Showing papers in "Resource Geology in 2007"
TL;DR: In this article, chemical analysis of biotite in representative granitic rocks in Japan shows that the total Al (TAl) content changes with the metal type of the accompanying hydrothermal ore deposits and increases in the following order: Pb-Zn and Mo deposits < Cu-Fe and Sn deposits < W deposits < non-mineralized granitic rock.
Abstract: Chemical analysis of biotite in representative granitic rocks in Japan shows that the total Al (TAl) content changes with the metal type of the accompanying hydrothermal ore deposits and increases in the following order: Pb-Zn and Mo deposits < Cu-Fe and Sn deposits < W deposits < non-mineralized granitic rocks. The TAl content of biotite in granitic rocks may be a useful indicator for distinguishing between mineralized and non-mineralized granitic rocks. A good positive correlation is seen between the TAl content of biotite and the solidification pressure of the granitic rocks estimated by sphalerite and hornblende geobarometers and the mineral assemblages of the surrounding rocks. These facts suggest that the TAl content of biotite can be used to estimate the solidification pressure (P) of the granitic rocks. The following empirical equation was obtained:
where TAl designates the total Al content in biotite on the basis O = 22. According to the obtained biotite geobarometer, it is estimated that Pb-Zn and Mo deposits were formed at pressures below 1 kb, Cu-Fe and Sn deposits at 1–2 kb, W deposits at 2–3 kb and non-mineralized granitic rocks were solidified at pressures above 3 kb.
167 citations
TL;DR: In this article, the authors analyzed gangue calcite to constrain the timing of mineralization in the Huize ore field, which consists of the Kuangshanchang and Qilinchang deposits.
Abstract: The Huize ore field, which is the most famous high-grade Mississippi Valley-type Zn-Pb ore field in China, consists of the Kuangshanchang and Qilinchang deposits. The Sm-Nd isotopic compositions of gangue calcite were analyzed to constrain the timing of mineralization. Eight calcite samples from the No. 6 orebody in the Qilinchang deposit have Sm and Nd concentrations of 1.82–25.93 and 15.25–79.02 ppm, respectively, and yielded an age of 225 ± 9.9 Ma with epsilon Nd =−10.6 and MSWD = 0.13. Five calcites from the No. 1 orebody in the Kuangshanchang deposit contain 2.37–2.90 ppm Sm and 8.18–16.85 ppm Nd, and yielded an age of 228 ± 16 Ma, with epsilon Nd =−10.6 and MSWD = 0.28. These two ages are close to the age of the Emeishan flood basalt, which is ca 255 Ma, and agree with the reported 40Ar/39Ar plateau and isochron ages of the native copper mineralization related to the Emeishan flood basalt in this region, which are 226–228 Ma. Moreover, previous study shows that the magnitude of uplift resulting from the Emeishan flood basalts is >1000 m. It indicates that the Kuangshanchang and the Qilinchang deposit formed during the same geological event and originated by fluid migration during uplift resulting from the Emeishan flood basalt.
62 citations
TL;DR: In this article, the chemical properties of the tonalite porphyries were analyzed by electron microprobe and 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.
Abstract: 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.
51 citations
TL;DR: The Dexing porphyry mine as discussed by the authors is located in a NE-trending magmatic belt along the southeastern margin of the Yangtze Craton and contains 1168 Mt of ores with 0.5% Cu and 0.01% Mo.
Abstract: The Dexing deposit is located in a NE-trending magmatic belt along the southeastern margin of the Yangtze Craton. It is the largest porphyry copper deposit in China, consisting of three porphyry copper orebodies of Zhushahong, Tongchang and Fujiawu from northwest to southeast. It contains 1168 Mt of ores with 0.5% Cu and 0.01% Mo. The Dexing deposit is hosted by Middle Jurassic granodiorite porphyries and pelitic schist of Proterozoic age. The Tongchang granodiorite porphyry has a medium K cal-alkaline series, with medium K2O content (1.94–2.07 wt%), and low K2O/(Na2O + K2O) (0.33–0.84) ratios. They have high large-ion lithophile elements, high light rare-earth elements, and low high-field-strength elements. The hydrothermal alteration at Tongchang is divided into four alteration mineral assemblages and related vein systems. They are early K-feldspar alteration and A vein; transitional (chlorite + illite) alteration and B vein; late phyllic (quartz + muscovite) alteration and D vein; and latest carbonate, sulfate and oxide alteration and hematite veins. Primary fluid inclusions in quartz from phyllic alteration assemblage include liquid-rich (type 1), vapor-rich (type 2) and halite-bearing ones (type 3). These provide trapping pressures of 20–400 ´ 105 Pa of fluids responsible for the formation of D veins. Igneous biotite from least altered granochiorite porphyry and hydrothermal muscovite in mineralized granodiorite porphyry possess δ18O and δD values of 4.6‰ and −87‰ for biotite and 7.1–8.9‰, −71 to −73‰ for muscovite. Stable isotopic composition of the hydrothermal water suggests a magmatic origin. The carbon and oxygen isotope for hydrothermal calcite are −4.8 to −6.2‰ and 6.8–18.8‰, respectively. The δ34S of pyrite in quartz vein ranges from −0.1 to 3‰, whereas δ34S for chalcopyrite in calcite veins ranges from 4 to 5‰. These are similar to the results of previous studies, and suggest a magmatic origin for sulfur. Results from alteration assemblages and vein system observation, as well as geochemical, fluid inclusion, stable isotope studies indicate that the involvement of hydrothermal fluids exsolved from a crystallizing melt are responsible for the formation of Tongchang porphyry Cu-Mo orebodies in Dexing porphyry deposit.
43 citations
TL;DR: The Qolqoleh gold deposit as discussed by the authors is located in the northwestern part of the Sanandai-Sirjan Zone, northwest of Iran and is almost entirely confined to a series of steeply dipping ductile-brittle shear zones generated during Late Cretaceous-Tertiary continental collision between the Afro-Arabian and the Iranian microcontinent.
