This thesis reports on the results of a study of the layered ultramafic rocks of the Selukwe Subchamber of the Great Dyke of Zimbabwe, and examines aspects of the field relations, petrology and geochemistry of the layered units. Special emphasis is placed on the uppermost pyroxenite layer (the P1 pyroxenite) and the sulphide-hosted platinum-group element (PGE) mineralization associated with it.
The Selukwe Subchamber is vertically divided into the upper Mafic Sequence, present only as gabbroic remnants, and the lower Ultramafic Sequence, which is further divided into the lower Dunite Succession and upper Bronzitite Succession. Field studies show that the layered cyclic units which characterize the Ultramafic Sequence typically comprise serpentinized dunite or harzburgite layers at the base (with or without a basal chromitite) and an overlying olivine bronzitite or bronzitite layer. The uppermost cyclic unit fCUD has a websterite layer overlyirfg the uppermost bronzitite layer – together these make up the P1 Pyroxenite.
There are a number of significant differences between the field geology on the eastern and western margins of the Selukwe Subchamber, particularly in CU1. The western margin is characterized by: compression of the stratigraphy, a lack of basal chromitites, and the presence of boulder beds and crescumulate-textured marginal bronzitites. Specifically in the CU1 layer on the western side there is development of transitional bronzitite layers, interdigitating bronzitite and websterite layers, boulder beds and the presence of ultramafic xenoliths at the Mafic/Ultramafic contact. The implications of these differences for variable environmental conditions in the chamber during the evolution of the magma are discussed in detail.
The main constituent mineralogy of the ultramafic rocks in the Selukwe Subchamber is represented by chromite, olivine, orthopyroxene and clinopyroxene, and plagioclase in the Mafic Sequence. The cumulus status of these minerals changes upwards through the cyclic units, giving rise to the typical observed sequence of lithologies. The crystallization sequence for the cumulus minerals through the Ultramafic and Mafic Sequences, deduced from textural relationships, is: chromite -> olivine -> bronzite -> augite -> plagioclase -> magnetite. The abundance and habit of the cumulus and postcumulus phases gives rise to characteristic textures in some lithologies, such as granular and poikilitic harzburgites, and “potato” reef in the websterite. Minor late-stage postcumulus minerals which occur in the interstitial spaces between the major phases include: biotite, quartz, potassic feldspar, micrographic quartz, primary amphiboles, opaque minerals (sulphide, magnetite, ilmenite) and the accessory minerals apatite, sphene and zircon. The sulphides with which the platinum-group minerals (PGM) are associated are dominantly associated with the late-stage minerals. Where sulphides are abundant, deuteric activity has resulted in alteration of the silicate assemblage to a hydrosilicate assemblage of fine-grained talc, tremolite-actinolite, chlorite and biotite.
The geochemistry of the Ultramafic Sequence has been studied in terms of the vertical and lateral variations in whole rock and pyroxene chemistry, and PGE mineralization, for the P1 Pyroxenite. LLess emphasis is placed on the geochemistry of the underlying layered sequence.
Vertically, the behaviour of the whole rock and mineral chemistry is consistent with that of an upwardly fractionating magma body. Cyclical reversals in the chemical behaviour, which corresond with zones of sulphide mineralization, may possibly represent periodic influxes of new magma. Variations in the concentrations of incompatible elements (P and Zr) reflect differences in the proportions of postcumulus phases which formed from trapped liquid. “Cryptorhythmic” units in the different borehole sections are identified by plotting P (whole rock) against orthopyroxene mg’-number.
Pyroxene geothermometry for co-existing ortho- and clinopyroxene in websterite yields an average equilibration temperature of 1122�C for the pyroxenes.
Lateral geochemical variations between the axial and marginal environments are most significant for: whole rock P and Zr contents, orthopyroxene chemistry, and sulphide, PGE and base metal mineralization in the MSZ and LSZ.
Two zones of sulphide mineralization can be delineated in the P1 pyroxenite – the broad, sparsely-mineralized, Lower Sulphide Zone (LSZ) and the upper, narrow zone of very high sulphide values – the Main Sulphide Zone (MSZ). In the sulphide zones, the base metals Cu, Ni and Zn behave sympathetically with sulphide, whereas the PGE are enriched at the bases of sulphide zones and are depleted upwards as sulphide increases. The order of enrichment of the PGE and base metals in the sulphide zones is Os-Ru,Rh-lr-Pd-Pt-Ni-Au-Cu. This order can be explained in terms of the chemical and physical affinity of the metals between a fractionating sulphide liquid and a silicate magma.
Microprobe investigations of the PGM in the MSZ and LSZ identified 14 minerals and mineral phases, viz. moncheite, maslovite, kotulskite, polarite, michenerite, sperrylite, platarsite, hollingworthite, cooperite, braggite, laurite, gold, gold and/or silver alloys, and Pt-Fe alloys. The PGM occur in three distinct textural environments: (i) at the boundary of silicates/hydrosilicates; (ii) entirely enclosed within silicate or hydrosilicate minerals; and (iii) wholly enclosed within sulphides. A multi-process model, incorporating both orthomagmatic and hydromagmatic processes, is proposed to account for the final stratigraphic distribution, and environmental and textural characteristics of the PGM.
The results of this study are used to propose an overall model for the petrogenesis and metallogenesis of the Selukwe Subchamber, including the origin of the layering, cumulate textures, and sulphide and PGE mineralization, and taking into account the role of postcumulus components in the P1 pyroxenite. Lateral variations in the field geology, geochemistry and mineralization between the axial and marginal environments are attributed to the influence of the chamber geometry on the prevailing thermal regime and conditions of crystallization.
|Degree Type||Doctoral degree|