Corbett and Leach 1997(LIBRO)

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Short course manual: Southwest Pacific rim gold-copper systems: Structure, alteration and mineralization, G Corbett & T Leach, 5/97 Edn.

Greg Corbett Terry Leach

Corbett Geological Services Terry Leach and Co

29 Carr St,

North Sydney, NSW 2060

Australia

Ph (61 2) 9959 3060

Fax (61 2) 9954 4834

E-mail [email protected] E-mail [email protected]

SOUTHWEST PACIFIC RIM

GOLD-COPPER SYSTEMS:

Structure, Alteration,

and Mineralization.

SHORT COURSE MANUAL

DRAFTas at 24 May 1997

G J Corbett andT M Leach


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Quartz sulfide gold + copper systems form proximal to magmatic source rocks, predominantly by the mixing ofmagmatic fluids with deep circulating cool and dilute meteoric waters. Carbonate-base metal gold systems format higher levels, mainly by reaction of magmatic-dominated fluids with low pH, CO2-rich waters. Epithermalquartz gold-silver systems form at the highest crustal levels and display the most distal relationship to the

magmatic source. Bonanza gold grades develop in these systems by the mixing of more dilute, boiling,magmatic-derived fluids with oxidizing ground waters. This latter group of deposits is transitional to the classicadularia-sericite epithermal gold-silver vein systems. Telescoping may overprint the varying styles of lowsulfidation gold mineralization upon each other or upon the source porphyry intrusion. Sediment hostedreplacement gold deposits are herein classified as genetically related to low sulfidation quartz-sulfide systems,but develop in reactive carbonate rocks.

Adularia-sericite epithermal gold-silver deposits form at elevated crustal settings in the absence of an obviousintrusion source for the mineralization. These systems vary with increasing depth from: generally barren surficialsinter/hot spring deposits, to stockwork vein/breccias, and fissure veins. Brittle basement rocks fracture well andso represent competent hosts for fissure veins within dilational structural settings. Boiling models account for thedeposition from meteoric waters of the characteristic gangue minerals comprising banded quartz, adularia andquartz pseudomorphing platy carbonate. However, precious and base metals are postulated to be magmatic-derived and are concentrated in thin sulfide-rich bands, commonly with low temperature clay minerals.Mineralization is therefore interpreted to have been deposited mainly by the mixing of upwelling, commonlyboiling, mineralized fluids with cool, oxidizing ground water.

The ore deposit models defined herein are useful in all stages of mineral exploration, from the recognition of thestyle of deposit, to the delineation of fluid flow paths as a means of targeting high grade ores, or porphyry sourcerocks. The exploration geologist may be aided by the use of conceptual exploration models which areinterpretative and so vary from the more rigorously defined deposit and exploration models. Conceptual modelsshould not be applied rigidly but modified using an understanding of the processes described herein to developmodels which are tailored to individual prospects.


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CONTENTS1. Characteristics of Gold-Copper Hydrothermal Systems 11

i) Introduction 11ii) Conceptual exploration models 11iii) Classification 12

iv) Fluid characteristics 13

2. Geothermal Environment for Pacific Rim Gold-Copper Systems 15i) Settings of active hydrothermal-geothermal systems 15ii) Silicic continental and volcanic arc hydrothermal systems 15iii) Characteristics of Philippine intrusion-related active hydrothermal systems 17

a) Physico-chemical zonations in Philippine geothermal systems 17b) Waning Stages of active geothermal systems 18c) Analogies to ore-forming systems 19d) Styles Philippine active hydrothermal systems 20

e) Evolution of active porphyry systems 22iv) Examples of active intrusion-related hydrothermal systems in the Philippines 22

a) Large disseminated systems in permeable structures or composite volcanic terrains 221. Young systems dominated by magmatic vapours 222. Circulating hydrothermal systems 243. Collapsing hydrothermal systems 24

b) Cordillera-hosted intrusion-related active hydrothermal systems 26v) Conclusions 28

1. Types of active porphyry systems 282. Evolution of active porphyry systems 29

3. Structure of Magmatic Ore Systems 30i) Introduction 30ii) Tectonic setting 30

a) Orthogonal convergence 31b) Oblique convergence 31c) Intra-arc rifts 32d) Back-arc extension 32

iii) Major structures 33a) Accretionary or arc parallel structures 33b) Transfer or arc normal structures 34c) Conjugate transfer structures 34d) Transform faults 35

iv) Fracture patterns in magmatic arcs 35a) Orthogonal convergence 36b) Oblique convergence 37

1. Framework from active systems 37

2. Application to oblique convergence 383. Riedel shear model 394. Tension veins 405. Drilling directions 40

c) Rock competency 41d) Fracture permeability 41

v) Changes in convergence 42a) Activation of arc parallel structures 42b) Activation of transfer structures 42c) Arc reversal 43

vi) Dilational ore environments 44a) Tension veins 44

b) Jogs 45


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c) Flexures 47d) Hanging wall splits 47e) Ore shoots 47

vii) Structures in time and space 48viii) Shear sense indicators 49

ix) Porphyry and intrusion-related fracture patterns 50a) Mechanism for fracture development 50b) Porphyry-related fracture systems 51

1. High level settings 512. Deeper level settings 52

x) Breccias 54a) Introduction 54b) Classification 55

1. Descriptive terms 552. Genetic terms 56

c) Primary non-hydrothermal breccias 56d) Ore-related hydrothermal breccias 57

1. Magmatic hydrothermal breccias 572. Phreatomagmatic breccias 603. Phreatic breccias 634. Dilational breccias 655. Magmatic hydrothermal injection breccias 656. Hydrothermal collapse breccias 667. Dissolution breccias 67

xi) Conclusion 67

4. Controls on Hydrothermal Alteration and Mineralization 68i) Introduction 68ii) Temperature and pH controls on alteration mineralogy 69

a) Silica group 70b) Alunite group 70c) Kaolin group 71d) Illite group 71e) Chlorite group 71f) Calc-silicate group 72g) Other mineral groups 72

iv) Alteration zones associated with ore systems 73v) Controls on deposition of gangue phases 74

a) Silica 74b) Carbonates 75c) Sulfates 75

vi) Controls on metal deposition 76

a) Gold 76b) Copper 78c) Lead and zinc 77d) Silver 78e) Gold fineness 78

5. Gold-Copper Systems in Porphyry Environments 80i) Porphyry copper-gold 80a) Structure 80

1. Setting 802. Fracture/vein patterns 80

b) Early models of alteration and mineralization zonation 81

c) Model of polyphasal overprinting events in southwest Pacific porphyry systems 82


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1. Stage I: Heat transfer and zoned alteration 832. Stage II: Exsolution of magmatic fluids 853. Stage III: Late stage cooling and metal deposition 884. Stage IV: Post-mineral phyllic, argillic and advanced argillic overprint 91

d) Summary 92

ii) Skarn deposits 93a) Introduction 93b) Processes of skarn formation 94

1. Prograde isochemical 942. Prograde metasomatic 953. Retrograde 96

c) Skarn ore deposits 96iii) Breccia-hosted gold deposit 98iv) Porphyry-related alkaline gold-copper deposits 98

6. High Sulfidation Gold-Copper Systems 100i) Characteristics 100

a) Classification 100b) Active analogues 101c) Two stage alteration and mineralization model 102

