Endogenic carbonate mineralization in the area of Oshurkovski massif
Burtseva M.V., Doroshkevich A.G., Ripp G.S.
Geological Institute SB RAS, Ulan-Ude, Russia
Oshurkovski massif of alkaline-basic rocks, which occupies the area more than 12 km2, is located in interfluve Utochkino and Oshurkovo. Host rocks are presented by gneiss and gneissoid granites. The massif is crossed by numerous dykes and veins of granitic pegmatites and aplites. The age of gabbroid formation (SHRIMP II, zircon) is 125.4±2 Ma.
A number of researchers attributed them to diorites (Andreev et al., 1972; Kuznetsov, 1980), others to gabbro-monzonite-dioritic (Polyakov et al., 1980) and monzodiorit-syenitic (Litvinovsky et al., 2002) series, to gabbro, gabbro-dioritic (Smirnov, 1971), gabbro-ultrabasitic (Kuznetsova et al., 1995), ultrabasitic alkaline (Yatsenko, 1972) series.
Melano-, mezo- and leucocratic varieties are separated among gabbriods. These are low silicate with high titanium and alkality rocks (42-46 wt. % SiO2, Na2O+K2O – 5-8 wt. %). There is nepheline in this rocks and high concentration of silica made for brazilite appearance at the starting crystallization stage. The rocks consist of varying amounts of amphibole, biotite, clinopyroxene, apatite, oligoclase (№ 16-18), potassium and potassium-sodic feldspars. Titanite, ilmenite and titanium-rich magnetite are presented regularly.
Syenites are minor presented in the massif. According to (Litvinovsky et al., 1998) they were formed by fractional crystallization. By other researches the syenites are related to metasomatic (Kuznetsov, 1980) and assimilatic (Smirnov, 1971) processes.
The presence of endogenic carbonate mineralization on a deposit was marked by many researchers. In most cases it has been carried to hydrothermal, assumptions of them carbonatitic nature were less often come out.
Among carbonate mineralization following types are established:
1. Veins of carbonatites with phlogopite, magnetite, barite, allanite, apatite, alkaline field spars, sphene, zircon.
2. Chlorite-calcitic mineralization which is a product of greenstone change by gabbroids.
3. Veinlets and breccia zone cemented by quartz and calcite-quartz mineralization with chlorite, pyrite.
4. Interstitial calcite in gabbroids.
5. Interstitial and nodular calcite in association with apatite, biotite and alkaline feldspars in dykes of alkaline gabbro.
Scales of occurrences of all listed carbonate mineralization types are insignificant, but in some cases veinlet mineralization zones to tens metres width are marked.
Carbonatites form low-width (up to 0.6-1.0 m) different oriented veins, traced on strike to 100 m. These are fine-, medium-grained calcitic rocks with striation, conformed to vein orientation. Their contacts with host rocks are sharp and often tectonized. The bodies of carbonatites also consist of splinters in fine-grained detritus floury matrix. Selvages are usually bordered by phlogopite and enriched by magnetite, sometime by alkali feldspar, titanite. The rocks contain apatite, high-strontium (up to 10-14 wt. % SrO) barite, accessory monazite, zircon, allanite. Their age (SHRIMP II, zircon) is 126±0.85 Ma. Mineral formation temperatures, estimated by isotopic-oxygen thermometers, are 932-946°C for a couple magnetite-phlogopite, and 625°C for a couple magnetite-calcite.
More late silicification and calcite recrystallization are founded in some veins. Magnetite is martitizated here, new formed allanite was appeared and SrO values were decreased in some calcites to less than 0.1 wt. % (1.5-2 et. % originally). Isotopic shift to weighting of oxygen (up to 16.4 ‰ δ18O SMOW) and reduction of weight of carbon (up to -13.1 ‰ δ13C PDB) is founded in hydrothermal recrystallized carbonates.
Chlorit-calcite mineralization is marked among gabbroids in the area of Oshurkovski massif sporadically. The amphiboles, pyroxenes replacement by calcite and chlorite is observed under a microscope. Besides, short irregular shape veinlets are widespread 1-2 cm width, combined by inequigranular calcite. Veinlets selvages are edged by leprose chlorite aggregates. The last corresponds to ripidolite in composition. Calcite does not contain impurity of magnesium, iron, manganese, barium and strontium. The isotope composition of oxygen varies from -2,8 to -7,3 ‰ δ18O, SMOW.
Calcite-silica mineralization. Some zones of the crushing are established in gabrroids 10 and more meters width. They are low-power veins and veinlets, parts of cementation brecciation rocks, combined by coarse-grained drusy quartz. Prominent feature of such mineralization is a presence of pyrite dissemination, which associates with quartz. Fluorite is founded in some cases as well. Calcite does not present in all bodies and it is connected with processes of reactionary interaction hydroterm with gabbroids. Decomposition of amphiboles, pyroxene is caused allocation calcite and chlorite. Calcite does not contain notable quantities of magnesium, iron, manganese, strontium and barium (less than 1 wt. %).
Interstitial allocations of calcite in gabrroids do not have reactionary borders with host minerals. The quantity of such calcite does not exceed 1-2 % of volume of rocks. It associates with sphene, hastingsite, biotite, plagioclase and К-Nа feldspars. This calcite is characterized by high abundances of Sr, Mg, Fe and close to calcite from carbonatites in composition.
Calcite from dykes of alkaline gabbro is founded in two parts having sharp contacts with host rocks. The dykes are commonly combined by diopside (15-25 % aegirinic minal) 60-70 %, alkaline feldspar. There is also apatite, sphene, calcite. It is supposed, that these dykes are comagmatic to carbonatites as conclude high quantities of disseminated calcite and nests of calcitic rocks with glomeroblastogenic allocations of alkaline feldspars and biotite.
Chemical compounds of dykes plot in the field of alkaline gabbroids. High concentration of alkalis is caused by presence of potassic feldspar, albite and biotite. The rock is characterized by high titanium, phosphorus and prevalence ferrous iron over oxidic.
Calcite from dykes is presented by several forms of allocation. One of them is disseminated in weight of silicate rock, carries out interstices between feldspar grains. The contents of such calcite reaches to 2-3 %. Reactionary borders between calcite and host minerals do not marked.
The part of calcite with alkaline feldspars and biotite form round allocation 10-15 cm in diameter. It consists from calcite (60-70 %), albite (25 %), biotite (5-7 %), potassic feldspars (5 %), apatite (2-3 %), sphene. Calcite composes a great bulk allocations, includes dissemination of biotite and alkaline feldspars. It is fine-grained. Its carbon isotope composition is a little facilitated concerning to carbon from Oshurkovski carbonatites and a mantle label, and oxygen is sharply enriched by a heavy isotope (18,64 ‰ δ18O, SMOW). These values both carbon, and oxygen, possibly, characterize an intensive isotope exchange between carbonate and fluid phases. The presence of hydrothermal calcite veinlets denote a fluid phase specifies.
This study was financially supported by RFBR grant 08-05-98028, NSch – 3848.2010.5, MK – 2873.2010.5.
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