Redox-controlled chalcophile element geochemistry of the Polaris Alaskan-type mafic-ultramafic complex, British Columbia, Canada

2021 ◽  
Vol 59 (6) ◽  
pp. 1627-1660
Author(s):  
Dejan Milidragovic ◽  
Graham T. Nixon ◽  
James S. Scoates ◽  
James A. Nott ◽  
Dylan W. Spence
Keyword(s):  
Parental Magma ◽  
Primitive Mantle ◽  
Sulfide Saturation ◽  
Element Geochemistry ◽  
Positive Dependence ◽  
The North ◽  
Economic Significance ◽  
Pge Mineralization ◽  
Fe Alloys ◽  
Platinum Group

ABSTRACT The Early Jurassic Polaris Alaskan-type intrusion in the Quesnel accreted arc terrane of the North American Cordillera is a zoned, mafic-ultramafic intrusive body that contains two main styles of magmatic mineralization of petrologic and potential economic significance: (1) chromitite-associated platinum group element (PGE) mineralization hosted by dunite (±wehrlite); and (2) sulfide-associated Cu-PGE-Au mineralization hosted by olivine (±magnetite) clinopyroxenite, hornblendite, and gabbro-diorite. Dunite-hosted PGE mineralization is spatially associated with thin discontinuous layers and schlieren of chromitite and chromitiferous dunite and is characterized by marked enrichments in iridium-subgroup PGE (IPGE) relative to palladium-subgroup PGE (PPGE). Discrete grains of platinum group minerals (PGM) are exceedingly rare, and the bulk of the PGE are inferred to reside in solid solution within chromite±olivine. The absence of Pt-Fe alloys in dunite of the Polaris intrusion is atypical, as Pt-enrichment of dunite-hosted chromitite is widely regarded as a characteristic feature of Alaskan-type intrusions. This discrepancy appears to be consistent with the strong positive dependence of Pt solubility on the oxidation state of sulfide-undersaturated magmas. Through comparison with experimentally determined PGE solubilities, we infer that the earliest (highest temperature) olivine-chromite cumulates of the Polaris intrusion crystallized from a strongly oxidized ultramafic parental magma with an estimated log f(O2) > FMQ+2. Parental magmas with oxygen fugacities more typical of volcanic arc settings [log f(O2) ∼ FMQ to ∼ FMQ+2] are, in turn, considered more favorable for co-precipitation of Pt-Fe alloys with olivine and chromite. More evolved clinopyroxene- and hornblende-rich cumulates of the Polaris intrusion contain low abundances of disseminated magmatic sulfides, consisting of pyrrhotite and chalcopyrite with minor pentlandite, pyrite, and rare bornite (≤12 wt.% total sulfides), which occur interstitially or as polyphase inclusions in silicates and oxides. The sulfide-bearing rocks are characterized by strong primitive mantle-normalized depletions in IPGE and enrichments in Cu-PPGE-Au, patterns that resemble those of other Alaskan-type intrusions and primitive arc lavas. The absolute abundances and sulfur-normalized whole-rock concentrations (Ci/S, serving as proxy for sulfide metal tenor) of chalcophile elements, including Cu/S, in sulfide-bearing rocks are highest in olivine clinopyroxenite. Sulfide saturation in the relatively evolved magmas of the Polaris intrusion, and Alaskan-type intrusions in general, appears to be intimately tied to the appearance of magnetite. Fractional crystallization of magnetite during the formation of olivine clinopyroxenite at Polaris resulted in reduction of the residual magma to log f(O2) ≤ FMQ+2, leading to segregation of an immiscible sulfide melt with high Cu/Fe and Cu/S, and high PGE and Au tenors. Continued fractionation resulted in sulfide melts that were progressively more depleted in precious and base chalcophile metals. The two styles of PGE mineralization in the Polaris Alaskan-type intrusion are interpreted to reflect the evolution of strongly oxidized, hydrous ultramafic parental magma(s) through intrinsic magmatic fractionation processes that potentially promote sulfide saturation in the absence of wallrock assimilation.