1. Volatile-rich event 1022. Liquid-rich event 103

d) Mineralization 103ii) High sulfidation systems formed as shoulders to porphyry intrusions 104

a) Characteristics 104b) Examples 105

iii) Lithologically controlled high sulfidation gold-copper systems 108a) Characteristics 108b) Examples 108

iv) Structurally controlled high sulfidation gold-copper systems 112a) Characteristics 112b) Examples 113

v) Composite structurally/lithologically controlled high sulfidation gold-copper systems 118a) Characteristics 118b) Examples 118

vi) Hybrid high-low sulfidation gold systems 121a) Characteristics 121b) Examples 122

vii) High sulfidation exhalative gold systems 124a) Characteristics 124b) Active analogues 124c) Example 125

7.Porphyry-Related Low Sulfidation Gold Systems 126i) Classification 126

a) Introduction 126b) Sequence of events 126

c) Types of porphyry-related low sulfidation gold systems 127ii) Quartz-sulfide gold + copper systems 128

a) Introduction 128b) Structural setting 129c) Alteration and mineralization 130d) Examples 131

iii) Carbonate-base metal gold systems 141

a) Introduction 141


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b) Definition 141c) Distribution 141d) Geological setting 142e) Structure 143f) Alteration and mineralization 144

g) Zonations in vein style and mineralization 145h) Fluid flow model 146i) Examples 146

j) Conclusion 162iv) Epithermal quartz gold-silver systems 163

a) Introduction 163b) Characteristics 163c) Structural setting 164d) Examples 165

1. Associated with intrusion-related mineralization2. Peripheral to intrusion-related mineralization 168

3. Associated with adularia-sericite epithermal gold-silver systems 170

e) Conclusions 175v) Sediment-hosted replacement gold deposits 176

a) Characteristics 176b) Example 177

8. Adularia-Sericite Epithermal Gold-Silver Systems 180i) Classification 180ii) Examples 181iii) Tectonic setting 181iv) Structure 182v) Fluid Characteristics and hydrothermal alteration 183vi). Mineralization 184

vii). Types of epithermal gold-silver deposits 185a) Sinter and hydrothermal (hot spring) breccia deposits 1851. Characteristics 1852. Examples 186

b) Stockwork quartz vein gold-silver deposits 1881. Characteristics 1882. Examples 188

c) Fissure veins or reefs 1891. Characteristics 1892. Examples 189

9. Conclusions 194i) Introduction 194ii) Gold-copper exploration models in project generation 194

iii) Gold-copper exploration models in reconnaissance prospecting 195iv) Gold-copper exploration models in project development 195v) Flexible models 196

Acknowledgements 198References cited 198Appendix I. List of Abbreviations 235

LIST OF FIGURES

1.1. Pacific rim gold-copper mineralization models1.2. Southwest Pacific rim gold-copper occurrences1.3. Size vs grade of some southwest Pacific rim copper-gold occurrences

1.4. Derivation of high and low sulfidation fluids


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2.1. Active geothermal systems and hydrothermal ore deposits2.2. Conceptual model for silicic back-arc rift geothermal systems2.3. Conceptual model - volcanic arc hydrothermal systems2.4. Conceptual Model - hydrology of shallow levels in geothermal systems

2.5. Philippines - geothermal fields, tectonic setting, copper-gold mines and prospects2.6. Tongonan geothermal field - structural setting2.7. Alto Peak - hydrological model2.8. Biliran geothermal field - thermal features2.9. Biliran geothermal system - conceptual cross section2.10. Tongonan geothermal field - conceptual cross section2.11. Southern Negros geothermal field - setting2.12. Southern Negros geothermal field - conceptual cross section2.13. Bacon-Manito geothermal field - conceptual cross section2.14. Bacon-Manito geothermal field - conceptual cross section2.15. Geothermal systems in Volcanic arc-Cordillera settings2.16. Amacan geothermal system - cross section

2.17. Daklan geothermal field - cross section2.18. Acupan - geological setting2.19. Baguio District, Philippines - structural controls and alteration

3.1. Pacific Rim plate boundaries3.2. Convergence and fracture systems as orthogonal versus oblique3.3. Southwest Pacific rim porphyry copper-gold settings3.4. Transfer structures and porphyry systems in Papua New Guinea3.5. Conjugate transfer structures, magmatism and changes in convergence3.6. Dilational fracture systems in settings of orthogonal convergence3.7. Structures formed in association with an earthquake in Iran3.8. Structures in settings of oblique convergence

3.9. Fracture systems in settings of oblique convergence3.10. Drill testing tension vein mineralization3.11. Central Tasman fold belt, eastern Australia3.12. Mt Muro, Kalimantan, Indonesia3.13. Dilational veins3.14. Extension and mineralization styles at different crustal levels3.15. Gympie pull-apart basin and goldfield3.16. Fracture/veins and porphyry intrusions3.17. Environments of breccia formation3.18. Magmatic-hydrothermal breccias - subvolcanic breccia pipe3.19. Magmatic-hydrothermal breccias - tourmaline breccia pipes, Chile3.20. Phreatomagmatic breccias3.21. Phreatic breccias

3.22. Magmatic-hydrothermal breccias - injection breccias

4.1. Common alteration mineralogy in hydrothermal systems4.2. Controls to quartz solubility4.3. Controls to calcite solubility4.4. Controls to barite and anhydrite solubility4.5. Solubility of Au, Cu and Zn relative to temperature and pH4.6. Controls to gold solubility4.7. Controls to zinc, lead and copper solubility4.8. Gold fineness

5.1. Sillitoe and Gappe porphyry copper model

5.2. Porphyry copper model stages I and II


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5.3. Porphyry copper model stages IIIa and IIIb5.4. Paragenetic sequence of alteration and copper-gold mineralization in porphyry systems5.5. Porphyry alteration mineralogy5.6. Spatial and temporal distribution of copper-gold minerals5.7. Evolution of pluton associated skarns

6.1. High sulfidation systems - styles6.2. High sulfidation systems - two stage fluid alteration and mineralization model6.3. High sulfidation systems - alteration mineralogy.6.4. High sulfidation systems - metal zonations6.5. Horse-Ivaal - surface alteration6.6. Horse-Ivaal - cross section of alteration6.7. Lookout Rocks - plan of alteration6.8. Lookout Rocks - cross section of alteration6.9. Vuda, Fiji - structure and alteration6.10. Vuda, Fiji - conceptual cross section6.11. Wafi-Bulolo region - structural setting

6.12. Wafi, Papua New Guinea - plan of alteration6.13. Wafi, Papua New Guinea - long section of alteration6.14. Raffertey's copper-gold conceptual cross section, Wafi6.15. Nansatsu deposits, Japan6.16. Miwah - alteration6.17. Miwah - conceptual model6.18. Frieda-Nena - setting6.19. Frieda-Nena - alteration and structure6.20. Nena - alteration6.21. Nena - cross section 4700N6.22. Nena - cross section 5200N6.23. Nena - alteration long section

