THE FORMATION OF THE PANGUNA PORPHYRY COPPER DEPOSIT, with an appendix on the. Frieda porphyry copper prospect, New Guinea.

Transcription

1 THE FORMATION OF THE PANGUNA PORPHYRY COPPER DEPOSIT, BOUGAINVILLE, PAPUA NEW GUINEA. with an appendix on the Frieda porphyry copper prospect, New Guinea. by C. J. EASTOE, B.Sc.(Hons.) Submitted in partial fulfilment of the requirements for the Degree of Doctor of Philosophy. UNIVERSITY OF TASMANIA HOBART 1979

2 This thesis contains no material which has been accepted f or the award of any other degree or diploma in any univer sity and, to the best of my knowledge and belief, contains no copy or paraphrase of material previously published or written by another person, except where due reference is made in the text of this thesis. University of Tasmania, January, 1979.

3 o-nautango Andesite.; leucocratic Quartz Dior The open pit at Panguna in November 1974.

4 I i ABSTRACT Various hydrothermal processes have been suggested as important in the formation of porphyry coppers, e.g. orthomagmatic evolution of salt-rich liquid, condensation of salt-rich liquid from magmatic vapour, convection of groundwater driven by magmatic heat, boiling of groundwater. A fluid inclusion study based on detailed two-dimensional sampling indicates that all of these processes appear to have contributed to the evolution of the Panguna deposit, but suggests that copper was deposited mainly by salt-rich liquid expelled direct from the magma. The deposit formed at the southern contact of the Kaverong Quartz Diorite with the Panguna Andesite. Three smaller porphyritic stocks, the Biotite Granodiorite, the Leucocratic Quartz Diorite and the Biuro Granodiorite, were emplaced in the deposit during mineralisation, which comprised three phases of hydrothermal activity. The first, phase A, took place when the southern part of the Kaverong Quartz Diorite was at 0 temperatures over 700 c. The Panguna Andesite was pervasively altered to an amphibole-magnetite-plagioclase assemblage, upon which was superimposed copper mineralisation and associated K-silicate alteration. The limit of copper deposition and quartz veining to the southwest coincides closely with a zone in which salt-rich liquid was cooled and diluted. A pyritic halo parallels this zone. The system cooled below 400 C before undergoing renewed mineralisation at temperatures over 400 C in two approximately concurrent but separate phases B and c. accompanied by the intrusion of porphyritic stocks. These phases were Phase B formed a well-defined cell bounded by a pyritic halo and centred on the Leucocratic Quartz Diorite. Phase C was expressed as veining of the Biotite Granodiorite, the Biuro Granodiorite and the area between them.

5 ii Copper mineralisation took place at a pressure near 300 bars and at temperatures between 350 C and 700 c or higher. Cu,Fe sulphides, quartz, anhydrite and hematite in veins, and potassium silicate alteration were formed from boiling salt-rich liquid, of density g/cm3, mostly of magmatic origin. The composition of these liquids (which nucleated both KCl and NaCl in fluid inclusions) in terms of the system NaCl-KCl-H2o varied between 76% salts (60% NaCl, 16% KCl) and 46% salts (30% NaCl, 16% KCl) by weight. Other liquids, apparently more dilute, nucleated only NaCl. The salt-rich liquids also contained Fe, Ca and S, and minor quantities of Mg, Cu, Mn and zn. A Cu concen.: tration of 1900 ppm has been estimated in one liquid. The atomic K/Na ratios of salt-rich liquids from three principal phases of vein mineralisation and from quartz phenocrysts conformed to a single trend, increasing from 0.17 to 0.46 as the NaCl content decreased. Groundwater, mainly of less than 5% salinity, inundated the orebody between phase A and phases B and c, and again after phases B and c, at. 0 temperatures below 400 c. Groundwater deposited quartz~pyrite and probably pyrite-clay and sphalerite-pyrite veins at temperatures near 300 C and caused loca2 phyllic alteration. Given a hydrostatic pressure regime in the groundwater system, the depth of formation was near 3 km. Fluids of groundwater composition, trapped as inclusions at or above their critical points, seem to bound the. regions in which two fluids coexisted during phases A and B, and possibly c. The evolution of fluid compositions and phase properties across the two-phase region is consistent with the predicted evolution of boiling salt-rich liquid expelled unsaturated from the magma, cooled to saturation and supersaturation by 500 c, then cooled and diluted by mixing with salt-rich liquid formed by the concentration of groundwater (as high as 45% salts)

