Petrogenesis and ore mineralization of taxitic gabbros from the Late Visean Khudolaz differentiated complex in the Southern Urals

   Research subject. Small taxitic gabbro intrusions that are widespread within the Khudolaz differentiated complex of the Southern Urals. This study is the first to investigate intrusions located outside the Khudolaz Trough.

   Aim. To reconstruct the petrogenetic conditions of taxitic gabbro formation, to identify the causes of their petrographic heterogeneity, and to determine their ore specialization.

   Methods. The bulk chemical composition of hand-selected samples collected from natural outcrops and drill core was determined by X-ray fluorescence analysis and inductively coupled plasma mass spectrometry. Sr and Nd isotopic compositions were measured by thermal ionization mass spectrometry using the isotope dilution method. Mineral compositions were studied using a scanning electron microscope and microprobes with wavelength-dispersive spectroscopy. Comagmat 3.75 software was used to reconstruct the parental melt, and FC- AFC- FCA and a mixing modeler were used to calculate the degree of crustal contamination.

   Results and conclusions. Based on petrographic characteristics, three types of taxitic gabbros were distinguished: (1) patchy, including schlieren varieties; (2) patchy-banded; and (3) hybrid. These types are virtually similar in mineral composition and bulk geochemistry, indicating a common melt source. The development of taxitic textures was largely associated with the water saturation of the initial melt and the dynamics of its emplacement along faults during strike-slip tectonics. The heterogeneous distribution of melanocratic and leucocratic segregations occurred during repeated melt inputs into a limited-sized intrusive chamber, creating domains with varying degrees of viscosity and crystallization. The bulk geochemical characteristics of the rocks, together with the isotopic composition of Sr (0.70319–0.70396) and Nd (εNd(330) from +4.59 to +6.72), indicate a mantle source of mixed origin between the primitive mantle and the suprasubduction wedge mantle. Calculations showed that the melt underwent up to 7 % contamination by host rocks of the Magnitogorsk island-arc terrane, a level insufficient to distort the crystallization conditions of the primary magma. The reconstruction of the primary melt and hornblende thermobarometry indicate crystallization at temperatures of approximately 1200–900 °C and pressures of 2–5 kbar with an initial water content exceeding 0.5 wt %. The taxitic gabbros of the Khudolaz complex exhibit limited ore potential with a specialization in Ti–Fe mineralization. This contrasts with the earlier homogeneous ultramafic–mafic rocks of the complex, which are prospective for sulfide Cu–Ni–PGE mineralization.

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