Deformation of host rocks and stratiform mineralization in the Hilton Mine area, Mt Isa

Abstract

The Hilton deposit is a deformed and metamorphosed Proterozoic stratiform Pb‐Zn‐Ag‐Cu deposit hosted by dolomitic and carbonaceous sediments of the Urquhart Shale of the Mt Isa Group. Rocks in the Hilton area show a history of folding and faulting which spans the time range recognized elsewhere in the Western Succession of the Mt Isa Inlier, though the effects of relatively late and brittle deformation are more pronounced in the Hilton area. The Hilton area shows intense faulting relative to similar rocks to the south in the Mt Isa‐Hilton belt. Faulting in the Hilton area has generally resulted in east‐west shortening and extension in both north‐south and vertical directions. This relatively intense late strain is attributed to the geometry of the Paroo Fault Zone, a major north‐trending zone that bounds the Hilton area to the west, and the Sybella Batholith, which formed a relatively rigid indenter during late deformation in the Hilton area. The structural history of the Hilton area is broadly consistent with ongoing east‐west shortening during progressive uplift from mainly ductile to more brittle conditions. Based on these observations, thinning of the Mt Isa Group which was previously attributed to synsedimentary faulting, can now be shown to be related to heterogeneous strain during late faulting. Sulphide layers show a history of folding which is similar to that of the surrounding rocks. Pyrite which is paragenetically associated with mineralization is overprinted by a bedding‐parallel foliation which predates all other structures in the area. This suggests that stratiform sulphide mineralization in the Hilton area predates deformation. Deformation has affected the Hilton orebodies at all scales. Changes in thickness and ‘fault windows’ in the orebody interval occur on the scale of the entire deposit. Mesoscopic ore thickness changes are often clearly related to extensional and contractional structures within sulphide layers. These macroscopic and mesoscopic ore‐thickness variations are spatially associated with cross‐cutting brittle faults, suggesting that strain incompatibility between brittle host rocks and more ductile ore layers played a major role in the present geometry and thickness of sulphide ores at Hilton.