Two-dimensional patterns of metamorphic fluid flow and isotopic resetting in layered and fractured rocks'pa

One-dimensional advection-dispersion models predict that characteristic δ¹⁸O vs. distance and δ¹⁸O vs. δ¹³C profiles should be produced during isothermal metamorphic fluid flow under equilibrium conditions. However, the patterns of isotopic resetting in rocks that have experienced fluid flow are often different from the predictions. Two-dimensional advection-dispersion simulations in systems with simple geometries suggest that such differences may be as a result of fluid channelling and need not indicate disequilibrium, high dispersivities, or polythermal flow. The patterns of isotopic resetting are a function of: (1) the permeability contrast between more permeable layers ('channels') and less permeable layers ('matrix'); (2) the width and spacing of the channels; (3) the width and spacing of discrete fractures; and (4) the orientation of the pressure gradient with respect to layering. In fractured systems, the efficiency of isotopic transport depends on the fracture aperture and the permeability of the surrounding rock. Resetting initially occurs along and immediately adjacent to the fractures, but with time isotopic resetting because of flow through the rock as a whole increases in importance. Application of the one-dimensional advection-dispersion equations to metamorphic fluid flow systems may yield incorrect estimates of fluid fluxes, intrinsic permeabilities, dispersivities, and permeability contrasts unless fluid flow occurred through zones of high permeability that were separated by relatively impermeable layers.     

Calcite veins as an indicator of fracture dilatancy and connectivity during strike-slip faulting in Toarcian shale (Tournemire tunnel,Southern France)

The reactivation of faults induced by natural/human induced fluid pressure increases is a major concern to explain subsurface fluid migration and to estimate the risk of losing the integrity of reservoir/seal systems. This study focusses on paleo-fluid migration in a strike slip fault with >100 m long, affecting a Toarcian shale (Causses Basin, France). A high calcite concentration is observed in a 5 cm thick zone at the boundary between the fault core and damage zone. Cumulated displacements in this zone are of millimeter-to-centimeter-scale offsets and different dilatant deformation textures are observed. The zone is affected by thin slip planes containing gouge. Cathodo-luminescence observations indicate that two phases of vein formation occurred. The first phase coincides with the fluid migration along this centimeter thick dilatant zone. The second one is associated to re-shear along the millimeter thick slip planes that results in more localized mineralization, but also in a better hydrologic connection through the shale formation. These results show that in shales fluids may migrate off a slipping surface in centimeter scale dilatant volumes, at first controlled by the intact shale anisotropy related to bedding and then favored by brecciating, structures re-orientation and strengthening processes induced by calcite sealing effects.     

断层影响下的页岩气储层水力压裂模拟研究

断层对页岩气储层压裂改造有重要影响,甚至诱发深部地震事件和近地表环境问题.本文采用多物理场耦合方法,基于渗流和应力耦合理论,研究储层水力压裂过程中断层以及封闭顶板中水力破坏区域的产生与演化机理,并分析讨论流体沿高渗通道运移扩散机理,研究结果表明:(1)断层改变储层水力破坏区域形态并且扩展了水力压裂破坏空间.较高注水压力...     

Local to regional scale structural controls on mineralisation and the importance of a major lineament in the eastern Mount Isa Inlier, Australia: Review and analysis with autocorrelation and weights of evidence

