Uchaly is a giant zinc and copper ore field in the southern Urals, within the Bashkyrtostan Republic of the Russian Federation: it lies about 115 km NNE of Magnitogorsk. First discovered in 1939, the (Old) Uchalinskoe orebody (Probable+Reserves of 113.6 mt ore) possesed a gossan with some expression at surface. Initially mined for residual gold and disaggregated barite "sand", open pit mining of hypogene ore started in 1958, followed by underground mining in 1986. The deeply buried Novo Uchalinskoe orebody (Reserves of 110 mt ore) was discovered in 1986, 1.5 km south of Uchaly and at a depth of 625m; it is currently (2006) not mined. Two other minor deposits (unmined) and several prospects are also known. Ore is processed in a local GOK (Gorno-obogatitel'nyi kombinat) which, while now privatised, has significant Bashkyrtostan Government equity. Copper and zinc concentrates are sent to smelters outside the district, mainly in Mednogorsk.
The Uralides is a complex orogenic system which sits between the East European Platform in the west and Siberian Platform to the east. During its long history, several increasingly mature island arc complexes developed between the Ordovician and Lower Carboniferous. These were accreted to the leading edge of the East European Platform prior to terminal collision of the platforms between the Devonian and Carboniferous. The Main Uralian Fault, a mélange zone containing slices of ophiolites and arc lithologies, marks the line of the suture. In the southern Urals, the Middle to Upper Devonian Magnitogorsk Arc (also called "Trough" or Metallogenic Belt) represents an outstanding metallogene credited with a pre-mining resource of 46 Mt of copper plus zinc. It includes at least four giant VMS clusters (viz Uchaly, Uzel'ga, Buribay and Gai), and at least fourty additional significant deposits. Approximately ten deposits are presently (2006) being mined, with about the same number awaiting development. The more recent discoveries are blind, concealed by up to 600 m of rock.
The early Devonian (Emsian) tholeiitic and boninitic assemblages evolved into: (1) younger tholeiitic complexes, followed by, (2) bimodal calc-alkaline complexes and, finally, (3) sequentially differentiated and mostly submarine volcanic complexes associated with submerged shield volcanoes and calderas, draped and fringed by volcaniclastics deposited in a forearc basin. The principal ore type in the Magnitogorsk Zone is volcanic-hosted massive sulphide (VMS). Deposits reflect the maturation trend of parent volcanic complexes, from simple but economically insignificant iron-copper (zinc, cobalt) "Cyprus-type" ores associated with basalts, through dominant "Urals-type" zinc + copper deposits related to the felsic component of bimodal suites, to the lead-zinc > copper "kuroko-type" in the most evolved calc-alkaline volcanics. In the southern Urals, more than 40 significant VMS deposits are congregated into the several "ore bearing zones" of Seravkin (2006); continuous stripes of coalescing volcanic edifices flanked by turbidites and partly interrupted by (relatively minor) syn- to early post-orogenic granitoids. The Uchaly ore field is included in the most northern, north-south trending, Uchaly-Aleksandrinsky ore-bearing zone.
The Uchaly ore field comprises two giant adjacent orebodies (the "Old" and Novo Uchalinskoe fields). Dominant in the field is the Middle Devonian (Eifelian) Karamalytash volcanic suite, the ore host, which is overlain by the younger (Givetian) Ulutau Suite. Both suites are intruded by several generations of gabbro to gabbrodiorite, of which the younger syn- to post-orogenic generation is of Carboniferous age, post-dating ore fornmation. The Karamalytash volcanic suite is bimodal, containing units of rhyolite and rhyodacite lavas, pyroclastic and volcaniclastic rocks enclosed by basalt to basaltic andesite units. The stratabound, lenticular, VMS orebodies sit on top of rhyodacite and rhyolite lavas and pyroclastics, partly extending into overlying volcanic fragmentals and volcano-chemical sediments which include "chert". Mean massive sulphide grades are 0.97% Cu, 3.8% Zn, 0.5% Pb, ~0.1% As, 150 ppm Cd, 70 ppm Te, 50 ppm Se, 14 ppm In, and at least 20 ppm Hg. Some of the minor metals are recovered. The fractured and brecciated host volcanics under the massive ore lens are pervasively silicified (up to "secondary quartzite") and sericite-altered. This style of alteration grades into chloritisation. The altered rocks are host to the usually encountered stringer-stockwork zone which is dominated by copper, carrying 0.6% Cu and 1.6% Zn. It grades into more sparsely disseminated mineralisation, a portion of which is sub-economic (0.39% Cu, 0.9% Zn) and not included in quoted reserves.
The (Old) Uchalinskoe orebody exhibits well-developed zonality of ore styles and mineralogy, matching the general VMS model. Uppermost are relatively minor clastic ores that formed on the sea floor palaeoslope; they contain sulphide and barite clasts in volcaniclastic breccias, conglomerates and arenites. At the top of the in-situ orebody is found brecciated metacolloform pyrite with lesser chalcopyrite, sphalerite and some galena, which grades downwards into thin-banded massive chalcopyrite, sphalerite, pyrite; massive chalcopyrite-pyrite; and bedded pyrite. Ultimately, this is underlain by altered felsics which host the chalcopyrite-pyrite footwall stockwork and breccias.
(Source: Peter Laznicka, 2006)