Abstract: The Qolqoleh gold deposit is located in the northwestern part of the Sanandai-Sirjan Zone, northwest of Iran. Gold mineralization in the Qolqoleh deposit is almost entirely confined to a series of steeply dipping ductile–brittle shear zones generated during Late Cretaceous–Tertiary continental collision between the Afro-Arabian and the Iranian microcontinent. The host rocks are Mesozoic volcano-sedimentary sequences consisting of felsic to mafic metavolcanics, which are metamorphosed to greenschist facies, sericite and chlorite schists. The gold orebodies were found within strong ductile deformation to late brittle deformation. Ore-controlling structure is NE–SW-trending oblique thrust with vergence toward south ductile–brittle shear zone. The highly strained host rocks show a combination of mylonitic and cataclastic microstructures, including crystal–plastic deformation and grain size reduction by recrystalization of quartz and mica. The gold orebodies are composed of Au-bearing highly deformed and altered mylonitic host rocks and cross-cutting Au- and sulfide-bearing quartz veins. Approximately half of the mineralization is in the form of dissemination in the mylonite and the remainder was clearly emplaced as a result of brittle deformation in quartz–sulfide microfractures, microveins and veins. Only low volumes of gold concentration was introduced during ductile deformation, whereas, during the evident brittle deformation phase, competence contrasts allowed fracturing to focus on the quartz–sericite domain boundaries of the mylonitic foliation, thus permitting the introduction of auriferous fluid to create disseminated and cross-cutting Au-quartz veins. According to mineral assemblages and alteration intensity, hydrothermal alteration could be divided into three zones: silicification and sulfidation zone (major ore body); sericite and carbonate alteration zone; and sericite–chlorite alteration zone that may be taken to imply wall-rock interaction with near neutral fluids (pH 5–6). Silicified and sulfide alteration zone is observed in the inner parts of alteration zones. High gold grades belong to silicified highly deformed mylonitic and ultramylonitic domains and silicified sulfide-bearing microveins. Based on paragenetic relationships, three main stages of mineralization are recognized in the Qolqoleh gold deposit. Stage I encompasses deposition of large volumes of milky quartz and pyrite. Stage II includes gray and buck quartz, pyrite and minor calcite, sphalerite, subordinate chalcopyrite and gold ores. Stage III consists of comb quartz and calcite, magnetite, sphalerite, chalcopyrite, arsenopyrite, pyrrhotite and gold ores. Studies on regional geology, ore geology and ore-forming stages have proved that the Qolqoleh deposit was formed in the compression–extension stage during the Late Cretaceous–Tertiary continental collision in a ductile–brittle shear zone, and is characterized by orogenic gold deposits.
37 citations
TL;DR: Baguio, in the Central Cordillera of Northern Luzon, is a district that displays porphyry copper and epithermal gold mineralization, associated with Early Miocene - Pliocene - Quaternary calc-alkaline and adakitic intrusions.
Abstract: Baguio, in the Central Cordillera of Northern Luzon, is a district that displays porphyry copper and epithermal gold mineralization, associated with Early Miocene - Pliocene - Quaternary calc-alkaline and adakitic intrusions. Systematic sampling, K-Ar dating, major and trace elements, and Sr, Nd, Pb isotopic analyses of fresh magmatic rocks indicate three magmatic pulses: an Early Miocene phase (21.2 - 18.7 Ma), a Middle - Late Miocene phase (15.3 - 8 Ma) and fi nally a Pliocene - Quaternary event (3 - 1 Ma). The fi rst phase emplaced evolved calc-alkaline magmas, essentially within the Agno Batholith complex, and is thought to be related to the westward-dipping subduction of the West Philippine Basin. After a quiescence period during which the Kennon limestone was deposited, magmatic activity resumed at 15.3 Ma, in connection with the start of the subduction of the South China Sea along the Manila Trench. It emplaced fi rst petrogenetically related and relatively unradiogenic low-K calc-alkaline lavas and intermediate adakites. Temporal geochemical patterns observed from 15.3 to 1 Ma include progressive enrichment in K and other large ion lithophile elements, increase in radiogenic Sr and Pb and corresponding decrease in radiogenic Nd. These features are thought to refl ect the progressive addition to the Luzon arc mantle wedge of incompatible elements largely inherited from South China Sea sediments. The origin of the long quiescence period, from 8 to 3 Ma, remains problematic. It might represent a local consequence of the docking of the Zambales ophiolitic terrane to Northern Luzon. Then, magmatic activity resumed at 3 Ma, emplacing chemically diversifi ed rocks ranging from low K to high K and including a large proportion of adakites, especially during the Quaternary (dacitic plugs). The authors tentatively relate this diversity to the development of a slab tear linked with the subduction of the fossil South China Sea ridge beneath the Baguio area.
34 citations
TL;DR: The Yinshan mine in Jiangnan tectonic belt in South China consists of Pb-Zn-Ag and Cu-Au ore bodies as mentioned in this paper, which are similar to that of the porphyry Cu mineralization at Dexing in the south-east of China.
Abstract: The Yinshan deposit in the Jiangnan tectonic belt in South China consists of Pb-Zn-Ag and Cu-Au ore bodies. This deposit contains approximately 83 Mt of the Cu-Au ores at 0.52% Cu and 0.8 g/t Au, and 84 Mt of the Pb-Zn-Ag ores at 1.25% Pb, 1.02% Zn and 33.3 g/t Ag. It is hosted by low-grade metamorphosed sedimentary rocks and mafic volcanic rocks of the lower Mesoproterozoic Shuangqiaoshan Group, and continental volcanic rocks of the Jurassic Erhuling Group and dacitic subvolcanic rocks. The ore bodies mainly consist of veinlets of sulfide minerals and sulfide-disseminated rocks, which are divided into Cu-Au and Pb-Zn-Ag ore bodies. The Cu-Au ore bodies occur in the area close to a dacite porphyry stock (No. 3 stock), whereas Pb-Zn-Ag bodies occur in areas distal from the No. 3 stock. Muscovite is the main alteration mineral associated with the Cu-Au ore bodies, and muscovite and chlorite are associated with the Pb-Zn-Ag ores. A zircon sensitive high-resolution ion microprobe U-Pb age from the No. 3 dacite stock suggests it was emplaced in Early Jurassic. Three 40Ar-39Ar incremental-heating mineral ages from muscovite, which are related to Cu-Au and Pb-Zn-Ag mineralization, yielded 179–175 Ma. These muscovite ages indicate that Cu-Au mineralization occurred at 178.2±1.4 Ma (2σ), and Pb-Zn-Ag mineralization at 175.4±1.2 Ma (2σ) and 175.3±1.1 Ma (2σ), which supports a restricted period for the mineralization. The Early Jurassic ages for the mineralization at Yinshan are similar to that of the porphyry Cu mineralization at Dexing in Jiangnan tectonic belt, and suggest that the polymetallic mineralization occurred in a regional transcompressional tectonic regime.
33 citations
TL;DR: The first three pulses correspond to the postcollision period between the North China and Yangtze cratons, an extension period during late-stage rotation of the principal compressional stress from N-S to E-W, and a dramatic thinning period of the lithosphere.
Abstract: Mesozoic epithermal gold deposits in eastern China are divided into calc-alkaline and alkaline magma-related gold deposits, and are also grouped as low-sulfidation, intermediate-sulfidation and high-sulfidation types, of which the first two predominate. These gold deposits are distributed in the Tianshan–Yinshan–Great Xing'anling Variscan fold belt of North China craton, Qinling-Dabie Indo-Sinian fold belt of Yangtze craton, and South China fold belt or Cathaysian block, from north to south along the eastern China continent. Most of the epithermal gold orebodies are hosted either in volcanic rocks or their related granitoids, and volcanic breccia pipes. These orebodies are mainly associated with adularia–chalcedony–sericite, and alunite–kaolinite–quartz alteration. These orebodies formed in four mineralization pulses at 175, 145–135, 127–115, and 110–94 Ma. The first three pulses correspond to the post-collision period between the North China and Yangtze cratons, an extension period during late-stage rotation of the principal compressional stress from N-S to E-W, and a dramatic thinning period of the lithosphere, respectively. The last mineralizing pulse was the result of another extension in South China. Although the mineralizing pulses occurred at different times, they all occurred in extensional settings and were accompanied by crust and the mantle interaction.
