Evaporite-bearing orogenic belts produce ligand-rich and diverse metamorphic fluids

Highlights

• Cl and SO₄-rich metamorphic scapolite imply saline and oxidised metamorphic fluids.

• Cl-rich rims on scapolite imply that fluid salinity increased during metamorphism.

• Increase in salinity likely due to phase separation of a complex NaCl-H₂O-CO₂ fluid.

• Metaevaporitic terranes maintain fertility through prolonged orogenesis.

• Metamorphic fluids from evaporites can generate a range of mineralisation styles.

Abstract

Detailed petrologic and chemical investigation of mid-amphibolite facies calcareous, scapolite-rich metasedimentary rocks from the Mount Isa region in northern Australia is used to explore changing fluid chemistry with prograde metamorphism. The presence of widespread scapolite with Cl- and variably SO₄-rich compositions in upper amphibolite facies rocks makes it unavoidable that the regional metamorphic fluids were locally highly saline and oxidised, and that high salinities persisted throughout metamorphism. Electron microprobe analyses and chemical maps of individual scapolite grains show zoning in Cl and S, likely to reflect buffering of the metamorphic fluid by scapolite during progressive metamorphism. The zoning in Cl and S demonstrates that scapolite has the potential to record changes in fluid chemistry during metamorphism. The variation in scapolite composition between samples, in combination with whole rock geochemistry, shows that different layers within this heterogenous rock package generated fluids of different chemistries. Interaction between scapolite-bearing rocks and externally-derived magmatic or metamorphic fluids that are out of equilibrium drives scapolite breakdown, releasing Cl to the fluid. In the Mount Isa region, metamorphic fluid production was enhanced by periods of magmatism, which promoted development of a regionally extensive and unusually saline fluid system that was active at multiple stages over a 250 million-year period. The highly saline and oxidised fluids formed through interaction with scapolite are well suited to transporting a broad range of metals, and may explain the diverse range of syn-orogenic mineral deposits in the Mount Isa Inlier. Metamorphic belts with large volumes of evaporitic material are ideal for generating a broad spectrum of syn-orogenic hydrothermal ore deposit types - including Fe oxide Cu-Au, Fe sulphide Cu-Au, Mo-Re and U-REE, but lacking the Au-only deposits found in typical orogenic belts. Unlike regions hosting traditional orogenic gold deposits, belts containing evaporitic sequences can preserve Cl-rich minerals such as scapolite in the metamorphosed source region, allowing them to remain active as ore forming systems through relatively high-grade metamorphism and multiple stages of tectonism. Periods of supercontinent breakup, such as the Mesoproterozoic, may have resulted in the formation of large, intracontinental basins well suited to the development of widespread evaporitic sequences. This, in combination with overprinting orogenesis and high temperature magmatism, may have provided the ingredients for widespread ore deposit formation at a global scale.

Introduction

It is widely accepted that regional metamorphism, and in some cases coincident magmatism, during orogenesis of greenstone belts has been responsible for generation of orogenic gold deposits around the world (e.g. Goldfarb and Groves, 2015, Phillips and Powell, 2009, Tomkins, 2013). Metamorphism coincides with compressional to transpressional tectonics, which creates a structural framework by which fluids migrate from mid/deep levels in the crust into transform and reverse fault controlled localised dilations above the brittle-ductile transition where the gold deposits form. In Archean cratons, the greenstone belts are dominated by hydrated mafic–ultramafic rocks, felsic volcaniclastics and fine- to coarse-grained clastic metasedimentary rocks, whereas Phanerozoic metamorphic belts are dominated by turbidite sequences, with hydrated mafic rocks present in some cases. Metamorphic dehydration of chlorite, and in some cases muscovite, is likely the main pathway by which to produce the large volumes of low salinity, moderately reduced, H₂S- and CO₂-bearing fluid needed to form orogenic gold deposits (e.g. Elmer et al., 2006, Phillips and Powell, 2010, Tomkins, 2010). Contributions from crystallizing felsic to intermediate magmas appear to have been important contributors in some cases (Kendrick et al., 2011b, Xue et al., 2013). These deposits typically contain minimal enrichment in base metals, thought to be because gold can be transported in low salinity fluids as hydrosulphide complexes, whereas base metals tend to be transported as chloride complexes (Stefánsson and Seward, 2004, Yardley, 2005). Generally, it is more common than not that Archean and Phanerozoic orogenic metamorphic belts contain at least some orogenic gold deposits, although the size and abundance vary considerably.

