CONTENTS TABLES ………………………………………………………………………… FIGURES ……………………………………………………………………….. ABSTRACT ……………………………………………………………………… ABSTRAK .………………………………………………………………………
iv v ix xii
I.
INTRODUCTION ……………………………………………………….. 1.1. Background ……………………………………………………… 1.2. Objectives …………………………………………………………. 1.3. The Study Area ……………………………………………………. 1.4. Previous Works ..………………………………………………… 1.5. Order of Presentation ……………………………………………... 1.5.1. Dissertation outline ………………………………………… 1.5.2. Nomenclatures and symbols ……………………………….
1 1 4 5 6 9 9 11
II.
REVIEW OF EPITHERMAL DEPOSIT ………………………………. 2.1. Definition …………………………………………………............. 2.2. Hydrothermal System in the Epithermal Environment ………….. 2.3. Classifications and Sulphide Assemblages ………………………. 2.4. Ore Body and Alteration Zones ….…..………………………….. 2.5. Deposit Form, Mineralization, Texture ..………….……………… 2.6. Tectonic Setting, Host Rock and Age ……………………………. 2.7. The Epithermal Base Metal Deposit ……...………………………. 2.8. Hypotheses ………………………………………………………...
13 13 14 15 17 19 21 22 25
III.
METHODS ……………………………………………………………… 3.1. Desk Study ………………………………………………………… 3.2. Field Works ……………………………………………………… 3.3. Laboratory Works ……………………………………………….. 3.3.1. Petrography and ore microscopy ………………………….. 3.3.2. XRD …………………………….………………………….. 3.3.3. XRF ……………………………………..………………….. 3.3.4. ICP-MS ……………………………………………….…….. 3.3.5. SEM-EDX ………………………………............................ 3.3.6. Fluid inclusion microthermometry ……………………….. 3.4. Data Processing and Interpretation ……………………………….. 3.4.1. Geology and hydrothermal alteration maps ………………. 3.4.2. Mineralogical and geochemical data ……………………… 3.4.3. Fluid inclusion microthermometric data …………………..
28 28 29 31 31 32 33 35 36 38 39 39 40 43
IV.
GEOLOGY ……………………………………..………………………. 4.1. Regional Geology ……………………………………………………. 4.1.1. Geology of Sulawesi ………………………………………. 4.1.2. Geology of South Sulawesi ................................................ 4.1.3. Volcanism and magmatism of South Sulawesi …………… 4.1.4. Regional geology of Baturappe area ………………………
45 45 45 49 50 52
i
4.2. Geology of the Study Area …………………………………………. 4.2.1. Geomorphology …………………………………………… 4.2.2. Lithology …………………………………………………… 4.2.2.1. Basaltic-andesitic lava ……………………………. 4.2.2.2. Gabbroic-dioritic stock …………………………… 4.2.2.3. Basaltic-andesitic dykes ………………………….. 4.2.3. Geological structure ……………………………………….. 4.2.4. Lithological and structural controls of the mineralizations . 4.2.5. Volcanic facies .…………………………………………….
54 55 58 59 64 66 69 74 78
V.
PETROLOGY AND GEOCHEMISTRY ……………………………… 5.1. Classification of Less-Altered Rocks ..…………………………… 5.2. Classification of Altered Rocks ………………………………….. 5.3. Determination of Magmatic Affinity ……………………………… 5.4. Volcano-Tectonic Setting and Magmatic Evolution ………………
80 82 90 95 96
VI.
HYDROTHERMAL ALTERATION ………………………………….. 6.1. Hydrothermal Alteration Zoning ……………………………………. 6.1.1. Chlorite zone ……………………………………………….. 6.1.2. Epidote-chlorite-calcite zone ……………………………… 6.1.3. Illite-quartz zone ………………………………………….. 6.1.4. Quartz-carbonate zone …………………………………….. 6.2. Relationships between Alteration Mineralogy and Lithogeochemistry 6.3. Mass Balance ………………………………………………………
106 107 112 114 119 122 127 134
VII.
MINERALIZATION …………………………………………………… 7.1. Bincanai Vein ……………………………………………………... 7.2. Baturappe Vein Zone ……………………………………………... 7.2.1. Baturappe vein-1 …………………………………….......... 7.2.2. Baturappe vein-2 ……………………………………......... 7.2.3. Bungolo vein ……………………………………………….. 7.2.4. Paranglambere vein ………………………………………... 7.3. Bangkowa Vein and Stringer Zone ………………………………. 7.3.1. Bangkowa vein …………………………………………….. 7.3.2. Bangkowa stringer …………………………………………. 7.4. Ritapayung Dissemination ……..………………………………….
141 142 153 153 158 162 165 169 169 172 174
VIII.
FLUID INCLUSION STUDY …………………………………………. 8.1. Fluid Inclusion Microscopy ………………………………………. 8.2. Microthermometry Results ……………………………………….. 8.3. Formation Temperature and Salinity of the Hydrothermal Fluid .. 8.4. Deposit Type ……………………………………………………… 8.5. Fluid Source and Precipitation Mechanism ……………………… 8.6. Formation Depth ………………………………………………….. 8.7. Pressure ……………………………………………………………. 8.8. Sulphur Fugacity …………………………………………………..
