Syntectonic discordant copper mineralization in the Hilton Mine, Mount Isa

Abstract

The stratiform Hilton Zn-Pb-Ag deposit contains significant amounts of subeconomic copper mineralization. The chalcopyrite-rich zone is localized in a structural thickening of stratiform orebodies, spatially associated with the Hangingwall fault, a steep syn- to post-D 2 fault. Chalcopyrite mineralization occurs in syndeformational sulfide breccias, syntectonic veins, and massive and disseminated replacement of dolomitic and pyritic host rocks. This suggests that discordant copper mineralization in the Hilton area is a syntectonic overprint of preexisting Zn-Pb-Ag mineralization. Relationships with folds and cleavage indicate a late D 2 to post-D 3 deformation timing for copper mineralization. Key timing relationships include (1) veins formed parallel to S 2 and S 3 , and partially overprinted by those cleavages, (2) veins formed parallel to S 2 and S 3 with alteration zones which display a variably developed cleavage, (3) chalcopyrite-bearing saddle reef veins associated with F 2 and F 3 folds, and (4) copper-dominant mineralization in a syn-D 2 dike intruding the Hilton mine. Unlike Zn-Pb-Ag mineralization hosted in late-stage structures, there is no obvious stratiform, laminated precursor to copper mineralization, suggesting that copper was introduced rather than remobilized during deformation.The primary large-scale focus for syntectonic mineralizing fluids was a zone of intense veining at the intersection between the Paroo fault zone and the Hangingwall fault. Structural focusing of fluid flow through preexisting orebodies was related to strain incompatibility between host rocks and sulfide-rich layers. The restriction of copper mineralization to areas of preexisting Zn-Pb-Ag mineralization suggests an additional strong chemical control of preexisting sulfides on the localization of syndeformational copper mineralization.Syntectonic Cu mineralization in the Hilton area highlights the roles of dilation related to displacement at fault intersections and to competency contrasts in the focusing of migrating metal-bearing fluids. Copper mineralization at Hilton and Mount Isa share a similar timing, but it would appear that discordant mineralization at Hilton may represent the outer portions of a Mount Isa-type Cu mineralizing system.