The role of sulfur in the formation of magmatic–hydrothermal copper–gold deposits

Abstract

Essential resources of many rare metals including copper, zinc, molybdenum, silver and gold occur in natural sulfide mineral deposits. Understanding the origin of these metal resources has been limited by a lack of data about the geochemistry of sulfur, the most important and abundant element of ore deposits. We report the first directly measured sulfur concentrations in high-temperature fluids, together with their ore-metal contents, using a new method for sulfur quantification in fluid inclusions by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Co-genetic brine and vapor inclusions from magmatic-hydrothermal ore deposits and granitic intrusions show an excess of sulfur over ore metals, as required for efficient oremineral precipitation. The results demonstrate that S, Cu and Au are highly enriched in vapor-like magmatic fluids, implying that such low-salinity fluids are the key agent for the formation of porphyry copper and epithermal gold deposits.

The link between reduced porphyry copper deposits and oxidized magmas

“…Recent high-pressure experimental studies show that the trisulfur S 3 À radical ion is an important sulfur species at high-pressure and high-temperature conditions (Fig. 2) (Pokrovski and Dubrovinsky, 2011), which are common for porphyry systems (Hedenquist and Lowenstern, 1994;Seo et al, 2009;Sillitoe, 2010). This requires reevaluation of related geological processes where sulfur is involved, and in particular, porphyry copper mineralization (Pokrovski and Dubrovinsky, 2011).…”

Geochemical differences between subduction- and collision-related copper-bearing porphyries and implications for metallogenesis

“…1), the majority of samples in this study (with the exception of a few ultrapotassic samples in the Bingham district; Maughan et al, 2002), including limited data for Miocene ultrapotassic rocks of the EPRIM (Kay et al, 1994;Redwood and Rice, 1997;Sandeman and Clark, 2004;Maria and Luhe, 2008;Gómez-Tuena et al, 2011) and southern Tibet (Miller et al, 1999;Ding et al, 2003Ding et al, , 2006Williams et al, 2004;Gao et al, 2007b;Zhao et al, 2009;Chen et al, 2012), do not contain high concentrations of Cu (<130 ppm). However, these mantle-derived potassic and ultrapotassic magmas are typically enriched in the LILE, LREE, and volatiles such as H 2 O, CO 2 , F, and Cl (e.g., Rock, 1987;Rock et al, 1990;Behrens et al, 2009), all of which likely enhance the solubility of chalcophile elements, such as Cu and Au, in high-temperature aqueous fluids (e.g., Heinrich et al, 1992;Pokrovski et al, 2005Pokrovski et al, , 2008Simon et al, 2005Simon et al, , 2006Zajacz et al, 2008Zajacz et al, , 2011Seo et al, 2009). This indicates that the generally high K 2 O concentrations (K 2 O/Na 2 O > 0.5) in magmas associated with porphyry Cu mineralization are most likely produced by mixing between melts derived from the underplated basaltic lower crust and ascending mantle-derived potassic and ultrapotassic magmas (e.g., Pettke et al, 2010;Yang et al, 2014).…”

The controls of post-entrapment diffusion on the solubility of chalcopyrite daughter crystals in natural quartz-hosted fluid inclusions

“…This explains why S-rich vapor inclusions commonly have Cu/S molar ratios that reflect the ~1:1 mass ratio of Cu and S in chalcopyrite (Seo et al, 2009). …”

“…This is likely to be due to the preferential complexation of Cu as highly soluble CuCl 0 and [CuCl 2 ] -species as proposed in a number of solubility Holland, 1984, 1986;Bai and Koster van Groos, 1999;Archibald et al 2002;Simon et al, 2006) and speciation studies (Mavrogenes et al, 2002;Berry et al, 2006Berry et al, , 2009. Despite this, Cu concentrations were also commonly elevated in several low salinity (<2.0 wt.% NaCl eq ), liquid or vapor inclusions at El Teniente (Vry, 2010 (Heinrich et al 1999;Mountain and Seward, 2003;Simon et al, 2006;Pokrovski et al, 2008;Seo et al, 2009;Landtwing et al, 2010). As a result, the solubility of chalcopyrite daughter crystals in quartz-hosted inclusions may be controlled by several different equilibria, some of which may be influenced by fH 2 and some of which may not.…”

“…This process was interpreted to continue until the system reached chemical equilibrium, explaining why S-rich vapor inclusions appear to be particularly prone to Cu gain (Lerchbaumer and Audétat, 2012) and why quartz-hosted inclusions commonly record 1:2 molar ratios of Cu and S (i.e. buffered to chalcopyrite stoichiometry by adding Cu until S content of the inclusion was exhausted; Seo et al, 2009).…”