Geology, mineral chemistry and tourmaline B isotopes of the Corrego Born Sucesso area, southern Serra do Espinhaco, Minas Gerais, Brazil: Implications for Au-Pd-Pt exploration in quartzitic terrain

The Palaeo-Mesoproterozoic metasiliciclastic rocks of the southern Serra do Espinhaco, Minas Gerais, Brazil, are host to historically important alluvial deposits of diamonds and gold. Detrital gold grains often comprise Au-Pd-Pt intermetallic compounds, with low Ag contents, which contain inclusions of tourmaline and titaniferous hematite (up to similar to 6 wt.% TiO2). The latter minerals connect the alluvial mineralisation to the rutile-hematite-quartz veins and tourmalinisation observed in the quartzitic country rocks of the alluvial gravel. The quartzite (Sopa-Brumadinho Formation of lacustrine to fan-deltaic origin) is affected by pervasive B metasomatism with F-bearing tourmaline replacing the recrystallised quartz fabric. The tourmaline belongs to the alkali group, with Mg/(Mg + Fe) and X/(X + Na) ratios in the ranges from 0.5 to 0.7 and 0.18 to 0.29, respectively, where X represents vacancies in the X site. Boron-isotopic values of tourmaline vary from similar to 1 to -10.4%,, delta B-11. The B-isotope range, in conjunction with the Na-Mg-rich tourmaline composition, and the widespread occurrence of tourmalinite in the Sopa-Brumadinho Formation suggest a derivation from non-marine evaporitic brines. Brines are capable of transporting otherwise immobile Ti and explain, under oxidising conditions, the fractionation of Ag from Pd to precipitate palladiferous gold with extremely high Pd/Ag ratios. Zirconium-in-rutile and Ti-in-quartz temperatures for a variety of hematite-rich veins suggest episodic vein emplacement over a temperature range from around 500 degrees C to similar to 350 degrees C. Cross-cutting relationships and episodic vein emplacement indicate a late-Brasiliano age. (C) 2011 Elsevier B.V. All rights reserved.