180 181 183 184 189 190 193 195 197
ii
CONCLUDING DISCUSSION …………………………………………
200
REFERENCES ………………………………………………………………….
224
APPENDICES : Appendix 1. Field data ………………………………………………………… Appendix 2. Orientation of dyke, fracture/fault, and vein ………………….. Appendix 3. Trench data ……………………………………………………… Appendix 4. Petrographic description ………………………………………… Appendix 5. Ore microscopic description ……………………………………. Appendix 6. XRD results ……………………………………………………… Appendix 7. XRF results ………………………………………………………. Appendix 8. XRF and ICP-MS results ………………………………………… Appendix 9. SEM-EDX results ……………………………………………….. Appendix 10. Fluid inclusion microthermometric data ………………………
231 241 244 257 269 274 277 284 286 303
CURRICULUM VITAE ………………………………………………………..
304
IX.
iii
TABLES 5.1. 5.2. 6.1. 6.2. 6.3. 6.4. 7.1. 7.2. 7.3. 7.4. 7.5. 7.6. 7.7. 7.8. 7.9. 7.10. 8.1. 8.2. 8.3.
8.4.
List of samples evaluated in classification of the volcanic rocks ……. Element concentrations in C1 chondrite, primitive mantle, N-type MORB, E-type MORB, ocean island basalts (OIB) and continental crust …….. Hydrothermal alteration mineral assemblage in altered rocks identified from microscopic observation ………………………………………….. Hydrothermal alteration mineral assemblage identified from XRD ……. Mineral assemblage in each hydrothermal alteration zone ……………. Ishikawa AI and CCPI values ………………………………………….. Chemical composition of the mineral specieses in sample TBC.1B …. Chemical composition of the unknown minerals in sample WBC.1A.V Paragenetic stages of the minerals in the Bincanai vein ………………. Paragenetic stages of the minerals in the Baturappe vein-1 …………….. Paragenetic stages of the minerals in the Baturappe vein-2 ……………..
Paragenetic stages of the minerals in the Bungolo vein ……………. Paragenetic stages of the minerals in the Paranglambere vein ………… Paragenetic stages of the minerals in the Bangkowa vein ……………… Field description of samples from the Ritapayung dissemination zone . Selected chemical composition of samples from the Ritapayung dissemination zone determined by XRF analysis ……………………… Fluid inclusion microthermometry results ………………………………. Estimated formation temperature of the veins in the study area and salinity of the responsible hydrothermal fluid …………………………………. Thermal profiles for selected compositions of NaCl-H2O liquids with corresponding depths, pressures and densities (Haas, 1971) that used to estimate the formation pressure of the veins in the study area ……….. Results of sulphur fugacity estimation for each vein using the fs2-T diagram of Einaudi et al. (2003) …………………………………………
81 98 108 110 126 128 148 149 152 158 161 164 168 172 176 178 183 184
196 199
iv
FIGURES 1.1.
1.2. 2.1. 2.2. 2.3. 2.4. 4.1. 4.2. 4.3. 4.4.
4.5. 4.6. 4.7. 4.8. 4.9. 4.10. 4.11. 4.12. 4.13. 4.14. 4.15. 4.16. 4.17. 5.1. 5.2. 5.3. 5.4.
Location of Baturappe area on the distribution and polarity map of mineralized Late Cretaceous to Pliocene magmatic arcs in Indonesia (Carlile and Mitchell, 1994) ……………………………………………. Study area and distribution of sampling points in geological mapping Cross-section showing environments of porphyry Cu, and high- and lowsulphidation epithermal deposits (Hedenquist et al., 1996) ………….. Section of a typical high-sulphidation ore body and its alteration zones (Hedenquist et al., 2000) ……………………………………………… Schematic section that generalizes patterns of alteration in a lowsulphidation system (Hedenquist et al., 2000) ……………………….. Mineralogic zoning of the two styles of epithermal deposit (Cooke and Simmons, 2000) ………………………………………………………… Geologic map of Sulawesi (after Hall and Wilson, 2000) …………… Distribution of magmatic affinity and significant hydrothermal ore mineralizations in the Western Sulawesi volcanic arc (Idrus et al., 2011) Geological sketch map of South Sulawesi area (modified after Yuwono et al., 1985) …………………………………………………………….. Cropped of geologic map of the Ujung Pandang, Benteng and Sinjai quadrangles, Sulawesi, and its representative cross-section (Sukamto and Supriatna, 1982) ……………………………………………………….. Satellite images of the Baturappe Volcanics (Caldera) ........................ Geological map of the study area ....................................................... Outcrops and photomicrographs of basaltic lava ............................... Outcrops and photomicrographs of andesitic lava ............................... Outcrops and photomicrograph of volcanic breccia ............................... Morphology, boulders, outcrop, hand specimen sample, and photomicrographs of gabbroic-dioritic stock unit ................................ Outcrops and photomicrographs of dykes ........................................... Three parallel N110oE dykes and a vein at Bangkowa river, and five units of dyke intruding basaltic lava and volcanic breccia at Ritapayung quarry Lineaments distribution in the study area ................................................ Rose diagram of structural elements trends in the study area and vicinity and rose diagram of fractures/joints at Bincanai area ......................... Controls of faults to the veins in the study area ………………………… Lithological controls of the mineralizations ………………………….. Conceptual model of the Baturappe volcanic facies in the study area (modified after Bogie and Mackenzie, 1998) ………………………… Distribution of lava member in the total alkali-silica diagram for volcanic rocks (Le Bas et al., 1986) ………………………………….…………. Hand specimen and photomicrographs of less-altered samples of lava Distribution of intrusions in the total alkali-silica diagram for plutonic rocks (Cox et al., 1979) ………………………………………………… Hand specimen and photomicrographs of less-altered samples of intrusions ………………………………………………………………..
