Hydrothermal fluids of apatite-magnetite ores of the Tomtor carbonatite massif (NE, Russia)

The Tomtor carbonatite complex, with an area of 250 km², is confined to the eastern framing of the Anabar Anteclise; it is located withtin the Ujinsky province of ultrabasic alkaline rocks and carbonatites (Northeast of Siberian Platform) (Erlich, 1964). The complex has a concentric zonal structure: the outer ring is composed of alkaline and nepheline syenites, the inner incomplete ring is nepheline-pyroxene rocks of the foidolite family, the core is represented by carbonatites. All rocks of the massif are intersected by dikes and explosion tubes of picrites and alneites. Onkuchakh apatite-magnetite deposit is located on the northeastern border of the carbonatite core. Apatite-magnetite ores (camaforites, phoscorites, nelsonites) form a series of ore steeply dipping (75-80^o) lenticular bodies of north-western strike. The resources of the apatite-magnetite ores of the Tomtor massif are about 1 billion tons of iron (Tolstov, 1994). Primary and pseudo-secondary fluid inclusions were studied in apatite, calcite and potassium feldspar of camaforites. Inclusions have isometric or elongated shapes up to 50 microns. Most of the studied inclusions have a negative crystal form located in the central parts and zones of apatite growth.

Apatite contains a multiphase (crystal-fluid) inclusions with gas, liquid and 1-5 visible crystalline phases. The gas phase is represented by CO₂, contains subordinate amounts of H₂O, H₂S and SO₂. The liquid phase is represented by H₂O with SO₄^2-, HSO^4- and HCO^3- ions. The solid phases in the inclusions are represented by mainly halite (NaCl) and sylvite (KCl), with strontianite (SrCO₃), barite (BaSO₄) and Ca-Sr-REE F-carbonate crystals. Complete homogenization occurs in the temperature range from 290 to 350 °C, the concentration is 30-45 wt. % of NaCl-eq. Calcite has the similar in composition fluid inclusions. The solid phases are mainly represented by halite (NaCl) and sylvite (KCl), as well as the dolomite (CaMg(CO₃)₂), strontianite (SrCO₃), REE phosphates and sulfates of Sr and Ba. Complete homogenization occurs at 250-300 °C, the concentration is 35-55 wt. % of NaCl-eq. The gas phase of the fluid inclusions in K-feldspar is predominantly CO₂; the liquid phase is H₂O. The solid phases are represented by witherite (BaCO₃) and calcite (CaCO₃). The homogenization temperature of fluid inclusions occurs at 350-375 °C.

The results show that the hydrothermal fluids of camaforites of the Tomtor massif are represented by the concentrated high-medium temperature sulfate-carbonate-chloride solutions of complex composition . The fluid composition is explained by the evolution of the carbonatite melt.

The work was supported by the Russian Science Foundation (RSF), project # 19-17-00013.

References

1. Erlich, E.N., 1964. The new province of alkali rocks on the north of Siberian platform and its geological aspects. Proc. All-Soviet Mineral.Soc.93,682–693.
2. Tolstov, A.V., 1994.Mineralogy and geochemistry of apatite-magnetite ores of the Tomtor Massif (NorthwesternYakutia). Russ.Geol. Geophys.35,76–84.