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.     

Mass Transfer, Oxygen Isotopic Variation and Gold Precipitation in Epithermal System: A Case Study of the Hishikari Deposit, Southern Kyushu, Japan

Abstract: Transportation of various kinds of elements occurred in wall rocks (Quaternary andesites) during the hydrothermal alteration accompanied by the Hishikari epithermal gold mineralization. For example, K₂O and MgO contents of wall rocks decrease away from the gold-quartz veins, while (CaO+Na₂O) content increases, and SiO₂ content is variable near the veins. Hydrothermal alteration zoning and bulk compositional variations in wall rocks suggest that the mixing of hydrothermal solution and acidic groundwater took place an important role as the cause for the hydrothermal alteration and bulk compositional variations. The relationship between dissolved silica concentration and temperature of hydrothermal solution mixed with groundwater is obtained based on precipitation kinetics-fluid flow–mixing model, and the computed results are compared with the distribution of SiO₂ minerals (quartz and cristobalite) in the hydrothermal alteration zones. This comparison suggests that the most reasonable flow rate of fluids migrating through hydrothermal alteration zones, and A/M (A: surface area of rocks interacting with fluid, M: mass of fluid) are estimated to be ca. 10^-4.2 m/sec, and ca. 0.10 m²/kg, respectively. The mixing of two fluids (hydrothermal solution and acidic groundwater) can also explain δ¹⁸O zoning in the altered country rocks, hydrothermal alteration zoning from K-feldspar through K-mica to kaolinite from the center (veins) to margin, and deposition of gold.     

胶东三山岛北部海域金矿蚀变特征与微量元素迁移规律

胶东地区是中国最重要的金矿矿集区,区内众多大型–超大型金矿集中产出,已探明金矿资源量占全国30%以上。构造蚀变岩型金矿是胶东区内重要的类型之一,三山岛北部海域金矿是该类型的典型代表。胶东三山岛北部海域金矿是近年来新发现的超大型金矿,对该矿床的蚀变岩石进行研究具有重要意义。通过详实地岩心编录与室内研究,查明了该矿床的主要蚀变类型及矿物组合特征,系统采集了典型蚀变岩石样品并进行了微量元素地球化学分析,运用质量平衡技术方法总结了热液蚀变过程中的元素迁移规律,同时探讨了黄铁绢英岩中微量元素特征。结果表明,在标高−1200～−1400 m 范围内蚀变强烈。蚀变类型有钾化、绢英岩化和黄铁绢英岩化等,其中与成矿关系最为密切的是绢英岩化和黄铁绢英岩化;从原岩到黄铁绢英岩的整个蚀变过程中,流体向围岩提供了大量（迁移量＞2）的As、Sb、Te等低温元素,中量（迁移量为1～2）的Pb、Zn、Cu等中温元素,少量（迁移量＜1）的Co、Ni、Cr等高温元素,表明在标高−1200～−1400 m处,流体以中低温元素组合为主,预测−1200～−1400 m处矿体仍处于中上部,深部还有很好的找矿潜力。     

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.     

Application of Chlorite Geothermometry to Hydrothermal Alteration in Toyoha Geothermal System, Southwestern Hokkaido, Japan

In this study, we applied chemical geothermometers to the estimation of formation temperatures of chlorites from various types of hydrothermally altered rocks in the Toyoha geothermal field, using core samples from six drill holes (TH-2 to TH-7) together with wasted ore samples from Toyoha vein-type ore deposit. Based on the preliminary examination of mineral assemblages by X-ray powder diffraction and optical microscopy, hydrothermal alteration observed through the drill holes was classified into four types of alteration zones: propylitic, mixed-layer minerals, kaolin minerals, and ore mineralized zones. The mineral assemblage of the ore mineralized zone observed through TH-2, TH-4, and TH-6 is similar to those of Toyoha ore veins reported previously. The Fe³⁺/ΣFe ratios of chlorites were determined by X-ray photoelectron spectroscopy (XPS), in addition to the usual microprobe analyses. The ratios ranged from 0.20–0.26 for chlorites from the propylitic alteration zone and from 0.13 to 0.17 for those from the ore mineralized zone associated with sulfide minerals. After correcting the Fe³⁺ contents in the octahedral sites of chlorite structures, we obtained acceptable temperatures of the chlorite formation by application of geothermometers, for instance, a similar range of 150–300°C for chlorites from either the propylitic zone or the ore mineralized zone developed through TH-2, TH-4, and TH-6. Chlorites from the ore mineralized zone proximal to the Toyoha deposit are characterized by high Fe and Mn contents compared to the propylitic chlorites, which is similar to the Toyoha vein-filling chlorites; the formation temperatures were close to both the homogenization temperatures of fluid inclusions and the present subsurface temperatures measured through drill holes. Chlorites from the Toyoha ore veins, however, gave slightly higher formation temperatures (180–350°C) than those of chlorites from the ore mineralized zone in the drill cores. This suggests that several types of hydrothermal alteration occurred at different stages in the Toyoha geothermal field and the composition of product chlorite was controlled not only by the temperature but also the composition of fluid related to the formation. Reliable estimation of temperature for the chlorite formation provides basic information on evaluating correctly other physicochemical conditions prevalent at the formation.     