2 7 15 17 18 19 46 48 50
53 56 57 61 62 63 65 67 68 70 71 73 75 79 83 84 85 86
v
5.5. 5.6.
5.7.
5.8. 5.9.
5.10. 6.1. 6.2. 6.3. 6.4. 6.5. 6.6. 6.7.
6.8. 6.9. 6.10. 6.11. 6.12. 6.13. 7.1. 7.2. 7.3. 7.4.
7.5. 7.6.
Distribution of lava and intrusions in the R1-R2 diagrams for volcanic and plutonic rocks (De la Roche et al., 1980) ……………………………… Distribution of altered volcanic rocks in the classification diagrams of Zr/TiO2 - SiO2 and Nb/Y - Zr/TiO2 (Winchester and Floyd, 1977; Floyd and Winchester, 1978) ………………………………………….. Sample and photomicrograph of very fine-grained porphyritic basalt; sample of altered porphyritic-trachytic andesite; and sample of altered dacite ................................................................................................. Plots of the five volcanic rock samples in SiO2-K2O diagram (Peccerillo and Taylor, 1976) ............................................................. Distribution of the three samples of basalt lava in the Zr – Zr/Y discrimination diagram of Pearce and Norry (1979). Square: TRP.4C, triangle: BK.03.C, circle: BK.09………………………………………. Primitive mantle-normalized spider diagram and REE chondritenormalized patterns of the four least-altered volcanic rocks …………… Hydrothermal alteration map of the Baturappe prospect .................... Photographs of chlorite- zone .................................................... Photographs of samples from epidote-chlorite-calcite zone around the Bincanai vein ...................................................................................... Indications of epidote-chlorite-calcite zone (vein-related propylitic alteration) at Bangkowa area, near the Bangkowa vein ....................... Manifestations of illite-quartz zone in the outcrops; clay surround and margin the veins at Baturappe and sulphide stringer at Bangkowa area.. Indications of quartz-carbonate alteration around the Bincanai vein ....... Plots of the 41 rock samples listed in Table 5.4 in the alteration box plot of Ishikawa AI vs CCPI (Large et al., 2001). The basalt-andesite least altered box is from Gemmell (2007) .......................................................... Behaviours of Na2O in each mineralization-related alteration zone .... Variation diagrams of the 41 rock samples listed in Table 6.4 ................ Isocon diagram of sample BCFW.1 (least-altered rock) vs WBC.2D (vein-related propylitic altered rock) .................................................. Isocon diagram of sample BCFW.1 (least-altered rock) vs WBC.2C (quartz-carbonate altered rock) .......................................................... Isocon diagram of sample BCFW.1 (least-altered rock) vs WBC.2B (quartz-carbonate altered rock) .......................................................... Enrichment-depletion diagrams of the three sample pairs .................. Distribution of the significant mineralizations in the study area ............. Outcrop view of the Bincanai vein .......................................................... Textures of the Bincanai vein ................................................................ Polished section view of sample TBC.1B from the Bincanai vein showing fine-grained unidentified minerals occupy fracture in galena and spots measured by SEM-EDX in the unknown minerals ..................... Spots measured by SEM-EDX on unknown minerals in sample WBC.1A.V ......................................................................................... Polished section photomicrographs showing the existing ore mineral assemblages and their textural relationships in the sulphide band of the Bincanai vein ……………………………………………………………..
88
92
93 95
97 99 111 113 116 118 120 123
129 132 133 136 136 137 137 142 143 145
148 149
151
vi
7.7. 7.8. 7.9.
7.10. 7.11.
7.12.
7.13. 7.14. 7.15. 7.16. 7.17. 7.18. 7.19. 8.1. 8.2. 8.3. 8.4. 8.5. 8.6. 8.7. 8.8. 8.9. 8.10. 8.11.
