Sulfides, Native Silver, and Associated Trace Minerals of the Skaergaard

1 2 3 4 5 6 7 8 Sulfides, native silver, and associated trace minerals of the Skaergaard 9 Intrusion, Greenland: Evidence of late hydrothermal fluids 10 11 Ben Wernette1†, Peishu Li1, and Alan Boudreau1 12 1Division of Earth and Ocean Sciences, Duke University, Durham, N.C., 27705, USA 13 †Corresponding Author: [email protected] 14 15 16 17 18 19 20 21 22 23 1 24 Abstract 25 Sulfide assemblages and accessory phases throughout the Skaergaard were characterized 26 to better understand the role of magmatic volatiles in modifying the Skaergaard metal budget and 27 distribution. Sulfides in and below the Platinova Reef are readily replaced by low-Ti magnetite; 28 in thin section, the ratio of the low-Ti magnetite mode)/(sulfide mode) reaches maximum 29 values at the Platinova Reef. Sulfide assemblages below the Reef are accompanied by trace 30 quantities of clinopyroxene, orthopyroxene, biotite, apatite, and minor calcite. Native Ag, 31 commonly accompanied by trace amounts of Cl, occurs both in and below the Platinova Reef. 32 Evidence of coexisting precious metal + brine assemblages exists where native metals are 33 accompanied by sylvite ± halite and Ag is accompanied by Ag-halides. Ag occurrences in the 34 Platinova Reef are of irregular morphology with trace Cl ± S ± calcite. Further evidence 35 supportive of a metal + brine assemblage is observed where Ag + quartz are found in an apparent 36 fluid inclusion consisting of Na, Si, Cl, Ca, K, and S. 37 In agreement with earlier studies, the observed assemblage is consistent with the 38 Skaergaard being a S-poor intrusion with S continually lost during cooling and crystallization. 39 Partitioning of Ag into an exsolving fluid phase is a function of Cl concentration. Numerical 40 modeling suggests that, in a sulfide bearing system, residual Ag concentrations and 41 concentrations in the exsolved fluid are most affected at the point where sulfide is lost to a 42 separating volatile fluid phase. It is suggested that, owing to the low S nature of the Skaergaard 43 system, fractional crystallization produced enrichment of Ag in the interstitial silicate much 44 higher than normal due to delayed sulfide saturation. As this interstitial liquid evolved, Ag was 45 lost to an exsolved volatile phase of high salinity and migrated upward along grain boundaries 2 46 and fluid pathways. A similar process likely occurred for Au and other elements with high 47 affinities for Cl such as platinum-group elements. 48 49 1. Introduction 50 In layered mafic intrusions (LMIs), evidence for the existence of volatile phases is 51 preserved in fluid inclusions, apatite composition, and halogen geochemistry (e.g., Hanley et al. 52 2008; Boudreau and McCallum 1989; Boudreau et al. 1997). Of debate however, is whether 53 these volatile fluids, play a significant role in the processes responsible for generating, or 54 modifying, the stratiform platinum-group element (PGE)-sulfide ore bodies for which LMIs are 55 known. Conventionally, these deposits are thought to form as the result of gravitationally driven 56 downward movement of immiscible sulfide liquid droplets. Large sulfide liquid/silicate liquid 57 partition coefficients (e.g., Brenan and Mungall 2014) and large silicate liquid/sulfide liquid 58 mass ratios (R-factor, Campbell and Naldrett 1979) are thought to allow for the extreme 59 enrichment of transition and noble metals as well as PGE in immiscible sulfide droplets. Indeed, 60 experimental results confirm that these are reasonable assumptions (e.g., Fleet et al. 1993). 61 Conversely, others have suggested that stratiform mineralization is the result of upward 62 moving exsolved fluid, rich in halides, allowing for the efficient transport of transition and 63 precious metals vertically through an igneous body (e.g., Boudreau and McCallum 1992). 64 Theoretical work (Shinohara 1994) supports this model and experimental work performed on 65 rhyolitic and granitic systems (Candela and Holland 1984; Simon et al. 2008; Frank et al. 2011) 66 and theoretical work done on LMIs (Boudreau and McCallum 1992; Muerer et al. 1999) suggest 3 67 the importance of exsolved halide-bearing volatiles. Together these end-member models are 68 referred to as the “orthomagmatic” and “hydromagmatic” models, respectively. 69 Complicating our understanding of LMIs is that the larger intrusions (i.e., Bushveld and 70 Stillwater Complex) show evidence of multiple magma injections and prolonged cooling 71 histories for which the preservation of original igneous textures and chemistry is uncertain. The 72 small, single-pulse intrusion of the Paleogene Skaergaard Complex of southeast Greenland 73 contains a zone enriched in Au and the PGEs known as the Platinova Reef. The Skaergaard 74 Intrusions provides a unique opportunity to examine for evidence supportive of one or both of 75 the competing genetic models without the complication of potential magma mixing events. In 76 this study, we examine sulfide assemblages and their associated phases and characterize the 77 occurrence of native silver to better understand the role of exsolved volatiles in the distribution 78 and modification of metals in LMIs. 79 80 2. Geologic Background and Summary of Previous Studies 81 2.1 Skaergaard Complex 82 For extensive reviews, readers are directed to the early studies of Wager and Deer (e.g., 83 Wager and Deer 1939) and the more recent work of Nielsen (2004) and references therein. 