Infrared spectral reflectance characterization of the hydrothermal alteration at the Tuwu Cu–Au deposit, Xinjiang, China

Short-wave infrared (SWIR) reflectance spectroscopy was used to characterize hydrothermal minerals and map alteration zones in the Tuwu Cu–Au deposit, Xinjiang, China. The Palaeozoic hydrothermal system at Tuwu is structurally controlled, developed in andesitic volcanic rocks and minor porphyries. Hydrothermal alteration is characterized by horizontally zoned development of quartz, sericite, chlorite, epidote, montmorillonite and kaolin about individual porphyry dykes and breccia zones, as is shown by changes outward from a core of quartz veining and silicification, through an inner zone of sericite + chlorite to a marginal zone of chlorite + epidote. The alteration system comprises several such zoning patterns. Silicification and sericitization are spatially associated with Cu–Au mineralization. Zoning is also shown by compositional variations such that Fe-rich chlorite and Al-rich sericite occur preferentially toward the core and the most intensely altered parts, whereas Mg-rich chlorite and relatively Al-poor sericite are present on the margin and the relatively weakly altered parts of the hydrothermal alteration system. The compositions of chlorite and sericite, therefore, can be potentially used as vectors to Cu–Au mineralization. Montmorillonite and kaolinite, of probable weathering origin, are located near the surface, forming an argillic blanket overlying Cu–Au mineralization. Sporadic montmorillonite is also present at depth in the hydrothermal alteration system, formed by descending groundwater. Presence of a well-developed kaolinite-bearing zone on the surface is an indication of possible underlying Cu–Au mineralization in this region. Epidote occurs widely in regional volcanic rocks, as well as in variably altered rocks on the margin of the hydrothermal mineralization system at Tuwu. The widespread occurrence of epidote in volcanic country rocks probably reflects a regional hydrothermal alteration event prior to the localized, porphyry intrusion-related hydrothermal process that led to the Cu–Au mineralization at Tuwu.     

Alteration and mineralization styles of the orogenic disseminated Zhenyuan gold deposit,southeastern Tibet: Contrast with carlin gold deposit

Orogenic disseminated and Carlin gold deposits share much similarity in alteration and mineralization.The disseminated orogenic Zhenyuan Au deposit along the Ailaoshan shear zone,southeastern Tibet,was selected to clarify their difference.The alteration and mineralization from the different lithologies,including meta-quartz sandstone,carbonaceous slate,meta-(ultra)mafic rock,quartz porphyry and lamprophyre were researched.According to the mineral assemblage and replacement relationship in all types of host rocks,two reactions show general control on gold deposition:(1)replacement of earlier magnetite by pyrite and carbonaceous material;(2)alteration of biotite and phlogopite phenocrysts in quartz porphyry and lamprophyre into dolomite/ankerite and sericite.Despite the lamprophyre is volumetrically minor and much less fractured than other host rocks,it contains a large portion of Au reserve,indicating that the chemically active lithology has played a more important role in gold precipitation compared to structure.LA-ICP-MS analysis shows that Au mainly occurs as invisible gold in fine-grained pyrite disseminated in the host rocks,with Au content reaching to 258.95 ppm.The diagenetic core of pyrite in meta-quartz sandstone enriched in Co,Ni,Mo,Ag and Hg is wrapped by hydrothermal pyrite enriched in Cu,As,Sb,Au,Tl,Pb and Bi.Different host rock lithology has much impact on the alteration and mineralization features.Carbonate and sericite in altered lamprophyre show they have higher Mg than those developed in other of host rocks denoting that the carbonate and sericite incorporated Mg from phlogopite phenocrysts in the primary lamprophyre during alteration.The ore fluid activated the diagenetic pyrite in meta-quartz sandstone leading the hydrothermal pyrite enriched in Cu,Mo,Ag,Sb,Te,Hg,Tl,Pb and Bi,but the hydrothermal pyrite in meta-(ultra)mafic rock is enriched in Co and Ni as the meta-(ultra)mafic rock host rock contain high content of Co and Ni.However,Au and As shear similar range in both types of host rocks indicating that these two elements most likely come from the deep source fluid rather than the host rocks.It was shown in the disseminated orogenic gold deposit that similar hydrothermal alteration with mineral assemblage of carbonate(mainly dolomite and ankerite),sericite,pyrite and arsenopyrite develops in all types of host rocks.This is different from the Nevada Carlin type,in which alteration is mainly dissolution and silicification of carbonate host rock.On the other hand,Au mainly occur as invisible gold in both disseminated orogenic and Carlin gold deposits.     

