Characteristics of the Cibaliung Gold Deposit: Miocene Low-Sulfidation-Type Epithermal Gold Deposit in Western Java, Indonesia

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.     

The Arinem Te‐Bearing Gold‐Silver‐Base Metal Deposit,West Java,Indonesia

The vein system in the Arinem area is a gold‐silver‐base metal deposit of Late Miocene (8.8–9.4 Ma) age located in the southwestern part of Java Island, Indonesia. The mineralization in the area is represented by the Arinem vein with a total length of about 5900 m, with a vertical extent up to 575 m, with other associated veins such as Bantarhuni and Halimun. The Arinem vein is hosted by andesitic tuff, breccia, and lava of the Oligocene–Middle Miocene Jampang Formation (23–11.6 Ma) and overlain unconformably by Pliocene–Pleistocene volcanic rocks composed of andesitic‐basaltic tuff, tuff breccia and lavas. The inferred reserve is approximately 2 million tons at 5.7 g t^?1 gold and 41.5 g t^?1 silver at a cut‐off of 4 g t^?1 Au, which equates to approximately 12.5t of Au and 91.4t of Ag. The ore mineral assemblage of the Arinem vein consists of sphalerite, galena, chalcopyrite, pyrite, marcasite, and arsenopyrite with small amounts of pyrrhotite, argentite, electrum, bornite, hessite, tetradymite, altaite, petzite, stutzite, hematite, enargite, tennantite, chalcocite, and covellite. These ore minerals occur in quartz with colloform, crustiform, comb, vuggy, massive, brecciated, bladed and calcedonic textures and sulfide veins. A pervasive quartz–illite–pyrite alteration zone encloses the quartz and sulfide veins and is associated with veinlets of quartz–calcite–pyrite. This alteration zone is enveloped by smectite–illite–kaolinite–quartz–pyrite alteration, which grades into a chlorite–smectite–kaolinite–calcite–pyrite zone. Early stage mineralization (stage I) of vuggy–massive–banded crystalline quartz‐sulfide was followed by middle stage (stage II) of banded–brecciated–massive sulfide‐quartz and then by last stage (stage III) of massive‐crystalline barren quartz. The temperature of the mineralization, estimated from fluid inclusion microthermometry in quartz ranges from 157 to 325°C, whereas the temperatures indicated by fluid inclusions from sphalerite and calcite range from 153 to 218 and 140 to 217°C, respectively. The mineralizing fluid is dilute, with a salinity <4.3 wt% NaCl equiv. The ore‐mineral assemblage and paragenesis of the Arinem vein is characteristically of a low sulfidation epithermal system with indication of high sulfidation overprinted at stage II. Boiling is probably the main control for the gold solubility and precipitation of gold occurred during cooling in stage I mineralization.     

Relationships between Volcanism, Hydrothermal Alteration and Epithermal Gold-Silver Mineralization in the Muka Mine Area in the Kitami Metallogenic Province, Hokkaido, Japan

Abstract: Neogene magmatism in the Muka mine area in the Kitami metallogenic province was characterized on the basis of K-Ar age data by felsic–to–mafic terrestrial extrusive and intrusive volcanism from Late Miocene to Early Pliocene. The geology of the Muka mine area comprises the Upper Cretaceous-Paleocene Yubetsu Group, consisting primarily of sandstone and shale; Upper Miocene Ikutahara Formation, consisting of clastic and felsic volcaniclastic rocks and Kane-hana Lava (rhyolite) of 7. 5 Ma; Upper Miocene Yahagi Formation, consisting of clastics, felsic volcaniclastics and rhyolite lavas; Late Miocene andesite and rhyolite dikes (Chidanosawa Rhyolite of 7. 2 Ma and Hon-Mukagawa Andesite of 6. 6 Ma); Lower Pliocene Hakugindai Lava (basalt: 4. 0 Ma); and Quaternary System. The volcanism consists of earlier Late Miocene felsic extrusive activity during the sedimentation of the Ikutahara Formation, later Late Miocene felsic extrusive and intrusive activities during the sedimentation of the Yahagi Formation and intermediate intrusive activity after the sedimentation of the Yahagi Formation and Early Pliocene mafic extrusive activity. The Muka gold-silver ore deposit occurs primarily in the felsic volcaniclastic rocks and Kanehana Lava of the Ikutahara Formation and in Hon-Mukagawa Andesite. These wall–rocks, the clastic rocks of the Ikutahara Formation and the clastic and felsic volcaniclastic rocks of the Yahagi Formation were affected to various extents by hydrothermal alteration. The hydrother-mal alteration can be divided into two stages (early and late) based on the modes of occurrence and mineral assemblages. Early hydrothermal alteration is characterized by regional and vein-related alterations associated with epithermal gold-silver mineralization in a near-neutral hydrothermal system. Regional alteration can be subdivided into a zeolite zone (mordenite+adularia±heulandite–clinoptilolite series mineral±smectite±quartz°Cristobalite±opal–CT) and a smectite zone (smec–tite±quartz±opal–CT). Vein-related alteration can be subdivided into a K-feldspar zone (quartz+adularia±illite±interstratified illite/smectite±pyrite), an illite zone (quartz+illite°Chlorite±interstratified illite/smectite±smectite±pyrite) and an interstratified illite/smectite zone (quartz+interstratified illite/smectite±smectite±pyrite). The adularization age of 6. 8 Ma in the K-feldspar zone that developed in Kanehana Lava hosting ore veins coincides well with the epithermal gold-silver mineralization age of 6. 6 Ma. Late hydrothermal alteration is characterized by a kaolinite zone (kaolinite±dickite±alunite±quartz°Cristobalite± tridymite±pyrite) in an acid hydrothermal system, and cuts early alteration zones such as the K-feldspar zone. Other modes of occurrence of acid alteration are a 7Å halloysite-kaolinite vein in the hydrothermal explosion breccia dike and smectite–kaoli–nite veins along joint planes of Kanehana Lava. The style of the gold-silver deposit associated with early near-neutral hydrothermal alteration is a low-sulfidation epithermal type. The low-sulfidation epithermal gold-silver mineralization of 6. 6 Ma in the vicinity of the Muka ore deposit was essentially accompanied by felsic volcanic activity during the sedimentation of the Yahagi Formation, and was closely related both temporally and spatially to the felsic intrusive activity of Chidanosawa Rhyolite of 7. 2 Ma. The related hydrother-mal activity of the gold-silver mineralization took place at intervals of approximately 0. 4–0. 6 Ma after the volcanic activity related to the mineralization.     

