Sediment-hosted gold mineralisation in the Ratatotok district, North Sulawesi, Indonesia

The Ratatotok district in the Minahasa Regency of North Sulawesi, Indonesia is an area of significant gold mineralisation. Gold has been mined in the district since at least the 1850s, and intensively by the Dutch between 1900 and 1921 with a recorded production of 5,060 kg of gold. Newmont began exploring the district in 1986, and has delineated a major sediment-hosted replacement-style deposit at Mesel, and other smaller deposits in an 8×5 km area. A total drill-indicated resource of over 60 metric tonnes of gold (+2 Moz) is reported for Mesel, and three of the smaller deposits. Approximately 80% of this resource is refractory. Silver grades are usually low (<10 g/t). The Mesel deposit is similar to many Carlin-type deposits in carbonate hostrocks, alteration, geochemical signature and ore mineralogy, but is distinct in tectonic setting. The discovery of replacement-style mineralisation at Mesel, in an impure limestone within a Tertiary island arc environment, demonstrates that deposits with outward characteristics similar to Carlin-type mineralisation are not restricted to a continental setting.Carbonate sediments in the Ratatotok district were deposited in a Late Miocene restricted basin. Later compressional tectonics caused uplift that resulted in karst development in the limestone and erosion of the adjacent volcanic arc with deposition of a thick epiclastic unit. This was followed by intrusion of shallow level pre-mineral andesite into the sequence. Mineralisation at Mesel, and probably elsewhere in the district, is synchronous with the late-stage reactivation of strike-slip faults. Mineralising fluids at Mesel were focussed along steep structures sympathetic to these faults, and trapped below a relatively impermeable andesite cap rock. Hydrothermal fluids caused decalcification of the silty, more permeable carbonate units with the formation of secondary dolomite, deposition of fine arsenian pyrite, silica veinlets and gold. Volume loss due to decalcification and dolomite formation caused collapse brecciation which enhanced fluid flow and further mineralisation. This locally culminated in total decarbonation and deposition of massive silica. Late-stage stibnite occurs in structural zones within the ore deposit, whereas arsenic (as realgar and orpiment) and mercury (as cinnabar) are concentrated on the periphery.Elsewhere in the Ratatotok district, gold mineralisation is restricted to replacement-style mineralisation in permeable zones along limestone-andesite contacts, open-space filling quartz-calcite veins and stockworks, and residual quartz-clay breccias. The residual breccias are developed in-situ, and are interpreted to form by dissolution of the wallrock limestone from around pre-existing mineralisation. This has resulted in widespread eluvial gold occurrences.     

岩石断裂作用的复杂性和混沌动力学

断裂是一个复杂的动力学体系,受到岩石结构、反应、流体迁移、应力、岩石变形和力学等多种地质因素和过程的耦合控制。本文建立了断裂体系的反应-输运-力学耦合动力学模型并编制了模拟程序。以湖南水口山矿区为例,通过动力学模拟表明不同地层岩性的断裂渗透率大小和演化特征存在显著差异,断裂作用促使岩石渗透率的空间非均匀性增强,从而有利于流体的局部汇聚和矿体的形成。断裂中压力随时间呈现出非周期振荡变化,反映了断裂演化的混沌特征。     

辽宁高家堡子大型银矿床流体包裹体特征及矿床成因

高家堡子银矿床为辽东地区近年来发现的一大型独立银矿床,它赋存于早元古代辽河群大石桥组大理岩地层之中,受层间破碎带构造控制。矿区发育含银-铅-锌碎裂大理岩型及硅化石英脉型两种主要类型矿石。流体包裹体研究表明,前者石英中主要发育气液两相、含CO2三相、CO2及单液相四种类型的原生流体包裹体,成矿流体属H2O-CO2-NaCl体系类型,与造山型矿床成矿流体相似;后者石英中主要发育气液两相及单液相包裹体,成矿流体属H2O-NaCl体系类型,其低温低盐度特点及氢、氧同位素数据表明成矿流体主要来自大气降水。综合分析表明,本区银的大规模富集成矿作用是印支期以来以大气降水来源为主的热液活动的产物。     

