Garnets of the Rudnogorskoe iron-oxide deposit (East Siberia): LA-ISP-MS data

Object. Garnet aggregates of layered hematite-magnetite ores and mineralized volcanoclastites of the Rudnogorskoe iron-oxide deposit in the Angaro-Ilimsky district, Irkutskiy region. Aim. To study mineralogical and geochemical features of garnets in different textural types of ores related to clarification of physico-chemical conditions of garnets formation and determination of the time of iron accumulation and scarns formation at the deposit. Methods. Chemical composition of the minerals of the garnet group was determined using Tescan Vega 3 sbu with the Oxford Instruments X-act and REMMA-202M. Infrared spectroscopy of garnets was performed by Fourier spectrometer Spectrum One with MultiScop microscope. Micro maps of the distribution of trace elements in the ooid, concentrations of chemical elements in the garnet, including rare earth elements, were determined by the LA-ISP-MS method on an Agilent 7700x spectrometer equipped with a New Wave Research UP-213 laser ablation device. Results. The garnet of layered ores has a constant composition with a few fluctuations of andradite (Adr. 50-65 mol. %) and grossular (Grs. 21–39 mol. %) end members, total FeO* (17,1–21,9 wt %) and a small amount of MgO and TiO2. The zonal individuals of grossular-andradite in the ooids of mineralized volcanoclastites are characterized by a variable (17–80 % andradite end member) composition: the content of FeO* in them varies from 5,8 to 24,8 wt %. The ooids contain zonal of the garnet graines of the shorlomite-andradite series with a morimotoite end member (Mrm. 4–15 mol. %) and TiO2 content up to 15,2 wt %. According to LA-ISP-MS data, in the minerals of the garnet group the increased amounts of light rare earth elements relative to heavy and a positive Europium anomaly were recorded. The presence of hydroxyl groups in the garnets of ooids from mineralized volcanoclastites was established by infrared spectroscopy. It is supposed that the formation of minerals of the garnet group occurred at relatively low or moderate temperatures (≥200–250°C) from a highly concentrated fluid. The garnets at the ooids of mineralized volcanoclastites and layered ores were overgrown on aggregates of hematite and magnetite and are replaced by chlorite and calcite, that not contradict halmyrolysis model of iron accumulation and subsequent skarns formation.