An isotopic study on the volcanics of the Rooiberg Group: age implications and a potential exploration tool

Many geochronological studies on silicic magmatic rocks associated with the Bushveld Complex (rhyolitic lavas of the Rooiberg Group and granites of the Lebowa Granite Suite) have shown evidence of open-system behaviour of the Rb-Sr and Pb-Pb isotopic systems until 1600–1000 Ma, many hundreds of million years after crystallisation of these rocks. This pervasive open-system behaviour has been attributed to sustained hydrothermal circulation driven by the high heat productivity of the Bushveld granites. New Sr and Pb isotopic data are presented for basaltic to rhyolitic volcanics from the Rooiberg Group of the Transvaal Sequence in the Dullstroom-Loskop Dam area of the eastern Transvaal. These data show little evidence of open-system behaviour after about 1950 Ma and many sample suites retain ages which could reflect the formation of the Rooiberg Group i.e. older than 2070 Ma. It is argued that this preservation is due to the absence of fractionated, fluid/vapour-rich Bushveld granites in the immediate vicinity of the volcanic occurrences. Rooiberg Group volcanics with extensively perturbed Rb-Sr and particularly Pb-Pb isotopic systems reflect the action of granite-derived hydrothermal fluids. As a consequence, the isotope systematics in these volcanics could prove a useful exploration tool for sites of granite-derived metal deposits.     

Genetic types and economic value of tin and tungsten deposits in Chukotka

The decisive factor in the genesis of a deposit of any particular formation is the development of a mobile folded zone in time and the change in character of the manifestation of intrusive magmatism and mineralization associated with it. The Chukotka deposits provide a convincing illustration of the connection between deposits of a particular stanniferous formation, of diverse composition, and granitoids at certain stages in the development of a mobile Mesozoic zone of folding. The geological-geochernical and structural factors in the Chukotka tin-tungsten province lead to the following classification, starting with: 1) stanniferous pegmatite; 2) stanniferous (tungsteniferous) skarn formation; 3) cassiterite quartz formation; 4) cassiterite-quartz-sulfide formation; 5) cassiterite silicate formation; 6) cassiterite sulfide formation. Each type and its subtype are discussed and details of the individual genetic types of stanniferous deposits are given in a table. Industrial tin mineralization in Chukotka is connected with deposits of cassiterite-silicate and cassiterite-quartz sulfide formations, while tin-tungsten mineralization is associated with cassiterite-quartz formation. — C.E. Sears.     

An overview of the geology of the Transvaal Sequence and Bushveld Complex,South Africa

The Late Archaean-Early Proterozoic Transvaal Sequence is preserved within the Transvaal, Kanye and Griqualand West basins, with the 2050 Ma Bushveld Complex intrusive into the upper portion of the succession within the Transvaal basin. Both Transvaal and Bushveld rocks are extensively mineralized, the former containing large deposits of iron, manganese, asbestos, andalusite, gold, fluorine, lead, zinc and tin ores, and the latter some of the World's major occurrences of PGE, chromium and vanadium ores. Transvaal sedimentation began with thin, predominantly clastic sedimentary rocks (Black Reef-Vryburg Formations) which grade up into a thick package of carbonate rocks and BIF (Chuniespoort-Ghaap-Taupone Groups). These lithologies reflect a carbonate-BIF platform sequence which covered much of the Kaapvaal craton, in reaction to thermal subsidence above Ventersdorp-aged rift-related fault systems. An erosional hiatus was followed by deposition of the clastic sedimentary rocks and volcanics of the Pretoria-Postmasburg-Segwagwa Groups within the three basins, under largely closed-basin conditions. An uppermost predominantly volcanic succession (Rooiberg Group-Loskop Formation) is restricted to the Transvaal basin. A common continental rift setting is thought to have controlled Pretoria Group sedimentation, Rooiberg volcanism and the intrusion of the mafic rocks of the Rustenburg Layered Suite of the Bushveld Complex. The dipping sheets of the Rustenburg magmas cut across the upper Pretoria Group stratigraphy and lifted up the Rooiberg lithologies to form the roof to the complex. Subsequent granitic rocks of the Lebowa and Rashoop Suites of the Bushveld Complex intruded both upper Rustenburg rocks and the Rooiberg felsites.     

