Platinum-group elements and gold in sulfide melts from modern arc basalt (Tolbachik volcano, Kamchatka)

Abstract

Sulfide melt inclusions entrapped in primitive olivine phenocrysts can be used to understand the compositions of early sulfide melts that may ultimately contribute to magmatic sulfide ore deposits. Sulfide globules hosted in olivine (86-92 mol% Fo) from the Tolbachik basalt (the 1941 eruption) are characterized in terms of their major and trace element abundances using electron microscopy and LA-ICP-MS analysis. Distribution of major elements within individual sulfide globules varies from homogeneous to heterogeneous. Phases include monosulfide solid solution (MSS) and intermediate solid solution (ISS) intergrowths and exsolved low-temperature minerals such as pyrrhotite, pentlandite, chalcopyrite and cubanite. Trace elements (platinum-group elements-PGE, Ag, Te, Au, Pb and Bi) are also present in solid solution in sulfide phases and as micronsized particles ("nuggets"). Such nuggets of dominantly Au, Pt, Au-Pd and Pd-Te are contained randomly within sulfide matrices or, more commonly, at phase boundaries. Nuggets are also attached to outer surfaces of sulfide globules. Concentrations of PGE in sulfides follow a log normal distribution over four orders of magnitude. The highest measured noble metal

Immiscible sulfide melts in primitive oceanic magmas: Evidence and implications from picrite lavas (Eastern Kamchatka, Russia)

“…In contrast to constraints from theoretical and experimental modelling (e.g., Naldrett, 2004;Zhang, 2015), the olivine-hosted sulfide globules in this and other studies (e.g., Ackermand et al, 2007;Czamanske and Moore, 1977;Francis, 1990;Zelenski et al, 2017) are not pure FeS, but contain substantial amounts of other metals ( Fig. 5; (Ni+Cu)/Fe = 0.47-0.59).…”

“…Unfortunately, the majority of continental rocks that host such deposits do not fit general criteria for primary/primitive melts (i.e., they feature <8 wt% MgO and the absence of high-forsteritic olivine phenocrysts). Even in the case of rare high-Mg rocks with primitive olivine in some large igneous provinces on continents (see review in Kamenetsky et al, 2017), a record of the early sulfide immiscibility is yet to be discovered. In contrast to continental magmas, sulfide globules in phenocrysts and glasses at mid-ocean ridge basalts (MORB) have been documented since the underwater samples became available in the 1960's (e.g., Czamanske and Moore, 1977;Francis, 1990;Kanehira et al, 1973;MacLean, 1977;Mathez, 1976;Moore and Calc, 1971;Peach et al, 1990).…”

Fluxing of mantle carbon as a physical agent for metallogenic fertilization of the crust

“…Subsequent transport of metals in ascending melts occurs either as nanoparticles or nanomelts, dissolved ions in the silicate melt, or, primarily, within sulfide liquid droplets 2 , 3 , 7 . Metal-rich alloys may be transported alongside sulfide droplets 8 and are entrained within the silicate melt, or collected by the sulfide liquid, as nano- to micrometer-size particles 8 – 10 . Essentially, this scenario can be viewed as a relatively “dry” magmatic system, with the fluxing of metals across the lithosphere being largely dominated by chemical processes.…”

Geochemistry, petrogenesis, and tectonic setting of the Los Tuxtlas Volcanic Field, Mexico

“…Average compositions of the ultrabasic and basic rocks as well as the intermediate rock from the LTVF (Figure 8) are compared with three basaltic compositions (average Icelandic basalt from Rehkämper et al, 1999; basaltic melt modelled from the asthenospheric upper mantle from Bockrath, Ballhaus, and Holzheid (2004); and modern arc basalt from the 1941 eruption of the Tolbachik volcano, Kamchatka from Zelenski et al (2017). The Icelandic basalt and the modelled basaltic melt had only four PGEs (Ir, Ru, Rh, and Pt) data.…”