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To Neoproterozoic Ghanzi Basin in Botswana and Namibia, and Implications for Copper-Silver Mineralization in the Kalahari Copperbelt

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To Neoproterozoic Ghanzi Basin in Botswana and Namibia, and Implications for Copper-Silver Mineralization in the Kalahari Copperbelt GEOCHRONOLOGY, MAGNETIC LITHOSTRATIGRAPHY, AND THE TECTONOSTRATIGRAPHIC EVOLUTION OF THE LATE MESO- TO NEOPROTEROZOIC GHANZI BASIN IN BOTSWANA AND NAMIBIA, AND IMPLICATIONS FOR COPPER-SILVER MINERALIZATION IN THE KALAHARI COPPERBELT By Wesley Scott Hall Copyright by Wesley S. Hall 2017 All Rights Reserved A thesis submitted to the Faculty and the Board of Trustees of the Colorado School of Mines in partial fulfilment of the requirements for the degree of Doctor of Philosophy (Geology). Golden, Colorado Date ____________________________ Signed: ________________________________ Wesley S. Hall Signed: ________________________________ Dr. Murray H. Hitzman Thesis Advisor Signed: ________________________________ Dr. Yvette Kuiper Thesis Advisor Golden, Colorado Date ____________________________ Signed: ________________________________ Dr. Merritt Stephen Enders Professor and Department Head of Geology and Geological Engineering ii ABSTRACT Despite a wealth of research on the Kalahari Copperbelt over the past 30 years, two crucial aspects of the mineralizing systems have remained elusive. First, the age of the rift sequence hosting the deposits and, second, the nature of the fluid pathways for the mineralizing fluids. Laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) U-Pb isotopic analysis on one igneous sample of the Makgabana Hills rhyolite (Kgwebe Formation) within the central Kalahari Copperbelt in Botswana constrains the depositional age of the unconformably overlying Ghanzi Group to after 1085.5 ± 4.5 Ma. The statistically youngest detrital zircon age populations obtained from the uppermost unit of the Ngwako Pan Formation (1066 ± 9.4 Ma, MSWD = 0.88, n = 3), the overlying D’Kar Formation (1063 ± 11, MSWD = 0.056, n = 3), and the lower Mamuno Formation (1056.0 ± 9.9 Ma, MSWD = 0.68, n = 4) indicate that the middle and upper Ghanzi Groups were deposited after ~1060 to ~1050 Ma. Lu-Hf analysis of detrital zircon suggests that the Mesoproterozoic and Paleoproterozoic rocks of the Namaqua Sector and the Rehoboth Basement Inlier were the primary sediment sources for the siliciclastic rocks of the Ghanzi Group and lesser material was derived from the basin-bounding footwall margin of the northwest Botswana rift, the Paleoproterozoic Magondi Belt and the Okwa Block, and possibly parts of the Limpopo Belt on the northern margin of the Kalahari Craton in southern Africa. A molybdenite Re-Os age of 981 ± 3 Ma provides a minimum depositional age constraint on D’Kar Formation sedimentation. Authigenic xenotime U-Th-Pb ages of ~925 and 950 Ma further the evidence for an earliest Neoproterozoic (Tonian) age for the D’Kar Formation. Re-Os ages of 549 ± 11.2 Ma (low-level highly radiogenic chalcocite-idaite) and 515.9 ± 2.4 Ma (molybdenite), and a U-Th-Pb age of 538.4 ± 8.3 Ma (xenotime inclusion in chalcopyrite) from several Cu-Ag deposits in the central Kalahari Copperbelt suggest prolonged mineralizing events during basin inversion related to the Pan- African (~600 to 480 Ma) Damara orogen. High-resolution aeromagnetic maps were utilized to define the stratigraphy and structure of the Ghanzi Group of the central Kalahari Copperbelt. Maps of the second vertical derivative transformation were compared with detailed stratigraphic data from drill holes. These data reveal previously unrecognized thinning of the Ngwako Pan Formation onto rocks of the underlying Kgwebe Formation and