Massive sulfide deposits in the south Urals: Geological setting within the framework of the Uralide orogen

Abstract

The south Urals is host to more than 80 Paleozoic volcanic-hosted massive sulfide (VMS) deposits developed in four distinct metallogenic zones. From west to east these are: the Sakmara zone, Main Uralian fault zone, and the east and west Magnitogorsk zones. In the Sakmara zone, the chemistry of host volcanic suites is consistent with development of the zone in a Silurian oceanic arc. The Main Uralian fault marks a line of paleosubduction and contains VMS deposits similar to those formed in modern mid-ocean ridge settings. The Magnitogorsk zones contain VMS deposits formed in a Devonian fore-arc, arc and inter-arc or proto-back arc setting. The earliest volcanics of the Magnitogorsk zone, the Baimak-Buribai formation, form a boninitic fore-arc sequence, evolving later to more calc-alkalic volcanics with evidence for a contribution from subducted slab to the volcanics. Later, and farther east of the subduction suture, a rifted, more mature arc setting formed where the Karamalytash formation volcanics developed in an inter-arc or proto-back arc setting. The Karamalytash formation shows little evidence of contribution from subducted sediment to the melt. Stratigraphically overlying the Baimak-Buribai formation, and partly time equivalent to the Karamalytash formation, is the Irendyk formation. The Irendyk formation is VMS-poor, but contains abundant epiclastic volcanosediments and epithermal-like gold-barite deposits, indicative of shallower sea conditions. The Irendyk formation appears to form a long linear geographic feature, perhaps marking the line of an emerging arc sequence behind which the Karamalytash formation developed in a rift. Previous authors suggest that the west and eastern Magnitogorsk zones developed as separate arcs, but the arc-like volcanics in the east Magnitogorsk zone may simply indicate the migration of the volcanic arc eastwards as the East European craton approached the Main Uralian fault.