SEDIMENT-HOSTED EXHALATIVE Pb-Zn DEPOSITS - PRODUCTS OF CONTRASTING ENSIALIC RIFTING.

Broken Hill- and Mt Isa-type deposits are regarded as two contrasting fundamental types of Proterozoic submarine exhalative Pb-Zn deposits. Examples of Broken Hill-type (BHD) are Broken Hill and Pegmont (Australia) and examples of Mt Isa-type (MID) are Mt Isa, Lady Loretta, Dugald River, and McArthur River (Australia). A number of deposits are regarded as intermediate between the two end members, examples being Gamsberg and Aggeneys (South Africa). Franklin and Sterling (USA), deposits of the Bergslagen area (Sweden) and the Rampura-Agucka deposit (India). BHD characteristically occur in high metamorphic grade, Early-Middle Proterozoic, deep water, distal, turbidite-metavolcanic sequences. MID occur in Middle Proterozoic, low metamorphic grade, shallow water, hypersaline, emergent shale-carbonate sequences and Palaeozoic, low metamorphic grade, shallow to deep water, shale-carbonate sequences. Both deposit types form immediately after sudden deepening of an ensialic rift.