Proceedingsof New Zealand Geothermal Workshop

SPATIALANDTEMPORALRELATIONSHIPSBETWEENHYDROTHERMALALTERATIONASSEMBLAGESAND INTRUSIONSAT

FOR PORPHYRYANDEPITHERMALOREDEPOSITSTHE GEOTHERMALFIELD,PHILIPPINES- IMPLICATIONS

A.J. D.R. D. C. C. D.

'Centre for Ore Deposit Research (CODES), University of Tasmania, AustraliaNoble GasLaboratory, School of Earth Sciences, University of Melbourne, Australia

Exploration and Mining, AustraliaDivision, Corporation,Philippines

Summary - Biotite, calc-silicate, propylitic, advanced argillic and illite alteration assemblages atare genetically associated with the Nasuji and Puhagan intrusions. Age of

hydrothermal biotite = 0.7-0.6 Ma) and alunite = 0.9-0.8 Ma) demonstrate that biotiteand hypogene advanced argillic assemblages formed contemporaneous with the Nasuji Pluton= 0.7-0.3 Ma). The 'blind' intrusion at Puhagan provides the heat source for present geothermal activity.

alterationassemblages(biotite, calc-silicate) have developed above this intrusionwith a halo of propylitic alteration. At an illite-bearing alteration assemblage is in thermalequilibrium with the present geotheml system. The intimate spatial and temporal relationship betweenintrusion emplacement and the styles of alteration at Palinpinon are characteristicof deposit styles such as

porphyry, HS and LS epithermal. However, mineralized zones have not been detected atPalinpinon, possibly due to poor fiacture permeability, insufficient fluid flux,or a lack of metals in thehydrothermal fluids.

1. INTRODUCTION

Palinpinon geothermal field, Southem Negros,Philippines, is a high temperature, liquid-dominated geothermal system in an active volcanicarc. Previous workers (Leach and Bogie, 1982;Celenk et al., 1987; Reyes, 1990; Mitchell andLeach, 1991; Corbett and Leach, 1998) have-drawn parallels between the types of alterationrecognised at Palinpinon and magmatic-hydrothermal ore deposits (specifically, porphyryCu deposits).

Hydrothermal alteration mineral assemblagesassociated with active and extinct hydrothermalsystems can provide important insights into theirfluid chemistry and temperature. The zonaldistribution pattern and overprinting relationshipsof alteration assemblages indicate how fluidchemistry has evolved both spatially andtemporally and can be used to understand geneticrelationships between mineral deposits within amineral district. Genetic relationshipsbetween anytwo of the four main styles of hydrothermal oredeposits related to arc-magmatism porphyryCu-Au, high sulfidation {HS) and low sulfidation{LS) Au, have beendemonstrated 1995) or inferred byprevious workers Sillitoe, 1989; Cooke andBloom, and such relationships have obviousimplications for mineral exploration throughoutthe circum-Pacific. However, in order tounderstand the spatial and temporal associationsbetween all four environments it is necessary toinvestigate their mineralogy, fluid chemistry andtimingwithin a single district.

This paper describes the spatial and temporaldistribution of hydrothermal alteration types atPalinpinon (biotite, calc-silicate, propylitic, illiteand advanced together with the associatedminor base and precious metal mineraloccurrences. Techniques employed include thin

section petrography, spectroscopy (PIMA,FTIR), X-ray fluorescence analyses,scanning electron microscopy (SEM), fluidinclusion microthermometry, proton-induced X-ray emission analyses andK-Ar andAr-Argeochronology.

2. GEOLOGY HYDROLOGY

The Palinpinon geothermal field is hosted byseveral volcano-sedimentary sequences (Figure 1).The lowermost drilled unit is the andesiticvolcaniclastic Lower Puhagan Volcanic Formation(Middle Miocene). The Okoy Formation (UpperMiocene-Early Pliocene) is a marine sequence ofcalcareous siltstones and sandstones. Theuppermost formations are the Southern Negros(Late Pliocene-Pleistocene) and Cuernos Volcanic

(Pleistocene-Recent) which bothconsist of volcanic and volcaniclastic rocks ofbasaltic andesite to andesitic composition. Dioriteto quartz diorite intrusions were emplaced intothese successions during Early Pliocene to Recenttimes. The Puhagan dikes = Ma)and the Nasuji Pluton = 0.7-0.3 Ma)intruded the Middle Miocene, Late Miocene andEarly-Late Pliocene rock formations (Rae et al.,submitted).