25 citations
TL;DR: The Penjom gold deposit lies on the eastern side of the Raub-Bentong Suture line within the Central Belt of Permo-Triassic rocks, near Kuala Lipis, Pahang, Malaysia.
Abstract: The Penjom gold deposit lies on the eastern side of the Raub-Bentong Suture line within the Central Belt of Permo-Triassic rocks, near Kuala Lipis, Pahang, Malaysia. The geology of the deposit is dominated by a sequence of fine- to coarse-grained rhyolitic to rhyodacitic tuff, tuff-breccia and a minor rhyolitic–rhyodacitic volcanic series, associated with argillaceous marine sedimentary rocks consisting of shale with subordinate shalely limestone of Padang Tungku Formation and Pahang Volcanic Series. Fine- to coarse-grained tonalite and quartz porphyry intruded this unit. The main structural features of the area are north–south-trending left-lateral strike-slip faults and their subsidiaries, which generally strike north–south and dip moderately to the east (350°–360°/40°–60°). Mineralization at the Penjom gold deposit is structurally controlled and also erratic laterally and vertically. The gold mineralization can be categorized as (i) gold associated with carbonate-rich zones hosted within dilated quartz veins carrying significant amount of sulfides; (ii) gold disseminated within stockwork of quartz–carbonate veins affiliated with tonalite; and (iii) gold often associated with arsenopyrite and pyrite in quartz–carbonate veins and stringers hosted within shear zones of brittle–ductile nature in all rock types and in brittle fractured rhyodacitic volcanic rocks. Sphalerite, chalcopyrite, tetrahedrite and pyrrhotite are the minerals accompanying the early stage of gold mineralization. These minerals also suffered from local brittle deformation. However, most of the gold mineralization took place after the deposition of these sulfides. Galena appears somewhat towards the end of gold mineralization, whereas tellurium and bismuth accompanied gold contemporaneously. The gold mineralization occurred most probably due to the metamorphogenic deformational origin concentrated mostly in the shear zone. The mineralization is strongly controlled by the wall rock (e.g. graphitic shale), the sulfide minerals and fluid–rock interaction.
23 citations
TL;DR: The Pongkor gold-silver mine is the largest low-sulfi dation epithermal precious metal deposit in Indonesia, and is of Pliocene age.
Abstract: The Pongkor gold – silver deposit is the largest low-sulfi dation epithermal precious metal deposit in Indonesia, and is of Pliocene age. The deposit consists of nine major subparallel quartz – adularia – carbonate veins with very low sulfi de content. Vein infi ll records fi ve paragenetic sequences, dominated by calcite in the early stage and quartz in the later stage of the hydrothermal evolution. Fluid inclusions in hydrothermal calcite and quartz of all stages indicate a temperature ranging from 180 to 220°C and a meteoric water origin (very low salinity close to 0 wt% NaCl equivalent). Carbon isotope data on calcite display a narrow range from �6.5 to �3.0 ‰ 13 C. The oxygen isotope values have a wider range of +4.6 to +10.1 ‰ 18 O. The broadly positive correlation of the 13 C versus 18 O plot suggests that the carbon species, which equilibrated during the formation of calcite, is dominated by H 2 CO 3 not far from equilibrium with HCO 3 � . The abundance of rare earth and yttrium (REY) in carbonate samples is very low ( REY mostly 250°C.
21 citations
TL;DR: In this paper, an exploration of epithermal gold mineralization in the Cibaliung area, southwestern part of Java Island, Indonesia, has been conducted and the authors have estimated a mineral resource of approximately 1.3 million tonnes at 10.42g/t gold and 60.7 g/t silver.
Abstract: Middle Miocene (11.18–10.65 Ma) low sulfidation-type epithermal gold mineralization occurred in the Cibaliung area, southwestern part of Java Island, Indonesia. It is hosted by andesitic to basaltic andesitic lavas of the Middle Miocene Honje Formation (11.4 Ma) and is covered by Pliocene Cibaliung tuff (4.9 Ma). The exploration estimates mineral resource of approximately 1.3 million tonnes at 10.42 g/t gold and 60.7 g/t silver at a 3 g/t Au cut-off. This equates to approximately 435,000 ounces of gold and 2.54 million ounces of silver. That resource resulted from two ore shoots: Cibitung and Cikoneng. Studies on ore mineralogy, hydrothermal alteration, geology, fluid inclusion, stable isotopes and age dating were conducted in order to characterize the deposit and to understand a possible mechanism of preservation of the deposit. The ore mineral assemblage of the deposit consists of electrum, naumannite, Ag-Se-Te sulfide minerals, chalcopyrite, pyrite, sphalerite and galena. Those ore minerals occur in quartz veins showing colloform–crustiform texture. They are enveloped by mixed layer clay illite/smectite zone, which grades into smectite zone outward. The temperature of mineralization revealed by fluid inclusion study on quartz in the veins ranges from 170 and 220°C at shallow and deep level, respectively. The temperature range is in agreement with the temperature deduced from the hydrothermal alteration mineral assemblage including mixed layered illite/smectite and laumontite. The mineralizing fluid is dilute, with a salinity <1 wt% NaCl equivalent and has stable isotopes of oxygen and hydrogen composition indicating a meteoric water origin. Although the deposit is old enough that it would have been eroded in a tropical island arc setting, the coverage by younger volcanic deposits such as the Citeluk tuff and the Cibaliung tuff most probably prevented this erosion.
TL;DR: The Selogiri area, situated in Wonogiri regency, Central Java, is one of several gold prospecting areas in the Southern areas Mountain Range in Java, Indonesia as mentioned in this paper.
Abstract: The Selogiri area, situated in Wonogiri regency, Central Java, is one of several gold prospecting areas in the Southern areas Mountain Range in Java, Indonesia. Three types of dioritic–andesitic intrusive rocks occur in the Selogiri area, namely, hornblende andesite porphyry, hornblende diorite porphyry and hornblende diorite, exposed in a half-circular depression where volcanic breccia and tuff are widely distributed. The occurrence of stockwork quartz veinlets and associated with magnetite and malachite coating along the cracks in the diorite porphyry suggests porphyry type mineralization. This is also supported by the occurrence of polyphase hypersaline fluid inclusions in the stockwork veinlet quartz. Small-scale miners are mining NS-trending quartz veins for gold associated with base metal sulfides. These veins are probably epithermal-type mineralization that overprinted porphyry-type mineralization. The Neogene intermediate to silicic hydrous magmatism in Java could have formed the porphyry-type mineralization in Selogiri, as in the rest of the Sunda–Banda arc.
