First data on the chemical composition and morphology of gold in the Tuba Kain deposit (South Urals)

Abstract

926 The Tuba Kain deposit discovered in the early 20th century is located in the southwestern part of the Eastern Baymak ore-bearing zone confined to the western slope of the Magnitogorsk megastructure of the Southern Urals (Fig. 1). The ore field is composed of volcanogenic and volcano-sedimentary rocks of the Devonian Irendyk, Yarlykap, and Ulutau formations intruded by subvolcanic bodies of dacite porphyrites [1–3]. The rocks of the Eifelian Irendyk Formation distributed in the northwestern and central parts of the ore field are represented by lapilli and agglomerate tuffs of mixed composition with subordinate intercalations of gravel to psammitic tuffs. The lithological– petrographic composition of rock fragments is highly variable imparting variegated coloration to rocks: quartzites, hematite–quartz varieties, quartz porphyries, amygdaloid basalt porphyrites, and diorites. The overlying cherts of the Upper Eifelian Yarlykap Formation are approximately coeval with the Karamyltash Formation and Bugulygyr Horizon of the eastern areas. The rocks of the Givetian–lower Frasnian Ulutau Formation are developed in the western and southern parts of the ore field. The rock composition is relatively uniform with the dominant share of tuffstones and, less commonly, tuffaceous siltstones. The subvolcanic bodies occurring in the central, northern, and eastern parts of the ore field are represented by massive compact plagioclase and hornblende–plagioclase andesite–dacite and dacite porphyrites. At the beginning of the Tuba Kain deposit exploitation, it represented a submeridional gold–barite vein up to 5 m thick occurring among foliated rocks. The vein became exhausted and the residual ore-bearing complex is now represented by secondary quartzites with tourmaline and chlorite–sericite–quartz metasomatites. Their composition is as follows (wt %): SiO 2, 26.74–58.38; TiO 2, 0.35–0.76; Al 2O3, 8.41– 12.18; F 2O3, 7.28–10.06; MnO, 0.0; MgO, 3.93–4.15; CaO, 0.12–0.16; Na 2O, 2.38–2.55; K 2O, 1.24–2.92; P2O5, 0.02–0.11; S tot , 0.32–5.02; Ba, 1.59–28.05; Zn, 0.0051–0.0218; Cr, 0.0–0.0092; V, 0.0142–0.170; Sr, 0.0023–0.0634; Zr, 0.0076–0.0270. The rocks confined usually to contacts of subvolcanic dacite porphyrites are characterized by the following composition (wt %): SiO 2, 69.91; TiO 2, 0.36; Al 2O3, 14.02; F2O3, 3.84; MnO, 0.05; MgO, 1.87; CaO, 1.54; Na 2O, 4.87; K 2O, 0.09; P 2O5, 0.08; S tot , 0.01; Ba, 00233; Zn, 0.0011; Cr, 0.0011; Ni, 0.0085; V, 0.0003; Sr, 0.0306; Zr, 0.0148, Nb, 0.0011; Rb, 0.0010. These rocks are accompanied by tuffs of the Irendyk and Ulutau formations with mixed compositions (wt %): SiO 2, 59.09–67.86; TiO 2, 0.32–0.45; Al 2O3, 11.91–19.23; F2O3, 2.05–5.24; MnO, 0.02–0.13; MgO, 2.19–5.39; CaO, 0.11–2.71; Na 2O, 1.10–4.26; K 2O, 1.80–3.63; P2O5, 0.05–0.10; S tot , 0.06–1.15; Ba, 0.57–9.51; Zn, 0.0069–0.0596; Cr, 0.0015–0.0110; Ni, 0.0005– 0.0032; V, 0.0024–0.0081; Sr, 0.0027–0.0620; Zr, 0.0112–0.0245; Nb, 0.0006–0.0010; Rb, 0.0012–