Economic potential of the Rooiberg Group: volcanic rocks in the floor and roof of the Bushveld Complex

Volcanic rocks of the Rooiberg Group are preserved in the floor and roof of the mafic Rustenburg Layered Suite of the Bushveld Complex. Field and geochemical characteristics of these volcanic rocks imply that they are genetically related to the Rustenburg Layered Suite. Four major ore-forming events are identified in the Rooiberg Group. The first phase was accompanied by volcanic hosted, fault controlled, hydrothermal copper mineralisation, which is found in the lowermost portion of the Rooiberg Group, underlying the Rustenburg Layered Suite. This type of mineralisation is tentatively linked to initial Rustenburg Layered Suite intrusions. Stratabound arsenic mineralisation that possibly formed in response to contact metamorphism, characterises the second phase, and occurred after extrusion of the Damwal Formation, possibly due to shallow granophyric intrusion. The third mineralising event occurred in response to contact metamorphism during the final stages of the Rustenburg Layered Suite, where especially Pb and Zn were introduced into the felsite roof rocks. This type of mineralisation affected the majority of the Rooiberg Group, but is most pronounced towards the contact with the Rustenburg Layered Suite. The fourth phase is restricted to the Rooiberg Group in the Nylstroom area and is linked to the granite intrusions of the Lebowa Granite Suite, from which Sn and F were introduced into the uppermost felsite succession. Mineralisation in the Rooiberg Group appears to be controlled by the character and intrusion level of the associated Bushveld magmas. Different styles of mineralisation in Rooiberg Group volcanic rocks are encountered at various stratigraphic levels. Major primary volcanogenic ore deposits appear to be absent.     

Application of discriminant function analysis to the felsic rocks of the Bushveld Complex,South Africa

The technique of multivariate analysis was used to investigate the geochemical relationships between the felsic rocks of the Bushveld Complex. The Bushveld granite and Rooiberg felsite form two distinct geochemical groups based on their major element compositions, possibly indicating that they originated from separate and genetically unrelated magmas. A discriminant function based on six major oxides was found to be 90 percent effective in distinguishing between the two groups. These conclusions have important implications for the petrogenesis of the Bushveld Complex.     

Some geochemical aspects of tin mineralization in the Tasman Geosyncline

Differentiation analysis using Köhler-Raaz indices has been applied to 284 granitic rocks from various parts of the Tasman Geosyncline in Queensland, New South Wales, Victoria and Tasmania. It was found that the majority (about 80%) of rocks known to be associated with tin mineralization (mostly leucocratic granites and adamellites) have a well defined range of Köhler-Raaz indices (qz=60–70%, F=20–25%, fm=5–15%) in agreement with findings in other parts of the world. Rocks associated with cassiterite deposits usually have a higher tin content than those not carrying cassiterite — but this information has to be used with caution because the form of tin in the rock (i. e. as an economic mineral e. g. cassiterite or in a host mineral, e. g. in biotite) is determined by several factors. All these factors (geochemical, physicochemical and geological) must be considered in an integrated approach to exploration for new tin deposits.     

On the relationships between the Bushveld Complex and its felsic roof rocks, part 1: petrogenesis of Rooiberg and related felsites

A major question concerning the Bushveld Complex is the relationship between the layered mafic rocks and the overlying Rooiberg Group felsites and related granophyres. Here, we assemble bulk-rock analyses to gain insight into this question and investigate the petrogenesis of the felsic rocks. The data indicate that the Rooiberg Group consists of distinctive magnesian and ferroan lavas. The former dominates the basalts to rhyolites of the basal Dullstroom Formation, while nearly all the dacites to rhyolites of the overlying Damwal, Kwaggasnek, and Schrikkloof Formations are ferroan. The ferroan rocks also include the Stavoren Granophyre, which exists regionally as a several-hundred-meter-thick concordant sheet between the Bushveld Complex and Rooiberg lavas. The compositions of the magnesian lavas are similar to calc-alkaline granitoids found in convergent margins, suggesting that the lavas could have originated in a mantle affected by previous Archean subduction events that are recorded by xenoliths and inclusions in diamonds from most Kaapvaal kimberlites. In contrast, the compositions of the ferroan lavas indicate formation by fractional crystallization of basaltic liquids and are essentially identical to ferroan rhyolites associated with mafic rocks from other settings. The hypothesis that these rocks are fractional crystallization products of Bushveld mafic liquids is consistent with published radiogenic and stable isotope data and known age relations. Based on compositional characteristics and geologic relations, the Stavoren Granophyre is the most likely candidate for the residual liquid that escaped from the top of the Bushveld Complex. Whether the bulk of the Bushveld Province ferroan rhyolites formed in the chamber of the extant layered mafic sequence or in a deeper, hidden crustal magma reservoir remains unclear.     