滇东南大庄锑矿床地质特征与成因探讨

滇东南大庄锑矿床地处华南褶皱系滇东南褶皱带个旧褶皱束,锑矿体主要产于上二叠统龙潭组上段及下三叠统永宁镇组下段破碎带及次生破碎带硅化含泥白云岩、白云质灰岩中,矿体明显受断层的控制。文章对大庄锑矿床地质背景、矿床地质特征以及土壤化探异常等方面进行了系统论述,并认为锑矿矿石类型主要为方解石-辉锑矿型,围岩蚀变主要表现为碳酸盐化、硅化、黄铁矿化及绿帘石化等;硫同位素特征表明成矿物质可能来源于深部物源区,成矿热液主要来源于岩浆水与大气降水的混合;矿床成因类型为受断裂控制的热液改造型矿床。     

Application of stress mapping in cross‐section to understanding ore geometry,predicting ore zones and development of drilling strategies

Stress mapping is a numerical modelling technique used to determine the distribution and relative magnitude of stress during deformation in a mineralised terrane. It is based on the general principle that fluid flow in the Earth's crust is primarily related to pressure gradients. It is best applied to epigenetic hydrothermal mineral deposits, where fluid flow and fluid flux are enhanced in dilational sections of structures and in sites of enhanced rock permeability due to high fracture density. These are defined by sites of low minimum principal stress (σ₃). Most stress mapping is carried out in two dimensions in plan view using geological maps. This is suitable for terranes with steeply dipping lithostratigraphy and structures in which the distribution of mineral deposits is largely controlled by fault structures portrayed on the maps. However, for terranes with gently dipping sequences and structures, and for situations where deposits are sited in and near the hinges of complex fold structures, stress mapping in cross‐section is preferable. The effectiveness of stress mapping is maximised if mineralisation was late in the evolutionary history of the host terrane, and hence the structural geometry of the terrane and contained deposits were essentially that expressed today. The orientation of syn‐mineralisation far‐field stresses must also be inferred. Two examples of orogenic gold deposits, which meet the above criteria, are used to illustrate the potential of stress mapping in cross‐section. Sunrise Dam, located in the Archaean Yilgarn Craton, is a lode‐gold deposit sited in a thrust‐fold belt. Stress mapping illustrates the heterogeneity of stress distribution in the complex structural geometry of the deposit, and predicts the preferential siting of ore zones around the intersections of more steeply dipping, linking thrusts and banded iron‐formation units, and below the controlling more gently dipping basal thrust, the Sunrise Shear. The Howley Anticline in the Pine Creek block hosts several Palaeoproterozoic gold deposits, sited in complex anticlinal structures in greywacke sequences. Stress mapping indicates that gold ores should develop in the hinge zones of symmetrical anticlines, in the hinge zones and more steeply dipping to overturned limbs of asymmetric anticlines, and in and around thrusts in both anticlines and parasitic synclines. The strong correlation between the predictions of the stress mapping, based on the distribution of low σ₃, and the location of gold ores emphasises the potential of stress mapping in cross‐section, not only as an exploration tool for the discovery of additional resources or deposits, but also as a test of geological models. Knowledge of the potential siting of gold ores and their probable orientations also provides a guide to drilling strategies in both mine‐ and regional‐scale exploration.     

湘中锡矿山锑矿床的Sr同位素地球化学

对湘中锡矿山锑矿床围岩灰岩、硅化灰岩、煌斑岩和脉石矿物进行了系统的Sr同位素研究。结果表明，矿区围岩发生了隐性蚀变，灰岩中Sr亏损，而^87Sr/^86Sr高于同时代的海相碳酸盐，这种隐性蚀变很可能是水／岩反应所致。矿体附近的硅化灰岩中Sr更加亏损，而^87Sr/^86Sr明显增加。成矿期方解石的^87Sr/^86Sr较高，成矿体系中变化的W／R比造成了方解石中^87Sr/^86Sr值的明显波动。成矿流体为一富放射成因^87Sr的溶液。成矿流体来自或流经基底地层，流体中的Sr由基底碎屑岩提供，矿质Sb也可能主要来自富Sb的元古宇基底。水／岩反应的理论模拟显示，锡矿山成矿流体中的Sr约为3．0μg/g，^87Sr/^86Sr为０．７１７；蚀变－成矿体系为一开放体系，矿石的沉淀机制主要为水／岩反应，成矿体系中W／R 比较高。     