suggest the presence of syn-sedimentary horst and graben and/or half-graben structures. Truncation of the aeromagnetics fabric of the uppermost Ngwako Pan Formation rocks suggests that the rocks were gently folded and eroded above paleotopographic highs prior to the ensuing basin-wide marine transgression and sedimentation of the unconformably overlying mixed marine siliciclastic and carbonate rocks of the D’Kar Formation. Detailed facies architecture derived from both drilling and magnetic data at the Zone 5 Cu-Ag deposit, located east of the Kgwebe and Makgabana Hills, suggests that its physical iii (stratigraphic) and chemical (organic material and in-situ bacteriogenic sulfide) nature were influenced by the underlying basin architecture, which was critical in development of trap sites and in funneling epigenetic hydrothermal fluids into those traps. The presented new data indicate that the basin architecture underlying the Kalahari Copperbelt strongly influenced the localization of deformation and hydrothermal fluid flow during epigenetic events. The results of this study can be used to help vector exploration for Cu-Ag deposits through the recognition of buried paleotopographic highs and associated favorable trap sites. iv TABLE OF CONTENTS ABSTRACT . iii LIST OF FIGURES . ix LIST OF TABLES . .xix ACKNOWLEDGEMENTS . xx CHAPTER 1 INTRODUCTION. 1 1.1 Introduction . 1 1.2 Dissertation organization. .5 1.2.1 Igneous and detrital zircon geochronology . .5 1.2.2 Xenotime U-Th-Pb geochronology. .6 1.2.3 Sulfide (Re-Os) geochronology. .6 1.2.4 Geophysics – magnetics . 7 1.2.5 General conclusions . .8 1.3 References cited. .8 CHAPTER 2 LITERATURE REVIEW. 11 2.1 Introduction . 11 2.2 The sedimentary rock-hosted stratiform copper ore system. 11 2.3 Geologic framework of the Kalahari Copperbelt in Botswana . .14 2.3.1 The Koras-Sinclair-Ghanzi rift system of southern Africa . .15 2.3.2 The Ghanzi basin. .17 2.3.3 The Okwa Group and the Damara Orogen in Botswana . .19 2.3.4 Post-Damara geology. .23 2.4 Geophysical exploration in the Kalahari Copperbelt. 23 2.5 Exploration and development history of the Kalahari Copperbelt . .24 2.6 Mineralization in the Kalahari Copperbelt . .26 2.6.1 Deposit characteristics. 26 2.6.2 Hydrothermal fluids . 29 2.6.3 Sulfur source . 30 2.6.4 Timing of mineralizing events. 32 2.7 References cited. 33 CHAPTER 3 IGNEOUS AND DETRITAL ZIRCON U-PB GEOCHRONOLOGY AND LU- HF ISOTOPE GEOCHEMISTRY OF THE LATE MESO- TO NEOPROTEROZOIC NORTHWEST BOTSWANA RIFT: IMPLICATIONS FOR THE TIMING OF DEPOSITION AND PROVENANCE OF THE GHANZI BASIN (KALAHARI COPPERBELT), EASTERN NAMIBIA AND NORTHWESTERN BOTSWANA. 36 v 3.1 Abstract . 36 3.2 Introduction . 37 3.3 Geologic background. 39 3.3.1 Stratigraphy and evolution of the Ghanzi basin . 41 3.3.2 Previous detrital zircon studies . 44 3.4 Samples . 45 3.5 Methods . 45 3.6 Geochronology results . 48 3.6.1 Porphyritic rhyolite, Makgabana Hills, east-central Ghanzi basin. 49 3.6.2 Detrital zircon U-Pb results. 49 3.6.3 Detrital zircon Lu-Hf results . 54 3.7 Discussion . 55 3.7.1 Implications for depositional ages and regional stratigraphic correlations . .56 3.7.2 Provenance of the Ghanzi Group. .57 3.8 Conclusions . 68 3.9 Acknowledgements . 69 3.10 References cited . 69 CHAPTER 4 RE-OS SULFIDE AND U-TH-PB XENOTIME GEOCHRONOLOGY OF SEDIMENTARY ROCK-HOSTED CU-AG DEPOSITS, KALAHARI COPPERBELT, BOTSWANA . .79 4.1 Abstract . 79 4.2 Introduction . 80 4.3 Geologic background . 82 4.3.1 Host rocks . 82 4.3.2 Sedimentary rock-hosted Cu-Ag deposits . .84 4.4 Sample descriptions and petrography . 86 4.4.1 Northeast Fold deposit, molybdenite (AIRIE Run # MD-1700). 86 4.4.2 Northeast Mango Two deposit, molybdenite (AIRIE Run #’s εD-1370 and MD-1380). ..
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