Structural and Stratigraphic controls on mineralization at the George Fisher Zn-Pb-Ag deposit, Northwest Queensland, Australia

Sediment-hosted stratiform Zn-Pb-Ag deposits have been interpreted as forming during exhalative hydrothermal activity synchronous with the deposition of host rocks and during either syn-diagenetic or syn-metamorphic veining and replacement of the hostrocks later in their orogenic history. This study analyzes the distribution metal and oretypes at the George Fisher deposit and investigates spatial and temporal relationships between high-grade mineralization and structures at all scales throughout the deformation history. The study aims to determine whether remobilization and upgrading of synsedimentary and/or syn-diagenetic proto-ore or primary syntectonic mineralization are involved in the formation of economic mineralization. Analysis of the kinematic controls on any Zn-Pb-Ag mineralization which is structurally controlled may enable other prospective structures in the near-mine region to be identified. The George Fisher Zn-Pb-Ag deposit is located 22km north of Mt Isa in Queensland, Australia; and is hosted by Proterozoic sedimentary rocks of similar age and lithology to the Mt Isa and adjacent Hilton Zn-Pb-Ag deposits. The host-rocks at George Fisher preserve a structural history comprising four distinct ductile deformations with concomitant faulting and younger brittle faults which cut the ore-bearing sequence. The first, D1 is manifest as an open fold (F1) with an approximately east-west striking axial plane and was overprinted by F2 folding and a pervasive slaty/solution cleavage (S2) during the regionally extensive D2 episode. An episode of sub-vertical shortening/subhorizontal extension (D3) followed this main phase of sub-horizontal shortening which formed folds with sub-horizontal axial planes and gently-dipping crenulations of the S2 cleavage and bedding. D3 features are overprinted by sub-vertical crenulations (S4) and minor folds (F4) with sub-vertical axial planes. Ore shoots that comprise high-grade and thicker mineralization plunge parallel to the F1 fold axes and are largely confined to the short-limb of this fold. Subsidiary ore shoots are coincident with areas of more intense F2 folding, and trend parallel to both F2 and F4 fold axes in longitudinal projection. Vein-hosted sphalerite and medium-grained galena breccia are the main sources of Zn and Pb metal in the deposit, respectively. Logged widths of each form shoots of thicker mineralization which are broadly coincident with high-grade shoots defined by the assaydata. Sulphide textural studies and vein - host-rock fabric cross-cutting relationships suggest that both postdate D2. Empirical relationships between ore shoot geometry and the structural framework of the deposit imply a D4 control on metal distribution. This is supported by the interpretation of a dominantly syn-D4 relative timing of the vein-hosted sphalerite and medium-grained galena breccia based on development of these mineralization-types in unique structural settings and the apparent lack of deformation of their constituent sulphides. Potential pre- F2 mineralization types include some disseminated sphalerite and fine-grained sphaleritegalena breccias which do not currently constitute economic mineralization and account for ca. 10% of the Zn+Pb in the deposit. Remobilization of proto-mineralization is supported by the Zn assay data which indicates that more than one population of Zn grades exists and that a higher grade population is unique to the economic ore-horizons. However, this qualitative observation does not discriminate between upgrading of a pre- F2 or syn-/post-F2 sulphide accumulation during D4. Re-Os isotopic analysis of sphalerite and galena at George Fisher define an isochron whose slope indicates an age of 1423±130Ma indicating closure of the Re-Os system postdates host-rock deposition by ~100-360Ma. A mantle source of Pb and Zn is interpreted from the Re-Os isotopic analysis based on the initial 187Os/188Os ratio of 0.077±0.071. This differs from previous studies of Proterozoic Zn-Pb-Ag deposits which infer scavenging of metal from within the sedimentary basin or from the basement rocks immediately underlying the sedimentary basin. Proximity to a regional fault zone such as the Mount Isa-Paroo Fault system, interpreted to be part of a fault-network linked to a major mid-crustal shear zone, is considered necessary to bring metal-bearing fluids from depth into contact with prospective host-lithologies at George Fisher. At the George Fisher deposit, it is inferred that the F1 fold focussed diagenetic hydrothermal activity and alteration and also acted as a heterogeneity focussing dilation and final sites of mineralization later in the deformation history. The F1 fold may be the upper level expression of a reactivated basement fault thereby accounting for the longevity of the hydrothermal system responsible for alteration and mineralization at George Fisher. It is possible that diagenetic processes prepared the host-rock for later mineralization at George Fisher.