Evidence that Broken Hill-type Pb-Zn deposits are metamorphosed SEDEX deposits
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LETTER
Evidence that Broken Hill-type Pb-Zn deposits are metamorphosed SEDEX deposits Donald F. Sangster 1 Received: 27 March 2018 / Accepted: 10 March 2020 # Springer-Verlag GmbH Germany, part of Springer Nature 2020
Introduction The genesis of Broken Hill-type (BHT) deposits has been an enigma for economic geologists for decades. Modeled after the Broken Hill deposit of New South Wales (Australia), BHT deposits are stratiform base-metal sulfide bodies hosted by clastic and chemical sediments. They are most commonly recognized for their occurrence in high-grade metamorphic terrains, a wide range of unusual metamorphic minerals, and an association with magnetite strata of variable thicknesses and lateral extents (Leach et al. 2005). Other characteristics typical of BHT deposits are their large size, elevated potassium values in the vicinity of sulfide ores, and the complex and varied nature of their chemical sediments. Electronic Supplementary Materials (ESM) Table 1 provides a list of important deposits exhibiting the most common characteristics of BHTs. Economic geologists are currently divided in their explanations for the genesis of BHT deposits. One group (e.g., Beeson 1990a; Parr and Plimer 1993; Walters 1996) argues that, based on seemingly unique characteristics, BHT deposits should be regarded as a type or sub-type of stratiform leadzinc mineralization distinct from SEDEX-type deposits. In support of their explanation, Parr and Plimer (1993) listed seven BHT genetic models proposed between 1922 and 1993. Other geologists (e.g., Sangster 1990; Goodfellow et al. 1993; Sangster and Hillary 1998) have viewed BHTs as being Donald F. Sangster: deceased Editorial handling: D. Huston Electronic supplementary material The online version of this article (https://doi.org/10.1007/s00126-020-00975-9) contains supplementary material, which is available to authorized users. * Donald F. Sangster
1
Ottawa, Canada
no more than metamorphosed SEDEX deposits. This contention has been challenged by Large et al. (1996) and others. For example, in regard to the Cannington BHT deposit of South Australia, Large et al. (2005, p. 259) conclude without equivocation that this deposit “is clearly not a metamorphosed SEDEX deposit”. In spite of this statement, and also Walters’ (1996) admonition that “any model which emphasizes the recognition of inferred pre-metamorphic relationships in amphibolite-granulite terranes is unrealistic”, the author has opted to examine evidence that BHTs are in fact metamorphosed SEDEX deposits. The controversial interpretation of BHT deposits as a unique mineral deposit type is further complicated by the curious condition that a post-ore event, metamorphism, would be one of the principal criteria defining this deposit type.
Selected BHT characteristics compared to those of SEDEX deposits High-grade metamorphism associated with BHTs While it can never be shown that BHTs are invariably associated with high-grade metamorphism, as one can presume not all BHTs have been discovered, this association seems
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