Age and tectonic setting of the Udokan sediment-hosted copper-silver deposit, Transbaikalia, Russia

Abstract

The Udokan Cu–Ag deposit in Transbaikalian Russia is one of the largest individual examples of the sediment-hosted stratiform type. Mineralization is hosted by the Udokan Complex, a ~12,000-m-thick metasedimentary sequence deposited between ~2.2 and 2.06Ga in a large intra- to peri-cratonic basin constructed over Archean basement of the Siberian craton. The basin was inverted and metamorphosed to dominantly greenschist facies at ~1.9Ga, as a consequence of the collision between the Aldan and Stanovoi domains.Titanite is a common constituent of the greenschist metamorphic assemblages of the Udokan Complex as well as being an integral part of the stratiform disseminated and associated veinlet Cu sulfide mineralization at Udokan. Titanite crystals were separated from a sample collected in the Medny area of the deposit where typical, high-grade, disseminated and veinlet chalcocite-bornite mineralization is well exposed. Titanite crystals from the disseminated and veinlet fractions were separately dated by the ID-TIMS U–Pb method, with the disseminated fraction returning a concordia age of 1895.3±9.7Ma and the veinlet fraction a concordia age of 1896.7±7.8Ma. Combination of both titanite fractions produced a concordia age of 1896.2±6.2Ma.The titanite age effectively dates the associated Cu sulfide mineralization. Since there is no evidence that titanite growth is appreciably younger than either the mineralization in the Medny area or the Udokan deposit as a whole, a ~1.9-Ga age for the stratiform Cu mineralization is considered likely. This age is undistinguishable from that of regional metamorphism during the collisional orogeny that led to final amalgamation of the Siberian craton and its incorporation into Columbia. The age also confirms Udokan as the world's oldest sediment-hosted Cu deposit and that its formation coincided with a continental-scale orogeny rather than with diagenesis and extensional basin development ~150m.y. earlier.