Fluid Inclusion Microthermometry and Implications for the Mechanisms of Ore-Grade Gold Mineralization in Epithermal System, Lalab Orebody, Sibutad, Zamboanga del Norte, Philippines

The Sibutad gold deposit has gold associated in quartz veins. The most important of these is the Lalab orebody, which contains ore‐grade gold, predominantly, in milky quartz veins and veinlets. Here, alteration quartz and fine‐grained crystalline clear and milky quartz were formed from hydrothermal fluids in three stages, namely stages I, II and III. Fluid inclusion microthermometry was carried out on stage I milky quartz, stage II fine‐grained alteration quartz and stage III milky quartz ± barite veins and veinlets. Homogenization temperatures (T_H) are >248°C in stage I, 214–232°C in stage II and 186–239°C in stage III. These fluid inclusions have salinity between 1 and 2 wt% NaCl equivalent. In terms of gold assay, stage I drill‐core samples have gold grades 0.53–0.76 g/ton Au, stage II samples have 1.12–3.70 g/ton Au and stage III samples have 9.06–23.88 g/ton Au. This correlation suggests that gold was precipitated from the stage II and III fluids.     

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.     

Tectonic setting, evolution and orogenic gold potential of the late Mesoarchaean Mosquito Creek Basin, North Pilbara Craton, Western Australia

The geology, evolution, and metallogenic potential of the Mesoarchaean Mosquito Creek Basin remains poorly understood, despite the presence of several orogenic gold deposits. The basin is dominated by medium- to coarse-grained, poorly sorted and chemically immature sandstone and conglomerates, characterised by very high Cr/Th, high Th/Sc, and low Zr/Sc relative to average continental crust. These features are consistent with the presence of significant mafic rocks in the source terrain(s), a limited role for sediment recycling, and deposition in an increasingly distal passive margin setting on the southeastern edge of the Palaeo- to Mesoarchaean East Pilbara Terrane.New U–Pb SHRIMP data on 358 detrital zircons indicate a conservative maximum depositional age of 2972 + 14/−37 Ma (robust median; 96.1% confidence). Zircon provenance spectra from conglomeratic rocks near the base of the unit are consistent with substantial derivation from the East Pilbara Terrane, but finer-grained sandstones higher in the stratigraphy appear to have been sourced elsewhere, as their zircon age spectra are not well matched by any of the exposed Pilbara terranes.The Mosquito Creek Basin was deformed before and during collision with the northern edge of the Mesoarchaean Kurrana Terrane, which resulted in the development of macroscopic north-verging folds, thrust faulting, and widespread sub-greenschist to greenschist facies metamorphism. This collisional event probably took place at ca. 2900 Ma, based on two identical Pb–Pb model ages of 2905 ± 9 Ma from epigenetic galena associated with vein-hosted gold–antimony mineralization. The metallogenic potential of the Mosquito Creek Basin remains largely unevaluated; however, the possibility of a passive margin setting and continental basement points to relatively limited potential for the formation of major orogenic gold deposits.     