6 iii by boiling. The salt-rich liquids were unsaturated near 430 C, and at lower temperatures the liquid and gas compositions converged to the critical composition at the boundary. Pressures fell sharply from lithostatic between the magma and the zone of supersaturated liquids of the ore-zone, and were hydrostatic in the lower-temperature unsaturated fluids. In the zone of supersaturation, pressures may have been lower than in the groundwater. Salt-rich liquid was pumped into the ore-zone by the lithostatic-hydrostatic pressure difference, then descended through the ore-zone because of its density. The transport of Fe and possibly CU in the vapour is insignificant under porphyry copper conditions, but Zn and Mo may undergo signific ant vapour transport. This may explain the separation of zn and Mo from Fe and Cu in porphyry copper systems. The absence of major sericite alteration (as opposed to the K-feldspar commonly associated with the salt-rich liquid) suggests that boiling removed excess HCl formed during the alteration of plagioclase and amphibole to biotite. The sulphate in anhydrite deposited by saltrich liquid probably originated from the decomposition of SOz. This mechanism does not account for increased sulphide deposition below 500 C because the liquid maintained a constant S0z:H2S ratio but the reduction 2+ of SOz by Fe may have become important at lower temperatures. The high oxidation state of magmatic fluids during copper mineralisation was due to the loss of H2 from the magma in those early-evolved volatiles that formed the amphibole-bearing assemblage. Chalcopyrites have a.s34s range of -1.6 to 1.5%., pyrites +0.5 to 3.l~and anhydrites +7.6 t~ 16.0% The salt-rich liquid that deposited anhydrite and chalcopyrite had o3 4 s = +1%.. The complexity of the hydrothermal processes indicates that there was not a simple relationship between these values and the o3 4 s values of sulphur in the magma ' ~

7 I iv TABLE OF CONTENTS page ABSTRACT TABLE OF CONTENTS LIST OF FIGURES LIST OF TABLES INTRODUCTION NOTATION ACKNOWLEDGEMENTS CHAPTER ONE : REVIEW CLASSIFICATION Ore localisation. Morphology and environment. Relative metal abundances. Composition of intrusive rocks. Classifications based on multiple criteria. Summary. CLASSIFICATION OF PORPHYRY COPPER DEPOSITS OF THE SOUTHWEST PACIFIC GENETIC ASPECTS: THE ROLE OF FLUIDS IN PORPHYRY COPPER FORMATION Models of fluid interaction. SELECTED STUDIES OF PORPHYRY COPPER FLUIDS CHAPTER TWO: Bingham, Utah. Sar Cheshmeh, Iran. Hydrogen and oxygen isotopes at Panguna. STRATIGRAPHY GEOLOGY The Kieta Volcanics. The Buka Formation. The Keriaka Limestopo. Unnamed Volcanics. The Bougainville Group. The Sohano Limestone. Quaternary Alluvium. Dioritic Intrusives. i iv xi xiv

8 I v page STRUCTURE MINERALISATION GEOLOGY OF THE PANGUNA AREA Panguna Andesite. Kaverong Quartz Diorite. Marginal phases of the Kaverong Quartz Diorite. The order of intrusion. Alteration. Mineralisation. Structure. Comment. CHAPTER THREE: (i) (ii) THE VEIN SYSTEM AMPHIBOLE-MAGNETITE-QUARTZ VEINS PART 1: QUARTZ-Cu,Fe SULPHIDE VEINS AND MINERAlOGY Copper-iron sulphides. Iron oxides. Rutile. Molybdenite. Other ore minerals. Gangue minerals. OCCURRENCE (ii) PART 2: ALTERATION ASSOCIATED WITH QUARTZ-Cu,Fe SULPHIDE VEINS Biotite. Sericite. Albite. K-feldspar. Chlorite. Epidote. Anhydrite. Opaque minerals in alteration assemblages. summary. (iii) ANHYDRITE VEINS (iv) QUARTZ~PYRITE "~TNS (v) (vi) (vii) THICK, MASSIVE PYRITE VEIN PYRITE ± BLEACHED SELVAGE DISCONTINUOUS SULPHIDE VEINLETS (viii) CHABAZITE-BEARING VEINS

9 r page vi (ix) (X) (xi) SPHALERITE-PYRITE VEINS PYRITE-CLAY VEINS GYPSUM VEINLETS (xii) BARREN QUARTZ VEINLETS ( xii:i) CALCITE (xiv) VIVI11NITE VEINLETS SUMMARY OF OBSERVATIONS ON VEIN ORDER CHAPTER FOUR: GENERAL EQUIPMENT TREATMENT OF DATA FLUID lind SOLID INCLUSION STUDIES CLASSIFICATION lind DESCRIPTION OF INCLUSIONS Solid inclusions. Fluid inclusions. Type I inclusions. Type II inclusions. Type III inclusions. Hematite.. Boiling. CHAPTER FIVE: TYPE I INCLUSIONS FLUID INCLUSIONS IN QUARTZ-CU,Fe SUIJ?HIDE VEINS Pressure corrections. TYPE II INCLUSION~ DENSITY lind COMPOSITION OF TYPE III INCLUSIONS BY VOLUME MEASUREMENT COMPOSITION OF TYPE III INCLUSIONS BY MICROTHERMOMETRY TEMPERATURES FROM TYPE III INCLUSIONS The behaviour of the inclusions. Solution of silica. Supersaturation. Interpretation of Th data. upper temperature limit. Lower temperature 10.:::0 t. Temperature estimates independent of the fluid inclusion data. LEACHING STUDIES OF VEIN QUARTZ SUMMARY