The two cross-cutting veins in Baturappe vein-1 trench ......................... Textures of the Baturappe vein-1 .......................................................... Polished section photomicrographs showing the existing of ore mineral assemblage and their textural relationships in quartz from the Baturappe vein-1 ...................................................................................................... Field views of the Baturappe vein-2 ................................................... Polished section photomicrographs showing the existing of ore mineral assemblage and their textural relationships in quartz from the Baturappe vein-2 ..................................................................................................... Sample TRP.2E from the quartz vein of Bungolo, showing finecrystalline quartz, vuggy texture, and very fine-grained disseminated sulphides ……………………………………………………………….. Polished section photomicrographs of sample TRP.2E from the quartz vein of Bungolo ..................................................................................... Sample BR.11 from the Paranglambere vein and its polished section photomicrographs ................................................................................... Field view of the Bangkowa vein ........................................................ Hand specimen and photomicrographs of samples from the Bangkowa vein .................................................................................................... Sulphide stringer with N120oE/70o orientation, enveloped by clay alteration, and hosted in propylitic-altered basalt at Bangkowa river .. Geological situation of Ritapayung quarry, where the disseminated sulphide occurred ................................................................................... Sample TRP.4F from Ritapayung showing disseminated sulphide in volcanic breccia and its photomicrographs ............................................ Several quartz chips of the veins in study area which polished to make wafers for fluid inclusion study ............................................................ Photomicrograph of primary two-phase liquid-vapor (liquid-rich) fluid inclusions in quartz of the Bincanai vein and Baturappe vein-2 samples Histograms of homogenization temperature and salinity of sample WBC.1A.V from the Bincanai vein ........................................................ Histograms of homogenization temperature and salinity of sample WBC.3A.V from the Bincanai vein ....................................................... Histograms of homogenization temperature and salinity of sample TBC.1A from the Bincanai vein ................................................................ Histograms of homogenization temperature and salinity of sample TBC.1B from the Bincanai vein ................................................................. Histograms of homogenization temperature and salinity of sample TRP.1G from the Baturappe vein-1 ........................................................... Histograms of homogenization temperature and salinity of sample TRP.1H from the Baturappe vein-1 ........................................................... Histograms of homogenization temperature and salinity of sample TRP.6D from the Baturappe vein-2 ............................................................ Histograms of homogenization temperature and salinity of sample TRP.6F from the Baturappe vein-2 ............................................................. Histogram of homogenization temperature of sample TRP.2E from the Bungolo vein .........................................................................................
154 155
157 159
160
162 163 166 170 171 173 175 177 181 182 185 185 185 186 186 187 187 188 188
vii
8.12. 8.13. 8.14. 8.15.
8.16.
8.17. 8.18. 8.19. 9.1.
9.2. 9.3. 9.4.
Histogram of homogenization temperature of sample BR.11 from the Paranglambere vein ............................................................................... Histogram of homogenization temperature of sample TBK.2A.3 from the Bangkowa vein ...................................................................................... Histogram of homogenization temperature of sample TBK.2F.3 from the Bangkowa vein ...................................................................................... Plot of ranges of homogenization temperatures and salinities of fluid inclusions in the veins in study area on the classification diagram of ore deposits of Wilkinson (2001) ............................................................. Diagram shows typical trends in Th vs salinity space due to various fluid evolution processes and precipitation mechanism (modified from Shepherd et al., 1985 and Wilkinson, 2001) ....................................... Homogenization temperature (Th) vs salinity diagrams of fluid inclusions from the Bincanai vein, Baturappe vein-1, and Baturappe vein-2 ...... Range of minimum formation depth of the veins estimated using the boiling point curve of Haas (1971) .................................................... Range of sulphur fugacity of each vein estimated using the fs2-T diagram of Einaudi et al. (2003) ………………………………………………… Inferred exposed level and range of formation of the veins in the Baturappe prospect based on their textural characteristics (after Morrison et al., 1990) .……………………………………………………………. Proposed genetic model of the Bincanai vein, including its recommended scout drilling ……………………………………………………………. Proposed genetic model of the Baturappe vein-1, including its recommended scout drilling …………………………………………… Proposed genetic model of the Baturappe vein-2, including its recommended scout drilling ……………………………………………
