Origin of Illites at Dexing Porphyry Copper Deposit,Jiangxi Province,East China：Implications for Alteration Zoning and Ore—Forming Fluid Evolution

According to differences in features of illites including spatial distribution, crystallinity index, volume of swelling layer, polytype and relationship between its index and copper grade, two typical kinds of illite can be classified within the Tongchang porphyry copper deposit, Dexing County, East China. One is a kind of hydrothermally altered minerals within the hydrothermal alteration zone, including altered granodiorite-porphyry and altered metamorphic tuffaceous phyllite near the contact zone with porphyry rockbody. The illite crystallinity and expandability are mainly affected by water/rock ratio or fluid flux, and hydrothermal illite is formed by illitization of plagioclase and/or micas during hydrothermal fluid evolution within the porphyry body and near the contact zone with wall rocks. The other is a product of low-grade metamorphism itself by illitization of smectite, whose crystallinity index is lower than the hydrothermal illite and which is of 2M₁ polytype with no swelling layer, in the altered metamorphic tuffaceous phyllite far from porphyry rockbody ( > 2 km). Moreover, the negative correlation between illite index and copper grade indicates that, within the alteration zone, the smaller the illite crystallinity, the stronger the alteration degree, and the higher the copper grade due to higher water/rock ratio. At lower levels of the porphyry body, however, the illite crystallinity (IC) values are controlled mainly by temperature and time.     

Illite occurrences related to volcanic-hosted hydrothermal mineralization in the Biga Peninsula,NW Turkey: Implications for the age and origin of fluids

A different approach to investigate the origin of fluids, temperature conditions, age of hydrothermal activity of mineralization in the Biga Peninsula, (Koru, Tesbihdere and Kumarlar) employed mineralogical (illite Kübler Index, b cell dimension, polytype) and geochemical (major, trace/REE, O–H stable isotope and Rb/Sr dating) methods. The Kübler Index (KI) values of illites indicate different temperature conditions, such as low temperature (high-grade diagenesis) for Koru deposit, and high temperature (anchizone) for the Tesbihdere and Kumarlar deposits. The textural, mineralogical and geochemical data from illites show that these have potential for estimating the age of hydrothermal activity and fluid characteristics. Both mineralogical (high grade diagenetic to anchizonal KI, 1M polytype, low d₀₆₀ values) and geochemical (similar major and trace element composition to host-rocks, low octahedral Mg + Fe contents, oxygen and hydrogen isotope composition) data are compatible with commonly known hydrothermal illites. Stable isotope data of illites are well matched to similar data from fluid inclusions, which indicate mainly magmatic fluids. The Rb/Sr age (22.4 ± 2.3 Ma: latest Oligocene and lowest Miocene) of the illites coincides with plutonic intrusions that are the main instigators of hydrothermal activities related to the extensional tectonic regime in the Biga Peninsula. The mineralogical and geochemical data of illites have some important advantages with respect to the use of fluid inclusions in determining δD of hydrothermal fluids thereby leading to better understanding ore-forming hydrothermal conditions.     

Mineralogical, geochemical and isotopic characteristics of hydrothermal alteration processes in the active, submarine, felsic-hosted PACMANUS field, Manus Basin, Papua New Guinea

