The role of Te, As, Bi, Sn and Sb during the formation of platinum-group-element reef deposits: Examples from the Bushveld and Stillwater Complexes
Introduction
It has been demonstrated that in magmatic Ni-Cu-PGE sulfide deposits the platinum-group elements are bimodally distributed (Godel et al., 2007, Holwell and McDonald, 2007, Dare et al., 2010b, Dare et al., 2014, Piña et al., 2012, Osbahr et al., 2013, Osbahr et al., 2014, Junge et al., 2015, Duran et al., 2016). The PGE are present at the ppm level in the base metal sulfides (BMS) pyrrhotite, pentlandite, and to a lesser extent chalcopyrite, and these host much of the PGE budget (Barnes and Ripley, 2016, and references therein). The balance of the PGE is hosted by platinum-group minerals (PGM). These PGM commonly consist of PGE plus one or more of the elements Te, As, Bi, Sb and Sn (TABS; Barnes, 2016), or S (O’Driscoll and González-Jiménez, 2016 and references therein). Understanding the influence of TABS on the distribution of PGE in magmatic systems is critical given the importance of PGE in the study of various fields of geology (Harvey and Day, 2016 and references therein). Moreover, there is an increasing interest in understanding the geochemical cycle of volatile elements such as TABS, and in these studies BMS are frequently proposed as controlling the TABS (Hattori et al., 2002, Lodders, 2003, Lorand and Alard, 2010, Wang and Becker, 2013, König et al., 2012, König et al., 2014, Lissner et al., 2014, Patten et al., 2016, Patten et al., 2017, Canali et al., 2017, D'Souza and Canil, 2018).
To date, the concentrations of TABS in Ni-Cu-PGE deposits has mainly been addressed in relatively sulfide-rich deposits, for example those from Sudbury (Canada), Noril’sk-Talnakh (Russia), the Platreef (South Africa) and Aguablanca (Spain) (Holwell and McDonald, 2007, Dare et al., 2010a, Dare et al., 2010b, Dare et al., 2014, Piña et al., 2012, Yudovskaya et al., 2017, Mansur et al., 2019c). In these deposits, the whole-rock concentrations of TABS, Pd and Pt are found to increase from the Cu-poor parts of the deposits, thought to represent monosulfide solid solution (MSS) cumulates, to the Cu-rich parts of the deposits, thought to represent the products of the fractionated sulfide liquid (Dare et al., 2010b, Dare et al., 2014, Chen et al., 2013, Chen et al., 2015, Duran et al., 2017). The TABS, Pd and Pt concentrations of the BMS show positive covariance with the whole-rock values (Mansur et al., 2019c). However, the bulk of the TABS and Pt are not hosted by the BMS. In the MSS cumulates, the PGM occur within BMS grains and appear to have formed predominantly by exsolutions (Dare et al., 2010b, Piña et al., 2012, Duran et al., 2017, Mansur et al., 2019c). In the Cu-rich portions of the deposits, some of the PGM occur as exsolutions, but predominately they occur among the sulfide grains, and are thought to have crystallized from the fractionated sulfide liquid (Dare et al., 2014, Duran et al., 2017, Mansur et al., 2019c). Alternatively, they could have crystallized from an immiscible TABS-rich liquid which segregated from the fractionated Cu-rich sulfide liquid (Helmy et al., 2007, Helmy et al., 2010, Holwell and McDonald, 2010, Piña et al., 2015, Cafagna and Jugo, 2016).
In the case of PGE-reef deposits, which contain disseminated sulfides, the distribution of TABS has not been well documented, although a number of roles listed below have been proposed for TABS. (i) In order to explain the very high PGE content of the PGE-dominated deposits, it has been proposed that TABS and PGE form pre-nucleation clusters (referred to as nanoparticles or nanoclusters). The clusters are incorporated in a magmatic sulfide liquid, and subsequently in the MSS and intermediate sulfide solid solutions (ISS) that crystalize from the sulfide liquid. These clusters could remain in the BMS, or coalesce to form PGM (Tredoux et al., 1995, Helmy et al., 2013, Wirth et al., 2013, Junge et al., 2015, Liang et al., 2019). (ii) Alternatively, the TABS and PGE could behave as in sulfide-rich deposits, but with the difference that the disseminated sulfides in reefs are generally assumed to represent sulfide liquid compositions, as the sulfides represent the product of equilibrium crystallization. In this case, a small portion of these elements would have partitioned into MSS and ISS as they crystallized, and PGM exsolved from the sulfides during cooling (Prichard et al., 2004, Godel et al., 2007, Godel and Barnes, 2008a). However, given the incompatible nature of Pd, Pt and TABS (Helmy et al., 2010, Patten et al., 2013, Liu and Brenan, 2015) the bulk of these elements would have partitioned into the fractionated sulfide liquid, and crystallized as PGM among the sulfide grains. (iii) A variation of this model is that the trapped fractionated sulfide liquid became saturated in a TABS-PGE rich liquid (Helmy et al., 2007, Helmy et al., 2010, Piña et al., 2015, Cafagna and Jugo, 2016). This liquid could migrate away from the sulfides, or crystallize among the sulfide grains (Holwell and McDonald, 2010). (iv) The TABS could have a role in fixing the PGE during dissolution of the sulfides by late magmatic or metamorphic fluids. Loss of S from sulfides leads to the exsolutions of PGM (Ballhaus et al., 1994, Peregoedova et al., 2004, Li and Ripley, 2006, Godel and Barnes, 2008a, Djon and Barnes, 2012). If most of the BMS are dissolved leaving TABS and PGE, the TABS serve to fix the PGE, as TABS-rich PGM (Wood, 2002, Scholten et al., 2018, Sullivan et al., 2018).