新疆阿舍勒铜矿容矿围岩蚀变的低钠晕及其意义

新疆阿舍勒铜矿的容矿围岩是一个套双峰式海相火山岩组合，围岩蚀变由多期次叠加和变质作用的影响而变得极为复杂，以细碧质碉石的青盘岩化、石英角斑质火山碎屑岩的黄铁绢英财化为主要特点，更有意义的是：原岩属钠质火山岩系，但容矿围岩无论是酸性次火山岩还是基性火山岩，蚀后都具有低钠的共同特点，这对于同类矿床具有找矿意义，低钠晕是海底火山喷气过程中高温气液交代而成，可作为海底火山喷发矿床的一个成因和找矿标志。     

Stable isotope evidences of jarosite–barite mineralization in the Rangan rhyolitic dome NE Isfahan,Iran

The Rangan area is part of Cenozoic magmatic belt of central Iran. Eocene volcanic flows and pyroclastic rocks are intruded by a Neogene rhyolitic dome along the major Qom–Zefreh fault. The dome is pervasively hydrothermally altered. The main mineral assemblage is jarosite+barite+pyrite+quartz+sericite. This assemblage indicates acid sulphate or advanced argillic alteration. Sulfur and oxygen isotope data (δ³⁴S & δ¹⁸O (SO₄)) obtained from jarosite and barite indicate a mixing episode during the evolution of hydrothermal system and reflect the overlapping of two distinct sources of acid sulphate alteration in Rangan, i.e., a magmatic–hydrothermal fluid reacting with steam-heated meteoric water. Considering the position of brittle–ductile transition and major fault movements, jarosite and barite seemingly precipitated from rapid injection of magmatic–hydrothermal fluids into the upper portions of a steam-heated environment.     

碰撞造山型斑岩铜矿蚀变分带模式--以西藏冈底斯斑岩铜矿带为例

岛弧环境斑岩铜矿蚀变分带模式已为人们所熟知 ,但碰撞造山环境的斑岩铜矿蚀变分带特征尚不清楚。对此 ,文中以西藏冈底斯斑岩铜矿带为例 ,选择驱龙、冲江、厅宫 3个典型斑岩铜矿 ,对其蚀变系统进行了系统研究。依据蚀变矿物组合可分为 3个蚀变带 ,呈环带状分布。从中心向外依次为钾硅酸盐化带、石英绢云母化带、青磐岩化带。泥化带不太发育 ,通常叠加在其它蚀变带之上。钾硅酸盐化带主要蚀变矿物为钾长石、黑云母、石英、硬石膏 ,伴有大量的黄铜矿与辉钼矿 ,是成矿物质的主要堆积区。石英绢云母化带与钾硅酸盐化带渐变过渡或叠加其上 ,是次于钾硅酸盐化带的储矿部位。蚀变矿物组合为绢云母 +石英 +钾长石 ,金属硫化物有黄铁矿、黄铜矿、辉钼矿、斑铜矿 ,少量的方铅矿、闪锌矿。主要的辉钼矿以石英 +辉钼矿脉的形式出现于本矿带。青磐岩化在斑岩体内不发育 ,矿化极微弱。蚀变岩石组分分析表明 ,岩石蚀变及其分带是岩浆流体 /岩石反应时K ,Na ,Ca ,Mg等组分迁移的结果 ,矿化伴随着蚀变发生。钾硅酸盐化带、石英绢云母化带和青磐岩化带的蚀变岩石与未 (弱 )蚀变斑岩具有一致的稀土配分模式 ,REE含量有规律地变化 ,说明蚀变岩石均经历了源于岩浆的流体的交代 ,不同的蚀变形成于岩浆流体演化的不同阶段。蚀?     