-2000

Figure 1.Geological cross-section(SW-NE)through Palinpinon geothermal field with the distribution ofisotherms.

1500

1000

PUHAGAN

AlterationTypes

Advanced argillic

Advanced argillic (Hypo)

Figure 2. Cross-section (SW-NE), showing the distributionof alteration types relative to the geology.Thin lines represent the geological contacts shown in Figure 1.Alteration type abbreviations: =

chlorite; epi= epidote; tre-act= tremolite-actinolite;Hypo=hypogene; = steam-heated.

Permeable zones related to faults and lithologicalcontacts are the dominant influence on the moderngeothermal hydrology at Palinpinon (Rae, 2002).The upflow zone is situated at a intersection(Ticala and Faults) and the mainnortheasterly outflow zone is parallel toNE-striking faults Ticala Fault) beneath theOkoy River (Figure 1). The reservoir currentlyconsists of neutral to slightly alkaline chloridewaters that have been affected by boiling, mixingand conductive cooling. Boiling occurs close to theregion of upflow and is mainly restricted to narrowpermeable zones. Mixing with steam-heated

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sulfate and meteoric waters occurs peripherally tothe upflow zone.

3. ALTERATION

3.1. Biotite Alteration

Biotite is the diagnostic mineral of thisassemblage, which occurs with magnetite, quartz,anhydrite, orthoclase, albite, tremolite-actinolite,hematite, pyrite chalcopyrite. Zones ofbiotite alteration occur in the upper regions of theNasuji Pluton, and in the Puhagan sector in theupflow zone beneath the Puhagan dikes (Figure 2).

3.2. C Alteration

This alteration type is defined by the presence ofgarnet, clinopyroxene andor scapolite. Otherminerals that can occur include tremolite-actinolite, epidote, wairakite, prehnite and albite.The most intense development of calc-silicatealteration occurs in the deepest parts of thegeothermal field in the upflow zone beneath thePuhagan dikes (Figure 2). In Nasuji-Sogongon,calc-silicate alteration occurs adjacent to theNasuji Pluton.

3.3. Propylitic Alteration

Propylitic alteration assemblages are recognised bythe presence of tremolite-actinolite, epidote andchlorite. These minerals occur with albite,orthoclase, quartz, pyrite chalcopyrite bornite

titanite leucoxene. This assemblage has threesubzones. The tremolite-actinolite subzone occursin the deepest parts of the geothermal field (Figure2). In Puhagan, it surrounds biotite and calc-silicate alteration zones. In Nasuji-Sogongon, ithas affected rocks in the deepest drilled portions ofthe Nasuji Pluton. The epidote subzone occursabove the tremolite-actinolite subzone (Figure 2)and propylitic (epidote)-altered rocks crop out inNasuji-Sogongon. The chlorite subzone occursabove the epidote subzone and in the outflow zone(Figure2).

3.4. IlliteAlteration

The occurrence of either illite or smectite definesthe illite alteration assemblage. Other minerals that-can be present include quartz, leucoxene,anhydrite, pyrite, chalcopyrite, sphaleriteand galena. This is the most extensively developedalteration type and occurs across the field theSurface to depths greater than 500 m below sealevel Figure 2).

3.5. Advanced Alteration

A hypogene subzone is characterised by tabularalunite, zunyite, quartz, aluminium-phosphate-

sulfate (APS) minerals, pyrophyllite, diaspore,andalusite pyrite marcasite

covellite. A steam-heated subzone is recognizedby quartz, or amorphous silica, kaolinite,pseudocubic alunite, minerals, native sulfur

diaspore pyrophyllite pyritemarcasite. Advanced argillic altered rocks crop out -

at the surface as a series of northeast-trendingridges along the Okoy Valley. The southwesternridges are altered to the hypogene assemblage, andthe northeastern ridges have a steam-heatedassemblage. Below surface in athick m) zone of advanced argillicalteration extends fkom the surface toapproximately the top of the Nasuji Pluton (Figure2). In Puhagan a zone of advanced argillicalteration extends to approximately 1000 m bsl

(Figure 2). There are sporadic ofandalusite, but most of the subsurface advancedargillic alteration consists of alunite, pyrophyllite,diaspore and

4. RADIOGENIC DATING

Two samples of hydrothermal biotite and alunitewere radiogenically dated by the PreciseRadiogenic Isotope Services group,

National University Canberra.The samples are the biotite and hypogeneadvanced argillic alteration zones spatiallyassociated with the Nasuji Pluton. Duplicateanalyses of the alunite were within the calculatedanalytical uncertainties, with an average age of0.85 0.02 Ma. The results of the biotite analysisyielded a flat spectrum with a plateau age of0.01Ma.