During ODP Leg 193, 4 sites were drilled in the active PACMANUS hydrothermal field on the crest of the felsic Pual Ridge to examine the vertical and lateral variations in mineralization and alteration patterns. We present new data on clay mineral assemblages, clay and whole rock chemistry and clay mineral strontium and oxygen isotopic compositions of altered rocks from a site of diffuse low-temperature venting (Snowcap, Site 1188) and a site of high-temperature venting (Roman Ruins, Site 1189) in order to investigate the water-rock reactions and associated elemental exchanges.The volcanic succession at Snowcap has been hydrothermally altered, producing five alteration zones: (1) chlorite ± illite-cristobalite-plagioclase alteration apparently overprinted locally by pyrophyllite bleaching at temperatures of 260-310°C; (2) chlorite ± mixed-layer clay alteration at temperatures of 230°C; (3) chlorite and illite alteration; (4) illite and chlorite ± illite mixed-layer alteration at temperatures of 250-260°C; and (5) illite ± chlorite alteration at 290-300°C. Felsic rocks recovered from two holes (1189A and 1189B) at Roman Ruins, although very close together, show differing alteration features. Hole 1189A is characterized by a uniform chlorite-illite alteration formed at ∼250°C, overprinted by quartz veining at 350°C. In contrast, four alteration zones occur in Hole 1189B: (1) illite ± chlorite alteration formed at ∼300°C; (2) chlorite ± illite alteration at 235°C; (3) chlorite ± illite and mixed layer clay alteration; and (4) chlorite ± illite alteration at 220°C.Mass balance calculations indicate that the chloritization, illitization and bleaching (silica-pyrophyllite assemblages) alteration stages are accompanied by different chemical changes relative to a calculated pristine precursor lava. The element Cr appears to have a general enrichment in the altered samples from PACMANUS. The clay concentrate data show that Cr and Cu are predominantly present in the pyrophyllites. Illite shows a significant enrichment for Cs and Cu relative to the bulk altered samples.Considerations of mineral stability allow us to place some constraints on fluid chemistry. Hydrothermal fluid pH for the chloritization and illitization was neutral to slightly acidic and relatively acidic for the pyrophyllite alteration. In general the fluids, especially from Roman Ruins and at intermediate depths below Snowcap, show only a small proportion of seawater mixing (<10%). Fluids in shallow and deep parts of the Snowcap holes, in contrast, show stronger seawater influence.     

Genesis of the hydrothermal gold deposits in the Canan area,Lepaguare District,Honduras

The Canan area (Honduras) is characterized by a gold-bearing ore deposit that is associated with quartz-veined shear zones. Gold mineralization occurs in low-to medium-grade metamorphic host-rocks (graphitic and sericitic schists). Hydrothermal fluids, which are associated with the emplacement of Cretaceous-Tertiary granodioritic intrusions, are responsible for the formation of quartz veins and the hydrothermal alteration of wall-rocks. Three main altered zones have been detected in the wall-rocks as far as 150 cm from the quartz veins. The distal zone (up to 50-cm thick) contains quartz, chlorite and illite. The intermediate zone is the thickest (up to 80 cm) and is marked by quartz, muscovite, sulphides, kaolinite and native elements such as Au and Ag. The proximal zone, which is close to the quartz veins, is rather thin (up to 25 cm) and contains clay minerals, Al-oxides-hydroxides and sulphides. The transition from the distal to the proximal zone is accompanied by the enrichment of SiO₂ and the depletion of all other major elements, except for Fe₂O_3(tot). Precious metals occur in the highest concentrations in the intermediate zone (Au up to 7.6 ppm and Ag up to 11 ppm). We suggest that gold was transported as a reduced sulphur complex and was precipitated from the hydrothermal solution by the reaction of the sulphur complexes with Fe²⁺ from the alteration of the mafic minerals of the host-rock. Fluid–wall-rock interactions seem to be the main cause of gold mineralization. Genetic relationships with a strike-slip fault system, hydrothermal alteration zones within the metamorphic wall-rocks, and an entire set of geochemical anomalies are consistent with orogenic-type gold deposits of the epizonal class.     

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.     

松树岗钽铌钨锡矿床石英脉的矿物学研究——云母和黑钨矿成分对热液成矿过程的制约

矿石矿物和脉石矿物的成分演化蕴含了热液成矿过程的详细信息。本文基于岩相学观察,从云母和黑钨矿着手,利用电子探针和LA-ICP-MS分析技术,对赣东北松树岗Ta-Nb-W-Sn矿床的浅部热液成矿过程开展了研究。结果表明,松树岗矿床浅部的钨锡矿体的石英脉,从早到晚,由深至浅,可以划分为黑钨矿石英脉、锡石石英脉、硫化物石英脉和贫矿石英脉。4类石英脉中都含有早期的铁锂云母和晚期的白云母与铁的氧化物集合体,深部早期脉中的云母以铁锂云母为主,而浅部晚期脉中的云母以白云母为主。与早期铁锂云母相比,晚期白云母具有明显较低的Ti、Na、Rb、Cs、W、Nb、Zn、Li_2O含量和明显较高的Pb、Cu、B含量。从深部早期脉到浅部晚期脉,云母成分存在如下演化趋势:Ti、Na、W、Nb含量降低,Pb、Zn、Cu、Li_2O、B含量增高。不同深度的黑钨矿石英脉中含有两种不同成分的黑钨矿,属同一期演化早晚形成。相对于热液流体早期沉淀的黑钨矿,晚期黑钨矿具有明显较低的Nb、Ta、Zr、Hf、Ti、Sn、U、In、Sc含量和明显较高的Mo含量和Fe O/MnO值。云母和黑钨矿主微量元素成分的演化揭示了在松树岗矿床浅部的热液成矿早期以岩浆热液为主,晚期由于水岩反应的加强有较多围岩物质贡献。     

