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Supergene Mineralisation of the Boyongan Porphyry Copper-Gold Deposit, Surigao Del Norte, Philippines

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Supergene Mineralisation of the Boyongan Porphyry Copper-Gold Deposit, Surigao Del Norte, Philippines Supergene Mineralisation of the Boyongan Porphyry Copper-Gold Deposit, Surigao del Norte, Philippines by Allan Maglaya Ignacio B.Sc. Geology, National Institute of Geological Sciences University of the Philippines Thesis submitted in partial fulfilment of the requirements of the Masters of Economic Geology Degree Centre for Ore Deposit Research, University of Tasmania December, 2005 DECLARATION OF ORIGINALITY This thesis contains no material which has been accepted for a degree of diploma by the University of Tasmania or any other institution, except by way of background information and duly acknowledged in the thesis, and contains no previous material previously pub- lished or written by another person except where due acknowledgement is given. Allan Maglaya Ignacio 01 December 2005 _________________________ STATEMENT OF AUTHORITY OF ACCESS This thesis may not to be made available for loan or copying for 1.5 years following the date this statement was signed. Following that time, the thesis may be available for loan and lim- ited copying in accordance with Copyright Act 1968. Allan Maglaya Ignacio 01 December 2005 _________________________ TABLE OF CONTENTS Page (s) LIST OF FIGURES …………………………………………………….. i - iii LIST OF APPENDICES ………………………………………………… iv ACKNOWLEDGMENTS ………………………………………………. v ABSTRACT ……………………………………………………………... vi - vii 1.0 INTRODUCTION ………………………………………………………. 1 - 8 1.1 Introduction …………………………………………………………. 1 1.2 Aims and Objectives ……………………………………………….. 1 1.3 Methods Employed …………………………………………………. 2 1.4 Location and Accessibility …………………………………………. 3 1.5 Climate ……………………………………………………………... 5 1.6 Previous Work ……………………………………………………… 5 2.0 GEOLOGICAL SETTING ………………………………………………. 9 - 37 2.1 Introduction ………………………………………………………. 9 2.2 Regional Tectonics …………….…………………………………. 9 2.3 Regional and Local Stratigraphy ………………………………... 11 2.3.1 Basement (Cretaceous-Paleogene) ………………………. 11 2.3.2 Bacuag Formation (Oliogocene-Miocene) .……………... 11 2.3.3 Motherlode Formation (Miocene) ……………………... 14 2.3.4 Mabuhay Formation (Pliocene) ………………………... 15 2.3.5 Timimana Limestone (Pliocene) ……………………….. 15 2.3.6 Tugunan Formation (Pleistocene) ……………………... 16 2.3.6.1 Maniayao Andesite (Quaternary) ………………………. 17 2.3.6.2 Paco Andesite (Quaternary) …………………………….. 22 2.3.7 Sediments (Quarternary) .……………………………….. 22 2.4 Boyongan Intrusive Complex ……………………………………… 22 2.4.1 Breccias ……………………………………………….. 25 2.4.2 Intrusions ……………………………………………….. 27 2.4.2.1 Pre-mineral Intrusions …………………………………. 27 2.4.2.2 Early-mineral Intrusions ……………………………….. 30 2.4.2.3 Inter-mineral Intrusions ………………………………... 32 Table of Contents Page (s) 2.4.2.4 Late-mineral Intrusions …………………………………. 33 2.4.3 Paleosol ………………………………………………... 33 2.5 Faults ………………………………………………………………. 34 2.6 Summary …………………………………………………………… 34 3.0 HYPOGENE ALTERATION & MINERALISATION …..……………... 38 - 48 3.1 Introduction ………………………………………………………. 38 3.2 Hydrothermal alteration ………………………..………………… 38 3.2.1 Potassic alteration ………………….…………………….. 38 3.2.2 Propylitic alteration ……………….……………………… 39 3.2.3 Intermediate argillic alteration ………..…………………. 39 3.2.4 Phyllic slteration ……………………….…………….….. 41 3.2.5 Calc-sodic alteration ……………………..………….…… 41 3.2.6 Advanced argillic alteration ………………..…...…..…… 41 3.2.7 Calc-silicate alteration …………………………….….…. 43 3.3 Veins ….…………………………………………………………… 43 3.4 Hydrothermal breccias …………………………………………… 45 3.5 Hypogene mineralisation ………………………………………… 46 3.6 Summary ………………………………………………..……….. 48 4.0 SUPERGENE ALTERATION & MINERALISATION ……………….. 49 - 91 4.1 Introduction ………………………………………………………... 49 4.2 Secondary copper sulfide mineralisation …………………………... 49 4.2.1 Chalcocite ………………...……………………………... 49 4.2.2 Digenite …………………………………………....……. 55 4.2.3 Pseudo-covellite …………………………………………. 55 4.3 Secondary Copper Oxide & Native Copper Mineralisation ……….. 56 4.3.1 Cuprite ….………………...……………………………... 56 4.3.2 Native copper ..……………………………..……………. 56 4.3.3 Malachite ………...………………………………………. 56 4.3.4 Pseudo-malachite………...………………………………... 61 4.3.5 Azurite ……….………...………………………………... 61 4.3.6 Chrysocolla ……………………………………………… 65 4.3.7 Pseudo-chrysocolla ……………………………………… 65 4.3.8 Pitch limonite/ cuprous goethite ..…………….……….. 65 4.3.9 Pseudo-neotocite ………………………………………… 66 Table of Contents Page (s) 4.4 Supergene iron oxides ………………………….………………… 66 4.5 Supergene gold ………………………………….……………….. 70 4.6 Spatial distribution of copper oxides and sulfides ………………. 70 4.7 Wallrock alteration (clay Mineralogy) …………..………………. 79 4.8 Supergene paragenesis …………………………………………… 84 4.9 Summary …………………………………………………………. 90 5.0 METAL ZONING ……………………………………………………….. 92 - 104 5.1 Introduction …………………………..