豫西前河金矿热液蚀变地球化学研究

含矿热液在迁移过程中与围岩发生了广泛的流体-岩石反应而引起热液蚀变.前河金矿区内主要的热液蚀变有钾长石化、青磐岩化、绢云母化、硅化、黄铁矿化、碳酸盐化和萤石化等7种类型;划分了6个蚀变-成矿阶段,即黑云母-钾长石阶段→青磐岩阶段→绢英岩阶段→黄铁矿-石英阶段→石英-黄铁矿阶段→碳酸盐-卤化物阶段,以及外、中、内3个蚀变带.5种典型蚀变岩的常量和微量元素分析表明,绢英岩化蚀变中,Si、K大量迁入,Pb含量大大增加,在∑REE有所降低的情况下HREE有所富集:δ Eu和δ Ce在各蚀变阶段均呈现负异常:这些现象可能是导致金沉淀的元素地球化学响应.各阶段成矿流体-岩石的交代特点在绿泥石单矿物的矿物化学成分变化上也有所反映.     

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.     

Fluvial Geochemistry through a Short-Duration, Tropical-Cyclone Induced Discharge event in the Burdekin River and Hann Creek, North Queensland, Australia

The chemical composition of river water integrates a number of factors such as weathering, land use, climate, vegetation cover and human activity that individually affect its chemistry. Short term variations may also be significant. The Burdekin River, NE Australia, is an example of a class of tropical streams which experiences two to four orders of magnitude variation in discharge in response to seasonal but erratic monsoonal and cyclonic rainfall. In these systems individual discharge events last for days to weeks. Given the inherent difficulty sampling these events published data on water chemistry (and thus calculated fluxes and global budgets) may tend to be biased to low flow conditions. One such discharge event in February 1996 has been investigated for its impact on the chemistry of the water. Major cations (Na, Mg, K, Ca) all decreased in concentration as the water level rose, as did the minor elements Sr, Ba and U. Some other trace elements, notably Rb, Cr, Pb and REE were enriched in the peak flow waters. The flux of all measured elements increased substantially during the seven days of the discharge event. Such short term but significant events will have a major impact on the annual fluxes of elements delivered to the oceans from the land and global discharge budgets may need to take them into account when refining databases in the future.     

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.     

西藏古堆地热田水热蚀变类型、分布特征及对勘探方向的启示

水热蚀变与泉华作为地热流体水-岩反应和化学沉积的产物,其相关研究可为水热活动发展史、恢复地热流体古温度及水岩反应过程等方面研究提供重要信息。古堆地热田位于错那—沃卡裂谷中部,是继羊八井地热田之后我国大陆最具发电潜力的非火山岩型高温地热田之一,极具开发潜力。为了解古堆地热田泉华和水热蚀变的类型、分布特征及判断有利的勘探方向,本文在野外地热地质调查的基础上,结合室内镜下鉴定和扫描电镜分析等工作,识别出绿泥石化、硅化、高岭石化和碳酸盐化等水热蚀变类型。研究发现,古堆地热田水热蚀变呈现出从蚀变中心向外由酸性蚀变过渡为弱酸性-中性蚀变的分带模式,蚀变中心常由强烈蚀变的硅化带或高岭石带组成;水热蚀变分布受断裂构造的控制,蚀变在断裂交汇处最为强烈,蚀变中心常沿着断裂延伸;根据水热蚀变的分布及蚀变强度,初步判断布雄朗古和杀嘎朗嘎的渗透性相对较好且热储温度高,具有较好的开发前景。在泉华沉积方面,古堆地热田西区主要为钙华区,东区为钙华和硅华叠加区;结合青藏高原动力学背景,认为古堆地热田水热发展史与陆陆碰撞的隆升过程及错那—沃卡裂谷活动密切相关且存在明显的耦合关系。     

Paleozoic Epithermal Au and Porphyry Cu Deposits in North Xinjiang, China: Epochs, Features, Tectonic Linkage and Exploration Significance

Abstract. Based on field investigation of large number of ore deposits including some latest discoveries and multidiscipline comprehensive research, we demonstrated the general features of metallic deposits and we suggest that Paleozoic archipelago-type collisional orogen at North Xinjiang, northwestern China show intimate similarity with the metallogenesis of Southeast Asia Cenozoic archipelago. We briefly described the characteristics of major porphyry-type, skarn-type Cu deposits and typical high-sulfidation type (HS-type) and low-sulfidation type (LS-type) epithermal Au deposits as well as some latest discoveries. Systematic isotopic age-dating on the Tuwu-Yandong superlarge porphyry Cu deposits revealed that they formed in Late Devonian to Early Carboniferous in an accretionary arc setting. The tectonic settings of epithermal Au deposits and its linkage with porphyry Cu deposits are further discussed. The formation condition for porphyry Cu deposits is more strict than epithermal Au deposits. The distribution width for porphyry Cu deposits in the orogenic belts is more limited than epithermal Au deposits. The discovery and prospecting progress of the Kalatage HS-type Cu-Au deposit were reported. The significance in further exploration was suggested.     