胶西北寺庄金矿床红化蚀变过程及其对金成矿贡献

胶东是我国最重要的金矿集区,其内金矿床均赋存于沿NE-NNE向断裂带展布的大规模红化蚀变带中;然而对红化蚀变是钾长石化还是赤铁矿-金红石化,及其对金成矿的贡献尚存争议。寺庄超大型金矿床的红化蚀变沿NE-NNE向焦家断裂带及其次级断裂-裂隙系统发育,占已探明资源储量70%的Ⅲ号矿体群即赋存于红化蚀变带内,是研究红化蚀变与金成矿关系的理想对象。本文以该金矿床红化蚀变花岗岩为研究对象,通过对比新鲜花岗岩与强、弱红化蚀变岩内矿物组合和地球化学组成,探讨红化蚀变对于金成矿的贡献。矿物学研究表明,弱红化蚀变岩内的蚀变发生在斜长石核部,以钠长石化为主,同时形成绢云母和少量热液钾长石,且赤铁矿在此阶段沉淀;而弱红化蚀变岩进一步水岩反应成为强红化蚀变岩的过程中出现大量热液钾长石。质量平衡计算表明,红化蚀变过程中SiO_2、K_2O迁入,而Na_2O、CaO、Al_2O_3、FeO~T、MgO迁出;红化流体由早期富Na向后期富K转变。岩石地球化学与氢氧同位素综合示踪显示,红化流体为高温、高氧逸度、富K的玲珑岩浆期后热液,与胶东金矿床中-低温、还原性、富CO_2成矿流体性质相反,表明红化流体未直接参与成矿过程。综合研究揭示,流体交代斜长石后形成贯通性孔隙提高红化蚀变岩的渗透性;热液钾长石交代斜长石导致岩石体积膨胀而破裂,降低岩石抗压强度;这些为成矿期断裂活动以及成矿流体的运移和成矿物质的沉淀提供了极为有利的围岩条件,可能是巨量金聚集成矿的关键因素之一。     

Geothermometry, geochronology, and mass transfer associated with hydrothermal alteration of a rhyolitic hyaloclastite from Ponza Island, Italy

A rhyolitic hyaloclastite from Ponza Island, Italy, was hydrothermally altered, producing four distinct alteration zones based on X-ray diffraction mineralogy and field textures: (1) nonpervasive argillic zone; (2) propylitic zone; (3) silicic zone; and (4) sericitic zone. The unaltered hyaloclastite is volcanic breccia with clasts of vesiculated obsidian in a matrix of predominantly pumice lapilli. Incomplete alteration of the hyaloclastite resulted in the nonpervasive argillic zone, characterized by smectite and disordered opal-CT. The other three zones exhibit more complete alteration of the hyaloclastite. The propylitic zone is characterized by mixed-layer illite-smectite (I-S) with 10 to 85% I, mordenite, opal-C, and authigenic K-feldspar (akspar). The silicic zone is characterized by I-S with ≥90% I, pure illite, quartz, akspar, and occasional albite. The sericitic zone consists primarily of I-S with ≥66% I, pure illite, quartz, and minor akspar and pyrite. K/Ar dates of I-S indicate hydrothermal alteration occurred at 3.38 ± 0.08 Ma.Oxygen isotope compositions of I-S systematically decrease from zones 1 to 4. In the argillic zone, smectite has δ¹⁸O values of 21.7 to 22.0‰ and I-S from the propylitic, silicic, and sericitic zones ranges from 14.5 to 16.3‰, 12.5 to 14.0‰, and 8.6 to 11.9‰, respectively. δ¹⁸O values for quartz from the silicic and sericitic zones range from 12.6 to 15.9‰. By use of isotope fractionation equations and data from authigenic quartz-hosted primary fluid inclusions, alteration temperatures ranged from 50 to 65°C for the argillic zone, 85 to 125°C for the propylitic zone, 110 to 210°C for the silicic zone, and 145 to 225°C for the sericitic zone. Fluid inclusion data and calculated δ¹⁸O_water values indicate that hydrothermal fluids were seawater dominated.Mass-transfer calculations indicate that hydrothermal alteration proceeded in a relatively open chemical system and alteration in the sericitic zone involved the most extensive loss of chemical species, especially Si. Systematic gains in Mg occur in all alteration zones as a result of I-S clay mineral formation, and systematic losses of Na, Ca, and K occur in most zones. With the exception of Ca, calculations of mass transfer associated with hydrothermal alteration on Ponza agree with chemical fluxes observed in laboratory experiments involving hydrothermal reactions of rhyolite and seawater. The anomalous Ca loss at Ponza may be due to hydrothermal formation of anhydrite and later low-temperature dissolution. On the basis of Mg enrichments derived from circulating seawater, we estimate the following minimum water/rock ratios: 9, 3, 6, and 9 for the argillic, propylitic, silicic, and sericitic zones, respectively. Hydrothermal fluid pH for the propylitic and silicic zones was neutral to slightly basic and relatively acidic for the sericitic zone as a result of condensation of carbonic and perhaps other acids.     