6.24. Lepanto/FSE - structural setting6.25. Lepanto/FSE - geology and mineralization6.26. Mt Kasi, Fiji - CSAMT/Structure6.27. Mt Kasi, Fiji - structure and mineralization model for open pit area6.28. Peak Hill - structure6.29. Peak Hill - paragenetic sequence6.30. Peak Hill - cross section6.31. Maragorik - setting6.32. Maragorik - alteration6.33. Maragorik cross section6.34. Bawone-Binebase, Sangihe Is, Indonesia6.35. Wild Dog - setting6.36. Wild Dog - geology

6.37. Wild Dog - conceptual cross section6.38. Masupa Ria - geology

7.1. Low sulfidation gold-copper systems - temporal and spatial zonations7.2. Low sulfidation gold-copper systems - classification and fluid flow model7.3. Low sulfidation gold-copper systems - alteration7.4. Ladolam gold deposit - geology7.5. Ladolam gold deposit - conceptual model7.6. Kidston - setting7.7. Kidston - geology7.8. Kidston - paragenetic sequence7.9. Kidston - distribution of gangue and ore phases

7.10. Bilimoia - structure


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7.11. Bilimoia - paragenetic sequence7.12. Bilimoia - conceptual model7.13. Arakompa - paragenetic sequence7.14. Arakompa - fluid inclusion data7.15. Carbonate-base metal gold systems - paragenetic sequence

7.16. Carbonate-base metal gold systems - fluid inclusion data7.17. Carbonate base metal gold systems - zonation7.18. Kelian - setting7.19. Kelian - geology7.20. Kelian - fluid flow vectors7.21. Kelian - carbonate species line 250 E7.22. Porgera - setting7.23. Porgera - structure7.24. Porgera - Waruwari structure7.25. Porgera - cross section7.26. Porgera - paragenetic sequence7.27. Porgera - distribution of carbonate species

7.28. Bulolo graben7.29. Morobe goldfield- vertical distribution of systems7.30. Wau diatreme-maar complex7.31. Kerimenge - composite cross section7.32. Woodlark Island - regional structure7.33. Busai, Woodlark Island - plan locations of cross sections7.34. Busai - mineralization cross section7.35. Busai - alteration cross section7.36. Busai - paragenetic sequence7.37. Maniape - structure7.38. Maniape - cross section7.39. Maniape - paragenetic sequence

7.40. Mt Kare - carbonate-base metal cross section7.41. Gold Ridge - carbonate-base metal cross section7.42. Karangahake - long section7.43. Misima - Geology7.44. Porgera Zone VII - paragenetic sequence7.45. Porgera Zone VII - alteration cross section7.46. Mt Kare - paragenetic sequence7.47. Coromandel Peninsula - structural setting7.48. The Thames goldfield, Ohio Creek Porphyry and Lookout Rocks alteration7.48. Arakompa-Maniape - fluid flow model7.50. Tolukuma - vein system7.51. Tolukuma - cross section7.52. Tolukuma - paragenetic sequence

7.53. Tolukuma - fluid flow model7.54. Cracow - structural setting7.55. Sediment hosted replacement deposits - conceptual model7.56. Mesel - structure7.57. Mesel - paragenetic sequence7.58. Mesel - conceptual fluid flow model

8.1. Models for low sulfidation epithermal vein systems8.2. Setting of Champagne Pool, Taupo Volcanic Zone8.3. Puhipuhi - Geology8.4. Golden Cross - structural setting8.5. Golden Cross - alteration cross section

8.6. Golden Cross - alteration long section


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1 CHARACTERISTICS OF GOLD-COPPER HYDROTHERMAL SYSTEMS

i) Introduction

This is the manual used for the short course of the same name presented at the SME/SEG Meeting in Phoenixin March 1996, with modifications in response to reviewers comments. The manual provides additional detailedinformation to the verbal presentation, and is designed so that the figures can be followed during the lectures, inwhich the slides of rocks, remote sensing images, etc, further support the concepts presented.

TerminologyWe have tried where possible to use terms which are technically correct and in common use by short courseattendees - predominantly field geologists working in mineral exploration. However, our terminology may notalways be in strict agreement with some geological literature, to which the readers are referred, and termsutilized in research studies.

ii) Exploration Models

This workshop demonstrates and describes ore deposit styles as well as exploration and conceptual explorationmodels as aids to the exploration and evaluation of southwest Pacific rim magmatic arc mineral resources.However, careful consideration of the nature of these exploration models is necessary before any reliance canbe placed upon them.

Exploration geologists compare, contrast and classify mineral occurrences in order to build up empirical patternsfrom data such as field observations. Deposit models are developed as empirical descriptions of individualdeposits, or of more use to the exploration geologist, styles of deposits. Exploration models are derived frominterpretations, focusing upon those characteristics of a deposit model which aid in the discovery of ore depositsof a particular style. Progressively lateral interpretations depart from rigorously reviewed science and so become

conceptual exploration models. Such a conceptualization may give the explorationist a competitive advantage(Henley and Berger, 1993) in the increasingly difficult search for ore deposits. These models may also assist inranking projects and aid in the abandonment of lower order targets.

Conceptual exploration models evolve through their application to exploration examples, and are refined byresearch, many being abandoned during this process. Although luck plays a part, the competitive nature of themodel. The very innovative nature which makes a conceptual exploration model of use to the explorationist,precludes the lengthy process of rigorous evaluation of many of the concepts by exhaustive research studies.

It is important that models must not be applied rigidly, but should be modified to become project-specific. Greatcare must be taken to abandon or modify inappropriate models. It is intended in this manual to outline to

explorationists, the processes involved in the derivation of conceptual exploration models, rather than in the rigidapplication of existing models. Structure and petrology are tools which the explorationist may use in thedevelopment of conceptual exploration models. This manual illustrates how the integration of these can lead tothe development of conceptual exploration models. Major structures localize intrusions and minor structuresprovide ground preparation. The study of petrology delineates styles of alteration and mineralization, fluidcharacteristics and mechanisms of ore deposition. The synthesis of structure and petrology may define fluid flowpaths in hydrothermal ore systems. search for ore bodies encourages explorationists to be the first to develop orutilise a conceptual exploration


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iv) Fluid Characteristics

The physico-chemical characteristics of the hydrothermal fluids control the:* type and quantity of metals transported,

* processes which produce mineralization,* location of the mineralization,

whereas the characteristics of the host rock control the mechanisms of fluid flow (Hedenquist, 1987). Aconceptual model for the transportation of fluids from a degassing magma to porphyry, high sulfidation and lowsulfidation systems is illustrated in Figure 1.4 and the characteristics of high and low sulfidation systemscompared in Table 1.2.

Country rocks become more competent (brittle) as a result of contact metamorphism during the initialemplacement of high level porphyry intrusions. Fracturing is initiated at the cooled margins of the intrusion andextends into the host country rocks. Cooling of the porphyry intrusion and the parent melt is accompanied by theprogressive exsolution of dissolved salts, magmatic volatiles (mainly H2O, SO2, CO2, H2S, HF and HCl), andmetals, and their transfer into the fractured carapace (Henley and McNabb, 1978). Dispersion and mixing ofthese magmatic fluids with circulating meteoric-dominated fluids, results in the zoned alteration andmineralization which characterizes porphyry copper deposits (Henley and McNabb, 1978; e.g., Grasberg andBatu Hijau in Indonesia; Ok Tedi and Panguna in Papua New Guinea). Skarns form where mineralizing porphyryintrusions are emplaced into calcareous host rocks (e.g., Ertsberg, Indonesia; Frieda River Copper, Papua newGuinea; Red Dome, eastern Australia).

