Platinum mineralization of the Svetly Bor and Nizhny Tagil intrusions, Ural Platinum Belt

This paper presents the results of a detailed mineralogical and micro analytical study of two Ural-Alaskan type intrusions in the Ural Platinum Belt: (1) the dunite-hosted mineralization of the Svetly Bor intrusion, and (2) the chromitite mineralization of the Nizhny Tagil intrusion. Two generations of platinum minerals are typical of both intrusions: magmatic Pt-Fe(Ni) alloys, and post-magmatic Pt(Fe,Ni,Cu) alloys. A trend from ferroan platinum to isoferroplatinum (Pt,Fe -> Pt3Fe) is shown for magmatic alloys of the Svetly Bor intrusion. Magmatic alloys of the Nizhny Tagil intrusion are represented by ferroan platinum (with Ni) only, varying in Fe. The magmatic Pt-Fe alloys of both intrusions were depleted in Fe during the evolution of ore-forming systems and crystallized during the entire magmatic process, generally as fine cubic crystals and anhedral grains hosted by dunite at the magmatic stage during and after the crystallization of dunite (platinum-dunite type of the Svetly Bor intrusion). The evolution of mineral paragenesis was accompanied by a temperature drop, as well as increases in fO(2) and fS(2). Most of the platinum was concentrated in the residual melts together with chromium, and crystallized in the final stage of the magmatic process (platinum-chromite ore of the Nizhny Tagil intrusion). Post-magmatic Pt(Fe,Cu, Ni) alloys formed during the serpentinization of dunite evolved according to a general compositional trend from tetraferroplatinum and ferronickelplatinum to tulameenite and later to Pt-Cu alloy and Pt-oxide (PtFe) -> Pt(Fe,Cu,Ni) -> Cu3Pt -> Pt-O. Platinum-rich mineralization of both intrusions was formed in the late magmatic stage from a melt rich in volatiles, regardless of their location in dunite or chromitites. Most probably, the rock structure (fracture systems) was the determining factor in the migration of PGE-rich residual melts and ore accumulation. (C) 2014 Elsevier B.V. All rights reserved.