Episodic mineralization of hydrothermal illite in the Soultz-sous-Forêts granite (Upper Rhine Graben, France)

Episodic and localized illite mineralization is documented in the hydrothermally altered Soultz-sous-Forêts granite (Upper Rhine Graben, France). Separated grain-size fractions of altered granite and argillite vein samples contain mixtures of 2M₁ and 1M trans-vacant illite varieties. The platy pseudohexagonal 2M₁ illite phases dominate the vein fillings, whereas the 1M illite occurs largely as a fibrous pore-filling variety, which is particularly abundant in the granite matrix. Multiple phases of fluid injections into the granite body have resulted in different illite assemblages, each sample containing a mixture of polytype generations formed during different crystal growth events. On the basis of mineralogical and K–Ar isotopic constraints, the ages of these vein-mineralizing events are determined by plotting the K–Ar values of the various grain-size fractions against polytype abundance and the fitted volume-weighted crystallite thickness distributions. The results suggest a Permian age for the formation of the studied argillite veins, characterized by successive injections of hydrothermal fluids. Secondary episodes of illite crystallization occurred during Jurassic and Cretaceous (or even younger times) in both the veins and the granite matrix. There are indications that the polytype structure and composition of illite were strongly influenced by variations in fluid chemistry and the degree of fluid–rock interaction as the granite was progressively sealed during post-Variscan, episodic hydrothermal activity.     

Structural control and hydrothermal alteration at the BIF-hosted Raposos lode-gold deposit, Quadrilátero Ferrífero, Brazil

In the Raposos orogenic gold deposit, hosted by banded iron-formation (BIF) of the Archean Rio das Velhas greenstone belt, the hanging wall rocks to BIF are hydrothermally-altered ultramafic schists, whereas metamafic rocks and their hydrothermal schistose products represent the footwall. Planar and linear structures at the Raposos deposit define three ductile to brittle deformational events (D₁, D₂ and D₃). A fourth group of structures involve spaced cleavages that are considered to be a brittle phase of D₃. The orebodies constitute sulfide-bearing D₁-related shear zones of BIF in association with quartz veins, and result from the sulfidation of magnetite and/or siderite. Pyrrhotite is the main sulfide mineral, followed by lesser arsenopyrite and pyrite. At level 28, the hydrothermal alteration of the mafic and ultramafic wall rocks enveloping BIF define a gross zonal pattern surrounding the ore zones. Metabasalt comprises albite, epidote, actinolite and lesser Mg/Fe–chlorite, calcite and quartz. The incipient stage includes the chlorite and chlorite-muscovite alteration zone. The least-altered ultramafic schist contains Cr-bearing Mg-chlorite, actinolite and talc, with subordinate calcite. The incipient alteration stage is subdivided into the talc–chlorite and chlorite–carbonate zone. For both mafic and ultramafic wall rocks, the carbonate–albite and carbonate–muscovite zones represent the advanced alteration stage.Rare earth and trace element analyses of metabasalt and its alteration products suggest a tholeiitic protolith for this wall rock. In the case of the ultramafic schists, the precursor may have been peridotitic komatiite. The Eu anomaly of the Raposos BIF suggests that it was formed proximal to an exhalative hydrothermal source on the ocean floor. The ore fluid composition is inferred by hydrothermal alteration reactions, indicating it to having been H₂O-rich containing CO₂ + Na⁺ and S. Since the distal alteration halos are dominated by hydrated silicate phases (mainly chlorite), with minor carbonates, fixation of H₂O is indicated. The CO₂ is consumed to form carbonates in the intermediate alteration stage, in halos around the chlorite-dominated zones. These characteristics suggest variations in the H₂O to CO₂-ratio of the sulfur-bearing, aqueous-carbonic ore fluid, which interacted at varying fluid to rock ratios with progression of the hydrothermal alteration.     