Volatiles may become overpressured where confined within the intrusion. Tectonic movements may fracture thecarapace and facilitate venting which causes the formation of breccia bodies (e.g., Kidston, eastern Australia)and fracture systems which host later mineralized magmatic fluids.

High sulfidation gold-copper deposits form if magmatic volatiles (SO2, CO2, H2S, HCl, HF) and brines are

channelled from the source intrusion up deep seated fracture/fault zones and rise rapidly with minimal rockreaction or mixing with circulating meteoric fluids. The progressive disproportionation of magmatic SO2into H2Sand H2SO4within the vapour plume, occurs at temperatures below approximately 400oC, and as thetemperature decreases, increasing amounts of H2SO4and H2S are produced (Rye et al., 1992). H2SO4and HClare inferred to start to dissociate at temperatures of around 300oC (Hedenquist and Lowenstern, 1994), andprogressively form hot acidic fluids as a result of the transition from SO2to H2SO4. Within dilational structuresand/or permeable lithologies, these hot acidic fluids mix with circulating meteoric waters and react with countryrock to form gold-copper deposits (Rye, 1993). Initially Hedenquist (1987) termed these systems 'highsulfidation' as sulfur is in a high oxidation state of +4, because of the dominance of magmatic SO 2. However,more recently (Hedenquist et al., 1994; White and Hedenquist, 1995), the term high sulfidation has been used toindicate the presence of the relatively high sulfidation state sulfide minerals such as enargite, luzonite and

tennantite in these systems. The abundance of sulfur cannot be used solely as a criteria to distinguish betweenlow and high sulfidation systems. Although sulfur species are commonly abundant in most southwest Pacifichigh sulfidation systems, some low sulfidation systems are sulfur-rich (e.g., Ladolam, Papua New Guinea).Examples of high sulfidation gold-copper deposits include: Lepanto, Philippines; Nena and Wafi, Papua NewGuinea; Mt Kasi, Fiji; Gidginbung and Peak Hill, eastern Australia.


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S i z e vs G r a deCIFIC RIMO U T H W E S T PAR O C C U R R E N C E SEPITHERMAL Q | . | sh kfl -05 040 K a r a n g a h P r g e r a ii i0__20- T ol u ku m ao

wE m p e r o r W fl i h io

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10 --.C

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l- l 1 o o z o o

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/ +IE

e r i c i t e e p i t h e r m a lA u A g0 e p i t h e r m a l q u a r t z A u - A gI c a r b o n a t e - b a s e m e t a l A u

Q ua i t z s u l t i d e A uq* h i g h s u l fi d a ti o n A u - C u

- A u 0 r i > h v f y G Ub o l s r e fl e cto m p o s i t e s y mt ra n s i ti o n a l s y s t e m s

G o l d E q u i v a l e n t a s :70g/ tAg =1g/tAu1 % C u = 1 . 2 5 g / l A uI l-H-H-la 4 5 67 2 o a o 4 o s o

SIZ E a s lo g M il li on oz gold equivalentFIG13

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M i - g i n aic u d

S f 1 s z r a e d i f h i

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9; s o *Oof p

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\ . \ 9 fa

+' \ ` ~ _ ' a * \ \ \ 9

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+ P + 4 - I 4- -L- I +% 4- - t - - t- 4- 4+ + -o -+ + + + H z o , 0 0 2 ,so2* ++ H Z S , H C I , HF, ,* ++ + + gases -i t- -t- - t - - t-

H i gh L ev e lIntrusion +

+ ++ + ++ ++ +4 - +4 - 4- L - L - l - + + - l - l - - l - + + + 4 - L - L + + + + +. ++ +

+

{ Bonanza

Dervationof High a n dL o w Su l f i da t i on Flu dsF I G . 1.4

1102

f/- ;e _ ` _ ` \

r a c k ; / C u d\

S

' Permeable Lithology` o r Subsid iary Structure

High sulf idation sulf ides(e.g. enarg i te , I uzon i tea nd t ennan t i t e )4 S02 + 4 H Q O -3H2SO4 + H Sdispropor t ionat ion ol magmat ic SO 2


20/317


21/317


Table 1-2

DISTINCTION b e t w e e n HIGH a n d LOWSULFIDATIONLow Sul f idat ion High Sul f idat ionFluidAlterat ion

Associatedmine ra l sMetals : EconomicAccessoryGold f ineness

Form o t _mineral izat ionStructure

11153

Dilute H 28-dominantGene ra lly neu t ra l a l te ra t ion a d ja c e n t t ost ructures d o m in a te d b ys e r i c i t efi l li t ic c l a y s - per i phe ra l propy l i t i cveins dominated by quar tz carbonateLo w % pyr i tega lena , spha le r i te , cha lcopyr it eA u A gP b , Z n, CuA s, T e , H g , Sb a t h i g h leve lsVar iab le f i n e n e s s w it h d e p thhigh f ineness (silver-poor) a t depthlo w f ineness (silver-r ich) at high leve lsVeins c ommon with crystal l inephases a t d e p th , b a nd e d a t sha l low levelsPre-exist ing f rac tures a t depthSubsidiary di la t ional s t ructures a t higher levelsM ag m a ti c, d ia t re m e a n d e r up ti on b r e c ci as

Saline, S 02 -dominantCharac te r i st ic z oned pe rvas ive acida l te r a t io n f ro m : r e s id u a l ( vu g h y )quartz-aIuni te - kaoI in minerals -i ll i te minerals -propyl i t icHig h % pyr i teengarite - IuzoniteA u C uA sT e a t h ig h le v e lsHigh f ineness (silver-poor)

Matrix to breociated hosted inc o m p e t e n t w a ll r o c k a l tera t ionDilat ional s t ructura l an d pe rm eab lel i tho logical cont ro lD ia t re m e b re c c ia s c o m m o n


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2 GEOTHERMAL ENVIRONMENT FOR SOUTHWEST PACIFIC GOLD-COPPER

i) Settings of Active Hydrothermal-Geothermal Systems

Geothermal systems studied over the past two decades provide an increased understanding of the processeswhich take place during the formation of hydrothermal ore deposits. Geothermal systems are encountered in awide range of geological settings and each one may be analogous to a distinct style of ore-forming system.These can be classified in terms of their crustal setting and probable heat source (e.g., Henley, 1985a; Fig. 2.1).

Magmatic-sourced oceanic geothermal systems occur in association with: oceanic crust along mid-ocean ridges,ocean island volcanoes formed in relation to hot spots, back-arc basins, and volcanic arcs along inter-oceanicsubduction zones (de Ronde, 1995). Exhalative features associated with sea floor geothermal systems, such assulfide-rich black smokers, are interpreted to be analogous to volcanogenic massive sulfide or Kuroko-style oredeposits (Binns et al., 1993, 1995).

Active hydrothermal systems which have a magmatic heat source may be associated with crustal rifting within acontinental crust, either in back-arc rift zones (e.g., Taupo Volcanic Zone, New Zealand), or in continental riftzones (e.g., East African Rift). As will be shown later in this section, these types of geothermal systems have ageological setting and fluid chemistry comparable to the circulating meteoric waters associated withadularia-quartz vein systems which host epithermal gold-silver deposits (e.g., Waihi and Golden Cross, NewZealand).