Geology,alteration and mesothermal Au-Ag-mineralization associated with a volcanic-intrusive complex at Mt. Shamrock-Mt. Ophir,SE Queensland

Au-Ag mineralization in the Mt-Shamrock — Mt. Ophir area of SE Queensland is related to a geographically-isolated calc-alkaline igneous centre consisting of high level plutonic and minor intrusions emplaced into the eroded remains of a silicic volcanic ediface and its basement. Mineralization occurs in both the igneous rocks and in the Permian siltstone country rocks and is controlled by a NE-trending structure parallel to Late Triassic lineaments. This structure is unrelated to, and younger than the exposed intrusions. Au-Ag-As-rich, Cu-Mo-poor mineralization occurs in breccias and veinlet networks within pervasively altered rocks characterized by silicification and H (sericite), CO₂ (calcite-ankerite), Na (albite), B (tourmaline), and S (pyrite) metasomatism. Secondary mineral compositions suggest that most of this alteration occurred at temperatures between 350 ° and 400 °C. The alteration was complex in detail and characterized by multiple hydrothermal events and space and/or time variations of physico-chemical conditions. Although some of these features are similar to prophyry deposits the chemical character of the alteration and mineralization is not typical of Cu-Mo-Au porphyries and has more in common with tectonometamorphic Au deposits formed at considerably greater depths.     

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 Geochemistry of the Buerkesidai and Kuoerzhenkuola Gold Deposits in the Sawuershan Region, Xinjiang Uigur Autonomous Region, Northwest China

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.     

Orogenic Gold Mineralization in the Qolqoleh Deposit, Northwestern Iran

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.     

The geochemistry of hydrothermal alteration at the Salgadinho copper deposit,Portugal

The recently discovered Salgadinho copper deposit, 7 km NNE of S. Luis, Portugal is located in the southernmost linear belt of outcropping low metamorphic grade deformed Palaeozoic rocks (Famennian) in the SW part of the Iberian Pyrite Belt. The stratabound replacement pyrite-chalcopyrite mineralisation is present in variably altered felsic pyroclastic rocks which are overlain by pyritic graphitic shales and tuffs which have undergone alteration in the lowermost 5m. The altered pyroclastic rocks are characterised by pale green celadonitic fluoro-muscovite and, in the most intense zone of alteration, quartz, ankerite, and ore minerals are present (pyrite, chalcopyrite, sphalerite, tetrahedrite, galena, bournonite). No exhalative Cu-Zn or Zn-Pb ore has been discovered associated with the submarine volcanic stratabound Cu-rich alteration zone. Alteration of feldspars and groundmass has involved a net loss from the system and gains in Fe²⁺, F, H₂O, Mg, Fe³⁺, Ca, Mn, P, Ti, S, Cu, Zn, Pb, As, Sb, Ag and Au at the expense of Si, K, Al and Na. The great enrichment of F in the altered rocks, the association of zonal alteration facies with coarse grained pyroclastic masses and the intimate association of pale green celadonitic fluoro-muscovite with mineralisation at Salgadinho and most other deposits of the Iberian Pyrite Belt represents a powerful exploration guide for submarine exhalative ores.Text of paper presented at the 26th I.G.C., Paris, July 1980     

Geological and Geochemical Characteristics of the Hydrothermal Clay Alteration in South Korea

Abstract: Hydrothermally altered areas forming pyrophyllite‐kaolin‐sericite‐alunite deposits are distributed in Chonnam and Kyongsang areas, Cretaceous volcanic field of the Yuchon Group. The Chonnam alteration area is located within depression zone which is composed of volcanic and granitic rocks of late Cretaceous age. The clay deposits of this area show the genetic relationship with silicic lava domes. The Kyongsang alteration area is mainly distributed within Kyongsang Basin comprising volcanic, sedimentary and granitic rocks of Cretaceous and Tertiary age. Most of the clay deposits of this area are closely related to cauldrons. Paleozoic clay deposit occurs in the contact zone between Precambrian Hongjesa granite gneiss and Paleozoic Jangsan quartzite of Choson Supergroup. Cretaceous igneous rocks of the both alteration areas belong to high K calc‐alkaline series formed in the volcanic arc of continental margin by subduction‐related magmatism. Chonnam igneous rocks show more enrichment of crustal components such as K, La, Ce, Sm, Nd and Ba, higher (La/Yb)cn ratio, and higher initial ⁸⁷Sr/⁸⁶Sr ratio (0. 708 to 0. 712) than those of Kyongsang igneous rocks. This might be due to the difference of degree of crustal contamination during Cretaceous magmatism. The most characteristic alteration minerals of Chonnam clay deposits are alunite, kaolin, quartz, pyrophyllite and diaspore which were formed by acidic solution. Those of Kyongsang clay deposits are sericite, quartz and pyrophyllite which were formed by weak acid and neutral solution. The formation ages of the clay deposits of two alteration areas range from 70. 1 to 81. 4 Ma and 39. 7 to 79. 4 Ma, respectively. The Daehyun clay deposit in Ponghwa area of Kyongsang province shows the alteration age range from 290 to 336 Ma. This result shows the different alteration episode from the hydrothermal alteration of Cretaceous to early Tertiary in the Kyongsang and Chonnam alteration areas. These data indicate, at least, three hydrothermal activities of Tertiary (middle to late Eocene), late Cretaceous (Santonian to Maastrichtian) and Paleozoic Carboniferous Periods in South Korea.     