188 189 189
190
191 192 194 198
208 221 222 223
viii
ABSTRACT The Baturappe epithermal silver-base metal prospect is situated in south of Sulawesi island, Indonesia. The prospect lies in the shoshonitic/alkaline southern arm of the Tertiary western Sulawesi plutono-volcanic arc. The Baturappe prospect is developed in the late Middle-Miocene Baturappe Volcanics which in the study area consists of respectively from the older to the younger: basaltic-andesitic lava, gabbroic-dioritic stock, and basaltic-andesitic dykes. Mineralizations (veins, sulphide stringer and disseminated sulphide) in the prospect mostly hosted in the basaltic-andesitic lava, including its volcanic breccia member. Two types of geological structure were developed in the prospect area, early radial volcanic structure and late tectonic-related northwest-southeast trend fault. The tectonic-related fault controlled the emplacement of significant epithermal base metal veins, whereas the radial structure mostly associated with thinner and barren quartz veins. The disseminated sulphide mineralization is controlled by the permeable pores of the volcanic breccia host. Petrochemical study revealed that the volcanics is potassic alkaline (shoshonitic) silica-undersaturated basaltic-andesitic in composition. The lava consists of trachy basalt, basaltic trachy-andesite, alkali basalt, tephrite, basanite, hawaiite, mugearite, and picrite. The composition of the volcanic-related intrusions generally correspond to those of their equivalent lava, mostly gabbroic, with a range from dominant alkali gabbro to less ultramafic, and andesite-trachy andesite to dacite. Behaviours of certain trace elements, particularly enrichments of Rb, Cs, Sr and LREE are consistent with the characteristics of the typical postcaldera K-rich volcanics. Interpretation of volcano-tectonic setting and magmatic evolution based on evaluation of major and trace elements suggested that the volcanics was generated in a within-plate (intraplate) environment, in extensional (rift) tectonic regime, which is not directly related to synchronous subduction event. The magma was originated from melting of previously metasomized upper mantle which is enriched in potassium and other incompatible elements. Hydrothermal alteration is zoned from distal to proximal: chlorite, epidotechlorite-calcite, and illite-quartz and quartz-carbonate zones. The chlorite zone is distributed at the periphery of the hydrothermal system. The epidote-chloritecalcite zone is a vein-related propylitic alteration which distributed in narrow zone around the veins, on the outer part of illite-quartz zone and quartz-carbonate zone. The illite-quartz zone (argillic zone) is developed within maximum 7 m from the structural-controlled veins at Baturappe and Bangkowa area; whereas the quartzcarbonate zone formed within maximum 0.4 m of the fault-controlled vein at Bincanai, as well as in host rocks of disseminated sulphide at Ritapayung area as a lithological-controlled alteration. The chlorite zone is inferred to be formed in temperature range of 220-300oC, whereas the last three mineralization-related alteration zones formed under temperature range of 220-320oC, generally from a near neutral-pH fluid. ix
There is a strong relationship between the increase of alteration intensity to the decrease of Na2O content in each mineralization-related alteration zones, which may related to breakdown of sodic plagioclase as a result of hydrothermal alteration processes. In general, there is also a negative correlation between MgO and FeO to Na2O and K2O, where increasing of MgO and FeO contents are accompanied by decreasing of Na2O and K2O contents; this may be related to the formation of Mg- and Fe-chlorite, particularly in the propylitic zone. In contrast, a general positive correlation is shown between Na2O and K2O to trace elements Rb and Sr, where the most positive correlation is shown between K2O and Rb. These imply that the formation of hydrothermal alteration minerals beside involves loss of Na2O and K2O (breakdown of Na- and K-bearing minerals), is also generally involves loss of Rb and Sr that may be previously contained in the pre-altered minerals. Mass balance calculation indicates that in the propylitic altered zone there is a little decrease in bulk composition of the altered rock with respect to the least-altered rock; the mass and volume changes is -0.95% and -8.75%, respectively. In contrast, in the quartz-carbonate zone the altered rocks show an increasing in bulk composition with respect to the least-altered rocks; the mass and volume changes are +15.56% to +20.93% and +6.09% to +33.87%, respectively. The gains and losses of major oxides and trace elements in the both alteration zones