Discrepant chemical differentiation and magmatic-hydrothermal evolution of high-silica magmatism associated with Pb–Zn and W mineralization in the Lhasa terrane

High-silica (SiO₂ > 70 wt.%) granites (HSGs) are the main source of W, Sn, and rare metals. However, abundant HSGs, temporally, spatially, and genetically associated with Pb–Zn mineralization, in the Lhasa terrane (LT), provided an ideal opportunity to study the key factors responsible for Pb–Zn enrichment, instead of W–Sn enrichment. Here we contribute to this topic through U-Pb dating of zircon and garnet, and whole-rock and Sr–Nd–Hf isotopic geochemistry of ore-related quartz porphyries in the Bangbule deposit and compared these results with published data from large and giant Pb–Zn and W deposits in the LT. The magmatism-alteration-mineralization event in the Bangbule deposit was recorded by robust zircon U–Pb ages of 77.3 ± 0.9 Ma and hydrothermal garnet U–Pb ages of 75.7 ± 4.8 Ma, which is 10–15 Ma earlier than the main Paleocene metallogenic event and the first record of late Cretaceous Pb–Zn polymetallic mineralization in the LT. The late Cretaceous-Paleocene magmatism and mineralization events are a response to the subduction of Neotethyan oceanic lithosphere, which occurred as a result of the collision of the Indian and Asian plates. These HSGs related to Pb–Zn mineralization, with high total-alkalis and low magnesian contents, are enriched in Ba, Th, and Rb, but depleted in Ti, Eu, Sr, and P. They belong to either the S-type, or I-type granites. The Sr–Nd–Hf isotopic compositions of the Pb–Zn mineralized granites demonstrate that they were generated by the partial melting of Proterozoic basement with or without mantle-derived melt input. This was consistent with the postulated source of W enrichment in the LT. The Pb–Zn and W related granites have similar zircon-Ti-saturation temperatures, comparable low whole-rock Fe₂O₃/FeO ratios, and zircon oxygen fugacity. This indicated that the Pb–Zn–W enrichment in the high-silica magma system could be attributed to a relatively reduced magma. The Pb–Zn related HSGs, abundant quartz and feldspar phenocrysts, and weak fractionation of twin-elements in whole-rock analysis, can be used to reconstruct a model of the magma reservoir. We postulate that these features could be reproduced by silica-rich crystal accumulation in a magma reservoir, with a loss of magmatic fluids. The magma associated with W mineralization exhibited a higher level of differentiation compared to the Pb–Zn related magma; however, different groups of zircon texture with varying rare earth elements and concomitance of rare earth elements tetrad effect and high fractionation of twin-elements in whole-rock are formed by a magmatic-hydrothermal transition in highly evolved system. As the source and oxygen fugacities of the Pb–Zn and W related magmas are similar, the absence of a giant W–Sn deposit in the LT may indicate that parent magmas with a low degree of evolution and magmatic-hydrothermal transition are not conducive to their formation. This implies that the rocks that originated as highly evolved silicate-rich parent magmas, with a high degree of magmatic-hydrothermal alteration, would need to be targeted for W–Sn mineral exploration in the LT. In summary, our results emphasize that variations in chemical differentiation and the evolution of high-silica magmatic-hydrothermal systems can lead to differences in Pb–Zn and W enrichment. This has implications for the evaluation of the mineral potential of high-silica granites and hence their attractiveness as targets for mineral exploration.     

德兴斑岩铜矿成矿流体来源及其演化的氧同位素证据

与原始岩浆组成 (δ1 8O=8.36× 1 0 - 3)相比 ,铜厂花岗闪长斑岩的氧同位素在纵向上存在着 3个特征区段 :1中段 (标高 0～ 80 m之间 )样品接近原始岩浆1 8O值 ;2上段 :在此之上的样品 δ1 8O值变化区间很大 ( 5.4 3～ 1 0 .86)× 1 0 - 3 ,有富集 1 8O的 ,也有亏损 1 8O的 ;3下段 :标高在 0 m以下深部未 /弱蚀变斑岩样品的δ1 8O值均一 ,亏损1 8O,集中在 ( 6.8± 0 .5)× 1 0 - 3范围内。研究表明 ,这种现象是岩浆流体、深部非岩浆流体和浅部大气降水等 3种不同来源的热液流体在不同温度、水 /岩比环境下发生水岩交换综合形成的     

滇西三江地区新生代碱性系列岩浆岩构造类型

滇西三江地区新生代碱性系列岩浆岩,其构造类型非大陆边缘之岛弧,也非大陆内部之裂谷,而属两大陆碰撞,在抬升板块陆内一侧巨型走滑构造体系。     

Olivine-rich units in the Fongen–Hyllingen Intrusion, Norway: implications for magma chamber processes