Geology and Alteration-Mineralization Characteristics of the Cibaliung Epithermal Gold Deposit, Banten, Indonesia

Abstract. The Cibaliung gold project is located at the central portion of the Neogene Sunda‐Banda magmatic arc. Gold‐silver mineralization in the area is hosted in a thick sequence of sub‐aqueous basaltic andesite volcanics with intercalated sediments intruded by sub‐volcanic andesite to diorite plugs and dykes, and subsequently cut by a cluster of diatreme breccias. These host rocks are unconformably overlain by dacitic tuffs, younger sediments and basalt flows. The gold prospects in Cibaliung occur within a NW‐trending structural corridor that is 3.5 km wide by at least 6 km long. It is fault‐bounded and is considered to be a graben. Two aligned NNW‐trending sub‐vertical shoots, Cikoneng and Cibitung, host the currently defined resource within the steeply dipping vein system with a minimum strike length of 1,300 m. As of July 2001, exploration has defined an inferred + indicated 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. Gold‐silver mineralization occurs as quartz veins characteristic of the low‐sulphidation epithermal adularia‐sericite type. Progressive dilation with a general increase in gold grade has produced multi‐stage veining and brecciation that grades from early to late stages as: pre‐mineral fluidized breccia, quartz vein stockwork, massive vein, crustiform vein, colloform‐crustiform vein with progressive increase in chloritic clay bands, clay‐quartz milled matrix breccias with a progressive increase in clay content, and synto post‐mineral fault gouge with vein clasts. Wall rock alteration is characterized by pro‐grade chlorite+adularia flooding that is locally overprinted by a low temperature argillic alteration (smectite, illite and mixed layered clays). Generally, the argillic alteration becomes weak with depth. The major mineral constituents of the veins are quartz, adularia and clay. In the early gold‐poor hydrothermal stages, quartz and adularia dominate with minor calcite and clay (smectite, poorly crystalline chlorite, interlayered chlorite‐smectite and illite‐smectite). In the later gold‐rich hydrothermal stages, clay with variable amounts of carbonate increases whereas the abundance of quartz and adularia decreases. Gold occurs mainly as electrum while silver occurs as argentite‐aguilarite‐naumannite and electrum, and rarely as native silver, sulphosalts and tellurides. Sulphides generally comprise <1 vol % of the vein, with pyrite as the most common species. Together with pyrite, traces of very fine‐grained base metal sulphides dominated by chalcopyrite, sphalerite and galena are in most cases intimately associated with electrum and silver minerals. Partial supergene oxidation generally extends down to about 200 m below the surface at Cikoneng and further down to more than 300 m at Cibitung. The hydrothermal system responsible for the gold‐silver mineralization in the area may be related to rhyolitic magmatism focused on a volcanic intrusive center during back arc rifting that formed a graben or pull‐apart basin. The dominant mechanism for the higher grade gold deposition is fluid mixing of up welling metal‐bearing hydrothermal solutions with relatively near surface cool, oxygenated condensate and/or steam‐heated meteoric fluids, as opposed to retrograde boiling. The strongly focused dilational structural environment is thought to have been the mechanism for focusing fluid flows, both up welling and descending, forming pipe‐like mineralized bodies in the rhomboidal dilation zones. It is interpreted that mineralization took place under low temperature conditions (<150–220d?C) at a minimum depth of around 200–250 m below the palaeo‐water table.     