江西会昌锡坑迳斑岩型锡矿床成矿系统发育特征与找矿模型

锡坑迳矿田位于南岭成矿带东段与武夷山成矿带交会部位,以早白垩世连续的岩浆喷发-侵入活动与多类型锡多金属成矿作用为特色。矿田内围绕似斑状花岗岩和花岗斑岩发育了岩背斑岩型、淘锡坝和矿背隐爆层间裂隙带型、苦竹岽和凤凰岽云英岩-破碎带蚀变岩型等大中型锡矿床。这些矿床的矿化-蚀变特征可与玻利维亚锡矿带、银岩、维拉斯托和洋滨等国内外斑岩型锡矿床类比,又独具特色,是研究斑岩型锡成矿系统发育特征与找矿模型的理想区域。文章在详细的野外地质调查基础上,系统总结了锡坑迳矿田内各类锡多金属矿化组合特征和蚀变结构,并对岩背矿床和淘锡坝矿床锡石开展了电子探针和LA-ICP-MS原位微区分析。研究结果显示,所有锡石样品均具有La、Pr、Gd和Tb的正异常、Ce与Eu的负异常、明显的Zr/Hf分馏和不规则的稀土元素配分模式,指示成矿热液早期阶段及锡石沉淀阶段经历了流体不相容。岩背锡石较淘锡坝锡石具有高的Ti/Zr和Ti/Sc比值,指示其更为靠近矿化中心。笔者研究认为,锡坑迳斑岩型锡成矿系统矿化类型包括细脉状、浸染状和细网脉状,围岩蚀变由早到晚、由成矿中心向外依次发育黄玉石英带、绿泥石-黄玉石英岩化带、绿泥石-绢云母化带和黏土化-碳酸盐化带。与斑岩型铜钼成矿系统相比,其成矿岩浆岩不仅有花岗斑岩,还有似斑状黑云母花岗岩,矿体在斑岩、花岗岩及上覆火山岩中均有赋存。相较于玻利维亚斑岩型锡成矿带发育的大面积石英-电气石化,锡坑迳斑岩成矿系统更富F,蚀变矿物中出现了大量黄玉、萤石和白云母等。对比国内外典型锡矿床的矿化-蚀变结构和锡石矿物学特征,文章建立了锡坑迳式斑岩型锡成矿系统的找矿模型。     

Mantle sources and magma evolution of the Rooiberg lavas,Bushveld Large Igneous Province,South Africa

