The First Report of Platinum-Group Minerals in Magnetite-Bearing Gabbro, Freetown Layered Complex, Sierra Leone: Occurrences and Genesis

Bowles, John F. W.; Prichard, Hazel Margaret; Suárez, Saioa; Fisher, Peter Charles

doi:10.3749/canmin.51.3.455

Cited by 21 publications

(30 citation statements)

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“…The Haladala gabbroic intrusion is one of the largest layered ultramafic‐mafic intrusions in the Southwest Tianshan Orogen, NW China (Figure ), and has the potential of economic V‐Ti magnetite deposits based on outcrops of magnetite‐rich layers [ Gao et al ., ; Lin and Deng , ; Long et al ., ]. Layered mafic‐ultramafic complexes occur in various tectonic settings, such as island arc (Alaskan type) [e.g., Beard , ], ophiolites [e.g., Christensen and Smewing , ], intracontinental rift system [e.g., Bowles et al ., ], or mantle plume [e.g., Zhou et al ., ; Cawthorn and Ashwal , ]. Nevertheless, V‐Ti magnetite‐bearing layered mafic‐ultramafic complexes are largely restricted in the intraplate igneous provinces related to mantle plumes [ Pirajno , ], and to a lesser degree related to mantle upwelling through a slab window in a subduction setting, such as Beja Igneous complex, Southern Iberia Variscan Belt [ Pin et al ., ].…”

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confidence: 99%

Plume‐orogenic lithosphere interaction recorded in the Haladala layered intrusion in the Southwest Tianshan Orogen, NW China

Huang

et al. 2016

JGR Solid Earth

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The plume‐orogenic lithosphere interaction may be common and important for the generation of large igneous provinces. The information regarding such a process is recorded by the Haladala gabbroic intrusion (~300 Ma), the largest layered ultramafic‐mafic intrusion hosting V‐Ti magnetite deposits in the Southwest Tianshan Orogen, NW China. The Haladala gabbros exhibit unfractionated chondrite‐normalized rare earth element patterns with negative Nb and Ta anomalies and positive Pb anomaly on the primitive mantle‐normalized multielement variation diagram. They are characterized by low initial Sr isotopes, slightly decoupled but high positive bulk rock εNd(t) and εHf(t), and high 207Pb/204Pb and 208Pb/204Pb relative to 206Pb/204Pb, delineating a DUPAL signature in the sources. The Haladala gabbros cannot be arc or postcollisional magmatism, given the lack of hydrous minerals and low K contents, respectively. This is further supported by the relatively low oxygen fugacity required for the gradual enrichment of V‐Ti magnetite during the magma fractionation and by an overall anhydrous mantle source suggested by troctolite mineral assemblage (olivine + plagioclase). The emplacement age of the Haladala gabbros is identical to that of the Wajilitag kimberlites in the Tarim's interior, which have been interpreted as the first magmatic expression of the Tarim mantle plume. We thus propose that the Haladala gabbroic intrusion was generated in a hybrid geodynamic setting in which the Southwest Tianshan Orogen was impacted by an upwelling mantle plume. In this sense, the Haladala layered gabbroic intrusion records the early phase of magmatism of the Tarim plume, which was preferentially emplaced in a lithospheric weak zone.

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Section: Introductionmentioning

confidence: 99%

Plume‐orogenic lithosphere interaction recorded in the Haladala layered intrusion in the Southwest Tianshan Orogen, NW China