Volcanic Activity, Hydrothermal Alteration and Epithermal Gold-Silver Mineralization in the Ryuo Mine Area in the Kitami Metallogenic Province, Hokkaido, Japan

Abstract: Characterization of Neogene magmatism in the Ryuo mine area in the Kitami metallogenic province was carried out on the basis of K-Ar data for felsic–to–mafic terrestrial extrusive and intrusive volcanism from Late Miocene to Early Pliocene. The Ryuo epithermal gold-silver deposit occurs primarily in the felsic volcaniclastic rocks of the Ikutahara Formation and in Ryuo Rhyolite. The Ryuo mineralization age of 7. 7 – 8. 1 Ma coincides well with the hydrothermal alteration age (7. 7 Ma) of Ryuo Rhyolite hosting ore veins. It is concluded that the Ryuo mineralization was essentially accompanied by felsic volcanic activity during the sedimentation of the Ikutahara Formation, and was closely related both temporally and spatially to the intrusive activity of Ryuo Rhyolite. Hydrothermal alteration related to the epithermal gold-silver mineralization of the Ryuo deposit is primarily characterized by early regional and vein-related alterations, and late steam-heated alteration. Early regional alteration consists of a smectite halo (smectite+pyrite±quartz±opal–CT±mordenite°Clinoptilolite–heulandite series mineral). Early vein-related alteration is primarily marked by potassic alteration. This alteration halo can be subdivided into a K-feldspar halo (quartz+adular–ia+pyrite±illite±interstratified illite/smectite±smectite), an illite halo (quartz+illite + chlorite + pyrite ± interstratified illite/smec–tite±smectite) and an interstratified illite/smectite halo (quartz + interstratified illite/smectite+pyrite±smectite). Late steam-heated alteration characterized by kaolinite or alunite locally overprints the early K-feldspar halo. The style of the Ryuo gold-silver deposit is a low-sulfidation epithermal type. The gold–silver–bearing quartz vein precipitates during boiling of ore fluid. The origin of the ore fluid might be meteoric water. The temperature and sulfur fugacity conditions during precipitation of electrum and acanthite are estimated to be 206°– 238°C and 10^-13.5 – 10^-11.6 atm, respectively.     

福建政和东际金-银矿床的热液蚀变特征及其勘探指示意义

福建东际金-银矿床产于中生代东坑火山岩盆地西缘的流纹质凝灰岩中,矿区及周边火山-沉积岩系遭受了广泛且强度不等的热液蚀变。本文工作采用红外反射光谱技术在东际矿区三个勘探剖面上对钻孔岩心进行系统性高密度采样分析,结果显示蚀变矿物组合及分带的大框架样式主要受原岩成分控制,具体表现为绢云母化趋向于发育于流纹质火山岩中,而绿泥石化则富集在安山质和英安质火山岩中。在更小的矿区局部空间范围内或单一岩性中,热液作用的物化条件作为次级控制因素决定着特定蚀变矿物的成分变化和蚀变类型的强度差异。从层状硅酸盐组合考虑,东际矿区热液蚀变以绢云母化和绿泥石化为最主要类型,而缺失发育良好的蒙脱石带,金-银矿化赋存在以伊利石为主的绢云母化带中,表明成矿环境属于低硫到中硫之间的浅成热液系统。含矿火山岩的热液蚀变组合和强度变化,以及金-银矿化的似层状特点,均指示成矿时流体是沿着南园组凝灰岩层内侧向流动,而蚀变分带细节显示在凝灰岩层中存在着二至三条流体主通道,金-银矿化则赋存在主通道中。流体主通道的热液蚀变标志是富铝绢云母,其与主通道之外的相对贫铝绢云母在红外光谱特征上反差明显。因此,采用红外光谱技术圈定整体热液蚀变系统的空间构型、解译成矿环境的物化条件及变化、并确定绢云母的铝含量变化用以判断成矿流体主通道位置,可以有效地缩小勘探目标和提高找矿工作的预测性。     

Hydrothermal activity in the Obiro deposit embedded in the Tagawa acidic rocks,Uetsu region,NE Japan

The Obiro deposit is located in the Tagawa Acidic Rocks (AR), Uetsu region, NE Japan. The Tagawa AR is composed of a volcanic phase of dacitic welded tuff and a plutonic phase of porphyritic granodiorite. Drill core and ore samples were collected from the deposit and examined by XRD, EPMA, and microthermometry. The drill core samples have suffered pervasively from sericite (illite) alteration, whereas pinkish K‐feldspar alteration halo occur close to veins. The results of EPMA and microthermometry is interpreted as that the magnatic‐hydrothermal fluids has changed as follows; the granodioritic magma intruded at about 1.0 kb and 700°C near the water‐saturated granite solidus; after cooling to about 500°C the fluids boiled according to a change in the pressure regime from lithostatic to hydrostatic; mixing with meteoric water led to sulfide mineralization at around 400°C or less. The main reasons for the mineralization in the Obiro deposit are as follows; the oxidized magma intruded at a shallower level, and thereafter hydrothermal fluids were boiled, resulting in a saline fluid. The saline fluid then dissolved metals such as Pb, Zn, Cu, and Bi, and these metals precipitated during cooling accompanied by dilution of the meteoric water and increasing pH, resulting in decreasing solubility.     

Origin of Ore-forming Fluids Responsible for Gold Mineralization of the Pongkor Au-Ag Deposit, West Java, Indonesia: Evidence from Mineralogic, Fluid Inclusion Microthermometry and Stable Isotope Study of the Ciurug–Cikoret Veins

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.     

