Bi-melt formation and gold scavenging from hydrothermal fluids: An experimental study Blake Tooth a , Cristiana L. Ciobanu a,b , Leonard Green c , Brian O’Neill d , Joe ¨l Brugger a,b,⇑ a Centre for Tectonics, Resources and Exploration (TRaX), School of Earth and Environmental Sciences, University of Adelaide, Adelaide, SA 5005, Australia b Department of Mineralogy, South Australian Museum, North Terrace, Adelaide, SA 5000, Australia c Adelaide Microscopy, University of Adelaide, Adelaide, SA 5005, Australia d School of Chemical Engineering, University of Adelaide, Adelaide, SA 5005, Australia Received 21 December 2010; accepted in revised form 9 June 2011onlinedt; available online 22 July 2011 Abstract The role of polymetallic melts in scavenging ore components has recently been highlighted in the context of fluid-poor meta- morphosed ore deposits. In contrast, the role of polymetallic melts in systems dominated by hydrothermal fluids remains poorly understood. Using a simple Au–Bi model system, we explored experimentally whether such polymetallic melts can precipitate directly from a hydrothermal fluid, and investigated the ability of these melts to scavenge Au from the solution. The experiments were conducted in custom-built flow-through reactors, designed to reproduce a hydrothermal system where melt components are dissolved at one stage along the flow path (e.g., Bi was dissolved by placing Bi-minerals along the fluid path), whereas melt pre- cipitation was caused further along the flow path by fluid–rock interaction. Bi-rich melts were readily obtained by reaction with pyrrhotite, graphite or amorphous FeS. When Au was added to the system, Bi–Au melts with compositions consistent with the Au–Bi phase diagram were obtained. In the case of fluid reaction with pyrrhotite, epitaxial replacement of pyrrhotite by magne- tite was observed, with textures consistent with an interface-coupled dissolution–reprecipitation reaction (ICDRR). In this case, the metallic melt precipitated as blebs that were localized at the replacement front or within the porous magnetite. Direct fractionation of Bi–Au melts from a hydrothermal fluid, or precipitation of a Bi-melt followed by partitioning of Au from ambient fluid, offer new pathways to the enrichment of minor ore components such as Au, without requiring fluid sat- uration with respect to a Au mineral. This mechanism can explain the strong geochemical affinity recognized between Au and low-melting point chalcophile elements such as Bi in many gold deposits. Examples of deposits where such a model may be applicable include orogenic gold deposits and gold skarns. Contrary to models involving migration of polymetallic melts to explain element remobilization, only small quantities (ppm) of polymetallic melts are required to affect the Au endowment of a deposit via interaction with a hydrothermal fluid. The experiments also show that micro-environments can play a critical role in controlling melt occurrences. For example, reaction fronts developing via ICDR reactions can promote melt formation as observed during the replacement of pyrrhotite by magnetite. The associated transient porosity creates space for the melt and promotes melt-fluid exchanges whereas the reaction front provides local geochemical conditions favorable to melt pre- cipitation (e.g., reduced, low aH 2 S(aq), and catalytic surface). Ó 2011 Elsevier Ltd. All rights reserved. 1. INTRODUCTION One of the recent breakthrough concepts in economic geology relates to the formation of melts by low melting chal- cophile elements (e.g., Bi, Te, and Pb; LMCE as outlined by 0016-7037/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.gca.2011.07.020 ⇑ Corresponding author at: Centre for Tectonics, Resources and Exploration (TRaX), School of Earth and Environmental Sciences, University of Adelaide, Adelaide, SA 5005, Australia. Tel.: +61 8 8207 7451; fax: +61 8 8207 7222. E-mail addresses: joel.brugger@adelaide.edu.au, Joel.Brugger@ samuseum.sa.gov.au (J. Brugger). www.elsevier.com/locate/gca Available online at www.sciencedirect.com Geochimica et Cosmochimica Acta 75 (2011) 5423–5443