Biogenic and abiogenic hydrothermal sulfides: controls of rare metal distribution in black shales (Yukon Territories, Canada)

Abstract

The black shales of the Selwyn basin contain relicts of worm tubes which are sulfurized and silificified. Nuclear microprobestudies on biogenic Fe Ni sulfides indicate the presence of several tenths to thousands ppm of Cu, Zn, Se, Mo, As, Sb and Tl. Several tenths of ppm of Ag. Cd and In were observed in hydrothermal sphalerite, and Pt and An of several hundreds of ppm occur as microsize inclusions in framboidal pyrite, rather than being incorporated in the sulfide structures. Mineralogical and chemical investigations prove the presence of hydrothermal fluids of acidic composition, pH of 8–8.5 and temperatures of about 260 °C which carried at least part of the metals. Nuclear reaction analyses indicate the presence of nitrogen mainly in hydrothermal feldspar.