Geology and ore genesis of the late Paleozoic Heijianshan Fe oxide–Cu (–Au) deposit in the Eastern Tianshan,NW China

The Heijianshan Fe–Cu (–Au) deposit, located in the Aqishan-Yamansu belt of the Eastern Tianshan (NW China), is hosted in the mafic–intermediate volcanic and mafic–felsic volcaniclastic rocks of the Upper Carboniferous Matoutan Formation. Based on the pervasive alteration, mineral assemblages and crosscutting relationships of veins, six magmatic–hydrothermal stages have been established, including epidote alteration (Stage I), magnetite mineralization (Stage II), pyrite alteration (Stage III), Cu (–Au) mineralization (Stage IV), late veins (Stage V) and supergene alteration (Stage VI). The Stage I epidote–calcite–tourmaline–sericite alteration assemblage indicates a pre-mineralization Ca–Mg alteration event. Stage II Fe and Stage IV Cu (–Au) mineralization stages at Heijianshan can be clearly distinguished from alteration, mineral assemblages, and nature and sources of ore-forming fluids.Homogenization temperatures of primary fluid inclusions in quartz and calcite from Stage I (189–370 °C), II (301–536 °C), III (119–262 °C) and V (46–198 °C) suggest that fluid incursion and mixing probably occurred during Stage I to II and Stage V, respectively. The Stage II magmatic–hydrothermal-derived Fe mineralization fluids were characterized by high temperature (>300 °C), medium–high salinity (21.2–56.0 wt% NaCl equiv.) and being Na–Ca–Mg–Fe-dominated. These fluids were overprinted by the external low temperature (<300 °C), medium–high salinity (19.0–34.7 wt% NaCl equiv.) and Ca–Mg-dominated basinal brines that were responsible for the subsequent pyrite alteration and Cu (–Au) mineralization, as supported by quartz CL images and H–O isotopes. Furthermore, in-situ sulfur isotopes also indicate that the sulfur sources vary in different stages, viz., Stage II (magmatic–hydrothermal), III (basinal brine-related) and IV (magmatic–hydrothermal). Stage II disseminated pyrite has δ³⁴S_fluid values of 1.7–4.3‰, comparable with sulfur from magmatic reservoirs. δ³⁴S_fluid values (24.3–29.3‰) of Stage III Type A pyrite (coexists with hematite) probably indicate external basinal brine involvement, consistent with the analytical results of fluid inclusions. With the basinal brines further interacting with volcanic/volcaniclastic rocks of the Carboniferous Matoutan Formation, Stage III Type B pyrite–chalcopyrite–pyrrhotite assemblage (with low δ³⁴S_fluid values of 4.6–10.0‰) may have formed at low fO₂ and temperature (119–262 °C). The continuous basinal brine–volcanic/volcaniclastic rock interactions during the basin inversion (∼325–300 Ma) may have leached sulfur and copper from the rocks, yielding magmatic-like δ³⁴S_fluid values (1.5–4.1‰). Such fluids may have altered pyrite and precipitated chalcopyrite with minor Au in Stage IV. Eventually, the Stage V low temperature (∼160 °C) and low salinity meteoric water may have percolated into the ore-forming fluid system and formed late-hydrothermal veins.The similar alteration and mineralization paragenetic sequences, ore-forming fluid sources and evolution, and tectonic settings of the Heijianshan deposit to the Mesozoic Central Andean IOCG deposits indicate that the former is probably the first identified Paleozoic IOCG-like deposit in the Central Asian Orogenic Belt.     