Abstract: The Asachinskoe epithermal Au-Ag deposit is a representative low-sulfidation type of deposit in Kamchatka, Russia. In the Asachinskoe deposit there are approximately 40 mineralized veins mainly hosted by dacite–andesite stock intrusions of Miocene–Pliocene age. The veins are emplaced in tensional cracks with a north orientation. Wall-rock alteration at the bonanza level (170–200 m a.s.l.) consists of the mineral assemblage of quartz, pyrite, albite, illite and trace amounts of smectite. Mineralized veins are well banded with quartz, adularia and minor illite. Mineralization stages in the main zone are divided into stages I–IV. Stage I is relatively barren quartz–adularia association formed at 4.7 ± 0.2 Ma (K-Ar age). Stage II consists of abundant illite, Cu-bearing cryptomelane and other manganese oxides and hydroxides, electrum, argentite, quartz, adularia and minor rhodochrosite and calcite. Stage III, the main stage of gold mineralization (4.5–4.4 ± 0.1–3.1 ± 0.1 Ma, K-Ar age), consists of a large amount of electrum, naumannite and Se-bearing polybasite with quartz–adularia association. Stage IV is characterized by hydrothermal breccia, where electrum, tetrahedrite and secondary covellite occur with quartz, adularia and illite. The concentration of Au+Ag in ores has a positive correlation with the content of K2O + Al2O3, which is controlled by the presence of adularia and minor illite, and both Hg and Au also have positive correlations with the light rare-earth elements. Fluid inclusion studies indicate a salinity of 1.0–2.6 wt% NaCl equivalent for the whole deposit, and ore-forming temperatures are estimated as approximately 160–190°C in stage III of the present 218 m a.s.l. and 170–180°C in stage IV of 200 m a.s.l. The depth of ore formation is estimated to be 90–400 m from the paleo-water table for stage IV of 200 m a.s.l., if a hydrostatic condition is assumed. An increase of salinity (>CNaCl≈ 0.2 wt%) and decrease of temperature (>T ≈ 30°C) within a 115-m vertical interval for the ascending hydrothermal solution is calculated, which is interpreted as due to steam loss during fluid boiling. Ranges of selenium and sulfur fugacities are estimated to be logfSe2 = −17 to −14.5 and logfS2 = −15 to −12 for the ore-forming solution that was responsible for Au-Ag-Se precipitation in stage III of 200 m a.s.l. Separation of Se from S-Se complex in the solution and its partition into selenides could be due to a relatively oxidizing condition. The precipitation of Au-Ag-Se was caused by boiling in stage III, and the precipitation of Au-Ag-Cu was caused by sudden decompression and boiling in stage IV.
TL;DR: The Pongkor gold-silver mine is located at the northeastern flank of the Bayah dome, which is a product of volcanism in the Sunda-Banda Arc.
Abstract: The Pongkor gold–silver mine is situated at the northeastern flank of the Bayah dome, which is a product of volcanism in the Sunda–Banda Arc. The hydrothermal alteration minerals in the Ciurug–Cikoret area are typical of those formed from acid to near-neutral pH thermal waters. On the surface, illite/smectite mixed layer mineral (I/Sm), smectite and kaolinite, and spotting illite, I/Sm and K-feldspar alteration occur at the top of the mineralized zone. Silicification, K-feldspar and I/Sm zones are commonly formed in the wall rock, and gradually grade outwards into a propylitic zone. The mineralization of precious metal ore zone is constrained by fluid temperatures between 180 and 220°C, and with low salinity (<0.2 wt% NaCl equivalent) and boiling condition. The minimum depth of vein formation below the paleo-water table is approximately 90–130 m for the hydrostatic column. Hydrogen and oxygen isotope data for quartz and calcite show relatively homogeneous fluid composition (−53 to −68‰δD and −5.7 to +0.3‰δ18O H2O). There is no specific trend in the data with respect to the mineralization stages and elevation, which suggests that the ore-forming fluids did not significantly change spatially during the vein formation. The stable isotope data indicate mixing between the hydrothermal fluids and meteoric water and interaction between the hydrothermal fluids and the host rock.
TL;DR: The Kingking deposit is a gold-rich porphyry copper deposit and the southernmost deposit at the eastern Mindanao mineralized belt, Philippines as discussed by the authors, where two dominant copper minerals, bornite and chalcopyrite, are associated with fine crystalline quartz veinlet stockworks in the mineralized diorites.
Abstract: The Kingking deposit is a gold-rich porphyry copper deposit and the southernmost deposit at the eastern Mindanao mineralized belt, Philippines. It is underlain by Cretaceous–Paleogene sedimentary and volcanic rocks that are intruded by mineralized Miocene diorite porphyries and by barren Miocene–Pliocene dacite and diorite porphyries. The main alteration zones in the deposit are the inner potassic zone and the outer propylitic zone. The biotite-bearing diorite and hornblende diorite porphyries are the primary host rocks of mineralization. Two dominant copper minerals, bornite and chalcopyrite, which usually occur as fracture fillings, are associated with fine crystalline quartz veinlet stockworks in the mineralized diorites. Minor secondary covellite, chalcocite and digenite are also observed. The primary Cu-Fe sulfide phases initially deposited from ore fluids consisted of bornite solid solution (bnss) and intermediate solid solution (iss), which decomposed to form the bornite and chalcopyrite. Peculiar bornite pods that are different from dissemination and are associated with volcanic rock xenoliths in biotite-bearing diorite porphyry are noted in a drill hole. These pods of bornite are not associated with quartz veinlet stockworks. Fluid inclusion analyses show three types of inclusions contained in Kingking samples: two-phase fluid-rich and vapor-rich inclusions and polyphase hypersaline inclusions from porphyry-type quartz veinlet stockworks. The liquid–vapor homogenization temperatures (TH) and the dissolution temperature of halite daughter crystals (TM) from the polyphase hypersaline inclusions predominantly range from 400°C up to >500°C. The wide range of TH and TM may be due to heterogeneous trapping of variable ratios of vapor and brine. For some inclusions, TH > TM and in some cases, TH < TM, indicating that some of the brine was supersaturated or saturated with NaCl at the time of entrapment. Calculated salinity of the polyphase hypersaline inclusions ranges from 40 to 60% NaCl equivalent. Temperature and vapor pressure of mineralized fluid were estimated to be 400°C and 16 MPa.
TL;DR: The Jusa and Barsuchi Log volcanogenic massive sulfide (VMS) deposits formed along a paleo island arc in the east Magnitogrosk zone of the Southern Urals between ca 398 and 390 Ma as discussed by the authors.