5. FLUID INCLUSIONS

5.1. Fluid InclusionTypes and Populations

Only primary and secondary fluid inclusions wereidentified in the sample suite (Roedder, 1984).Based on observations made at room temperature,six fluid inclusion types are recognised (Table 1).

to the associated types, the fluidinclusions are grouped into three populations(Table 2). Population A occurs in quartz veins,associated with biotite alteration and primaryquartz from the Nasuji Pluton. Population B inquartz veins associated with illite and propylitic(epidote + tremolite-actinolite) alterationassemblages. Population C occurs in quartz veinsassociated with illite and propylitic (chlorite)alteration zones.

Fluid InclusionsType Subtypelalb23a 3al

3ad3b 3bl

3bd4

Table 1.Fluid inclusion types with behaviour upon homogenisation.Phases(at 25°C) Homogenisation

Number Dominant Types Behaviour2 liquid liquid+vapor vapordisappears

liquid liquid+vapor+ opaque unknown) vapordisappears2 vapor vapor+ liquid noresponseonheating3 liquid liquid+vapor+halite vapordisappears3 liquid liquid+vapor halite salt dissolution3-5 liquid liquid+vapor+halite+ opaque unknown) vapordisappears3-6 liquid liquid+vapor halite+ opaque unknown) salt dissolution4-7 daughter liquid+vapor+halite+ sylvite+ opaque salt dissolution

minerals unknown)

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B

C

Table 2. Fluid inclusionpopulations with associated vein alteration and inclusiontypes andthe range of homogenisation temperatures and salinities Population Ainclusions also occur as secondaryfluid inclusions in primary q in the Nasuji Pluton.Inclusion VeinMineralogy AlterationType Inclusion Homog. Temp. NaClPopulation Types

A anh bio mt biotitela+ :11.8 to 19.9)

anh illite,wai py propylitic(epidote+ga tremolite-actinolite)

anh cc wai illite, . l a 189" to 345°C 0.0 to 4.5

propylitic(chlorite)

3a+3b+2+ 267' to 34.3 to 61.6

la+2 214" to 356°C 0.0 to 10.5:3.8)

Abbreviations: adu=adularia;anh=anhydrite;bio=biotite; cc=calcite; =chalcopyrite; epidote; gagalena; mt=magnetite;py=pyrite; qtz=quartz; = sphalerite; = tremolite-actinolite;Homog. Temp.=homogenisationtemperature.

1240 480

Figure 3. Histogramshomogenisation temperature (A) and calculatedsalinities NaCl, type 4 fluid inclusionsare NaCl

5.2. Microthermometry AnalyticalMethods

Fluid inclusion heating and freezing measurementson 239 inclusions were made on quartz in veins

zones of illite, propylitic, and biotitealteration. included were primary quartz fromporphyritic and Nasuji Plutonhornblende quartz diorite. Measurements werecarried out on USGS-type and

stages. The precision of measuredtemperatures is for heating and for

5.3. Microthermometric

Homogenisation temperatures for population Ainclusions range 267°C to above 600°C(Figure 3A). Type 3 and 4 inclusions have saltdissolution temperatures that correspond tosalinities between 34.3 to 61.6 equivalent weightpercent NaCl and 11.8 to 19.9

(Figure 3B). Population B inclusions have

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homogenisation temperatures between 214" and356°C(Figure 3A) and apparent salinitiesbetween0.0 and 10.5 NaCl (Figure 3B). However,observed first melting temperatures indicate thatsome inclusions containsolutions. In such cases, the observed meltingsequence indicates solutions with 1.6NaCl and 3.8 compositions.Population C inclusions have homogenisationtemperatures between 189" and 345°C andsalinities between 0.0 to 4.5 NaCl(Figures3A 3B).

5.4. PIXEResults

Fluid inclusions from one sample were analysedby using the CSIRO-GEMOC NuclearMicroprobe, Sydney. Those analysed were fivepopulationA inclusions (four type 3b; one type 2)in primary fiom the Nasuji Pluton. Thesehave appreciable metal contents. The type 3binclusions have average concentrations of 6.0weight percent Fe, 0.7 0.3Pb and 930 ppm Cu. However there is a widerange of Cu concentrations, with one inclusioncontaining as much as 0.2 Cu. The type 2inclusion also contains very high concentrationsofFe (3.4 (0.22 and Cu (654 ppm).

