The mineral system approach applied to magmatic Ni–Cu–PGE sulphide deposits

Most magmatic Ni–Cu–PGE sulphide deposits occur within long-lived magma pathways fed by high degree partial melts of the mantle. Holistic mineral-system analysis for such deposits has some parallels with dominantly hydrothermal systems, but also some important differences.Major provinces are associated with large volumes of magma erupted at margins of ancient Archaean cratons, and are associated with small intrusions through which large volumes of magmas have passed. There is no demonstrable association with any particular magma type, although in most provinces the ores are found associated with the most primitive available magmas, whatever these may be. Ore-bearing intrusions tend to form early in the evolution of the host province, although exceptions exist to this rule, and these intrusions typically account for very small proportions of the volumes of the province as a whole.Ore deposition is favoured by prolonged high-volume flow over a horizontal floor. This floor may take the form of the base of a channelized sill, tube or blade-shaped dyke, which account for most of the known host igneous bodies to significant ore deposits. Deposition mechanisms may be chemical or physical, but large high-grade deposits require a major component of transported sulphide liquid, initially carried as droplets. Late stage migration of sulphide liquid as gravity currents within intrusion networks, coupled with infiltration and melting of floor rocks, accounts for the common observation in mafic intrusion hosted deposits of cross cutting relationships between massive sulphides, host intrusions and country rocks.The following set of criteria is proposed in targeting and evaluating Ni–Cu–PGE sulphide systems: 1) nature of magmatism and relationship to pre-existing cratonic architecture; 2) magmatic and structural controls on the development of protracted-flow magma conduits; 3) access to crustal S sources at some point along the pathway; 4) favourable intrusion geometry and emplacement style for deposition, reworking and upgrading of sulphide magmas, and 5) favourable structural history and erosional level for preservation and detectability.These components can be translated into mappable geological criteria. At the predictive targeting scale, the key features are proximity to ancient cratonic boundaries and long-lived, trans-crustal structures, and relationship to voluminous mafic or ultramafic magmatism typically with high Mg and low Ti contents, but otherwise lacking distinctive characteristics. At the detection scale, there are two distinct approaches: recognition of high volume magma pathways with prolonged flow-through operating at length scales of km based on morphological, petrological, geophysical and structural observations; and identification of the petrographic and geochemical signals of accumulation or extraction of sulphide liquid.     

福建无岩浆热源的深循环水热系统

近年来对福建省存在具有岩浆热源的高温水热系统的呼声甚高。因而在福建是否可能存在具有岩浆热源的高温热田再次成为必须回答的重要问题。在综述前人研究成果的基础上,论述了福建省地热活动的地质构造背景、温泉分布状况、地热系统的特征、深部是否存在岩浆热源以及地壳的热结构,系统论述了福建深循环水热系统的特征。认为由于地质构造条件所决定,福建有大量的温泉出露,其中大部分为中温水热系统(90~150 ℃)和低温水热系统(＜90 ℃),均属受断裂控制的深循环水热系统,皆为温水热储。极少数高温热水系统(＞150 ℃)也仅仅是地下水沿断裂循环的深度较大所致。笔者认为福建不具备形成“有岩浆热源的水热对流系统”的条件。     

The Mazarrón basin,SE Spain: a study of mineralization processes,evolving magmatic series,and geothermal activity