10 page vii CHAPTER SIX: FLUID INCLUSION STUDIES IN HOSTS OTHER THAN QUARTZ-Cu,Fe SULPHIDE VEINS QUARTZ PHENOCRYSTS AMPHIBOLE-MAGNETITE-QUARTZ VEINS QUARTZ-PYRITE VEINS Salinity. Temperatures. Relationship with type I inclusions from quartz-cu,fe sulphide veins. PYRITE ± BLEACHED SELVAGE SPHALERITE-PYRITE VEINS QUARTZ HEALING BRECCIATED SPHALERITE as CHAPTER SEVEN: PRESSURE ESTIMATES QP ASSOCIATION TYPE II INCLUSIONS TYPE III INCLUSIONS SUMMARY CHJU>TER EIGHT: THE HISTORY OF THE DEPOSIT PHASE C PHASE B RELATIONSHIP OF B AND C PHASE A CONDITIONS DURING PHASE B EVENTS PRIOR TO PllASE A SUMMARY CHAPTER NINE: FLUID SYSTEMS AND PORPHYRY COPPER FORMATION THE WATER IN TYPE I INCLUSIONS THE ORTHOMAGMATIC MODEL AND THE VJU>OUR-PL!JME MODEL Quantities of salt-rich liquid. K/Na ratios. FLUID EVOLUTION ACROSS THE TWO-PHASE REGION Predictions. Comparison of predictions and observation. The hydrothermal system at Sar Cheshmeh. CONDITIONS IMPOSED BY THE REAL SYSTEM The presence of KCl. The role of groundwater

11 viii page THE RELATIONSHIP OF FLUID DISTRIBUTION WITH METAL AND ALTERATION ZONING 119 TEMPERATURES AND PRESSURES IN THE TWQ-PHASE REGION 1 Temperatures. 1 Pressures. 1 KjNa RATIOS IN THE SALT-RICH LIQUIDS 122 K/Na fractionation during boiling. 122 The precipitation of halite. 123 K/Na fractionation due to differential diffusion. 123 Mixing of two liquids. 123 Rock-fluid interaction. 125 summary, implications and comparison with other deposits. 126 ADDING THE VERTICAL DIMENSION 128 Vertical and horizontal flow. 128 The top of the system. 130 The system below the exposed ore-zone. 131 Summary. 132 COMPARISON OF PANGUNA WITH PREVIOUSLY-SUGGESTED FLUID MODELS 133 CHAPTER TEN: CHEMICAL CONSIDERATIONS THE BUFFERING OF K/Na IN SALT-RICH LIQUIDS THE FUGACITIES OF OXYGEN AND SULPHUR. DURING COPPER MINERALISATION Oxygen fugacity. Sulphur fugacity. THE CHEMISTRY OF THE VAPOUR PHASE. State of water. Fugacity of HCl. Fugacity of NaCl. sulphur species. BASE METAL. TRANSPORT IN THE VAPOUR Enhanced volat:ility in salt mixtures. Chloride volatility versus hydroxide volatility. IRON SPECIES Ferrous species5 Ferric species. COPPER SPECIES The polymers of CuCl. Na-bearing species

12 i page ix Fe-bearing species. Summary. ZINC SPECIES Na-bearing species. Fe-bearing species. znc1 2. MOLYBDENUM SPECIES DISCUSSION OF VAPOUR CHEMISTRY THE ACIDITY OF THE SALT-RICH LIQUID SULPHIDE AND SULPHATE DEPOSITION METAL ZONING THE OXIDATION STATE OF MAGMATIC FLUIDS CHAPTER ELEVEN: INTRO[)UCTION THERMOMETRY SULPHUR ISOTOPE STUDY Sphalerite-pyrite veins. Anhydrite-chalcopyrite VARIATION OF o34s IN TIME AND SPACE 178 Pyrite. 178 Chalcopyrite. 178 FURTHER EXAMINATION OF ANHYDRITE-CHALCOPYRITE FRACTIONATION 179 o 34s IN THE SULPHIDES, SULPHATES & FLUIDS AS A FUNCTION OF foz 180 COMPARISON WITH C':.'HER DEPOSITS 182 THE ISOTOPIC COMPOSITION OF MAGMATIC SULPHUR 184 Coroments. 187 CONCLUSIONS REFERENCES APPENDIX 1; LIST OF SPECIMENS FROM PANGUNA APPENDIX 2: FLUID INCLUSION DATA FROM'PANGUNA 0 APPENDIX 3: A RECONNAISSANCE FLUID INCLUSION AND SULPHUR ISOTOPE STUDY OF THE FRIEDA PORPHYRY COPPER PROSPECT, PAPUA NEW GUINEA INTRODUCTION Scope of the study. Ac~nowledgements. A note on geographic nomenclature

13 X page REGIONAL GEOLOGY The Salumei Formation. The Wogarnush Beds. The April uitramafics. The Frieda Complex. Structure. INTRUSIVE ROCKS ALTERATION Potassic alteration. Phyllic alteration. Propylitic alteration. Other notes. MINERALISATION FLUID INCLUSION STUDIES IN QUARTZ VEINS Type I inclusions. Type III inclusions. FLUID INCLUSION STUDIES IN ANHYDRITE VEINS SULPHUR ISOTOPES CONCLUSIONS