Although major crustal lineaments may play an important role in mineralisation, the relationship between lineaments and mineral deposits can be quite cryptic, and structural controls may vary as a function of scale along lineaments. Major lineaments alone may be of limited use for detailed target generation. The Cloncurry Lineament in the Eastern part of the Mount Isa Inlier is a crustal scale structure defined by potential field-derived ‘worms’. Weights-of-evidence quantifies the association between mineral occurrences and this lineament. Autocorrelation is used to recognise structural controls on mineralisation at different scales, by progressively limiting the lengths of the vectors between mineral occurrence points in the autocorrelation plot. The weights-of-evidence analysis shows that Au, Au–Cu, Cu–Au and Cu deposits have a positive spatial correlation to the Cloncurry Lineament, which suggests it that acted as a primary crustal scale control on the localisation of Cu and Au through focussing mineralisation systems on a broad scale. However, autocorrelation defines a variety of local structural controls, which can be interpreted as shear zones, variably oriented fault sets, en echelon fault arrays, and potentially the orientation of bedding and/or iron formations which localise fluid flow and mineral deposition at finer scales. The results suggest that major lineaments defined by geophysical contrasts can be used in conjunction with techniques of spatial analysis for targeting structurally controlled mineralisation in areas under thin cover adjacent to mineralised terrains such as the Mt Isa Inlier.     

断层内部结构及其对封闭性的影响

先前的研究多考虑断层封堵和开启的2种极端状态,近来的研究认为,在多数情况下断层处于2种之间的状态,只有在静止期具有封闭能力的断层,才有可能对油气起封堵作用。分析断层对流体运移的影响,需要分析断层在演化过程中的内部结构特征。断层可以划分出破碎带、诱导裂缝带和围岩3部分,断层岩和伴生裂缝构成破碎带的主体部分。常见的断层岩包括断层角砾岩、断层泥和部分碎裂岩,它们充填在断层裂缝空间中,断层内部结构受断层形成时的构造应力性质、断层活动强度和围岩岩性因素的控制。从动态角度看,随着断距增加,断层活动伴随着裂缝的发育和岩石的破碎混杂,可用泥质源岩层厚度和断距的比值来划分不同的发育阶段。断层活动期为油气运移通道,在静止时表现出差异性的封闭,通常用断层渗透率和排替压力2个参数来定量评价断层的封闭程度。断层岩渗透率主要受断距、泥质含量、埋深等因素的控制;断层排替压力的预测方法有2种:一种是从断层岩成岩角度分析的"等效埋深法",另一种是分析实测排替压力与主控地质因素的"拟合法"。通过简化的断层模型,建立了渗透率、排替压力与主控因素的预测关系。和储层类似,流体在断层中的运移遵循多孔介质的渗流特征。利用断层两侧的流体压力和油气柱高度并不能直接评价封闭性能,还必须考虑油气充注史和流体压力变化历史。     

北方岩溶区断裂带水文地质性质及结构模型

断层带结构和内部流体流动特性是水文地质研究领域的难点问题。石油地质领域,在油气运移与成藏方向已形成较成熟的断层封闭性定量评价技术手段。相比较,断裂的水文地质性质研究尚停留在断裂的力学性质对断层导水、阻水特性的定性评价阶段,尚未详细开展断裂带结构、渗透性各向异性等方面的研究工作。文章梳理总结国外断裂带水文地质性质研究中关于结构组成、断裂带演化、渗透率影响因素等方面的研究成果,引入断裂带渗透率结构模型,并以中国北方岩溶区碳酸盐岩与碎屑岩互层含水岩组为例,构建断裂带水文地质结构模型。断裂带研究尺度和精度不同、断裂带发育部位不同,导致其结构及水文地质性质亦不相同,如何建立起精确、典型的断裂带水文地质结构模型,需要各领域数据共享及多学科融合共同开展研究工作。      

