O, C, S isotopic composition of the minerals from copper-ore associations of Neoproterozoic Volhynia-Brest Province traps
Kuzmenkova O.F., Kolosov I.L.
Belorussian Geological Exploration Research institute, Minsk, Belarus
Major Volhynia-Brest Trap igneous province (VBP) Neoproterozoic age (about 550 mln.y) is located on the southwestern of the East European platform. In VBP structure are dominated subalkaline basalt series and their tuffs (lower part of the section volcanic sequence) as well as basalts of the normal series and their tuffs (upper section). Slightly are developed pikrobazalts of normal series; intrusive dolerites, gabbro-dolerites of subalkaline series and acidic volcanites of the normal and subalkaline series. Copper mineralization is confined to the dolerite, basalt and their tuffs. Copper and silver have of primary magma genesis and are shown of geochemical feature of the magma chamber. The outpouring of basaltic lava happened on the ground at low oxygen fugacity (ƒO2 = -12.1 - -6.45) under QFM buffer, which contributed to the excretion of copper in the native form (4). It partially composed to the rock-forming minerals of basalts, partially remained in the melt. In the rocks of VBP had been identified three copper-ore mineral associations: late-magmatic, post-magmatic native-copper and copper-sulphide hydrothermal.
1. Late-magmatic. Native copper in dendrit, crystal (0,5 – 1,5 mm) form is formed at late crystallization stage of basaltic lava in association with ilmenite, hloropfeite, analcime, volcanic glass, chalcedony. The presence of chalcedony in basic rocks of province is due to contamination of melts by crustal material. The water-bearing residual melt has been mobile for some time, and grouped, served the degassing channels (vesicles) in the not fully consolidated basalts under active fluid regime.
2. Post-magmatic. This mineral association is the result of autometasomatic conversion of rocks under the influence of VBP hydrotherms. Chlorophaeite, analcime, chalcedony continue to allocate; to divide clearly late-magmatic and post-magmatic stages of their forming are not possible due to gradual transition from interstitial segregations of these minerals to the "shadow", and then to clear veins. Olivine has been replaced by serpentine, hloropfeite, iddingsite-boulingite, opal, sphene; zoisite, palagonite, opal, quartz, pyrite has been formed, glass saponitization, plagioclase albitization, mesostasis chloritization, etc. has been occurred. At a late stage were formed zeolites (stellerite in subalkaline basalts; mordenite, heulandite - in tholeittes), minerals, silica, chlorite, hydromica, hematite, carbonates, sulfides, copper, etc. The removal and redeposition of copper mainly in the native form occurs in the form of leaves, scales, dendrites (1 - 5 mm). The temperature of homogenization of gas-liquid inclusions (GLI) in quartz from tholeittes from Rafalivka quarry is 265 - 335 îÑ, the isotopic composition of oxygen (δ18Î = 6,05 – 8,25 ‰ – here and then relatively SMOW) consistent with the composition of magmatic water (1).
The isotopic composition of pyrite sulphur from calcite-pyrite vein (2 mm) of fresh tholeitte from Rafalivka quarry is abnormally heavy (δ34S = 30,7 ‰ – here and then relatively CD). The equilibrium with basaltic melt fluid in the buffer QFM (ƒO2 < -15), 1 kbar, contains sulphur predominantly in the form of H2S (6). Since the situation of crystallization of VBP melts was more oxidative, sulphur was contained primarily in the form of SO2. Such sulphur form is poorly soluble in melts and is easily removed, that could be explained the almost complete absence of sulphides in the basalts of VBP. A small part of the sulfur in the H2S condition can crystallize in the sulphide form and have a weighted isotopic composition (Δ 34S = 4 ‰) relatively melt δ34S under separating fluid has been cooled in a rapidly reducing ƒO2. In this case δ34S melt should be 26 – 27‰, which looks for basaltic magmas unrealistic. At the same time basites with abnormally high (up to +17‰) and abnormally low (up to -13,4‰) values δ34S primary sulphides are known in the world, which are explained by other researches as melt contamination of sedimentary sulfides and sulfates (6). Contamination of VBP melts by crustal material, and the presence of lithospheric component in magmatic source is shown by our studies (5). To create such heavy isotopic composition of sulphur, the possible contaminates should be act sedimentary sulfates. The isotopic composition of calcite carbon associated with pyrite (δ13Ñ = -0,8 ‰ – here and then relatively PDB) is similar to that in marine carbonates, which also confirms heavy isotopic composition of oxygen (δ18Î = 27,5 ‰). Hydrotherms could be formed by mixing of fluids, separated from the contaminated by marine sedimentary rocks melts, (sulphates, carbonates, clay), and actually “metamorphic” waters, activated by hot melts in the VBP crystal basis. Rocks with a similar composition of oxigen and carbon are known in the basement of Belarus – metacarbonates of Rudma unit (δ13Ñ from 0,8 to -1,7 ‰; δ18Î from 14.8 to 23,4‰) (7) and Ukraine - calciphyres, marbles of Bugskaya and Teterevskaya unites (δ13Ñ from 3,5 to -1,9‰; δ18Î from 15,5 to 24,5‰) and others (2).
