The Western Pacific is featured by its many island arc chains and marginal sea basins tectonically positioned between the Eurasia, Pacific, and India-Australian plates. These sea basins (e.g.,

This paper discusses the mineralogy, whole-rock geochemistry and elemental mass balance of the hydrothermal alteration zones within the Batu Hijau porphyry copper-gold deposit, Sumbawa Island, Indonesia. The hydrothermal alteration and mineralisation developed in four stages, namely (i) the early stage consisting of a central copper-gold-bearing biotite (potassic), proximal actinolite (inner propylitic) and the distal chloriteepidote (outer propylitic) zones; (ii) the transitional stage represented by the chlorite-sericite (intermediate argillic) zone; (iii) the late stages distinguished into the sericite-paragonite (argillic) and pyrophylliteandalusite (advanced argillic) zones; and (iv) the very late stage typified by the illite-sericite zone. In general, major elements (particularly Ca, Mg, Na and K) and some minor and rare earth elements decrease from the least altered rocks towards the late alteration zones as a consequence of the breakdown of Ca-bearing hornblende, biotite and plagioclase. Chemical discrimination by means of millicationic R1-R2 diagram indicates that R1 [4Si -11(Na + K) -2(Fe + Ti)] increases while R2 [6Ca + 2Mg + Al] decreases with increasing alteration intensity, from least-altered, through early, transitional, to late alteration zones. Rare earth elements-chondrite (C1) normalised patterns also exhibit the depletion of the elements through the subsequent alteration zones. These results are consistent with the elemental mass balance calculation using the isocon method which shows that the degree of mass and volume depletion systematically increases during alteration. A decrease of the elements as well as mass and volume from early, through transitional to late alteration stages may imply a general decrease of the element activities in hydrothermal fluids during the formation of the alteration zones.

Copper -gold mineralization at the world-class Batu Hijau porphyry deposit, Sumbawa Island, Indonesia, is closely related to the emplacement of multiple stages of tonalite porphyries. Petrographic examination indicates that at least two texturally distinct types of tonalite porphyries are currently recognized in the deposit, which are designated as "intermediate tonalite" and "young tonalite". They are mineralogically identical, consisting of phenocrysts of plagioclase, hornblende, quartz, biotite and magnetite ± ilmenite, which are set in a medium-coarse grained groundmass of plagioclase and quartz. The chemical composition of the rock-forming minerals, including plagioclase, hornblende, biotite, magnetite and ilmenite in the tonalite porphyries was systematically analyzed by electron microprobe. The chemical data of these minerals were used to constrain the crystallization conditions and fl uorine -chlorine fugacity of the corresponding tonalitic magma during its emplacement and crystallization. The crystallization conditions, including temperature (T), pressure (P) and oxygen fugacity ( f O 2 ), were calculated by applying the hornblende -plagioclase and magnetite -ilmenite thermometers and the Al-in-hornblende barometer. The thermobarometric data indicate that the tonalite porphyries were emplaced at 764 ± 22°C and 1.5 ± 0.3 × 10 5 kPa. If the pressure is assumed to be lithostatic, it is interpreted that the rim of hornblende and plagioclase phenocrysts crystallized at depths of approximately 5.5 km. As estimated from magnetite -ilmenite thermometry, the subsolidus conditions of the tonalite intrusion occurred at temperatures of 540 -590°C and log f O 2 ranging from −20 to −15 (between Ni-NiO and hematite-magnetite buffers). This occurred at relatively high f O 2 (oxidizing) condition. The fl uorine -chlorine fugacity in the magma during crystallization was determined on the basis of the chemical composition of magmatic biotite. The calculation indicates that the fl uorine -chlorine fugacity, represented by log ( f H 2 O)/ ( f HF) and ( f H 2 O)/( f HCl) in the corresponding tonalitic magma range from 4.31 to 4.63 and 3.62 to 3.79, respectively. The chlorine fugacity (HCl) to water (H 2 O) is relatively higher than the fl uorine fugacity (HF to water), refl ecting a high activity of chlorine in the tonalitic magma during crystallization. The relatively higher activity of chlorine (rather than fl uorine) may indicate the signifi cant role of chloride complexes (CuCl 2 − and AuCl 2 − ) in transporting and precipitating copper and gold at the Batu Hijau deposit.

Batu Hijau is a world-class gold-rich porphyry copper deposit, situated in Sumbawa Island, Indonesia. Deep drilling indicates that several intervals of calc-silicate rock were intersected, where they are apparently interbedded with volcaniclastic rocks. The calc-silicate rocks occur at the contact with copper-gold-bearing tonalite porphyries. The rocks are fine-grained and granular with green, reddish-brown and white layers. The green layers consist mostly of fine-grained clinopyroxene (diopside and hedenbergite) and the reddish-brown layers consist mostly of garnet (andradite), whereas the white layers are commonly composed of calcite and zeolite (chabazite). The calc-silicate rocks were formed by contact metasomatism of andesitic volcaniclastic rocks, as it is calcic in composition. Paragenesis study reveals at least two stages of calc-silicate mineralization. Stage 1 (prograde) is characterized by the presence of garnet (andradite), clinopyroxene (diopside and hedenbergite), anorthite and quartz at 340-360 C (high salinity 35-45 NaCl wt percentage eqn.). Stage 2 (retrograde) is characterized by chlorite and rare epidote at 280-300 C (low salinity 1-10 NaCl wt% eqn.). Late calcite Ϯ quartz veinlets and calcite + chabazite veins/veinlets may also be related to this stage and cross cut the oldest mineral assemblages. Mineralization (magnetite, chalcopyrite and pyrite) may occur during the retrograde stage. Clinopyroxene and garnet were modified by Fe-rich hydrothermal fluid (oxidizing condition) indicated by increase of Fe from core to rim of both the cogenetic minerals. The presence of the calc-silicate rocks associated with massive magnetite-chalcopyrite-pyrite assemblage indicates the occurrence of calcic-exoskarn surrounding the Batu Hijau porphyry copper-gold deposit.

