Spatial and temporal distribution of gold deposits in the urals

During the past 10 years the Paleozoic Urals mountain belt has been the sub ject of internationally based, deep seismic, and ancillary geologic studies aimed at improving our knowledge of the lithospheric evolution of the orog enic belt, in general, and its prominent crustal root, in particular. In vi ew of its wide-ranging mineral endowment, especially for gold, this work al so provides important keys to help clarify relationships between collisiona l tectonic processes and gold mineralization. Following an outline of curre nt ideas oil the tectonic evolution of the Urals, we present an overview of the distribution of the majority of known gold deposits. This information, together with the available isotopic and geologic age data of associated a lteration assemblages, is compared to available geochronological data for v arious magmatic and deformation episodes. Spatial and temporal relationship s suggest that hydrothermal mineralizing processes and the geodynamic evolu tion of the orogen are linked. In an attempt to analyze the deep structural framework of the orogen, down to the mantle, we have also examined a numbe r of subcontinental- and regional-scale continuous geophysical datasets. The Urals have been subdivided traditionally into a series of north-south-s triking tectonic zones but more recently into three principal tectono-magma tic sectors, which comprise a Suture sector along the Main Uralian fault zo ne and two sectors with tectonically imbricated island-are, transitional (v iewed as active continental margin), and continental zones. Gold deposits a re found in all tectono-magmatic sectors of the orogen. Minor gold occurs i n Siluro-Devonian volcanic-hosted massive sulfide deposits. magnetite(-copp er) skarns. and porphyry copper deposits. In a number of cases, younger str uctures (shear zones, faults, and their intersections) have controlled the deposition of higher gold grades. In the Urals, this has led to the concept of progressive concentration of gold during later tectonic overprinting, a nd the understanding that regional deformation and hydrothermal fluid-rock interaction in the Late Carboniferous to Early Triassic upgraded gold conte nts of earlier deposits. During the Late Carboniferous to Early Triassic, g old-bearing quartz vein lodes, which effectively encompass the majority of the larger gold deposits in the Urals, were also formed in structural-chemi cal traps. Some deposits, such as Mindyak that is hosted by tectonic melang es along the Main Uralian fault zone, are clearly typical late-orogenic lod e gold deposits. Others occur within and along the margins of early to midd le Carboniferous older granite and, although local orthomagmatic relationsh ips have been described, most recent observations now favor a strong struct ural control on the lodes and no direct genetic association with granite in trusion. In the two largest gold deposits, Kochkar and Berezovskoe, gold mi neralization was controlled by structural and combined structural-chemical traps in dilational jogs in both shear and contact zones. Locally, gold min eralization has been shown to occur during a change from orthogonal to tran spressional compression. The inferred ages of the lode gold deposits suggest they are mostly coeval with the generally undeformed Permian-Early Triassic younger granites. Howe ver, a spatial relationship with gold mineralization has not been observed. The younger granites were formed in an extensional regime following Ural-w ide magma generation, caused by a thermal flux attributed to underplating o f the crust by mantle-derived mafic magmas. The widespread distribution of this late thermal event, during changes in stress regime, and the involveme nt of magma-generating mantle processes suggest regional delamination in th e lithosphere and concomitant upwelling of the asthenosphere. These process es may have largely occurred prior to the formation of the prominent crusta l root of the Urals, which formed during subsequent transpressional converg ence. Another, Triassic, Ural-wide phase of hydrothermal alteration has bee n recognized, but associated gold deposits have been documented in only two cases. We tentatively attribute this phase to destabilization of the litho sphere of the wider Altaids, reflected by the formation of the West Siberia n and Central Asian basins, east of the Urals, during the Mesozoic. The integrated analysis of magnetic, Bouguer gravity, heat flow, and crusta l thickness data suggests that a major indenter of the East European craton is responsible for a singular, transcrustal, sidewall ramp in the subsurfa ce of the middle Urals. This ramp may extend into the mantle and formed the principal conduit zone for mineralizing fluids. The world-class Kochkar an d Berezovskoe gold deposits are adjacent to this proposed conduit.