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Akira Imai

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Author-ID : 4242812

Civil Engineering, Building, Construction & Architecture Earth & Planetary Sciences Energy Engineering Environmental Science

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The mineralogy of gold-copper skarn related porphyry at the Batu Hijau deposit, Sumbawa, Indonesia May Thwee Aye; Subagyo Pramumijoyo; Arifudin Idrus; Lucas Donny Setijadji; Akira Imai; Naoto Araki; Johan Arif
Journal of Applied Geology Vol 3, No 1 (2011)
Publisher : Geological Engineering Department Universitas Gadjah Mada

Clacic gold-copper bearing skarn in the Batu Hijau porphyry deposit is located in the western part of Sumbawa Island, Indonesia. Skarn mineralizations were found at the deep level of the deposit (-450m to -1050mL) by drilling program 2003. No evidence around Batu Hijau has limestone although most skarn are metasomatiz ed from carbonate-rich rock as limestone or marble. Most skarn-type metasomatic alteration and mineralization occurs at the contact of andesitic volcanic rock and intermediate tonalite porphyry intrusion and within intermediate tonalite in some. Although both endoskarn and exoskarn can be developed, it has no clear minerals to known the endoskarn. Exoskarn is more principle skarn zone. The formation of skarn occurred two min stages: (1) prograde and (2) retrograde. The prograde stage is temporally and spatially divided into two sub-stages as early prograde (sub-stage I) and prograde metasomatic (sub-stage II). Sub-stage I begin immediately after the intrusion of the tonalite stock into the calcium rich volcanic rocks. Then, sub-stage II originated with segregation and evolution of a fluid phase in the pluton and its invasion into fractures and micro-fractures of host rocks developed during sub-stage I. The introduction of considerable amount of Fe, Si and Mg led to the large amounts of medium- to coarse-grained anhydrous calc-silicates. From the texture and mineralogy, the retrograde metasomatic stage can be divided into two sub-stages: (a) early retrograde and (sub-stage III) and (b) late retrograde (sub-stage IV). During sub-stage III, the previously formed skarn zones were affected by intense multiple hydro-fracturing phases in the gold-copper bearing stocks. Therefore, the considerable amounts of hydrous calc-silicates (epidote), sulfides (pyrite, chalcopyrite, sphalerite), oxides (magnetite, hematite) and carbonates (calcite) replaced the anhydrous calc-silicates. Sub-stage IV was coexisting with the intrusion of relatively low temperature, more highly oxidizing fluids into skarn system, bringing about partial alteration of the early-formed calc-silicates and developing a series of very fine-grained aggregrates of chlorite, clay, hematite and calcite.

Study on mineralogy and chemistry of the saprolitic nickel ores from Soroako, Sulawesi, Indonesia: Implication for the lateritic ore processing Sufriadin Sufriadin; Arifudin Idrus; Subagyo Pramumijoyo; I Wayan Warmada; Akira Imai
Journal of Applied Geology Vol 3, No 1 (2011)
Publisher : Geological Engineering Department Universitas Gadjah Mada

An investigation of mineralogy and chemistry of saprolitic nickel ores developed on ultramafic rock with different serpentinization degree from Soroako, Sulawesi has been conducted using X ray diffraction, thermal analysis, FTIR, and ICP-AES. The implication for the processing of these ores under acidic media was also studied. Weathering of unserpentinized peridotite in the Soroako west block produces saprolitic ore containing minerals such as relict olivine, goethite, quartz, talc with minor serpentine and smectite; whereas the weathered materials overlaying serpentinized peridotite in the Petea area are mainly composed of residual serpentine with lesser chlorite, maghemite, and remnant pyroxene and amphibole. Chemical analysis determined by ICP-AES demonstrates that west ore is higher in Si, Mg, and Ni, as compared to that Petea ore. Conversely, Fe and Al concentrations are higher in Petea ore than in west block ore. SEM-EDX examination reveals that olivine, talc, serpentine and goethite are the Ni-bearing phases occurring in west block ore; while serpentine is the principal host for Ni in the Petea ore. Chemical leaching under sulfuric acid reveals that olivine has highest dissolution rate in the west ore followed by serpentine; while talc, pyroxene, and iron oxides have slow dissolution rates. In contrast, serpentine in Petea ore is easily dissolved and is followed by chlorite; whereas amphibole, pyroxene, and maghemite are difficult to leach. Quartz is present in both ores and it seems to be undissolved during the chemical leaching. It is shown that Ni recovery from Petea saprolitic ore is higher than that of West Block ore.