The Miocene to Present Mazarrón basin provides a window on the metallogenic role of an evolving magmatic series. High-K calc–alkaline magmas gave rise to an important cluster of Pb–Zn–Ag–Cu vein and stockwork deposits emplaced in dacitic to rhyodacitic domes, part of a complex volcanic–metallogenic province (Au, Hg, Pb–Zn–Cu–Ag, Sn) stretching for ～150 km along the Mediterranean coast of SE Spain. By Pliocene time the former magmatic series had been replaced by intraplate alkaline basaltic volcanism, thus becoming the southern branch of the Western/Central Europe alkaline province. In terms of base metal sulphide deposits, this European province is barren, although it triggered widespread, CO₂-rich geothermal activity. Modern geothermal activity at El Saladillo (Mazarrón) resulted in the deposition of carbonate sinter deposits and formation of microbial mats. Proximal facies consist of millimetric to centimetric multicoloured layers of microbial mats, including yellow-orange thin bands of calcified bacteria and mineral growths of aragonite and calcite; green layers of live thermophilic Lyngbya-type cyanobacteria; black, degraded organic matter; and pyrite as the sole sulphide phase. Except for arsenic (37–63 μg g^?1), all of the studied trace elements (Ag, Ba, Bi, Cd, Cu, Pb, Sb, Se, and Sn) appear in remarkably low concentrations in the geothermal sinters. This is consistent with compositional data for the El Saladillo waters, with no significant metal concentrations. We present the first conceptual model (Miocene to Present) for the ore-forming processes, magmatism, CO₂ degasification, and geothermal activity for this realm. We argue that the time- and space-limited character of this volcanism (small, scattered outcrops), the deep magma emplacement level, the metal sulphide behaviour in alkaline basaltic magmatic chambers, and the dry character of these magmas prevented any metallogenic interaction between the chambers and the much shallower meteoric waters that drove the El Saladillo geothermal system and others of the same kind in Spain and elsewhere in Europe.     

The role of granites in volcanic-hosted massive sulphide ore-forming systems: an assessment of magmatic�Chydrothermal contributions

Assessment of geological, geochemical and isotopic data indicates that a significant subgroup of volcanic-hosted massive sulphide (VHMS) deposits has a major or dominant magmatic?Chydrothermal source of ore fluids and metals. This group, which is typically characterised by high Cu and Au grades, includes deposits such as those in the Neoarchean Doyon-Bousquet-LaRonde and Cambrian Mount Lyell districts. These deposits are distinguished by aluminous advanced argillic alteration assemblages or metamorphosed equivalents intimately associated with ore zones. In many of these deposits, ??³⁴S_sulphide is low, with a major population below ?3??; ??³⁴S_sulphate differs from coexisting seawater and ??³⁴S_{sulphate?Csulphide}????20?C30??. These characteristics are interpreted as the consequence of disproportionation of magmatic SO₂ as magmatic?Chydrothermal fluids ascended and cooled and as a definitive evidence for a significant magmatic?Chydrothermal contribution. Other characteristics that we consider diagnostic of significant magmatic?Chydrothermal input into VHMS ore fluids include uniformly high (>3 times modern seawater values) salinities or very ¹⁸O-enriched (??¹⁸O?>?5??) ore fluids. We do not consider other criteria [e.g. variable salinity, moderately high ??¹⁸O_fluid (2?C5??), ??³⁴S_sulphide near 0??, metal assemblages or a spatial association with porphyry Cu or other clearly magmatic-hydrothermal deposits] that have been used previously to advocate significant magmatic?Chydrothermal contributions to be diagnostic as they can be produced by non-magmatic processes known to occur in VHMS mineral systems. However, in general, a small magmatic?Chydrothermal contribution cannot be excluded in most VHMS systems considered. Conclusive data that imply minimal magmatic?Chydrothermal contributions are only available in the Paleoarchean Panorama district where coeval seawater-dominated and magmatic?Chydrothermal systems appear to have been physically separated. This district, which is characterised by chloritic and sericitic alteration assemblages and lacks aluminous advanced argillic alteration assemblages, is typical of many VHMS deposits around the world, suggesting that for ??garden variety?? VHMS deposits, a significant magmatic?Chydrothermal contribution is not required. Other than deposits associated with advanced argillic alteration assemblages, the only deposit for which we ascribe a major magmatic?Chydrothermal contribution is the Devonian Neves Corvo deposit. This deposit differs from other deposits in the Iberian Pyrite Belt and around the world in being extremely Sn-rich, with the Sn closely associated with Cu and in having formed from high ¹⁸O-rich fluids (??¹⁸O_fluid ??8.5??). We consider these characteristics, particularly the last, as diagnostic of a significant magmatic hydrothermal contribution. Our analysis indicates that two subgroups of VHMS deposits have a major magmatic?Chydrothermal contribution: Cu/Au-rich deposits with aluminous alteration assemblages and reduced, very Sn-rich deposits in which Sn was introduced in a high-temperature ore assemblage. Comparison with ??normal?? VHMS deposits suggests that these subgroups of VHMS deposits may form in specialised tectonic environments. The Cu/Au-rich deposits appear to form adjacent to magmatic arcs, an environment conducive to the generation of hydrous, oxidised melts by melting metasomatised mantle in the wedge above the subducting slab. This contrasts with the back-arc setting of ??normal?? VHMS deposits in which relatively dry granites (In this contribution, we use the term granite sensu latto) formed by decompression melting drive seawater-dominated hydrothermal circulation. The tectonic setting of highly Sn-rich VHMS deposits such as Neves Corvo is less clear; however, thick continental crust below the ore-hosting basin may be critical, as it is in other Sn deposits.     