Gold and copper mineralization at the El Indio deposit, Chile

A Middle Tertiary volcanic belt in the High Andes of north-central Chile hosts numerous precious- and base-metal epithermal deposits over its 150 km north-south trend. The El Indio district, believed to be associated with a hydrothermal system in the late stages of development of a volcanic caldera, consists of a series of separate vein systems located in an area of 30 km² which has undergone intense argillic-sericitic-solfataric alteration. The majority of the known gold-copper-silver mineralization occurs within a structural block only 150 by 500 m in surface area, with a recognized vertical extent exceeding 300 m. This block is bounded by two high-angle northeast-trending faults oriented subparallel to the mineralized veins.Hypogene mineralization at El Indio is grouped into two main ore-forming stages: Copper and Gold. The Copper stage is composed chiefly of enargite and pyrite forming massive veins up to 20 m wide, and is accompanied by alteration of the wall rocks to alunite, kaolinite, sericite, pyrite and quartz. The Gold stage consists of vein-filling quartz, pyrite, native gold, tennantite and subordinate amounts of a wide variety of telluride minerals. Associated with this stage is pervasive alteration of the wall rocks to sericite, kaolinite, quartz and minor pyrophyllite. The transition from copper to gold mineralization is marked by the alteration of enargite to tennantite and by minor deposition of sphalerite, galena, huebnerite, chalcopyrite and gold. Mineral stability relations indicate that there was a general decrease in the activity of S₂ accompanied by variations in the activity of Te₂ during the Gold stage.Fluid-inclusion data show homogenization temperatures ranging from about 220 to 280°C, with salinities on the order of 3–4 eq. wt. % NaCl for the Copper stage. The Gold-stage inclusions indicate a similar range in homogenization temperatures, but significantly lower salinities (0.1–1.4 eq. wt. % NaCl). Fluid inclusions of transition minerals show a weak inverse relationship between homogenization temperatures (190–250°C) and salinities (3.4–1.4 eq. wt. % NaCl), which may represent mixing of hotter Gold-stage fluids with cooler late-Copper-stage fluids. No evidence of boiling was found in fluid inclusions, but CO₂ vapor-rich inclusions were identified in wall-rock quartz phenocrysts which pre-date copper and gold mineralization.Mineral stability calculations indicate that given a fairly restricted range of solution compositions, the Copper-, Transition- and Gold-stage minerals at El Indio could have been deposited from a single solution, with constant total dissolved sulfur which underwent reduction through time. Limited sulfur-isotope data indicates that pyrite from the Copper stage was not in isotopic equilibrium with Copper-stage alunite or Transition-stage sphalerite. The sulfur-isotope and fluid-inclusion data indicate that two fluids with comparable temperatures but different compositions flowed through the El Indio system. The earlier fluid deposited copper attended by sericite-alunite-kaolinite alteration, and later epithermal fluids deposited gold with quartz-sericite-kaolinite-pyrite alteration.     

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。     

甘肃李坝金矿围岩蚀变与金成矿关系

西秦岭地区是目前国内造山型和卡林型金矿找矿的热点地区之一,已发现的甘肃李坝造山型金矿为超大型规模。以李坝金矿6号矿带为例,系统地研究了其蚀变矿物组合、近矿围岩蚀变分带及相应的金矿化特征,总结了矿床(带)的蚀变分带模式。该模式具典型的中心式环带结构,可分为3个蚀变带,由中心向外依次为黄铁绢英岩化带、绢云母化带和绿泥石化带。蚀变矿物组合分别为黄铁矿+绢云母+石英±毒砂±白云母±电气石±方解石、绢云母+绿泥石+石英+黄铁矿±黑云母及绿泥石+黑云母±绢云母±黄铁矿;与这3个蚀变带相对应的是金的富集带、矿化带和无矿带。蚀变岩石物质组分迁移分析表明,围岩蚀变及其分带是热水流体/岩石反应时岩石化学组分发生迁移的结果,矿化伴随着蚀变发生,且金矿化与黄铁矿化和浸染状硅化关系最为密切。     

