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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 mag- matic 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 Leach and Bogie 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 Leach and Bogie 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.
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