are generally consistent either with the hydrothermal alteration mineral assemblages, indications of the hydrothermal alteration processes such as destruction of primary minerals and absorption of certain trace elements in altered minerals, and behaviours of early metal-bearing sulphides. Six significant veins were reviewed in this study, namely: Bincanai vein, Bangkowa vein, Baturappe vein-1, Baturappe vein-2, Bungolo vein, and Paranglambere vein. The last four veins are clustered in Baturappe area. The veins are generally occurred in north-northwest to west-northwest orientation, with dominant steeply to less gently southwest-ward dipping. The veins mostly exhibit the typical texture of epithermal vein, i.e. crustiform banding texture. Most of the veins show crystalline texture of quartz and sulphides (dominated by galena), from very fine- to coarse-grained crystals. From the evaluation of the textural characteristics, it is inferred that the veins were generally formed by a fluctuated and episodic, liquid dominated fluid and deposited below boiling level, minimum about 350 m below surface and in temperature about > 225oC. Fluid inclusion study on quartz of the veins resulted that the Bincanai main vein was formed in temperature range of minimum about 230 to 240oC which equivalent to minimum depth of 300 to 370 m below paleo-water table and hydrostatic pressure of 27 to 33 bars, from a 2.0 to 2.5 wt.% NaCl eq. salinity fluid, with a range of sulphur fugacity of 10-11 to 10-14. The Baturappe vein-1 formed in temperature range of minimum about 260 to 270oC which equivalent to minimum depth of 520 to 620 m below paleo-water table and hydrostatic pressure of 46 to 53 bars, from a more saline fluid; maximum 5.7 wt.% NaCl eq., with a x
range of sulphur fugacity of 10-8 to 10-13. The Baturappe vein-2 formed in temperature range of minimum about 270 to 280oC which equivalent to minimum depth of 620 to 740 m below paleo-water table and hydrostatic pressure of 53 to 62 bars, also from a more saline fluid; maximum 5.0 wt.% NaCl eq., with a range of sulphur fugacity of 10-9 to 10-12. In comparison to the other veins, the Paranglambere- and Bungolo vein formed in relatively lower temperatures, minimum about 170 to 180oC and 200 to 210oC, respectively. In contrast, the Bangkowa vein formed in the most higher temperature relative to the other veins, minimum about 280 to 290oC. The ranges of sulphur fugacity of the last three veins are 10-15 to 10-16, 10-13 to 10-15 and 10-9 to 10-11, respectively. From evaluations of their mineral (sulphide and gangue) assemblages combined with the results of fluid inclusion microthermometry, it is concluded that in general at least three generations of hydrothermal fluid have affected the study area. Bangkowa vein formed from a relatively barren fluid (this is the first generation of the veins); Bincanai, Baturappe vein-2, Bungolo and Paranglambere veins formed from a ±silver-base metal-rich intermediate-sulphidation fluid; and the Baturappe vein-1 formed from a copper-rich intermediate- to highsulphidation fluid. Generally all of the veins formed from fluids that resulted from mixing of hotter-deeper magmatic-origin fluid with cooler-less saline meteoric water. From the trends of plots in the Th vs salinity diagrams, it is concluded that in general no indications of boiling mechanism responsible for the formation of the veins; this is consistent with the result of the textural characteristics evaluation which indicated that the veins were formed below boiling level. For the development of the prospect to next stages of exploration, based on the comprehensive results of this study, it is recommended to conduct a subsurface exploration programs in form of geophysical survey and scout drilling, with a priority on the Bincanai vein, and the Baturappe- vein-1 and vein-2. The Bincanai vein has a more significant economic metal assemblages with respect to the other veins in the prospect. Beside the base metals, silver and bismuth minerals are also contained in the vein. The average grade of the Bincanai vein is: Pb 17.51 %, Zn 0.35 %, Cu 0.66 %, Ag 713 g/t and Bi 308 g/t. The scout drilling program is recommended to conduct inclinely 45o to the ground surface at about 90 m southwest side of the vein, with drilling depth of 100 m. The Baturappe vein-1 is also considered important to be followed-up to scout drilling as the vein has a wide range of sulphide assemblage, from intermediate- to high sulphidation minerals; and so as the Baturappe vein-2 which contained an economic important sulphide, molybdenite. The scout drilling program for the Baturappe vein-1 is recommended to conduct inclinely about 50o to the ground surface (perpendicular to the dip of the vein) at about 15 m southwest side of the vein, with minimum 10 m depth. For the Baturappe vein-2, the scout drilling is recommended to conduct inclinely about 62o to the ground surface (perpendicular to the dip of the vein) at about 16 m southwest side of the vein, with minimum 8 m depth. xi
