Тезисы международной конференции

Рудный потенциал щелочного, кимберлитового

 и карбонатитового магматизма

Abstracts of International conference

Ore potential of alkaline, kimberlite

and carbonatite magmatism

Rare metal granites and ongonites of the Badzhalsky ore area (the Lower Amur region)

Alekseev V.I.

National university of mineral resources «Mining», Saint-Petersburg, Russia

wia59@mail.ru

The discovery of both rare metal granites with accessory minerals such as Nb, Ta, Sn, W etc., and their shallow analogues - ongonites, elvanites, kalgutytes, xianghualingites was made in the latter half of the 20^th century through research of A.A. Beus, A.I. Ginzburg, V.I. Kovalenko, N.V. Vladykin, Yu.B. Marin, V.S. Antipin, V.B. Dergachev, A.G. Vladimirov, to name but a few. It gives us an insight into the special ongonite magmatism characteristic of the postorogenic stage of continental earth crust development (Vladimirov et all., 2007). In the last twenty years the ongonitic bodies in form of small intrusions of zinnwaldite granites relating to the lithium-fluoric geochemical type were found in the Far East in close proximity to the largest tin ore deposits - Pyrkakajsky, Pravourmijsky, Xianghualing (Brusnitsyn et all., 1993; Alekseev, 2011; Zhu et all., 2011).

The Badzhalsky volcanic zone is located in the upper part of the Amgun river. It represents a nidal structure born during a Mesozoic activization of the junction zone between the Hercynian Sikhote-Alin folded area and the Precambrian Bureinsky massif. The unique feature of the area is an outstanding scale of acid and plutonic rocks cluster, the origin of which is hard to explain even from the earth crust palingenesis point of view: the area of the Badzhalsky zone makes up 5350 km², the thickness of extrusive rocks reaches 1,5–2 km. By the nineties the idea of the genetic relation between Badzhalsky granitoids and extrusive rocks as well as of the existence of andezite-granodiorites and ryolite-granites volcano-plutonic associations was introduced. At an early Late Cretaceous stage of geological history (110-80 million years) the intrusion of the major part of rhyolitic and younger trachyandesitic melts occurred, which led to the formation of the Badzhalsky rhyolite-biotite granite complex and the Silinsky (Levoyarapsky) complex of subvolcanic trachyandesite and monzonitoide respectively (Gonevchuk, 2002). In 1987-1990 by efforts of the Leningrad mining institute expedition under the direction of Yu.B. Marin a special mapping of the Verhneurmijsky ore knot was carried on. In the east exocontact of the Verhneurmijsky massif fine-grained subalkaline albitic granites dykes breaking through all earlier known granitoids were established. A conclusion was drawn that ore-bearing lithium-fluoric granites are present on the depth of the dome (Marin et all., 1990). Rare metal granites composition data was published in 1993 (Brusnitsyn et all., 1993).

Outcroppings of zinnwaldite-albitic granites, which form an independent, the youngest, Pravourmiysky intrusive complex, were located. The complex area is associated with «the Urmijsky alkaline zone», marked of be V.G. Kryukov in the east exocontact of the Verhneurmijsky massif (Kryukov, Shcherbak, 1987), and is situated in the epicenter of a Badzhalsky criptobatholite ledge in the middle reaches of the Irungda-Makit river. The area structure includes a poorly eroded Dozhdlivy massif of medium-grained zinnwaldite granites and low-thickness ongonite dykes. The area of ongonitic body emergence is less than 1% of the Badzhalsky zone area. The area zonality is determined: the dome of zinnwaldite granites which might be an eroded ledge of sublatitudinal ridge-like intrusion is bordered by ongonite dykes field. Enclosing rhyolite ignimbrite in the near exocontact area of the intrusion is under the processes of an intense recrystallization, microclinization and greisenization. As concerns zinnwaldite granites, along the contacts with biotite granites and rhyolites a banding is developing as well as zones of pegmatoids – block, graphic types and stockscheiders. Ongonitic intrusive-dykes belt is limited on flanks and in the north by cross-sectional dyke swarms of monzogranite- and granosyenite-porphyries of the Silinsky complex, which are distributed in heavy pitching rupture zones of the north-north-west strike. The material peculiarity of rare metal granites and ongonites of the Badzhalsky area consists in the leading role of albite, white microcline and zinnwaldite. They differ from the Mongolian and Transbaikalian analogues by the absence of amazonite and a low content of topaz. The accessory complex of these rocks is unusual, including minerals of niobium, tungsten, yttrium, and rare-earth elements: Y-fluorite, cyrtolite, monazite-(Ce), samarskite-(Yb), ishikawaite, ferberite, wolframoixiolite, scheelite, xenotime-(Y), allanite-(Y), fluocerite, tveitite-(Y), struverite, chernovite-(Y), fergusonite-(Y), aeschynite-(Y).