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.     

Sulfur isotope distribution in Late Silurian volcanic‐hosted massive sulfide deposits of the Hill End Trough,eastern Lachlan Fold Belt,New South Wales

Volcanic‐hosted massive sulfide (VHMS) deposits of the eastern Lachlan Fold Belt of New South Wales represent a VHMS district of major importance. Despite the metallogenic importance of this terrane, few data have been published for sulfur isotope distribution in the deposits, with the exception of previously published studies on Captains Flat and Woodlawn (Captains Flat‐Goulburn Trough) and Sunny Corner (Hill End Trough). Here is presented 105 new sulfur isotope analyses and collation of a further 92 analyses from unpublished sources on an additional 12 of the VHMS systems in the Hill End Trough. Measured δ³⁴S values range from ‐7.4% to 38.3%, mainly for massive and stockwork mineralisation. Sulfur isotope signatures for polymetallic sulfide mineralisation from the Lewis Ponds, Mt Bulga, Belara and Accost deposits (group 1) are all very similar and vary from ‐1.7% to 5.9%. Ore‐forming fluids for these deposits were likely to have been reducing, with sulfur derived largely from a magmatic source, either as a direct magmatic contribution accompanying felsic volcanism or indirectly through dissolution and recycling of rock sulfide in host volcanic sequences. Sulfur isotope signatures for sulfide mineralisation from the Calula, Commonwealth, Cordillera and Kempfield deposits, Peelwood mine and Sunny Corner (group 2) are similar and have average δ³⁴S values ranging from 5.4% to 8.1%. These deposits appear to have formed from ore fluids that were more oxidising than group 1 deposits, representing a mixed contribution of sulfur derived from partial reduction of seawater sulfate, in addition to sulfur from other sources. The δ³⁴S values for massive sulfides from the John Fardy deposit are the highest in the present study and have a range of 11.9–14.5%, suggesting a greater component of sulfur of seawater origin compared to other VHMS deposits in the Hill End Trough. For barite the sulfur isotope composition for samples from the Commonwealth, Stringers and Kempfield deposits ranges from 12.6% to 38.3%. More than 75% of barite samples have a sulfur isotope composition between 23.4 and 30.6%, close to the previously published estimates of the composition of seawater sulfate during Late Silurian to earliest Devonian times, providing supporting evidence that these deposits formed concurrently with the Late Silurian volcanic event. Sulfur isotope distribution appears to be independent of the host rock unit, although there appears to be a relation linking the sulfur isotope composition of different deposits to defined centres of felsic volcanism. The Mt Bulga, Lewis Ponds and Accost systems are close to coherent felsic volcanic rocks and/or intrusions and have sulfur isotope signatures with a stronger magmatic affinity than group 2 deposits. By contrast, group 2 deposits (including John Fardy) are characterised by ³⁴S‐enrichment and a lesser magmatic signature, are generally confined to clastic units and reworked volcanogenic sediments with lesser coherent volcanics in the local stratigraphy, and are interpreted to have formed distal from the magmatic source. An exception is the Belara deposit, which is hosted by reworked felsic volcanic rocks and has a more pronounced magmatic sulfur isotope signature.     