Abstract: The Jusa and Barsuchi Log volcanogenic massive sulfide (VMS) deposits formed along a paleo island arc in the east Magnitogrosk zone of the Southern Urals between ca 398 and 390 Ma. By analogy with the VMS deposits of the west Magnitogrosk zone, they are considered to be Baimak type deposits, which are Zn-Cu-Ba deposits containing Au, Ag and minor Pb. Detailed mapping and textural analysis of the two deposits shows that they formed as submarine hydrothermal mounds which were subsequently destroyed on the sea floor under the influence of ocean bottom currents and slumping. Both deposits display a ratio of the length to the maximum width of the deposit >15 and are characterized by ribbon-like layers composed mainly of bedded ore and consisting principally of altered fine clastic ore facies. The Jusa deposit appears to have formed in two stages: deposition of colloform pyrite followed by deposition of copper–zinc–lead sulfides characterized by the close association of pyrite, chalcopyrite, sphalerite, galena, tennantite, arsenopyrite, marcasite, pyrrhotite, bornite, native gold and electrum and high concentrations of gold and silver. The low metamorphic grade of the east Magnitogorsk zone accounts for the exceptional degree of preservation of these deposits.
TL;DR: The Sawayaerdun gold mine in Wuqia county, Southwest Tianshan, China as mentioned in this paper has been identified as a gold mine with an inferred resource of 123 tonnes.
Abstract: The Sawayaerdun gold deposit, located in Wuqia County, Southwest Tianshan, China, occurs in Upper Silurian and Lower Devonian low-grade metamorphic carbonaceous turbidites. The orebodies are controlled by a series of NE-NNE-trending, brittle – ductile shear zones. Twenty-four gold mineralized zones have been recognized in the Sawayaerdun ore deposit. Among these, the up to 4-km-long and 200-m wide No. IV mineralized zone is economically the most important. The average gold grade is 1 – 6 g/t. Gold reserves of the Sawayaerdun deposit have been identifi ed at approximately 37 tonnes and an inferred resource of 123 tonnes. Hydrothermal alteration is characterized by silicifi cation, pyritization, arsenopyritization, sericitization, carbonatization and chloritization. On the basis of fi evidence and petrographic analysis, fi ve stages of vein emplacement and hydrothermal mineralization can be distinguished: stage 1, early quartz stage, characterized by the occurrence of quartz veins; stage 2, arsenopyrite – pyrite – quartz stage, characterized by the formation of auriferous quartz veinlets and stockworks; stage 3, polymetallic sulfi de quartz stage, characterized by the presence of auriferous polymetallic sulfi de quartz veinlets and stockworks; stage 4, antimony – quartz stage, characterized by the formation of stibnite – jamesonite quartz veins; and stage 5, quartz – carbonate vein stage. Stages 2 and 3 represent the main gold mineralization, with stage 4 representing a major antimony mineralization episode in the Sawayaerdun deposit. Two types of flinclusion, namely H 2 O – NaCl and H 2 O – CO 2 – NaCl types, have been recognized in quartz and calcite. Aqueous inclusions show a wide range of homogenization temperatures from 125 to 340°C, and can be correlated with the mineralization stage during which the inclusions formed. Similarly, salinities and densities of these fl uids range for each stage of mineralization from 2.57 to 22 equivalent wt% NaCl and 0.76 to 1.05 g/cm 3 , respectively. The ore-forming fl uids thus are representative of a medium- to low-temperature, lowto medium-salinity H 2 O – NaCl – CO 2 – CH 4 – N 2 system. The 34 S CDT values of sulfi des associated with mineralization fall into a narrow range of �3.0 to +2.6 ‰ with a mean of +0.1 ‰ . The 13 C PDB values of dolomite and siderite from the Sawayaerdun gold deposit range from �5.4 to �0.6 ‰ , possibly refl ecting derivation of the carbonate carbon from a mixed magmatic/sedimentary source. Changes in physico-chemical conditions and composition of the hydrothermal fl uids, water – rock exchange and immiscibility of hydrothermal fl uids are inferred to have played important roles in the ore-forming process of the Sawayaerdun gold – antimony deposit.
TL;DR: The Yerranderie epithermal silver-gold-base metal vein system has significant mineral and alteration zonation as discussed by the authors, which is part of a fault-controlled, intermediate sulfidation, epithermically silver-and gold-base-metal vein system.
Abstract: Felsic volcanic units of the Early Devonian Bindook Volcanic Complex host the Yerranderie epithermal silver–gold–lead district 94 km west–southwest of Sydney. Mineralization in the district forms part of a fault-controlled, intermediate sulfidation, epithermal silver–gold–base metal vein system that has significant mineral and alteration zonation. Stage 1 of the mineral paragenesis in the veins developed quartz and carbonate with early pyrite, whereas stage 2 is a crustiform banded quartz–pyrite–arsenopyrite assemblage. Stage 3, the main stage of sulfide deposition, comprises early sphalerite, followed by a tetrahedrite–tennantite–gold assemblage, then a galena–chalcopyrite–native silver–pyrite assemblage, and finally a pyrargyrite–polybasite–pearceite assemblage. Stage 4 involves the deposition of quartz veins with minor (late) pyrite and stage 5 is characterized by siderite that infilled remaining voids. Mineral zonation occurs along the Yerranderie Fault, with bornite being restricted to the Colon Peaks–Silver Peak mine area, whereas arsenopyrite, which is present in both the Colon Peaks–Silver Peak and Wollondilly mine areas, is absent in other lodes along the Yerranderie Fault. The Yerranderie Fault, which hosts the major lodes, is surrounded by a zoned alteration system. With increasing proximity to the fault the intensity of alteration increases and the alteration assemblage changes from an outer quartz–muscovite–illite–(ankerite) assemblage to a quartz–illite–(pyrite–carbonate) assemblage within meters of the fault. 40Ar/39Ar dating of muscovite from the alteration zone gave a 372.1 ± 1.9 Ma (Late Devonian) age, which is interpreted to be the timing of the quartz–sulfide vein formation. Sulfur isotope values for sulfides range from 0.1 to 6.2‰ with one outlier of −5.6 δ34S‰. The results indicate that the initial ore-forming fluids were reduced, and that sulfur was probably sourced from a magmatic reservoir, either as a direct magmatic contribution or indirectly through dissolution and recycling of sulfur from the host volcanic sequence. The sulfur isotope data suggest the system is isotopically zoned.
TL;DR: The Waisoi East and West porphyry Cu deposits as mentioned in this paper were separated into two deposits: the waisoi east and the wisoi west porphyries, which are characterized by the bornite-chalcopyrite-pyrite assemblage associated with traces of molybdenite and native gold.
Abstract: The porphyry Cu deposits at Waisoi in Namosi district, Viti Levu are separated into two deposits: the Waisoi East deposit and the Waisoi West deposit. In the Waisoi East deposit, quartz porphyry is exposed and in the Waisoi West deposit, diorite porphyry is sporadically exposed in addition to a small body of quartz porphyry. The mineralization in the Waisoi East deposit is characterized by the bornite–chalcopyrite–pyrite assemblage associated with traces of molybdenite and native gold. Polyphase fluid inclusions in stockwork quartz veinlets show homogenization temperatures ranging from 210 to >500°C. The high-grade Cu mineralization in the Waisoi West deposit is characterized by the bornite–chalcopyrite–pyrite assemblage accompanied with sheeted and stockwork quartz veinlets. Polyphase fluid inclusions occasionally containing hematite flakes in quartz veinlets in the center of the Waisoi West deposit homogenize at temperatures ranging from 450°C to >500°C. However, fluid inclusions in stockwork quartz veinlets in the periphery, homogenize at lower temperatures around 210°C. Both in the Waisoi East and Waisoi West deposits, primary bornite–chalcopyrite–pyrite assemblage in the high Cu-grade zone was deposited at the upper stability limit of chalcopyrite with respect to sulfur fugacity. Thus, the principal Cu mineralization at the Waisoi deposits occurred at a relatively high sulfur fugacity, that is, in a high-sulfidation environment.