Kübler Index and K-Ar Ages of Illite in the Yinshan Polymetallic Deposit, Jiangxi Province, South China: Analyses and Implications

Abstract. The Yinshan polymetallic deposit is a hydrothermal vein-type deposit closely related to Late Jurassic felsic-inter-mediate volcanic-subvolcanic activity in Jiangxi Province, South China. Illite is a major alteration mineral observed in the deposit. Our study shows that the Kübler index of the illite has a close relation to ore-forming fluids of different stages of hydrothermal alteration and mineralization. The early Pb-Zn-Ag mineralization dated at 130–136 Ma is characterized by relatively low water/rock ratios and diffusive fluid movement within phyllite, whereas the later Cu-Au-S mineralization at 122 -125 Ma was accompanied by higher water/rock ratios and localized fluid flow through fractures and channels. Illite formed in the early Pb-Zn-Ag mineralization stage contains swelling layers while the illite formed in the later Cu-Au-S mineralization stage has no swelling layers but was associated with intensive chloritization. The last stage of mineralization (at 104 Ma) was minor and did not produce significant amounts of illite.     

南澜沧江带勐满热泉型金矿床成矿作用:热液蚀变与元素地球化学制约

勐满金矿床是西南三江特提斯造山带迄今为止报导的为数不多的热泉型金矿床之一,也是南澜沧江带唯一发现的该成因类型的金矿床。然而目前对勐满金矿床热液蚀变特征的分析不足和地球化学数据的缺乏,一直制约着对其成矿过程的深入理解。勐满金矿床原生矿体产于早古生代澜沧群曼来组片岩和侏罗纪花开左组碎屑岩不整合面附近,断裂构造导流、控矿作用显著。矿区两类围岩均发生强烈蚀变,但蚀变类型简单,仅为硅化和高岭土化,与金成矿密切相关。热液高岭土化的大量发育,反映围岩中的长石等含铝矿物与呈酸性的流体发生作用。全岩微量元素组成对比研究表明,近矿围岩蚀变过程中未发生明显的微量元素迁移。镜下观察到Au与黄铜矿等金属硫化物共生,元素相关性分析显示Au与Ag、Cu、Pb、As、S、Sb等元素有正相关趋势,表明它们由统一热液系统携带并发生卸载。在弱酸性成矿流体中,Au主要以金硫络合物的形式进行迁移。当含Au流体运移至地层不整合面附近时,与围岩反应并发生强烈高岭土化,导致流体中的SiO_2和高岭石含量急剧增加,逐渐在矿区导流断裂中沉淀下来。断裂变窄甚至封闭,流体内压持续升高,最终发生爆破,成矿流体强烈减压沸腾,引发金硫络合物失稳,Au发生卸载并沉淀。该过程反复多次发生,形成了矿区含金硅质角砾岩及蚀变岩型矿石。     

