NEW DATA ON AGE OF PRE-ORE MAGMATISM AT ERMAKOVKA DEPOSIT

Lykhin D.A.*, Salnikova E.B.**

*Institute of Ore Deposit, Geology, Petrography, Mineralogy and Geochemistry RAS, Moscow, Russia

**Institute of Pre-Cambrian Geology and Geochronology RAS, St.Petersburg, Russia

liha@igem.ru

The Ermakovka bertrandite-phenacite-fluorite deposit that localizes in West Transbaikalia is unique by content, reserves and mineral diversity of beryllium minerals. It belongs to the bertrandite-phenacite-fluorite ore formation. The most complete data on geological structure of the deposit, main ore mineral types of ores and their genesis are presented in papers (Gal′chenko 1968; Ginsburg 1977; Novikova 1994; Bulnaev 1996; Reif 1999). The deposit is associated with the leucogranite massif of the Small Kunalei Complex that formed in interplate conditions in the Early Mesozoic (Yarmolyuk 2000; Lykhin 2001). Based on the detailed geological observations, three magmatic stages have been distinguished at the deposit by us, i.e. pre-ore, syn-ore and post-ore ones. The age characteristics obtained for granite dykes of the pre-ore stage 283 ± 30 Ma needed more precision. Therefore, additional geochronological studies of those dykes have been carried out by us.

The pre-ore dykes of granites in the deposit are represented by fine- and middle-grained rocks of light grey or pink colours with porphyry-like structure. The main rock-forming minerals are quartz (30%), K-Na feldspar (35-45%), plagioclase that is represented by oligoclase (15-20%). Dark-coloured minerals are represented by biotite and amphibole, their quantities being varied from 5 to 15%. The accessory minerals are zircon and apatite. In petrochemical respect, the dykes of the deposit belong to subalkaline granites, alkaline leucogranites, granosyenites; amphibole-potasssium feldspar pegmatites occur. In dykes, the concentrations of magnesium, calcium and aluminum decrease with the increasing silica. All granitoids are practically alike by distribution of admixture elements. These are comparatively hardly differentiated rocks with domination of light rare earths (LREE) over heavy ones (HREE), (La/Yb)_N = 16.91 without europium minimum in fact (Eu/Eu* = 0.94) and with deficit of cesium, niobium, strontium and titanium. Let us note that the dykes of the Angara-Vitim batholith spreading to the north of the deposit are represented by fine-grained granites, aplites, granite-porphyries, syenite-porphyries and diorite porphyrites. Geochemical data on granitoids of the Angara-Vitim batholith in the Uda-Vitim zone largely coincide with data on pre-ore dykes of the Ermakovka deposit, being differed from the latter ones by the lower yttrium and higher cesium and lithium.

We carried out Rb-Sr isotope studies on four samples to make the age of granitoids in the pre-ore magmatic stage more precise. The obtained isochrone is 302.7± 24.5 Ma, (⁸⁷Sr/⁸⁷Sr)₀ = 0.7058 ± 2, SKVO = 0.16. Since the SKVO value obtained on isochrone is significantly less than 1, it can be considered that Rb-Sr isotope system in granitoids closed at ~302.7± 24.5 Ma. The obtained age 302.7± 24.5 Ma indicates that the ore process and young magmatism in the deposit (~224 Ma, Lykhin et al. 2001) did not result in rejuvenation of Rb-Sr system in early granitoids.

We also carried out isotope geochronological studies of amphibole-K feldspar pegmatites. Amphibole was extracted from pegmatites and analyzed by ⁴⁰Ar/³⁹Ar method. Clear age plateau of 302.5± 0.9 Ma is distinguished in amphibole spectrum on six medium-high temperature steps. The isochrone age is 302.6± 1.6 Ma on all steps at the primary one (⁴⁰Ar/³⁹Ar)₀ = 290 ± 1 and SKVO = 2.3. The insignificant lowering of low-temperature part of spectrum testifies to little effect of later processes. The obtained isochrone age 302.6± 1.6 Ma corresponds to the age of amphibole ⁴⁰Ar/³⁹Ar isotope system closure.

We also selected a large sample for zircon from one dyke analyzed by Rb-Sr technique. The morphological peculiarities of zircon indicate its magmatic origin. Points of studied zircon isotope composition are on concordia of age 325± 3 Ma, SKVO = 0.07, probability = 0.79. We consider the obtained age as the most exact estimate of formation of melts that are primary for dykes of pre-ore magmatic stage granites.

The new geochronological data obtained on dykes of granitoids and pegmatites by Rb-Sr and Ar-Ar techniques completely coincide within analytical error (302.7± 24.5 and 302.6± 1.6 Ma respectively). The U-Pb data 324.7± 3.2 Ma within analytical error coincide with the Rb-Sr data. However, since the U-Pb system in accessory zircons is the most stable to isotope disturbance caused by secondary processes, we consider the age of the dykes as 325± 3 Ma. The younger age (~303 Ma) obtained on the dykes can testify to the later closure of Rb-Sr and Ar-Ar isotope systems in them.

The geochronological studies carried out on dykes of granites that belong to the pre-ore magmatic stage in the Ermakovka deposit demonstrate that their age is 325± 3 Ma. This age coincides with the data obtained during the dating the Angara-Vitim granite batholith (285-320 Ma) by other researchers (Litvinovsky 1992; Budnikov 1995; Yarmolyuk 1997; Rytsk 1998). Based on geochronological and geochemical data, we consider the dykes of granitoids in the pre0ore magmatic state at the Ermakovka deposit belonging to the Barguzin complex. Some age divergence can be caused by the fact that the dykes formed in the initial stage of the Angara-Vitim batholith formation and in one of its most southern marginal zones.

The work has been carried out with financial support of Russian Federation for Basic Research (grant 06-05-64217).

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