TL;DR: The Bilimoia mine in Papua New Guinea is composed of fault-controlled, metamorphic-hosted, intrusion-related mesothermal to low sulfidation epithermal quartz-Au-Te-Bi vein system as discussed by the authors.
Abstract: The Bilimoia deposit (2.23 Mt, 24 g/t Au), located in the eastern Central Mobile Belt of mainland Papua New Guinea, is composed of fault-hosted, NW–NNW-trending Irumafimpa–Kora and Judd–Upper Kora Au-quartz veins hosted by Middle–Late Triassic basement that was metamorphosed to medium-grade greenschist facies between Middle–Late Triassic and Early–Middle Jurassic. Mineralizing fluids were introduced during crustal thickening, rapid uplift, change of plate motions from oblique to orthogonal compression, active faulting and S3 and S4 events in an S1–S4 deformation sequence. The Bilimoia deposit is spatially and temporally related to I-type, early intermediate to felsic and late mafic intrusions emplaced in Late Miocene (9–7 Ma). Hydrothermal alteration and associated mineralization is divided into 10 main paragenetic stages: (1) chlorite–epidote-selvaged quartz–calcite–specularite vein; (2) local quartz–illite–pyrite alteration; (3) quartz–sericite–mariposite–fuchsite–pyrite wall-rock alteration that delimits the bounding shears; (4) finely banded, colloform-, crustiform- and cockade-textured and drusy quartz ± early wolframite ± late adularia; (5) hematite; (6) pyrite; (7) quartz ± amethyst-base metal sulfides; (8) quartz–chalcopyrite–bornite–Sn and Cu sulfides–Au tellurides and Te ± Bi ± Ag ± Cu ± Pb phases; (9) Fe ± Mn carbonates; and (10) supergene overprint. Fluid inclusions in stage 4 are characterized by low salinity (0.9–5.4 wt% NaCl equivalent), aqueous–carbonic fluids with total homogenization temperatures ranging from 210 to 330°C. Some of the inclusions that homogenized between 285 and 330°C host coexisting liquid- and vapor-rich (including carbonic) phases, suggesting phase separation. Fluid inclusions in quartz intergrown with wolframite have low salinity (0.9–1.2 wt% NaCl equivalent), aqueous–carbonic fluids at 240–260°C, defining the latter’s depositional conditions. The ore fluids were derived from oxidized magmatic source initially contaminated by reduced basement rocks. Wall-rock alteration and involvement of circulating meteoric waters were dominant during the first three stages and early part of stage 4. Stage 5 hematite was deposited as a result of stage 4 phase separation or entrainment of oxygenated groundwater. Gold is associated with Te- and Bi-bearing minerals and mostly precipitated as gold-tellurides during stage 8. Gold deposition occurred below 350°C due to a change in the sulfidation and oxidation state of the fluids, depressurization and decreasing temperature and activities of sulfur and tellurium. Bisulfides are considered to be the main Au-transporting complexes. The Bilimoia deposit has affinities that are similar to many gold systems termed epizonal orogenic and intrusion-related. The current data allow us to classify the Bilimoia deposit as a fault-controlled, metamorphic-hosted, intrusion-related mesothermal to low sulfidation epithermal quartz–Au–Te–Bi vein system.
TL;DR: In this paper, an isotope study on fluid inclusions of pyrites from ores of Kuoerzhenkuola and Buerkesidai gold deposits produces 40Ar/36Ar and 3He/4He ratios in the range of 282-525 and 0.6-9.4 R/Ra, respectively, indicating a mixed source of deep-seated magmatic water (mantle fluid) and shallower meteoric water.
Abstract: The Sawuershan region, one of the important gold metallogenic belts of Xinjiang, is located in the western part of the Kalatongke island arc zone of north Xinjiang, NW China. There are two gold deposits in mining, namely the Kuoerzhenkuola and the Buerkesidai deposits. Gold ores at the Kuoerzhenkuola deposit occur within Carboniferous andesite and volcanic breccias in the form of gold-bearing quartz–pyrite veins and veinlet groups containing native gold, electrum, pyrite, pyrrhotite and chalcopyrite. Gold ores at the Buerkesidai deposit occur within Carboniferous tuffaceous siltstones in the form of gold-bearing quartz veinlet groups and altered rocks, with electrum, pyrite and arsenopyrite as major metallic minerals. Both gold deposits are hosted by structurally controlled faults associated with intense hydrothermal alteration. The typical alteration assemblage is sericite + chlorite + calcite + quartz, with an inner pyrite–sericite zone and an outer chlorite–calcite–epidote zone between orebodies and wall rocks. δ34S values (0.3–1.3‰) of pyrite of ores from Kuoerzhenkuola deposit are similar to those (0.4–2.9‰) of pyrite of ores from Buerkesidai deposit. δ34S values (1.1–2.8‰) of pyrite from altered rocks are similar to δ34S values of magmatic or igneous sulfide sulfur, but higher than those from ores. 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb data of sulfide from ores range within 17.72–18.56, 15.34–15.61, and 37.21–38.28, respectively. These sulfur and lead isotope compositions imply that ore-forming materials might originate from multiple, mainly deep sources. He and Ar isotope study on fluid inclusions of pyrites from ores of Kuoerzhenkuola and Buerkesidai gold deposits produces 40Ar/36Ar and 3He/4He ratios in the range of 282–525 and 0.6–9.4 R/Ra, respectively, indicating a mixed source of deep-seated magmatic water (mantle fluid) and shallower meteoric water. In terms of tectonic setting, the gold deposits in the Sawuershan region can be interpreted as epithermal. These formations resulted from a combination of protracted volcanic activity, hydrothermal fluid mixing, and a structural setting favoring gold deposition. Fluid mixing was possibly the key factor resulting in Au deposition in the gold deposits in Sawuershan region.
TL;DR: In this article, a series of numerical models has been constructed to simulate coupled mechanical-hydrological processes, using Fast Lagrangian Analysis of Continua (FLAC), a finite difference computer code.