Epithermal Gold Mineralization in the Trenggalek District,East Java,Indonesia

Gold‐mineralized quartz veins at the Trenggalek district of the Southern Mountains Range in East Java, Indonesia, are hosted by Oligo‐Miocene volcaniclastic and volcanic rocks, and are distributed close to andesitic plugs in the northern prospects (Dalangturu, Suruh, Jati, Gregah, Jombok, Salak, and Kojan) and the southern prospects (Sentul and Buluroto). The plugs are subalkaline tholeiitic basaltic‐andesite to calc‐alkaline andesite in composition. ⁴⁰Ar–³⁹Ar dating of a quartz‐adularia vein at the Dalangturu prospect yielded an age of 16.29 ± 0.56 Ma (2σ), and a crystal tuff of a limestone‐pyroclastic rock sequence at the southwest of the Dalangturu prospect was determined as 15.6 ± 0.5 Ma (2σ). Statistic overlap of ages suggests that the gold mineralization in the northern prospects took place in a shallow marine to subaerial transitional environment. Hydrothermal alteration of the host rocks is characterized by the replacement of quartz, illite and adularia. Quartz veins in surface outcrops are up to 50 cm wide in the northern prospects and up to 3 m wide in the southern prospects, showing a banded or brecciated texture, and are composed of quartz, adularia, carbonates with pyrite, electrum, sphalerite, galena, and polybasite. Gold contents of quartz veins are positively correlated with Ag, Zn, Pb, and Cu contents in both the northern and southern prospects. The quartz veins at the Jati, Gregah, and Sentul prospects have relatively lower gold‐silver ratios (Ag/Au = 23.2) compared to those at the Kojan, Dalangturu, Salak, and Suruh prospects (Ag/Au = 66.8). The quartz veins at the Dalangturu prospect are relatively rich in base metal sulfides. Ag/(Au+Ag) ratios of electrum in the Dalangturu prospect range from 45.2 to 65.0 at%, and FeS contents of sphalerite range from 1.2 to 6.4 mol%. Fluid inclusion microthermometry indicates ore‐forming temperatures of 190–200°C and 220–230°C at the Sentul and Kojan prospects, respectively. Widely variable vapor/liquid ratio of fluid inclusions indicates that fluid boiling took place within the hydrothermal system at the Sentul prospect. Salinities of ore‐fluids range from 0 to 0.7 wt% (av. 0.4 wt% NaCl equiv.) and from 0.5 to 1.4 wt% (av. 0.9 wt%) for the Sentul and Kojan prospects, respectively. The boiling of hydrothermal fluid was one of the gold deposition mechanisms in the Sentul prospect.     

Hydrothermal Gold Mineralization at the Rodnikovoe Deposit in South Kamchatka, Russia

Abstract. The Rodnikovoe gold deposit situated in a presently active hydrothermal system located north of the Mutnovsko-Asachinskaya geothermal area in southern Kamchatka, Far Eastern Russia, consists of typical low-sulfidation quartz-adularia veins in a host rock of diorite. The age of the mineralization was dated by the K-Ar method as 0.9 to 1.1 Ma based on adular-ia collected from the veins. Representative ore minerals in the deposit are electrum, argentite, aguilarite, polybasite, pearceite and lenaite. Dominant alteration minerals are adularia, α-cristobalite, chlorite, illite and kaolinite. Hydrothermal solutions of neutral pH were responsible for the mineralization, which is divided into six stages defined by tectonic boundaries. Gold mineralization occurred in stages I and III. Hydrothermal brecciation occurred during stages III, IV and VI. Stages II, IV, V and VI were barren. The estimated ore formation temperature based on a fluid inclusion study is 150 to 250 °C at a depth of approximately 170 m below the paleo-water table. Boiling of hydrothermal fluids is hypothesized as the cause of the intermittent deposition of gold ore. The sulfur and oxygen fugacities during the deposition of anhydrite prior to the hydrothermal brecciation were higher than those during the gold mineralization stages. The occurrence in the hydrothermal breccia of fragments of high grade Au-Ag and polymetallic ores suggests that higher grade mineralization of these metal ores might have occurred in a deeper portion of the deposit.     

Geology,Alteration, and Mineralization of the Kay Tanda Epithermal Gold Deposit,Lobo, Batangas,Philippines