ABSTRAK Daerah penelitian, prospek mineralisasi perak - logam dasar epitermal Baturappe, terletak di bagian selatan Pulau Sulawesi, Indonesia. Prospek ini berada pada lengan selatan busur plutono-volkanik Sulawesi barat yang berumur Tersier dan berafinitas shoshonitik/alkalik. Prospek mineralisasi ini berkembang pada Formasi Volkanik Baturappe yang berumur akhir-Miosen Tengah, yang di daerah penelitian disusun oleh satuan-satuan batuan, dari yang tertua ke yang termuda: lava basaltik-andesitik, stok gabroik-dioritik, dan retas-retas basaltikandesitik. Mineralisasi (urat dan sulfida tersebar) di prospek ini umumnya diwadahi oleh batuan-batuan anggota satuan lava basaltik-andesitik, termasuk anggota breksi volkaniknya. Dua jenis struktur geologi berkembang di daerah penelitian, yaitu struktur-volkanik radial yang terbentuk lebih dahulu, dan struktur-tektonik berupa patahan berarah baratlaut-tenggara yang terbentuk kemudian. Patahan ini mengontrol proses alih-tempat dan deposisi urat-urat logam dasar epitermal yang signifikan, sedangkan struktur-volkanik radial umumnya hanya berasosiasi dengan urat-urat kuarsa tipis dan tak-termineralisasi (barren). Di sisi lain, mineralisasi sulfida tersebar dikontrol oleh porositas permeabel dari batuan wadahnya; breksi volkanik. Berdasarkan studi geokimia batuan, diketahui bahwa batuan volkanik di daerah penelitian berkomposisi basaltik-andesitik, potasik-alkalik (shoshonitik) tak-jenuh silika. Satuan lava disusun oleh batuan-batuan traki basal, traki-andesit basaltik, basal alkali, tefrit, basanit, hawaiit, mugearit, dan pikrit. Komposisi batuan-batuan intrusi umumnya sejenis dengan komposisi lava ekuivalennya, umumnya gabroik, dengan kisaran batuan mulai dari gabro alkali hingga sedikit batuan ultramafik, dan andesit-traki andesit hingga dasit. Perilaku geokimia unsur-unsur minor dan jejak, terutama pengayaan Rb, Cs, Sr dan LREE, umumnya konsisten dengan ciri tipikal batuan-batuan volkanik potasik (kaya-K) pasca-kaldera. Hasil interpretasi tatanan volkano-tektonik dan evolusi magma berdasarkan evaluasi unsur-unsur mayor dan jejak, mengindikasikan bahwa volkanisme di daerah penelitian tergenerasi pada lingkungan dalam-lempeng (intraplate), pada rezim tektonik ekstensi (rift), yang tidak berhubungan langsung dengan peristiwa subduksi. Sumber magma berasal dari hasil peleburan mantel bagian atas yang sebelumnya telah mengalami proses metasomatisme berupa pengayaan potasium dan unsur-unsur inkompatibel lainnya. Alterasi hidrotermal terzonasi dari bagian luar ke dalam (distal ke proksimal): zona klorit, zona epidot-klorit-kalsit, dan zona illit-kuarsa dan kuarsa-karbonat. Zona klorit terdistribusi di bagian tepi sistem hidrotermal dan tersebar luas di daerah penelitian. Zona epidot-klorit-kalsit merupakan zona alterasi propilitik yang berhubungan spasial-genetik dengan mineralisasi urat, dan terdistribusi secara sempit di sekitar urat, di bagian luar zona illit-kuarsa dan zona kuarsakarbonat. Zona illit-kuarsa (argilik) terbentuk pada daerah berjarak maksimum 7 m dari urat-urat yang dikontrol struktur di daerah Baturappe dan Bangkowa; xii
sedangkan zona kuarsa-karbonat terbentuk pada daerah berjarak maksimum 0,4 m dari urat yang dikontrol patahan di daerah Bincanai, dan pada batuan wadah mineralisasi sulfida tersebar di daerah Ritapayung, sebagai alterasi yang dikontrol oleh litologi. Zona klorit diestimasi terbentuk pada kisaran temperatur sekitar 220-300oC, sedangkan ketiga zona alterasi lainnya sekitar 220-320oC, dari fluida ber-pH hampir netral. Terdapat korelasi yang kuat antara peningkatan intensitas alterasi dengan penurunan konsentrasi Na2O pada setiap zona alterasi yang berhubugan dengan mineralisasi, yang diinterpretasi disebabkan oleh destruksi plagioklas sodik akibat proses alterasi hidrotermal. Secara umum terdapat korelasi negatif antara MgO dan FeO terhadap Na2O dan K2O, di mana peningkatan konsentrasi MgO dan FeO umumnya disertasi dengan penurunan konsentrasi Na2O dan K2O; yang diduga berhubungan dengan pembentukan mineral-mineral klorit-Mg dan klorit-Fe, terutama di zona propilitik. Sebaliknya, korelasi positif ditunjukkan oleh hubungan antara Na2O dan K2O terhadap unsur-unsur minor Rb dan Sr, di mana korelasi positif terkuat terjadi antara K2O dan Rb. Korelasi positif ini mengindikasikan bahwa pembentukan mineral-mineral alterasi hidrotermal di samping menyebabkan penurunan konsentrasi Na2O dan K2O (destruksi mineralmineral pra-alterasi kaya sodium dan potasium), juga menyebabkan penurunan konsentrasi Rb dan Sr yang terkandung dalam mineral-mineral pra-alterasinya. Hasil kalkulasi kesetimbangan massa (mass balance) menunjukkan bahwa batuan yang teralterasi propilitik mengalami sedikit penurunan komposisi kimia total dibandingkan batuan yang tak teralterasi; terjadi penurunan massa sebesar -0.95% dan penurunan volume sebesar -8.75%. Sebaliknya, batuan-batuan teralterasi di zona kuarsa-karbonat mengalami pengayaan komposisi kimia total dibandingkan batuan tak teralterasinya; peningkatan massa berkisar +15.56% hingga +20.93% dan peningkatan volume +6.09% hingga +33.87%. Pengayaan dan penurunan konsentrasi oksida-oksida mayor dan unsur-unsur minor/jejak di kedua zona alterasi tersebut umumnya konsisten dengan: kumpulan mineral alterasi hidrotermal yang terbentuk, indikasi-indikasi proses alterasi hidtrotermal yang terjadi seperti destruksi mineral-mineral primer dan absorpsi unsur-unsur jejak tertentu pada mineral-mineral