云南中甸红牛-红山矽卡岩型铜矿床矿物学特征与成矿作用

红牛-红山矿床位于西南三江成矿带的中甸岛弧,是形成于晚燕山期的矽卡岩型铜矿床。矿区与成矿作用密切相关的石英二长斑岩中角闪石和黑云母斑晶的出现以及较高的含F量(分别为1.49%和2.62%),表明其岩浆为富H2O富挥发分熔体;石英斑晶具有港湾状、浑圆状的溶蚀表面和钾长石细晶外壳,并且显示了典型的骸晶状结构指示了其岩浆经历了快速上升侵位过程和岩浆热液的自交代作用;钻孔中岩浆热液角砾岩和大量石英细脉的出现暗示了岩浆在快速上侵过程中发生了隐爆作用,形成并出溶了含有大量F、Cl等组分的高盐度超临界流体。矽卡岩阶段石榴子石和透辉石具有明显的三个期次:早期细粒的钙铝榴石(And22-57)和角岩中的透辉石(Hd7-27)形成于少量高温气液岩浆流体与围岩的扩散交代作用;中期粗粒的钙铁榴石(And75-98)和次透辉石-钙铁辉石(Hd10-99)形成于大量高温、低氧逸度的岩浆流体与围岩的渗滤交代作用;晚期的钙铝榴石脉(And14-60)和钙铁辉石脉(Hd31-58)形成于低温、高氧逸度的早期交代残留溶液。矽卡岩矿物的生成,使碳酸盐围岩丢失CO2,矿物体积减少,孔隙度和渗透性增加,为成矿提供了条件。退化变质阶段的透闪石、阳起石、绿帘石、绿泥石等交代早期矽卡岩矿物,消耗了成矿流体中大量的CO2和H2O,生成含水矿物以及石英、方解石,使围岩裂隙愈合,孔隙流体压力增加,导致成矿流体沸腾,形成大量黄铜矿、磁黄铁矿、黄铁矿、辉钼矿化。石英-硫化物阶段,由于成矿流体超压→流体沸腾,裂隙生成→减压排泄,裂隙愈合→流体超压的循环,在此过程中围岩经历了多次破裂和裂隙的愈合,直至整个成矿体系完全开放,并与大气水发生混合,使成矿流体中剩余金属最终沉淀。     

Evidences of felsic volcanism and hydrothermal activities from clays associated with the Palaeoproterozoic Porcellanite Formation of the Vindhyan Supergroup,Central India

Chemico-mineralogical attributes of authigenic clays associated with the altered volcanic tuffs that occur in the Palaeoproterozoic Porcellanite Formation contain evidences of hydrothermal alteration and diagenetic processes in a marine environment. Previous sedimentological and geochemical studies on Porcellanite Formation were restricted to the Chopan area, but, the details related to provenance, nature and source of volcanism archived in these clays have not been ascertained. In order to understand these aspects, present study on these authigenic clays were carried out. Clay minerals represent dominance of illite with subordinate amount of montmorillonite. Moreover, low abundance of kaolinite is also noticed. The illite fibers and plates associated with the kaolinite indicate illitization. The kaolinite to illite transformation is favoured by incorporation of K⁺ ions, derived from the K-feldspar dissolution and its overgrowth. Major oxide contents of these clays and their ratios when plotted over diagrams marked with standard illite, kaolinite, smectite and chlorite compositional fields show clustering within or close to the illite field. Thermodynamic components calculated for these clays when plotted over AR₂³⁺AlSi₃O₁₀(OH)₂ − R₂³⁺Si₄O₁₀(OH)₂ − AR²⁺R³⁺Si₄O₁₀(OH)₂ ternary diagram, data plots lie within the illite, mixed layer I/S and smectite fields. Binary major oxide data plots between bulk rock and authigenic clay compositions showed felsic affinity. Montmorillonite and illite predominated in the eastern and western marginal areas of the Vindhyan Basin, respectively. However, former resulted from the hydrothermal alteration of volcanic glass associated with the ferruginous breccia and altered tuffs and remnants of the volcanic vents, whereas, later is associated with the tuffaceous beds. Owing to the adsorption, Ba, Rb and Sr is enriched in clays comparing to the bulk rock composition. Low (< 15 ppm) Sc values suggested major contribution from the felsic component. Also, low Rb/Sr and Th/U values revealed moderate insitu weathering. The dominance of K-feldspar alteration and insitu weathering is also evident from clustering of clay data plots in the A-CN-K ternary diagram. Pronounced negative Eu anomaly together with higher LREE/HREE values associated with these clay minerals implied proximity to source and their possible derivation from the silicified felsic tuffs available in the provenance.     

Subseafloor hydrothermal alteration during the Early Proterozoic at Garpenberg,Central Sweden

The Early Proterozoic sulfide deposit at Garpenberg is located in the metallogenetic province of central Sweden. It is a strata-bound massive sulfide deposit contained in a supracrustal sequence of mainly acid metavolcanic rocks. Stratiform Zn-Pb-Cu mineralization is underlain by Cu-bearing stockwork ore and an extensive alteration zone. The sulfide ores and their altered wall rocks were formed by subseafloor hydrothermal activity. The alteration pattern observed in the wall rocks of this deposit is consistent with a hydrothermal system where the fluid consists mainly of seawater and a high water/rock mass ratio predominates. The hydrothermal activity caused the destruction of the primary mineralogy, mainly feldspars, and a general redistribution of the chemical elements in the altered wall rocks which were principally depleted in Ca, Na and Eu and enriched in Mg. Eu was redeposited with the ore metals near or at the seafloor and Ca was deposited as limestone. Most of the major and trace elements show large mobility during the alteration; only Ti, Zr, Y and REE (excluding Eu) behaved as relatively immobile elements.     