Geothermal systems encountered in magmatic arcs associated with subducting oceanic crust (e.g., Philippines,Indonesia) are actively forming porphyry-related systems. These systems form porphyry and skarn copper-gold(+ molybdenum), high sulfidation gold-copper, and mesothermal to epithermal precious and base metaldeposits.

Geothermal systems are also encountered in continental environments in the absence of any obvious magmaticheat source. Rapid uplift results in high geothermal gradients which may facilitate the leaching of metals from athick sedimentary pile by circulating meteoric waters. Fluids migrate along major fault zones associated withplate collisions (e.g., along the Alpine fault, South Island, New Zealand), and deposit gangue minerals andmetals in dilational structural settings as post-metamorphic gold veins (e.g., Macraes Flat, South Island, NewZealand). Overpressuring in response to rapid deposition in thick sedimentary basins (e.g., southeast USA)results in the establishment of circulating hydrothermal systems and these may be related to Mississippi Valley-style massive sulfide deposits (Henley, 1985a).

ii) Silicic Continental and Volcanic Arc Hydrothermal Systems

There are considerable differences in the geological setting and fluid characteristics between geothermalsystems formed in silicic continental rift environments (e.g., New Zealand), and volcanic arc environments (e.g.,Philippines; Henley and Ellis, 1983; Reyes, 1995). The geochemistry of representative wells and surface springsfrom selected Philippine and New Zealand geothermal systems is given in Table 2.1.

In geothermal systems typical of those encountered in silicic rift environments, the heat source is considered tobe deeply buried (>5-6 km) batholiths (Hedenquist, 1986), of inferred granite/granodiorite composition, whichformed from melted continental crust (Henley, 1985b; Fig. 2.2). Water recharge is derived from meteoric groundwaters and the intrusion supplies heat, chloride, some gases, and possibly other elements. The resultant fluid iscommonly termed chloride reservoir or chloride hydrothermal fluids in geothermal sciences. Boiling occurs atshallow levels in response to reduced pressure and forms near-surface two phase zones. The upwelling


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H E A TSOURCE

HOST

GEOTHERMASYSTEM

O F I EDEPOSIT

EA G M A T IC > AMAGMATICO C E A N IC C R US T P L A TE M A F IG I N C O N T I N E N T A LC R U S Tig y

. _ , J;f$H O T OCEANIC M A G M A T I C BACK A F I C & CRUSTAL INTERIORSPOT F I I F T I N G AR C C O g ; | E : | _ I } I | E I I T A L F A U L T I N G B A S I NL

V O L C A N O G E N I CMA SSI V ESULFIDEPORPHYHY Cu-Au-MoSKARNI N T F I U S I O N - R E L A T E DCu-Au-Zn-Pb

EPITHERMAL P O S T - MISSISSIPPIAu-A g METAMORPH IC VALLEYG F I A N I T I G O - A u B A S ES n - W - M o S H E A F I M E T A L SZONES

A c t i v e g e o t h e r m a l sys tem s a n d h y d r o t h e r m a l o re de pos i t s 94-tdaF I G . 2 .1

e

/_,_, W / o \\_ fi` _ O 0 $0 \, , \y /\ , ,\ , / _ \2 'i b a s e m e n t\ /i / , / \ ,b a s e m e n t \\ ' / , I / i`\

\ Iz k m \ \ `l i/ i f T ` \ \ Meteor ic/ / , M a g m a t i /\ r e c h a r g e1 60/ f luid ,< = / (flggolso\ _


24/317


chloride hydrothermal fluid, or chloride reservoir, generally reaches the surface as boiling springs, which depositsilica sinters either above the main upflow zone associated with hydrothermal eruption craters (e.g., ChampagnePool, Waiotapu, and Ohaaki Pool, Broadlands in New Zealand; Hedenquist, 1990: Table 2.1), or in outflowzones (e.g., Orakeikorako, New Zealand; Sheppard and Lyon, 1984; Simmons et al., 1992). Minor zones of acid

sulfate fluids result from the oxidation of H2S in surface ground waters (e.g., Rotokawa, New Zealand; Kruppand Seward, 1987: Waimangu, New Zealand; Simmons et al., 1992). CO2-rich waters form at shallow levelswhere the gas is absorbed into cool hydrothermal fluids or ground water, and the recharge of these waters todepths of up to 1200 m has been recorded at the margins of the Broadlands geothermal field, New Zealand(Hedenquist, 1990).

Active hydrothermal systems associated with volcanic arc terrains display a number of characteristics which aresignificantly different from those in continental silicic environments (Henley and Ellis, 1983; Reyes, 1995; Figs.2.2, 2.3). In these systems meteoric recharge is typically heated by multiple shallow (


25/317


26/317


27/317


C_P h r e a z o n e

T w oh a s e z o n e

C o n v e c t v e z o n e

C o n d u c f v e z o n e

m a s M u l t ip l e h i g h l e v e l i n t r u s i o n s

@SoI fa ta r ah IG e o t e rm a A c i d - s u l t a t eI We" _ _ 7 - ( N F a I t s p r m g S u l f a t e - c a r b o n a t e$ 0 9 'ai E ., . . . s p r m g< a f ~ \ \ x \ D $ "C H 0 N e u t r a l - c h l o r i d en -\_ n\B0 I I S U L F A T E B pringZ - 1 - . . ^ R B 0 N A s `""':::j:j:' '1j::'"" A Q U / F E / - 7ZONEOFMWNG I* op f hYdrotherma| ?` M e a n s e a l e v e l^ [ , ~ E D P g ' f , f f , ; L , B o m r w t p h a s e 9 (Y8 f Q , H f a n d g a s s e p a r a h o n L A T E R A L

, , _ f 4 \ _

K \ //M /

\lP` I un n nunpu n\ < v tu ti f - -

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F I G . 2 .3

4 | fIU T F | - 0 W Z Q N E P i e z o m e t r i s u r f a c e o fX w 4t r/ I fl d

X \ I / X RECHARGE O F / \ > < > < ] X A / \ // / x\ / X J x\ $4%/ \ x \q/A X x X/X / // V H \x \ X x X I

/ X \ \/r H \|\ * X

d e e p c h l o n d e r e s e r v e

METEORIC FLUIDSDrawn wim re fe renoe to Bagie e t a L (1987)

C O N C E P T U A L M O D E LVolcamc A rcH y d r o t h e r m a l System

h

\,/'

Vadose /9 '2Zone//m

H d r o th e r m a l s o l ta t a r aH 2S + 1 / O 2 - > S +H 2 0x l p e r c h ed aera ted ~ ~ ~ | f = ~ = , , , ; : ; , ; ; ; ; : s s s r ; ; > s . s > \ @ f ~ M ~ e fatersPhreat icZone Q wgte/\YHa/ \

\ o > /Ch lo r i dero the rmald _l`eSeI'V0|rAcid-sul fatewatersC02- r i c h w a t e r sMixed C 0 2 - richan d sul fate WatersSil ic ificat ionR e c ha rg e o fsur f ic ia l WatersU p flo w o fchlor ide wate rsG a s esS u m m a r y r e a c ti o n sonly

d @ : e - u . \ \ $.\

e Y \ < ' e \ ''

