V-NB-W-bearing rutile from Karalveem gold deposit as a potential indicator for ore deposits

The Karalveem gold deposit is part of the Karalveem ore field and is located 18 km west of the Bilibino city (Chukotka Autonomous Area, Russia). Gold mineralization is located in the Triassic sedimentary rocks and gabbro-dolerites. Ores are represented by two types: quartz veins and metasomatic rocks after gabbro-dolerite. The paper presents the results of the study of accessory rutile from both ore types of the Karalveem deposit. It has been determined that the formation of gold-bearing metasomatic rocks and quartz veins took place at the stage of hydrothermal transformation of gabbro-dolerites, with the formation of the ore association of minerals such as arsenopyrite, pyrite, chalcopyrite, rutile, cobaltin, glaucodot, sphalerite, marcasite, native gold and galena. Mineralogical and petrographic studies have shown that the rutile replaces ilmenite (the primary mineral from gabbro-dolerites), forms individual crystals and their assemblages in association with quartz, carbonates (calcite, dolomite) and sulfides (pyrite, chalcopyrite, sphalerite), and fills the interstitials between arsenopyrite grains. Raman spectroscopy has been used to diagnose the crystal structure of rutile of both ore types. The spectra of the studied minerals show distinct peaks at 240, 445, and 611 cm-1, which are similar to those for rutile from the RRUFF database (http://rruff.info). Rutile crystals in both ore types are characterized by a heterogeneous internal structure, which is related to variations in the concentrations of trace elements. A feature of the chemical composition of the rutile in both ore types is the presence of tungsten (up to 5- 8 wt. % WO3), niobium (up to 4 wt. % Nb2O5), iron (up to 3 wt. % FeO) and vanadium (up to 1 wt. % V2O5) The incorporation of W6+ and Nb5+ into the structure of rutile is balanced by the presence of Fe3+ and V3+, and most likely V4+, according to the following heterovalent isomorphism schemes [Urban et al., 1992; Scott, Radford, 2007; Reznitsky et al., 2016]: (1) (Fe, V, Cr, Al)3+ + (Nb, Sb, Ta) 5+ <=> 2Ti4+ (2) (Fe, V, Cr, Al)3+ + W6+ <=> 3Ti4+ (3) V4+ <=> Ti4+ The presence of V4+ in the composition of rutile from the Karalveem ores suggests that hydrothermal solutions were relatively oxidized. The presence of tungsten and niobium is typical for the mineral of gold deposits in the world [Graham, Morris, 1973;Harris, 1989; Urban et al., 1992; Rice et al., 1998; Clark, Williams-Jones, 2004; Scott, Radford, 2007; Mironov et al., 2008; Scott et al., 2011; Agangi et al., 2019]. It was determined that rutile can be used for prospecting and exploration of new gold deposits in northeastern Russia, since the mineral is a typical accessory mineral of the Karalveem ores and always contains the tungsten, niobium and vanadium admixtures.

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