Trace-element budget of the Que River shale: Exploration implications of hanging-wall modification of sulfide minerals at the Hellyer deposit, Tasmania

Category Petrology
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Layton-Matthews, Daniel۱; Gemmell, Bruce۲; Large, Ross۲; Peter, Jan۳
Holding Date 07 October 2008

A project was undertaken to examine pyrite trace element compositional variations in the hanging wall altered black shales from the Hellyer and Que River volcanogenic massive sulphide (VMS) deposits, Tasmania. This research was aimed at investigating the trace element addition/depletion to pyrite during hydrothermal fluid interaction with post-ore, seafloor sediments in the hanging wall. Field observations of the immediate hanging wall, comprising pillowed-lava sequences (PLS) overlain by a thick succession of argillite (Que River Shale - QRS), indicate an increase in the total pyrite content with proximity to the Hellyer and Que River ore bodies and a decrease in pyrite abundance stratigraphically up-hole from the PLS-QRS contact. Ten diamond drill holes were sampled from distal through proximal intersections of the QRS. Ninety-two samples were taken from the QRS and from the PLS near the QRS-PLS contact. Pyrite morphologies were classified into three broad categories based on host lithologies, bed-planar fabrics and grain shapes, including: 1) primary sedimentary, 2) primary igneous, and 3) secondary hydrothermal pyrite groupings. 330 LA-ICP-MS pyrite analyses indicate an order of decreasing trace element abundance of: Ni, As, Mn, Pb > Co, Sb, Zn, Se, Ba, Ti, Mo > Ag, Cr, Cd, Tl, Te, Sn, V > Bi, Zr, La, Au > Th, W, U, and Pt. The time-integrated signal of LA-ICP-MS data allowed the recognition of mineral inclusions from elements incorporated into the pyrite mineral lattice as either solid-solution or micro-inclusions. Ni, As, Co, Mn, Co, Sb, Se and Bi all display smooth profiles indicative of pyrite lattice substitution, whereas Cu, Pb, Zn, Mo, Te, Cd, Ag, and Au have both smooth profiles indicative of pyrite lattice substitution and spike-like profiles indicative of the presence of mineral inclusions. Ba, Ti, V, Cr, Tl, Zr, Th, W, U, and Pt all have spike-like profiles indicative of mineral inclusions.
There is an increase in the total trace element content and an increase in base-metal mineral inclusions in pyrite with proximity to the Hellyer and Que River ore bodies. The late recrystallization and growth of pyrite related to the passage of late fluids is reflected in a decrease in the total trace element contents of pyrite and a shift in the δS34 from +24 per mil in the early pyrite up to +50 per mil in late pyrite.