CODES – Centre for Ore Deposit and Earth Sciences
MELT-FLUID EVOLUTION, MAGMATIC IMMISCIBILITY AND BUDGET OF CHALCOPHILE AND NOBLE METALS IN BASALTIC MAGMAS
LEADER: | |||
Vadim Kamenetsky | |||
TEAM MEMBER: | |||
Maya Kamenetsky | |||
STUDENT: | |||
Adam Abersteiner | |||
COLLABORATORS: | |||
Kathy Ehrig Michael Zelenski Ilya Chaplygin | BHP Institute of Experimental Mineralogy, Russia Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry (IGEM), Russia | ||
Alexander Belousov Pavel Nesterenko Liudmila Zhitova | Institute of Volcanology and Seismology, Russia Moscow State University, Russia Novosibirsk State University, Russia | ||
Victor Sharygin | Sobolev Institute of Geology and Mineralogy, Russia |
PROJECT SUMMARY
2019
This research aims to establish the initial metal abundances in common primitive magmas, and the mechanisms of separation of immiscible liquids and fluids from the silicate melt, through studies of melt and fluid inclusions in minerals.
Subvolcanic environments in supra-subduction zones are renowned for hosting epithermal deposits that often contain electrum and native gold, including bonanza examples. Our study published during 2019 in Terra Nova examines mineral assemblages and processes occurring in shallow-crust volcanic settings using materials from recent eruptions (2012–2013) of the basaltic Tolbachik volcano in the Kamchatka arc. The Tolbachik eruptive system is characterised by an extensive system of lava tubes. After cessation of magma input, the tubes maintained the flow of hot oxidised gases that episodically interacted with the lava surfaces and sulfate-chloride fumarolic assemblage on these surfaces. The gas-rock interaction had strong pyrometamorphic effects that resulted in the formation of molten salt, oxidised (tenorite, hematite, Cu-rich magnesioferrite) and skarn-like silicate mineral assemblages. By analogy with experimental studies and metallurgical practices we propose that this naturally occurring smelting process was responsible for extraction of metals from the basaltic wall rocks and deposition of Cu-, Fe- and Cu-Fe-oxides and native gold.
Near-magmatic temperature and efficiency of metal smelting in the post-eruptive environment are facilitated by elevated oxygen fugacity, which is caused by ingress of air into episodically emptied magmatic plumbing systems and consequent oxidation of fluids and metals. We further argue that metal build-ups, although initially sub-economic, accompany and postdate every stage of magma injection and eruption. Metal accumulations scattered vertically and laterally in shallow magmatic conduit can be upgraded by coupled dissolution and re-deposition in successive volcanic cycles. It is also anticipated that long-lived volcanic systems, processing tens to hundreds of cubic km of magmas to the surface, are capable of attaining high gold endowments by scavenging precursor, fumarole-style accumulations of metals.
2018
This research aims to establish the initial metal abundances in common primitive magmas, and the mechanisms of separation of immiscible liquids and fluids from the silicate melt, through studies of melt and fluid inclusions in minerals.
Magma unmixing into separate sulfide and silicate melts is a key process in the formation of magmatic sulfide ore deposits. Our paper in Lithos (Zelenski et al.) presents sulfide inclusions in olivine from the Tolbachik volcano (Kamchatka) that preserve the original chemical composition of the segregated sulfide melt. Studied sulfide globules display a variety of textures ranging from homogeneous to fine-grained to coarse-grained and lamellar. They depend on the cooling rates of olivine phenocrysts that are estimated to vary by five orders of magnitude. The Fe:Ni:Cu ratio also controls the resulting texture, whereas the size of the globules does not have any significant effect. The morphology and textural patterns of individual grains indicate that fine-grained textures resulted from the rapid breakdown of a homogeneous solid phase, which the sulfide melt solidified into during extremely rapid quenching. The presence of large or abundant small pores in sulfides indicates the separation of an appreciable amount of dissolved fluid from the sulfide melt during crystallisation. Sulfides in the form of thin foils and planar swarms in healed cracks can be assigned to cyclic pressure changes that resulted in phenocrysts rupturing and healing.
