Use of geochemical features subalkaline granites of the Severniy pluton (Chukotka) for a petrological and metallogenical estimation

Alekseev V.I.

Saint-Petersburg state mining institute named after G.V. Plekhanov (technical university), Saint-Petersburg, Russia

In the report at the previous conference (Alekseev, 2008) it was stated that the cause of extremely wide spread occurrence of rocks rich in rare elements in the Severny massif are the intrusion of subalkaline leucogranites of lithium-fluoric geochemical type and associated volume metasomatic transformation of biotite granites of the main phase. The problem of selection of intrusive bodies which are capable to be a petrogeochemical benchmark in regional petrological and metallogenic investigations has arisen because of the convergence of chemical characteristics of the granites of earlier and late complexes. Ongonite magmatism events can be used as such benchmark in folded areas (Kovalenko, 1977; Dergachev, 1991; Vladimirov e.a., 2007).

We studied granitoids of the chukchi complex which are notable for wide range of rock composition – from granodiorites and adamellites to biotite granites and leucogranites. Vein phase and dykes consist of granite porphyry and granodiorite porphyry, granite aplites, pegmatites and lamprophyre. For the first time in Chukchi Peninsula we discovered ongonite dykes among coarsegrained biotite granites of Shelagski hogback (Chauncky district) (Alekseev, 2005). Twenty six thin dykes were described within the area where Ichuveemsky dyke belt crosses the central dome of the Severny pluton. These dykes are thin and have submeridian strike and group in two vein series 3 and 1 km wide which extend more than 5 km through the central dome in upper reaches of the Apapelhin and the Glubokaya.

The main petrochemical features of the chukchi ongonites are high alumina content (Shand ratio á1₂ï₃/ë₂ï+NÁ₂ï+óÁï = 1.11÷1.22) and superalkalinity (K₂O+Na₂O = 8,05÷9,47). Prevalence sodium over potassium

is typical, the latter is in its clarke concentration. Ongonites differ from granitoids of the region by their leucocratic character and from lithium-fluoric granites they differ by their alkalinity and alumina content (fig.).

Fig. Chemical composition of the Severny pluton granitoids and granites of the Chauncky district.

1, 2 – rare metal granitoids of the Severny pluton: ongonites(1) and microcline-albite leucogranites (2); 3 – biotite granites of the chukchi complex (numerals on the diagram: 1 – Peveksky, 2 – Yanranaisky, 3 – Inroginaisky, 4 – Puirkanayansky, 5 – Paliansky, 6 – Puitliansky, 7 – Kukeneisky massifs); 4 – granite according R. Daly.

Geochemically ongonites are characterized by high concentration of F, Li, Rb, Sn, Nb and lower concentration of Sr and Ba (table). According to rare metal content they refer to ultrararemetal granitoids (Kozlov, 1985). Elemental formula:

.

Among regional peculiarities of rock composition we should note a high content of Be, Cs, B and abnormally low Zr content. Thereby they resemble kalgutites – ultrararemetal ongonites which were described in Altai (Dergachev, 1991) but they differ from kalgutites by the absence of abnormal phosphorus content and not so intensive accumulation of caesium. Stable boron accumulation in all granitoid varieties of the region is a significant factor. Probably that is what caused the formation of tin-bearing tourmaline metasomatites in the hydrothermal phase. In addition the structures of geochemical relations of rare elements in ongonites and tourmalinites differ sharply.

In general chemical composition of Chukchi ongonites is analogous to similar rocks but it has special regional features. Identified geochemical features of ongonites are proposed to use as indicators of rare metal magmatism. It is especially significant for Kuiviveem-Puirkakaisky tin ore region which granitoids are characterized by high rare metal content.

References:

Alekseev V.I. About an origin lithium-fluoric granites of the Severny massif (Chukotka) // Zapiski RMO, 2005. ¹ 6. P. 19-30. (in Russia).

Alekseev V.I. The reasons petrochemical similarities subalkaline lithium-fluoric and normal granites of the Severny pluton (Chukotka) // Materials of the All-Russian seminar "Geochemistry of magmatic rocks". The scientific school «Alkaline magmatism of the Earth», Saint-Petersburg, 23-26 May 2008. M.: GEOKHI RAS, 2008. P. 3-4. (in Russia).

Vladimirov A.G., Annikova I.Yu., Antipin V.S. Ongonit-elvanovyj magmatism of Southern Siberia // Litosfera, 2007. ¹ 4. P. 21-40. (in Russia).

Dergachev V.B. Ongonites and elvanites // Izvestiya RAS. A series geological. 1991. ¹ 10. P. 34-43. (in Russia).

Kovalenko V.I. Petrology and geochemistry of rare metal granitoids. Moscow: Nauka, 1977. 206 p. (in Russia).

Kozlov V.D. Geochemistry of ore-bearing granitoids of rare metal provinces. Moscow: Nauka, 1985. 304 p. (in Russia).

Note. Severny pluton: 1 – ongonites (6 tests), 2 – microcline-albite leucogranites (7 tests), 3 – biotite granites of the main phase (10 tests); Standard samples: 4 – microcline-albite granite, Zhanchivlansky massif, Mongolia; 5, 6 – ongonites, Baga-Gazruinsky dykes, Mongolia (5), Arui-Bulaksky massif, Transbaikalia (6). Dash – no information.