COMPLEXITY AND CHAOTIC DYNAMICS OF ROCK FAULTING

Fault is a complex dynamic system controlled by the coupling of rock texture, reaction, fluid flow, stress, and rock deformation mechanism. A coupled reaction-transport- mechanical dynamic model for fault system is established and described in this paper. An example is presented for the Shuikoushan deposit, Hunan. The results of dynamic simulation indicate that the evolution and magnitude of fracture permeability of different rocks are different, and that faulting can enhance the spatial heterogeneity of rock permeability and facilitate fluid flow and mineralization in local fault zone. The pressure for a fault usually shows a variation mode of aperiodic oscillation with time, which reflects the chaotic behavior of the evolution of a fault.     

辽宁高家堡子银矿床矿石晶洞构造及其成因机制

高家堡子银矿床是在辽东青城子铅锌矿田内发现的一大型独立银矿床,赋存于辽河群大石桥组第三段以大理岩为主的地层之中,矿体形态及产状受层间破碎带构造控制。矿区矿石分为硅化石英脉型及碎裂大理岩型两类,其中前者为矿区主要的富银矿石。该类矿石晶洞构造极其发育,根据地质及流体包裹体研究成果,认为矿石中的晶洞构造是岩浆来源富CO2流体溶蚀赋矿的大理岩围岩形成的溶洞,被晚期以大气降水为主流体充填形成晶簇状石英。     

Contrasting coeval paragenesis of gold and scheelite in an orogenic hydrothermal system,Macraes mine,New Zealand

The Macraes deposit (> 10 Moz resource) is a Cretaceous orogenic system hosted in the Hyde-Macraes Shear Zone (HMSZ) which was mineralised under lower greenschist facies during later stages of lower greenschist facies metamorphism of host metasedimentary schists. Gold is encapsulated primarily in sulphides that have replaced silicates in ductile shears that are focussed in micaceous rocks. The shears anastomose around structurally competent lenses, and were enhanced by hydrothermal graphite deposition and alteration of albite to muscovite. In contrast, scheelite with minor auriferous sulphides occurs in multigenerational quartz veins that filled fractures in competent lithologies. Hence, scheelite was deposited coevally with gold, from the same hydrothermal fluid, but in different structural settings from most gold at all scales from millimetres to hundreds of metres. Consequentially, there is weak correlation between Au and W at all scales in the deposit. Multigenerational gold and scheelite mineralisation occurred during progressive deformation in the shear zone in two contrasting structural and mineralogical styles in syn-deformationally weakening gold-bearing micaceous shears, and in syn-deformationally hardened competent rocks that became silicified and veined with quartz and scheelite. Hydrothermal fluid flow in the gold-bearing shears occurred at the grain boundary, microshear, and microfracture scales, and was slow (< 1 m/year), continuous, and pervasive. In contrast, vein formation in more competent lithologies was episodic, locally rapid (> hundreds of m/year), and was controlled by fracture permeability. The Au and W enrichment in the Macraes deposit resulted from regional scale metal mobility, driven by coeval recrystallisation in higher-grade (upper greenschist to amphibolite facies) metamorphism that persisted structurally below the Macraes deposit for at least 10 Ma after mineralisation ceased.     

