Elsevier

Ore Geology Reviews

Volume 114, November 2019, 103128
Ore Geology Reviews

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

https://doi.org/10.1016/j.oregeorev.2019.103128Get rights and content

Highlights

  • Structurally compatible elements (Co, Ni) remain in pyrite at amphibolite facies.

  • Remobilization of base metals from Fe sulfides restricted to distances of a few cm.

  • Sulfides veins (<few m long) formed from remobilization of sedimentary sulfides.

  • Nairne Pyrite Member likely contributed some Cu, Pb, and Zn to syngenetic deposits.

Abstract

The trace element composition of pyrite has been used to explore for hydrothermal ore deposits and to understand ore-forming processes. However, the effects of metamorphism on the trace element distribution in pyrite have received relatively limited attention. In this study, laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analyses of pyrite and pyrrhotite, along with minor amounts of sphalerite, chalcopyrite, and galena, are used to evaluate the effects of amphibolite facies metamorphism on the trace element distribution and remobilization of metals in iron sulfides in the clastic sediment-hosted Cambrian Nairne Pyrite Member (NPM), South Australia. The NPM and the Mt. Torrens Pb-Zn-Ag prospect, which occur near the base of the Kanmantoo Group, preserve irregularly zoned subhedral to euhedral metamorphic pyrite (Py1) and anhedral pyrrhotite (Po1), along with relatively minor quantities of remobilized anhedral pitted and cataclastic pyrite (Py2a) in quartzofeldspathic rocks and anhedral inclusion-poor pyrite (Py2b) in calc-silicate rocks that armor or cross-cut earlier formed Py1. Rare anhedral secondary melnikovite pyrite (Py3) locally formed on the margins of Py1 and Py2a. Trace element studies show that Py1 in the NPM at Brukunga and Ironstone Ridge contains mean values of 1254 ppm Co, 123 ppm Ni, 2167  ppm As, 16 ppm Se, 10 ppm Cu, 25 ppm Zn, and 15 ppm Pb, whereas Py1 in the Mt. Torrens prospect contains mean values of 2312 ppm Co, 263 ppm Ni, 1835  ppm As, 95 ppm Se, 9 ppm Cu, 6 ppm Zn, and 9 ppm Pb. Rare inclusion-rich cores of Py1 show higher concentrations of trace elements than inclusion-free rims with minor amounts of chalcopyrite, galena, and sphalerite forming along grain boundaries or in fractures within pyrite. This is interpreted to be the result of the release of Cu, Pb, and Zn from pyrite as it recrystallized. Remobilization of these elements then formed discrete sulfides at the millimeter to centimeter scale, with some exceptions at the meter scale where chalcopyrite, sphalerite, and galena, along with other sulfides and sulfosalts, formed in veins and tension gashes. These observations suggest that remobilization of trace metals, including Cu, Pb, and Zn, did not migrate more than a few meters at most. Moreover, despite the recrystallization of pyrite and pyrrhotite and subsequent remobilization of some trace elements (i.e., Co, Ni, As), which were structurally bound in these Fe sulfides, still retain elevated concentrations at amphibolite facies conditions.

Remobilization of metals from the NPM during metamorphism to form Cu-Au (e.g., Kanmantoo) and Pb-Zn-Ag-(Cu-Au) (e.g., Angas, Wheal Ellen) deposits in the Tapanappa Formation stratigraphically higher in the Kanmantoo Group seems unlikely. However, it is possible that one source of metals for these deposits could have been leached from the NPM and carried in large hydrothermal cells prior to metamorphism. Such a scenario is consistent with previously published sulfur isotope data for sulfides from the NPM, and Cu-Au and Pb-Zn-Ag-(Cu-Au) deposits, which indicate that sulfur derived from the NPM and pyritic schists in the Kanmantoo Group was a likely source of sulfur for the base and precious metal deposits.

Introduction

Questions regarding the mobility of metallic elements as a result of recrystallization of pyrite during amphibolite facies metamorphism and whether such elements can subsequently form Cu-Pb-Zn-(Au-Ag) deposits are, as of yet, largely unresolved. These questions are addressed by studying the trace element composition of pyrite and pyrrhotite in the Nairne Pyrite Member (NPM), South Australia, one of the longest iron sulfide-bearing units in the world (extends intermittently for at least 100 km). The Kanmantoo Group, which hosts the NPM, consists of a structurally thickened package (∼7–8 km thick; Haines et al., 2001) of Cambrian quartzites, greywackes, pyritic units, and siltstones metamorphosed from the lower to upper amphibolite facies. The base of the NPM hosts the small Mt. Torrens Pb-Zn-Ag deposit, whereas the Angas, Scotts Creek and Wheal Ellen Pb-Zn-Ag-(Cu-Au) and Kanmantoo Cu-Au deposits, as well as minor pyritic schists occur higher up in the Kanmantoo Group within the Tapanappa Formation (Seccombe et al., 1985, Both, 1990, Gum, 1998, Toteff, 1999).