A complementary study has been published in American Mineralogist (Savelyev et al.). It is the first detailed report of the crystallised sulfides melts in the Cretaceous oceanic picrites in the Kamchatsky Mys ophiolite in Eastern Kamchatka (Far East Russia). Sulfide melts are present as inclusions in olivine (87.1–89.6 mol% Fo) and interstitial to the groundmass minerals (clinopyroxene, plagioclase and Timagnetite). The sulfide melt inclusions in olivine and the groundmass are composed of several sulfide phases that correspond to the monosulfide (Fe–Ni; Mss) and intermediate (Fe–Cu–Ni; Iss) solid solutions. Several types of micron-sized Pd–Sn, Pt– and Au–Ag phases are recorded within the matrix sulfides, commonly along phase boundaries and fractures. Major elements (S, Fe, Cu, Ni, Co), platinum group elements (PGE) and gold analysed in the homogenised olivine-hosted sulfide melt inclusions, and phases identified in the matrix sulfides record the range of magmatic sulfide compositions. The most primitive sulfide liquids are notably enriched in Ni and Cu ((Ni+Cu)/Fe > 0.5), continuously evolve with crystallisation of (e.g., increasing Cu/Ni and Au/PGE) and demonstrate metal fractionation between Mss and Iss. The compositions of individual sulfide phases are strongly affected by the noble metal (PGE, Au) ‘nuggets’ that exsolve at subsolidus temperatures and form during serpentinisation of the rocks. We conclude that the budget of noble metals in the studied picrites is controlled by sulfides, but the abundances of Pt and Au are influenced by mobility in post-magmatic alteration. Our data can also be used for modelling sulfide saturation at crustal pressures and understanding behaviour of the noble metals in primitive oceanic magmas.
2017
This research aims to establish the initial metal abundances in common primitive magmas, and the mechanisms of separation of immiscible liquids and fluids from the silicate melt, through studies of melt and fluid inclusions in minerals.
Compositions of 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. In 2017 our publication in Lithos (Zelenski et al.) characterised sulfide globules hosted in olivine (86–92 mol% Fo) from the Tolbachik basalt (the 1941 eruption) in terms of their major and trace element abundances.
Trace elements (platinum-group elements – PGE, Ag, Te, Au, Pb and Bi) are present in solid solution in sulfide phases and as micron-sized particles (‘nuggets’). Such nuggets of dominantly Au, Pt, Au–Pd and Pd–Te compositions are contained randomly within sulfide matrices or, more commonly, at phase boundaries. The highest measured noble metal concentrations in the analysed globules (436 ppm Au + PGE) are 13.3 ppm Au, 115 ppm Pt and 299 ppm Pd, whereas 40% of globules have < 15 ppm of noble metals. Although the individual Tolbachik sulfide globules have variable PGE abundances, their mean composition resembles those of major PGE-sulfide ore deposits (e.g., Norilsk, Sudbury, Platreef and Merensky Reef).
Our publication in Geochimica et Cosmochimica Acta (Park et al.) aimed at understanding the factors controlling the enrichment of Rh and IPGEs in Cr-spinels in magmas in different geodynamic environments. We estimated partition coefficients between Cr-spinel and silicate melts, and investigated the role of Cr-spinel fractional crystallisation on the PGE geochemistry of primitive magmas during the early stages of fractional crystallisation.
The extent to which water and halogens in Earth’s mantle have primordial origins, or are dominated by seawater-derived components introduced by subduction was explored in our study, published in Nature Geoscience (Kendrick et al.).
In the study published in Geology (Prokofiev et al.) we report for the first time fortuitously preserved, largesized fluid inclusions in chalcedony in the basalts belonging to the Siberian large igneous province. We propose that the colloidal nature of fluids forming chalcedony lends strong support to the natural existence of experimentally predicted ‘silicothermal fluids’.
2016
This research aims at establishing the initial metal abundances in common primitive magmas, and the mechanisms of separation of immiscible liquids and fluids from the silicate melt, through studies of melt and fluid inclusion in minerals.
Results in 2016 revealed that some lava flows and scoria cones of the historic basaltic eruptions of the Tolbachik volcano (Kamchatka arc) are unusually gold-rich. Based on whole rock analyses, Tolbachik basalts contain up to 11.6 ppb gold, nuggets of gold (electrum) up to 900μm in size, native gold droplets, and numerous vapour-deposited gold crystals within fumarolic incrustations and directly on surfaces of basaltic lapilli. The occurrence of native gold of magmatic origin is extremely rare, and only a few finds of micron-sized gold particles in unaltered basalts have been documented.
The results, published in Earth and Planetary Science Letters, demonstrate that the gold nuggets in the Tolbachik basalt are of hydrothermal origin, and were physically scavenged from epithermal veins hosted by country rocks during intrusion of mafic magmas. Depending on the melt temperature, and/or the time span of the melt-rock interaction, gold was either ejected by the erupting volcano in the form of abraded nuggets or liquid droplets, or fully assimilated into the shallow, long-lived magma chamber to provide a fourfold increase in gold content over the background concentrations of 2.7 ppb Au. After the eruption, the continued discharge of volcanic gas enriched in gold led to the deposition of abundant crystals of gold on cooling lava and scoria.