水口山矿田主要金矿床类型及其找矿方向

水口山矿田的主要金矿床类型有：（１）爆发角砾岩复合型金矿床；（２）热液硅化角砾岩型金矿床；（３）风化淋滤铁帽型金矿床；（４）黑土夹角砾型金矿床。本文介绍了它们的特征及其找矿方向，指出了找矿有利部位。     

A metasomatic origin for the iron-oxide Au-Cu Starra orebodies, Eastern Fold Belt, Mount Isa Inlier

The Starra ironstone-hosted Au-Cu deposit in northwest Queensland, Australia, displays evidence of a polyphase history of late-post metamorphic, structurally controlled metasomatism. The mineralisation is hosted within several massive ironstone units which occur at a lithological contact of metamorphosed sandstone and siltstone sequences and foliated chlorite-biotite-magnetite schists. The ironstones strike north-south and occur as discontinuous lenses that cross cut the host rocks in places. Magnetite ironstone textures are generally massive in appearance, but relict deformed breccia and foliation textures are locally preserved. Sulphide ore textures are more brittle and consist of fine, pervasive brecciation of the ironstones and host rocks. Petrological and textural observations support a hydrothermal origin for both ironstone and mineralisation. Three dominant post-peak metamorphic paragenetic stages of alteration and mineralisation are recognised: (1) early widespread Na-Ca metasomatism consisting of albite-quartz-actinolite-scapolite, (2) localised K-Fe metasomatism consisting of biotite-magnetite-hematite- quartz-pyrite and (3) the mineralising stage consisting of anhydrite-calcite-hematisation of magnetite-pyrite-chalcopyrite-gold and extreme chloritisation of biotite. The Na dominant alteration occurs throughout the Eastern Fold Belt and for at least 50 km to the east and 150 km to the north of Starra. The ironstones were products of localised Fe-metasomatism associated with shearing and brecciation of previously Na-metasomatised host rocks. Gold and copper mineralisation resulted from the interaction of magnetite, a late oxidising fluid and an increase in pH. This produced variable hematisation of magnetite and caused the solubility of chloride-complexed gold to decrease dramatically. High gold:copper ratios can be explained in terms of the solubility differences of gold and copper in the thermochemical conditions that existed at Starra. Received: 15 May 1996 / Accepted: 17 January 1997     

Structural and diagenetic origin of breccias in the carbonate-hosted Polaris Zn–Pb deposit,Nunavut, Canada

The Polaris deposit, located on Little Cornwallis Island in the Canadian Arctic, was a Mississippi Valley-type Zn–Pb deposit hosted by brecciated carbonate rocks of the Upper Ordovician Thumb Mountain Formation. Mapping indicates that strike-slip faults on the east side of the Polaris deposit were active during the last stage of the Late Devonian Ellesmerian Orogeny. Polaris is on a jog in the north-oriented, Early Devonian Boothia fault system and was the site of localized extension during south-directed Late Devonian Ellesmerian compression. This structural setting elsewhere in the district may be prospective for Zn–Pb mineralisation.Ore fluids rising in the Late Devonian interacted with the host rock causing dissolution, brecciation and collapse. Carbonate beds are thinned, indicating widespread removal of carbonate material. Five breccia types (crackle, pseudo, cobble, mega and collapse) are present in the vicinity of the deposit. Crackle breccia is preserved around the periphery of the deposit and is indicative of structural dilation or the early stages of mineralisation. Dolomite and pseudobreccias are also preserved around, and extend beyond, the periphery of the deposit. These are considered an early stage of alteration directly related to the mineralising fluids and could act as a vector to the centre of the mineralising system where collapse, mega and cobble breccias occur intimately with massive mineralisation.     

Structural controls on fluid flow and related mineralization in the Xiangshan uranium deposit, Southern China

The Xiangshan uranium deposit of Southern China is a Mesozoic volcanic uranium deposit where mineralization is closely associated with regional rock deformation and fluid flow. To better understand the genesis of this deposit, we have carried out coupled deformation and fluid flow modeling to explore the relationship between mineralizing fluid flow and structural deformation processes. The numerical modeling results demonstrate that fluid transport and focusing are predominantly controlled by deposit scale structures and associated rock deformation.     