云南兰坪金满中生代沉积岩中的铜矿成矿作用

兰坪－思茅中新生代盆地沉积岩中产出一系列热液型铜矿床（矿点），以石英、铁白云石与含铜硫化物（包括黄铜矿、斑铜矿、黝铜矿和辉铜矿）为主，呈脉状产于砂岩、页岩中，而单独的黝铜矿、方解石和重晶石脉则产于底部碳酸盐岩中．红色碎屑岩中的浅色还原层内的细脉和裂隙中也具有铜矿化。矿石构造以角砾状、脉状最为发育，矿脉多位于褶皱与走向断裂破碎带的叠加部位。金满铜矿的Ｓｒ、Ｓ、Ｐｈ同位素证据、稀土元素地球化学及液体包裹体资料表明，成矿元素主要来自沉积岩层，合矿卤水属于盆地卤水，流来自于硫酸盐的还原作用及沉积硫化物的淋滤溶解作用。含矿流体是Ｈ。Ｏ－ＮａＣＩ～ＣＯ。体系卤水，成矿温度为１５０～３００Ｃ，流体盐度为５％～２０％ＮａＣＩ。据含ＣＯ２包裹体的测温资料估算成矿压力大于６Ｘ１０＇Ｐａ，相当于３ｋｍ深的静岩压力。根据铜矿床成因研究，笔者提出了含矿热卤水储备与突发成矿作用的成矿模式。成矿前，矿化卤水象石油天然气一样，首先集中到一定的构造部位，在构造活动期突然爆炸成矿。成矿期由３个阶段组成，即爆炸充填、渗透充填与改造阶段，热液活动主要在前两个阶段。     

Major factors influencing proppant behaviour and proppant-associated damage mechanisms during hydraulic fracturing

The ultra-low permeability of shale reservoirs necessitates engineering applications such as hydraulic fracturing to enable the extraction of economically viable amounts of gas. In this process, a high-pressure fluid is injected into the reservoir to create a network of fractures. Proppants are solid, spherical, high-strength particles with size range between 8 and 140 mesh (105 μm–2.38 mm), which are injected into the reservoir simultaneously with fracturing fluid to prompt the opening of the fractures created, and they play a major role in the hydraulic fracturing process. As a result, appropriate management of proppants in shale reservoirs based on precise identification of their behaviour in shale reservoirs is necessary, because unexpected proppant performance or behaviour, commonly known as proppant damage mechanisms, can greatly reduce fracture conductivity. Therefore, it is essential to determine the major factors affecting proppant behaviour in order to maintain constant fracture conductivity. Numerous factors have been found in previous studies, and they can be summarized into three major groups: proppant properties, reservoir properties and hydraulic fracturing production, which affect proppant damage mechanisms. In the present paper, case studies have been provided on the determination of potential factors influencing proppant behaviour, followed by a discussion of their effects on fracture conductivity. The aim of this study is to present current opinions on potential factors influencing proppant behaviour based on a comprehensive literature review.     

Fluid flow in extensional environments; numerical modelling with an application to Hamersley iron ores

The mechanical feasibility of focusing both surface- and basinal-derived fluids towards sites of iron ore genesis during Proterozoic deformation in the Hamersley Province is tested here by computer simulation. Finite difference modelling of porous media flow during extensional deformation of a mountain range shows that surface fluids are drawn towards areas of failure and focus into the centre of the mountain. The addition of permeable structures such as a normal fault provides focused fluid pathways in which mechanical and geological conditions are particularly conducive to both upward and downward flow. Upward flow from the base of the fault within the model overall is favoured by low permeability basement materials and supra-hydrostatic pore pressures. Downward migration of fluids becomes more prominent as extension progresses and upward fluid flow from the base diminishes. The introduction of sedimentary layering into the models allows lateral fluid flow, such that sites of potential fluid mixing may then occur within permeable iron formation units close to the fault zone. Allowing parts of the stratigraphy to become more permeable as a function of high fluid flux simulates permeability enhancement by silica dissolution as a mechanism for iron ore genesis. The involvement of both basinal and surficial fluids in the genesis of the ore deposits is supported by the mechanical models and in addition provides an explanation for a progression from relatively reduced to oxidised conditions at the Mt Tom Price deposit (and possibly other large deposits) with time.     