We report a new whole-rock dataset of major and trace element abundances and ⁸⁷Sr/⁸⁶Sr–¹⁴³Nd/¹⁴⁴Nd isotope ratios for basaltic to rhyolitic lavas from the Rooiberg continental large igneous province (LIP). The formation of the Paleoproterozoic Rooiberg Group is contemporaneous with and spatially related to the layered intrusion of the Bushveld Complex, which stratigraphically separates the volcanic succession. Our new data confirm the presence of low- and high-Ti mafic and intermediate lavas (basaltic—andesitic compositions) with >?4 wt% MgO, as well as evolved rocks (andesitic—rhyolitic compositions), characterized by MgO contents of <?4 wt%. The high- and low-Ti basaltic lavas have different incompatible trace element ratios (e.g. (La/Sm)_N, Nb/Y and Ti/Y), indicating a different petrogenesis. MELTS modelling shows that the evolved lavas are formed by fractional crystallization from the mafic low-Ti lavas at low-to-moderate pressures (~?4 kbar). Primitive mantle-normalized trace element patterns of the Rooiberg rocks show an enrichment of large ion lithophile elements (LILE), rare-earth elements (REE) and pronounced negative anomalies of Nb, Ta, P, Ti and a positive Pb anomaly. Unaltered Rooiberg lavas have negative εNd_i (??5.2 to ??9.4) and radiogenic εSr_i (6.6 to 105) ratios (at 2061 Ma). These data overlap with isotope and trace element compositions of purported parental melts to the Bushveld Complex, especially for the lower zone. We suggest that the Rooiberg suite originated from a source similar to the composition of the B1-magma suggested as parental to the Bushveld Lower Zone, or that the lavas represent eruptive successions of fractional crystallization products related to the ultramafic cumulates that were forming at depth. The Rooiberg magmas may have formed by 10–20% crustal assimilation by the fractionation of a very primitive mantle-derived melt within the upper crust of the Kaapvaal Craton. Alternatively, the magmas represent mixtures of melts from a primitive, sub-lithospheric mantle plume and an enriched sub-continental lithospheric mantle (SCLM) component with harzburgitic composition. Regardless of which of the two scenarios is invoked, the lavas of the Rooiberg Group show geochemical similarities to the Jurassic Karoo flood basalts, implying that the Archean lithosphere strongly affected both of these large-scale melting events.     

The economic mineral potential of the mid-Proterozoic Waterberg Group, northwestern Kaapvaal craton, South Africa

The 1900–1700 Ma Waterberg Group in the main Waterberg fault-bounded basin consists of dominantly coarse siliciclastic red beds with minor volcanic rocks. The sedimentary rocks were deposited mainly by alluvial fans, fluvial braidplains and transgressive shallow marine environments, with lesser lacustrine and aeolian settings. Uplifted, largely granitic source areas were located along the Thabazimbi-Murchison lineament (TML) fault system in the south, and along the Palala shear zone in the northeast. Palaeoplacer titanomagnetite-ilmenite-zircon heavy mineral deposits, best developed in the Cleremont Formation in the centre of the basin, reflect initial fluvial reworking and subsequent littoral marine concentration. Coarse alluvial cassiterite placer deposits are found in the Gatkop area in the southwest of the basin, and appear to have been derived from stanniferous Bushveld Complex lithologies south of the TML. Hydrothermal zinc and U-Cu mineralisation in the Alma lithologies in the same area appears to be related to the TML fault system. Small manganese deposits and anomalous tungsten values occur in the south of the basin, where they are again closely spatially associated with the TML. Copper-barium mineralisation is found associated with dolerite dykes, and in stratigraphically controlled, inferred syngenetic settings. The most interesting of these apparently syngenetic occurrences is found within green coloured reduced mudrocks and inferred volcanic rocks, at an unconformity developed within the overall red bed sequence of the Waterberg Group, adjacent to the TML in the southwest of the basin. The most important potential mineralisation in the main Waterberg basin thus encompasses shoreline placer Ti and the possibility of substantial sediment-hosted copper deposits. Received: 31 May 1996 / Accepted: 17 February 1997     

广东紫金铁嶂锡矿床地质特征及矿床成因探讨

铁嶂锡矿床赋矿围岩为下侏罗统金鸡组下部的砂页岩,其锡含量较高,为矿源层。矿体分布于北东向和北西向断裂构造中,可分为大脉型与细脉带型矿体。锡石有两类:一类为沉积的胶状锡石和由胶体变晶生成的微晶锡石,分布于富动植物化石和碳质的千枚状页岩和富锡的砂岩结核中;第二类为粒状锡石,与石英、硫化物一起呈脉状产出。矿床系沉积变质热液改造成因。     