贵州水银洞金矿构造蚀变体稀土元素地球化学特征

水银洞金矿构造蚀变体(SBT)为产出于茅口租(P2m)和龙潭组(P3l)之间不整合面上的一套强硅化灰岩、灰岩角砾岩、硅化粘土岩组合.呆用ICP-MS测定钻孔岩芯中构造蚀变体样品稀土元素组成,对比研究SBT围岩、区域岩浆岩及现代海底热水系统流体稀土元素组成.结果显示,SBT的轻重稀土分馏明显[LREE/HREE=4.92～17.51,(La/Yb)N=5.94～38.37],曲线右倾型;轻稀土分异明显,曲线右倾程度大;重稀土分异不明显,曲线平坦;负Eu(0.61～0.94)、Ce(0.52～1.07)异常明显;SBT及围岩均具有明显W型稀土元素四分组效应,而不同于区域岩浆岩和现代海底热水系统流体,表明热液流体来源以壳源为主.     

陕西凤县地区九子沟铜矿的特征、成因及意义

九子沟铜矿是凤县-太白铅锌多金属矿集区近年来发现的新矿床。矿化类型主要表现为硅质岩型、硅化灰岩型和石英-方解石脉型。矿体主要呈层状、似层状、透镜状与围岩整合产出,且多被后期顺层或切层含矿石英-方解石脉交代,形成富矿体。综合地质特征和地球化学特征,认为该矿床成因类型为喷流沉积-改造型铜矿。九子沟等铜矿床（点）与区域上的铅锌矿床构成了热水喷流沉积矿床的元素分带模式。这一发现,指示今后该地区可在热水喷流沉积矿床元素分带模式的基础上,结合区带内背斜控矿的地质特征开展找矿工作。     

Significance of strain localization and fracturing in relation to hydrothermal mineralization at Mount Isa, Australia

The rheology of layered meta-sedimentary rocks, and their orientation and position relative to major fault systems were the key controls on Proterozoic hydrothermal copper mineralization at Mount Isa, Australia. Compositional layering in the host rock partitioned mechanical behavior and strain, leading to selective permeability generation and the focusing of fluid flow. Shale layers preferentially failed by plastic shearing, whereas meta-siltstones remained elastic or failed in tension depending on magnitude of deformation and fluid pressure. Numerical simulations support the hypothesis that the orientation of layering and the proximity to major fault systems controlled fracturing and permeability increase in the Urquhart shale. The dilating shale provided a pathway for an upward-flowing, reduced basement fluid, from which quartz was precipitated during cooling. During a later event, the reactivation of steep structures provided access to surface derived oxidized metal-bearing brine, causing the precipitation of dolomite followed by chalcopyrite ore in the brecciated silicified shale.     

Tectonic deposition of mercury in the Almadén district,Las Cuevas deposit,Spain

The Las Cuevas mercury deposit is located in the northern part of the Almadén district, Ciudad Real, Spain. It displays characteristic epigenetic features. A reinterpretation of the geological context of the deposit and a detailed structural analysis indicate that the host rocks at Las Cuevas belong to the same stratigraphic units that host the old Almadén mine, but that they experienced a different tectonic evolution. Two types of ore are distinguished at Las Cuevas: (1) hydraulic breccias in a black quartzite, identical to the San Francisco horizon of the Almadén mine. This ore has been interpreted as syngenetic and is associated with an alkaline explosive volcanic event; and (2) a stockwork composed of horizontal subcritical tension cracks which developed along the contact between black shales and volcanic rocks in the hinge of a drag fold, and which is associated with advanced argillic alteration. Mobilization of mercury from type 1 to type 2 ore was related to a local inversion of the stress field associated with an increase of permeability by self-sealing, which probably occurred during uplift. Las Cuevas is representative of a whole class of mercury deposits hosted within basement rocks.     

