251 GEOTHERMAL ASSESSMENT OF THEFIJI ISLANDS : AN OVERVIEW Malcolm E. Cox Geothermal Institute, Geology Department, University of Auckl and ABSTRACT L I This report attempts to summarise geothermal investigations i n Fiji and come to some conclusions PACIFIC i n regard to the utilisation potential of geothermal PLATE resources i n that country. The work carried out, institutions involved and survey results are rised. Some dffferences i n philosophy o f approach and i n methods o f interpretation are apparent, but a l l studies have introduced some aspects worth considering. Useof a silica mixing model geothermometer for dilute waters provided excessively high temperature estimates. The concl sions reached i n this report are that the Savusavu area o f Vanua Levu has the greatest potential for AUS TR ALIAN utilisation, with reservoir temperature o f PLATE which could produce by a binary system. ternatively, substantial direct heat isation potential exists such as for drying or freezing plants. The next greatest potential lies in small scale direct utilisation of a fairly - extensive reservoir i n the Labasa area. Geothermal systems on Viti Levu and other localities within the Fiji group are indicated to be of low through- put with subsurface temperatures o f 90-115°C. Utilisation of such systems i s limited to very small scale use i n this environment. INTRODUCTION FIG. Location o f Fiji Islands i n S.W. Pacific. Tectonic features are shown, and zones o f The islands o f Fiji are located i n the S.W. shallow 70 km) seismicity (after Pacific between 16" and and consist of two Johnson and nar, 1972). major volcanic islands and numerous smaller islands and atolls. They a r e situated within the southern As a compari geothermal occurrences of the part of the belt of volcanism and seismicity which S.W. Pacific area can be found summarised in borders the western Pacific and which i s related to another report (Cox, major crustal structures and subduction. Fiji, however, i s within a tectonically complex, low RECORDED HISTORY AND UTILISATION seismicity section of this belt (Fig. and has a volcanic history from Eocene to Recent times, with Thefirst recorded description of thermal most activity during the Miocene-P1iocene period springs i n Fiji wasof those at Savusavu on the 1967). About 60 locations of thermal southern coast o f Vanua Levu i n about 1840 (Wilkes. springs are distributed throughout the two main 1845). During the next years most o f the islands and on five of the smaller islands of the other springs were located and described. Fiji group (Fig. 2). These springs are the only Traditionally, many of the springs had been used surface features of geothermal activity and most have for bathing, and the springs are still temperatures i n the range. Boiling springs used f o r the cooking of root crops. During the do occur, however, i n two areas on the island of 1940's small thermal baths were constructed a t Vanua Levu. This report attempts to summarise the Savusavu springs. geothermal investigations carried out in Fiji and come t o some conclusion regarding the potential I n 1951 the colonial government conducted f o r development o f the geothermal resources. 252 COX I pilot experiments into the feasibility of producing salt from seawater by evaporation using geothermal Vonuo Leru heat (Munro, 1961; 1961 ) . These experiments were so a t Savusavu, and used a 1800 1 tre main supply tank and various combinations of settling pans (2.2 and 4.8 set into the ground. The . final pilot plant produced a crude evaporite o f Levu 95.8% NaCl a t about 13.6 per day. The experi- ments were disbanded largely due to problems with temperature variations. (A sample of the salt was, 0 however, used i n the Suva gaol to bake a successful . batch o f bread). The local timber yard a t Savusavu 0 attempted some crude experiments of drying timber LOU i n an open-ended drum set into hot ground; other, Group similar experiments were made drying copra, e .. but none were further. 0 OUTLINE OF GEOTHERMAL INVESTIGATIONS I I I assessment o f Fiji eothermal FIG. 2: Locations o f thermal springs i n the Fiji potential began i n 1956 when J. Healy Geological ands. Survey) visited many o f t h e thermal springs on Viti Levu and Vanua Levu. Healy (1960) described and sampled the occurrences, measured flow rates and temperatures, related the systems to local geology 1974-75: Detailed descriptions of thermal features. and sumnarised the literature to that time. He Spring sampling and measurements. concluded that the geothermal systems i n the Geological r photo interpretation. Labasa and Savusavu areas o f Vanua Levu warranted Ground temperature surveys (1 m depth). further work and noted that the characteristics . Rechecking I.R. anomalies. of the Savusavu springs