Origin of light hydrocarbon gases from the Matsukawa geothermal area in Japan

Light hydrocarbon and isotope compositions of methane were analyzed in well steam samples from the Matsukawa vapour-dominated type geothermal system. Alkanes (C₁-C₄) and alkene (C₂) were detected in all samples. Light hydrocarbon contents of CO₂-type steam are slightly higher than those of CO₂-H₂S-type steam. The isotope composition of methane and the relationship between methane/ethane ratio and δ¹³C-value of methane suggest that these light hydrocarbon gases are mixtures of thermogenic and abiogenic components. The abiogenic hydrocarbon may be attributed to magmatic hydrocarbon gases equilibrated with carbon dioxide at f_o2 defined by the fayalite-magnetite-quartz buffer (FMQ).     

长白山天池火山区深部流体成分及其稳定同位素组成

天池火山区深部流体成分和稳定同位素组成特征指示,该区地下有相对独立的四个含水层。2.2km以上为冷水层,2.2-3.4km、3.5-3.9km、4km以下分别为上部、中部和深部热水层。现代水热活动均伴随强烈的深源气体释放,碳和氦同位素比值揭示,这些气体属幔源气体,其中仅混入少量大气,壳源物质混染不明显。大规模幔源气体释放主要集中在天池火山湖周围,这表明该区地壳浅部可能存在一定规模的热的幔源岩浆体。据碳同位素地质测温估算,该岩浆体的热变质带顶部距地面约5km.气体动态变化显示该岩浆体目前处在不稳定期,值得引起重视。     

Changes in fluid geochemistry and physico-chemical conditions of geothermal systems caused by magmatic input: The recent abrupt outgassing off the island of Panarea (Aeolian Islands, Italy)

Hydrothermal systems and related vents can exhibit dramatic changes in their physico-chemical conditions over time as a response to varying activity in the feeding magmatic systems. Massive steam condensation and gas scrubbing processes of thermal fluids during their ascent and cooling cause further compositional changes that mask information regarding the conditions evolving at depth in the hydrothermal system. Here we propose a new stability diagram based on the CO₂-CH₄-CO-H₂ concentrations in vapor, which aims at calculating the temperatures and pressures in hydrothermal reservoirs. To filter gas scrubbing effects, we have also developed a model for selective dissolution of CO₂-H₂S-N₂-CH₄-He-Ne mixtures in fresh and/or air-saturated seawater.This methodology has been applied to the recent (November 2002) crisis that affected the geothermal field off the island of Panarea (Italy), where the fluid composition and fluxes have been monitored for the past two decades. The chemical and isotopic compositions of the gases suggest that the volatile elements originate from an active magma, which feeds a boiling saline solution having temperatures of up to 350°C and containing ≈12 mol% CO₂ in vapor. The thermal fluids undergo cooling and re-equilibration processes on account of gas-water-rock interactions during their ascent along fracture networks. Furthermore, steam condensation and removal of acidic species, partial dissolution in cold air-saturated seawater and stripping of atmospheric components, affect the composition of the geothermal gases at shallow levels. The observed geochemical variations are consistent with a new input of magmatic fluids that perturbed the geothermal system and caused the unrest event. The present-state evolution shows that this dramatic input of fluids is probably over, and that the system is now tending towards steady-state conditions on a time scale of months.     

A new sulphur isotopic study of some Iberian Pyrite Belt deposits: evidence of a textural control on sulphur isotope composition

The sulphide deposits of the Iberian Pyrite Belt (IPB) represent an ore province of global importance. Our study presents 113 new sulphur isotope analyses from deposits selected to represent the textural spectrum of ores. Measured ³⁴S values range from −26 to +10‰ mostly for massive and stockwork ores, in agreement with data previously published. In situ laser ³⁴S analyses reveals a close correlation of ³⁴S with texture. Primary diagenetic textures are dominated by relatively low ³⁴S (−8‰ to −2‰), whereas stockwork feeder textures are dominated by higher ³⁴S (∼+3‰ to +5‰). Intermediate textures (mainly coarse textures in stratiform zones) have intermediate ³⁴S, although they are mostly dominated by the high ³⁴S component. Rare barite has a homogeneous ³⁴S around +18‰, which is consistent with direct derivation from Lower Carboniferous seawater sulphate. A dual source of sulphide sulphur in the IPB deposits has been considered. A hydrothermal source, derived from reduction of coeval seawater sulphate in the convective systems, is represented by sulphide in the feeder zones. Here variations in ³⁴S are caused by variations in the extent of the sulphate reduction, which governs the SO₄:H₂S ratio. The second end-member was derived from the bacterial reduction of coeval seawater sulphate at or near the surface, as reflected in the primary textures. A distinct geographical variation in ³⁴S and texture from SW (more bacteriogenic and primary textures) to NE (more hydrothermal textures and ³⁴S) which reflects a variation in the relative input of each source was likely controlled by local geological environments. Given that the sulphur isotope characteristics of the IPB deposits are unlike most VMS and Kuroko deposits, and noting the dominance of a mixed reduced sedimentary and volcanic environment, we suggest that the IPB could represent an ore style which is intermediate between volcanic and sedimentary hosted massive sulphide types. Received: 8 October 1997 / Accepted: 14 May 1998     

