PODIFORM CHROMITE AT VOSKHOD, KAZAKHSTAN Caroline Johnson Submitter in partial fulfilment of the requirements for the degree of Ph.D. November, 2012 Somewhat unconventionally, this thesis is for me. Caroline Johnson “I love deadlines. I like the whooshing sound they make as they fly by.” Douglas Adams (1952-2001) 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 i 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'. ii ACKNOWLEDGEMENTS Sincere thanks to Iain McDonald for his patience, time and encouragement. Thank you to my supervisors, Chris Neary, Julian Pearce and Hazel Prichard as well as my industry mentor Nic Barcza for their help and advice. I owe a large debt of gratitude to; Nic Barcza, Chris Powers, Matt Boyes, Simon Apps, Kevin Alexander and Lisa Pereira for all their technical help, logistical support and kindness – it was a pleasure to work with you. Without their efforts this project would not have been possible. Peter Fisher has been of invaluable assistance and equally patient during the long periods of time I have spent on the SEM. Lawrence Badham and Peter Greatbatch were responsible for making hundreds of polished thin sections between them. Iain McDonald and Ley Woolley were efficient at running many sample solutions on the mass spectrometers at Cardiff and very kind to teach me the methods for sample preparation in the geochemistry laboratory. A big thank you to Ian Parkinson and Andy Tindle, who supervised my use of the EMP at the Open University. Extra thanks to Millie and Anna, who “on-the-day” agreed to accommodate me during my time at the O.U and were really very lovely. Thank you to; Matthew Minifie, Tracy Aze, Tom Gregory, Alan Hastie, Bryan Hatton, Sarah Dare, Freddie Spyre, Aggie Georgeopoulou, Ruth Liley, Namaste Nicolai and David Thornalley for your unending moral support and more importantly - the laughter. Thanks is also expressed to my magma process group officemates Chris Brough, Kerry Howard and Iain Neill. I acknowledge the funding for this project from Cardiff University, Oriel Resources Plc. and an additional annual contribution from SRK. Travel grants from the SEG, GSSA, MDSG and IOM3 Andrew Carnegie fund to attend conferences and field trips have been greatly appreciated. Finally, thank you to my parents, Geoffrey and Gillian as well as to my sisters, Bertie and Minnie, who have been gracious enough to express continual faith in my ability. iii Table of Contents Page Abstract i Acknowledgements iii Contents iv Chapter 1. Introduction 1 Chapter 2. Ophiolites 3 2.1 Ophiolite stratigraphy 4 2.1.1 Crustal units 5 2.1.2 Mantle units 6 2.1.3 Chromite in the ophiolite sequence 7 2.2 Ophiolite formation and emplacement 8 2.2.1 Tectonic settings: Mantle and melt geochemistry 9 2.2.1.1 Mid-Ocean Ridge Basalt (MORB) 11 2.2.1.2 Boninite 12 2.2.1.3 Island-Arc Tholeiite (IAT) and Calc-Alkaline Basalt (CAB) 13 2.2.2 Mantle peridotite types 14 2.2.2.1 Abyssal peridotite 14 2.2.2.2 Forearc Peridotite 15 2.2.3 Ophiolite mantle peridotite 15 2.2.4 Limitations relating mantle peridotites and crustal lavas 16 2.3 Geochemistry: A tool for tectonic reconstruction 17 2.3.1 Monogenetic and polygenetic tectonic settings 18 2.3.1.1 Melt-rock reaction within monogenetic tectonic settings 19 2.3.1.2 Melt-rock reaction within polygenetic tectonic settings 19 2.4 Ophiolite types 20 2.5 Ophiolites of the Urals 21 Chapter 3. Chromite 24 3.1 Chromium 24 3.1.1 Uses of chromium 25 3.2 Chromite deposits 26 3.2.1 Stratiform chromitite 27 3.2.1.1 Chromite crystallisation models: Stratiform chromitite 29 iv 3.2.2 Ophiolitic chromite 31 3.2.2.1 Ophiolite chromite: Crustal cumulates 32 3.2.2.2 Ophiolite chromite: Mantle hosted podiform chromitite 33 3.3 Podiform chromitite genesis models 38 3.3.1.1 The chromitite-dyke hypothesis 40 3.3.1.2 Mixing multistage-melts 42 3.3.1.3 The melt-rock reaction models 43 3.3.1.4 Water and chromite formation 46 3.4 Chromite geochemistry 47 3.4.1 Conditions that impact the composition of chromite crystallised from a melt 48 3.5 Geochemical variation and trends in chromite 55 3.5.1 The Fe-Ti Trend 56 3.5.2 The Cr-Al Trend 57 3.5.2.1 Sub-solidus re-equilibration between chromite and olivine 58 3.5.3 The Rhum Trend 58 3.5.4 Mg#-Cr# relationships and understanding chromite genesis 59 3.5.4.1 Trend A 60 3.5.4.2 Trend B 62 3.5.5 Cyclic layering and chromitite layers 63 3.6 Chromite alteration 63 3.6.1 Accessory chromite morphologies: Proposed mechanisms of formation 65 3.6.2 Silicate inclusions in chromite 65 3.6.3 Chromite ore types 66 3.6.4 Densification of chromite: Mechanisms and features 68 3.6.4.1 Mechanical separation: Drifting velocity 69 3.6.4.2 Overgrowth – Postcumulus reaction with a chrome-rich liquid 69 3.6.4.3 Post-cumulus reaction between chromite, plagioclase and a late-stage liquid 70 3.6.4.4 Compaction - Sintering 70 3.6.4.5 Deformation: Grain boundary features 72 Chapter 4.