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), and magmatic models (e.g., Kimpton et al., 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 that can be released to form metamorphogenic gold deposits during prograde metamorphism (e.g., Pitcairn et al., 2006, Tomkins, 2010), metamorphic fluids 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). Instead, Pb and Zn can be incorporated into the structures of K-feldspar and biotite, respectively (Finch and Tomkins, 2017).

Pyrite is the most common sulfide found in nature and is ubiquitous in most hydrothermal ore deposits (Rickard, 2015). Its cubic structure allows pyrite to incorporate significant quantities of trace elements (e.g., Cu, Pb, Zn, As, Se, Co, Ni). Trace element studies of pyrite provide information on ore paragenesis, the source of metals, the origin of various ore deposits, and how trace elements can be used to vector to hydrothermal ore deposits (e.g., Large et al., 2007, Gregory et al., 2015, Mukherjee and Large, 2017). Previous studies of the trace element composition of pyrite have largely focused on sedimentary and diagenetic pyrite (e.g., Large et al., 2007, Large et al., 2014, Large et al., 2017), as well as pyrite found in rocks metamorphosed to low-grades (greenschist facies). However, there is debate whether recrystallization and metamorphism to higher grades expels trace elements from the structure of pyrite or whether trace elements remain in its structure. Utilizing data obtained from proton microprobe analyses of pyrite, chalcopyrite, and sphalerite from volcanogenic massive sulfide (VMS) deposits in eastern Australia, some of which were metamorphosed (i.e., Rosebery – greenschist facies; Dry River South – amphibolite facies), Huston et al. (1995) showed that metamorphic growth and subsequent recrystallization of pyrite liberated some trace elements (e.g., Bi, Pb, Mo, Cu, Ag) to grain boundaries to either form new minerals or to be incorporated in coexisting sulfides. Large et al. (2007) evaluated the trace element distribution of pyrite in rocks metamorphosed to the chlorite-sericite subfacies of the greenschist facies in the sediment-hosted Sukhoi Log Au deposit, Russia, and proposed that the trace element content of recrystallized and metamorphic pyrite (e.g., Pb, Cu, Zn, Ag, Te, and Au) was lower than earlier formed hydrothermal and diagenetic pyrite because certain trace elements were partitioned into other sulfides rather than being incorporated into the structure of pyrite. Genna and Gaboury (2015) subsequently demonstrated that in evaluating the composition of pyrite in the Bracemac-McLeod Zn-Cu-Ag-Au VMS deposit, which was also metamorphosed to the sericite subfacies of the greenschist facies, there was expulsion of some base and precious metals from pyrite during metamorphism, while there was retention or even enrichment of other metallic elements including Ni, Co, As, Sb, Tl, and Se. George et al. (2018) demonstrated that metamorphic recrystallization of pyrite at the greenschist facies in the pyrite-dominated orebodies in the southern Apuan Alps, Italy, mobilized and concentrated some trace metals to form various sulfides and sulfosalts, while Kampmann et al. (2018) showed that pyrite from the Falun Zn-Pb-Cu-(Au-Ag) deposit metamorphosed to a higher metamorphic grade (lower amphibolite facies) liberated Pb, Bi, Au, and Se to form discrete grains of sulfides, sulfosalts, and native gold.

Skinner, 1958, LaGanza, 1959, George, 1969a, and Nenke (1972) proposed the NPM to be a metamorphosed sedimentary Fe sulfide unit. Given the light δ34S values of −20 to −12‰ for pyrite in the NPM and pyritic schists stratigraphically higher in the Kanmantoo Group, sulfur was considered to have been produced by biogenic reduction of seawater sulfate in anoxic basin waters (Jensen and Whittle, 1969, Seccombe et al., 1985, Gum, 1998). Sulfur isotope compositions of sulfides from the base metal deposits are isotopically heavier (δ34S = −9 to +15‰; Seccombe et al., 1985, Gum, 1998) than those from the Fe-S units. This is compatible with sulfur in the Cu-Au and Pb-Zn-Ag-(Cu-Au) deposits being, in part, derived from the pyritic units (Seccombe et al, 1985). However, it is unknown whether these same pyritic units were a possible source of Cu, Pb, Zn, Ag, and Au for the base metal deposits. Trace element analyses of pyrite, pyrrhotite, chalcopyrite, and sphalerite from the NPM and other pyritic schists in the Kanmantoo Group are compared here to the composition of the same minerals in the Kanmantoo Cu-Au, and the Aclare, Wheal Ellen, and Angas Pb-Zn-Ag deposits (Fig. 1). The main focus of the study is on the Brukunga Fe-S deposit in the NPM, located ∼45 km east of Adelaide and 6 km north of Nairne, on the eastern edge of the Mount Lofty Ranges, which was mined from 1955 to 1972 for the production of sulfuric acid, and the Mt. Torrens prospect. In this part of the NPM, rocks were metamorphosed to the lower-middle amphibolite facies.