Insights to magmatic-hydrothermal processes in the Manus back-arc basin as recorded by anhydrite

Microchemical analyses of rare earth element (REE) concentrations and Sr and S isotope ratios of anhydrite are used to identify sub-seafloor processes governing the formation of hydrothermal fluids in the convergent margin Manus Basin, Papua New Guinea. Samples comprise drill-core vein anhydrite and seafloor massive anhydrite from the PACMANUS (Roman Ruins, Snowcap and Fenway) and SuSu Knolls (North Su) active hydrothermal fields. Chondrite-normalized REE patterns in anhydrite show remarkable heterogeneity on the scale of individual grains, different from the near uniform REE_N patterns measured in anhydrite from mid-ocean ridge deposits. The REE_N patterns in anhydrite are correlated with REE distributions measured in hydrothermal fluids venting at the seafloor at these vent fields and are interpreted to record episodes of hydrothermal fluid formation affected by magmatic volatile degassing. ⁸⁷Sr/⁸⁶Sr ratios vary dramatically within individual grains between that of contemporary seawater and that of endmember hydrothermal fluid. Anhydrite was precipitated from a highly variable mixture of the two. The intra-grain heterogeneity implies that anhydrite preserves periods of contrasting hydrothermal versus seawater dominant near-seafloor fluid circulation. Most sulfate δ³⁴S values of anhydrite cluster around that of contemporary seawater, consistent with anhydrite precipitating from hydrothermal fluid mixed with locally entrained seawater. Sulfate δ³⁴S isotope ratios in some anhydrites are, however, lighter than that of seawater, which are interpreted as recording a source of sulfate derived from magmatic SO₂ degassed from underlying felsic magmas in the Manus Basin. The range of elemental and isotopic signatures observed in anhydrite records a range of sub-seafloor processes including high-temperature hydrothermal fluid circulation, varying extents of magmatic volatile degassing, seawater entrainment and fluid mixing. The chemical and isotopic heterogeneity recorded in anhydrite at the inter- and intra-grain scale captures the dynamics of hydrothermal fluid formation and sub-seafloor circulation that is highly variable both spatially and temporally on timescales over which hydrothermal deposits are formed. Microchemical analysis of hydrothermal minerals can provide information about the temporal history of submarine hydrothermal systems that are variable over time and cannot necessarily be inferred only from the study of vent fluids.     

洋中脊热液系统中的锇及其同位素

洋中脊热液系统是将相对富集在深部的Os运移到海底表面的重要媒介,同时该过程也是全球Os循环的重要组成部分.在归纳总结洋中脊热液系统各物源组分和产物中Os的化学形态、含量及其同位素组成特征的基础上,探讨了Os在洋中脊热液活动各阶段中的分布演化规律及物源贡献特征.在缺乏沉积物覆盖的洋中脊区域,热液系统中的Os及其同位素组成特征主要受控于海水和不同构造环境下洋壳组分特征的差异以及这两种物源组分混合比例的不同.经历了海底之下的水岩反应后,围岩会将下渗海水中的部分放射性成因Os固定,而将自身富集的非放射性成因Os释放进入热液流体中.堆积在海底之上的各种热液产物中的Os大多来自海水,而海底之下的热液产物则因为海水下渗深度以及海水与热液流体混合程度的差异而体现出宽泛的Os含量和187Os/188Os比值变化范围.      