Hydrothermal Alteration and Fluid Geochemistry of the Tongonan Geothermal Field, Philippines

Abstract: The alteration mineralogy, the present-day fluid chemistry, and some fluid inclusion data are used to make inferences on the chemical changes that have occurred in the fluids during the history of the Tongonan Geothermal Field. Thermal activity in the Tongonan area began in the Miocene when emplacement of many plutons forming a batholith contact metamorphosed the overlying volcanics to hornblende hornfels assemblages. In the early Pliocene, when tectonic uplift occurred along the Philippine Fault, about 2 mole % of mainly carbon dioxide and sulfur gas was released to a geothermal fluid and condensed in groundwater with geothermal steam. The condensate intensely altered the reservoir rock and formed an acid mineral assemblage, which was overprinted by a later, lower temperature, neutral-pH assemblage. Some chlorite, epidote and illite in the reservoir rock formed at temperatures up to 100°C lower than present-day temperatures possibly during the Plio-Pleistocene uplift period, i.e., the system was heating up. The assemblage garnet-anhydrite formed in fractures from a condensate after the gas had nearly completely separated from the deep, CO₂-rich fluid during vigorous boiling possibly during hydrothermal eruptions. The output of gas to the geothermal fluid decreased, while the salinity (10,000 mg/kg or - 2 wt% NaCl) and the temperature of the geothermal fluid remained nearly constant throughout the Quaternary. When this neutral-pH, alkali chloride fluid boiled, it initially precipitated albite or epidote on the rims then anhydrite at the center of fractures at high temperatures (-250–300°C). At lower temperatures (-150–250°C), adularia or wairakite and later calcite were deposited as the proportion of gas in the steam condensate increased. The origin of solutes is also discussed.     

Numerical Simulation of Hydrothermal Alteration Chemical Reactions During Ore-forming Process of the Jiaojia Gold Deposit,Jiaodong Peninsula,China;

The Jiaojia gold deposit is featured by obvious alteration zonation, and is one of the typical altered-type gold deposits in the Jiaodong gold province. However, the formation conditions of hydrothermal alteration zonation and the spatial location of chemical reaction associated with gold precipitation are still unclear. To quantitatively discuss the hydrothermal process, we used the TOUGHREACT software to simulate the chemical reactions between ore-bearing fluids and wall rocks of the Jiaojia gold deposit. First, we constructed a conceptual mineralization model related to hydrothermal alteration by discussing the chemical reaction between ore-bearing fluids and wall rocks. Subsequently, we simulated the chemical equilibrium concentration of ore-forming fluids and pH changes under different temperature and pressure conditions, which was used to study the dissolution and precipitation mechanism of hydrothermal minerals during ore-forming processes. The simulation results show that the chemical equilibrium concentration of Au+ significantly decreased with temperature from 280 ℃ to 180 ℃, and the chemical equilibrium concentration of Fe2+ showed similar trend, indicating that the favorable gold metallogenic temperature range is 180 ℃ to 280 ℃, and Fe2+ in ore-forming fluids reacts with [Au(HS)2]− to promote gold precipitation. The temperature and pressure conditions influence the chemical equilibrium concentrations of ore elements, nevertheless, temperature is a more critical factor controlling gold precipitation, while the influence of pressure is relatively weak. The pH simulation results show that the pH values of ore-forming fluids increased during the hydrothermal alteration reaction, which means that the ore-forming environment changes from acidic to neutral and/or alkaline. The pH variation during the ore-forming process is consistent with conventional geological observations, and thus validated the simulation results. The above results indicate that the temperature driving lateral migration of ore-forming fluids along the fault resulted in the continuous overprinting alteration. The pyrite-sericite-quartz alteration in the footwall of the fault zone is conducive to gold precipitation and enrichment due to frequent overprinting of various alteration and changes of temperature and pH. © 2023 Science Press. All rights reserved.     