In the current work we examine the distribution of PGE and TABS in whole-rock samples, and in disseminated BMS from the Bushveld and Stillwater Complexes. The samples comprise the main sulfide-related PGE reefs of the intrusions, the Merensky Reef (Bushveld), and the J-M Reef and Picket Pin deposit (Stillwater), and also barren sulfide-bearing samples, from outside the reef intervals from both intrusions. This allows assessment of the distribution of PGE and TABS in PGE reef type deposits, and investigation whether TABS play a significant role during the formation of PGE-dominated deposits.
This contribution will show that the whole-rock concentrations of Se and TABS (except for As) correlate with S and PGE, and thus their distribution is controlled by BMS. The distribution of As, and to a lesser extent Sb, are controlled by both the amount of liquid fraction in cumulate rocks, and the amount of sulfides. The study will also show that the concentrations of PGE are the highest in BMS from the reef samples, in contrast the concentrations of TABS are the lowest in BMS from the same samples. The highest concentrations of TABS in BMS were found in samples with the lowest whole-rock PGE contents. The formation of the PGM by pre-nucleation clusters is considered, and discarded. The hypotheses that PGM are formed by exsolutions, and by crystallization from the fractionated sulfide liquid is favoured. To explain the contrast in the behaviour of TABS and PGE in BMS of the reefs, it is proposed that the high concentration of PGE in BMS from the reef leads to the diffusion of the TABS from the BMS into the PGM. Therefore, the BMS from the reefs are depleted in TABS, although the whole-rock concentrations of TABS in the reefs are high.
Section snippets
Description of studied samples
The samples of the Merensky Reef (Bushveld Complex, South Africa) are from the Rustenburg and Impala mines (Fig. 1a). The samples from the JM-Reef (Stillwater Complex, USA) are from the East Boulder and Stillwater mines (Fig. 1b). These rocks have been previously studied documenting petrography, microstructures, PGM distribution, whole-rock major and trace element, PGE and S contents of the reefs and surrounding rocks (Barnes and Maier, 2002, Prichard et al., 2004 - Impala section; Godel et
Analytical methods
Tellurium, As, Bi, Sb and Se analyses were carried out at LabMaTer, Université du Québec à Chicoutimi (UQAC). A slightly modified version of the Hydride Generation-Atomic Fluorescence Spectrometry (HG-AFS) technique described by Mansur et al. (2019b) was used to determine these elements. The modification was that sample size was increased from 0.2 g to 0.4 g. This modification was introduced to lower the dilution factor, and consequently lower the limit of detection of the method. International
Whole-rock concentrations of TABS in the PGE reefs
The whole-rock concentrations of TABS plus the previously published S, Ni, Cu and PGE concentrations can be found in the electronic supplementary material (ESM – Table 3).
The concentrations of S, PGE, Se, Te, Bi, As and Sb in the Impala (Fig. 3a) and Rustenburg (Fig. 3b) sections were plotted against the sample height to assess their distribution through the Merensky Reef. The distributions of TABS in the Merensky Reef at both the Impala and Rustenburg sections are similar. Selenium, Te and Bi
Discussion
Our results show that BMS from PGE-rich samples (i.e. from the reef intervals) have high concentrations of PGE, however, they have low concentrations of TABS. In contrast, BMS from PGE-poor samples (i.e. from outside the reef intervals) have low concentrations of PGE, but the highest concentrations of TABS. Consequently, in samples from outside the reefs the BMS account for greater proportion of whole-rock budget of TABS compared to BMS from the reef intervals. These variations suggest that the
Conclusions
This contribution provides insights into the possible roles of Te, As, Bi, Sb and Sn (TABS) during the formation of PGE deposits. Our main findings are:
- 1.
The ratio of TABS to PGE in reef rocks (0.01–0.3) is too low for the TABS to have acted as stabilizers of pre-nucleation clusters (nanoclusters), and thus TABS nanoclusters are not essential to form reefs.
- 2.
Whole-rock analyses reveal that the concentrations of TABS (except As) correlate positively with S and PGE. This suggests that all the
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by a Canada Research Chair program grant to Sarah-Jane Barnes (215503). We would like to thank Dany Savard and Audrey Lavoie (LabMaTer, UQAC) for their assistance with LA-ICP-MS analyses. This manuscript benefited from insightful comments from Steve Barnes, Rubén Piña, and one anonymous reviewer, and careful editorial handling by the associate editor Edward Ripley.
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