Gold mineralisation and alteration of Penakacherla schist belt, India, constraints on Archaean subduction and fluid processes

In the Dharwar Craton, southern India, gold deposits are found mostly along the six arcuate shear zones passing through late Archaean greenstone belts (2.7 Ga). One such shear zone complex extends for about 400 km within and along the Ramagiri–Hungund schist belt. The Penakacherla sector of this shear zone is excellently exposed, enabling a detailed investigation of synorogenic gold mineralisation and its relationship to associated hydrothermal alteration.Metamorphism and deformation under NE–SW compression associated with Archaean subduction processes converted mafic volcanic rocks into amphibolites and intermediate to felsic volcanic rocks into quartz mica schists. Continued compression generated a 50–100-m-wide shear zone complex consisting of mafic phyllonites. Advection of hydrothermal fluids through this shear zone and reaction between fluids and the mafic phyllonites resulted in a silicified, K-metasomatic assemblage mainly consisting of chlorite, amphibole, K-mica, plagioclase, ankerite, quartz, Fe-oxides, pyrite, chalcopyrite and arsenopyrite. Networks of quartz and carbonate veinlets, a few millimeters to a few centimeters thick, formed along the foliation planes giving rise to microscopic alteration envelope, in which individual veinlet systems are merged into one another to form a composite alteration system. Gold is found within these quartz veinlets, mafic phyllonites and at their mutual contacts.Hydrothermal fluids have modified the primary major, minor, trace and LREE compositions of host rocks such that their mutual behaviour became non-systematic. Some HFSE and HREE also show minor mobility but the overall REE pattern generally resembles that of the precursor mafic volcanic rocks. Mass and volume loss/gain by Si and Ca has made significant impact on Al, Ti and Zr abundances, which are generally immobile during hydrothermal alteration. However, element pairs such as Zr–Hf, V–Sc and Nb–Ta maintain primary inter-element ratios, although their absolute abundances are drastically diluted. Similarly, ΣREE in highly silicified and carbonatised samples are reduced, but patterns remain similar to those of relatively least altered mafic phyllonites with (La_N/Yb)_N between 1 and 3. In some samples, LREE enrichment is observed elevating in (La_N/Yb)_N from 3 to 11. Pathfinder elements and base metals such as As, Cd, Cu, Pb, Zn and Sb have been added along with the Au and Ag.δ¹³C of carbon varies from −16‰ to −21‰ suggesting a biogenic origin, whereas coexisting pyrite δ³⁴S ranges from 1‰ to 3‰, pointing towards the involvement of magmatic or average crustal sulphur. Overall concentrations of K, Rb, Sr, Ba, Nb, Ta, Ti, Cs, Cr, Co, V, Y and Sc and many of the ratios such as K/Rb, La/Sc, La/Yb indicate that metamorphism, devolatilisation and dehydration of an oceanic subducting slab might have partially contributed the mineralising fluids and generated the alteration assemblage observed in the host rocks. Fluid sources were mantle and greenstone belt dehydration and devolatilisation generating observed compositional and alteration diversity.     

Geology and volcanic stratigraphy of the Canatuan and Malusok volcanogenic massive sulfide deposits,southwestern Mindanao,Philippines

