Abstract

Phase compositions and microtextures of ore minerals in intrusive traps of the western part of the Siberian Platform have been studied using scanning electron microscopy. Oxide and sulfide solid solutions crystallize at the magmatic stage; their grain and aggregate shapes are determined by the cooling rate of magmatic bodies. We have revealed a gradual transition of oxides from fine-grained texture in the quenching zone, through skeleton, case, and frame forms, to isometric aggregates of mixed crystals in the holocrystalline silicate matrix. Sulfide spheroids (either associated with oxides or separated from them) are changed by dissemination and nests. The chemical compositions of both oxides and sulfides are correlated with the petrochemical types of rocks. Chrome-spinels or chrome-enriched ulvospinels crystallize first in the most magnesian dolerites. Iron and titanium oxides with Mn, V, Mg, and Al impurities prevail in the rest rock varieties. As temperature decreases, ilmenite, ulvospinel, and titanomagnetite crystallize after chrome-spinels. Exsolution structures are very intricate for titanium and iron oxides and depend on the oxidation conditions and on the assemblage of impurities and their quantities. The first exsolution particles of ilmenite are more magnesian, while the following ones are more manganese. Subsolvus exsolution is accompanied by the release of impurities, grain stripping, and rearrangement and natural enrichment of ore material. Conjugate transformation of silicates and ore minerals results in aggregate pseudomorphs and minerals, such as titanite, zircon, and baddeleyite. Nickel-containing sulfides formed at the magmatic stage prevail in more magnesian rocks. Copper minerals are more diverse. These are polymorphic modifications of chalcopyrite and cubanite in ore solid solutions formed at the magmatic stage, chalcopyrite in paragenesis with monoclinic pyrrhotite in zones of hydrothermal metasomatites, and chalcopyrite in solid solutions with bornite and chalcosine and in assemblage with low-temperature sulfides. The obtained data on mineral structures and assemblages can be used as indicators to classify the genesis and formation types of ores.

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