The Fongen–Hyllingen Intrusion (FHI) is considered to have crystallised from stratified magma residing in a bowl-shaped magma chamber. Seven olivine-rich units, representing the most primitive cumulates in the central part of the intrusion, are associated with compositional reversals and are interpreted as having formed at the lowest part of the magma chamber floor. Based on phase-relationships, the crystallisation order is explained in terms of magma mixing and fractional crystallisation. Repeated influxes of small volumes of dense, primitive magma at the base of the chamber had a major impact on the crystallising assemblage on the local floor and a decreasing effect towards the flanks of the chamber. This was due to the small volume of replenishing magma, the geometry of the chamber and the consequent restriction of magma mixing to the deepest part of the chamber where the new magma was emplaced. It is estimated that the chamber floor sloped as little as 1–2°, but this was sufficient to give widely different cumulate sequences near the bottom of the chamber and on the flanks.     

秦岭凤太矿田金属成矿系列初探

以八卦庙金矿和八方山－ 二里河铅锌矿为例, 重点论述了凤太矿田金属矿床的主要特征, 并将其分为两个不同的成矿系列,即热水沉积型Ｐｂ、Ｚｎ （Ｃｕ） 成矿系列和热水沉积－岩浆热液叠加型金成矿系列。这两个成矿系列在成矿地质环境、含 （赋） 矿层特征、成矿物质来源、成矿作用方式等方面均有较大的差异。     

Sillimanite and andalusite produced by base-cation leaching and contact metamorphism of felsic igneous rocks

Abstract Contact metamorphism adjacent to a porphyritic quartz-monzodiorite at Kentucky, New South Wales, Australia has produced hornfelses in porphyritic leucogranite at a peak temperature of about 650–700° C and a maximum confining pressure of about 2 kbar (200 MPa). A gradation appears to exist from normal slightly peraluminous to modified strongly peraluminous metagranite hornfelses, which have also been enriched in sulphur. The strongly peraluminous hornfelses, containing cordierite, andalusite, sillimanite, biotite, pyrite and pyrrhotite, retain residual porphyritic igneous microstructures. These rocks appear to have been formed by leaching of base cations, during and possibly just before the contact metamorphism. Folia of fibrous sillimanite anastomose between lenticular grains of quartz and feldspar and truncate igneous zoning in plagioclase grains, suggesting that cation leaching and solution transfer occurred during growth of the sillimanite. Fibrous sillimanite also grew in grain boundaries of polygonal aggregates formed by the contact metamorphism. Therefore, at least some of the cation leaching appears to have occurred at the highest metamorphic grade. Metasandstones that are locally strongly peraluminous adjacent to the monzodiorite stock also, have probably undergone similar leaching.     

冲绳海槽中部Jade热液活动区中块状硫化物的稀土元素地球化学特征

用ICP－MS对冲绳海槽Jade热液活动区中6个块状硫化物样品进行了稀土元素分析。除一个样品表现出极微弱的正Eu异常外(δEu=1.09)，其他样品均表现出负Eu异常(δEu=0.58-0.71)和LREE相对富集((La/Yb)N=1.29-47.87)的球粒陨石标准化配分模式。块状硫化物样品之间稀土元素组成的变化是由于热液流体－岩石的相互作用以及海水和热液流体不同程度混合的结果，与深海沉积物，火山岩和海水稀土元素配分模式对比，表明块状硫休物的REE部分来自沉积物和火山岩，海水的混合作用对块状硫化物的REE配分模式具有一定的影响。海底热液硫化物稀土元素组成的变化可以一定程度的反映在出热液流体的演化特征。     

金川铜镍硫化物矿床岩浆成矿作用的偏在性

偏在性是指形成超大型、大型矿床的特殊成矿条件和成矿位置。在论述金川岩体母岩浆的形成、演化、深部熔离和分期上侵就位的全过程成矿作用的基础上，通过和国内外同类矿床对比，阐明了金川矿床的偏在性。这种偏在性主要表现为，它既具备形成超大型岩浆硫化物矿床的６项条件，又具备小侵入岩体成大矿的３项条件。所以，金川矿床的“偏在性”构成了世界上最主要的岩浆硫化物矿床类型之一：元古代与大陆边缘裂谷有关的二辉橄榄岩小侵入体矿床     

The Haymiliyah Sulphide Ores (Haylayn Massif,Oman Ophiolite): In-situ Segregation of PGE-poor Magmatic Sulphides in a Fossil Oceanic Magma Chamber

