The Elura Ag-Pb-Zn mine in Australia; ore genesis in a slate belt by syndeformational metasomatism along hydrothermal fluid conduits

Abstract

The Ag-Pb-Zn ore deposit at the Elura mine, New South Wales, Australia, is situated in a Devonian slate belt and essentially consists of two vertically elongated, ellipsoidal bodies of massive sulfide. These are located above a zone of subeconomic vein mineralization. From the boundary of each orebody toward its core, the host sediment is progressively incorporated (as inclusions) in a deformed matrix of massive sulfide, but continuity of bedding attitudes is retained (across separate inclusions). Preserved bedding patterns and the sulfide deformation show that each orebody formed by metasomatism in the hinge of a developing anticline. The two anticlines are doubly plunging structures with superposed smaller folds, in which four generations of fold axial plane cleavages (S 1 -S 4 ) represent polyphase deformation (D 1 -D 4 ).The sulfide distribution in bedding-parallel outliers of the orebodies is controlled by the S 2 cleavage, but the style of D 2 folding within such horizons has been influenced by the presence of sulfide. Also, there are mesoscopic replicas of the orebodies with a shape and orientation that reflect the vertically elongated D 2 strain ellipsoid. Most replicas lie across hinges of D 2 folds, but locally their presence has affected the fold pattern. These, and other, complex relationships are explained as a consequence of relatively rapid metasomatism by sulfide during D 2 deformation.Synkinematic (syn-D 2 ) porphyroblasts of siderite mark an alteration halo around the ore. Alteration patterns indicate a coeval origin for the ore and its halo, which is further marked by vein-hosted breccias within a swarm of tension veins. Crack-seal microstructures in the latter indicate repeated microcracking during vertical stretching of the dome and basin folds at lithostatic pore fluid pressures. Structural relationships between the crack-seal veins and breccia veins are complex but suggest periodic release of the overpressured fluids by brecciation of sediment. The drops in fluid pressure triggered metasomatism by carbonates and sulfides in the two largest fold domes. The brecciation is attributed to dome collapse, following dilational rupture due to faulting in a system defined by shear zones and domains of bedding disruption.