Volume 26, Nº 3 (2018)

Numerical modeling of the generation and evolution of parental melts of the komatiite–tholeiite association of the Uraguba structure was carried out using previously obtained geochemical and isotope data. It was established that komatiite, komatiite and tholeiite basalts depleted in LREE and having ε_Nd(Т = 2.79) = +2.9…+3.2 were generated by equilibrium partial melting (F > 15%) of a depleted source (garnet-bearing Ol_0.63 + Opx_0.22 + Cpx_0.06 + Grt_0.09 mantle peridotite) at 4–8 GPa, while the genesis of primary melts of LREE-enriched komatiites (La_N/Sm_N ~ 1.2–1.6) with ε_Nd(Т = 2.79) = +2.5…+2.2 was related to the equilibrium partial melting (F > 20%) of an “enriched mantle peridotite” (EM–Ol_0.60 + Opx_0.20 + Cpx_0.08 + Grt_0.12) at pressure of 2.5–4 GPa. Coexistence in space and time of two types of melting products of mantle peridotites formed at different depths is explained by melting of different parts of adiabatically ascending mantle plume.

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).

Rocks with P-bearing olivine were found in soil samples delivered by the “Luna-20” automated station. They are ascribed to the highland anorthosite–norite (more rarely, gabbro-norite)–troctolite rock series enriched in phosphorus and other incompatible elements, but are not related to typical KREEP rocks enriched in incompatible elements. Their source is presumably of hybrid origin and related to primary high- Mg suite (HMS) rocks. The occurrence of high- and low-Cr populations of P-bearing olivine in different structural rock types can be attributed to the annealing-related more rapid chromium diffusion (relative to that of phosphorus) in olivine from metamorphosed rocks. This assumption is supported by stoichiometric formula calculations of these olivines. An alternative explanation for these olivine populations is their derivation from at least two different sources. Disequilibrium crystallization of the P-bearing olivines, which is confirmed by an intricate phosphorus zoning, excludes the existence of P-rich melts, which is consistent with previous observations. At the same time, olivine fractionation can be responsible for the phosphorus content in lunar melts. The incorporation of phosphorus in olivine of the “Luna-20” anorthosite troctolites is presumably controlled by a coupled substitution mechanism of divalent cations and silicon for phosphorus and chromium in the tetrahedral and octahedral sites (Milman-Barris et al., 2008). Another possible mechanism is the substitution of divalent cations in octahedral sites by phosphorus and chromium, which provides the possible presence of P³⁺.