Podiform chromite at Voskhod, Kazakhstan

Johnson, Caroline 2012. Podiform chromite at Voskhod, Kazakhstan. PhD Thesis, Cardiff University.

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Abstract

The Voskhod podiform chromitite is one of more than 30 chromitite deposits that collectively form the Main Ore Field (MOF) within the Kempirsai Massif, in Kazakhstan. The MOF is the largest podiform chromitite ore-field in the world. The Voskhod deposit, encased in a serpentinised dunite halo, is situated within harzburgite units that comprise the mantle sequence of the Kempirsai ophiolite. This study arose from a unique opportunity to work on drill core samples through an un-mined podiform chromite deposit and investigate its internal structure, composition and genesis. The 18Mt ore-body has a strike of 600 m, is 170 m to 360 m wide and has an average thickness of 39 m. It has an immediate dunite halo between 1 m and 5 m thick. The ore body is made up of multiple stacked chromitite layers. Mineralised layers are separated by barren dunite or by weakly disseminated dunite lenses ranging from <1 m to 50 m. The style of mineralization varies throughout the ore body; the central region is dominated by thick (>5 – 45 m) units of massive chromite (>80% chromite), with progression towards the south west disseminated chromite (10 – 40% chromite) becomes increasingly abundant. Drill core logging and cross-section profiling of the internal structure of the ore body has identified an intricately connected network of what appear to be chromite-filled channel-ways. Outside of the halo the host rocks are inter-layered harzburgite and dunite. Accessory chromite in harzburgite has an average Cr# of 0.31 compared to Cr# 0.49 in the dunite. The harzburgites are depleted, having formed from intermediate degrees of partial melting (~15 – 18 %) of a fertile mantle source at a mid-ocean ridge (MOR) setting. The dunite units have transitional geochemical fingerprints that imply they formed from the interaction of MOR mantle harzburgite with both mid ocean ridge baslt-melt and an arc derived-melt. They are not the products of extremely high degrees of partial melting. The encasing dunite halo is extensively serpentinised (>80%). Chromite is only present as an accessory phase having an average Cr# of 0.62. The dunite has a geochemical signature indicating that it formed by reaction between residual harzburgite and a boninite melt in supra-subduction zone (SSZ) tectonic setting. A variety of geochemical fingerprints have been identified; residual MOR harzburgite, reacted-MOR dunite, reacted-SSZ dunite and harzburgite, indicating that the mantle section has had a ii polygenetic tectonic evolution, recording both ocean basin opening (MOR setting) and closing (SSZ setting) events. Trace element and REE whole rock geochemistry of the chromitites and associated host rocks provide evidence of depletion and a later-stage LREE-enrichment event. LREE-enrichment is most intense within and immediately adjacent to the chromitite. Chromites from the ore zone are at the Cr-rich extreme for podiform chromites (Cr# ave. 0.80-0.85) and are TiO2 poor (ave. 0.16 wt%), similar to chromite in boninite worldwide and nearby. Al/Ti ratios have been used to calculate the composition of the parent melts from which the Voskhod podiform chromitite crystallised: compositions that are synonymous with a boninite melt composition. Chemical variation in chromite is systematic and on a much smaller scale than was anticipated. Even variations in a single thin section provide key evidence for different magmatic processes. An apparent melt-rock reaction in harzburgite has been examined in freeze-frame. The chromite chemistry has been investigated at 50 cm, 1 cm and 1 mm scales. Compositional differences were identified on the basis of MgO% and FeO(t)% compositions. Diagrams FeO-Fe2O3 and Cr# - Mg# were used to demonstrate the variations and identify relationships. Broad cryptic layering on a 50 cm scale has been found as well as fine-cryptic layering on a 1 – 8 cm scale. The variations are interpreted to reflect differences in the mineral phases crystallised from the melt; periods when on chromite only crystallised are distinguished from periods when both chromite with olivine crystallised. It seems likely that the deposit is made up of thousands of episodes of chromite accumulation that formed in an intermittently replenished open-system. It also seems likely that the conduit was never a single melt-filled cavity; instead melt flow was focused through the mantle over an extended period. The conduit appears to be comprised of multiple branches, as chromite (± olivine) crystallised from the melt the channel-way became blocked and the melt was forced to deviate and make a new pathway through the mantle. As time elapsed the process resulted in the formation of stacked chromitite lenses, creating an orebody that has an internal arrangement of chromitite and dunite unites which resemble a stacked braided 'delta'.

Item Type:	Thesis (PhD)
Status:	Unpublished
Schools:	Earth and Environmental Sciences
Subjects:	Q Science > QE Geology
Date of First Compliant Deposit:	30 March 2016
Last Modified:	16 Oct 2019 14:05
URI:	https://orca.cardiff.ac.uk/id/eprint/40714

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