TRACE ELEMENTS IN MAGNETITE AS INDICATORS OF FORMATION CONDITIONS OF IRON ORE OF AKTASH DEPOSIT, WESTERN KARAMAZAR, TAJIKISTAN

Abstract

The relevance. In recent years, the interest in study of the magnetite composition as an indicator of the conditions for forming iron ore deposits has increased due to the mass spectrometry with inductively coupled plasma and laser ablation. The application of mass spectrometry with inductively coupled plasma and laser ablation analysis for the study of magnetite is especially important in Western Karamazar, where magnetite deposits bearing polymetallic mineralization were preserved. The models of the origin of these deposits are still debatable. To solve this problem, we studied the textural and structural features of magnetite ores and the composition of trace elements (per 32 elements) of magnetite types. The aim of the research is to obtain new mineralogical and geochemical data of magnetite types to develop a model for the evolution of Aktash type iron ores. The objects of study are eight morphological varieties of magnetite in sulfide-magnetite ore bodies of the Aktash deposit at the Kansai ore field from Western Karamazar. Methods. The mineral composition of ores was studied in polished sections using an Olympus BX51 optical microscope equipped with an Olympus DP12 digital camera. Minerals were examined using a Tescan Vega 3 sbu scanning electron microscope (Institute of Mineralogy SU FRC MG UB RAS). The contents of trace elements in magnetite were determined by the mass spectrometry with inductively coupled plasma and laser ablation on an Agilent 7700x mass spectrometer with the MassHunter software package and a New Wave Research UP-213 laser (Institute of Mineralogy SU FRC MG UB RAS). International standards were used for calibration and calculation: USGS NIST-610 and USGS GSD-1g glasses. The calculation was carried out in the Iolite program using 56Fe as an internal standard. Results. At the Aktash deposit, magnetite is subdivided into several morphological varieties: apohyaloclastic (Mt-1h), dendritic (Mt-1d), filamentous (Mt-1f), collomorphic kidney-shaped (Mt-1c) and radial-radiant (Mt-1r) magnetite-1 aggregates, zonal subhedral grains of magnetite-2 (Mt-2s), elongated lamellar crystals of magnetite-3 («mushketovite», Mt-3m), and nonzonal euhedral metacrystals of magnetite-4 (Mt-4e). We assumed that the magnetite-1 types were formed at the stages of halmyrolysis and diagenesis of calcareous volcanic-sedimentary deposits in the zones of gas seeps and bacterial chemosynthesis. The apohyaloclastic and collomorphic magnetites were formed after hyaloclasts, the other three types – by bacteriomorphic structures. Relict inclusions of aluminosilicates and accessory minerals are identified by elevated contents of Mg, Al, Zr, Cr, and V in apohyaloclastic magnetite-1h relative to those in bacteriomorphic magnetite (Mt-1d, 1f, 1r) at similar amounts of As. Bacteriomorphic magnetite is characterized by elevated contents of As, Sb, Mo, and W and low contents of Al, Ti, V, Cr, Mn, Ni, and Zn compared to magnetite-2s and -4e. At the stage of late diagenesis, the early aggregates of magnetite-1f, c, r were overgrown with zoned crystals of magnetite-2s. Magnetite-2s is characterized by the lowest contents of As, Sb, Mo, W with increasing concentrations of Al, Ti, Cr, Mn, Ni, Cu, Zn, and Bi due to partial replacement of skarn carbonates, aluminosilicates, sulfides, and pyrophanite with corroding aggregates. Similar to hematite, magnetite-3m formed after hematite crystals and concentrates W, Zn, and Mo. The replacement of early magnetite types by skarn minerals resulted in the formation of magnetite-4e, which isomorphically implaced by the maximum amounts of Ti, V, Cr, Mn, Zn, that indicates the high temperature formation.

Keywords

• trace elements
• magnetite
• skarns
• aktash deposit
• western karamazar