Sulfosalt melts and heavy metal (As‐Sb‐Bi‐Sn‐Pb‐Tl) fractionation during volcanic gas expansion: the El Indio (Chile) paleo‐fumarole

Abstract

2012). Sulfosalt melts and heavy metal (As-Sb-Bi-Sn-Pb-Tl) fractionation during volcanic gas expansion: The El Indio (Chile) paleo-fumarole. Geofluids, 12 (3),[199][200][201][202][203][204][205][206][207][208][209][210][211][212][213][214][215] Sulfosalt melts and heavy metal (As-Sb-Bi-Sn-Pb-Tl) fractionation during volcanic gas expansion: The El Indio (Chile) paleo-fumarole AbstractHigh-sulfidation vein gold deposits such as El Indio, Chile, formed in fracture arrays <1000m beneath paleosolfatara in volcanic terranes. Stable isotope data have confirmed a predominance of magmatic vapor during the deposition of arsenic-rich sulfide-sulfosalt assemblages in this deposit. These provide a unique opportunity to analyze the processes and products of high-temperature volcanic gas expansion in fractures that form the otherwise inaccessible infrastructure deep inside equivalent present-day fumaroles. We provide field emission scanning electron microscope and LA-ICP-MS micro-analytical data for the wide range of>heavy, semi-metals and metalloids (arsenic, antimony, bismuth, tin, silver, gold, tellurium and selenium) in the complex pyrite-enargite-Fe-tennantite assemblages from Copper Stage mineralization in the El Indio deposit. These data document the progressive fractionation of antimony and other heavy metals, such as bismuth, during crystallization from a sulfosalt melt that condensed from expanding vapor at about 15MPa (150bars) and >650°C following higher temperature vapor deposition of crystalline pyrite and enargite. The sulfosalt melt aggressively corroded the earlier enargite and pyrite and hosts clusters of distinctive euhedral quartz crystals. The crystallizing sulfosalt melt also trapped an abundance of vugs within which heavy metal sulfide and sulfosalt crystals grew together with K-Al silicates and fluorapatite. These data and their geologic context suggest that, in high-temperature fumaroles on modern active volcanoes, over 90% of the arsenic content of the primary magmatic vapor (perhaps 2000mgkg -1) was precipitated subsurface as sulfosalt. ABSTRACTHigh-sulfidation vein gold deposits such as El Indio, Chile, formed in fracture arrays <1000 m beneath paleo-solfatara in volcanic terranes. Stable isotope data have confirmed a predominance of magmatic vapor during the deposition of arsenic-rich sulfide-sulfosalt assemblages in this deposit. These provide a unique opportunity to analyze the processes and products of high-temperature volcanic gas expansion in fractures that form the otherwise inaccessible infrastructure deep inside equivalent present-day fumaroles. We provide field emission scanning electron microscope and LA-ICP-MS micro-analytical data for the wide range of heavy, semi-metals and metalloids (arsenic, antimony, bismuth, tin, silver, gold, tellurium and selenium) in the complex pyrite-enargite-Fe-tennantite assemblages from Copper Stage mineralization in the El Indio deposit. These data document the progressive fractionation of antimony and other heavy metals, such as bismuth, during crystall...

Sulfur isotope and trace element systematics of zoned pyrite crystals from the El Indio Au–Cu–Ag deposit, Chile

“…2): (1) pre-ore massive pyrite vein filling ('early pyrite'); (2) enargitetennantite mineralization (containing up to 10 % Cu and 120 ppm Ag), followed by; and (3) 'Au-stage' (>100 ppm Au) quartz-pyrite-sulfosalt veins (containing 'late pyrite') (Jannas et al 1990;Deyell et al 2004). Jannas et al (1990Jannas et al ( , 1999 observed that massive enargite and pyrite fill 'Cu-stage' veins; however, Henley et al (2012) clarify that deposition of massive pyrite veins preceded sulfosalt mineralization at El Indio, as veins of sulfosalt fill fractures crosscutting massive pyrite. Hence, we refer to this massive pyrite as 'early pyrite,' Sulfosalt phase relations and trace element chemistry suggest that sulfosalt assemblages condensed from one-phase magmatic vapor 1 above 665 °C and below 694 °C (the melting point of the crystalline enargite that precedes Fe-tennantite) (Henley et al 2012).…”

“…Jannas et al (1990Jannas et al ( , 1999 observed that massive enargite and pyrite fill 'Cu-stage' veins; however, Henley et al (2012) clarify that deposition of massive pyrite veins preceded sulfosalt mineralization at El Indio, as veins of sulfosalt fill fractures crosscutting massive pyrite. Hence, we refer to this massive pyrite as 'early pyrite,' Sulfosalt phase relations and trace element chemistry suggest that sulfosalt assemblages condensed from one-phase magmatic vapor 1 above 665 °C and below 694 °C (the melting point of the crystalline enargite that precedes Fe-tennantite) (Henley et al 2012). Since the stability limit of pyrite is 742 °C and no major break in the paragenetic sequence is observed, it is reasonable to conclude that pyrite deposition occurred between 665 and 742 °C.…”

“…Within this region, vein mineralization is restricted to an area ~2 km 2 hosted within a syn-volcanic array of faults and fractures that overprint wallrock alteration and silicification ( Fig. 1) (Berger and Henley 2011;Henley et al 2012).…”

“…The El Indio Au-Ag-Cu high-sulfidation deposit is located 500 km north of Santiago in the high Andes near the Chilean-Argentinian border (Jannas et al 1990;Henley et al 2012). The principal ore minerals are enargite (Cu 3 AsS 4 ), tennantite (Cu 6 [Cu 4 (Fe,Zn) 2 ]As 4 S 13 ) and native Au, hosted in a complex network of veins within a sequence of altered ignimbrites and brecciated tuffs (Jannas et al 1990).…”

Platinum-group minerals in the Skouries Cu-Au (Pd, Pt, Te) porphyry deposit

“…In principle, co-existing polymetallic melts in hydrothermal systems can partition metals from the fluid and concentrate them as melt components (Douglas, 2000), and sulfosalt melts have been shown to Ore Geology Reviews 99 (2018) 344-364 have condensed from expanding magmatic vapour at temperatures of ∼650°C in the El Indio paleo-fumarole (Henley et al, 2012;Henley and Berger, 2013;Mavrogenes et al, 2010). Semi-metals like Bi and Te at high concentrations can act as powerful fluxes, lowering the melting point of metals such as Au and the PGE -for example the Au-Bi melt has a eutectic at 241°C (Gather and Blachnik, 1974;Okamoto and Massalski, 1983).…”