Huang

et al. 2016

JGR Solid Earth

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“…Examples such as the Duluth Complex (USA) and the Sudbury Igneous Complex (Canada) are more appropriately discussed below in the section on PGE-Ni-Cu sulfide-rich deposits, in terms of the style of PGM mineralization. Other published PGM studies include work on LMI such as the River Valley Intrusion (Canada; Holwell et al 2014), the Freetown Layered Complex (Sierra Leone, Africa; Bowles et al 2013), the Konttijaervi (associated with the basal contact of the Suhanko intrusion) and Kevitsa (also a Ni-Cu-PGE deposit) intrusions (northern and central Finland, respectively; Vuorelainen et al 1982;Gervilla and Kojonen 2002), the Bird River Sill (Canada; Talkington et al 1983), the Imandra layered intrusion (northwestern Russia; Barkov and Fleet 2004), the Ivrea-Verbano Basic Complex (lower crustal LMI, Italy; Garuti and Rinaldi 1986;Ferrario and Garuti 1990), the Munni Munni layered intrusion (western Australia; Mernagh and Hoatson 1995) and the Sompujärvi PGE Reef in the Penikat intrusion, which is not associated with chromitite, sensu stricto (Finland; Barkov et al 2005). Supplementary Table 1 contains additional details of these intrusions.…”

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“…It comprises four major cyclic units, associated with magma replenishment events; within one of these units (Zone 3), four PGEbearing horizons (B, C, D, and M; in order of descending stratigraphy) have been reported. Bowles et al (2013) studied 21 thin sections of Horizon B and one of each of the others and documented a total of 169 PGM grains ( Fig. 8).…”

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Petrogenesis of the Platinum-Group Minerals

O’Driscoll¹,

Gonzáléz-Jiménez²

2015

Reviews in Mineralogy and Geochemistry

151

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“…Platinum-group minerals (PGM) have been located in these rocks and shown to have been altered by superficial weathering [1,2]. Alluvial PGM have been recovered from the streams draining the intrusion and they are notably different in mineral assemblage, composition, and size from the PGM in the unaltered host rocks [3,4].…”

Section: Introductionmentioning

confidence: 99%

C14-22 n-alkanes in Soil from the Freetown Layered Intrusion, Sierra Leone: Products of Pt Catalytic Breakdown of Natural Longer Chain n-alkanes?

Bowles¹,

Bowles²,

Giże³

2018

Preprint

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:Soil above a platinum-group element (PGE)-bearing horizon within the Freetown Layered Intrusion, Sierra Leone contains anomalous concentrations of n-alkanes (CnH2n+2) in the range C14 to C22 not readily attributable to an algal or lacustrine origin. Longer chain nalkanes (C23 to C31) in the soil were derived from the breakdown of leaf litter beneath the closed canopy humid tropical forest. Spontaneous breakdown of the longer chain n-alkanes to form C14-22 n-alkanes without biogenic or abiogenic catalysts is unlikely as the n-alkanes are stable. In the Freetown soil, the catalytic properties of the PGE (Pt in particular) may lower the temperature at which oxidation of the longer chain n-alkanes can occur. Reaction between these n-alkanes and Pt species such as Pt 2+ (H2O)2(OH)2 and Pt 4+ (H2O)2(OH)4 can bend and twist the alkanes, and significantly lower the Heat of Formation. Acknowledging the possibility of microbial catalysis and the difficulty of identifying a direct organic geochemical source of the lighter n-alkanes, this paper explores the theoretical potential for abiogenic Pt species catalysis as a mechanism of breakdown of the longer n-alkanes to form C14-22 alkanes. This novel mechanism could substantiate the presence of the PGE in solution predicted by soil geochemistry and illustrate processes involving the PGE. Graphical AbstractThe anomalous proportion of C14 to C22 n-alkanes with a C31 molecule distorted by Pt 4+ (H2O)2(OH)4

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The First Report of Platinum-Group Minerals in Magnetite-Bearing Gabbro, Freetown Layered Complex, Sierra Leone: Occurrences and Genesis

Cited by 21 publications

References 48 publications

Plume‐orogenic lithosphere interaction recorded in the Haladala layered intrusion in the Southwest Tianshan Orogen, NW China

Plume‐orogenic lithosphere interaction recorded in the Haladala layered intrusion in the Southwest Tianshan Orogen, NW China

Petrogenesis of the Platinum-Group Minerals

<strong>C<sub>14</sub>-<sub>22</sub> n-alkanes in Soil from the Freetown Layered Intrusion, Sierra Leone: Products of Pt Catalytic Breakdown of Natural Longer Chain n-alkanes?</strong>

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