断层侧向封闭性及对断圈油水关系的控制

张性正断层断裂带内部结构具有二分性:即断层核和破碎带,断层核由多个滑动面和不同类型的断层岩组成,其渗透性取决于带内泥质含量所决定的断层岩的类型,破碎带表现为高密度裂缝切割围岩,总体表现为高渗透的特征,因此,断裂带渗透能力取决于断层核与破碎带相对发育程度,侧向封闭能力主要取决于断裂带中泥质含量.存在5种侧向封闭类型:即对...     

Effects of mechanical layering on hydraulic fracturing in shale gas reservoirs based on numerical models

Generally, induced hydraulic fractures are generated by fluid overpressure and are used to increase reservoir permeability through forming interconnected fracture systems. However, in heterogeneous and anisotropic rocks, many hydraulic fractures may become arrested or offset at layer contacts under certain conditions and do not form vertically connected fracture networks. Mechanical layering is an important factor causing anisotropy in sedimentary layers. Hence, in this study, with a shale gas reservoir case study in the Longmaxi Formation in the southeastern Chongqing region, Sichuan Basin, we present results from several numerical models to gain quantitative insights into the effects of mechanical layering on hydraulic fracturing. Results showed that the fractured area caused by hydraulic fracturing indicated a linear relationship with the neighboring layer’s Young’s modulus. An increase of the neighboring layer’s Young’s modulus resulted in better hydraulic fracturing effects. In addition, the contact between two neighboring layers is regarded as a zone with thickness and mechanical properties, which also influences the effects of hydraulic fracturing in reservoirs. The initial hydraulic fracture was unable to propagate into neighboring layers under a relatively low contact’s Young’s modulus. When associated local tensile stresses exceeded the rock strength, hydraulic fractures propagated into neighboring layers. Moreover, with the contact’s Young’s modulus becoming higher, the fractured area increased rapidly first, then slowly and finally became stable.     

页岩气富集带地质控制因素及地震预测方法

首先对页岩气富集带的地质控制因素及各家的评价指标进行了探讨,认为页岩气的评价指标可以分为两大类：一类是评价页岩气量的多少,即资源因素,它决定了该区页岩气资源潜力及储量的多少,是页岩气资源评价、有利区块优选的关键指标。主要包括页岩厚度、有机质（TOC）丰度、干酪根类型、成熟度(Ro)、天然气含量及状态（游离、吸附）等。另一类决定了页岩气能否经济地开采出来,以及产量的高低,即所谓的“核心区”或“甜点”区。包括埋藏深度、页岩本身的矿物成分、脆度、孔隙度（裂缝）、渗透率以及原始地应力大小、方向及差异等。这两类指标共同控制了页岩气的富集和最终产量。因此,在页岩气勘探开发过程中,应首先利用地质和地球化学方法对第一类指标进行评价,从中优选出有利区块。在有利区块内,可以利用地震相关技术对第二类指标预测和评价,从中优选出适合钻井和开采的页岩气富集带或“甜点”区。根据对页岩气富集带（甜点）地质控制因素的讨论,可以将页岩气划分为“裂隙” 型页岩气藏及“脆性”型页岩气,两种类型均需位于弱应力各向异性区,以便于压裂过程中得到理想的复杂网状裂缝系统。笔者认为,对于“裂隙”型页岩气藏,几何地震属性分析技术（包括相干体、蚂蚁体、曲率体等）可以很好地刻画断层、隐伏断层及裂缝发育带,对这些成果的解释可以以玫瑰图的方式展示,从而可以揭示该区域的裂缝的主要发育方向及次要方向,进而推断该区的局部主应力方向,为水平井位部署提供依据。而对于“脆性”型页岩气藏,叠前反演技术、叠后反演技术可以揭示低泊松比(υ),纵横波速比等来进行有利目标（甜点）识别。曾经在常规油气勘探中发挥重要作用的地震技术,在页岩气等非常规油气资源勘探开发中,仍然是一种不可或缺的关键技术。     

A Fully Coupled Chemo-Poroelastic Analysis of Pore Pressure and Stress Distribution around a Wellbore in Water Active Rocks