The Kay Tanda epithermal Au deposit in Lobo, Batangas is one of the Au deposits situated in the Batangas Mineral District in southern Luzon, Philippines. This study aims to document the geological, alteration, and mineralization characteristics and to determine the age of the mineralization, the mechanism of ore deposition, and the hydrothermal fluid characteristics of the Kay Tanda deposit. The geology of Kay Tanda consists of (i) the Talahib Volcanic Sequence, a Middle Miocene dacitic to andesitic volcaniclastic sequence that served as the host rock of the mineralization; (ii) the Balibago Diorite Complex, a cogenetic intrusive complex intruding the Talahib Volcanic Sequence; (iii) the Calatagan Formation, a Late Miocene to Early Pliocene volcanosedimentary formation unconformably overlying the Talahib Volcanic Sequence; (iv) the Dacite Porphyry Intrusives, which intruded the older lithological units; and (v) the Balibago Andesite, a Pliocene postmineralization volcaniclastic unit. K‐Ar dating on illite collected from the alteration haloes around quartz veins demonstrated that the age of mineralization is around 5.9 ± 0.2 to 5.5 ± 0.2 Ma (Late Miocene). Two main styles of mineralization are identified in Kay Tanda. The first style is an early‐stage extensive epithermal mineralization characterized by stratabound Au‐Ag‐bearing quartz stockworks hosted at the shallower levels of the Talahib Volcanic Sequence. The second style is a late‐stage base metal (Zn, Pb, and Cu) epithermal mineralization with local bonanza‐grade Au mineralization hosted in veins and hydrothermal breccias that are intersected at deeper levels of the Talahib Volcanic Sequence and at the shallower levels of the Balibago Intrusive Complex. Paragenetic studies on the mineralization in Kay Tanda defined six stages of mineralization; the first two belong to the first mineralization style, while the last four belong to the second mineralization style. Stage 1 is composed of quartz ± pyrophyllite ± dickite/kaolinite ± diaspore alteration, which is cut by quartz veins. Stage 2 is composed of Au‐Ag‐bearing quartz stockworks associated with pervasive illite ± quartz ± smectite ± kaolinite alteration. Stage 3 is composed of carbonate veins with minor base metal sulfides. Stage 4 is composed of quartz ± adularia ± calcite veins and hydrothermal breccias, hosting the main base metal and bonanza‐grade Au mineralization, and is associated with chlorite‐illite‐quartz alteration. Stage 5 is composed of epidote‐carbonate veins associated with epidote‐calcite‐chlorite alteration. Stage 6 is composed of anhydrite‐gypsum veins with minor base metal mineralization. The alteration assemblage of the deposit evolved from an acidic mineral assemblage caused by the condensation of magmatic volatiles from the Balibago Intrusive Complex into the groundwater to a slightly acidic mineral assemblage caused by the interaction of the host rocks and the circulating hydrothermal waters being heated up by the Dacite Porphyry Intrusives to a near‐neutral pH toward the later parts of the mineralization. Fluid inclusion microthermometry indicates that the temperature of the system started to increase during Stage 1 (T = 220–250°C) and remained at high temperatures (T = 250–290°C) toward Stage 6 due to the continuous intrusion of Dacite Porphyry Intrusives at depth. Salinity slightly decreased toward the later stages due to the contribution of more meteoric waters into the hydrothermal system. Boiling is considered the main mechanism of ore deposition based on the occurrence of rhombic adularia, the heterogeneous trapping of fluid inclusions of variable liquid–vapor ratios, the distribution of homogenization temperatures, and the gas ratios obtained from the quantitative fluid inclusion gas analysis of quartz. Ore mineral assemblage and sulfur fugacity determined from the FeS content of sphalerite at temperatures estimated by fluid inclusion microthermometry indicate that the base metal mineralization at Kay Tanda evolved from a high sulfidation to an intermediate sulfidation condition.     

Hydrothermal alteration and K–Ar ages of Neogene‐Quaternary magmatic‐hydrothermal systems at Toyoha‐Muine area in southwest Hokkaido,Japan

This paper presents a review of hydrothermal alteration and K–Ar age data from the Toyoha‐Muine area (TMA), where the Toyoha polymetallic (Ag–Pb–Zn–Cu–In) deposit is located near the Pliocene andesitic volcano that formed Mt Muine. Systematic prospect‐scale mapping, sampling, X‐ray analysis and microscopic observation show that hydrothermal alteration is divided into two groups: acid‐pH and neutral‐pH alteration types. The former is further divided into mineral assemblages I, II and III, while the latter into mineral assemblages IV and V. Different mineralogical features in five mineral assemblages are summarized as follows: (I) Quartz (silicified rock); (II) Pyrophyllite or dickite; (III) Kaolinite or halloysite ± alunite; (IV) Sericite or K‐feldspar; and (V) Interstratified minerals (illite/smectite and chlorite/smectite) and/or smectite. K–Ar radiometric ages determined on twenty‐eight K‐bearing samples (whole volcanic rocks and separated hydrothermal minerals) mainly fall into one of three periods: Early Miocene (24.6–21.4 Ma), Middle–Late Miocene (12.5–8.4 Ma) and Pliocene–Pleistocene (3.2–0 Ma). These three periods are characterized as follows. Early Miocene: A minor hydrothermal activity, which might be genetically related to the intermediate or felsic magmatic activities, formed mineral assemblage IV at 24.6 Ma in the northern part of the TMA. Middle to Late Miocene: The basaltic intrusion, andesitic eruption, and granodiorite intrusions induced hydrothermal activities between 12.5 and 8.4 Ma, resulting in the formation of a mineral assemblage IV with some base metal mineralization. Pliocene–Pleistocene: An andesitic eruption formed Mt Muine between 3.2 and 2.9 Ma. The andesitic activity was associated with acid‐pH mineral assemblages I, II and III locally around the volcano. Latent magmatic intrusions subsequent to the andesitic eruption generated hydrothermal activities that formed mineral assemblages IV and V between 1.9 and 0 Ma in the southern and southeastern parts of Toyoha deposit at depth, overprinting the Middle to Late Miocene alteration. The hydrothermal activities also formed mineral assemblages I, II and III along the Yunosawa fault (east of the Toyoha deposit) and assemblage III in the south and southeast of the Toyoha deposit near the surface.     

