The Panzhihua intrusion is one of several large Fe-Ti oxide ore bearing intrusions related to the major flood volcanism of the Emeishan Large Igneous Province (ELIP), SW China. The Panzhihua intrusion in particular has recently become the focus of numerous studies owing to the excellent exposure in large open pit mining operations. The formation of Fe-Ti oxide ore layers has been the focus of these studies and has become a somewhat controversial topic with three separate models currently proposed for ore formation. The gabbroic Panzhihua intrusion extends for ± 19 km along strike, has a maximum thickness of 3000 m and hosts extensive (up to 60 m thick) Fe-Ti oxide ore layers in the lower portions of the intrusion. The intrusion has been divided into five zones: marginal zone (MGZ), lower zone (LZ), middle zone A (MZa), middle zone b (MZb) and the upper zone (UZ). The gabbroic rocks are comprised of plagioclase, clinopyroxene and interstitial Fe-Ti oxides with minor olivine. Apatite is present within the MZb only and shows no correlation with Fe-Ti oxide ore layers. Fe-Ti oxides are present throughout the stratigraphy of the intrusion. This is unlike typical layered intrusions where significant Fe-enrichment through fractionation of Fe-poor silicate phases (i.e. plagioclase) is required before Fe-Ti oxide saturation. There are no oxide-free cumulate rocks at the Panzhihua intrusion, implying either an evolved parent magma or very high Fe content of the source rocks. I present here new mineral composition data, whole-rock major and trace element geochemistry along with whole-rock Sr-Nd isotopes and PGEs in order to constrain the evolution of the Panzhihua parent magma en route from source to chamber and the formation of Fe-Ti oxide ore layers. Furthermore an initial pilot study using O-isotope data is conducted on Ti-magnetite and plagioclase separates from gabbroic vs. ore rocks. Results are coupled with detailed thermodynamic modeling using the software PELE in order to further constrain Fe-Ti oxide ore layer formation. The intrusion is characterised by extreme depletion of PGEs relative to the coeval flood basalts and picrites. High Cu/Pd and Pd/Pt imply two separate stages of S-saturated and S-undersaturated depletion of PGEs. Pd is highly compatible in sulphide and is quickly scavenged by sulphide liquids resulting in an increase in Cu/Pd of the residual liquid. Furthermore decoupling of Pd and Pt can be achieved by either late stage hydrothermal alteration or through S-undersaturated stage of PGE depletion where Pt is scavenged by Pt-rich alloys or oxide minerals. I show that the latter is more likely. Fractionation modeling suggests that the Panzhihua parent magma formed at depth from original picritic magma. This is consistent with several other recent studies on other layered intrusions of the ELIP. Sr-Nd isotopic ratios indicate very little crustal contamination has occurred en route to the current chamber. Sr and Nd concentrations of footwall rocks are too low to produce any significant change in initial Sr and Nd isotopic ratios of the intruding basaltic magmas, indicating that crustal contamination will not be indicated by Sr-Nd isotopic ratios. Gradational change in the Sr-Nd isotope ratios across the MGZ provides strong evidence for formation in an open system by multiple replenishments of progressively less contaminated magmas from depth. Contamination is difficult to constrain but must be occurring prior to emplacement at the current level (low Sr and Nd contents of footwall). A gradational upward decrease in highly incompatible element across the MGZ can then be explained by continuous magma flow, which effectively removes the evolved intercumulus liquids from the growing cumulate pile at the base of the chamber. The initial stages of formation of the Panzhihua intrusion are interpreted to result from prolonged low volume pulses of magma into a slowly opening chamber. The timing of Fe-Ti oxide crystallisation is fundamental in the understanding of the petrogenesis of ore layers. Distinct geochemical variation in whole-rock Fe2O3/TiO2 and Zr/Nb indicates that Timagnetite is the dominant oxide within the lower ± 270 m of the intrusion whereas above this level both Ti-magnetite and ilmenite are present as cumulus phases. This is interpreted to indicate a variation in the fO2 where the lower intrusion crystallises at higher fO2 relative to that above this level. Silicates within the ore layers, in particular plagioclase, are highly embayed and resorped where in contact with Fe-Ti oxides. This characteristic of the silicate grains implies early crystallisation prior to Fe-Ti oxides with subsequent disequilibrium conditions resulting in resorption. Furthermore distinct reaction rims of kaersutite amphibole, Fo-enriched olivine, An-enriched plagioclase and pleonaste are observed. The abundance of amphibole suggests H2O involvement in this reaction and consumption of silicates. A model for parent magma crystallisation at various H2O contents indicates that plagioclase crystallisation temperature is very sensitive to H2O content of the parent magma. Plagioclase crystallises early for “dry” compositions but significantly later for “wet” compositions. Fe-Ti oxide ore layers are generally well layered, contain gabbroic xenoliths and are observed raversing/cross-cutting the cumulate stratigraphy. I present here a new model for ore layer formation in order to account for these distinct features of the ore layers. A model invoking multiple replenishments of magma with variable oxide microphenocryst content, H2O content and volume is proposed. Magma evolving in the plumbing system and fed to the Panzhihua chamber is variably enriched in H2O, which results in significantly different crystallisation paths. High H2O magmas (> 2 wt %) crystallise Fe-Ti oxides early whereas low H2O magmas (< 1 wt %) crystallise oxides late. Early pulses of H2O-poor magma crystallise a sequence of plag+cpx+Fe-Ti oxide (±ol). Later pulses of H2Orich magma subsequently intrude the partially crystallised cumulate sequence incorporating and consuming previously crystallised silicates with subsequent early crystallisation of Ti-magnetite and formation of ore layers. H2O-rich magmas likely have suspended Ti-magnetite microphenocrysts as well, which crystallise at depth in the plumbing system. This model can account for the various characteristic features of the Fe-Ti oxide ore layers at the Panzhihua intrusion as well as other Fe-Ti oxide ore bearing intrusions in the region.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:rhodes/vital:4915 |
| Date | January 2013 |
| Creators | Howarth, Geoffrey Hamilton |
| Publisher | Rhodes University, Faculty of Science, Geology |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis, Doctoral, PhD |
| Format | 224 leaves, pdf |
| Rights | Howarth, Geoffrey Hamilton |
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