The isotopic composition of oxygen δ18Î (17,1 - 27,5‰) and carbon δ13Ñ (-9,9 - -12,9‰) of calcite from veins (2 – 30 mm) in basalts and gabbro-dolerites are easier. The isotopic composition of calcite carbon from Rafalivka quarry tholeittes (δ13Ñ from -7,7 to -8,7‰) by (1) is near to our data, but the oxygen composition (δ18Î from 6,04 to 7,1‰) is magmatic. Similar values have calcite from basalts of well 5871-Zhirichi (δ13Ñ from -5.41 to -4,58‰; δ18Î from -3.03 to 10,66‰) (8); the homogenization temperature GLI (60 - 233îÑ) indicates the mineral deposition under the basalt cooling. According to (8), the formation of stratiform bodies of native copper in the gray-green tuffs of Zhirichi ore shoot occurred as a result of reduction of iron from three- to the bivalent form, with the participation of chlorine-anion in the Riphean strata groundwater level fluctuations, with the ore-forming solutions were not genetically related to the magma source. However, to assume their sterility of magmatic fluids in an active volcanic area is not possible.
Considerable variations in the calcite oxygen isotopic composition can not be explained only by the fractionation of isotopes with decreasing the temperature of the fluid. The carbon isotopic composition of calcite is mantle, worsening to -0,8 ‰ and relief to -12,9 ‰ due to the interaction of the fluid respectively, with the sedimentary carbonates and biogenic carbon. Postmagmatic hydrothermal formed with the involvement of magmatic and metamorphic water, to a lesser degree - the meteor. Its composition was determined: by the composition of the melt from the fluid was separated (formed by the composition of the mantle source and crustal contaminate), and composition of fluids from the crystalline basement and Riphean clastic strata.
3. Hydrothermal. Polymineral streaks of 10-30 mm (30 cm) are composed of quartz, chlorite, carbonates, zeolites, sulfides of copper, pyrite, copper green, etc., rarely – only calcite, iceland spar. Pyrite from the vein in tholeitte from Yanova Valley quarry has a lighter isotopic composition of sulfur (δ34S = -14,5‰). Magmatic fluids are not as low values δ34S even at low temperatures, and water-marine origin fluids at low temperatures can produce a wide range of values (δ34S = 30 – -15‰) (6). The "metamorphic" origin of the hydrothermal water also indicates the isotopic composition of oxygen (δ18Î = 14,9‰) and significantly lighter composition of calcite carbon (δ13Ñ = -23,5‰) associated with pyrite. Such organic carbon might be originated from the graphite of crystalline basement rocks of the province: graphitic gneisses of Rudma unit of Belarus (δ13Ñ from -19.0 to -27,4‰) (3), graphite-biotite gneisses, crystal shales of Teterevskaya unite (δ13Ñ = -27,4 – -32,8‰); graphite-biotite gneisses of Bugskaya unite (δ13Ñ = -22,7 – -35,8‰) (2). Detection of calcite with a similar carbon composition (δ13Ñ = -20,5‰) in tholeitte from borehole 776 within the Central Belarus Zone, southwestern the Rudma unit rocks development, may indicate a possible continuation of their band development here.
Copper sulphide mineralization in association with chlorite, analcime, calcite is confined to tuffs in the basalt lack areas of VBP – Stolinskoe and Ozernitskoe ore shoots in Belarus in the latitudinal zones of intense fracturing, Shepetovsky ore unit in Ukraine. Water calcite-forming solutions has been metamorphic (δ18Î = 9,3 – 16,9‰), carbon isotopic composition varies from mantle (δ13Ñ = -2,1 – -5,6‰) to organic (δ13Ñ = -20, 4‰) in Shepetovsky ore unit (8).
Copper removing from the traps and its redeposition in the sulphide form occurred during superimposed Hercynian activation, as indicated by the attachment of veins to active in these period submeridional and sublatitudinal faults. The northeastern and the northwestern deep faults transported magma in Neoproterozoic. The crystalline basement and platform cover rocks of the province were the fluids source.
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