Zn and Pb mobility during metamorphism of sedimentary rocks and potential implications for some base metal deposits

Abstract

Comprehension of the genesis of Pb-Zn ore systems is currently limited by a poor understanding of where these metals are sourced from. Our study of metal mobility during regional metamorphism in the Mt. Lofty Ranges, South Australia, demonstrates that in staurolite-absent siliciclastic metasedimentary rocks, biotite contains >80 % of the bulk rock Zn, as well as a considerable proportion of the total Pb. Fluid flow through these metasedimentary rocks led to a continuous depletion of Pb and Zn on a mineral and bulk rock scale during prograde regional metamorphism. We calculate that ∼80 % of the bulk rock Zn and ∼50 % of the bulk rock Pb were mobilised, mainly through reactions involving biotite. These reactions led to a calculated Pb and Zn Bloss^of ∼2.7 and 27 Mt, respectively, in the high-grade metamorphic zone. Halogen contents of apatite and biotite and bulk rock Zn isotope data provide evidence that Cl-rich metamorphic fluids were important for metal transport. Hence, fluid flow accompanying prograde metamorphism of typical sedimentary rocks can mobilise base metals to the degree required to potentially supply significant Pb-Zn ore systems.

Porphyroclasts: Source and Sink of Major and Trace Elements During Deformation-Induced Metasomatism (Finero, Ivrea-Verbano Zone, Italy)

“…The lack of significant deposition of synkinematic sulfides within the ultramylonites does not support the large presence of S-complexes in the second fluid, suggesting alternatively the involvement of a silicate crustal component as source of the fluid. It has been demonstrated that Zn-bearing biotite and chlorite can host large amounts of bulk Zn content in siliciclastic metasedimentary rocks [62] and altered/metasomatized mafic rocks [63], respectively. In order to explain the Zn enrichment together with the large K, Al, Ti, and Fe content recorded by the most deformed sectors of the shear zone, it is argued that dissolution of crustal biotite more likely governed the composition of the second fluid phase.…”

The effects of amphibolite facies metamorphism on the trace element composition of pyrite and pyrrhotite in the Cambrian Nairne Pyrite Member, Kanmantoo Group, South Australia

“…The Syncline) and created an (S 3 ) crenulation (Offler and Fleming, 1968;Mancktelow, 1990;. Metamorphism is of the low-P and high-T type ranging from the greenschist facies (~350-400 °C) in the north and west to the upper amphibolite facies (~540-630 °C) in the south and east (Fleming and White, 1984;Mancktelow, 1990;Sandiford et al, 1990;Dymoke and Sandiford, 1992;Oliver et al, 1998;Foden et al, 1999), with local areas of migmatite around syntectonic igneous intrusions (Hammerli et al, 2015). The syntectonic granites are mostly confined to the core of the orogen (eastern Mt Lofty Ranges, Fleurieu Peninsula and Kangaroo Island) where heat provided by these intrusions formed a metamorphic gradient (Foden et al, 1999).…”

“…However, it is important to acknowledge that non-sulfides could have potentially been a sink for these base metals as well. Hammerli et al (2015) evaluated the potential mobilization of Pb and Zn in staurolite-and sulfide-absent metasedimentary rocks in the eastern part of the Kanmantoo Trough during metamorphism.…”

“…Recent studies have suggested various origins for the Cu-Au (e.g., Kanmantoo) and Pb-Zn-Ag-(Cu-Au) deposits in the Tapanappa Formation, including syngenetic (e.g., Seccombe et al, 1985;Toteff, 1999;Pollock et al 2018), syn-metamorphic (e.g., Oliver et al, 1998), post-peak metamorphic (e.g., Foden et al, 1999;Schmidt Mumm et al, 2009), and magmatic models (Rolley, 2018). Mass-balance calculations by Hammerli et al (2015) suggest that considerable amounts of Pb and Zn can be released from staurolite-absent metasedimentary rocks during prograde metamorphism in the Kanmantoo Group, with Cu showing no appreciable release. However, unlike Au, which can be released to form metamorphogenic gold deposits during prograde metamorphism (e.g., Pitcairn et al, 2006;Tomkins, 2010), other studies have interpreted their data to suggest that silicates such as biotite and feldspar retain Pb and Zn to mid-crustal conditions and do not carry Cu, Pb, and Zn in solution effectively to form orogenic base metal deposits (e.g., Pitcairn et al, 2006;Yardley and Cleverley, 2015;Zhong et al, 2015;Finch and Tomkins, 2017).…”

Basinal Brines at the Origin of the Imiter Ag-Hg Deposit (Anti-Atlas, Morocco): Evidence from LA-ICP-MS Data on Fluid Inclusions, Halogen Signatures, and Stable Isotopes (H, C, O)

“…Several studies have established that likely metal sources for Pb-Zn in base metal deposits include (1) sedimentary basins (Leach et al, 2005), (2) crystalline basements (Russell, 1978;Oliver et al, 2006), and (3) the leaching of metals by fluid flow during regional metamorphism (Lebedev and Nagaytsev, 1980;Haack et al, 1984;Hammerli et al, 2015). For Imiter, 187 Os/ 188 Os values lower than 0.2 are considered by Levresse et al, (2004) as representative of a mantle source.…”