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e 1 \ . 1 9 ` ` \ u \ \. 9 f \ d < ' `e \ \ . \ \ * \ , f - I

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\ A` f T ' ` - l= \ ` *\1 fe tfi ~ ~ *A _ \\.; C02+H2->H2Q3 'V771/ _ /7 * Y .;____ _A s".\ , .. .I .HAt / f k _ _ _ r

- -\ / ? . ' . f\ \ 1, \_ _ \ _ , ,t; H a sr ( + 9 0 2 )Z o ne s o fen t ra inmentan dfluid mix ing

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FIG. 2 .4


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Biliran Island, has encountered magmatic-derived acidic fluids at around 1 km from the surface. Recent drillingat Alto Peak (Reyes et al., 1993) and Mt. Pinatubo (Ruaya et al., 1992) intersected zones which were dominatedby magmatic acid fluids.

b) Waning stages of the active Philippine systems

As intrusions cool and hydrothermal systems wane, the decrease in temperature and reservoir pressure resultsin draw down of surficial waters deep into the hydrothermal system. Cool, low pH CO 2-rich and acid sulfatewaters have been encountered at depths up to 2000 m in some Philippine geothermal fields (Reyes, 1990b).Down-hole pressure/temperature (Leach et al., 1986), geochemical (Lawless et al., 1983), and stable isotopeanalyses (Robinson et al., 1987) of these waters have confirmed that they are derived from perched aquifers inthe phreatic and vadose zones.

Mixing of cool, descending, low pH sulfate and/or CO2-rich surficial fluids with hot silica-saturated deephydrothermal fluids (Fig. 2.4) results in deposition of carbonates and sulfates (in response to increasingtemperatures) and silicification (in response to cooling; Leach et al., 1986). As the hydrothermal systems areinvariably located in tectonically active areas, major fracture/fault systems may be continually reopened,permitting descent of surficial fluids to progressively deeper levels. The overall effect of this is to seal mostpermeable features and form impermeable caps in the upper levels of these hydrothermal systems (Bogie andLawless, 1987).

Acid sulfate waters have been encountered at significant depths channelled down permeable structures (e.g., upto 1500 m below the surface in the Bacon Manito geothermal field; Reyes, 1990a). The waters maintain, tosome extent, their acidity to these depths due to isolation from the deep neutral chloride fluids, possibly bybrought about by wall rock alteration and mineral deposition within the structures. The vertical zonation withinthese structures, from shallow to deeper levels of: gypsum to anhydrite, and of alunite --> alunite + kaolinite -->dickite --> pyrophyllite + diaspore in these structures (Leach et al., 1986; Reyes, 1990a), is indicative of

progressive heating and neutralization of descending acid sulfate waters.

Evidence for the incursion of CO2-rich waters into the chloride hydrothermal system is provided by theoccurrence of dolomite, siderite and ankerite at depth in some Philippine geothermal systems (Leach et al.,1983; Reyes, 1990b). In New Zealand, the identification of siderite + kaolinite at depths of up to 600-1200 m onthe margins of the Broadlands geothermal field (Hedenquist, 1990) and of Mn-carbonates at shallow levels inthe Rotokawa geothermal field (Krupp and Seward, 1987), are also indicative of the draw down of CO 2-richwaters into the chloride reservoir.

The near-neutral chloride hydrothermal fluid is depleted in magnesium (generally


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30/317


FIG. 2.5

Scales deposited from deep chloride reservoir fluids in back-pressure plates in surface pipework from theTongonan geothermal field have graded up to tens of percent of copper (mainly as tennantite), percents of leadand zinc, thousands of ppm silver, and hundreds of ppm gold (R. Harper and T. Leach; unpubl. PNOC-EDCreports; Mitchell and Leach, 1991). Up to 1 percent Cu + Pb + Zn have been deposited downstream in drainsand channels at Tongonan (Arevalo, 1986).

Drilling for geothermal energy in the Philippines has enabled the investigation of porphyry-style hydrothermalsystems at depths of greater than 3.5 km below surface, over areas of up to 20-50 km2, and at temperatures of

120A L e p a n t o / F S E = LuzonU - ^G u r n a o a n g S a n t o N i oB a g u i o D i s tr ic t ( A c u p a n , A n t a m o k )S t T o m a s II / \D a k l a n4/Mt i rS a s ,e/Didf

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2. Magmatic volatile plumeOn Biliran Island (Vulcan region) and at Alto Peak, plumes of magmatic volatiles are currently being emplacedinto pre-existing zoned skarn and contact hydrothermal alteration assemblages (Reyes et al., 1993; Lawless andGonzalez, 1982). Down hole measurements indicate that these magmatic vapours are possibly derived from a

very hot (>300-400o

C) fluid which is interpreted by the authors to have exsolved during early crystallization of ahigh level melt (below), and has locally vented directly to the surface as magmatic solfataras (e.g., Vulcan,Biliran Is.).

3. Convective hydrothermal alterationRelease of heat and fluids from the high level intrusions establishes deep circulating meteoric hydrothermalsystems into which magmatic fluids are entrained (Henley and Ellis, 1983; Hedenquist, 1987). These circulatingsystems create zoned hydrothermal alteration which grades from an inner potassic zone dominated by biotite toperipheral propylitic alteration (Henley and McNabb, 1978; Gustafson and Hunt, 1975). The Tongonangeothermal system is interpreted herein to currently occur at this stage of development. The high fluidtemperatures (>320oC; Reyes, 1990b) and salinities (>15,000 ppm Cl-; Table 2.1) suggest that a significant inputof magmatic brine from the cooling melt has been entrained into the convecting hydrothermal system. Althoughonly minor base metal mineralization has been deposited from this hot, moderately saline system (Leach andWeigel, 1984; Arevalo, 1986), significant lead-zinc mineralization has been produced by flashing fluids fromdepths of >2.5 km to near ambient conditions within surface pipework (Mitchell and Leach, 1991). It hastherefore been interpreted (Arevalo, 1986) that the circulating chloride brine at Tongonan is substantiallyundersaturated with respect to base and precious metals. However, the deposition of significant base andprecious metal mineralization can be induced under extreme artificial conditions as outlined above.

4. Draw down of surficial watersCooling of the intrusive heat source is herein interpreted to induce a pressure draw down of cool dilute meteoricwaters and shallow, moderately low pH acid sulfate and CO2-rich waters to considerable depths within thechloride reservoir (e.g., Palinpinon, Bacon Manito). This draw down of shallow-derived waters is interpreted

(e.g., Reyes, 1990b) to result in the formation of a zoned phyllic and later argillic overprints on pre-existinghydrothermal alteration. The draw down of these fluids can result in mineral deposition and subsequentprogressive sealing of permeable channels at shallow levels, and the development of an impermeable cap onthe system (Lawless et al., 1983). Although systems such as Palinpinon are dilute relative to Tongonan (Table2.1) and interpreted herein to have been drilled at a late stage in its evolution, the most significant coppermineralization intersected to date in a Philippine geothermal system is from Palinpinon.

iv) Examples of Active Intrusion-Related Hydrothermal Systems in the Philippines

a) Large disseminated systems in permeable host rocks in composite volcanic terrains

1. Young Systems Dominated by Magmatic Vapours

i) Alto Peak Geothermal System(summarised from Reyes et al., 1993)

The Alto Peak geothermal system, in northern Leyte, is hosted in a volcanic arc which extends along the easternmargin of the Philippine fault (Figs. 2.5, 2.6). Drilling within the Alto Peak geothermal system intersected aspatially restricted magmatic vapour plume sourced from a degassing high level intrusion at depth (Fig. 2.7).