Geology and Hydrothermal Alteration of the Milyang Pyrophyllite Deposit, Southeast Korea

Abstract: The Milyang pyrophyllite deposit, which is embedded in the Late Cretaceous Yuchon Group of the Kyongsang Supergroup, is one of the largest hydrothermal clay deposits in the Kyongsang basin, southeast Korea. Host rocks of the deposit are porphyritic andesite lava and minor andesitic lapilli tuff. In the Milyang district, a hydrothermally altered zone is about 2 × 3 km in extent; we can recognize the concentric arrangement of advanced argillic, propylitic, and sericitic alteration zones from the central to peripheral parts of the zone. The Milyang pyrophyllite deposit forms a part of the advanced argillic alteration zone. The Milyang pyrophyllite deposit is subdivided into the following four zones based on mineral assemblages: the pyrophyllite zones 1, 2, 3, and the silicified zone. The pyrophyllite zone 1, which occupies the central part of the deposit, comprises mainly pyrophyllite, kaolinite, and diaspore without quartz. Diaspore nodules often concentrate in beds 40–50 cm thick. Andalusite, dumortierite, and tourmaline locally occur as network veins, crack‐filler, or small spherulitic spots. The Al₂O₃ content of the ore ranges from 27 to 36 wt%. The pyrophyllite zone 2, which constitutes a major part of the deposit, comprises mainly pyrophyllite, kaolinite, and quartz. The Al₂O₃ content of the ore ranges from 15 to 24 wt%. The pyro‐phyllite zone 3 is the hematite‐rich marginal facies of the deposit. The silicified zone, which occurs as beds and septa, is mostly composed of quartz with minor pyrophyllite and kaolinite; the SiO₂ contents range from 79 to 90 wt%. Comparing chemical compositions of the high‐Al ores with those of unaltered host andesite, the Fe, Ca, alkalis, HFSE, and HREE contents are significantly depleted, whereas S, B, As, Sr, and LREE are enriched. The hydrothermal alteration of the Milyang pyrophyllite deposit can be classified into the following four stages: 1) extensive sericitic and propylitic alteration, 2) medium‐temperature (200–250°C) advanced argillic alteration, 3) high‐temperature (250–350°C or more) advanced argillic alteration, and 4) retrograde low‐temperature alteration. The heat and some volatile components such as B and S would be derived from the Pulguksa Granite intruded underneath the deposit.     

毕力赫金矿床围岩蚀变及其与金矿化关系

本文对毕力赫金矿床Ⅱ矿带围岩蚀变及其与金矿化关系进行了研究.矿床主要蚀变类型为硅化、钾化、黄铁矿化、绢云母化、电气石化、绿泥石化、高岭土化、碳酸盐化,其中硅化、绢云母化、黄铁矿化与金矿化关系密切;蚀变分带明显,由地表向下,依次为青磐岩化带→绢英岩化带→钾质蚀变带,绢英岩化带与金矿化关系最为密切.     

伊犁京希—伊尔曼德金矿床的热液蚀变及成矿流体演化特征

京希－伊尔曼德金矿床的热液蚀变在空间上有明显的分带性，中心蚀变带以强烈的硅化为主，典型的蚀变矿物组合为石英或玉髓和地开石，中间带为高级泥化带，以地开石－高岭石－石英或玉髓为特征；外带为以蒙脱石－高岭石－伊利石－其他粘土矿物等矿物组合为主的泥化带，蚀变强度和矿物组合的分带性是温度、压力和化学梯度的反映，是流体在不断的水或流体－岩石反应和成分交换的产物。该矿床成矿流体演化过程为：早期酸性（pH=2-3)含矿流体在沿断裂上升过程中，受围岩灰岩中的流体（pH为中性）缓冲，在其进入高渗透性的碎屑岩层时，流速和水－岩石或流休－岩石反应大大加快，并在与大气降水的混合作用下，pH值逐步升高（3－5），产生了流体的温度及成分梯度，在温度和压力迅速下降的条件下，金及蚀变矿物沉淀、结晶生长，形成了蚀变空间分带，中心带保存完好的多孔状石英和地开石等高级泥化矿物组合说明该矿床是高硫化热液体系作用下的产物。