LITHOOEOCHEMISTRY AND ORIGIN OF METASEDIMENTS HOSTING THE BROKEN HILL DEPOSIT, AGGENEYS, SOUTH AFRICA, AND IMPLICATIONS FOR ORE GENESIS by RAEL DESMOND LIPSON Town Cape of DOCTOR OF PHILOSOPHY University IN THE DEPARTMENT OF GEOCHEMISTRY FACULTY OF SCIENCE UNIVERSITY OF CAPE TOWN APRIL 1990 The University of Cape Town has been given the right to reproduce this thesis in whole ' or kl part. Copyright is held by the author. I The copyright of this thesis vests in the author. No quotation from it or information derived from it is to be published without full acknowledgementTown of the source. The thesis is to be used for private study or non- commercial research purposes only. Cape Published by the University ofof Cape Town (UCT) in terms of the non-exclusive license granted to UCT by the author. University ABSTRACT The thin metasedimentary suite (< 1 OOOm) of supracrustal schist and quartzite at Broken Hill, Aggeneys, forms part of the Proterozoic Aggeneys Subgroup within the Namaqua Province of South Africa. The structural succession at Broken Hill comprises gneisses (not investigated in this thesis), which overlie red-coloured, nodular magnetite-bearing Namies Schist that grades downwards into mature Broken Hill Quartzites. The latter host heavy mineral layers containing detritally rounded zircons. Ore Schist lying below Broken Hill Quartzites, has similar characteristics to Namies Schist, and hosts the banded iron formation-associated Broken Hill Pb-Ag-Zn-Cu orebody. Biotite garnet schist and stringers of Garnet Quartzite within Ore Schist represent an Ore Equivalent Horizon at the same stratigraphic level as the two superposed lenses comprising the Broken Hill orebody. A Biotite Graphite Zone marks the sharp contact between Ore Schist and the green-coloured, pyrite- and graphite- bearing Shaft Schist. Lenses of Cale Silicate Rock plus Manganese Quartzite within Shaft Schist lie close to the Biotite Graphite Zone. Analyses of an extensive suite of spatially and lithologically controlled borehole core and surface outcrop samples by X-ray fluorescence spectrometry reveals a contrast between sporadically high base metal values in Namies Schist, and Shaft Schist which is chemically anomalous only in its elevated Pb content. Namies Schist at Broken Hill is therefore interpreted as predating the orebody, whereas Shaft Schist postdated the deposit. An overturned sequence is thus inferred at Broken Hill, and it can be correlated unit-for-unit with sequences at Namiesberg and Wortel (termed regional rocks) lying 25 - 30km to the east and north. A voluminous basement of potassic, siliceous, biotite gneisses underlie the supracrustal rocks. The low MnO and high S content of the homogeneous Shaft Schist stand in contrast to the high MnO and low S character of the chemically variable Namies Schist, and viewed together with rock colour and absence or presence of magnetite, indicate that Namies Schist and the related, detrital Broken Hill Quartzites, formed in oxygenated, agitated, aqueous conditions whereas Shaft Schist developed as a black shale in an anoxic, meromictic, lagoon-like environment. Lenses of Cale Silicate Rock plus Manganese Quartzite primarily represent the precipitates from elements which had accumulated at the thermocline boundary prior to destabilization of the quiet, lagoonal cond}tions. The high S content of Shaft Schist and negative Ce anomaly of Cale Silicate Rock plus Manganese Quartzite show that Shaft Schist formed beneath sea water. A passive continental margin environment may be deduced from the extreme silica purity and sheet-like morphology of the quartz arenitic Broken Hill Quartzites, and from the elevated alumina concentrations of Namies Schist and Shaft Schist. Shaft Schist and Broken Hill Quartzites were probably the lagoon and beach barrier sediments, respectively, of a prograding beach barrier system, and the upward coarsening Namies Schist represents the shoreface pelites. Ore Schist probably formed as washover sediments derived from Namies Schist. The tendancy for quartzite to thin southwards through Aggeneys is matched by progressively cleaner separation in that direction, of silica in quartzite from alumina in schist, indicating south-directed progradation over tens of kilometres. Shapes of rare earth element patterns, Eu/Eu* ratios from Namies Schist, Broken Hill Quartzites and Shaft Schist, and major and trace element modelling suggest that the metasediments were derived from a weathered, granodioritic provenance. Ca removed during early provenance weathering was probably precipitated distally to form the calcic rocks south and east of Aggeneys, while removed Na remained in solution. Detrital transport of the granodioritic residuum caused the formation of Ca- and Na- depleted, and