Gold Mineralization and Occurrence of Sinter in the Hoshino Area, Fukuoka Prefecture, Japan

Abstract. Several epithermal gold deposits occur in the Hoshino area, which is located in the western end of the late Cenozoic Hohi volcanic zone, north‐central Kyushu, Japan. The area is characterized by intermediate to felsic extrusive rocks of Pliocene age. In the Hoshino area, the shallow manifestation of the hydrothermal activity is exposed on the surface. Several outcrops of sinter are still preserved on the top of hydro thermally altered volcanic rocks, and high‐grade gold‐bearing quartz veins occur on the surface at lower levels. The hydrothermal alteration resulted into well‐developed alteration zones. The zonal alteration pattern, primarily of near‐neutral pH type, is characterized by the outer smectite zone of a lower temperature, and the inner mixed layer minerals zone of a higher temperature. Quartz vein‐related or fracture‐controlled alteration, is represented by the occurrence of interstratified illite/smectite and K‐feldspar, suggesting a potassium‐enriched alteration. The mineralization in the Hoshino area is recognized on the surface by the occurrence of gold‐bearing quartz veins distributed mainly in the mixed layer minerals zone. The fracture system related to the gold mineralization is mainly characterized by NW‐SE trend. The alteration pattern and the mineralogical composition of the veins suggest that the mineralizing fluids had near‐neutral pH and the mineralization is of low‐sulfidation‐type. Fluid inclusion data and textures observed in quartz veins indicate that gold precipitated during boiling. The chemical composition of quartz veins shows that high‐grade gold‐bearing quartz veins are characterized by higher content of Al₂O₃, K₂O and Rb. Several outcrops of silica‐sinters are distributed on the top of the mixed layer minerals zone. Although their structures are not very well preserved, because of later silicification, the Hoshino sinters still show characteristic textures identical to those observed in modern sinters, such as the presence of plant fossil incorporated into the sinters, the strongly developed depositional laminations and the columnar structures perpendicular to the depositional surfaces. Quartz is the only silica mineral constituting the Hoshino sinters presently. The conversion of amorphous silica into quartz was probably governed by higher temperatures resulting from later hydrothermal activity. This later hydrothermal activity is reflected by the intense silicification affecting mainly the lower parts of the sinters and also by the presence of quartz veins cutting the sinters. The distribution of sinters in the Hoshino area is very significant. The presence beneath the sinters of concealed high‐grade gold‐bearing quartz veins should be highly considered and exploration work is strongly suggested.     

40Ar/39Ar and K–Ar geochronology of magmatic and hydrothermal events in a classic low-suphidation epithermal bonanza deposit: El Peñon, northern Chile

The epithermal El Peñon gold–silver deposit consists of quartz–adularia veins emplaced within a late Upper Paleocene rhyolitic dome complex, located in the Paleocene–Lower Eocene Au–Ag belt of northern Chile. Detailed K–Ar and ⁴⁰Ar/³⁹Ar geochronology on volcano–plutonic rocks and hydrothermal minerals were carried out to constrain magmatic and hydrothermal events. The Paleocene to Lower Eocene magmatism in the El Peñon area is confined to a rhomb-shaped basin, which was controlled by N–S trending normal faults and both NE- and NW-trending transtensional fault systems. The earliest products of the basin-filling sequences comprise of Middle to Upper Paleocene (～59–55 Ma) welded rhyolitic ignimbrites and andesitic to dacitic lavas, with occasional dacitic dome complexes. Later, rhyolitic and dacitic dome complexes (～55–52 Ma) represent the waning stages of volcanism during the latest Upper Paleocene and the earliest Eocene. Lower Eocene porphyry intrusives (～48–43 Ma) mark the end of the magmatism in the basin and a change to a compressive tectonomagmatic regime. ⁴⁰Ar/³⁹Ar geochronology of hydrothermal adularia from the El Peñon deposit yields ages between 51.0±0.6 and 53.1±0.5 Ma. These results suggest that mineralization occurred slightly after the emplacement of the El Peñon rhyolitic dome at 54.5±0.6 Ma (⁴⁰Ar/³⁹Ar age) and was closely tied to later dacitic–rhyodacitic bodies of 52 to 53 Ma (K–Ar ages), probably as short-lived pulses related to single volcanic events.     

Geological,geochemical, stable isotope,and fluid inclusion characteristics of epithermal gold mineralization,Velvet District,Nevada

Gold mineralization in the Velvet District occurs in an eastward dipping sequence of late Tertiary rhyolitic ash-flow tuffs, flows, and tuffaceous sediments in northwestern Nevada. Minor gold and silver concentrations are associated with irregular zones of brecciation, argillic alteration, and quartz veining along north-northeast trending normal faults. Reaction of mineralizing fluids with wallrock produced an argillic alteration assemblage of illite, mixed-layer clays, smectite, and kaolinite. Illite alteration and highest gold concentrations appear to be associated with zones of high water/rock ratios. Kaolinite, smectite, alunite, and opal are postulated to have formed during a steam-dominated episode of alteration.Fluid inclusion studies indicate that the quartz veins were deposited in the temperature range 230 to 280°C from fluids which had salinities equivalent to 0.2–0.8 weight percent NaCl. δ ¹⁸O of quartz veins varies from ?2.5 to $\text{+6.7 ‰}$ and indicates that the ore fluid must have been Tertiary meteroric water. Stable isotope data appear to define a zone of concentrated fluid flow and potential subsurface mineralization in the southeastern part of the district. Fluid inclusion and isotope studies can be used in combination with more standard geochemical, geophysical, and geological information to provide site-specific targets for epithermal metal concentrations.     

