Noble gas isotopic constrains on mineralization of the Jinchuan Cu-Ni-PGE sulfide deposit,West China
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Abstract:
Noble gases are recognized as being exceptional tracers of volatile sources during mineralization He, Ne and Ar abundance and isotopic compositions from 23 silicate and sulfide separates in the Jinchuan Cu-Ni-PGE sulfide deposit, West China, have been investigated by melting extraction MM5400 mass spectrometer The results show that (3)He/(4)He ratios in silicate minerals (av 0 239Ra) are slightly lower than those in sulfide (av 0 456Ra), and decrease from olivine (av 0 291Ra), orthopyroxene (av 0 215Ra) to clinopyroxene (av 0 174Ra) (20)Ne/(22)Ne and (21)Ne/(22)Ne ratios are plotted Into the ranges between MORB line and continental crust line (3)He/(4)He and (40)Ar/(36)Ar of olivine (Olv) and orthopyroxene (Opx) deducted by radiogenic He and Ne are close to the He and Ar isotopic ranges of subcontinental lithosphere mantle (SCLM) He, Ne and Ar isotopic compositions suggested that SCLM, continental crust (CC) and air saturated water (ASW) three end-members were involved into ore-forming magma of the Jinchuan Cu-Ni-PGE sulfide deposit The segregation of immiscible sulfide liquid from magma took place at the early stage of magma evolution The primary ore forming magma had been formed by partial melting of SCLM, and undergone two stages of evolution It had been assimilated possibly with wall rock which resulted in the sulfur supersaturating in silicate magma and segregation of immiscible sulfide melt and formed a mixed component (MC) with SCLM and continental crust (CC) signatures The mixed component (MC) emplaced into upper chamber and mixed with a high proportion of deep circulatory ASW which induced sulfur supersaturating of ore-forming magma and in-situ formed the disseminated ore in upper magma chamberKeywords:
Radiogenic nuclide
Magma chamber
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Platinum group
Fractional crystallization (geology)
Igneous differentiation
Peridotite
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δ18O
Isotopes of argon
Crustal recycling
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Felsic
Isotopes of argon
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Incompatible element
Primitive mantle
Peridotite
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Radiogenic nuclide
Baltic Shield
Isochron dating
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Platinum group
Fractional crystallization (geology)
Magma chamber
Large igneous province
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Lithophile
Primitive mantle
Igneous differentiation
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Trace element
Country rock
Ultramafic rock
Petrogenesis
Baddeleyite
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Isotopes of argon
Isotopes of helium
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The Palaeoproterozoic Penikat layered ultramafic–mafic intrusion in northern Finland is one of the most richly mineralized layered intrusions on Earth, containing at least six platinum-group element (PGE) enriched horizons exposed along >20 km of strike, amongst them the SJ reef, which at ∼3–7 ppm Pt + Pd over a width of ∼1–2 m is surpassed by few other PGE reefs globally in terms of its endowment in PGE. Important PGE enrichments also occur in the PV reef (average 2·6 ppm Pd, 4 ppm Pt over 1·1 m) and AP1 reef (average 6·2 ppm Pd, 1·7 ppm Pt over 0·7 m). Here we present new major and high-precision trace element and Nd isotope data from a traverse across the intrusion, and a new U–Pb age of 2444 ± 8 Ma for the intrusion. We show that the PGE reefs formed by predominantly orthomagmatic processes as, for example, reflected by well-defined positive correlations between Pt + Pd and Os + Ir + Ru contents. Late-magmatic fluids played no significant role in concentrating PGE. There are at least six cyclic units in the intrusion, displaying a progressive upward decrease in differentiation indices Mg# and Cr/V. Subdued stratigraphic variations in incompatible trace element ratios (Ce/Sm mostly 5–10) and Nd isotope compositions (εNd –3 to –1) indicate that mixing of magmas of distinct lineage, or in situ contamination with country rocks, was not required to form the PGE reefs. There is also no evidence for addition of external sulphur to the magma, based on S/Se ratios at, or below, primitive mantle levels. Instead, we propose that sulphide melt saturation at Penikat was reached in response to fractionation of a siliceous, high-magnesium basalt, and that the sulphides were concentrated through hydrodynamic phase sorting, consistent with bonanza-style PGE grades in large potholes.
Platinum group
Geochronology
Trace element
Ultramafic rock
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