Ingress of magmatic Ni-Cu sulphide liquid into surrounding brittle rocks: Physical & structural controls

Field observations and theoretical considerations suggest that dense magmatic sulphide liquids are injected into and/or passively infiltrate surrounding wall rocks late in the emplacement history of intrusion-hosted magmatic ore systems. Using analogies with other engineering and mineral systems, we evaluate the structural and physical controls on the ingress of sulphide liquid into brittle host rocks. Gravity-driven infiltration of sulphide liquid out of the host igneous intrusion into country rock may be analogous to the behaviour of dense immiscible fluids in groundwater systems (Dense Non Aqueous Phase Liquids; DNAPL), which is controlled by the body force of the liquid itself. Downward penetration of sulphide liquid into micro-fractures may occur once a sufficient thickness of sulphide, known as the critical accumulation height, is reached in the overlying intrusion. Upward and lateral injection of sulphide liquid is a similar process to the behaviour of crustal hydrothermal fluids, where active deformation, fluid pressure and host-rock anisotropy play critical roles. Such physical controls must be considered and integrated with well-established petro-geochemical concepts in order to better understand the genesis of magmatic Ni-Cu-PGE sulphide deposits.     

http://www.sciencedirect.com/science/article/pii/S1674987113000856

Sulphide inclusions, which represent melts trapped in the minerals of magmatic rocks and xenoliths, provide important clues to the behaviour of immiscible sulphide liquids during the evolution of magmas and the formation of NieCueFe deposits. We describe sulphide inclusions from unique ultramafic clots within mafic xenoliths, from the mafic xenoliths themselves, and from the three silica-rich host plutons in Tongling, China. For the first time, we are able to propose a general framework model for the evolution of sulphide melts during the evolution of mafic to felsic magmas from the upper mantle to the upper crust. The model improves our understanding of the sulphide melt evolution in upper mantle to upper crust magmas, and provides insight into the formation of stratabound skarn-type FeeCu polymetallic deposits associated with felsic magmatism, thus promising to play an important role during prospecting for such deposits.     

Derivation of gold by oxidative metamorphism of a deep ductile shear zone: Part 1. Conceptual model

Quartz-carbonate gold deposits were emplaced in shear zones at or above the brittle-ductile transition. Some of the largest deposits are known to have formed along major, long-lived, transcurrent shears. Shears of this type widened downwards in the ductile regime, as a result of decreasing rock viscosity with depth; some were as wide as 40 km at depths of granulite facies metamorphism. Ductile shears are permeable and, since the permeability is along microfractures, fluid flow was pervasive, providing the opportunity for extensive chemical reaction. Reaction rates were enhanced by shear heating and by deformation-induced stress gradients in minerals, and reductions in grain size. Fluid flow tended to be upwards, because of pressure drop into the brittle portion of the shear. Given the wedge-shaped profile of ductile shears, fluids that had passed through a large volume of lower crust would have been focused at the brittle-ductile transition. Thus, if processes existed to selectively remove elements during fluid movement through the lower crust, these elements would also have been focused at this transition.One of the most constant features of quartz-carbonate lodes is carbonate alteration, which may extend kilometers out from major deposits. The ¹³C signature of this is consistent with a mantle source for the CO₂. Upward-moving CO₂ vapour of probable mantle origin has been implicated in the dehydration of amphibolite facies rocks to granulites and the concomitant depletion of large ion lithophile elements (LILE). The best documented cases of modification of the lower crust by CO₂ are from major shear zones. CO₂ streaming at depth could only have occurred under conditions more oxidizing than that required for graphite stability. These conditions favour solubility of gold by (a) oxidizing Au⁰ to Au⁺; (b) by dissolving sulphide from the rocks to complex with Au⁺. Recent work has shown that some major Archean gold deposits were derived from relatively oxidized fluids.A conceptual model is outlined for the genesis of at least some quartz-carbonate gold deposits. CO₂ permeating deep ductile shear zones dehydrated amphibolite facies rocks. A relatively oxidized CO₂-H₂O fluid was produced, which dissolved sulphide and gold from large volumes of lower crust. Gold was carried upwards in the narrowing shear, to be focused and precipitated at or above the brittle-ductile transition.     