The Canatuan and Malusok massive sulfide deposits are located near Siocon, Zamboanga del Norte, in southwestern Mindanao, Philippines. The Canatuan–Malusok area is underlain by the Jurassic–Cretaceous Tungauan schists, which form much of the Zamboanga Peninsula. The volcanic strata at Canatuan and Malusok can be traced for >7 km along strike and is host to at least three discrete massive sulfide bodies: Canatuan, Malusok and SE Malusok. Basal basaltic andesite volcanic rocks are generally chemically uniform and show only moderate alteration. The massive sulfide deposits occur in overlying rhyolitic to rhyodacitic volcanic rocks that are altered to a schistose assemblage of quartz, sericite, chlorite and pyrite. The alteration is texturally destructive but graded clastic beds are locally observed. Despite tropical saprolitic weathering, four lithogeochemical subunits of the felsic package are identified. Stratigraphic interleaving, however, has made correlation of these units over any significant distance difficult. The sulfide lenses are overlain by a few metres of felsic schists which locally contain manganese-bearing silicates and oxides that serve as a stratigraphic marker. Hangingwall andesitic volcaniclastic rocks are discontinuously preserved, although where present, they consist of regularly bedded mafic volcanic sandstones. The lateral continuity of a manganese-bearing marker and flanking felsic volcaniclastic intervals indicate that locally the volcanic strata form a homoclinal sequence. The Canatuan Au–Ag–Cu–Zn deposit consists of a gossan overlying a massive sulfide lens. The sulfides and gossan are flat lying and hosted within felsic volcanic rocks. The gossan is gold–silver-rich, and was formed by a combination of oxidation and volume collapse of the original sulfide lens. The sulfide minerals present below the current water table, are auriferous massive pyrite with base metal sulfides, with some supergene chalcocite. The transition from gossan to sulfides is very sharp, occurring at the water table. Massive sulfide deposits at Malusok are hosted in the same felsic sequence as Canatuan and they have similar base and precious metal contents. Only limited gossan has been found at Malusok. The bimodal nature of the volcanic rocks at Canatuan, together with their low HFSE contents, near-flat REE patterns and tholeiitic affinities, suggest that they formed in an intra-oceanic arc setting above a depleted mantle source. Mafic and felsic volcanic rocks of similar composition have been recovered from the Tonga-Kermadec and Izu-Bonin-Marianas island-arc systems in the western Pacific. Mafic rocks at Canatuan show no evidence for LILE enrichment that characterizes melts derived from metasomatized mantle under more mature arcs, suggesting that they are the product of a nascent, rather than a mature arc. There is no evidence from the REE, or other incompatible trace elements, that continental crust or evolved arc crust was involved in the generation of the Canatuan-Malusok volcanic rocks. Although it has been proposed that the Zamboanga metamorphic complex comprises microcontinental fragments of Eurasian affinity, our data do not support an evolved crustal setting for the Canatuan-Malusok volcanic rocks, which we suggest were derived from an intra-oceanic arc and subsequently accreted to the eastern Mindanao terrane.Electronic Supplementary Material Supplementary material is available in the online version of this article at http://dx.doi.org/10.1007/s00126-003-0350-7Editorial handling: R.R. Large     

The Crater Mountain Deposit,Papua New Guinea: Porphyry‐related Au–Te System

Ore mineralization and wall rock alteration of Crater Mountain gold deposit, Papua New Guinea, were investigated using ore and host rock samples from drill holes for ore and alteration mineralogical study. The host rocks of the deposit are quartz‐feldspar porphyry, feldspar‐hornblende porphyry, andesitic volcanics and pyroclastics, and basaltic‐andesitic tuff. The main ore minerals are pyrite, sphalerite, galena, chalcopyrite and moderate amounts of tetrahedrite, tennantite, pyrrhotite, bornite and enargite. Small amounts of enargite, tetradymite, altaite, heyrovskyite, bismuthinite, bornite, idaite, cubanite, native gold, CuPbS₂, an unidentified Bi‐Te‐S mineral and argentopyrite occur as inclusions mainly in pyrite veins and grains. Native gold occurs significantly in the As‐rich pyrite veins in volcanic units, and coexists with Bi‐Te‐S mineral species and rarely with chalcopyrite and cubanite relics. Four mineralization stages were recognized based on the observations of ore textures. Stage I is characterized by quartz‐sericite‐calcite alteration with trace pyrite and chalcopyrite in the monomict diatreme breccias; Stage II is defined by the crystallization of pyrite and by weak quartz‐chlorite‐sericite‐calcite alteration; Stage III is a major ore formation episode where sulfides deposited as disseminated grains and veins that host native gold, and is divided into three sub‐stages; Stage IV is characterized by predominant carbonitization. Gold mineralization occurred in the sub‐stages 2 and 3 in Stage III. The fS₂ is considered to have decreased from ～10^?2 to 10^?14 atm with decreasing temperature of fluid.     

