Geological and Atmospheric Sciences
Journal or Book Title
Ore Geology Reviews
Broken Hill Hill-type Pb-Zn-Ag (BHT) deposits constitute some of the largest and highest grade ore deposits in the world. The Broken Hill deposit, Australia, is the largest accumulation of Pb, Zn, and Ag on Earth and the Cannington deposit is among the largest silver deposits. Characteristic features of BHT deposits include: 1. High Pb + Zn + Ag values; 2. Metamorphism from upper greenschist to granulite facies; 3. Sulfides in Paleoprotorozoic oxidized clastic metasedimentary host rocks in the absence of metamorphosed carbonates/calcsilicate horizons; 4. Sulfides that are spatially associated with bimodal (felsic and mafic) intrusive and volcanic rocks, and stratabound gahnite- and garnet-bearing rocks and iron formations, 5. Stacked orebodies with characteristic Pb:Zn: Ag ratios and skarn-like (generally pyroxenoid dominated) Fe-Mn-Ca-F gangue assemblages, and the presence of Cu, Au, Bi, As, and Sb; 6. Sulfur-poor assemblages, 7. A spatial relationship to continental rifts. 8. A spatial association with alteration zones despite previous proposals to the contrary, and 9. Sulfur isotope compositions of sulfides generally centered around zero per mil.
Previous classification schemes of BHT deposits have included deposits that are hosted in marbles/calcsilicates, however this is inappropriate because the type example of this class of deposit (Broken Hill) is not hosted in marble or calc-silicate rocks. Multiple deposits have previously been proposed in the literature as BHT deposits but of the larger deposits (i.e., ~2 Mt or greater), only deposits in the Diamantina Orogen in eastern Australia, which includes those in the three major Proterozoic provinces (Curnamona, Georgetown Inlier, Eastern Succession of the Mt. Isa Inler) are considered to be BHT deposits (Broken Hill, Cannington, Mount Misery (Chloe and Jackson), Pegmont, and Pinnacles). Within the Curnamona Province, which hosts hundreds of minor BHT deposits, there are some occurrences that resemble BHT deposits but can be sub-classified into Parnell- and Corruga-types. Several deposits that have previously been referred to as BHT deposits are instead metamorphosed clastic sedimentary exhalative (SEDEX), marble/calc-silicate-hosted SEDEX, Bergslagen-type bedded, stratiform Zn-Pb-Ag rich mineralization hosted in rhyolitic ash-siltstone (SAS-type)/stratabound volcanicassociated, limestone-skarn (SVALS-type), or are transitional deposits between BHT and these metamorphosed classes of deposits. Although the non-sulfide zinc deposits at Franklin and Sterling Hill, New Jersey, have been considered in the past as possible non-sulfide versions of BHT deposits they do have some affinities to BHT deposits but are essentially Pb-, Ag- and S-free carbonate-hosted SEDEX deposits. In addition, the Menninnie Pb- Zn deposit, South Australia, is not a BHT deposit since mineralization formed post-peak metamorphism.
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Spry, Paul G. and Teale, Graham S., "A classification of Broken Hill-type deposits: A critical review" (2021). Geological and Atmospheric Sciences Publications. 352.
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