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This vapour plume penetrates into a weak, circulating, moderately saline (7500 ppm Cl-) hydrothermal system,and at depth has caused a localized advanced argillic alteration assemblage to overprint on zonedpotassic-propylitic alteration.

The active hydrothermal system at Alto Peak (Fig. 2.7) is hosted in Pliocene to Recent andesite-dacite volcanicsand subvolcanic quartz diorite dikes, which pass down into a thick sequence (>2000 m) of Late Miocene toPleistocene, locally calcareous, marine sedimentary breccias, siltstones, mudstones and hyaloclastites(Binahaan Formation). Basement rocks comprise Cretaceous harzburgite and pyroxenite.

Composite volcanic centres, domes and collapse calderas are developed within dilational NW trendingsegments (Alto and Cental faults) of the Philippine fault system. Additional permeability within the volcanic-sedimentary sequence is also provided by subsidiary EW, NS, and NE trending faults. Alteration mappingindicates that earlier low temperature clay alteration has been locally overprinted by vertically zoned epidote-amphibole-biotite-pyroxene mineralogy, indicative of a later influx of considerably hotter fluids. Locally, skarnswhich formed at the contacts of calcareous sediments and high level quartz diorite dikes, display the zonation:garnet-pyroxene --> wollastonite-vesuvianite --> biotite-pyroxene-amphibole --> quartz-biotite-anhydrite +epidote.

Two wells intersected a near vertical magmatic-derived vapour-rich 'chimney', 1 km wide and 2-3 km deep,which connects a deep vapour-dominated zone at depth to a shallow zone of steam heated ground water. Gasgeochemistry, fluid isotope, and fluid inclusion data suggest that the vapour plume contains up to 40-50 percentmagmatic component, and is derived from a very hot (>400oC), saline (>17,000 ppm Cl-) fluid. It is interpretedherein that the source of this fluid and the quartz diorite dikes is a degassing recent intrusion at depth. Alterationat depth (1700-1800 m below surface) within this magmatic vapour-rich chimney is localized along fractures andconsists of quartz-pyrophyllite-alunite + diaspore-anhydrite and minor apatite, zunyite and topaz.

The vapour 'chimney' is dominated by CO2as the main gas phase. Reyes et al. (1993) interpreted that the lack

of Cl-SO4in the magmatic waters (despite the local occurrence of magmatic-derived advanced argillic alteration)indicates that either the conversion of acidic oxidizing magmatic to neutral pH fluids is now complete, or it islimited to regions of the geothermal field which have not yet been drilled. Recent drilling has in fact intersectedpermeable fracture zones which produce hot acidic, magmatic-derived acidic fluids (A.G. Reyes pers. commun.,1995).

ii) Biliran(data from: Mitchell and Leach, 1991; Lawless and Gonzalez, 1982)

The Vulcan thermal area is aligned for 3-4 km along a NE trending suture zone (Vulcan fault) which transectsthe island of Biliran, north of Leyte as a possible arc normal structure formed perpendicular to the Philippine

trench (Fig. 2.8). The presence of super-heated steam, SO2gases and HCl condensates in the Vulcan thermalarea has been interpreted (Lawless and Gonzalez, 1982) to indicate that the Vulcan fault is directly venting hotmagmatic volatiles to the surface. The Biliran system is therefore herein inferred to represent an active analogueof high sulfidation systems.

The geothermal system at Biliran is hosted in basement metamorphics which are overlain by 300 m ofcalcareous sediments and then by a 1.5-2 km thick sequence of andesitic volcanic and volcaniclastic rocks (Fig.2.9). There are no recent volcanics at the surface which might represent the extrusive rock equivalents of anintrusive source for the heat and magmatic fluids within the currently active hydrothermal system.


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Titley et al., 1978).

3. Collapsing Hydrothermal Systems

i) Southern Negros Geothermal Field(data from: Seastress, 1982; Leach and Bogie, 1982; Zaide, 1984; Reyes, 1990b; Mitchell and Leach, 1991)

Two geothermal systems occur within the Cuernos de Negros volcanic centre. These are the northernPalinpinon and the southern Baslay-Dauin fields (Fig. 2.11). The volcanic centre comprises twin peaks, dacitedomes, and parasitic cones to the south and east, and a pyroclastic plateau to the north. The most recentpyroclastic flows (14,000 years BP) are dacitic and were erupted from a vent 2 km north of the summit craters.

The Palinpinon geothermal system is hosted in a 1.5 km thick sequence of Miocene-Recent volcanics whichoverlie Eocene-Miocene calcareous sediments and early Eocene volcanics and volcaniclastics (Fig. 2.12). Alarge Miocene monzonite pluton was emplaced into the volcanic and sedimentary rocks in the western portion ofthe field. Recent porphyry stocks have been emplaced along the eastern margin of the monzonite, facilitated bymovement along a parallel set of NS trending structures which also controlled the venting of the recentpyroclastic flows. The porphyry stocks, or more likely their deeper equivalents, are the heat source for thecurrently active hydrothermal system.

Two distinct phases of hydrothermal alteration are encountered (Leach and Bogie, 1982). A relict phase ofpotassic and advanced argillic alteration occurs in the west, with propylitic alteration to the east, and there is acurrent phase of phyllic alteration in the central regions of the system (Fig. 2.12). The potassic zone grades froman inner assemblage of clinopyroxene-biotite, outward to zones of biotite-quartz and actinolite-biotite. In places,zoned hornblende-pyroxene-biotite hornfels occur at intrusion-volcanic contacts. Elsewhere, zoned skarns areintersected within calcareous sediments adjacent to porphyry intrusions. Alteration grades vertically and laterally(west to east) as zones of: potassic, epidote-chlorite, and shallow chlorite-zeolite alteration assemblages.

Advanced argillic alteration was intersected from surface to 1300-1500 m depth in the western Sogongonregion, and crops out as prominent 2-3 km long ridges aligned NS along regional structures (Fig. 2.12). Thevolcanic rocks have undergone alteration to intense silicification and zoned kaolinite, alunite, andpyrophyllite-diaspore + tourmaline mineral alteration assemblages. Trace hypogene covellite mineralization isassociated with this advanced argillic alteration. These zones of intense silicification and advanced argillicalteration are comparable to high sulfidation alteration which occurs marginal to porphyry copper deposits(Section 6.II), and are interpreted herein to have formed from magmatic vapour plumes similar to those whichare currently venting at Alto Peak and Vulcan, Biliran Is. Similar porphyry-related silicified ridges extend for 3-4km along major structures in the Amlan River area 5-6 km north of the Palinpinon geothermal system, as well asin a belt extending north along the island of Negros and into Masbate Island (Mitchell and Leach, 1991).