84 Briefly, the Skaergaard Complex is a ~ 55 Ma (Brooks and Gleadow 1977; Hirschman et al. 85 1997) intrusion located in southeast Greenland (Fig. 1). The intrusion is related to the Paleogene 86 rifting of Greenland from Eurasia (Nielsen 1975). The Complex is hosted uncomfortably in 87 Archean gneiss and is principally comprised of the Layered, Upper Border, and Marginal Border 88 Series. The Layered Series is thought to have crystallized from the floor of the intrusion while 4 89 the Upper Border and Marginal Border Series crystallized from the roof and the walls of the 90 intrusion, respectively. The Layered Series is subdivided into the Lower, Middle, and Upper 91 Zone according to the presence of index minerals. The Platinova Reef is a diffuse (~ 60 m in 92 thickness, Anderson et al. 1998) metal-rich zone with distinct PGE and Au rich subzones hosted 93 in the upper part of the Middle Zone. 94 Wager et al. (1957) describe a Middle Zone Cu-rich sulfide assemblage consisting of 95 chalcopyrite and bornite. Anomalously high Cu/S ratios observed throughout the stratigraphy 96 have been attributed to either early shallow-level degassing and S loss (Li and Boudreau 2017), 97 late S loss to hydrothermal fluids (Andersen 2006), or anomalous Cu/S ratios in the Skaergaard 98 parental magma source region (Keays and Tenger 2016). Andersen (2006) concluded that sulfide 99 texture and mineralogy in the Lower Zone is well explained by an exsolved hydrothermal fluid 100 replacing preexisting sulfides with low-Ti magnetite and redistributing Cu, S, and precious 101 metals to different stratigraphic levels. Li and Boudreau (2017) arrived at a similar conclusion 102 after conducting a modal analysis of sulfides found in the Lower Zone and Marginal Border 103 Series. Conversely, Godel et al. (2014) note that LZ sulfides are generally not accompanied by 104 low-Ti magnetite, ultimately suggesting that late hydrothermal fluids did not oxidize magmatic 105 sulfide assemblages. 106 Detailed studies of precious metal distribution through the Skaergaard stratigraphy abound 107 (Holwell et al. 2016; Keays and Tenger 2016; Nielsen et al. 2015; Godel et al. 2014). These 108 studies make little to no mention of Ag, understandably focusing largely on the volumetrically 109 significant Cu, Au, Pd, and Pt. In the Platinova Reef, researchers have observed the occurrence 110 of Ag in Au-Cu-Ag alloys (Bird et al. 1991 and Cabri et al. 2005) as well as Au-PGE alloys 111 (Andersen et al. 1998). Additionally, Holwell et al. (2014) note silver concentrations of ~ 250 5 112 ppm in sulfides from the Triple Group. Moreover, Ag has been reported in Au-Cu-Ag alloys 113 found in marginal basement schists (Andersen et al. 2017). 114 115 3. Sampling and Methods 116 Samples were collected in 1990 by A. Boudreau. Two samples, SK90-5 and SK90-13, 117 were published in a previous study (Li and Boudreau 2017) and were re-examined and included 118 in this study for completeness. Individual polished thin-sections were systematically scanned 119 using the Cameca CAMEBAX microprobe at Duke University. Sulfide assemblages, and 120 accessory phases were characterized using energy dispersive spectrometry (EDS) and X-ray 121 composite maps. The same methods were used to identify and characterize the occurrences of 122 native Ag. Sulfides, Ag, and accessory phases were imaged and imported into the photo 123 processing software IMAGE-J (Schneider et al. 2012) for area and perimeter measurements. 124 Because trace assemblages are those that do not occur in large quantities, extreme caution was 125 used when characterizing occurrences of native Ag. Ag without associated trace minerals or 126 elements (e.g., calcite, Cl, K, Na, S) were only included in the dataset when found with other late 127 crystallizing phases (apatite) or associated with silver halides. This filtering removed 128 approximately two thirds of the original observations of Ag. 129 130 131 132 6 133 4. Results 134 4.1 Summary of Stratigraphic Trends 135 As summarized in Fig. 2 and discussed in detail below, stratigraphic trends show a 136 general decrease in chalcopyrite (and a corresponding increase in bornite) from the Lower Zone 137 through the Middle Zone before chalcopyrite and digenite join the sulfide assemblage in the 138 Upper Zone. Similarly, the relative modal abundance of low-Ti magnetite as a fraction of the 139 sulfide assemblage is highest in the Middle Zone.

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