DIVERSE VHMS DEPOSITS IN THE PALEOZOIC OF THE SOUTH URALS, A REFLECTION OF DIVERSE SETTINGS AND SEAFLOOR EVOLUTION

The south Urals hosts more than 80 Paleozoic volcanic-hosted massive sulfide (VHMS) deposits classified by Russian authors into Cyprus-type, Urals type, Baimak-type, Besshi-type and ‘Atlantic’-type. Four distinct geographic zones host the deposits which are from west to east respectively: the Sakmara, Main Urals Fault and the East and West Magnitogorsk zones. Sakmara zone volcanics probably formed in an early oceanic arc west of the Main Urals Fault during Silurian times. The Main Urals fault marks the collision between the East European craton and oceanic arc sequences to the east and contains VHMS deposits in slivers of fore-arc rocks. The Magnitogorsk zones contain VHMS deposits formed in Devonian fore-arc, arc and inter-arc or proto-back arc settings. The earliest volcanics are boninitic fore-arc rocks, evolving later to more calc-alkalic volcanism. Later, farther east from the subduction suture, a rifted, more mature arc setting formed where volcanics developed in an inter-arc or proto-back arc setting. Between these sequences, the arc rocks are largely free of large VHMS deposits, indicating the development of a trench-parallel shallow or even emergent arc, unfavourable for VHMS development. Mafic hosted Cyprus and Atlantic VHMS types are related to fore-arc settings whilst the Baimak and Urals type deposits are hosted in arc and inter-arc settings respectively. Analysis of the distribution of deposits in the Magnitogorsk arc together supports a relatively simple model of oceanic arc architecture over an eastwards subducting ocean during the mid Devonian. Many of the Urals deposits yield seafloor chimney and biota fragments, key evidence for formation on the open seafloor. Delicately laminated and fine-grained sulfide textures are also found, interpreted in terms of post precipitation reworking of seafloor systems. The Urals deposits range from mound-like sulfide bodies, of low length to thickness ratios to extensive sheet-like bodies of high length to thickness ratio. This range in morphology can be shown in many cases to relate to processes after primary seafloor hydrothermal activity. Such features reflect the degree of seafloor degradation and reworking of sulfides on the seafloor coupled with pre-diagenetic chemical changes occurring in the clastic sulfide layers.