火山-侵入杂岩带的成岩-成矿专属性

成岩成矿关系问题,特别是成矿作用与岩浆活动的专属性问题,一直是矿床学研究中的重要课题。粤东地区热液脉状锡、钨（多金属）矿床产于中生代花岗质火山岩、次火山岩和侵入岩组成的火山侵入杂岩带中,成岩年龄与成矿年龄一致,但矿床地质、矿床矿物流体包裹体和同位素地球化学没有相对标准来区分这些来源于岩浆作用不同阶段、不同产出状态岩浆岩的成矿物质特征,使得人们对于区内矿床成因的认识不一。文中运用稀土元素地球化学方法,着重探讨和对比了矿床矿石、蚀变岩石及与成矿有关的岩浆岩的稀土元素特征和配分型式,并进一步根据在不同压力条件下Ｃｌ－在熔体和热液间的分配系数实验数据和不同压力条件下热液与相应熔体平衡时的稀土元素分配系数实验结果,分别计算了厚婆坳锡矿床和莲花山钨矿床与矿区有关岩浆岩熔体平衡的热液中的稀土元素浓度并制出配分型式,再与矿床矿石及矿石矿物稀土元素配分型式比较,从而确定成矿与侵入阶段的花岗岩岩浆活动关系密切。区内矿床为岩浆热液矿床。研究结果表明,稀土元素作为热液流体来源的示踪剂能有效地确定火山侵入杂岩带的成岩成矿专属性。     

Geochemical analyses of termite mounds as a prospecting tool for tin deposits in Bastar,MP—a preliminary study

Tin deposits in the form of cassiterite occur in precambrian zoned pegmatite bodies intrusive into Archean metabasic and metasedimentary rocks in Bastar, MP. The associated economic minerals include columbite-tantalite, lepidolite, beryl etc. As a major part of the area is covered by a thick overburden, termite mounds have been used as geochemical sampling medium in view of their significant potentiality in tropical areas with overburden, and also because of the low cost and ease of sampling. Though many elements have been used as pathfinders for tin, tin itself acts as the best indicator of its deposits. The present work has given encouraging results, and therefore can be applied for tin prospecting and possibly for heavy metals like Nb, Ta, W, etc., in this and other areas where termite mounds have a good spatial distribution.     

广西姑婆山锡矿田矿床地质特征及矿床成因

姑婆山锡矿田锡矿床类型丰富，其中以矽卡岩型、矽卡岩-蚀变断裂破碎带复合型锡矿床为主，矿床主要产于燕山早期姑婆山花岗岩西南接触带2km范围内，且具一定的分带性，从接触带往外，依次为矽卡岩型→矽卡岩与蚀变断裂破碎带复合型→锡石硫化物型→石英脉型。矿床受成矿花岗岩、有利赋矿层位及构造复合等因素的控制，与成矿花岗岩关系尤为密切，成岩、成矿时间接近，成矿物质主要来自岩浆，矿床为与花岗岩有关的岩浆期后热液矿床。     

The geochemistry of the Bushveld granites in the Potgietersrus tin-field

An area of 324 km² was selected for a pilot-study of the geochemical characteristics of the Bushveld granites in the Potgietersrus tin-field. Two hundred and sixty nine samples were collected from a regular grid of stations. Additional samples collected from selected sample stations provide data on “about station” variance to test the validity of conclusions based on regional variation. The samples were analysed for Ba, Ce, Co, La, Li, Nb, Ni, Rb, Sc, Sn, Sr, Ti, Y, Zn and Zr together with the major elements Ca, Na and K.The stratiform Bushveld granites have mean concentrations for these elements that compare closely with those in low Ca-granites. Ce, La, Rb, Y, Zn and Zr are enriched relative to low Ca-granites while Sr is markedly impoverished. The stanniferous Bobbejaankop granite stock is distinguished from the stratiform granites with respect to Ba, Sr, Ti and Zn. The median tin concentration in the stratiform granites is 6.5 ppm and in the Bobbejaankop granite it is 9 ppm. Economically viable deposits of tin disseminated through the upper 120 m of the Bobbejaankop granite stock have been exploited for over 70 years.It is concluded that the Bobbejaankop granite crystallized at low temperatures from a hydrous residual melt that had collected beneath a roof of felsite and early crystallization products of the original granitic magma. The presence of this concentration of volatile-rich, hydrous liquid depressed the melting temperatures of the quasi-solid, granitic crystal much below the ambient temperature initiating re-mobilization. The enrichment in Sn of the stratiform granites in the Potgietersrus area is compared with the “normal” (i.e., ± 3 ppm) Sn-content of the Bushveld stratiform granites building the Sekukuni Plateau in the eastern lobe of the Bushveld Complex where no tin mineralization has been reported.     