6. DISCUSSIONAND CONCLUSIONS

6.1. Evolution of the Palinpinon HydrothermalSystem

The evolution of hydrothermal activity atPalinpinon, conspicuous- by the variety of itsalteration types is interpreted to have been drivenby two intrusive events: the Nasuji Pluton, and aninferred 'blind' intrusion below drilled depths inthe Puhagan area.

The most extensive areas of biotite and hypogeneadvanced argillic alteration are in the Nasuji-Sogongon region, associated with the intrusion ofthe Nasuji Pluton. Also present in this region arecalc-silicate, propylitic, illite and steam-heatedadvanced argillic alteration assemblages.dating of primary hornblende from the NasujiPluton has established its emplacement between0.7-0.3 Ma (Rae et al., This age is

close to the formation of hydrothermalbiotite0.6 Ma) and alunite (0.9-0.8 Ma) from proximalzones of biotite and hypogene advanced argillicassemblages. This implies that a hydrothermalalteration imprint was established during intrusionof the Nasuji Pluton. However, the older advancedargillic alteration is probably the better estimatefor the of the intrusion emplacement andthe establishment of the associated magmatic-

system with the deeper samplestaking longer to cool below their argon closuretemperatures.

Based on the fluid inclusion microthermometry byRae et al. (submitted), during the latter stages ofmagma crystallisation, a two phase, aqueous salinefluid from the siliceous melt underlithostatic pressures at temperatures in excess of600°C and minimum depths of 2-2.5 This twophase fluid consisted of a low density vapor and ahigh density saline (60-80 liquid.Under lithostatic pressures and ductiledeformation, these aqueous fluids escaped into thesurrounding rocks to form high temperature

biotite alteration assemblages atapproximately 1.5 depth (Rae, 2002).

With cooling to approximately there was achange in pressure conditions due to hydrostaticpressures lithostatic load. This causedthe vestiges of liquid magma to quench and formthe porphyritic quartz diorite of the Nasuji Pluton,but also and brecciation ofsolidified parts of the host intrusion and proximalcountry rocks. causedpermeability to develop breccia zones and quartzveins, both associated the biotite alterationassemblages. analyses show that these fluidswere endowed with base metals up to 0.2

Cu), but the lack of sulfide accumulations atPalinpinon implies either low fluid volumes andorlack of an effective trap for sulfide deposition.

Coeval with the biotite alteration assemblage, azone of hypogene advanced argillic alterationformed at shallow levels above the Nasuji Pluton.Magmatic volatiles escaped fiomthe magma, and ascended toshallower levels along near-vertical zones ofpermeability (faults joints). Near the surfacethese gases mixed with groundwaters, to formacidic 2; Hedenquist, 1986)that developed the hypogene advanced argillicalteration assemblages. The depth of formation ofthis alteration zone remains unknown, however the

of high temperature mineralsassociated with propylitic (epidote) and advanced

alteration assemblages in surfaceoutcrops implies at least 450 m of erosion (Reyes,1990) over the past 0.9-0.8 Ma.

As the Nasuji Pluton cooled the proportion of fluidsourced from the intrusion was reduced andhydrothermal circulation became dominated bydilute, meteoric-derivedgroundwaters (Rae, 2002).

These moved into the pluton where they wereheated and caused propylitic (epidote + tremolite-actinolite) alteration of the intrusive rocks,overprinting earlier-formed biotite alterationassemblages. At shallower levels, hydrothermalcirculation of meteoric groundwaters probablyresulted in the formation of illite and propylitic(chlorite) alterationmineral assemblages in Nasuji-Sogongon and extending eastwards into Puhagan.In regions above the paleo-water table, gasdissolved into perched aquifers and may havecaused the foxmation of steam-heated advancedargillic alteration assemblages.

The erosion of at least 450 m of rock in the Nasuji-Sogongon area sometime within the last 0.8 Ma,exposed the Nasuji-Sogongon zone of 'HS-style'advanced argillic alteration as a series ofnortheasterly striking alunite ridges along theOkoy Valley. During this period, an wasemplaced beneath the Puhagan dikes kmdepth) at the intersection of the Ticala and

Faults. This shifted the focus ofhypogene alteration (calc-silicate and biotite)eastwards and caused the development of thehydrothermal convection cell that defines thepresent-day geothermal system. Since the calc-silicate assemblage formed, this region has cooledby at least while parts of the biotitealteration assemblage have remained inequilibriumwith present-day fluids.