The aims of this contribution are to determine: 1. The trace element concentration of pyrite and pyrrhotite (and to a lesser extent sphalerite, galena, and chalcopyrite) in three groups of deposits (pyrite-pyrrhotite units, Cu-Au, and Pb-Zn-Ag-(Cu-Au)) in the Kanmantoo Group to evaluate how lower to middle amphibolite facies metamorphism affects trace element distribution and concentration in these sulfides; and 2. Whether or not the pyritic units could have been a source of base and precious metals to the Cu-Au and Pb-Zn-Ag deposits.

Section snippets

Regional geology

The NPM occurs within the Cambrian Kanmantoo Group, which is a package of metamorphosed pelitic, psammitic and minor carbonate rocks deposited into the fault-controlled Kanmantoo Trough that occurs uncomformably above the Early Cambrian Normanville Group (Belperio et al., 1998, Jago et al., 2003). East-directed subduction along the palaeo-Pacific margin of Gondwana relating to the Ross Orogen created this extensional tear basin, which is bound by steep growth faults (Belperio et al., 1998,

Sample collection

One hundred and fifty four polished thin sections were prepared from samples obtained from diamond drill core: Nairne Pyrites 13 (n = 30) and Nairne Pyrites 10 (n = 26) from the NPM at Brukunga, Mt. Torrens (DD84KA1 (n = 26), DD84KA2 (n = 17), DD84KA3 (n = 7), 81mt-D1 (n = 12), 77MTDD1 (n = 12), 77MTDD2 (n = 7), 77MTDD4 (n = 5), and 77MTDD5 (n = 4)), and Wheal Ellen Pb-Zn-Ag prospect (W3A (n = 3), and W6 (n = 5)). Thirty polished-thin sections derived from drill hole Nairne Pyrite DDH-9 (

Nairne Pyrite Member

Although intermittent exposures of the NPM occur for ∼100 km and were folded by a regional N-S trending south plunging F2 fold (Fig. 1), stratiform iron sulfide-bearing horizons are thickest in the Brukunga Fe-S deposit (Skinner, 1958, Gum, 1998). The orientation of the NPM at Brukunga, which dips ∼70° east and strikes north–south, was controlled by a series of anticlines and overturned synclines that formed during D2 (Skinner, 1958, LaGanza, 1959, Mason, 1968). The NPM consists of a sequence

Nairne Pyrite Member

The metallic minerals in the bedded units are predominantly pyrite and pyrrhotite but minor amounts of chalcopyrite, sphalerite, galena, arsenopyrite, and marcasite are also present. Trace amounts of sulfides, sulphosalts, native elements (including graphite), and alloys, which were identified by LaGanza, 1959, Graham, 1978, and Gum (1998) are locally abundant in tension gashes and remobilized veins up to 2 m in length. The textural relationship between the Fe-sulfides and silicates in the

Pyrite

Unless otherwise stated, comparisons made here are between mean compositions of various elements in pyrite (Py1, Py2a, Py2b, and Py3), although mean (with one standard deviation), maximum, and detection limits are reported in Table 3. Pyrite1 from the NPM, Mt. Torrens, and pyritic schist contain mean Co values of 1254 ppm, 2235 ppm, and 6785 ppm, and mean As values of 3167 ppm, 1762 ppm, and 1883 ppm, respectively (Table 3, Figs. 6a–b and 7a–d). Cobalt and As concentrations of Py1 from the

Effects of metamorphism on sulfides in the Kanmantoo Group

Sulfides in the NPM and Mt. Torrens Pb-Zn deposit, and higher in the stratigraphic sequence in the Tapanappa Formation (e.g., Wheal Ellen and Angas Pb-Zn-Ag-(Cu-Au) deposits, Kanmantoo Cu-Au deposit) were metamorphosed to the amphibolite facies. Spry et al. (1988) suggested that the effects of metamorphism on the sulfide deposits in the Kanmantoo Group were reflected by the coarse nature of the sulfides (including the formation of euhedral pyrite and arsenopyrite porphyroblasts), the

Conclusions

  • 1.

    Despite recrystallization and annealing, the concentrations of structurally compatible elements in pyrite (Co, Ni, Se) and As from the Kanmantoo Group during amphibolite facies metamorphism were not expelled from its structure.

  • 2.

    The Co:Ni ratio of pyrite from the Nairne Pyrite Member (NPM) and Mt. Torrens are higher than those for pyrite in unmetamorphosed sedimentary rocks reported elsewhere in the literature, which could be due to: 1) the high abundance of pyrrhotite (∼50%), which acts as a

Acknowledgements

Funding for this study was provided by the Geological Survey of South Australia (Department for Energy and Mining) and a Society of Economic Fellowships Grant to Conn. We kindly thank David and Keryn Groom for assisting us with access to drill core at the South Australian Drill Core Library. The help of Tony Milnes is greatly appreciated for providing copies of University of Adelaide theses on the Nairne pyrite deposit and for access to archived samples. The comments and support of Steve Hill,

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