179 Overprinting of Hydrothermal Regimes I N

179

OVERPRINTING OF HYDROTHERMAL REGIMES IN THE PALIMPINON GEOTHERMAL F I E L D , SOUTHERN NEGROS, PHILIPPINES

T.M. L e a c h and I.Bogie

Kingston Reynol ds Thom 1ardice Limited (KRTA)

ABSTRACT major hydrothermal regimes are evident from the teration mineralogy in the Palimpinon Geothermal Field. A relict mineral zonation consisting potassic, advanced argill and propyl itic zones appears to have formed in response to the intrusion of a large in the western section of the field. A recent mineral zonation, that is interpreted t o have formed during the current geothermal system, is superimposed on the relict system and appears to be centered around the eastern portion of the field. The ict assemblages have many characteristics of a failed or barren porphyry copper system. The ict advanced ic mineralogy is interpreted to be of magmatic fluid origin and probably has not formed from the present geothermal regime with its Figure la: Well locations and cross section line predominant meteoric fluid component. The abundant Palimpinon Geothermal anhydrite being deposited in this geothermal system Field. Resistivity contours are shown is interpreted to have originated by redistribution in ohm-metres = 500 m) of anhydrite formed initially during the relict magmatic hydrothermal system. Most Philippine systems are similar.

INTRODUCT I0N Elevation An of the location and development of the Palimpinon field is given by Maunder et al. (1982). A general stratigraphy and subsurface geology derived from well geology is given in The youngest formation, the Cuernos Volcanics .) consists of an upper dacite unit (with a age of 14,000 years B.P.) and a lower clinopyroxene andesite. These are underlain by the Southern Negros Formation (SNF). a 1200 m thick sequence of hornblende andesites, andesite breccias and tuffs, and minor In the eastern Puhagan sector the SNF is underlain by the Okoy Sedimentary Formation (OSF) a thick sequence of calcareous tstones and sand- stones interspersed with minor volcanic breccias. This sequence is interrupted by approximately 200 m of andesites and breccias; (the lower Puhagan Volcanics) and is cut by an Cuernos Okoy andesite sill. The OSF is absent in the western formation sector where the intruded by a monzodiorite, the Nasuji Pluton The contact between the intrusive and overlying volcanics in some wells consists of a hornfels, in others a zone of microdiorites, porphyritic dikes Figure Subsurface geology o the Palimpinon and andesites. field along line A-A . 180

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HYDROTHERMAL ALTERATION for the current system.

At least two episodes o f hydrothermal activity (ii) The advanced argillic zone is disting- have been recognized. A relict hydrothermal regime uished by the presence of either diaspore, and/or which appears t o be spatially related to the N.P. ite and of kaol and/or unite. This and a second regime which i s superimposed on the zone is usually accompanied by extensive earlier assemblages and i s in approximate cation with varying abundances of anhydrite and equil ibrium with the current geothermal system. pyrite, and rare tourmaline (Agnes Reyes, pers. comn. 1982). The original mineralogy is totally obscured.

The advanced teration is most extensively developed i n SG-2 from the surface t o 1300 m, elsewhere it i s not being more abundant i n the NJ-SG sector than the Puhagan area. This zone, as well as the other relict mineral zones, does not extend into the Cuernos Volcanics, indicating that the i c t hydrothermal system predates this formation.

(iii) The phyllic zone i s characterized by the presence o f extensive illite andis accompanied by widespread icification and varying abundances of muscovite, anhydrite, pyrite and chlorite.

