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Characteristics and Genesis of Gold Mineralization in the Salu Bulo Prospect , Sulawesi, Indonesia
Get PDF秋田大学博士（工学）Several significant gold deposits hosted in metamorphic rocks have been discovered in Sulawesi, Indonesia, namely Paboya, Awak Mas, Palopo and Bombana. The Salu Bulo prospect is one of the gold prospects in the Awak Mas project in the central part of the Western Metallogenic Province, Sulawesi, Indonesia. The four domains in the Salu Bulo prospect (Biwa, Lelating, Bandoli and Freddie) has been explored by Placer Dome Inc., in 1999 and One Asia Resource Ltd., in 2011 to 2013 through 132 drilled holes with an average length of 100 m. The resource was measured and indicated about 5.6 million tons at 2.2 g/t Au with a cut-off grade at 0.5 g/t Au as well as an additional 0.5 million tons at 1.1 g/t Au as an inferred resource, reported by Tetra Tech in 2013. The Salu Bulo prospect is hosted by the cover sequence of Latimojong Metamorphic Complex. Within the prospect, the Latimojong Metamorphic Complex consists of meta-dark (graphitic), green (chloritic) and red (hematitic) mudstone, siltstone, sandstone and intercalated meta-volcanic and volcaniclastic rocks along with phyllite. The ore bodies in the Salu Bulo prospect are north-south trending and dipping steeply eastward, approximately three meters thick which are associated with veins, stockwork and breccias with an orientation sub-parallel and discordant to the foliation of the host rocks. The veins can be classified in three stages namely early, main and late stages and gold mineralization is related to the main stage. The veins formed during the main stage are composed of quartz, carbonate (mainly ankerite) and albite. The breccia formed mainly during the main stage of mineralization. The breccia is associated with quartz, carbonate (mainly ankerite), albite and pyrite.
Sulfide minerals, native gold and electrum are lesser abundant or absent in the veins and stockwork. High grade gold ores in the Salu Bulo prospect are accompanied with intense alteration along the main stage veins and breccia. Alteration mineral assemblage includes ankerite ± calcite, quartz, albite and pyrite along with minor sericite. Pyrite is the most abundant sulfide mineral, which is spatially related to gold (<2-42 μm in size). It is more abundant as dissemination in the altered host rocks than those in veins, suggesting that waterrock interaction played a role to precipitate pyrite and gold in the Salu Bulo prospect. Pyrite shows several different morphologies and textures: fine-grained, porous, deformed and massive pyrites. Lesser amounts of tennantite-tetrahedrite, chalcopyrite, bornite, galena, hematite and rutile are also present as inclusions in pyrites, and as discrete disseminated minerals in altered host rocks, veinlets and matrix of breccia. Covellite and chalcocite occur on the rim of some chalcopyrite.
Correlation coefficients of whole-rock Ag, Ni, Mo and Na contents to whole-rock Au content are above 0.5 indicating that they are strongly correlated with gold. The Mo, Ni, Sb, Hg, Se, Sr and W contents are relatively elevated in altered host rocks and ores than in unaltered host rocks, whereas the Cs, Rb and V contents are relatively depleted. The K₂O content is depleted in hydrothermally altered host rocks and ores indicating absence of potassic alteration and decomposition of K-bearing mineral in host rocks during hydrothermal alteration. Various elevated and depleted of MgO, Fe₂O₃, Al₂O₃ and MnO contents in hydrothermally altered host rocks and ores are related to inherited host rocks composition (i.e. mineral composition). PAAS normalized REE plots of unaltered and altered host rocks and ores are relatively similar patterns which are depleted of LREE and subhorizontal HREE with various Eu positive anomalous (Eu/Eu* varying from 0.1 to 0.9). ƩREE of unaltered host rocks is relatively similar and higher than hydrothermally altered host rocks and ore indicating evidence for mobility and fractionation of REE during hydrothermal alteration. Gold was introduced as electrum and native gold with Au / (Au + Ag) ratio ranging from 66.2 to 78.5 atomic % and from 81.4 to 82.3 atomic %, respectively as fracture filling and inclusion in pyrites. The Ni and Co concentrations of pyrite are high, 10 - 7780 ppm and 390 - 2710 ppm, respectively, in edges, cores and rims of pyrite with Co/Ni ratio of pyrite ranging from 0.09 to 63.