Platinum-group element geochemistry of intraplate basalts from the Aleppo Plateau, NW Syria

2012 ◽  
Vol 150 (3) ◽  
pp. 497-508 ◽  
Author(s):  
GEORGE S.-K. MA ◽  
JOHN MALPAS ◽  
JIAN-FENG GAO ◽  
KUO-LUNG WANG ◽  
LIANG QI ◽  
...  
Keyword(s):  
Partial Melting ◽  
Middle Miocene ◽  
Primitive Mantle ◽  
Element Geochemistry ◽  
Alkali Basalts ◽  
Melt Extraction ◽  
Mid Ocean Ridge ◽  
Order Of Magnitude ◽  
Platinum Group ◽  
Ocean Ridge

AbstractEarly–Middle Miocene intraplate basalts from the Aleppo Plateau, NW Syria have been analysed for their platinum-group elements (PGEs). They contain extremely low PGE abundances, comparable with most alkali basalts, such as those from Hawaii, and mid-ocean ridge basalts. The low abundances, together with high Pd/Ir, Pt/Ir, Ni/Ir, Cu/Pd, Y/Pt and Cu/Zr are consistent with sulphide fractionation, which likely occurred during partial melting and melt extraction within the mantle. Some of the basalts are too depleted in PGEs to be explained solely by partial melting of a primitive mantle-like source. Such ultra-low PGE abundances, however, are possible if the source contains some mafic lithologies. Many of the basalts also exhibit suprachondritic Pd/Pt ratios of up to an order of magnitude higher than primitive mantle and chondrite, an increase too high to be attributable to fractionation of spinel and silicate minerals alone. The elevated Pd/Pt, associated with a decrease in Pt but not Ir and Ru, are also inconsistent with removal of Pt-bearing PGE minerals or alloys, which should have concurrently lowered Pt, Ir and Ru. In contrast, melting of a metasomatized source comprising sulphides whose Pt and to a lesser extent Rh were selectively mobilized through interaction with silicate melts, may provide an explanation.

Ophiolite-related associations of platinum-group minerals at Rudnaya, western Sayans and Miass, southern Urals, Russia

2018 ◽  
Vol 82 (3) ◽  
pp. 515-530 ◽  
Author(s):  
Andrei Y. Barkov ◽  
Nadezhda D. Tolstykh ◽  
Gennadiy I. Shvedov ◽  
Robert F. Martin
Keyword(s):  
Southern Urals ◽  
Chromian Spinel ◽  
Type Alloy ◽  
Chelyabinsk Oblast ◽  
The Southern Urals ◽  
Platinum Group Minerals ◽  
Pge Mineralization ◽  
Fe Alloys ◽  
Platinum Group ◽  
Western Sayan

ABSTRACTWe describe similar assemblages of minerals found in two placers in Russia, both probably derived from an ophiolitic source. The first is located along the River Rudnaya in the western Sayan province, Krasnoyarskiy kray, and the second pertains to the Miass placer zone, Chelyabinsk oblast, in the southern Urals. The platinum-group element (PGE) mineralization in both cases is mostly (at least 80%) represented by alloy minerals in the system Ru–Os–Ir, in the order of occurrence osmium, ruthenium and iridium. The remainder consists of Pt–Fe alloys and species of PGE sulfides, arsenides, sulfarsenides, etc. The associated olivine and amphiboles are supermagnesian, and the chromian spinel has a high Cr# value. The observed enrichment in Ru, typical of an ophiolitic source, may be due to high-temperature hydrothermal equilibration and mobilization in the ophiolite, as is the enrichment in Mg and Cr. Low-temperature replacement of the alloys led to the development of laurite, sulfoarsenides and arsenides. Some placer grains in both suites reveal unusual phases of sulfo-arsenoantimonides of Ir–Rh, e.g. the unnamed species (Rh,Ir)SbS and (Cu,Ni)1+x(Ir,Rh)1–xSb, wherex= 0.2, and rhodian tolovkite, (Ir,Rh)SbS. Two series of natural solid-solutions appear to occur in the tolovkite-type phases. Among the oddities in the Rudnaya suite are globules of micrometric PGE sulfides, crystallites of platinum-group minerals, amphibole, and chalcopyrite bearing skeletal micrometric monosulfide-like compounds (Cu,Pt,Rh)S and (Pd,Cu)S1–x. Pockets of fluxed evolved melt seem to have persisted well below the solidus of the host Pt3Fe-type alloy.

