The Blendevale Ore-Body

Abstract The Blendevale deposit is hosted by the platform facies of a Devonian (Givetian to Frasnian) reefal atoll in Pillara Limestone. It contains «20Mt of ore with an average grade of 8% Zn, 2.5% Pb and 17g/t Ag and is the largest of the Zn-Pb deposits on the Lennard Shelf (Murphy 1990). At Blendevale, mineralization occurs in the limestones along several north- to northnortheast-trending normal faults which form a small graben (Fig. 5.1). The Blendevale ore-body is hosted in the lower section of the Pillara stratigraphy by the platform facies Unit 4 Middle Stromatoporoid Limestone, the Unit 5 Upper Fenestral Limestone and the Unit 6 Upper Stromatoporoid Limestone. In addition to the platform facies limestones, marginal slope and reef subfacies limestones crop out to the west of Blendevale. The area has been extensively drilled, and as such provides the ideal opportunity to test structural hypotheses made from surface observations, with factual sub-surface data. The two main faults (Figs 5.1 and 5.2; Western Fault and Eastern Fault) hosting the majority of the reserves, dip towards each other at 60° to 70°, and have vertical displacements of stratigraphic marker horizons between 40 and 200m. The majority of sulfide mineralization occurs in breccias consisting of matrix-supported clasts of limestone with colloform sphalerite, minor galena and radiaxial calcite, and symmetrically zoned veins of marcasite, colloform to massive sphalerite with galena and spar calcite. Multiple brecciation is common with fragments coated with new mineralization, indicating that mineralization was in part contemporaneous with faulting. Mineralization and brecciation occurred after early diagenesis and stylolitization but prior to most porosity occlusion by spar calcite (Vearncombe et al., in press). Sphalerite fluid inclusion homogenization temperatures of 70 to 110X and salinities over 20wt% equivalent NaCI as well as hydrocarbons have been reported (Etminan and Hoffmann, 1989), typical of many MVTs (Roedder, 1984).