Three mechanisms of ore re-mobilisation during amphibolite facies metamorphism at the Montauban Zn–Pb–Au–Ag deposit

The relative importance of mechanical re-mobilisation, hydrothermal dissolution and re-precipitation, and sulphide melting in controlling redistribution of metals during concurrent metamorphism and deformation is evaluated at the middle amphibolite facies Montauban deposit in Canada. As at many other deposits, ductile deformation was important in driving mechanical re-mobilisation of massive sulphides from limb regions into hinge regions of large-scale folds and is thus the most important for controlling the economics of Pb and Zn distribution. Two possible stages of hydrothermally driven re-mobilisation are discussed, each of which produces characteristically different alteration assemblages. Prograde hydrothermal re-mobilisation is driven by pyrite de-sulphidation and concurrent chlorite dehydration and is thus an internally driven process. At Montauban, the H₂S-rich fluid generated through this process allowed re-mobilisation of gold into the wall rock, where it was deposited in response to sulphidation of Fe Mg silicates. Retrograde hydrothermal re-mobilisation is an externally driven process, whereby large volumes of fluids from outside the deposit may dissolve and re-precipitate metals, and cause hydration of silicate minerals. This second hydrothermally driven process is not recognised at Montauban. Sulphide melting occurred as temperatures neared the peak metamorphic conditions. Melting initiated in the massive sulphides through arsenopyrite breakdown, and a small volume of melt was subsequently re-mobilised into the wall rock. Trace element partitioning and fractional crystallisation of this melt generated a precious metal-rich fractionate, which remained mobile until well after peak metamorphism. Thus, prograde hydrothermal re-mobilisation and sulphide melting were the most important mechanisms for controlling the distribution of Au and Ag.     

Very low-grade metamorphic study in the pre-Late Cretaceous terranes of New Caledonia (southwest Pacific Ocean)

Abstract   Pre-Late Cretaceous terranes from the central part of New Caledonia have been metamorphosed under very low-grade conditions by two high-pressure/low-temperature events. The present study investigates the metamorphic patterns with phyllosilicate crystallinities, electron microprobe analyses and petrography. The first metamorphic event is of Late Jurassic age and is characterized by very low (anchizone) to low-grade (epizone) conditions with a decrease of the illite Kübler Index (KI) and the chlorite Árkai Index (ÁI) values from northeast to southwest. This trend is also confirmed by chlorite thermometry. In the south of the area, un-metamorphosed sediments (diagenetic KI values) are observed in the Senonian 'formation à charbons', post-dating the metamorphism in this region. The second metamorphism is an Eocene high-pressure event, which overprints the Late Jurassic metamorphism in the northern part of the studied area. In this zone, the pattern of KI and ÁI indicates another gradient with increasing metamorphic conditions from southwest to northeast. Temperatures calculated by chlorite thermometry also indicate an evolution from southwest to northeast with slight increase of temperature from 298 ± 8°C to 327 ± 16°C. In both metamorphic zones, the K-white mica b cell dimension calculated on micas analyzed at electron microprobe are in good agreement with high-pressure/low-temperature metamorphic conditions (b₀ > 9.04 Å). A combination of chlorite thermometry and K-white mica b cell dimension allows estimation of a minimum pressure of 1.3 GPa in the Eocene zone (in excellent agreement with the 1.5 GPa registered in the northern part of New Caledonia) and a minimum of 1.1 GPa in the Late Jurassic metamorphic part.     

湘中锡矿山式锑矿成矿地质条件分析

湘中锡矿山式锑矿形成于晚白垩-古新世,空间上与岩脉关系密切,各锑矿床、矿化点都伴有或附近发育有煌斑岩及中-酸性岩脉群,在锑矿成矿同期地质事件中,还有周缘一些中-新生代红色盆地的形成及基性火山岩喷发,据之,提出湘中锡矿山式锑矿成矿与燕山晚期拉张构造-岩浆活化作用有关.锑矿床(点)基本上都产出于两组或两组以上断裂的交汇点附近,3组断裂的交汇部位对应于最主要的锑矿床(点).矿体具体受断裂交汇部位附近的次级短轴背斜轴部、倾伏背斜的倾伏端及其翼部被纵向陡倾角断裂构造所切穿的部位控制,是断裂导矿与背斜构造圈闭的体现.岩性组合控矿表现为易于硅化蚀变交代的砂质碳酸盐岩与隔挡层泥质岩所构成的岩性圈闭.根据包裹体均-温度和盐度测定成果,推算成矿压力为(200～300)×10⁵Pa,成矿深度约为1 km.     

Metamorphism of the Basement of the Qilian Fold Belt in the Minhe-Ledu Area, Qinghai Province, NW China

The basement of the central Qilian fold belt exposed along the Minhe-Ledu highway consists of psammitic schists, metabasitic rocks, and crystalline limestone. Migmatitic rocks occur sporadically among psammitic schist and metabasitic rocks. The mineral assemblage of psammitic schist is muscovite + biotite + feldspar + quartz ± tourmaline ± titanite ± sillimanite and that of metabasitic rocks is amphibole + plagioclase + biotite ± apatite ± magnetite ± pyroxene ± garnet ± quartz. The migmatitic rock consists of leucosome and restite of various volume proportions; the former consists of muscovite + alkaline feldspar + quartz ± garnet ± plagioclase while the latter is either fragments of psammitic schist or those of metabasitic rock. The crystalline limestone consists of calcite that has been partly replaced by olivine. The olivine was subsequently altered to serpentine. Weak deformations as indicated by cleavages and fractures were imposed prominently on the psammitic schists, occasionally on me     