Alteration mineralogy,lithochemistry and stable isotope geochemistry of the Murgul (Artvin,NE Turkey) volcanic hosted massive sulfide deposit: Implications for the alteration age and ore forming fluids

The Murgul (Artvin, NE Turkey) massive sulfide deposit is hosted dominantly by Late Cretaceous calc-alkaline to transitional felsic volcanics. The footwall rocks are represented by dacitic flows and pyroclastics, whereas the hanging wall rocks consist of epiclastic rocks, chemical exhalative rocks, gypsum-bearing vitric tuff, purple vitric tuff and dacitic flows. Multi-element variation diagrams of the hanging wall and footwall rocks exhibit similar patterns with considerable enrichment in K, Rb and Ba and depletion in Nb, Sr, Ti and P. The chondrite-normalized rare earth element (REEs) patterns of all the rocks are characterized by pronounced positive/negative Eu anomalies as a result of different degrees of hydrothermal alteration and the semi-protected effects of plagioclase fractionation.Mineralogical results suggest illite, illite/smectite + chlorite ± kaolinite and chlorite in the footwall rocks and illite ± smectite ± kaolinite and chlorite ± illite in the hanging wall rocks. Overall, the alteration pattern is represented by silica, sericite, chlorite and chlorite–carbonate–epidote–sericite and quartz/albite zones. Increments of Ishikawa alteration indexes, resulting from gains in K₂O and losses in Na₂O and the chlorite–carbonate–pyrite index towards to the center of the stringer zone, indicate the inner parts of the alteration zones. Calculations of the changes in the chemical mass imply a general volume increase in the footwall rocks. Abnormal volume increases are explained by silica and iron enrichments and a total depletion of alkalis in silica zone. Relative K increments are linked to the sericitization of plagioclase and glass shards and the formation of illite/smectite in the sericite zone. In addition, Fe enrichment is always met by pyrite formation accompanied by quartz and chlorite. Illite is favored over chlorite, smectite and kaolinite in the central part of the ore body due to the increase in the (Al + K)/(Na + Ca) ratio. Although the REEs were enriched in the silicification zone, light REEs show depletion in the silicification zone and enrichment in the other zones in contrast to the heavy REEs' behavior. Hydrothermal alteration within the hanging wall rocks, apart from the gypsum-bearing vitric tuffs, is primarily controlled by chloritization with proportional Fe and Mg enrichments and sericitization.The δ¹⁸O and δD values of clay minerals systematically change with increasing formation temperature from 6.6 to 8.7‰ and − 42 to − 50‰ for illites, and 8.6 and − 52‰ for chlorite, respectively. The O- and H-stable isotopic data imply that hydrothermal-alteration processes occurred at 253–332 °C for illites and 136 °C for chlorite with a temperature decrease outward from the center of the deposit. The positive δ³⁴S values (20.3 to 20.4‰) for gypsum suggest contributions from seawater sulfate reduced by Fe-oxide/-hydroxide phases within altered volcanic units. Thus, the hydrothermal alteration possibly formed via a dissolution–precipitation mechanism that operated under acidic conditions. The K–Ar dating (73–62 Ma) of the illites indicates an illitization process from the Maastrichtian to Early Danian period.     

Hydrothermal Alteration Related to Silver Mineralization at the Iwami Silver Deposit,Shimane Prefecture,Japan

The Iwami epithermal silver deposit consists of Ag-Cu veins in a dacitic intrusive body at the deep portion of the Eikyu area, and veinlets with disseminated Ag mineralization in dacitic tuff breccia at a shallow portion of the Fukuishi area. Hydrothermal alteration associated with the silver mineralization is characterized by intense potassium metasomatism with oxidizing conditions. An illite zone occurs around the pathways of uprising fluids in both the Eikyu and Fukuishi areas. It grades laterally into the illite/smectite zone, which is surrounded by a broad smectite zone. Because of boiling, abundant adularia associated with silver mineralization overlaps on the altered tuff breccia in the Fukuishi area. The alteration zoning suggests that the western Eikyu area and the eastern Fukuishi area belong to a single hydrothermal system. The data of fluid inclusion microthermometry indicate that the temperatures range 220–270°C, and salinities range 5–7 wt percent NaCl equivalent for the silver mineralization at the upper portion in the Eikyu area and the lower portion in the Fukuishi area. Radiometric ages for volcanic rocks in the area range from 2.19 to 1.64 Ma, and the dacitic intrusion formed at approximately 1.6 Ma. The silver-dominant mineralizing hydrothermal fluids system was active around 1.44 to 1.07 Ma, which formed the Eikyu Ag-Cu veins at depth, and the Fukuishi Ag ores at the shallower portion.     