alterasi, serta perilaku geokimia sulfidasulfida logam awal yang terdapat pada batuan. Enam urat signifikan di wilayah prospek telah dievaluasi pada studi ini, yang masing-masing dinamakan: urat Bincanai, urat Bangkowa, urat Baturappe-1, urat Baturappe-2, urat Bungolo, dan urat Paranglambere; ke-empat urat terakhir terdistribusi secara rapat di zona Baturappe. Secara umum urat-urat tersebut berarah utara-baratlaut hingga barat-baratlaut, miring ke arah baratdaya, dominan terjal, dan sedikit landai. Secara keseluruhan urat-urat tersebut memperlihatkan tekstur khas urat epitermal: crustiform banding. Ciri umum lain dari urat-urat tersebut adalah tekstur kristalin, baik pada kuarsa maupun sulfidanya (didominasi oleh galena), mulai dari kristal-kristal berukuran sangat halus hingga sangat kasar. Berdasarkan evaluasi karakteristik tekstur urat tersebut, diinterpretasikan bahwa xiii
secara umum urat-urat epitermal ini terbentuk dari fluida yang didominasi oleh cairan (liquid) dan terbentuk secara episodik dan fluktuatif di bawah zona pendidihan (below boiling level), pada kedalaman minimum sekitar 350 m di bawah permukaan, dan pada temperatur > 225oC. Studi inklusi fluida pada kuarsa menunjukkan bahwa urat Bincanai terbentuk pada temperatur minimum sekitar 230-240oC yang ekuivalen dengan kedalaman minimum 300-370 m di bawah muka air tanah purba dan tekanan hidrostatik 27-33 bar, dari fluida bersalinitas 2,0-2,5 wt.% NaCl ekuivalen, dengan kisaran fugasitas sulfur 10-11 hingga 10-14. Urat Baturappe-1 terbentuk pada temperatur minimum sekitar 260-270oC yang ekuivalen dengan kedalaman minimum 520-560 m di bawah muka air tanah purba dan tekanan hidrostatik 46-53 bar, dari fluida yang bersalinitas lebih tinggi; mencapai 5,7 wt.% NaCl ekuivalen, dengan kisaran fugasitas sulfur yang juga lebih tinggi: 10-8 hingga 10-13. Urat Baturappe-2 terbentuk pada temperatur minimum sekitar 270-280oC yang ekuivalen dengan kedalaman minimum 620-740 m di bawah muka air tanah purba dan tekanan hidrostatik 53-62 bar, dari fluida yang bersalinitas lebih tinggi; mencapai 5,0 wt.% NaCl ekuivalen, dengan kisaran fugasitas sulfur 10-9 hingga 10-12. Dibandingkan dengan urat-urat lainnya, urat Paranglambere dan urat Bungolo terbentuk pada kisaran temperatur yang lebih rendah, berturut-turut minimum sekitar 170-180oC dan 200-210oC. Sebaliknya, urat Bangkowa terbentuk pada temperatur yang paling tinggi dibandingkan urat-urat lainnya, minimum sekitar 280-290oC. Kisaran fugasitas sulfur dari ketiga urat terakhir tersebut berturut-turut adalah 10-15 - 10-16, 10-13 - 10-15 dan 10-9 - 10-11. Berdasarkan evaluasi himpunan mineral (sufida dan gangue) dan hasil-hasil studi mikrotermometri inklusi fluida, secara umum disimpulkan bahwa sedikitnya tiga generasi fluida hidrotermal telah bekerja di daerah penelitian. Urat Bangkowa terbentuk lebih dahulu, dari fluida yang relatif tak-termineralisasi; urat-urat Bincanai, Baturappe-2, Bungolo dan Paranglambere terbentuk dari fluida sulfidasi-menengah yang kaya logam dasar ± perak; dan urat Baturappe-1 terbentuk dari fluida sulfidasi- menengah hingga tinggi yang kaya tembaga. Uraturat tersebut terbentuk dari fluida yang merupakan hasil percampuran (mixing) antara fluida magmatik bersuhu tinggi dengan air meteorik bersalinitas dan bersuhu rendah. Berdasarkan trend inklusi fluida pada diagram Th vs salinitas, disimpulkan bahwa secara umum urat-urat epitermal di daerah penelitian tidak terbentuk dari mekanisme pendidihan, di mana kesimpulan ini konsisten dengan hasil evaluasi karakteristik teksturnya yang mengindikasikan pembentukan di bawah zona pendidihan. Untuk pengembangan prospek mineralisasi ke tingkat eksplorasi yang lebih detail, berdasarkan hasil studi yang dilakukan, direkomendasikan untuk melaksanakan program eksplorasi bawah permukaan dalam bentuk survei geofisika dan terutama pemboran perintis (scout drilling), dengan prioritas pada urat Bincanai, urat Baturappe-1 dan urat Baturappe-2. Urat Bincanai secara relatif mengandung kumpulan logam ekonomis yang lebih signifikan dibandingkan xiv
dengan urat-urat lainnya di wilayah prospek. Di samping logam dasar, perak dan bismut juga dijumpai pada urat Bincanai. Kadar rata-rata urat Bincanai adalah: Pb 17,51 %, Zn 0,35 %, Cu 0,66 %, Ag 713 g/t and Bi 308 g/t. Pemboran perintis pada urat ini disarankan dilakukan pada jarak 90 m di sebelah baratdaya urat, dengan kemiringan pemboran 45o terhadap level permukaan, dengan kedalaman 100 m. Urat Baturappe-1 juga direkomendasikan untuk ditindaklanjuti ke tahap pemboran perintis karena mengandung kisaran kumpulan logam sulfida yang lebar, mulai dari sulfidasi menengah hingga tinggi; demikian juga dengan urat Baturappe-2 yang mengandung salah satu mineral sulfida penting, yaitu molibdenit. Pemboran perintis pada urat Baturappe-1 disarankan dilakukan pada jarak 15 m di sebelah barat daya urat, dengan kemiringan pemboran sekitar 50 o terhadap level permukaan (tegaklurus terhadap kemiringan urat), dengan kedalaman pemboran minimum 10 m. Sedangkan pada urat Baturappe-2 pemboran perintis disarankan dilakukan pada jarak 16 m di sebelah baratdaya urat, dengan kemiringan sekitar 62o terhadap level permukaan (tegaklurus terhadap kemiringan urat), dengan kedalaman pemboran minimum 8 m.
xv