PGE Geochemical Characteristics of the Huangshanxi Magmatic Ni-Cu Sulfide Deposit, East Tianshan, Xinjiang, and its Significance for the Mineralization

2014 ◽  
Vol 962-965 ◽  
pp. 164-167
Author(s):  
Yong Fu ◽  
Tao Sun
Keyword(s):  
Total Concentration ◽  
Crustal Contamination ◽  
Parental Magma ◽  
Platinum Group Elements ◽  
Geochemical Characteristics ◽  
Sulfide Deposit ◽  
Average Value ◽  
The North ◽  
Deep Crust ◽  
Platinum Group

The Huangshanxi sulfide-bearing intrusion, is located in the centre segment of Tudun-Huangshan-Jing`erquan-Tulargen mafic-ultramafic rock belt, the eastern part of the North Tianshan, controlled by the Kangurtag-Huangshan ductile sheer zone, and it is a multiple intrusion which composes of lherzolite, amphibole peridotite, wehrlite, pyroxenite, norite, gabbro, and diorite. The disseminated sulfides and sideronitic sulfides are the mainly ore types, the scale of the ore body is large and the grade is stable relatively. The total concentration of platinum-group elements (PGEs) in rocks and ores is very low, which average value is 0.93ppb and 8.8ppb respectively and it increases with sulfur content increases in ores. The PGE concentration appeared two peaks in the range of 200~300m and 880~980m depth in the drill core, consistent with the content of the sulfides. Rocks and ores samples have similar mantle-normalized PGE patterns which are shown as the PGE flat and slightly fall to the left, and the lower fractionation between IPGE and PPGE. The primitive magma may be the high MgO tholeiitic magma which should be undepleted in PGE and derived from partial melting of a metasomatized mantle source. Compared with continental tholeiite, simulating result reveals that the parental magma is visibly depleted in PGE, possibly duing to the sulfide pre-segregation of initial magma in deep crust. The platinum-group elements geochemical characteristics and petrochemical data show that the crustal contamination and the fractionation of olivine and pyroxene may be the main factors leading to S-saturation and sulfide segregation in deep crust.

scholarly journals Trace element geochemistry and PGE potential of noritic bodies from Uauá block, Bahia - Brazil

1990 ◽  
Vol 13 ◽  
pp. 1-7
Author(s):  
Elson Paiva de Oliveira
Keyword(s):  
Trace Element ◽  
Incompatible Trace Element ◽  
Platinum Group Elements ◽  
Primitive Mantle ◽  
Trace Element Geochemistry ◽  
Element Geochemistry ◽  
Platinum Group ◽  
Two Bodies ◽  
Field Relationship

Preliminary field relationship and incompatible trace element geochemistry on two bodies of noritic rocks from Uauá area are discussed with the aim of evaluating their metallogenic potential. The rocks show primitive mantle normalized and chondrite normalized patterns very similar to chilled margins and parental magmas to the Bushveld and Insizwa complexes (RSA) which host major platinum group elements deposits. The data suggest the Uauá noritic bodies may be potential for ore mineralisation.

Platinum-group minerals of the F and T zones, Waterberg Project, far northern Bushveld Complex: implication for the formation of the PGE mineralization

2018 ◽  
Vol 82 (3) ◽  
pp. 539-575 ◽  
Author(s):  
Matthew J. G. McCreesh ◽  
Marina A. Yudovskaya ◽  
Judith A. Kinnaird ◽  
Christian Reinke
Keyword(s):  
Bushveld Complex ◽  
Magmatic Sulfides ◽  
Platinum Group Minerals ◽  
Pge Mineralization ◽  
Base Metal Sulfides ◽  
Fe Alloys ◽  
Platinum Group ◽  
Current Stage ◽  
Southern Marginal Zone ◽  
Sulfide Assemblage