The application of P–T–X(CO2) modelling in constraining metamorphism and hydrothermal alteration at the Damang gold deposit,Ghana

Orogenic gold mineralization at the Damang deposit, Ghana, is associated with hydrothermal alteration haloes around gold‐bearing quartz veins, produced by the infiltration of a H₂O–CO₂–K₂O–H₂S fluid following regional metamorphism. Alteration assemblages are controlled by the protoliths with sedimentary rocks developing a typical assemblage of muscovite, ankerite and pyrite, while intrusive dolerite bodies contain biotite, ankerite and pyrrhotite, accompanied by the destruction of hornblende. Mineral equilibria modelling was undertaken with the computer program thermocalc , in subsets of the model system MnO–Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–CO₂–H₂O–TiO₂–Fe₂O₃, to constrain conditions of regional metamorphism and the subsequent gold mineralization event. Metapelites with well‐developed amphibolite facies assemblages reliably constrain peak regional metamorphism at ~595 °C and 5.5 kbar. Observed hydrothermal alteration assemblages associated with gold mineralization in a wide compositional range of lithologies are typically calculated to be stable within P–T–X(CO₂) arrays that trend towards lower temperatures and pressures with increasing equilibrium fluid X(CO₂). These independent P–T–X(CO₂) arrays converge and the region of overlap at ~375–425 °C and 1–2 kbar is taken to represent the conditions of alteration approaching equilibrium with a common infiltrating fluid with an X(CO₂) of ~0.7. Fluid‐rock interaction calculations with M–X(CO₂) diagrams indicate that the observed alteration assemblages are consistent with the addition of a single fluid phase requiring minimum fluid/rock ratios on the order of 1.     

热液流体活动规律高光谱遥感分析示范研究——以新疆白杨河铀矿床为例

高光谱遥感技术可以有效地用于分析热液流体活动规律,进而指导找矿预测。利用国际先进的CASI/SASI航空高光谱遥感技术、并结合ASD便携式地面高光谱技术手段,文章从航空、地面、深部钻孔岩芯等不同尺度对白杨河铀矿区及周围地表和深部的热液蚀变类型和热液活动规律等进行了立体识别与研究。研究表明,白杨河铀矿区及周围航空高光谱遥感识别的蚀变矿物在空间上可分为北部蚀变区、矿区蚀变区、南部蚀变区等3个蚀变带。北部蚀变区发育一套以叶腊石、明矾石、高铝绢云母为主的酸性热液蚀变组合;矿区蚀变区地表发育高铝绢云母、中铝绢云母、赤铁矿、硅化等热液蚀变,矿区深部钻孔岩芯主要发育高铝绢云母、中铝绢云母、低铝绢云母、高岭石、赤铁矿、褐铁矿、绿泥石等蚀变矿物及其混合蚀变,且绢云母蚀变矿物的Al-OH吸收峰波长位置存在"单向递减"和"先减后增"等2种明显变化规律;南部蚀变区发育低铝绢云母和中铝绢云母蚀变。分析表明,北部蚀变区是相对的区域热液流体活动中心,铀矿区蚀变区是流体活动中心旁侧的一个明显的热液流体活动区,南部蚀变区是更加远离流体活动中心的,温度进一步降低的流体活动区。矿区北缘接触带的热液蚀变温度要比南缘接触带蚀变温度高,北缘接触带热液流体蚀变温度具有由中西部向东部不断降低、由接触带向接触带外围不断降低的特点。总之,白杨河铀矿区深部热液流体活动至少存在"直流型"和"分流型"2种典型的形式,前者反映热液流体活动温度自深部→接触带→浅部不断降低的特点,后者反映热液流体活动温度自接触带分别向上和向下逐渐降低的特点。此外,文章还对热液流体活动与铀矿化关系,以及矿区外围铀矿找矿方向进行了分析。