With convection, zones of propylitic alterationdeveloped as halos to the Puhagan hypogene, calc-silicate and biotite alteration assemblages. Thesepropylitic zones are in thermal equilibrium withthe present-day system. The tremolite-actinoliteassemblage is restricted to the upflow zone,proximal to the hypogene alteration assemblages.Away from this center, the subzones vary fromepidote to chlorite assemblages. Such a systematicvariation away fiom a core of biotite alteration hasbeen recognised in the Tintic Cu system

et al., 1991).

Above the zones of propylitic alteration, illitealteration assemblages overprint biotite, propyliticand advanced argillic alteration zones associatedwith the Nasuji-Sogongon hydrothermal system.Dissolved gases that separated fromboiling zones, and redissolved into perchedaquifers, have formed steam-heated acid sulfatewaters with associated advanced alterationzones. Descent of acid sulfate waters down near-vertical permeable structures faults), results insteeply dipping zones of advanced argillicalteration in the Puhagan area).

6.2. Implicationsfor Mineral Exploration

The hydrothermal alteration assemblages atPalinpinon are interpreted to record a long-lived(0.8 0.1 Ma) hydrothermal system that evolvedwith respect to: temperature (600" to 300°C); fluidsources (dominantly magmatic to dominantly

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meteoric); acidity (acid to near neutral); andpressures kb lithostatic to

0.1-0.2 kb hydrostatic). There is an intimate spatialand temporal relationship between the intrusionemplacement and the styles of alteration that arecharacteristic of deposit styles such as porphyry,

HS and LS epithermal. This work showsthat all four alteration system can occur in asingle mineral district and may form as aprotracted, evolving hydrothermal system with LSepithermal assemblage postdating the hypogeneporphyry-HS epithermal alteration assemblages.

Despite the parallels that can be drawn betweenPalinpinon and mineralized porphyry and

ore deposits, assay results have failed todetect any significant mineralized zones of copper,zinc, lead, gold and silver at Palinpinon (Rae et al.,submitted). It may be that wells drilled to datehave to intersect ore zones, or that suchzones have since eroded. Until any evidence to thecontrary is obtained is interpreted to bea barren hydrothermal system. Reasons for couldbe: (1) poor permeability necessary to focus fluidflow and ore deposition; (2) insufficient fluid fluxto accumulate large amounts of baseprecious metals; andor (3) a lack of enoughmetals. Important features that characteriseporphyry, HS and LS deposits are apparentlyabsent at Palinpinon. These include, high densityquartz stockwork veining (porphyry), a core zoneof vuggy silica epithermal) and vein swarms,massive veins or mineralized brecciacomplexes (LS epithermal). All of these featuresintimate that high grade ores at these deposit typeshave been promoted large volumes of fluid andhighly permeable The absence of thesefeatures at Palinpinon indicates that permeability

fluid flux have been insufficient and that asurge in magmatic activity, faulting, orcatastrophic decompression and breccia formationare to promote ore deposition.

7. ACKNOWLEDGEMENTS

This studywas done aspart of the senior authors'sdissertation at University of Tasmania, for

which the receipt of a Tasmanian Researchis acknowledged.

EDC, North Ltd. MineralsMining Ltd.and

Mining provided additional and support.

8. REFERENCES

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(Ed).. Mineralogical of Canada,Short Course Series. 23. pp. 419-454.

Celenk, H., Flores, and Dela A.P.,1987. Geochemical characterisation ofepithermal alteration in southeast Negros,Philippines. Journal GeochemicalExploration 27, 189-211.

Cooke, D.R. and Bloom, M.S., 1990. Epithermaland subjacent porphyry mineralization,Acupan, district, a fluidinclusion and paragenetic study. JournalGeochemicalExploration 35,297-340.

Corbett, G.J. and Leach, T.M.,1998. SouthwestPacific Rim gold-copper systems: structure,alteration and mineralization. Society ofEconomic Geologists Special Publication 6,Kansas City, pp.237.

Leach, T.M. and Bogie, I., 1982. ofhydrothermal regimes in the Palinpinongeothermal field, Southern Negros,Philippines. 4th New Zealand GeothermalWorkshop,Auckland, New Zealand, 179-183.

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Sillitoe, R.H., 1989. Gold deposits in westernPacific island arcs; the magmatic connection.Economic GeologyMonograph 6,266-283.

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