Sediment geochemistry and base metal sulphide mineralisation in the Quidong area,Southeastern New South Wales,Australia

Geochemically anomalous, pyritic sediments occur directly above a Mid Silurian unconformity in the Quidong area of southeastern New South Wales. The composition of these sediments reflects derivation from a mixture of: (a) feldspar- and mica-depleted detritus reworked from underlying quartz-rich flysch; (b) Mg-rich clay or chlorite precipitated from hydrothermal exhalations; and (c) pyrite formed by reaction of iron in clays or oxides with reduced sulphur derived largely from sea-water sulphate and possibly a magmatic source. Three types of base metal sulphide mineralisation occur at Quidong including: (a) weak syngenetic concentrations in the pyritic sediments; (b) stratabound and fault-controlled bodies of massive sulphides hosted by the pyritic sediments and containing higher grade Pb, Zn and Cu; and (c) small vein and cavity fillings of galena, barite and other minor sulphides in overlying limestones. All types of mineralisation are related to hydrothermal activity which occurred during and after deposition of the pyritic facies. The geochemistry of the immediately underlying basement rocks and Pb isotope data indicate that the source of the metal-bearing fluids was deeper in the crust and probably related to widespread partial melting and magmatic processes. The sulphidic sediments and stratabound sulphide deposits represent syngenetic-epigenetic, sediment hosted mineralisation developed in a shallow marine environment, distal from major volcanic centers. This style of mineralisation has not previously been described from the region. It has some similarities to the Irish-Alpine type spectrum of deposits best known in Europe.     

Spatial-temporal Frame,Evolution and Mineralization of the Northern Qilian Metallogenic Province

Four metallogenic epochs occurred in different tectonic environments during theevolution of the Northern Qilian metallogenic province through the geological time. The Mid-dle Proterozoic metallogenic epoch witnessed the tectonic environment of crustal breakupcaused by mantle diapirism, in which ultramafic-mafic rocks were intruded along beep faultbelts and the superlarge Jinchuan magmatic Cu-Ni sulphide deposit was formed. In theMiddle-Late Proterozoic metallogenic epoch the crust was further broken to form anintracontinental rift, in which the Chenjiamiao style massive Cu-Fe sulphide deposits hosted bybasic volcanic tuff were formed in the lower volcano-sedimentary sequence, while the largesedex type Jingtieshan style Fe-Cu deposits were formed within the upper abyssal carbon-richargillaceous sedimentary sequence. The Early Palaeozoic saw the aulacogen environment, with-in which the Baiyinchang style superlarge massive base and precious metal sulphide depositshosted by quartz keratophyric tuff were formed in the Middle-Late Cambrian rifted island arcand the massive Cu-Zn sulphide deposits and magmatic chromite deposits associated with theophiolite suite were formed in the Early-Middle Ordovician, and the Honggou style massiveCu-Fe sulphide deposits hosted by spilite were formed in the Late Ordovician back-arc basinenvironment. In the Late Palaeozoic-Meso-Cenozoic, the metallogenic province went into anintracontinental orogenic stage characterized by compressive tectonic environment, in whichthere occurred carbonate-quartz vein type and tectono-alteration gold deposits associated withductile-shear structures.     

