Abstract

The Mount Isa base metal deposits show a definite spatial separation between stratiform Ag-Pb-Zn lenses, generally regarded as syngenetic, and Cu ores within a recrystallized and brecciated host lithology, the silica dolomite. Microstructural evidence indicates that the Cu orebodies and the silica dolomite formed during early syndeformational (syn-D 3 ) replacement processes in a dolomitic shale sequence. Three stages of alteration are distinguished: (1) dolomite recrystallization-silicification, (2) dolomitization, and (3) chalcopyrite + or - Fe sulfide + or - quartz + or - chlorite deposition. Structural timing constraints suggest a post-D 2 to pre-S 3 interval for the first stage, whereas the last two stages occurred during (early) D 3 . The formation of the silica dolomite with the Cu orebodies is separated in time from the adjacent syngenetic, stratiform Ag-Pb-Zn ores by a period including at least two regional deformation events.

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