Variations in Chemical Composition of Clay Minerals and Magnetic Susceptibility of Hydrothermally Altered Rocks in the Hishikari Epithermal Gold Deposit, SW Kyushu, Japan

Hydrothermal alteration, involving chiefly chlorite and illite, is extensively distributed within host rocks of the Pleistocene Hishikari Lower Andesites (HLA) and the Cretaceous Shimanto Supergroup (SSG) in the underground mining area of the Hishikari epithermal gold deposit, Kagoshima, Japan. Approximately 60% of the mineable auriferous quartz‐adularia veins in the Honko vein system occur in sedimentary rocks of the SSG, whereas all the veins of the Yamada vein system occur in volcanic rocks of the HLA. Variations in the abundance and chemical composition of hydrothermal minerals and magnetic susceptibility of the hydrothermally altered rocks of the HLA and SSG were analyzed. In volcanic rocks of the HLA, hydrothermal minerals such as quartz, chlorite, adularia, illite, and pyrite replaced primary minerals. The amount of hydrothermal minerals in the volcanic rocks including chlorite, adularia, illite, and pyrite as well as the altered and/or replaced pyroxenes and plagioclase phenocrysts increases toward the veins in the Honko vein system. The vein‐centered variation in mineral assemblage is pronounced within up to 25 m from the veins in the peripheral area of the Honko vein system, whereas it is not as apparent in the Yamada vein system. The hydrothermal minerals in sandstone of the SSG occur mainly as seams less than a few millimeters thick and are sporadically observed in halos along the veins and/or the seams. The alteration halos in sandstone of the SSG are restricted to within 1 m of the veins. In the peripheral area of the Honko vein system, chlorite in volcanic rocks is characterized by increasing in Al in its tetrahedral layer and the Fe/Fe + Mg ratio toward the veins, while illite in volcanic rocks has relatively low K and a restricted range of Fe/Fe + Mg ratios. Temperature estimates derived from chlorite geothermometry rise toward the veins within the volcanic rocks. The magnetic susceptibility of tuff breccia of the HLA varies from 21 to less than 0.01 × 10^?3 SI within a span of 40 m from the veins and has significant variation relative to that of andesite (27–0.06 × 10^?3 SI). The variation peripheral to the Honko vein system correlates with an increase in the abundance of hematite pseudomorphs after magnetite, the percentage of adularia and chlorite with high Fe/Fe + Mg ratios, and the degree of plagioclase alteration with decreasing distance to the veins. In contrast, sedimentary rocks of the SSG maintain a consistent magnetic susceptibility across the alteration zone, within a narrow range from 0.3 to 0.2 × 10^?3 SI. Magnetic susceptibility of volcanic rocks of the HLA, especially tuff breccia, could serve as an effective exploration tool for identifying altered volcanic rocks.     

Geology and geochronology of Naruo large porphyry-breccia Cu deposit in the Duolong district,Tibet

The Duolong district is located in the south Qiangtang terrane of Tibet and is the most significant ore cluster within the Bangongco-Nujiang metallogenic belt. Duolong contains one giant, three large and two medium to small-sized porphyry (±epithermal ± breccia) copper deposits and several other mineralized porphyry bodies. All deposits are closely associated with early Cretaceous (123–115 Ma) intermediate-felsic intrusions. Naruo is a poorly studied porphyry-breccia copper deposit in the north of the Duolong district. Hydrothermal alteration surrounding the ore-bearing granodiorite at Naruo is characterized by an inner potassic zone and an outer propylitic zone, overlapped locally by minor phyllic and argillic alteration assemblages. A detailed paragenetic study has identified five distinct hydrothermal veins (M, A, B, C, D) within the porphyry system. Hydrothermal B veins are strongly related to copper mineralization. Strong propylitic alteration is also observed throughout the hydrothermal breccias identified at Naruo. Sandstone breccia, diorite-bearing breccia and granodiorite-bearing breccia were identified according to the distribution and composition of clasts. U-Pb zircon dating has determined the ages of the ore-bearing granodiorite (121.6 ± 1.3 Ma) and a barren intrusion (115.5 ± 1.1 Ma) within the porphyry system, diorite clasts (122.3 ± 0.9 Ma) and later diorite matrix (120.5 ± 1.0 Ma) in the hydrothermal breccia system, suggesting that with the exception of the late barren intrusion, they all belong to the same mineralizing event at Duolong. The geological and geochemical evidence presented in this study suggest that the porphyry and breccia systems may have originated from the same magma source, but are now spatially independent.     