…………….………………. 92 5.2 Copper and gold distribution …..………..…………….……………. 92 . 5.3 Metal zoning …....………………………………………………….. 93 5.4 Discussion ..….….………………………………………………….. 100 5.5 Summary …………………………………..………….……………. 103 6.0 CARBON AND OXYGEN ISOTOPE GEOCHEMISTRY ……………... 105 - 112 6.1 Introduction …………………………………………………………. 105 6.2 Methodology ……………………………………………………….. 105 6.3 Results ……………………………………………………………... 106 6.4 Discussion ………………………………………………………….. 106 6.5 Stable isotope geothermometry …………………………………….. 110 6.6 Summary ……………………………………………………………. 111 7.0 SUPERGENE ORE GENESIS & IMPLICATIONS FOR EXPLORATION 113 - 124 7.1 Supergene ore genesis ………………...…………………………. 113 7.2 Summary ……………….………………………………………... 117 7.3 Implications for exploration ……………………………………… 122 8.0 REFERENCES ………………………………………………………….. 125 - 132 i LIST OF FIGURES Page Figure 1.1: Location of Boyogan in the Philippines …………………………… 4 Figure 1.2: Map showing creeks, rivers, roads, trails, and selected localities within the vicinity of Boyongan, Surigao del Norte, Norteastern Mindanao, Philippines …………………………………….……….. 6 Figure 2.1: Major tectonic features of Mindanao ……………………………… 10 Figure 2.2: Geology and structures of Surigao del Norte, Northeastern Min- danao, Philippines ………………………………………………….. 12 Figure 2.3: Quarternary and Pre-Quarternary geological map of Boyongan …... 13 Figure 2.4: Schematic graphic log showing stratigraphy for the Surigao gold dis- trict …………………………………………………………………. 16 Figure 2.5: Drillhole location at Boyongan ……………………………………. 18 Figure 2.6: Geological cross-section along 1062200N (looking north) ………... 19 Figure 2.7: Photos of core samples: Bacuag Formation, Motherlode Formation, Mabuhay Formation, Timamana Formation, Tugunan Formation, Maniayao Andesite ………………………………………………… 20 Figure 2.8: Geological cross-section along 779700E (looking west) …………. 21 Figure 2.9: Photos of fine-grained diorite breccia, polymict breccia, mudstone- rich breccia, skarn, diatreme float, and paleosol ………………….. 24 Figure 2.10: Geological plan along –100RL …………………………………….. 26 Figure 2.11: Core photos of mottled diorite, “Bird’s Eye” porphyry, medium- grained diorite, and fine-grained diorite …………………………… 28 Figure 2.12: Core photos of first and second early-mineral diorite porphyries, in- ter-mineral diorite, late diorite porphyry …………………………... 31 Figure 2.13: Schematic space-time diagram of alteration and vein stages relative to the intrusive history of the Boyongan Intrusive Complex ……… 35 Figure 3.1: Plan –100RL and section 1062200N showing quartz stockwork zone, outer limit of potassic alteration, and inner limit of pyrite halo overlain on geology ………………………………………………... 40 Figure 3.2: Core photos of potassic alteration, quartz-sericite halo, early- actinolite-magnetite alteration, wispy/quartz-magnetite veinlets, banded or ribbon quartz veins, comb quartz vein ………………….. 42 Figure 3.3: Core photos of chalocopyrite±bornite vein, free gold from panned rock saw cuttings, hydrothermal breccia 1, and hydrothermal breccia 2 ……………………………………………………………... 45 List of Figures ii Page Figure 3.4: Core photos of hypogene sulfide occurrences at Boyongan ………... 47 Figure 4.1: Core photos showing remnant sulfides in the oxide zone at Boyongan …………………………………………………………… 50 Figure 4.2: Styles of supergene mineralization at Boyongan (oxides) ………….. 51 Figure 4.3: Ore microscopy sample location along 1062200N (looking north) …. 52 Figure 4.4: Styles of supergene mineralisation under the microscope (sulfides) .... 53 Figure 4.5: Styles of supergene mineralisation (sulfides) and gold occurrences under the microscope ……………………………………………….. 54 Figure 4.6: Styles of supergene mineralisation under the microscope (sulfides and oxides) …………………………………………………………. 57 Figure 4.7: Styles of supergene mineralisation at Boyongan (oxides) ………….. 58 Figure 4.8: Styles of supergene mineralisation at Boyongan (oxides) ………….. 59 Figure 4.9: Styles of supergene mineralisation under the microscope (oxides) ….. 60 Figure 4.10: Styles of supergene mineralisation under the microscope (oxides) …. 62 Figure 4.11: Styles of supergene mineralisation at Boyongan (oxides) ………….. 63 Figure 4.12: Styles of supergene mineralisation at Boyongan (oxides) and acid tests …………………………………………………………………. 64 Figure 4.13: Core photos of limonite (goethite) textures and occurrences ………. 67 Figure 4.14: Styles of goethite occurrences under the microscope ………………. 68 Figure 4.15: Core photos showing evidences of copper remobilisation …………. 69 Figure 4.16: Section 1062200N (looking north) showing how copper zones were generated …………………………………………………………..... 71 Figure 4.17: Section 1062200N (looking north) showing spatial distribution of su- pergene and hypogene mineral assemblages ………………………. 72 Figure 4.18: Section 1062200N (looking north) showing quartz stockwork contour and limonite contour …………………………………………….….. 73 Figure 4.19: Section 779700E (looking west) showing
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