Phyllic alteration (illite/muscovite-quartz-pyrite + anhydrite + carbonate) occurs at shallow levels within atwo-phase zone dominated by mixed CO2-rich and acid sulfate waters, and at depth overprinting earlier potassicand propylitic alteration (Leach and Bogie, 1982; Reyes, 1990b). It is speculated by the authors that the deepphyllic overprint has been caused by the draw down of shallow waters. Elsewhere, localized cool acid sulfateand dilute meteoric recharge fluids have been encountered at depths up to 2 km, where they have migrateddown permeable structures and laterally along contacts with sills and dikes (Reyes, 1990b).

The hydrothermal system at Palinpinon exhibits all the major features encountered in southwest Pacific rimporphyry copper deposits, including early formed skarns, hornfels, and potassic alteration zones proximal to thesource intrusions, and more distal propylitic alteration zones. Advanced argillic alteration and silicification also


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Miocene age and locally containing outcropping porphyry copper deposits), and Mesozoic metamorphicbasement complexes (Mitchell and Leach, 1991).

Recently emplaced shallow level plugs and stocks, commonly along contacts of pre-existing intrusions, are

herein interpreted to represent the heat sources for hydrothermal systems. Dacite domes, diatreme breccias andassociated pyroclastic rocks form extrusive phases of these plugs and stocks. Meteoric recharge originates atelevations higher than the active hydrothermal system. The restriction of vertical permeability to structures, anddome and diatreme contacts, causes the chloride fluids to approach the surface within confined upflow features,thereby facilitating the mixing of the upwelling fluids with surficial CO2-rich and acid sulfate fluids. Distal outflowsfollow dilational structures and form dilute neutral chloride springs at lower elevations.

These active hydrothermal systems associated with volcanic arcs in cordillera settings are analogous toporphyry-related quartz-sulfide gold + copper, carbonate-base metal gold, and epithermal quartz gold-silverdeposits described herein (Section 7).

i) Amacan Geothermal Field(data from: Barnett et al., 1985; PNOC-EDC, 1985a; Mitchell and Leach, 1991)

The Amacan geothermal field in North Davao, east Mindanao (Fig. 2.5) is hosted in Mesozoic metamorphic andEarly Tertiary sedimentary rocks into which have been emplaced multiple Miocene intrusions ranging from earlyquartz diorite to later microdiorite porphyry (Fig. 2.16). The adjacent North Davao porphyry copper deposit,occurs within quartz diorite porphyry intruded into the metamorphic rocks. A number of Pliocene to Recentvolcanic plugs and domes are aligned in a NS trend parallel to regional structures formed in relation tosubduction along the Philippine trench, to the east. Lake Leonard caps a maar volcano/diatreme brecciacomplex rimmed by pyroclastic material dated at 1800 years BP (Barnett et al., 1985).

The heat source for the active hydrothermal system is interpreted to be felsic magmas which are the sources for

the domes, plugs and maar volcano pyroclastic rocks. Thermal manifestations and deeper circulatinghydrothermal fluids are restricted to the margins of domes and diatremes, with outflows extending 6-12 kmnorthward, controlled by NS regional structures associated with the Philippine fault. Hydrothermal solfatarasoccur at the contact of the Ugos dome and around the margins of the Leonard diatreme. Neutral chloride springsoccur 100 m below the Ugos steam vents. Abundant CO2-rich/bicarbonate springs in the region depositspectacular travertine deposits. The high gas content of the hydrothermal system (approx. 4 wt % CO 2; Table 3,AM-1) is indicative of either incursions to depth of the CO2-rich surficial waters, or a significant input of magmaticvolatiles into the circulating hydrothermal system.

ii) Daklan Geothermal Field(data from: PNOC-EDC, 1982; Reyes, 1990b; Mitchell and Leach, 1991)

The Daklan geothermal field, 30 km north of Baguio in the central Cordillera of Luzon (Fig. 2.5), is hosted inMiocene andesite overlying a thick (


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The Acupan gold mine, 15 km southeast of Baguio (Figs. 2.5, 2.19) exploited gold mineralization hosted in NEtrending structures which cut the 1 Ma Balatoc diatreme and associated dacite plug (Fig. 2.18). The diatremeand plug (an endogenous dome) were emplaced into a series of older Miocene to Pliocene intrusions andandesite volcanic rocks. Hot water seepages were common throughout the upper levels of the Acupan gold

mine, and steam and boiling water discharges at deeper levels. Geothermal drilling encountered only minorpermeability within the diatreme at around 1000-1200 m depth.

A comparison of down hole conditions (PNOC-EDC, 1985b) and fluid inclusion data (Sawkins et al., 1979;Cooke and Bloom, 1990) suggests that the water and steam encountered in the mine are manifestations of thedying phase of a once extensive hydrothermal system which deposited the gold mineralization (Cooke andBloom, 1990). The styles of alteration and mineralization at the Acupan mine are described in more detail inSection 7.iii on carbonate-base metal gold deposits.

v) Conclusions

1. Types of Active Porphyry Systems

Active geothermal systems in the Philippines provide insights into the process of formation of porphyry copperand porphyry-related gold deposits. Drilling to depths of over 3.5 km, over more than 20-30 squ km, enables thedevelopment of comprehensive alteration, mineralization, and fluid flow models for these active porphyrysystems.

Intrusion-related active hydrothermal systems in the Philippines are of two main types (Reyes 1990b), either:i) large diffuse systems adjacent to, or flanked by, composite andesitic volcanic complexes (e.g., Tongonan,Palinpinon, Bacon-Manito), orii) smaller systems associated with dacite or silicic andesite plugs or domes hosted in cordillera settings (e.g.,Amacan, Daklan, Acupan).

The large geothermal systems at Tongonan, Palinpinon, and Bacon-Manito, are associated with permeabilityprovided by either regional structures, or composite volcanic environments. Shallow (


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Subsequent cooling of the intrusions has been accompanied by exsolution of fluids and the formation ofmagmatic vapour plumes (e.g., Alto Peak and Biliran). Fluid flow may be controlled by the same dilationalstructures which facilitated the emplacement of the intrusion. These vapours can vent to the surface as

magmatic solfataras (e.g., Vulcan). The progressive disproportionation of magmatic SO2, and the dissociation ofHCl and H2SO4are inferred to form acidic fluids which locally produce an advanced argillic overprint on theearlier high temperature contact alteration (e.g., Alto Peak).

Convective hydrothermal systems develop when the heat provided by an intrusion at depth, and possibly also byventing magmatic fluids, results in the formation of circulating meteoric-dominated hydrothermal systems athigher crustal levels. Early convective systems are generally poorly developed and have a high magmaticcomponent (e.g., Alto Peak and Biliran), whereas later ones are better developed and more saline (e.g.,Tongonan). The convective transfer of heat from the source intrusion causes zoned alteration composed ofpotassic (dominated by biotite) alteration at deeper, hotter levels, grading to propylitic alteration (actinolite,epidote, and zeolites) at shallower and cooler levels, and finally to near surface clay alteration (e.g.,Tongonan;Reyes, 1990b).

Cooling of the source intrusion creates pressure draw down and collapse in the hydrothermal system. Thisresults in the descent of cool meteoric and relatively low pH, CO 2-rich and acid sulfate waters (e.g., Palinpinon,Tongonan, Bacon Manito) to depths up to 2 km below the surface,

Corbett and Leach 1997(LIBRO)

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