Mineral paragenesis and fluid inclusions of the Bincanai epithermal silver-base metal vein at Baturappe area, South Sulawesi, Indonesia Irzal Nur; Arifudin Idrus; Subagyo Pramumijoyo; Agung Harijoko; Akira Imai
Journal of Applied Geology Vol 3, No 1 (2011)
Publisher : Geological Engineering Department Universitas Gadjah Mada

The Baturappe prospect located at southern part of Sulawesi island, Indonesia, is a hydrothermal mineralization district which is characterized by occurrence of epithermal silver-base metal deposits. The mineralization is hosted in basaltic-andesitic volcanic rocks of the late Middle-Miocene Baturappe Volcanics. More than 20 units of quartz – base metal veins are distributed in the area, and one of the most significant is the Bincanai vein. This study is aimed to characterize the mineral paragenesis and to elucidate the physicochemical conditions of the formation of the deposit on the basis of mineral assemblage and fluid inclusion mictrothermometry. Sulphide assemblages in the vein indicate an intermediate sulfidation state epithermal; beside galena and sphalerite as the early stage minerals, chalcopyrite, tennantite, and tetrahedrite are also identified as the later stage. Microthermometric study of fluid inclusions in quartz indicates formation temperature of the vein ranges from about 230 to 280°C Histogram of homogenization temperature suggests that there are two generations of hydrothermal fluid responsible for the ore mineralization in the vein; the higher temperature range represents formation temperature of the base metal (galena, sphalerite), while the lower temperature range is correlate with the precipitation of the rest relatively lower temperature sulphides (chalcopyrite, pyrite, tetrahedrite, tennantite, polybasite, and Bi-Ag-Cu-Fe-bearing sulfide). The sequence is also consistent with the mineral paragenetic. The mean of salinity (2.0–2.5 wt.% NaCl eq.) indicates that fluid responsible for the mineralization in the Bincanai vein is relatively low-salinity fluid.

Lateritization process of peridotites in Siruka, Choiseul, Solomon Islands Christopher V. Sagapoa; Akira Imai; Takeyuki Ogata; Kotaro Yonezu; Koichiro Watanabe
Journal of Applied Geology Vol 3, No 2 (2011)
Publisher : Geological Engineering Department Universitas Gadjah Mada

The lateritic weathering crusts exposed in Siruka, Choiseul Islands, Solomon Islands, were developed on the expense of serpentinized peridotite underlain by Siruka schists and Voza lavas with a subhorizontal contact. The lateritic profiles consist of three generalized zones: bedrock, saprolitic zone (weathered and decomposed zone) and the limonitic zones. The profiles demonstrate variations in depths and continuity but illustrate mineralogy and geochemical affinity down profile and are analogous to saprolitic nickel laterite deposits. Silica and magnesia in the bed rock and the saprolitic zones have been removed and only the residual elements (iron, chromium, aluminium, manganese, cobalt and nickel) remain in the limonitic zone. These elements are relatively concentrated as a result of the removal of the soluble elements. Nickel is associated with silica and magnesia, as lizardite ormixed gels (garnierite nickel ore) at the weathering fronts. On the other hand, nickel, with generally low concentrations (

MERCURY AND ARSENIC CONTAMINATION FROM SMALL SCALE GOLD MINING ACTIVITIES AT SELOGIRI AREA, CENTRAL JAVA, INDONESIA Agung Harijoko; Tin May Htun; Rodhie Saputra; I Wayan Warmada; Lucas Donny Setijadji; Akira Imai; Koichiro Watanabe
Journal of Applied Geology Vol 2, No 1 (2010)
Publisher : Geological Engineering Department Universitas Gadjah Mada

Small scale gold mines discussed here are located at Selogiri area, Central Java, Indonesia which was mined by local community mainly during gold rush in 1990s. This Selogiri gold deposit genetically is characterized by porphyry mineralization overprinted by epithermal system. The ore minerals assemblage consists of pyrite, sphalerite, chalcopyrite, galena, chalcocite and rare arsenopyrite. Chemical analysis of soil and stream sediment sampled over 1.5 km across at the Selogiri gold extraction site indicates that the site has been contaminated with mercury due to mining activities. The mercury concentrations in soil and stream sediments collected during dry season range from 0.01 to 481 ppm and 0.01 to 139 ppm, respectively, higher than background value of 0.05 ppm. In contrast, mercury concentration in stream sediments collected during rainy season from the same location as dry season sampling ranges from 0.01 to 13.42 ppm, and one sample has anomalous value of 331 ppm. This result show that rain water may disperse and decrease mercury concentration in stream sediments. In case of arsenic, although the ore contains rare arsenic minerals, arsenic concentration in bulk rock and ore is high ranging from 8 to 59 ppm, while the arsenic concentration in tailing is much higher ranging from 5.8 to 385 ppm. Chemical analyses on pyrite reveal that the pyrite grains contain arsenic and might be the source of arsenic in Selogiri mine site. However, analysis of dug-well water demonstrates that the mercury and arsenic content is still lower than the maximum allowable concentration. Keywords: Mercury, arsenic, contamination, Selogiri, gold mine

Co-Authors Agung Harijoko Arifudin Idrus Christopher V. Sagapoa I Wayan Warmada Irzal Nur, Irzal Johan Arif Koichiro Watanabe Kotaro Yonezu Lucas Donny Setijadji May Thwee Aye Naoto Araki Rodhie Saputra Subagyo Pramumijoyo Sufriadin, Sufriadin Takeyuki Ogata Tin May Htun

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