Evolution of Ore-Forming Metasomatic Processes at Large Skarn Iron Deposits Related to the Traps of the Siberian Platform

The paper presents systematized and synthesized data on the parameters and evolutionary sequence of metasomatic processes that accompanied interaction between Permian–Triassic trap complex and rocks of the sedimentary cover of the Siberian Platform at the large skarn iron deposits. Relations of the textural–compositional, morphological, and genetic diversity of the skarns and ores with the phases and stages of the origin of ore-bearing volcano-tectonic edifices are demonstrated with reference to the Korshunovskoe and Rudnogorskoe deposits. The genetic reconstructions are based on survey materials and data on the mineralogy of the rocks and ores (obtained by optical and scanning electron microscopy, microprobe analysis, EPR, Raman and IR spectroscopy, and by studying inclusions in minerals). A principally important feature of the volcano-tectonic edifices of the large mineral deposits is their multistage evolution and combinations of fluid-conducting zones, which are related to (1) volcanic apparatuses, (2) shallow-depth magmatic chambers (laccoliths) hosted in carbonate–salt rocks, and (3) multistage fracture structures produced by the collapse of the leached space. The major ore-bearing structures were formed simultaneously with the development of an intermediate magmatic chamber hosted in Cambrian carbonate–salt rocks beneath a seal of terrigenous sedimentary rocks. Magmatic-stage magnesian skarns with disseminated ores in them and in the calciphyres were produced during the prograde stage in the apical parts of the laccoliths, at contacts between the dolerites and dolomites. During the early prograde stage, skarn–ore bodies developed around injection bodies of globulated dolerites, laccoliths, and sills; stockworks and steep bodies of fragmentary magnesian and calcic skarns and ores were formed within the diatremes; and conformable bodies and veins were produced in the splay fracture zones. The later reactivation of faults and fractures and the involvement of connate brines and solutions from the evaporite complex triggered the redeposition of the ore masses, crystallization of the mineral assemblages of hydrated skarns, development of large domains of serpentine–chlorite–epidote–amphibole rocks, calcic skarns, and ores. Data on multiphase fluid inclusions in the forsterite, apatite, and halite indicate that the mineral-forming fluid initially was a highly concentrated solution–melt (total salinity of 60%) with high-density reduced gases. The magnesian skarns were formed during the following stages: (1) forsterite + fassaite + spinel + first-population magnetite (820–740°C); (2) phlogopite + titanite + pargasite + second-population magnetite (600–500°C), and (3) clinochlore + serpentine + tremolite + pyrrhotite + chalcopyrite (≥450°C).