Evidence for synchronous extrusive and intrusive Bushveld magmatism

The intracratonic, 2.06 Ga volcanic rocks of the Rooiberg Group of southern Africa consist of nine magma types, varying in composition from basalt to rhyolite. Basalts and andesites, intercalated with dacites and rhyolites, are found towards the base; rhyolite is the chief magma composition in the upper succession. The absence of compositions intermediate to the magma types and variations in major and trace element concentrations suggest that fractional crystallization was not prominent in controlling magma compositions. REE patterns are comparable for all magma types and concentrations increase for successively younger magmas; LREE show enriched patterns and HREE are flat. Elevated Sr_i-ratios and high concentrations of elements characteristically enriched in the crust suggest that the Rooiberg magmas were crustally contaminated or derived from crustal material. Some Rooiberg features are related to the intrusive events of the Bushveld complex.Petrogenesis of both the Rooiberg Group and the mafic intrusives of the Bushveld complex is linked to a mantle plume, melting at progressively higher crustal levels. The basal Rooiberg magmas have undergone a complex history of partial melting, magma mixing and crustal contamination. Crustal melts extruded as siliceous volcanic flows to form the Upper Rooiberg Group, simultaneously intruding at shallow levels as granophyres. Crustally contaminated plume magma synchronously intruded beneath the Rooiberg Group to produce the mafic rocks of the Rustenburg Layered Suite. Granite intrusions terminated the Bushveld event. The Bushveld plume was short-lived, which conforms, together with other features, with younger, voluminous plume environments.     

大兴安岭南段锡的成矿作用：以黄岗-甘珠尔庙地区为例

大兴安岭南段是我国重要的锡多金属成矿集中区,为促进区域锡多金属找矿突破,以在黄岗-甘珠尔庙地区多年的野外调查研究工作为基础,运用年代学测试、化学分析及地质模型等方法,分析了区域锡的成矿作用。认为二叠统基底地层及锡林郭勒杂岩是成矿物质的初始来源,经历的变质过程具有进一步富集的作用,大部分锡矿均遭受了多期次的火山-岩浆活动,最终决定锡多金属成矿的是燕山期酸性岩浆-热液成矿作用。同时成矿作用还受到控岩控矿构造、围岩性质、氧化-还原条件、开放-封闭环境的制约,认为在氧化性花岗岩浆侵入钙质地层条件下成矿作用以铁锡为主;还原性岩浆在侵入具开放性特征的围岩时,成矿作用以花岗岩外带围岩中的热液型脉状锡矿为主,而在侵入具封闭性特征的围岩时,成矿作用以花岗岩内带岩浆凸起部位的花岗岩型锡矿为主。因此,建议将主攻矿种由铅锌调整为锡银,部署重砂及化探方法圈定异常,针对硫化矿体部署电法,针对隐伏岩体部署磁法与重力,还可部署烃、汞、氧气测量等非常规化探方法预测隐伏岩体。     

San Rafael, Peru: geology and structure of the worlds richest tin lode

The San Rafael mine exploits an unusually high grade, lode-type Sn–Cu deposit in the Eastern Cordillera of the Peruvian Central Andes. The lode is centered on a shallow-level, Late Oligocene granitoid stock, which was emplaced into early Paleozoic metasedimentary rocks. It has a known vertical extent exceeding 1,200 m and displays marked vertical primary metal zoning, with copper overlying tin. The tin mineralization occurs mainly as cassiterite–quartz–chlorite veins and as cassiterite in breccias. The bulk of it is hosted by a K-feldspar megacrystic, biotite- and cordierite-bearing leucomonzogranite, which is the most distinctive phase of the pluton. Copper mineralization occurs predominantly in the veins that straddle the metasedimentary rock–intrusion contact or are hosted entirely by slates. Both tin and copper mineralization are associated with strong chloritic alteration, which is superimposed on an earlier episode of sericitization and tourmaline–quartz veining. Based on the distribution of alteration and ore mineralogy, cassiterite deposition and subsequent chalcopyrite precipitation are believed to have been the result of a single, prolonged hydrothermal event. The source of the metals is inferred to be a highly evolved, peraluminous magma, related to the leucomonzogranitic phase of the San Rafael pluton. Preliminary fluid inclusion microthermometry suggests that ore deposition took place during the mixing of moderate and low salinity fluids, which were introduced in a series of pulses. Several large fault-jogs, created by sinistral-normal, strike-slip movement, are interpreted to have focused synkinematic magmatic fluids and permitted their effective mixing with meteoric waters. It is proposed that this mixing led to rapid oxidation of Sn (II) chloride species and caused supersaturation of the fluids in cassiterite, resulting in the development of localized, high-grade ore shoots. A favorable structural regime that promoted large-scale mixing of two fluids originating under very different physico-chemical conditions appears to have been the key factor responsible for the unusual richness of the deposit.     

