Three-D Mineralogical Mapping of the Kovdor Phoscorite–Carbonatite Complex, NW Russia: I. Forsterite

Abstract

The Kovdor alkaline-ultrabasic massif (NW Russia) is formed by three consequent intrusions: peridotite, foidolite-melilitolite and phoscorite-carbonatite. Forsterite is the earliest mineral of both peridotite and phoscorite-carbonatite, and its crystallization governed evolution of magmatic systems. Crystallization of forsterite from CaFe rich peridotite melt produced Si-Al-Na-K-rich residual melt-I corresponding to foidolite-melilitolite. In turn, consolidation of foidolite and melilitolite resulted in Fe-Ca-C-P-F-rich residual melt-II that emplaced in silicate rocks as a phoscorite-carbonatite pipe. Crystallization of phoscorite began from forsterite, which launched destruction of silicate-carbonate-ferri-phosphate subnetworks of melt-II, and further precipitation of apatite and magnetite from the pipe wall to its axis with formation of carbonatite melt-III in the pipe axial zone. This petrogenetic model is based on petrography, mineral chemistry, crystal size distribution and crystallochemistry of forsterite. Marginal forsterite-rich phoscorite consists of Fe 2+-Mn-Ni-Ti-rich forsterite similar to olivine from peridotite, intermediate low-carbonate magnetite-rich phoscorite includes Mg-Fe 3+-rich forsterite, and axial carbonate-rich phoscorite and carbonatites contain Fe 2+-Mn-rich forsterite. Incorporation of trivalent iron in the octahedral M1 and M2 sites reduced volume of these polyhedra; while volume of tetrahedral set has not changed. Thus, trivalent iron incorporates into forsterite by schema (3Fe 2+) oct → (2Fe 3+ +) oct that reflects redox conditions of the rock formation resulting in good agreement between compositions of apatite, magnetite, calcite and forsterite.

Three-D Mineralogical Mapping of the Kovdor Phoscorite-Carbonatite Complex, NW Russia: II. Sulfides

“…Increase of mean equivalent circular diameter of pyrrhotite grains reflects a decrease of the rock crystallization rate from the pipe wall to its axis. This trend is similar to that of co-existing rock-forming and accessory minerals, in particular, forsterite, magnetite and baddeleyite [22,26,27].…”

“…The Kovdor phoscorite-carbonatite complex has gradual concentric zonation in terms of the rocks modal and chemical composition, as well as grain size, chemical composition, crystallochemical features and properties of rock-forming and accessory minerals [11,21,22,24,[26][27][28][29]. This zonation mainly results from gradual change in chemical composition of the residual phoscorite melt from silicate-rich to carbonate-rich during crystallization from the pipe margins to its axial zone [22].…”

“…This zonation mainly results from gradual change in chemical composition of the residual phoscorite melt from silicate-rich to carbonate-rich during crystallization from the pipe margins to its axial zone [22].…”

“…mainly results from gradual change in chemical composition of the residual phoscorite melt from silicate-rich to carbonate-rich during crystallization from the pipe margins to its axial zone [22]. Geochemically, nickel behaves like Mg and Fe 2+ , and readily substitutes them in most crystalline phases including olivine [71].…”

Editorial for Special Issue “Arctic Mineral Resources: Science and Technology”

“…The formation mechanisms of zeolite deposits in carbonatites and their geochemical and mineralogical constraints are examined by Zozulya et al [10] using the Breivikbotn deposit (Northern Norway) as an example. Three following papers of the issue [11][12][13] are devoted to 3-D mineralogical mapping of the Kovdor phoscorite-carbonatite complex (Kola peninsula, Russia). On the basis of detailed mineralogical studies (including crystal-structure analysis) of forsterite [11], sulfides [12] and pyrochlore-supergroup minerals [13], the authors construct a 3-D model of the deposit and try to decipher major features of its genesis and evolution.…”