The phyllic type alteration is most abundant in Nasuji wells, and extends from about 1000 m above sea level to 400 m below sea level. It is interspersed with, and appears to post-date, the propyl itic teration. Figure 2: Cross section of ict teration zones i n the Palimpinon field. It i s estimated that phyllic alteration occurred a t temperatures above i n near-neutral to (a) The i c t Hydrothermal Mineral Zonation slightly acidic conditions. This type o f alteration occurs near the surface where it is The relict hydrothermal regime has formed obviously ict. However at greater depths, where teration zones very similar those associated measured temperatures exceed 230% t h e i c t nature with porphyry copper deposits (Lowell and Gilbert o f the mineralogy i s questionable. 1970, Rose and Burt 1979). is no base metal mineralisation with pyrite being the The propylitic zone i s characterized by the sole sulphide recognised t o date. appearance o f epidote, with varying abundances o f bite, chlorite, anhydrite, pyrite and calcite. Potassic, advanced argil1ic, phyll and pro- It i s encountered i n the SNF o f the Puhagan wells pylitic alteration zones can be recognised. Their and sporadical el sewhere. It i s generally distribution throughout the field is shown i n accepted that epidote occurs a t temperatures above Figure 2. These zones a r e considered relict since i n near-neutral low conditions. As the temperatures and/or the fluid chemistry with the phyllic zone, propylitic alteration is interpreted for their formation are significantly definitely i c t i n near surface conditions. different from current conditions. However, where downhol e temperatures exceed i c t nature o f the propyl itic teration i s ( i ) The potassic zone i s characterised by questionable. abundant biotite and magnetite plus minor actinolite. It extends about 200 m into the pluton (b) Recent Hydrothermal Mineral Zonation (see Fig.3) and up t o 600 m into the contact zone. It is thought that this zone has formed a t temperatures There i s some evidence from most Puhagan wells greater than about In places biotite occurs that the i c t propyl itic and phyll alterations i n a hornfels and i s o f contact metamorphic origin. i n the SNF, have been superseded i n part by a lower temperature mineralogy which i s i n approximate Some o f t h e permeability and production i n the equil ibrium with measured temperatures. The Nasuj sector appears t o be associated teration is characterized variously by montmorill- with the contact zone, except i n wells where horn- onite, interlayed montmorillonite- ill ite, chlorite, fels is encountered. It i s thought that fracturing calcite and ites (stilbite, heulandite and associated with the intrusion of the N.P. wairakite). though not well developed, this facilitated sufficiently rapid dispersion o f heat alteration appears to be widespread and extends into to locally inhibit the formation o f a hornfels, and the overlying Cuernos Volcanics. i c t propyl itic favour a hydrothermal potassic zone. Renewed alteration extends throughout the SNF i n t h e Puhagan tectonic movement i n t h e subsequently sector, but i n the underlying, inherently impermeable reopened these fractures, forming permeable channels siltstones of OSF, the alteration is confined largely 181

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t o the more permeable units of volcanics, volcanic relict and present hydrothermal regimes are breccias, volcaniclastics and sills. Fluid inclus- unrelated. If the former is postulated it is very ion temperatures i n associated veins i n the OSF are difficult to form a sufficiently hot acid fluid to close to measured downhol e temperatures, indicating produce the mineralogy observed under conditions that the propylitic mineralogy a t these depths may similar to those currently prevailing. Some possible be current. In particular the appearance o f garnet, mechanisms are: tremolite and actinolite, which may be referred to as a lower propyl itic zone, are in reasonable agreement ( i ) Mixing o f oxygenated meteoric water with geo- with measured temperatures o f up to Biotite thermal fluid resulting in oxidation o f to occurs a t the bottom o f a few wells and, a t these produce acid sulphate water. This mechanism i s temperatures, may indicate the formation o f a present severely limited by the solubility of i n water. day potassic zone. At meteoric water contains sufficient to produce 22 ppm sul phate (Truesdell 1975) as the solubil ity of in drops significantly with I increasing temperature the amount o f acid- sulphate water which can be formed by this mechanism will be even a meteoric water temperature o f 6M hence have little affect upon the