The Mesoproterozoic Mount Isa Inlier in northern Australia appears to record the ideal scenario for orogenic gold, including high thermal gradient metamorphism, compressional/transpressional tectonics, fertile source rocks and extensive crustal-scale fluid-rock interaction systems. This region contains some of the most extensively metasomatised rocks in the world and is one of the most highly endowed metallogenic provinces in Australia (e.g. Oliver et al., 2008, Williams, 1998). However, enigmatically, there are very few gold-only deposits. Instead, there are thousands of Cu-Au prospects and deposits of different varieties (including some large deposits) that are interpreted to have formed synchronously with metamorphism and magmatism during the ca. 1610–1500 Ma Isan Orogeny (Fig. 1). Syn-orogenic mineralisation varies from the classic iron-oxide Cu-Au type (IOCG; Ernest Henry, Osborne; e.g. Gauthier et al., 2001, Mark et al., 2006), to iron sulphide Cu-Au (ISCG; Eloise; Baker, 1998), a large Mo-Re deposit (Merlin; Babo et al., 2017), and uranium-REE skarn deposits (Mary Kathleen, Elaine Dorothy; Oliver et al., 1999, Spandler et al., 2016).

The fluids responsible for the metasomatism and associated mineral occurrences across the Mount Isa Inlier had unusually high salinity and CO₂ contents (e.g. de Jong and Williams, 1995, Oliver et al., 2004, Kendrick et al., 2006, Kendrick et al., 2008, Kendrick et al., 2011a). The current paradigm posits that magmatic fluids were responsible for formation of most of the syn-orogenic metasomatism and mineralisation in the Mount Isa region (cf. Mark et al., 2006, Oliver et al., 2004). This notion is consistent with the widely held view amongst economic geologists that Cl-rich fluids are either derived from felsic-intermediate intrusions, such as those associated with porphyry Cu(-Au-Mo) deposits (e.g. Richards, 2011, Sillitoe, 2010), or evaporative basins (e.g. Leach et al., 2010), but not metamorphism (Goldfarb and Groves, 2015). However, the Eastern Fold Belt (Fig. 1) also contains thick sequences of carbonate-rich, scapolite-bearing (containing Cl^– and SO₄^–) metasedimentary rocks that underwent greenschist to amphibolite facies metamorphism during the Isan Orogeny (Oliver et al., 1992). The majority of previous studies have proposed the abundant scapolite to be a result of regional saline fluid flow, rather than the source (Ramsay and Davidson, 1970, Oliver et al., 1992, Hammerli et al., 2014). However, no studies have attempted to investigate whether there is a systematic change in scapolite chemistry or scapolite zoning during prograde metamorphism in the region. The saline, high CO₂ metamorphic fluids generated from scapolitic calc-silicate rocks would be distinctly different to the typical low salinity orogenic fluids of Archean and Phanerozoic orogenic belts, and thus should result in different styles of mineralisation. High salinity increases the solubility of base metals and gold, as well as other metal-complexing ligands such as sulphur, fluorine and phosphorus (Newton and Manning, 2004, Tropper and Manning, 2007), so we hypothesise that this significantly broadens the range of metals that can be carried in the fluid. This may explain the diversity of mineral systems and lack of Au-only deposits in the Mount Isa region.

This study focuses on characterising the chemistry of scapolite from a sequence of mid-amphibolite facies calcareous metasediments from the Mount Isa region as a function of changing metamorphic grade and host rock composition. We use our detailed geochemistry to frame a discussion of the role that scapolite plays both in the formation of highly saline and oxidised regional fluids and as a recorder of evolving fluid chemistry in complex NaCl–H₂O–CO₂ systems. The resulting insights are applicable globally, and add to the foundation of our understanding of the role of metamorphic processes in ore deposit formation. There are important implications for mineral exploration: if scapolite plays a key role in the genesis of Cl-rich metamorphic fluids, then regions with thick sequences of metaevaporites are likely to be prospective for a diverse range of mineral deposit styles, regardless of their metamorphic grade.