Lead isotope systematics of the giant massive sulphide deposits in the Iberian Pyrite Belt

Lead isotope analyses were performed on 26 polymetallic massive sulphide deposits of the Iberian Pyrite Belt, as well as on overlying gossans and associated volcanic rocks. All the massive sulphide deposits (except for Neves-Corvo), and nearly all the volcanic rocks show very similar isotopic compositions grouped around 18.183 (²⁰⁶Pb/²⁰⁴Pb), 15.622 (²⁰⁷Pb/²⁰⁴Pb) and 38.191 (²⁰⁸Pb/²⁰⁴Pb), indicating that most of the ore deposit lead was derived from the same continental crust environment as the associated volcanic rocks. The isotopic compositions are representative of the average south Iberian crust during the Devonian to Early Carboniferous (Dinantian), and their constancy implies a homogenization of the mineralizing fluids before the deposition of the massive sulphides from hydrothermal fluids circulating through interconnected regional fracture systems. This isotopic constancy is incompatible with multiple, small, independent hydrothermal cells of the East Pacific Rise type, and fits much better with a model of hydrothermal convections driven by “magmatic floor heating”. Neves-Corvo is the only south Iberian massive sulphide deposit to have a heterogeneous isotopic composition with, in particular, a highly radiogenic stanniferous ore (²⁰⁶Pb/²⁰⁴Pb of the cassiterite is >18.40). A model of lead mixing with three components is proposed to explain these variations: (1) one derived from the Devonian to Early Carboniferous (Dinantian) continental crust that generated all the other massive ores; (2) an Eohercynian stanniferous mineralization partly remobilized during the formation of the massive sulphides, but independent of them; and (3) a Precambrian continental crust component. The juxtaposition of three different sources places Neves-Corvo in a specific paleogeographic situation that could also explain its mineralogical specificity. The geodynamic context that best explains all the obtained isotopic results is one of an accretionary prism. The fact that lead isotope signatures of the gossans are almost identical to those of the underlying massive sulphides means that this technique could be a useful exploration tool for the Iberian Pyrite Belt.     

湖南香花岭矽卡岩中锡的矿物学行为与成矿过程

湖南香花岭矽卡岩型锡矿床是南岭地区一个重要的锡多金属矿床,发育有丰富的含锡矿物。在野外和显微镜下观察 基础上,文章利用电子探针技术系统分析了香花岭矽卡岩中含锡矿物的矿物学特征,探讨了锡的成矿过程、成矿流体以及 锡的来源。研究结果表明,香花岭矽卡岩中含锡矿物由锡矿物（锡石、尼日利亚石、孟宪民石等）和富锡矿物（韭闪石、 尖晶石、葡萄石、塔菲石等）组成。锡的成矿有三个阶段：矽卡岩早阶段,Sn进入尖晶石、韭闪石等造岩矿物中,形成富 锡矿物;氧化物阶段,锡矿物如锡石、尼日利亚石、孟宪民石等逐渐晶出;晚期热液阶段,早期含锡矿物热液蚀变原位析 出锡石,或富Sn热液交代早期矿物形成了富锡环边。矽卡岩中成矿流体富含F,CO2,Li等挥发组分,控制了Sn的富集、迁 移、结晶等过程。香花岭矽卡岩中Sn根本上来源于地层,锡的成矿过程反映了Sn在地壳中的地球化学循环过程。