Fluid inclusions in quartz in the veins of the main stage and the matrix of breccia are mainly two-phase liquid-rich inclusions with minor two-phase vapor-rich and single-phase liquid or vapor inclusions. CO₂ and N₂ are detected in the fluid inclusions by Laser Raman microspectrometry. Raman spectrum of CO₂ clearly shows strong bands at ∼ 1285 cm-1 (v1) cm−1 and ∼ 1388 cm−1 (2v2) in mineralized vein and at ∼ 1282 cm-1 (v1) and ∼ 1385 cm−1 (2v2) in the matrix of breccia, N₂ and graphite at ∼ 2328 cm−1 and ∼ 1617 cm−1 respectively in mineralized vein and at ∼ 2327 cm−1 and ∼ 1606 cm−1 respectively in matrix of breccia. Homogenization temperature (Th) of fluid inclusions in the veins ranges from 132 to 357 ℃ and that in the matrix of breccia ranges from 148 to 368 ℃, which homogenized into a liquid phase. Salinity of fluid ranges from 3.5 to 8.0 wt% (average 6.3 wt%) NaCl equivalent in the veins and from 3.9 to 8.5 wt% (average 6.0 wt%) NaCl equivalent in the matrix of breccia. The wide range of homogenization emperature of fluid inclusions and the co-existence of two-phase liquid-rich and two-phase vapor-rich inclusions suggest boiling of fluid when they were trapped. The trapping temperature was 190 to 210 ℃. Fluid boiling probably occurred when the fluid was trapped at approximately 120 to 190 meters below the paleo water table. δ¹⁸Osmow values of fluid, +5.8 ‰ and +7.6 ‰ calculated from δ¹⁸Osmow of quartz (+17.2 ‰ and +19 ‰) at 205 ℃ from the main stage vein indicate oxygen isotopic exchange with wall rocks during deep circulation. δ³⁴Scdt of pyrite narrowly ranges from -2.0 to +3.4 ‰ suggesting a single source of sulfur. Gold mineralization in the Salu Bulo prospect occurred in an epithermal condition, after the metamorphism of the Latimojong Metamorphic Complex and emplacement of Palopo Granite or Enrekang Volcanic Series in the Late Miocene to Pliocene. It was formed at relatively shallow depth from CO₂-bearing sodic mineralizing fluid with low to moderate salinity (3.0 - 8.5 wt% NaCl equiv.). Temperature and pressure of ore formation range from 190 to 210 ℃ and 1.2 to 1.9 MPa, respectively
Geology, mineralogy, and sulfur isotopic studies of proximal volcanic products at the Tangkuban Parahu Volcano, Indonesia.
Get PDF秋田大学博士（理学）Tangkuban Parahu volcano is one of the most active volcanoes in West Java, Indonesia, although most of the recent eruptions were relatively mild (e.g., 2013 eruption). However, there is still little information from the volcanic products in the proximal area. Here, we provide new documentation from the proximal volcanic succession, including tephra-stratigraphy, componentry analysis, and petrography of volcanic products. Detailed mapping of the proximal area shows that the volcanic products are predominantly composed of an alternation fine-clay and coarse ash, lapilli tuff, and pyroclastic breccia within 10 tephra units. Componentry of ash particles revealed the presence of five components, which associated with hydrothermally altered lithics, oxidized lithics, coherent crystalline lithics, magmatic juvenile and free crystal in entire eruptive products. These indicate that the subvolcanic hydrothermal system has been developed since the Holocene and associated with a continual introduction of magmatic intrusion. Petrographic observation shows the hydrothermal minerals of quartz or silica accompanied by alunite and kaolinite, which represents acidic alteration within the crater-conduit. The presence of the silicified zone indicates that the subvolcanic hydrothermal system plays an essential role as a cap-rock of pressurized gas and steam at depth (200-500m), whereas magmatic injection caused the vapor plume expansion. Our observation concluded that the proximal volcanic succession captured the evidence of coupled phreatic and phreatomagmatic activities during the latest development of Mt.Tangkuban Parahu.
In order to understand the subvolcanic hydrothermal system condition, we carried out the mineralogy and sulfur isotopic signature of volcanic products from the proximal area of Tangkuban Parahu volcano. X-ray diffraction analysis reveals the majority of the hydrothermal mineral constituent. Silica polymorph (quartz, cristobalite, and tridymite) and alunite are dominant, whereas kaolinite randomly presents in some stratigraphic sections. Jarosite and goethite are abundant in the upper section of composite stratigraphy. The temporal mineralogical changing could reflect the depth of explosion or hydrothermal alteration development below the active craters, where some jarosite and possibly represent highly oxidized conditions after the emplacement process. From petrographic observation, hydrothermally altered lithics consist of two primary ash particle types; 1) partly altered magmatic glass and 2) selectively to pervasively altered ash fraction. The later ash particles comprise four types hydrothermal mineral assemblages along with two evidence of hydrothermal veinlet feeders. Hydrothermal mineral assemblages are opal, opal + cristobalite + alunite ± kaolinite, quartz + alunite ± kaolinite, and oxidation of alunite + jarosite, whereas the veinlets are silica+alunite-kaolinite-goethite±APS and quartz-pyrite. The accessory minerals (pyrite, chalcopyrite, and enargite) also present in many ash particles. These alteration assemblages are classified as acid-sulfate alteration and argillic alteration, which mostly present at near-surface of geothermal field in calcalkaline stratovolcanoes. Their presence also possibly rooted in high-temperature magmatic-vapor zone underneath the volcano
A novel three-dimensional/four-dimensional ultrasound analysis of the effect of the transfer position in in vitro fertilization and embryo transfer on the pregnancy rate
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An analysis of factors associated with personal and perceived stigma against talking about suicide in a rural Japanese community
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Effects of eldecalcitol and ibandronate on secondary osteoporosis and muscle wasting in rats with adjuvant-induced arthritis
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Development of a Rehabilitation Robot Combined with Functional Electrical Stimulation Controlled by Non-disabled lower Extremity in Hemiplegic Gait
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Skin grafting cases by negative pressure wound therapy for the treatment of skin tumors and intractable cutaneous ulcers
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