K-, Al- and total rare earth element-enriched shales and sandstones. Ion microprobe Pb isotope dating of detrital zircons from Broken Hill Quartzites return maximum ages of 2.0Ga which confirm the post-Archaean signatures given by low La/Th ratios and negative Eu anomalies of the metasediments. The age constraints and oxygen isotope composition of Broken Hill Quartzites are compatible with a provenance of Vioolsdrif granitoids, which crop out to the north and north-west of Aggeneys. The average base metal and associated element concentrations of Namies Schist and Broken Hill Quartzites at Broken Hill are greater than for their correlative regional rocks, indicating the presence of a geochemical halo around the orebody. The Ore Equivalent Horizon displays a localized primary geochemical halo measureable for up to lOkm distance, and most clearly defined by increasing Zn and Ba concentrations progressively nearer the orebody. "Ore Factor" scores from the Factor Analysis of Ore Schist, also increase progressively nearer ore. Together with positive heavy rare earth element anomalies of many Ore Equivalent Horizon samples, and their unusual petrography, these observations suggest that many component elements of the Ore Equivalent Horizon were of exhalative origin. Within the Ore Equivalent Horizon, higher concentrations of base metals and related elements on the west side of the Broken Hill orebody relative to the east side, and a maximum MnO concentration lying some 300m east of the orebody, suggest that the hydrothermal fluid from which ore formed moved eastwards along the sea floor. Thermochemical modelling of Zn/Cu and Pb/Cu ratios of hydrothermal solutions in equilibrium with sphalerite, galena and chalcocite shows that higher solution pH favours greater relative Cu concentrations in the ore fluid. Whereas the higher temperature fluids (> 150°C) have relatively low Zn/Cu ratios, maximum relative Zn concentrations occur in the lowest pH and lowest temperature fluids. The four orebodies in the Aggeneys-Gamsberg area were likely to have had similar metal ratios to those present in their parent hydrothermal fluids. Using the gross Zn/Pb, Zn/Cu and Pb/Cu ratios of each deposits and thermochemical data from both this investigation and from Huston and Large (1987), it is shown that the Pb-Zn-Cu-rich Broken Hill deposit formed from fluids at ca. 280°C and a pH of 4.7 - 5.4. The Zn-rich Gamsberg deposit formed from fluids at 100 - 150°C and a pH of 4.0 - 4.5. The acidic and reduced, S-bearing waters in the enclosed \ Gamsberg basin are contrasted with the oxidizing, SO,-bearing and more alkaline environment in which the Broken Hill (and Black Mountain) orebodies formed. The nature of individual basin sea water which was convected kilometres down into the crust, as well as the depth of convection, ultimately determined the proportions of metals leached from a basement which probably becomes more maf ic at depth. The higher temperature, more deeply circulating, alkaline fluids which gave rise to the Black Mountain and Broken Hill deposits, leached relatively more Cu, whereas low temperature, shallowly circulating, l more acidic and HiS-rich fluids generated the Gamsberg and Big Syncline orebodies. Unrealistically large volumes of illite would be required at Aggeneys to produce a deposit of the size of Broken Hill, solely by the dehydration of sediment. Localization of ore to the general Aggeneys-Gamsberg area where amphibolite bodies and wedges of elastic conglomerates occur, point to the likely existence of synsedimentary rift processes. Such rift faults probably focussed fluids on their return journey from depth, and sulphur and metals precipitated at or near the vents in response to falling temperature, to form stratiform orebodies on the sea floor. The passage of hydrothermal fluid caused slight metal contamination of Namies Schist and Broken Hill Quartzites immediately below the ore time horizon. The Broken Hill orebody and geochemical halo were preserved by progradational deposition of the overlying Shaft Schist. LITHOGEOCHEMISTRY AND ORIGIN OF METASEDIMENTS HOSTING THE BROKEN HILL DEPOSIT, AGGENEYS, SOUTH AFRICA, AND IMPLICATIONS FOR ORE GENESIS TABLE OF CONTENTS Page No. LIST OF FIGURES IN VOLUME 2 vii LIST OF TABLES IN VOLUME 2 xvii DEDICATION xx ACKNOWLEOOEMENTS xxi ABBREVIATIONS xx iv PART A. INTRODUCTION 1. HISTORY AND LOCATION 1 2. STUDY CONTEXT, OBJECTIVES AND METHODOLOGY 3 2.1 Study context 3 2.2 Objectives 3 2.3 Methodology 4 PART B. GEOLOGY AND PETROGRAPHY 3. REGIONAL GEOLOGICAL SETTING 6 4. BROKEN HILL GEOLOGY AND PETROGRAPHY 12 4.1 Structure, metamorphism and lithostratigraphy 12 4.2 Broken Hill schists 14 4.2.1 Namies Schist (NS) 14 4.2.2 Shaft Schist (SS) 16 4.2.2.1 Biotite Graphite Zone (BGZ) 17 4.2.2.2