Many terrestrial silicate reservoirs display a characteristic depletion in Nb, which has been explained in some studies by the presence of reservoirs on Earth with superchondritic Nb/Ta. As one classical example, K-rich lavas from the Sunda rear-arc, Indonesia, have been invoked to tap such a high-Nb/Ta reservoir. To elucidate the petrogenetic processes active beneath the Java rear-arc and the causes for the superchondritic Nb/Ta in some of these lavas, we studied samples from the somewhat enigmatic Javanese rear-arc volcano Muria, which allow conclusions regarding the across-arc variations in volcanic output, source mineralogy and subduction components. We additionally report some data for an along-arc sequence of lavas from the Indonesian part of the Sunda arc, extending from Krakatoa in the west to the islands of Bali and Lombok in the east. We present major and trace element concentrations, Sr–Nd–Hf–Pb isotope compositions, and high-field-strength element (HFSE: Nb, Ta, Zr, Hf, W) concentrations obtained via isotope dilution and MC-ICP-MS analyses. The geochemical data are complemented by melting models covering different source compositions with slab melts formed at variable P–T conditions. The radiogenic isotope compositions of the frontal arc lavas in combination with their trace element systematics confirm previously established regional variations of subduction components along the arc. Melting models show a clear contribution of a sediment-derived component to the HFSE budget of the frontal arc lavas, particularly affecting Zr–Hf and W. In contrast, the K-rich rear-arc lavas tap more hybrid and enriched mantle sources. The HFSE budget of the rear-arc lavas is in particular characterized by superchondritic Nb/Ta (up to 25) that are attributed to deep melting involving overprint by slab melts formed from an enriched garnet–rutile-bearing eclogitic residue. Sub-arc slab melting was potentially triggered along a slab tear beneath the Sunda arc, which is the result of the forced subduction of an oceanic basement relief ~ 8 Myr ago as confirmed by geophysical studies. The purported age of the slab tear coincides with a paucity in arc volcanism, widespread thrusting of the Javanese basement crust as well as the short-lived nature of the K-rich rear-arc volcanism at that time.

No abstract

-This paper discusses characteristics of some key features of the primary Buru gold deposit as a tool for a better understanding of the deposit genesis. Currently, about 105,000 artisanal and small-scale gold miners (ASGM) are operating in two main localities, i.e. Gogorea and Gunung Botak by digging pits/shafts following gold-bearing quartz vein orientation. The gold extraction uses mercury (amalgamation) and cyanide processing. The field study identifies two types/generations of quartz veins namely (1) Early quartz veins which are segmented, sigmoidal, discontinous, and parallel to the foliation of host rock. The quartz vein is lack of sulfides, weak mineralized, crystalline, relatively clear, and maybe poor in gold, and (2) Quartz veins occurred within a 'mineralized zone' of about 100 m in width and ~1,000 m in length. The gold mineralization is strongly overprinted by an argillic alteration zone. The mineralization-alteration zone is probably parallel to the mica schist foliation and strongly controlled by N-S or NE-SW-trending structures. The gold-bearing quartz veins are characterized by banded texture particularly colloform following host rock foliation and sulphide banding, brecciated, and rare bladed-like texture. The alteration types consist of propylitic (chlorite, calcite, sericite), argillic, and carbonation represented by graphite banding and carbon flakes. The ore mineralization is characterized by pyrite, native gold, pyrrhotite, and arsenopyrite. Cinnabar, stibnite, chalcopyrite, galena, and sphalerite are rare or maybe absent. In general, sulphide minerals are rare (<3%). Fifteen rock samples were collected in Wamsaid area for geochemical assaying for Au, Ag, As, Sb, Hg, Cu, Pb, and Zn. Eleven of fifteen samples yielded more than 1.00 g/t Au, in which six of them are in excess of 3.00 g/t Au. It can be noted that all high-grade samples are originally or containing limonitic materials, that suggest the role of supergene enrichment. Interestingly, most of the high-grade samples contain also high grade As (up to 991ppm), Sb (up to 885 ppm), and Hg (up to 75 ppm). Fluid inclusions in both quartz vein types consist of four phases including L-rich, V-rich, L-V-rich, and L1-L2-V (CO 2 )-rich phases. Mineralizing hydrothermal fluid is typified by CO 2 -rich fluid, moderate temperature of 300 -400 ºC and a typical low salinity (0.36 to 0.54 wt.% NaCl eq). Based on those key features, gold mineralization in Buru Island meets the characteristics of LS epithermal or orogenic gold deposit types; however, it tends to be fitter with orogenic gold deposit rather than another type.

This study is dealing with geology and characteristics of mineralogy, geochemistry and physicochemical conditions of hydrothermal fluid respon-