ABSTRACTThis study provides the first detailed mineralogical data on platinum-group element (PGE) mineralization of the Waterberg Project, in a previously unknown segment of the Bushveld Complex located in the Southern Marginal Zone of the Limpopo Belt. The lower ultramafic F zone is dominated by sperrylite (up to 82 area%) with minor Pt–Pd bismuthotellurides, Pd–Ni arsenides, Au–Ag alloy, Rh–Pt sulfoarsenides and rare Pt–Fe alloys. The upper more felsic-rich gabbroic T zone is dominated by Pt–Pd bismuthotellurides (up to 90 area%), Pd tellurides and Au–Ag alloy with rare sperrylite, braggite, Pd stannides and antimonides. The platinum-group minerals (PGM) of the F zone are associated mainly with magmatic base-metal sulfides (pyrrhotite, troilite, chalcopyrite and pentlandite), that have undergone alteration during significant serpentinization, accompanied by the formation of the secondary sulfide assemblage. The T zone in a leucogabbroic sequence contains relics of magmatic sulfides and is characterized by the development of the indicative chalcopyrite-millerite-pyrite assemblage, which is associated with widespread hydrothermal quartz and hydrous silicates (amphiboles, phlogopite, epidote and chlorite). The fluid-induced style of PGM remobilization, the high Au/PGE and the high proportion of native gold in the high-grade T zone ores in the magnetite-bearing leucogabbroic rocks are unique to the Bushveld Complex. The genesis of the T ores is interpreted as a result of primary PGE enrichment in the zone of interaction between the first influxes of the Upper Zone fertile melt and a resident gabbroic melt at the top of the Troctolite-Gabbronorite-Anorthosite (TGA) fractionated sequence with subsequent fluid remobilization. Whether the hydrothermal overprint facilitated the PGE sequestration in a favourable setting or dispersed the pre-existing magmatic concentrations along fluid pathways remains essentially unresolved at the current stage.

scholarly journals Parental magma, magmatic stratigraphy, and reef-type PGE enrichment of the 2.44-Ga mafic-ultramafic Näränkävaara layered intrusion, Northern Finland

2019 ◽  
Vol 55 (8) ◽  
pp. 1535-1560 ◽  
Author(s):  
Ville Järvinen ◽  
Tapio Halkoaho ◽  
Jukka Konnunaho ◽  
Jussi S. Heinonen ◽  
O. Tapani Rämö
Keyword(s):  
Parental Magma ◽  
Layered Intrusion ◽  
Border Zone ◽  
Primitive Mantle ◽  
Layered Intrusions ◽  
Sulfide Saturation ◽  
Forming Process ◽  
Layered Series ◽  
Reef Type ◽  
High Sulfide

AbstractAbout 20 mafic-ultramafic layered intrusions in the northern Fennoscandian shield were emplaced during a widespread magmatic event at 2.5–2.4 Ga. The intrusions host orthomagmatic Ni-Cu-PGE and Cr-V-Ti-Fe deposits. We update the magmatic stratigraphy of the 2.44-Ga Näränkävaara mafic-ultramafic body, northeastern Finland, on the basis of new drill core and outcrop observations. The Näränkävaara body consists of an extensive basal dunite (1700 m thick), and a layered series comprising a peridotitic–pyroxenitic ultramafic zone (600 m thick) and a gabbronoritic–dioritic mafic zone (700 m thick). Two reversals are found in the layered series. The composition of the layered series parental magma was approximated using a previously unidentified marginal series gabbronorite. The parental magma was siliceous high-Mg basalt with high MgO, Ni, and Cr, but also high SiO2 and Zr, which suggests primary magma contamination by felsic crust. Cu/Pd ratio below that of primitive mantle implies PGE-fertility. The structural position of the marginal series indicates that the thick basal dunite represents an older wallrock for the layered intrusion. A subeconomic reef-type PGE-enriched zone is found in the border zone between the ultramafic and mafic zones and has an average thickness of 25 m with 150–250 ppb of Pt + Pd + Au. Offset-type metal distribution and high sulfide tenor (50–300 ppm Pd) and R-factor (105) suggest reef formation by sulfide saturation induced by fractional crystallization. The reef-forming process was probably interrupted by influx of magma related to the first reversal. Metal ratios suggest that this replenishing magma was PGE-depleted before emplacement.

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