Subduction of Continental Crust in the Early Palaeozoic North Qaidam Ultrahigh-Pressure Metamorphic Belt, NW China:Evidence from the Discovery of Coesite in the Belt

Coesite was discovered as inclusions in zircon separates from pelitic gneiss associated with a large eclogite body in the North Qaidam ultrahigh-pressure (UHP) terrane. Some graphite inclusions were also found. This finding suggested the occurrence of in-situ UHP metamorphism and that the terrane was most likely recrystallized at pressures below the diamond stability field. It supported other previous indirect UHP evidence, such as polycrystalline quartz inclusions in eclogitic garnet, quartz lamellae in omphacite and P-T estimates for both eclogite and garnet peridotite. The U-Pb and Sm-Nd ages of the North Qaidam eclogite indicated that subduction of continental crust occurred in the Early Palaeozoic, which probably recorded a collision between the Sino-Korean and Yangtze plates.     

On Continent-Continent Point-Collision and Ultrahigh-Pressure Metamorphism

Up to now it is known that almost all ultrahigh-pressure (UHP) metamorphism of non-impact origin occurred in continent-continent collisional orogenic belt, as has been evidenced by many outcrops in the eastern hemisphere. UHP metamorphic rocks are represented by coesite- and diamond-bearing eclogites and eclogite facies metamorphic rocks formed at 650-800℃ and 2.6-3.5 GPa, and most of the protoliths of UHP rocks are volcanic-sedimentary sequences of continental crust. From these it may be deduced that deep subduction of continental crust may have occurred. However, UHP rocks are exposed on the surface or occur near the surface now, which implies that they have been exhumed from great depths. The mechanism of deep subduction of continental crust and subsequent exhumation has been a hot topic of the research on continental dynamics, but there are divergent views. The focus of the dispute is how deep continental crust is subducted so that UHP rocks can be formed and what mechanism causes it to be subducte     

内蒙古锡林郭勒杂岩岩石学特征及其变质作用的p—t条件

锡林郭勒杂岩出露于华北地台北缘晚古生代褶皱带内，形成于1．3－1．0Ga，主要由黑云斜长片麻岩、角闪斜长片麻岩、斜长角闪片岩以及变粒岩夹透镜状或脉状斜长角闪岩组成，具角内岩相变质，原岩以钙碱性系列火成岩为主。通过对其中角闪岩及片麻岩中的长石、角闪石的矿物化学研究，计算出其变质作用发生的温度为540－550℃，压力为0.5-0.6GPa。     

略论煤的变质问题

我国煤的变质作用根据受热的主要热源及其作用方式和煤质特征，可归纳为三种基本类型。即：深成变质作用、岩浆热变质作用和动力变质作用。此外，1988年中国地质大学潘治贵等，通过对青海热水矿区研究发现而命名为一种新变质作用类型-热水变质作用。在前人研究的基础上，笔者拟就影响变变质的主要因素谈谈看法，供讨论。     

大别-苏鲁超高压变质带P-T-t轨迹的动力学模拟

综合现有的地质、地球化学资料以及同位素年龄等研究成果，同时结合新西兰南岛北端陆壳俯冲的最新发现，提出了超高压变质岩的形成四阶段演化模式：板片俯冲形成增生楔、板片俯冲驱动角落流、板片拆离浮力抬升至Moho深度和后期上地壳伸展阶段.以此为定量模拟的出发点，利用二维有限元对大别-苏鲁超高压变质带的形成演化进行了动力学和热演化模拟，追踪超高压变质岩形成演化过程中的质点路径以及对应的P-T-t轨迹.计算的P-T-t轨迹及其空间分布特征均能与实测结果较好吻合.     

岩浆流体在热液矿床形成中的作用

岩浆流体在浅部分离为岩浆卤水和蒸汽相 ,CO2 、SO2 的加入将增加不混溶区间。Ag ,Zn ,Pb ,Sn等在高盐度卤水中呈氯化络合物的形式搬运 ,Cu、Au呈I价态的二硫化络合物的形式在富硫的蒸汽相中搬运。岩浆流体与大气水混合的稀释和热效应 ,是导致Sn元素沉淀的主要机制 ,流体混合需要长期稳定的抽送系统 :( 1)对流体界面混合 ;( 2 )两组裂隙处相遇混合。斑岩Cu矿床早期以岩浆流体为主导 ,晚期大气水普遍存在。反应性强、富含金属的岩浆流体从侵入体往外运移并且与主岩反应 ,形成带状分布的蚀变矿物组合。高硫化浅成热液矿床的早期以流体对主岩的广泛淋滤为特征 ,流体呈酸性和氧化性。密度差使得低盐度液体与深处高盐度卤水在空间上分离。低硫化浅成热液矿床的成矿流体呈低盐度、中性pH值和处于还原性、静水压力条件 ,流体沸腾是成矿卸载的主要机制。富Au型矿床与低盐度富气相流体有关 ,富Ag型矿床与较高盐度的流体有关。在热液系统的寿命中 ,导致矿化的流体活动仅在短暂的时期内存在。热液系统之间在岩浆标志上的变异是由于岩浆流体的间歇性贡献或缺失造成的。