Water active rocks consist of minerals that hold water in their crystalline structure and in pore spaces. Free water from drilling fluid can be attracted by the formation depending on the potential differences between pore space and drilling fluid. The fluid movement into the formation or out of the formation can lead to a change in effective stress, thus causing wellbore failures. In all previous studies it is found that the solute transport from or to the formation is primarily controlled by diffusion process and the effect of advection on solute transfer is negligible for a range of very low permeable shale formations (>10⁻⁵ mD). In this study a range of permeable shale formations (10⁻⁵ to 10⁻³ mD) commonly encountered in drilling oil and gas wells are considered to investigate the solute transfer between drilling fluid and formation due to advection. For this purpose a finite element model of fully coupled chemo-hydro-mechanical processes was developed. Results of this study revealed that the solute transfer between the drilling fluid and the shale formation is controlled primarily by permeability of the shale formations. For the range of shale formations studied here, there exists a threshold permeability below which the solute transfer is dominated by diffusion process and above which by fluid in motion (fluid flow). Results from the numerical experiments have shown that when the permeability of shales is greater than this threshold permeability, the chemical potential gradient between the pore fluid and drilling fluid reaches equilibrium faster than that when the permeability of shales is below this threshold value. Also it has been found that when advection is taken into account, effective radial and tangential stresses decrease around the wellbore, particularly near the wellbore wall where the solute concentration has reached near equilibrium.     

Maintaining the stability of deviated and horizontal wells: Effects of mechanical,chemical and thermal phenomena on well designs

Wellbore instability, particularly in shale formations, is regarded as a major challenge in drilling operations. Many factors, such as rock properties, in-situ stresses, chemical interactions between shale and drilling fluids, and thermal effects, should be considered in well trajectory designs and drilling fluid formulations in order to mitigate wellbore instability-related problems. A comprehensive study of wellbore stability in shale formations that takes into account the three-dimensional earth stresses around the wellbore as well as chemical and thermal effects is presented in this work. The effects of borehole configuration (e.g. inclination and azimuth), rock properties (e.g. strength, Young's modulus, membrane efficiency and permeability), temperature and drilling fluid properties (e.g. mud density and chemical concentrations) on wellbore stability in shale formations have been investigated. Results from this study indicate that for low-permeability shales, chemical interactions between the shale and water-based fluids play an important role. Not only is the activity of the water important but the diffusion of ions is also a significant factor for saline fluids. The cooling of drilling fluids is found to be beneficial in preventing compressive failure. However, decreasing the mud temperature can be detrimental since it reduces the fracturing pressure of the formation, which can result in lost-circulation problems. The magnitude of thermal effects depends on shale properties, earth stresses and wellbore orientation and deviation. Conditions are identified when chemical and thermal effects play a significant role in determining the mud-weight window when designing drilling programmes for horizontal and deviated wells. The results presented in this paper will help in reducing the risks associated with wellbore instability and thereby lowering the overall non-productive times and drilling costs.     

微支撑剂对页岩油气的增产机理及选配原则

水力压裂是页岩油气增产的主要手段,其核心是向储层连续泵入压裂液迫使储层产生水力裂缝/缝网形成油气运移高效通道,从而达到油气增产的目的。相较人工主缝,微裂缝具有数量大、缝宽小、走向复杂等特点,常规支撑剂无法对其形成有效支撑,如何提高微裂缝的导流能力是进一步提高页岩油气产能的关键。微缝导流能力实验表明:微支撑剂嵌入页岩裂缝表面导致一定程度的渗透率损害,但支撑微裂缝的渗透率仍比页岩基质渗透率高出2～3个数量级,仍可为页岩油气运移提供有效渗流通道。本文系统分析了微支撑剂对微裂缝的支撑特性、微支撑剂的长距离输运性能及辅助降滤等特征,提出了微支撑剂对致密油气增产的内在机制。结合我国致密油气储层基质孔隙尺度,形成了考虑微裂缝开度及基质孔隙尺寸的微支撑剂选配原则。     

