Тезисы международной конференции
Abstracts of International conference
Chemical composition of ore-forming fluids of Fe-F-REE carbonatite deposits
of the Central Tuva region, Russia
Prokopyev I.R., Borisenko A.S., Borovikov A. A.
Institute of Geology and Mineralogy SB RAS, Novosibirsk, Russia
Determination of the conditions of generation of magmatogene fluids and their composition and metal-bearing capacity is the crucial fundamental problem of endogenous ore formation. The recent wide application of modern instrumental analytical methods (LA-ICP-MS, scanning electron microscopy, IR and Raman spectroscopies, etc.) combined with thermobarogeochemical investigations has yielded new data concerning the generation conditions, composition, and metal-bearing capacity of magmatogene fluids released at different stages of crystallization of granitoid, basic, and alkali-basic melts.
Geology and mineralogy
Carbonatites of Tuva are part of the Late Mesozoic Central Asian carbonatite province widespread in Russia and Mongolia. Fe-F-REE carbonatite deposits are localized in the Central Tuva carbonatite-REE belt of N–S strike, and in general can be divided into three ore groups (from north to south): Chailyukhem, Karasug, and Ulatai-Chezsk. Carbonatite ore-bodies are confined to the deep structural sublatitudinal zone, crossing Kurtushibinsky ophiolite belt, Khemchiksko-Systyghemsky synclinorium and Tuvinian depression. The Rb-Sr age of the carbonatites is 118 ± 9 Ma (Sugorakova et al, 2004). Late Mesozoic carbonatites associate with intrusions (stocks and dikes) of gabbros, gabbro-dolerites, granosyenites, syenite-porphyries, diorite porphyrites, and lamprophyres (kersantites, spessartites).
Carbonatites are represented by two mineralogical types: ankerite-calcite and fluorite-barite-siderite ones. Fe-F-REE carbonatites are lenses, tube-like and intricately shaped bodies composed of siderite, barite, fluorite, ankerite, calcite, and scarcer hematite, magnetite, bastnaesite, parisite, Ba-celestite, apatite, K-feldspar, muscovite, biotite, and rare sulphides. In general, ores have brecciform texture owing to the presence of fragments of the host sandstones, schists, syenite-porphyry, granosyenites, and other rocks, produced by fluid-explosion processes. On the earth surface, the ores are strongly oxidized and replaced by goethite, hydrogoethite, and hematite.
Thermobarogeochemical studies of Fe-F-REE carbonatites from the Central Tuva region (Bredikhina, Mel’gunov, 1989; Ontoev, Kandinov, 1980; Prokopiev et al., 2010, Borisenko et al., 2011) showed that they are formed from highly concentrated melts-brines of chloride-carbonate (±CO2) composition at rather high temperatures (400–700°C) and 2.5–3.5 kbar. At the later stages of rock formation, the temperatures of mineral formation decreased to 300–480°C, and the salinity dropped to 40–50 wt.% NaCl eq. At the late hydrothermal stage (calcite–fluorite–celestine), these parameters were as low as 140–290°C and 40–25 wt.% NaCl eq., respectively.
Multiphase fluid inclusions from the Karasug deposit were found in idiomorphic well-shaped colourless crystals of quartz or morion and in violet cubic crystals of fluorite which occur among ankerite-calcite or siderite matrix of carbonatite ores. Inclusions in quartz (Fig. 1) mostly contain a large cubic crystal of halite, often with a violet tint, which occupies most of the vacuole volume, as well as a small sylvite crystal, three or four anisotropic crystalline phases, and fine crystals of ore phases. The gas phase contains liquid CO2. Multiphase inclusions in fluorite are of similar phase composition, but the gas phase lacks or it is poor in liquid CO2.
Figure 1. Multiphase fluid inclusion in morion without analyzer (a) and with analyzer (b).
Multiphase fluid inclusions in quartz and fluotite from carbonatite ores of the Ulatay-Chezsk group deposits (Teeli-Orgudyd, Ulatai) also contain chlorides of Na and K in different proportions which occupy a larger vacuole volume of the inclusion, usually 1-2 solid phases and/or particles of ore minerals (eg. hematite). Liquid CO2 prevails in the gas phase of early fluid inclusions, whereas the inclusions in quartz of late generations are characterized by the CO2–N2 composition of the gas phase.
Raman spectroscopy revealed calcite CaCO3, Ce-ancylite Sr(Ce,Ca,La)[CO3]2(OH)·H2O, anhydrite CaSO4, thenardite Na2SO4, ferricopiapite Fe5(SO4)6(OH)2·20H2O, and gaylussite Na2Ca(CO3)2·5H2O among the solid phases of multiphase inclusions. Using scanning electron microscopy we identified REE-carbonate, probably bastnaesite (Nd,Ce,La)CO3·(F,OH), galena PbS, barite-cellestite (Ba,Sr)SO4, anhydrite CaSO4, carbonate ankerite Ca(Mg, Fe)[СО3]2 in uncovered vacuoles of multiphase inclusions.
All this evidences that the ore-forming fluids were highly concentrated melts-brines or brines of carbonate-sulphate-chloride composition. These fluids were oxidized; their redox potential corresponded to the sulphate-sulphide balance, as evidenced by the coexistence of sulphates (thenardite, anhydrite, ferricopiapite) and sulphides (galena) in the inclusions. The oxidized state of fluids is also confirmed by the presence of Fe(III) sulphates (Fe-copiapite) and hematite in the inclusions. The high concentrations of CO2 and Fe(III) in the inclusions in early quartz and fluorite suggest that the mineral-forming melts had low pH values. According to Raman spectroscopic data, the inclusions in quartz of late generations had a CO2–N2 composition of the gas phase (N2 = 86.7–72.6, CO2 = 13.3–27.4 mol.%), and the solid phases included hydrous Na and Ca carbonates. This may suggests the alkaline type of such melts.
Analysis of multiphase inclusions by LA-ICP-MS revealed high concentrations (n 0.1to n1 wt.%) of Fe, Mn, Sr, Ba, Ca, Cu, Pb, and Zn, and significant contents (n1 to n100 ppm) of As, Rb, Sb, Co, Au, Hg, Mo, Bi, W, and rare earth elements Y, Cs, La, Ce, Nd (table).
Table. Concentration of elements in the multiphase fluid inclusions in quartz determined by LA-ICP-MS.
* - according to the thermometric data.
The work was supported by grant RFBR #10-05-00730 and SS #65458.2010.5.
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