Potassium–Argon Ages of the Epithermal Gold–Silver Mineralization in the Haenam–Jindo Area,Southwestern Korea

The Haenam–Jindo area, located on the southwestern margin of the Korean Peninsula, was the site of vigorous volcanic activity during the Late Cretaceous and Early Tertiary periods. Large parts of the area record strong hydrothermal alteration, and there exist many clay–alunite and gold–silver deposits. We undertook potassium–argon (K–Ar) age dating of five mineral samples (including adularia, sericite and alunite) from the Eunsan, Moisan and Gasado epithermal gold–silver deposits in this area. The purities of the samples were confirmed by X‐ray diffraction analysis. The K–Ar ages of adularia from the Eunsan deposit and adularia and sericite from the Moisan deposit (related to gold–silver mineralization) are 75.0 ± 1.6, 74.7 ± 1.6 and 75.1 ± 1.6 Ma, respectively. The similarity of these ages, combined with the close proximity and similar geochemical characteristics of the deposits, indicates that the mineralization occurred as part of a single hydrothermal system. The K–Ar ages of alunite at the surface and adularia at depth within the Gasado deposit are 82.2 ± 1.9 and 70.7 ± 1.9 Ma, respectively, revealing that the clay–alunite and gold–silver mineralization formed at different ages. K–Ar age data indicate that the gold–silver mineralization in this area occurred mainly at 75–70 Ma, resulting from hydrothermal activity in the Haenam–Jindo area (82–70 Ma). This is the first time that the mineralization of precious metals in Korea has been identified during this period.     

Mineralization,alteration, structure,and Re–Os age of the Lanjiagou porphyry Mo deposit,North China Craton

Lanjiagou is a porphyry Mo deposit in terms of its alteration zonation and mineralization associated with granitic intrusions and predominance of quartz vein-hosted molybdenum mineralization. It is the largest Mo deposit in North China Craton (404,000 t). There is an intimate spatial/temporal association between all stages of mineralization and Early Jurassic granitic intrusions at Lanjiagou. Most of the molybdenum was emplaced during the principal hydrothermal (PH) stage (184.6 ± 1.3 – 185.6 ± 1.4 Ma), contemporaneously with intrusion of fine-grained porphyritic granite (188.9 ± 1.2 Ma) into a granite batholith (193 ± 3 Ma). The PH mineralization stage is mainly hosted by a quartz-dominated stockwork associated with phyllic alteration in the fine-grained porphyritic granite. This stage was followed by the late hydrothermal (LH) activity. Thick Mo-rich quartz veins were emplaced during the LH stage and cut the porphyry ore bodies. A ring breccia zone formed during the last hydrothermal stage and apparently cuts both the porphyry and the quartz vein ore bodies. The main hydrothermal vein stages have predominantly concentric and radial vein orientations centred on the ring breccia zone. Most of the concentric veins have shallow dips, whereas the radial veins are subvertical. The LH veins have predominantly NEE and NW orientations in the deposit and are moderately inclined. We surmise that the veining was controlled by the local stress regime generated by the intrusion of a large, deep pluton that we interpreted to be the source of the granites, the breccia zone, and the molybdenum mineralization. Resurgence within the magma chamber reactivated the steep concentric structures in a reverse sense, and accumulation of magmatic and/or fluid pressure resulted in explosive brecciation, producing the ring breccia zone. A predominantly late set of NW-trending, post-ore felsic dikes, associated with the regional structures, are a consequence of far-field stresses exceeding local stresses in the deposit.     

Characteristics of Gold Mineralization at the Ciurug Vein, Pongkor Gold-Silver Deposit, West Java, Indonesia

Abstract. The Pongkor Gold‐Silver Mine, Bogor district, West Java, is approximately 80 km southwest of Jakarta. The gold and silver mineralization in the area is present in a deposit consisting of an epithermal vein‐system named individually as the Pasir Jawa, Gudang Handak, Ciguha, Pamoyanan, Kubang Cicau, and Ciurug veins. In the area studied, rocks of basaltic‐andesitic composition are dominated by volcanic breccia and lapilli tuff, with andesite lava and siltstone present locally. The hydrothermal alteration minerals in the Ciurug area are typical of those formed from acid to near‐neutral pH thermal waters, where the acid alteration is distributed from the surface to shallow depth, while the near‐neutral pH alteration becomes dominant at depths. The Ciurug vein shows four main mineralization stages where each discrete stage is characterized by a specific facies; these are, from early to late: carbonate‐quartz, manganese carbonate‐quartz, banded‐massive quartz and gray sulfide‐quartz facies. The major metallic minerals are pyrite, sphalerite, chalcopyrite and galena; they occur in almost each mineralization stage. Bornite was observed only in the southern part of the Ciurug vein at a depth of 515 m, and the occurrence of this mineral is reported here for the first time. Electrum and silver sulfides (mostly acanthite) are minor, whereas silver sulfosalts, stromeyerite and mckinstryite, and covellite are in trace amounts. The silver sulfosalts have compositional ranges of pearceite, antimon‐pearceite and polybasite. Most of the electrum occurs coexisting with other sulfide minerals, as inclusions in pyrite grains, with very little as inclusions in chalcopyrite or sphalerite. Gold grades within the Ciurug vein vary from 1.2 to hundreds of ppm, where the highest gold grade occurs in the latest mineralization stage in a thin sulfide band in vein quartz. Fluid inclusion microthermometry of calcite and quartz indicates deposition throughout the mineralized veins in the range from 170 to 230d?C and from low salinity fluids (predominantly lower than 0.2 wt% NaCl equiv.). Fluid inclusions occur with features of boiling.