克拉通边缘岩石圈金属再富集与金-钼-稀土元素成矿作用

克拉通是大规模成矿的重要构造环境,其边缘产出了众多世界级规模的金、钼、稀土元素矿床。然而,克拉通如何控制巨型矿床的形成与分布尚不十分清楚。文章基于作者和前人的研究成果,探讨了扬子和华北克拉通岩石圈早期金属富集与后期金属活化问题。在全球范围,多数克拉通在其形成之后长期保持稳定,但部分克拉通(如华北、扬子)在克拉通化之后又经历了早期(元古代)增生与晚期(中生代—新生代)改造。在克拉通化及其之后,处于克拉通边缘的大洋岩石圈或克拉通块体间的有限洋盆发生板片俯冲,释放出含金属组分(REE、Cu、Au)的富CO2流体,交代亏损的大陆岩石圈地幔(SCLM),并使之发生交代和金属再富集。俯冲诱发的弧岩浆在大陆下地壳底侵可形成新生下地壳,伴随着少量硫化物的堆积而发生金属(Au、Cu)再富集。由于克拉通相对稳定,新生下地壳在进变质脱水过程中仍能保存部分金属,释放的(含Au)变质流体很可能被封存或固结在地壳的某个部位。在克拉通破坏改造期,软流圈上涌改变克拉通SCLM热结构并诱发其部分熔融,产生富REE的碳酸岩熔体和富水的基性岩浆(如煌斑岩)。前者在浅部地壳侵位并出溶成矿流体,形成碳酸岩型REE矿床;后者在深部地壳脱挥发分(H2O+CO2),诱发新生下地壳重熔和含Au硫化物(和/或含Au流体囊)活化,形成富Au岩浆系统或流体系统。这些深地壳熔/流体沿克拉通边界或岩石圈不连续运移至上部地壳,岩浆系统直接出溶成矿流体,形成以斑岩体为中心的斑岩型Au矿,含Au富CO2流体流沿断裂网络系统活动并沉淀金属,形成石英脉型和蚀变岩型Au矿。伴随克拉通破坏改造,克拉通边界断裂或基底断裂重新活化,并诱发古老下地壳熔融,产生含Mo岩浆系统。这个理论框架不同于已有的造山带成矿理论模式,它解释了克拉通边缘异常富集Au、Mo、REE矿床及其成矿规律,可用于类似克拉通地区的成矿预测。     

水文地球化学方法在赣地横径地区地热水成因分析中的应用

本文根据从横径地区获得的温泉水、气体和同位素化学成分资料，系统地研究了该区地热水的成因。水、气体、同位素地球化学研究表明，横径地区热水的形成与岩浆和火山活动无关；地热水主要起源于大气降水，其中气体组分起源于大气、地壳和地幔的混合；地热水处在中低温的地热储中；地热水的补给区位于乌泥障地区，其补给区可能的高程为337．5－743．6m。     

Thickness and geothermal gradient of Neoarchean continental crust: Inference from the southeastern North China Craton

The thickness and geothermal gradient of Archean continental crust are critical factors for understanding the geodynamic processes in Earth's early continental crust. Archean tonalite-trondhjemite-granodiorite (TTG) gneisses provide one of the potential indicators of paleo-crustal thickness and geothermal gradient because crust-derived TTG melts are generally thought to originate from partial melting of mafic rocks at the crustal root. In the Western Shandong Province (WSP) of the North China Craton (NCC), two episodes of Neoarchean TTG magmatism are recognized at ~2.70 Ga and ~2.55 Ga which were sourced from partial melting of juvenile crustal components. The ~2.70 Ga TTG gneisses show highly fractionated rare earth element (REE) patterns (average (La/Yb)_N = 39), whereas the ~2.55 Ga TTG gneisses have relatively less fractionated REE patterns (average (La/Yb)_N = 18). Petrogenetic evaluation suggest that the magmatic precursors of the TTG gneisses of both episodes originated from partial melting of juvenile crustal materials at different crustal depths with residual mineral phases of Grt, Cpx, Amp, Pl and Ilm. Together with the garnet proportion in the residue, the P–T pseudosections of equilibrium mineral assemblages, and the temperature calculated from Titanium-in-zircon thermometer, we estimate the Neoarchean crustal thicknesses as 44–51 km with geothermal gradients of 17 to 20 °C/km for the ~2.70 Ga TTG gneisses whereas the ~2.55 Ga TTG gneisses show lesser crustal thicknesses of 35–43 km with geothermal gradients of 19 to 26 °C/km, with an approximately 10 km difference in crustal thickness. Our estimates on the thicknesses and geothermal gradients of the Neoarchean crust are similar to those (~41 km, ~20 °C/km) of the modern average continental crust, indicating that a modern-style plate tectonic regime may have played an important role in the formation and evolution of the Neoarchean continental crust in the NCC.