In the Badzhalsky area two consecutive petrochemical series of granitoides are found side by side: 1) normal biotite granites of the standard geochemical type, intruded in comagmatic rhyolites in form of large plutons; 2) subalkaline biotite granosyenites and zinnwaldite granites of latite and plumasite geochemical types respectively, which form small intrusions. The geochemical evolution of normal granites is oriented on an increase of agpaite features: normal granites are combined with subalkaline leucogranites. A transition to a rare metal stage is accompanied by a sharp decrease of agpaite features, connected with an intrusion of granosyenite-porphyritic dykes. The subalkaline granitoides evolution follows the line of increase of agpaite and aluminous features, accumulation of rare elements - W, Nb, Y, F, Sn, Li, Rb and others (fig., a). It is characteristic, that ongonites differ from lithium-fluoric granites by decreased aluminous features that reflect in a lowered content of topaz. Yttrium and niobium distribution in granites gives evidence of the formation of biotite granites and monzonitoides in the conditions of an active continental margin; the latter show formation signs of passive margins. Rare metal granites and ongonites are formed in within-plate conditions and correspond to granites of A-type with a mainly crust substance source (fig., b). However accumulation of niobium and a close association with monzonitoides, enriched by K, P, Ba and granitophylic elements (Li, Rb, F, W), point at the presence of mantle component in an initial melt (Eby, 1992).

Figure. Geochemical features of rare metal granites and ongonites of the Badzhalsky area.

a – aluminous – agpaite features in the diagram (Maniar, Piccoli, 1989) (A = Al₂O₃, N= Na₂O, K = K₂O, C = CaO (mol.cont.)), b – geodynamic types in the diagram (Pearce et al., 1984). Granitoides: 1 – biotite granites of the Badzhalsky complex, 2 – granosyenites of the Silinsky complex, 3, 4 – zinnwaldite granites (3) and ongonites (4) of the Pravourmijsky complex.

Thus, in the Mesozoic history of the granite magmatism of the Amur River region the newest ongonitic stage is specified as a result of tectono-magmatic activization of the Sikhote Alin fold system during a subduction of the Pacific plate under the continental margin of the East Asia. Ongonite magmatism of the Amur River region had a mantle and crustal origin, a nidal character, differed by the sodium petrochemical and rare metal geochemical evolution of melts and eventuated in the formation of zinnwaldite-microcline-albite granites and ongonites with a complex of accessory minerals such as W, Nb, F, Y, REE. The Pravourmijsky ongonite complex controls the largest tungsten-tin ore deposits of the Badzhalsky area and is similar to rare metal complexes of Chukotka, Yakutia, Primorsky Krai and China by geological-tectonic and petrological-geochemical characteristics.

Researches are executed at financial support of the Russian Federal Property Fund (the project 11-05-00868-а) and the Ministry of Education and Science of the Russian Federation (the state contract № 14.740.11.0192).

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