Hydromechanical reactivation of natural discontinuities: mesoscale experimental observations and DEM modeling

Fracture interaction mechanisms and reactivation of natural discontinuities under fluid pressurization conditions can represent critical issues in risk assessment of caprock integrity. A field injection test, carried out in a damage fault zone at the decameter scale, i.e., mesoscale, has been studied using a distinct element model. Given the complex structural nature of the damage fault zone hydraulically loaded, the contribution of fracture sets on the bulk permeability has been investigated. It has been shown that their orientation for a given in situ stress field plays a major role. Based on these results, a simpler model with a fluid-driven fracture intersecting a second fracture has been set up to perform a sensitivity analysis. It is in presence of a minimum differential stress value with a minimum angle with the maximum principal stress that the second fracture could be both, hydraulically and mechanically reactivated. Results also showed that in the vicinity of the fluid-driven fracture, a natural fracture will offer contrasted hydromechanical responses on each side of the intersection depending on the stress conditions and its orientation with respect to the stress field. In this case, we show that a hydromechanical decoupling can occur along the same plane. These results provide insights into fracture-controlled permeability of fault zones depending on the properties of the fractures and their hydromechanical interactions for a given in situ stress field.

     

A model of regional fluid flow: Sand concentration factors and effective lateral and vertical permeabilities

Permeabilities of sand and shale differ by five to seven orders of magnitude. In the depositional pattern characteristic of the Gulf Coast, lenticular sand bodies occur randomly dispersed within a shale matrix, even though they can be grouped into stratigraphic sequences, contributing to net permeability of the sand-shale system and thus facilitating upward flow of pore water. In addition, a substantial lateral flow develops as fluid flow is directed toward sand bodies, thus concentrating fluid from a wide source area into sand bodies of limited lateral and vertical extent. Calculations reported here determine the effect of randomly distributed sands on both vertical and lateral permeabilities of composite sandshale systems. Effective permeabilities depend only on mean sand percent and local variance about the mean, because the effects of sand body shapes and their orientation and dip are not incorporated in the analysis. Results indicate that at a mean sand percentage of 20–50% a large lateral migration of fluid flow develops as sand bodies attract flow from a region two to seven times the radius of the sand body itself. For a mean sand percentage in excess of 50–60%, large vertical migration results and thus hydrocarbon trapping should not be expected for sand percentages in excess of 50–60%. Results of these calculations are in accord with observations that most larger oil accumulations on the Gulf Coast are found in regions having sandiness between about 20 and 50%.     

Impact of water and nitrogen fracturing fluids on fracturing initiation pressure and flow pattern in anisotropic shale reservoirs

A numerical model is proposed to investigate the impact of water and nitrogen fracturing fluids on the fracturing initiation pressure and the flow pattern in anisotropic shale reservoirs. This model considers the anisotropy of shale deformation and permeability, the compressibility of fracturing fluid, and the fluid moving front. A crack initiation criterion is established with the stress intensity factor of mode I crack. Both crack initiation pressure and seepage area are verified and analyzed. These results show that shale deformation and permeability anisotropy, fracturing fluid compressibility, viscosity, and pressurization rate have significant impacts on fracturing initiation pressure and seepage area.     

活化断层对加拿大阿萨巴斯卡盆地不整合型铀矿的控制

先存断层的活化对许多热液矿床的形成起到至关重要的作用。加拿大阿萨巴斯卡盆地的不整合型铀矿是一个受活化断层控制矿床的典型例子。该铀矿产于基底与盆地砂岩之间的不整合面附近,并与根植于基底的断层密切相关。这些控矿基底断层切穿并错动了盆地的不整合面。一系列证据表明这些基底断层以韧性的方式形成于盆地之前,但在盆地形成之后又发生脆性活化,而正是这种断层活化作用控制铀矿的产出。先存断层作为完整岩石中的薄弱位置,在后期构造运动中,其活化比产生新断层更容易发生。数值模拟表明在后期挤压构造运动中,有先存基底断层的不整合面被显著错动,而无先存断层的不整合面并没有错动。基底断层的脆性活化,不仅在活化过程中为流体提供了驱动力,而且由于活化导致岩石渗透率的提高,为后期的流体流动提供了通道以及容矿场所,形成阿萨巴斯卡盆地的不整合型铀矿。     