The Semail ophiolite in Oman is one of the few ophiolitic complexes that may display locally magmatic sulphide ores in layer 3 of the crustal section. The ores found in the wadi Haymiliyah plutonic sequence, are composed of low Ni-pyrrhotite, chalcopyrite, pyrite and pentlandite. They are located at the bottom of a thick two-pyroxene and noritic gabbro unit, the Main Laminated Noritic Gabbro Unit (MLNGU) that crystallized from evolved tholeiites according to a calc-alkaline liquid line of descent. The MLNGU rests on coarse-grained layered gabbros of the Main Layered Gabbro Unit (MLGU) crystallized from more primitive MORB-like magmas. A detailed mineralogical study coupled with analyses of S, Se, chalcophile transition metals (Cu, Ni, Platinum-group elements, PGE) and Au allows two stages to be distinguished in the precipitation of the sulphides. Sulphide modal abundances start to increase at the top of the MLGU (up to 4%) where orthopyroxene becomes a major phase. Sulphides in the MLGU are only intercumulus and Cu-rich, indicating a S-undersaturated regime. The fine-grained two-pyroxene gabbros at the bottom of the MLNGU contain up to 16% sulphides (monoclinic pyrrhotite, pyrite, chalcopyrite). However, their precious metal contents are very low and far from economic grade ( PGE+ Au <15 ppb except one concentration at 230 ppb in the richest layer). Sulphide droplets in cumulus silicates indicate that sulphides precipitated in the fine-grained gabbros from a S-saturated magma. Sulphide liquid immiscibility was likely triggered by a set of unusual circumstances, such as a progressive S enrichment in high f_O2 environments resulting from the closure of the Haymiliyah magma subchamber, the lack of a strong Fe-enrichment trend, and possibly a sudden temperature drop. The abundance of disseminated ores suggests that gravity segregation was inefficient; sulphide droplets probably nucleated at the crystallization front of the silicates. This hypothesis (of in-situ crystallization of sulphides) would also account for the very low precious metal contents. All the S present in the magma chamber was mobilized into the ores since the underlying MLNGU and isotropic gabbros are almost devoid of sulphides.     

Architecture and emplacement of the Nebo–Babel gabbronorite-hosted magmatic Ni–Cu–PGE sulphide deposit,West Musgrave,Western Australia

The Nebo–Babel Ni–Cu–platinum-group element (PGE) sulphide deposit in the West Musgrave Block, Western Australia, is the largest nickel sulphide discovery in the last 10 years. The deposit is hosted within a concentrically zoned, olivine-free, tube-like (chonolithic), gabbronorite intrusion associated with the, approximately, 1,078-Ma Giles Complex-layered intrusions in the Warakurna large igneous province. Emplaced into sulphide-free amphibolite facies orthogneiss, the fault-offset Nebo–Babel chonolith extends for 5 km and has a cross-section of 1 × 0.5 km. Igneous mineralogy, fabrics, and textures are well preserved. The lithostratigraphy includes variably textured leucogabbronorites (VLGN) that form an outer shell around mineralised gabbronorite (MGN), with barren gabbronorite (BGN) and oxide–apatite gabbronorite (OAGN) in the middle and lower parts of the chonolith. Mineral and whole-rock geochemistry indicate that the units become progressively evolved in the order: VLGN, MGN, BGN, and OAGN, and that incompatible trace-element concentrations increase downwards within the MGN and BGN. The mineralisation, which is confined to the early, more primitive units (VLGN and MGN), occurs as massive sulphide breccias and stringers and as disseminated gabbronorite-hosted sulphides. The massive sulphides were emplaced late in the intrusive sequence, have different PGE chemistry and Cu tenor to the disseminated sulphides, and have undergone sulphide fractionation. The distribution of disseminated sulphides, which are primary magmatic in origin, is related to chonolith geometry and magma flow regimes, rather than to gravitational settling. Sulfur-bearing country rocks are absent in the Nebo–Babel deposit area, and thus, local crustal S addition was unlikely to have been the major mechanism in achieving sulphide immiscibility. The Nebo–Babel intrusion is part of an originally continuous magma chonolith with multiple and related magma pulses. The parental magma was medium- to low-K tholeiite with 8–9 wt% MgO. The initial magma pulse (VLGN), the most primitive and sulphide saturated, was probably emplaced along a linear weakness in the country rock. After crystallisation of VLGN, marginally more fractionated, sulphide-saturated magma was injected through the thermally insulated core of the conduit, forming the MGN. Successive pulse(s) of more fractionated magma (BGN) were emplaced in the core of the intrusion. After magma flow ceased, closed system crystal fractionation produced consistent mineral and chemical fractionation trends within BGN and OAGN. After crystallisation, the intrusion was overturned and then offset by the Jameson Fault resulting in the apparent ‘reverse’ chemical and mineral trends in Nebo–Babel.