Regional fluid flow and the genesis of Irish Carboniferous base metal deposits

The Irish carbonate-hosted base metal deposits have long been an enigmatic subclass of deposit. Some of the Irish deposits (Harberton Bridge, Allenwood) are clearly epigenetic, hosted in breccia pipes and have close affinities to Mississippi Valley-type deposits. Others, are characterised by stratabound and sometimes stratiform mineralisation (Abbeytown, Navan), while a third group is associated closely with concordant dolomitic breccias and cavity fill mineralisation at the base of carbonate mud mounds (Lisheen, Ballinalack). When the stratigraphic and textural evidence is reviewed, it is apparent that all of the base metal mineralisation occurred in already compacted sediments. Hence, mineralisation probably occurred at depths of at least several 100 m depth below the sea bed. The mineralisation occurred either during transtensional Lower Carboniferous basin development, or subsequently, during the onset of Variscan shortening. Fluids may have been derived from dewatering of Variscan-driven deformation to the south of the carbonate platform, with fluid flow through major fracture zones and basal clastic aquifers. Alternatively, high heat flow produced by Lower Carboniferous extension may have driven the mineralising system. Syn-genetic models are extremely difficult to sustain.     

Interpretation of a set of faults across the hinge and a limb of a large-scale flexure in the Mount Isa district, Queensland, Australia, in terms of fractures related to the folding process

A revised interpretation of a number of faults across the hinge and western limb of a large-scale anticlinal flexure in the Mount Isa district has been made in terms of the faults following earlier-formed be joints. Such joints often develop in weakly or moderately folded competent sediments, as a result of either tensile stresses that were active at a late stage during folding or the influence of residual stresses generated during tectonic uplift. The joints are oriented such that on a stereographic projection their poles plot parallel to the a axis of a fabric cross and at 90° to the fold axis (b). bc joints are thus approximately normal to bedding and contain the fold axis, and hence they fan around the axial plane of the fold containing them. Across the hinge and western limb of a steeply N-plunging large-scale F₂ flexure in the Mount Isa district, a number of faults at high angles to bedding fan about the axial plane. Making use of the fold geometry and local bedding orientation it is possible to predict the orientation of ideal bc fractures at locations within the fold. These predictions fit well with the observed fault pattern. The movement on the faults, although apparently complex, appears consistent with continued shortening perpendicular to an axial-plane cleavage during the D₂ deformation or as part of a later D₂ deformation.     

The Chaqupacha Mississippi Valley-type Pb–Zn deposit,central Tibet: Ore formation in a fold and thrust belt of the India–Asia continental collision zone

The newly discovered Chaqupacha Mississippi Valley-type (MVT) Pb–Zn deposit in central Tibet has been found to be helpful for understanding MVT ore formation relative to tectonic evolution of a foreland fold and thrust belt. The deposit lies in the Tuotuohe area of the western Fenghuo Shan-Nangqian fold and thrust belt of the India–Asia continental collision zone. It contains NNW-striking and folded Late Permian strata including an upper clastic unit and an underlying limestone unit. The strata overlie late Oligocene clastic rocks through a south-dipping reverse fault that is associated with regional northward thrusting during the Paleogene. The Late Permian and late Oligocene strata are unconformably overlain by flat-lying early Miocene marl and mudstone of the Wudaoliang Formation. Lead and zinc ores are mainly hosted by pre-ore dissolution and collapse breccias in the Late Permian limestone. The style of mineralization is epigenetic, as shown by replacement of the pre-ore dissolution breccia matrix and open-space-fill by galena, sphalerite, calcite, and minor barite and pyrite. δ³⁴S values of the main sulfide galena range from − 27.5‰ to + 12.6‰. These features, together with the lack of magmatic activity during the mineralization, suggest that Chaqupacha is an MVT deposit. Subordinate mineralization is also present in the early Miocene Wudaoliang Formation marl and the paleokarst breccia which contains matrix compositionally equivalent to strata of the Wudaoliang Formation. The mineralization shares similar mineral associations and textures with the pre-ore dissolution breccia-hosted mineralization. Thus, the Pb and Zn mineralization in the entire deposit probably resulted from the same mineralizing event, which is younger than the youngest ore-hosting rocks (i.e., the early Miocene Wudaoliang Formation). Considering that thrusting in the Tuotuohe area had ceased prior to deposition of the Wudaoliang Formation host rocks, the mineralization at Chaqupacha post-dated the regional deformation. The Chaqupacha deposit thus provides a good example of MVT mineralization in a foreland fold and thrust belt that post-dates regional thrusting.