Iron oxide-copper-gold deposits: an Andean view

Iron oxide-copper-gold (IOCG) deposits, defined primarily by their elevated magnetite and/or hematite contents, constitute a broad, ill-defined clan related to a variety of tectono-magmatic settings. The youngest and, therefore, most readily understandable IOCG belt is located in the Coastal Cordillera of northern Chile and southern Peru, where it is part of a volcano-plutonic arc of Jurassic through Early Cretaceous age. The arc is characterised by voluminous tholeiitic to calc-alkaline plutonic complexes of gabbro through granodiorite composition and primitive, mantle-derived parentage. Major arc-parallel fault systems developed in response to extension and transtension induced by subduction roll-back at the retreating convergent margin. The arc crust was attenuated and subjected to high heat flow. IOCG deposits share the arc with massive magnetite deposits, the copper-deficient end-members of the IOCG clan, as well as with manto-type copper and small porphyry copper deposits to create a distinctive metallogenic signature.The IOCG deposits display close relations to the plutonic complexes and broadly coeval fault systems. Based on deposit morphology and dictated in part by lithological and structural parameters, they can be separated into several styles: veins, hydrothermal breccias, replacement mantos, calcic skarns and composite deposits that combine all or many of the preceding types. The vein deposits tend to be hosted by intrusive rocks, especially equigranular gabbrodiorite and diorite, whereas the larger, composite deposits (e.g. Candelaria-Punta del Cobre) occur within volcano-sedimentary sequences up to 2 km from pluton contacts and in intimate association with major orogen-parallel fault systems. Structurally localised IOCG deposits normally share faults and fractures with pre-mineral mafic dykes, many of dioritic composition, thereby further emphasising the close connection with mafic magmatism. The deposits formed in association with sodic, calcic and potassic alteration, either alone or in some combination, reveal evidence of an upward and outward zonation from magnetite-actinolite-apatite to specular hematite-chlorite-sericite and possess a Cu-Au-Co-Ni-As-Mo-U-(LREE) (light rare earth element) signature reminiscent of some calcic iron skarns around diorite intrusions. Scant observations suggest that massive calcite veins and, at shallower palaeodepths, extensive zones of barren pyritic feldspar-destructive alteration may be indicators of concealed IOCG deposits.The balance of evidence strongly supports a genetic connection of the central Andean IOCG deposits with gabbrodiorite to diorite magmas from which the ore fluid may have been channelled by major ductile to brittle fault systems for several kilometres vertically or perhaps even laterally. The large, composite IOCG deposits originated by ingress of the ore fluid to relatively permeable volcano-sedimentary sequences. The mafic magma may form entire plutons or, alternatively, may underplate more felsic intrusions, as witnessed by the ore-related diorite dykes, but in either case the origin of the ore fluid at greater, unobserved depths may be inferred. It is concluded that external 'basinal' fluids were not a requirement for IOCG formation in the central Andes, although metamorphic, seawater, evaporitic or meteoric fluids may have fortuitously contaminated the magmatic ore fluid locally. The proposed linkage of central Andean and probably some other IOCG deposits to oxidised dioritic magmas may be compared with the well-documented dependency of several other magmatic-hydrothermal deposit types on igneous petrochemistry. The affiliation of a spectrum of base-metal poor gold-(Bi-W-Mo) deposit styles to relatively reduced monzogranite-granodiorite intrusions may be considered as a closely analogous example.Editorial handling: B. Lehmann