Abstract: Mount Isa is a major Australian and world Pb-Zn-Ag mineral province. The wide varieties of mineralization in the province are believed to be closely related to the geodynamic processes of Isan Orogeny, which occurred between ca 1500 and 1620 Ma. In order to understand the geodynamic processes associated with the Isan Orogeny and the giant mineralization systems in the Mount Isa district, a series of numerical models has been constructed to simulate coupled mechanical–hydrological processes, using Fast Lagrangian Analysis of Continua (FLAC), a finite difference computer code. The numerical modeling results have demonstrated that the most probable far-field stress orientation during the Isan Orogeny is the asymmetrical E–W shortening, which led to greater easternward tectonic movement at the west boundary of the district in comparison with westward movement at the east boundary. During the initial and early stage of the Isan Orogeny, the mechanical and hydrological conditions in the Leichardt Fault Trough of the West Fold Belt are much more favorable for fluid accumulation and mineralization than in the East Fold Belt. The Mount Isan fault zone developed as a high dilation shear zone where the fluids were focused. As the asymmetrical shortening progressed, shortening deformation and shear strain localization became intensified in the eastern part of the orogenic district. The eastern region therefore became a more favorable locality for hydrothermal mineralization. This structural development feature seems to explain why mineralization in the East Fold Belt is generally later than in the West Fold Belt. Fluid production from the Williams–Naraku granites could result in fluid over-pressuring, and this probably contributed to the extensive brecciation and related mineralization in the East Fold Belt.
TL;DR: The Angelica copper deposit is located at the southernmost sector of the Jurassic Tocopilla plutonic complex in the North Chilean Coastal Cordillera as discussed by the authors, and is characterized by impregnation of supergene copper products of atacamite and minor amounts of chrysocolla, lavendulan and black copper.
Abstract: The Angelica copper deposit is situated at the southernmost sector of the Jurassic Tocopilla plutonic complex in the North Chilean Coastal Cordillera. This deposit occurs in monzonitic to monzodioritic rocks, and has platelike orebodies with no appreciable hydrothermal alteration nor sulfide mineralization. The mineralized zones are located in the western side of the two main normal faults with NE and NW orientations, and are characterized principally by impregnation of supergene copper products of atacamite and minor amounts of chrysocolla, lavendulan and “black copper”. Generally, chrysocolla is more abundant at a distal NE sector of the deposit. The black copper is Cu-Fe-Mn-Si-Cl-rich multimineral aggregates composed of atacamite with minor amounts of quartz, pseudomalachite, dioptase, neotocite, gypsum, paratacamite and melanothallite, and its surface exhibits nanometer-sized cylindrical morphologies. All these characteristics suggest an exotic origin for the Angelica copper deposit. A few vein-type copper deposits situated at the southwestern sector along the NE-oriented fault are inferred as the possible source of the Angelica copper deposit.
TL;DR: The El Zancudo mine in Colombia has been investigated in this paper to analyze the genetic processes of the ore minerals and to examine the genesis of the deposit, which consists mainly of banded ore veins hosted in greenschist and lesser disseminated ore in porphyritic rocks.
Abstract: The occurrence and the chemical compositions of ore minerals (especially the silver-bearing minerals) and fluid inclusions of the El Zancudo mine in Colombia were investigated in order to analyze the genetic processes of the ore minerals and to examine the genesis of the deposit. The El Zancudo mine is a silver–gold deposit located in the western flank of the Central Cordillera in Antioquia Department. It consists mainly of banded ore veins hosted in greenschist and lesser disseminated ore in porphyritic rocks. The ore deposit is associated with extensive hydrothermally altered zones. The ores from the banded veins contain sphalerite, pyrite, arsenopyrite, galena, Ag-bearing sulfosalts, Pb-Sb sulfosalts, and minor chalcopyrite, electrum, and native silver. Electrum is included within sphalerite, pyrite, and arsenopyrite, and is also partially surrounded by pyrite, arsenopyrite, sphalerite, and tetrahedrite. Native silver is present in minor amounts as small grains in contact with Ag-rich sulfosalts. Silver-bearing sulfosalts are argentian tetrahedrite–freibergite solid solution, andorite, miargyrite, diaphorite, and owyheeite. Pb-Sb sulfosalts are bournonite, jamesonite, and boulangerite. Two main crystallization stages are recognized, based on textural relations and mineral assemblages. The first-stage assemblage includes sphalerite, pyrite, arsenopyrite, galena and electrum. The second stage is divided into two sub-stages. The first sub-stage commenced with the deposition and growth of sphalerite, pyrite, and arsenopyrite. These minerals are characterized by compositional growth banding, and seem to have crystallized continuously until the end of the second sub-stage. Tetrahedrite, Pb-Cu sulfosalts, Ag-Sb sulfosalt, and Pb-Ag-Sb sulfosalts crystallized from the final part of the first sub-stage and during the whole second sub-stage. However, one Pb-Ag-Sb sulfosalt, diaphorite, was formed by a retrograde reaction between galena and miargyrite. The minimum and maximum genetic temperatures estimated from the FeS content of sphalerite coexisting with pyrite and the silver content of electrum are 300°C and 420°C, respectively. These estimated genetic temperatures are similar to, but slightly higher than the homogenization temperatures (235–350°C) of primary fluid inclusions in quartz. The presence of muscovite in the altered host rocks and gangue suggest that the pH of the hydrothermal solutions was close to neutral. Most of the sulfosalts in this deposit have previously been attributed as the products of epithermal mineralization. However, El Zancudo can be classified as a xenothermal deposit, in view of the low pressure and high temperature genetic conditions identified in the present study, based on the mineralogy of sulfosalts and the homogenization temperatures of the fluid inclusions.
TL;DR: In this article, the authors analyzed sediment samples from Jebel Chaker in the eastern part of the Ypresian basin in central-northern Tunisia using various geochemical methods in order to decipher their organic signature.
Abstract: Sedimentary rock samples from Jebel Chaker in the eastern part of the Ypresian basin in central-northern Tunisia were analyzed using various geochemical methods in order to decipher their organic signature. Examination of the distribution of total organic carbon (1.04–1.82%) suggests that the petroleum potential of Ypresian facies is not ignored in such area. The Ypresian episode permitted the accumulation of organic matter, which is typically marine plankton as indicated by the unimodal distribution of N-alkane at nC18 and by the predominance of the aliphatic hydrocarbons compared to the aromatics. It is concluded that the Ypresian organic matter is relatively immature, as indicated by the high content of polar compounds (3–67%) in bitumen. This conclusion is supported by the relative low Tmax values (433–438°C), suggesting that the organic matter is located towards the end of diagenesis and beginning of catagenesis. These new results testify to the establishment of suboxic conditions that led to the accumulation and preservation of good quantities of organic matter in central-northern Tunisia during the Ypresian. Due to their geochemical characteristics, the Chaker facies represent new potential source rocks in central-northern Tunisia.
TL;DR: The Bumo gold deposit as mentioned in this paper is a low-sulfide quartz vein type deposit, situated in the west of Hainan Island, approximately 25 km southeast of Dongfang City (termed Basuo in alias).