Geology and Hydrothermal Alteration of the Duobuza Gold‐Rich Porphyry Copper District in the Bangongco Metallogenetic Belt,Northwestern Tibet

The Duobuza gold‐rich porphyry copper district is located in the Bangongco metallogenetic belt in the Bangongco‐Nujiang suture zone south of the Qiangtang terrane. Two main gold‐rich porphyry copper deposits (Duobuza and Bolong) and an occurrence (135 Line) were discovered in the district. The porphyry‐type mineralization is associated with three Early Cretaceous ore‐bearing granodiorite porphyries at Duobuza, 135 Line and Bolong, and is hosted by volcanic and sedimentary rocks of the Middle Jurassic Yanshiping Formation and intermediate‐acidic volcanic rocks of the Early Cretaceous Meiriqie Group. Simultaneous emplacement and isometric distribution of three ore‐forming porphyries is explained as multi‐centered mineralization generated from the same magma chamber. Intense hydrothermal alteration occurs in the porphyries and at the contact zone with wall rocks. Four main hypogene alteration zones are distinguished at Duobuza. Early‐stage alteration is dominated by potassic alteration with extensive secondary biotite, K‐feldspar and magnetite. The alteration zone includes dense magnetite and quartz‐magnetite veinlets, in which Cu‐Fe‐bearing sulfides are present. Propylitic alteration occurs in the host basic volcanic rocks. Extensive chloritization‐silicification with quartz‐chalcopyrite or quartz‐molybdenite veinlets superimposes on the potassic alteration. Final‐stage argillic alteration overlaps on all the earlier alteration. This alteration stage is characterized by destruction of feldspar to form illite, dickite and kaolinite, with accompanying veinlets of quartz + chalcopyrite + pyrite and quartz + pyrite assemblages. Cu coexists with Au, which indicates their simultaneous precipitation. Mass balance calculations show that ore‐forming elements are strongly enriched during the above‐mentioned three alteration stages.     

Characteristic Features of REE and Pb–Zn–Ag Mineralizations in the Na Son Deposit,Northeastern Vietnam

The Na Son deposit is a small‐scale Pb–ZnPb–Zn–Ag deposit in northeast Vietnam and consists of biotite–chlorite schist, reddish altered rocks, quartz veins and syenite. The biotite–chlorite schist is intruded by syenite. Reddish altered rocks occur as an alteration halo between the biotite–allanite‐bearing quartz veins and the biotite–chlorite schist. Allanite occurs in the biotite–allanite‐bearing quartz veins and in the proximal reddish altered rocks. Rare earth element (REE) fluorocarbonate minerals occur along fractures or at rim of allanite crystals. The later horizontal aggregates of sulfide veins and veinlets cut the earlier reddish altered rocks. The earlier Pb–Zn veins consist of a large amount of galena and lesser amounts of sphalerite, pyrite and molybdenite. The later Cu veins cutting the Pb–Zn veins include chalcopyrite and lesser amounts of tetrahedrite and pyrite. The occurrences of two‐phase H₂O–CO₂ fluid inclusions in quartz from biotite–allanite‐bearing quartz veins and REE‐bearing fluorocarbonate minerals in allanite suggest the presence of CO₂ and F in the hydrothermal fluid. The oxygen isotopic ratios of the reddish altered rocks, biotite–chlorite schist, and syenite range from +13.9 to +14.9 ‰, +11.5 to +13.3 ‰, and +10.1 to +11.6 ‰, respectively. Assuming an isotopic equilibrium between quartz (+14.6 to +15.8 ‰) and biotite (+8.6 ‰) in the biotite–allanite‐bearing quartz vein, formation temperature was estimated to be 400°C. At 400°C, δ¹⁸O values of the hydrothermal fluid in equilibrium with quartz and biotite range from +10.5 to +11.7 ‰. These δ¹⁸O values are consistent with fluid that is derived from metamorphism. Assuming an isotopic equilibrium between galena (+1.5 to +1.7 ‰) and chalcopyrite (+3.4 ‰), the formation temperature was estimated to be approximately 300°C. The formation temperature of the Na Son deposit decreased with the progress of mineralization. Based on the geological data, occurrence of REE‐bearing minerals and oxygen isotopic ratios, the REE mineralization is thought to result from interaction between biotite–chlorite schist and REE‐, CO₂‐ and F‐bearing metamorphic fluid at 400°C under a rock‐dominant condition.