?MI- .- (ii) Oxidation of geothermal fluids above the . groundwater table to produce acid sulphate water which then infiltrates into the system. This - mechanism has been thought to occur i n a number o f fields, Well 402 a t Tongonan, Philippines. teration will tend to become - . less intense with depth due to reaction with rocks, decreased dissociation of acids with increasing - - temperature and dilution with near neutral chloride waters. There is however no decrease i n the - intensity of acid alteration in the Sogongon area f o r 1200-1400 m. Furthermore,the presence of diaspore throughout this zone would require heating of fluid to a temperature greater than (lower stability range o f diaspore Hemley e t To Figure 3: Cross section (A-A1 ) o f current obtain this temperature would significant heat extraction by rock interaction or teration zones mixing with a hotter fluid, both which will increase the Therefore a low and high temperature In the Nasuji-Sogongon area, the relict potassic zone, i n many places, i s replaced by a are mutually exclusive. lower temperature phyll o r propyl itic teration. I n deeper parts of the pluton the monzodiorite Gas separation due to boiling with subsequent accumulation and condensation below an impermeable exhibits propyl itic teration, the distribution of which i s controlled by deep-seated fractures which feature to produce an overpressured gas pocket i n provide production from within the intrusion. which and are significantly concentrated uid inclusion measurements a r e consistent with with respect to water so as to form an acidic fluid. these phyll ic and propyl itic alteration assemblages There i s insufficient information to fully evaluate being formed from present day fluids. this process, however, gas kicks occur during geothermal drilling,indicating overpressured gas The i c t advanced argil1 zone i s impermeable, pockets. To what extent these gases go into solution Any acid due to extensive icification and clay deposition. t o produce acidic fluids is unknown. It i s important to note that diaspore and/or teration associated with them however would be quite local ised whereas the observed acid teration pyrophyllite are not commonly found with kaolinite and/or alunite. Since the former assemblage is widespread. requires acidic conditions above about and All three of these mechanisms have distinct the latter acid conditions below (Hemley 1imitations on providing an aggressive enough fluid et 1980) the kaol unite assemblage --. with which to explain the nature and distribution appears t o be retrograde. ternatively since the of the advanced teration. To do so temperature o f formation i s within present day conditions the kaol unite association could produce a low high temperature fluid) it is proposed that there has been a magmatic input. be current. The absence o f present day acid conditions a t inconsistent with As there is no volcanic activity in Southern Negros and as the limited amount o f unpublished isotopic this. data available show no obvious magmatic component i n the water o f the current system, any magmatic INTERPRETATION OF THE ALTERATION PATTERN fluids are probably unrelated to the present system. Such fluids could have been derived from the degass- Either the i c t assemblages represent an ing of the Nasuji Pluton. Initially such fluids earlier, hotter phase o f the present system with would form a convective system within the fractured the presence o f acidic fluids at depth o r the 182