Abstract: The Bumo gold deposit is a low-sulfide quartz vein type deposit, situated in the west of Hainan Island, approximately 25 km southeast of Dongfang City (termed Basuo in alias). Tectonically, the SW-NE-oriented Gezhen Ductile-Shearing Zone was largely attenuated in Bumo. It is hosted by gneisses and schists of Proterozoic age (locally called the Baoban Group), but the radiometric age of ores mainly is Jurassic–Cretaceous, being coeval to the regional metallogenesis along the Eurasian margin. The affinity with the Phanerozoic orogenic gold deposits is indicated by low salinity (2.4 wt% NaCl–10.3 wt% NaCl), intermediate temperature (150–300°C) and appropriate δ18O value (approximately 7–11‰). The gold metal tonnage, proved previously, within an area smaller than 1.0 km2 in the gold belt II amounted to 11.59 t, with the average gold grades for the major Au quartz veins (7 and 8) in this belt of 52.7 and 71.1 g/t, respectively. This shows significant enrichment of gold and an unusual “nugget effect”. A ring structure, mainly represented by ridges, valleys, dikes, big bull quartz veins and lithological borders or pertinent faults, marks the gold field, and its origin was linked to a possible magmatic diapir. It was viewed as macroscopic evidence that the dip angles of strata tend to decline gradually from deeper ones in the center area to nearly horizontal in the outskirts, and certain intrusion-affiliated elements such as Bi, As and W are considerably abundant in soil samples. Two areas, namely the area A (3.06 km2) in the outskirts of the ring structure or in northeastern Bumo and the area B (1.32 km2) in the field center or to the west of the gold belts I and II, were selected for the geochemical survey in a 50 × 50-m sampling grid. The analytical results of the samples from horizon C and in a −80 mesh size showed that the average contents of Bi, As and W amounted to 0.55, 19.3 and 3.67 ppm in the area A (1225 samples) and to 0.79, 68.3 and 4.14 ppm in the area B (529 samples), respectively. This geochemical exploration led to the discovery of several Au quartz veins in both areas A and B, which also manifested an Au nugget effect (the average Au grades between 50 and 100 g/t) in spite of their limited scales (from a couple of centimeters to 50 cm wide and from several meters to 15 m long). A mylonite zone in area A was also found by this study. An intimate spatial correlation between a lithological border (gneiss and schist or pegmatite) or the mylonite zone and Au anomalies indicates an important prospect for further gold exploration.
TL;DR: In this article, the evaluation of the relatively fresh host rock and altered rock samples associated with the Pantingan Gold System exposed in Mount Mariveles, Bataan yield several notable observations that are useful in pinpointing potential gold pathfinder elements.
Abstract: The evaluation of the relatively fresh host rock and altered rock samples associated with the Pantingan Gold System exposed in Mount Mariveles, Bataan yield several notable observations that are useful in pinpointing potential gold pathfinder elements. Geochemical and petrologic analysis showed that the altered rocks can be subdivided into rocks that underwent propylitic alteration (group 1), argillized rocks with silica contents similar to those of the fresh host rocks (group 2), argillized but not strongly silicified rocks (group 3) and argillized and strongly silicified rocks (group 4). Selected element ratio patterns in the altered rocks and gold concentrations in gold-bearing quartz veins vary between the rock groups. Moreover, mass balance calculation also reflected the geochemical observations pertaining to the gains and losses of SiO2, Fe2O3+ MgO, CaO + Na2O and K2O, which are believed to be chemical reactions (i.e. breakdown of plagioclase, silica inundation or leaching, sulfide and calcite formation) caused by the influx of hydrothermal fluids.
TL;DR: Papua New Guinea's petroleum sector has substantially grown in the last 2 years as discussed by the authors and there is an increase in exploration expenditure, which also sees a corresponding increase in the number of exploration licences.
Abstract: Papua New Guinea’s petroleum sector has substantially grown in the last 2 years. There is an increase in exploration expenditure, which also sees a corresponding increase in the number of exploration licences. Two new petroleum projects have been commissioned and these have added to production of the existing projects. The oilfields are small by world standards. The sector is the second major contributor to the export income and taxation revenue of the country, after minerals.
TL;DR: Two ore and three alteration types were identified in the Lascogon Project of Philex Gold Philippines, in Surigao del Norte, Mindanao Island, Philippines as discussed by the authors.
Abstract: Two ore and three alteration types were identified in the Lascogon Project of Philex Gold Philippines, in Surigao del Norte, Mindanao Island, Philippines. The jasperoid ore is the host to the Carlin-like gold mineralization in the Lascogon and Danao prospects. The ore occurs in a decalcified and silicified horizon, with minor chlorite and goethite, stibnite, pyrite and quartz crystals ranging from cryptocrystalline to botryoidal. The stringer–stockwork type Cu-Au mineralization in the Suyoc prospect is hosted in argillized andesitic rocks of the Mabuhay Formation. The primary ore minerals are chalcopyrite with minor amounts of sphalerite. The alteration types identified are propylitic alteration, argillic alteration and silicification. The propylitized basaltic and andesitic flows of the Bacuag Formation bound the jasperoid mineralization in the Lascogon prospect. Stratigraphically, the relationship between propylitized basalts and stringer–stockwork Cu-Au is not clear but a lateral change can be inferred from jasperoid in the center and stringer–stockwork towards the east.
TL;DR: In order to elucidate the formation mechanism of low-sulfidation epithermal gold deposit, the adsorption of [Au(S2O3)2]3− (a model compound for gold(I) complex ion) on alumina gel was investigated as a basic model experiment.
Abstract: In order to elucidate the formation mechanism of low-sulfidation epithermal gold deposit, the adsorption of [Au(S2O3)2]3− (a model compound for gold(I) complex ion) on alumina gel (a model compound for the aluminum-bearing minerals) and change in chemical state of [Au(S2O3)2]3− after adsorption on the surface of alumina gel were investigated as a basic model experiment. In the pH range from 4 to 6, the amount of [Au(S2O3)2]3− adsorbed on alumina gel decreased with increasing pH and decreased drastically between pH 6 and 7, and then approached zero above pH 8 at 30°C. At 60°C, the amount of gold adsorbed above pH 7 was enhanced compared with that at 30°C. This adsorption tendency indicates that [Au(S2O3)2]3− is mainly adsorbed by electrostatic interaction between negative charges of [Au(S2O3)2]3− and positive charges of alumina gel because of its isoelectric point around pH 9. The chemical state of gold after adsorption of [Au(S2O3)2]3− on alumina gel was examined using X-ray absorption near edge structure (XANES). The result showed that [Au(S2O3)2]3− was spontaneously reduced to elemental gold even in the absence of specific reducing agents after adsorption on alumina gel. This reduction reaction might occur by two steps: (i) disproportionation of the adsorbed [Au(S2O3)2]3− at the surface of alumina gel, and (ii) spontaneous reduction of the resulting gold(III) complex ions on the surface of alumina gel. The experimental results suggest that aluminum plays an important role in the concentration of gold(I) complex ions and subsequent reduction of gold during the formation of low-sulfidation epithermal gold deposits.