suggested an intrusive Shallow resistivity soundings and traver- neat source. ses (Savusavu only; i n conjunction with UNDP geophysicist, S. During the many o f t h e spring groups were further described by Fiji Mineral 1976 : Spring and groundwater 1 sampling. Resources Department (MRD) geologists i n geological Geophysical surveys a t Labasa-Tabia: mapping surveys. Before the Savusavu salt resistivity soundings and bipole- dipole experiments, Ibbotson (1960) carried out a shallow mapping (equipment on loan from Hawaii soil temperature survey around these springs to Institute of Geophysics and initially determine the distribution of near-surface heat. set up by HIG geophysicist, J. Kauahikaua); Some additional thermal spring sampling was done ground magnetics; S.P. traversing. during the early 1970's by H. Colley (MRD) with a 1977: Stable isotope study o f Vanua Levu and bias to mineral isation associated with hydrothermal Viti Levu springs (in conjunction with systems . J.R. Hulston, Institute of Nuclear Sciences, DSIR, N.Z.). I n 1972 black and white thermal infra- red Reinterpretation of existing airborne imagery surveys were flown over the main geothermal magnetic surveys. areas o f Vanua Levu (Savusavu and Labasa) and the Sampling o f springs i n other parts of small area of Wainunu i n S.W. Vanua Levu. The Fiji. surveys were flown by Canadian Aero Service under Preliminary resistivity survey o f Sabeto contract to MRD, and used a Bendix unit area, N.W. Viti Levu (incomplete due to (sensitivity 11.5 - 12.5 m and resolution of 2.5 equipment failure). mrad.) at flight elevations of 610 and 1220 m. The surveys were initially interpreted by the Subsequent t o the above studies, i n 1978-79, contractor (Moreton, 1973) with subsequent ground regional reconnaissance surveys were conducted by checking by MRD geologists (Colley, 1975). Results Pacific Energy and Minerals Ltd (PEM), as part of o f the surveys were later reinterpreted i n respect a work commitmentto the Fiji Governmentfor t o measurements o f surface temperatures (Cox , 1980b; options on a petroleum exploration 1icence o f f the The I.R. surveys provided 1ittle additional north coast o f Viti Levu. These included: information about the geothermal areas, though several minor, previously unrecorded seepages Interpretation of magnetic surveys f o r were located. depths t o Curie Point temperatures. Sampling o f many spring groups and From MRD personnel began detailed interpretation of chemistry. studies o f Fiji's geothermal resources, initially Reconnaissance soil mercury surveys on in the more active areas o f Savusavu and Labasa- Viti Levu and Vanua Levu. Tabia. 253 cox In 1979, K.H. Williamson (IGS, UK) visited (Cox and Hulston, which show the thermal Fiji to carry out an appraisal of geothermal waters to be isotopically similar to local surface prospects and surveys done to that time. This was water and to have nil enrichment. at the request of H. (Director, to Overseas Development Administration, UK . Regional assessment techniques were employed Will iamson visited thermal spring localities by PEM (Anderson and Austin, 1979). Using in northern Viti Levu, Labasa and Savusavu on data (apparently largely offshore Vanua Levu and collected samples for chemical and northern Viti Levu with some onshore overlap) stable isotope their contractors(Eureka Resource Associates) deduced depths to the Curie Point and constructed Following by PEM, temperature heat flow maps. (The Curie Point is that where gradient studies were conducted during 1980 and the crust has lost its magnetisation due to 1981 in northern Viti Levu, and a t Savusavu. elevated temperatures). They deduced an average Measurements were made by N. Skinner (University of thermal gradient for N. Viti Levu of the South Pacific, Suva) in holes drilled by MRD and for the Ba thermal springs From as well as existing holes. Thermal gradients had these results .they selected the Ba area of NW been previously measured in 1972 by J. Sass (U.S. Viti Levu a s a primary target for geothermal Geological Survey) in 4 shallow mineral exploration exploration. Some question, however, apparently holes in northern Viti Levu, 2 a t Emperor Gold exists in regard to the method of interpreting Mine and 2 MRD holes. the depth to Curie Point (Williamson, 1980). From reconnaissance soil Hg surveys over major RESULTS OF GEOTHERMAL SURVEYS roads (sample spacing 2-5 km with closer spacings over anomalous areas) Anderson and Viti Levu - Healy (1960) concluded that the Austin (1979) considered the Ba area as being geothermal systems on Viti Levu were of non-volcanic anomalous in respect to heat flow. They reported origin with low temperature surface discharges values of 35-1105 ppb with backgrounds of 20-70 and probably had 1imi ted subsurface ppb.