of veinlet are locally noted. It testifies to the fact that the veinlets rather formed by filling in the fissures. The thickness of veinlets varies from 0.2 to 1.5 cm, the borders with host rocks are distinct, even in most cases and more rarely winding. Molybdenite forms accumulations of flakes, sometimes radial aggregates. Its amount averagely equals 5-10%, and it decreases to less than 5% in some veinlets or reaches 15%. Molybdenite distribution is irregular. In some cases, it is confined to the central part, and to selvages of veinlets – in other ones or disseminated through the whole veined quartz. Flakes of molybdenite are much more rarely get outside the veinlet into the host rocks.

The sample was cut into series of plates 10 mm wide each being parallel to the veinlet to clarify scales and character of interacting the ore host granites with solutions that flowed along the fissures. The samples 30-40 g by weight obtained in such a way were grinded and used for spectral analysis for Mo, W, Sn, Be, Cu, Zn, Pb, Ag, Bi, Co, Ni, V, Mn, Fe in the Central laboratory of the Buryat Geological Survey. The samples about 60 g by weight were prepared from several samples by the same technique for silicate analysis in the laboratory of Geological Institute SB RAS (11 petrogenic oxides + S and F). The molybdenite-quartz veinlets are divided into two groups by presence of near fissure metasomatites. In most cases, margins are not observed either visually or in polished sections. The veinlets with weakly expressed small zones around them nearly 2-10 mm thick distinguished by sharp increase (to 95%) of K-feldspar amount are less spread.

By the results of chemical analysis, regular change in granite composition is fixed in narrow zones (1-3 cm) that attach to the ore veinlets. It testifies to diffusional-metasomatic nature of those margins. Among the petrogenic components, only TiO₂, FeO, MnO, MgO do not discover significant deviations from background values that are specific of the unaltered granites. By behavior of other microelements, the molybdenite-quartz veinlets of the distinguished groups are notably differed. The first group accompanied by the margins with F background that are sharply depleted in SiO₂ (62-63 % in contrast to 72-73 %) and Na₂O (0.2-0.4% in contrast to 2.7-3.3 %), enriched in К₂O (Fig.1), sometimes Al₂O₃, Fe₂O₃ and S.

The margins are not visually distinguished near the second group veinlets, but their presence is confirmed by host granite alterations near the veinlets. Desilication of granites and increase in К₂O in these margins are not specific, and their depletion in Na₂O и Al₂O₃ is observed in cases of fluorite higher contents in veined material. The enrichness in F and S is specific of the near veinlet margins of this group. By K.Z.Stelmachonok’s data (1994, 1995), the homogenization temperatures of most primary fluid inclusions range within the narrow interval 220-260С in veinlets of both groups. The revealed differences suggest the formation of the first group veinlets by high K solutions of higher alkalinity, and those of the second one – by the near neutral solutions with higher concentrations of S and F, as the stockwork is localized in homogenous granites of the Pervomaisky massif, by composition.

Mo content in veinlets of the first group (0.0n–0.1%), is as a rule ten times lower than the second one (0.n–n%). Near ore metasomatites of the first group are usually enriched in Mo, W, Sn, Li, обеднены Zn, and the higher contents of Mo and Sn are less specific the near veinlet margins of the second group. Changes in Pb contents are limited by signs.

The determined regularities suggest the efficiency dependence of transfer and concentrated Mo, W и Sn sedimentation from pH of ore forming solutions, activity of K, F и S.

The work has been carried out with financial support of Russian Foundation for Basic Research (grant 03–05–64857), Minpromnauka of RF (grant NSH 2284.2003.5), grant of RF President (№ МК-4543.2007.5) and grant "Foundation for Russian science assistance".

References

Stelmachonok K.Z. (1994) About synchronousness of ore host fissures formation and molybdenum mineralization development in the Pervomaisky stockwork deposit and reasons of fissure formation // Doklady of RAS. V.337. N3. P.382-385. (in Russian)

Stelmachonok K.Z. (1995) About nearly simultaneous formation of the one system veinlets in molybdenum stockwork ore body of the Dzhida deposit // FAS. V.341. N5. P.399-402. (in Russian)