Leach and Bogie margin of the intrusion to form the potassic zone, solubility of anhydrite. This would cause the acidity of the fluid being controlled by the precipitation of anhydrite in the outer parts of high temperature which will limit the hydrolysis of the system which is not observed in the Palimpinon and the dissociation of any H produced field. (Burnham 1979). This fluid may outward along It is more likely that the anhydrite fractures in the country rock and, with cooling, is being redistributed. As the deep recharge water become progressively more acid as there will be for the system will be initially undersaturated increased hydrolysis of SO and dissociation of with respect to anhydrite (Truesdell 1975) and to form the advance3 argillic zone. providing that permeability is created by Neutral ized fluid moving out from the advanced fracturing postdating the formation of the ict argillic zone will continue to react with the assemblages, this recharge water will dissolve the rock to form the zones. The propyl itic anhydrite. Providing the fluid comes close teration and possibly some of the phyll to saturation with anhydrite, precipitation else- alteration may be due to an associated meteoric where at lower temperature can be achieved by convective system (Henley and McNabb 1980). The boiling. As is abundant in solution apparent lack of base-metal mineral isation could 80 ppm in Okoy wells) it is the concentration possibly be due to the premature collapse of this of which is critical. Although the increase magmatic system with the influx of meteoric fluids. in ionic strength of the solution may result in The Nasuji pluton can therefore be considered to be increa ed sulphate complex formation, the formation a barren Cu-porphyry. uid inclusion evidence of SO4 due to the increase in with from 1971) indicates that the gas separation, may sufficiently offset this to temperatures of format ion of the potassic zone result in anhydrite precipitation. The mechanism may be as high as this is correct the can operate where there is a pressure drop, as may temperature of formation of the ict minerals, be produced by increased permeability along a flow in particular those of the potassic zone, will be channel. In this instance anhydrite will be limited considerably higher than similar minerals forming to high temperature permeable zones. Scal ing in under the current regime. well was found to be layered calcite- anhydrite, indicating anhydrite has formed upon boiling. This The Philippines is a typical Cu-porphyry however does not imply that anhydrite is a reliable environment with at least 10 deposits being permeability indicator as it may be relict or have currently mined. The nearest to the Palimpinon blocked off the permeability by its deposition. geothermal field are at Siaton 20 km to W and at Sipalay 50 km to the NW (Motegi 1975). Anhydrite is also common inother Philippine geothermal fields, in particular Tongonan and Anhydrite Bacon-Manito which so have intrusives with associated ict high temperature potassic and A further aspect o f applying a Cu-porphyry advanced ic alteration. model t o account f o r the relict alteration is that it also explains the widespread occurrence o f CONCLUSION anhydrite i n the deeper (up t o 3 km) hotter parts o f the field. The Palimpinon geothermal field has a complex alteration history much o f which i s unrelated to Anhydrite is a common mineral of Cu-porphyries current conditions. Initial teration can be (Norton 1972) where reacting hydrolyses interpreted i n a Cu-porphyry type model where with decreasing temperature t o form and alteration i s in response to the intrusion o f the which reacts with primary plagioclase N.P. Whilst the N.P. may contribute some heat to the system the major upflow and hence the present + heat source o f the system does not appear t o be associated with the N.P. The current geothermal (anhydrite) (anorthite) system has produced an teration overprinting some o f which i s distinguishable from the initial 2 A10 + 2 + (OH) teration. Petrological interpretations i n this (diaspore) (quartz) situation must be made with extreme care, i n particular differentiating between current and ict to produce anhydrite, quartz and a high temperature teration. acid alteration mineral , in this case diaspore. ACKNOWLEDGEMENTS Whil anhydrite is an abundant mineral within the relict advanced argillic zone it i s also found The authors acknowledge the use o f many in the other zones i n the deeper hotter parts of unpublished reports by KRTA and the Philippine the field where fluid inclusion data have shown its National 0 1 Energy Development Corporation deposition to be associated with the current system. (PNOC-EDC) d u r i n g t h e writing of this paper. In addition gratitude i s given for many fruitful Since the OSF i s relatively impermeable it seems discussions with the members o f t h e staff of both unlikely that much of the sulphate within the system these organizations, i n particular Frank Studt and could be derived by leaching of sedimentary rocks. Brian Maunder o f KRTA a n d A g n e s R e y e s o f PNOC-EDC. To produce sulphate by oxidation o f which then This work forms part o f a technical co-operation percolates downwards, is restricted by the inverse between the New Zealand and Philippines governments, the executing agency for which i s KRTA. 183

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REFERENCES

Burnham, C.S. (1979). , Magmas and Hydrothermal Motegi, M., Mineral isation of the fluids, Geochemistry o f Hydrothermal Ore Philippines (A Review) Journal Deposits Chap. 3 pp. o f the Geological Society o f the Philippines, . 29 20-44. Hemley, J.J., J.W., Marinenko, J.W. and R.W. (1980). Equilibria in the System Norton, D., Concepts Relating Anhydrite A120 and Some General ications Deposition t o Solution Flow i n Hydrothermal for isation Processes. Systems. 24th International Geological Economic Geology Vol. 75 pp Conference. Section 10, 237-244.

Henley, R.W. and McNabb, A. Magmatic Roedder, E. Fluid inclusion studies on Vapor umes and Groundwater Interaction the porphyry-type ore deposits a t Bingham, i n Porphyry Copper Emplacement. Economic Utah; Butte, Montana; and imax, Colorado: 73 Econ. Geol. 66, 98-120. Lovelock, B.G., Cope, and Rose, A.W. and Burt, D.M. Hydrothermal A Model o f the Tongonan Field. teration, Chap. 5, Geochemistry of Proceedings o f 1982 Pacific Geothermal Hydrothermal Ore Deposits, 2nd Ed. Conference. Truesdell, A.H. Summary o f Section Lowell, J.D. and Guilbert, J., Lateral and Vertical Geochemical Techniques i n Exploration. 2nd teration Mineral isation zoning i n Porphyry U.N. on the Development and Use of Ore Deposits. Economic Geology. 65 Geothermal Resource. Vol. 1, pg pp 373-408. Maunder, e t . (1982). , The Pal impinon Geothermal Resource, Negros, Philippines: A Case History. Proceedings of 1982 Pacific Geothermal Conference,