Abstract

A comparison of three stratiform lead-zinc-silver deposits from northern Australia, at Mount Isa, Hilton, and McArthur River, reveals significant differences between them, but many critical features in common. As suggested many times in the past, they appear to share a common origin. This common origin, however, seems not to be that of the current paradigm whereby they form integral components of the sequences in which they occur. Instead, all are better interpreted as replacement deposits, formed during an east-west-shortening event, which is at least the second deformation event that each sequence underwent. The McArthur River deposit is distinctly different from the others, in that sphalerite and its included galena has directly replaced laminated bituminous bands. In contrast, sphalerite and galena in the other two deposits have replaced either bedding-parallel or breccia-matrix metasomatic alteration minerals, mostly dolomite.All deposits occur adjacent to major reverse faults rather than, as commonly supposed, normal growth faults. They are associated with major carbonate alteration systems that are dolomitic in the immediate deposit environment. The degree of deformation of the host rocks associated with each deposit is variable, with McArthur River being in the least deformed sequence, Mount Isa significantly more so, and Hilton associated with a quite deformed sequence of a different style.The interpretation that these deposits share a similar origin, in spite of significant differences in many characteristics, reinforces the necessity to reexamine other stratiform deposits around the world to see whether they are integral components of the sediment, or whether they may also be of late replacive origin.

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