大别山地区沙坪沟斑岩型钼矿床蚀变及矿化特征研究

沙坪沟钼矿床是大别山地区新近发现的、世界第二大的斑岩型钼矿床。本文在前期以及前人的工作基础上,通过详细的野外地质观察和系统的岩相学、矿相学以及探针测试分析工作,详细研究了沙坪沟钼矿床的蚀变与矿化特征。结果表明,沙坪沟钼矿床的围岩蚀变类型主要有钾硅酸盐化、青磐岩化和绢英岩化,绢英岩化又可细分为以石英为主和以绢云母为主。矿床中产出16种脉体类型,根据矿床的脉体类型划分了4个成矿阶段:(1)石英-钾长石阶段;(2)石英-硫化物阶段;(3)石英-绢云母阶段;(4)石英-萤石-石膏阶段。蚀变及矿化特征显示,沙坪沟钼矿床的钼矿化开始于石英-钾长石阶段晚期,结束于石英-绢云母阶段早期,石英-硫化物阶段是辉钼矿主要的形成阶段。沙坪沟钼矿床经历了多期次脉动式成矿流体的蚀变与矿化过程,在不同蚀变-矿化阶段中,成矿流体的物化条件和组分的变化是控制各阶段蚀变类型和脉体中矿物组合的主要因素。产于以伸展为主的板内环境的沙坪沟钼矿床与其他构造环境下的斑岩钼矿床在与成矿关系密切的岩石、蚀变分带等方面相似,但由于围岩性质的差异,矿体赋存位置及产状不同;而在矿物种类、矿化产出位置及产状、与矿化关系最为紧密的蚀变类型等方面存在差异。斑岩型钼矿床的构造背景可能控制了其岩浆的形成、演化以及含矿性,而岩浆岩最终定位的深度、围岩等条件也是控制蚀变和矿化特征的重要因素。     

蚀变岩帽的特征、成因以及在华南的分布探讨

Sillitoe（1995）蚀变岩帽（Lithocap）的定义为大范围富黄铁矿的硅化、高级泥化和泥化蚀变,在地质环境上位于古地表和浅成中-酸性岩浆侵入体之间。蚀变岩帽往往显示为突出的正地形,有助于寻找隐伏的斑岩矿化体。但蚀变岩帽在地表的范围往往多达几十个平方千米,又常常掩盖下覆斑岩矿床的蚀变矿化特征及其地球化学印记,因此大型的蚀变岩帽又给勘探工作带来一定的挑战。蚀变岩帽相关矿床的勘探需以地质填图为基础,结合近红外光谱分析（SWIR）进行蚀变填图,以及全岩地球化学以及矿物地球化学表现的元素或元素组合异常,来帮助定位热源或深部斑岩体。遥感和地球物理中的激电响应,也可以辅助定位岩体。华南地区的蚀变岩帽主要分布于长江中下游成矿带和东南沿海火山岩带。前人对安徽庐枞盆地中的矾山蚀变岩帽进行了系统研究,确定了矾山蚀变岩帽形成于白垩纪,与围岩砖桥组火山岩年龄一致。同位素和流体包裹体工作证明了形成矾山蚀变岩帽的流体主要为深部岩浆热液中的酸性气体与浅部大气降水的混合,在浅部高渗透率的火山岩及其岩性界面反应,广泛发育了一套硅化和高级泥化蚀变,指示与矾山相关可能存在斑岩和高硫型浅成低温热液铜金矿床。福建紫金山地区有中国最大的高硫型浅成低温热液矿床,主要赋存于紫金山蚀变岩帽中。紫金山蚀变岩帽的地质特征和蚀变分带已经研究的较为详细,但目前深部的侵入体还没有发现。浙江的蚀变岩帽是中国非金属矿产的重要来源,包括明矾石矿、地开石矿和红柱石矿等,这些蚀变岩帽与金属矿化的关系尚未有相关研究。根据目前的资料总结,有较多的蚀变岩帽分布在中国华南,这些蚀变岩帽特征典型,但目前的研究程度尚浅。现有的研究结果表明,华南的蚀变岩帽的成矿潜力巨大,可能存在一条巨型的斑岩-浅成低温矿床成矿带,具有广阔的找矿勘查前景,建议加强蚀变岩帽及相关矿床的找矿与研究工作。     

安徽庐枞盆地矾山酸性蚀变岩帽形成时代及其地质意义

酸性蚀变岩帽是岩浆热液流体和围岩在近地表相互作用的产物,是斑岩-浅成低温热液成矿系统的重要指标。发育在长江中下游成矿带庐枞盆地内的矾山酸性蚀变岩帽产出面积较大( 20km~2)。前人对该酸性蚀变岩帽中的明矾石矿床的地质和地化特征进行了相关研究,但详细的年代学研究工作尚未开展。为精确厘定矾山酸性蚀变岩帽的形成时代,本文开展了明矾石~(40)Ar-~(39)Ar法和金红石原位U-Pb法定年。矾山酸性蚀变岩帽中明矾石共有三种类型:ⅠA型明矾石主要呈交代蚀变发生在热液蚀变早阶段,与石英、粒状黄铁矿或赤铁矿、少量金红石共生;ⅠB型明矾石形成于热液蚀变晚阶段,主要呈叶片状集合体充填在开放空间中,与石英、星点状赤铁矿、粒状金红石集合体共生,少量金红石和赤铁矿沿明矾石解理裂隙分布;Ⅱ型明矾石是表生明矾石,主要呈细粒集合体沿裂隙分布,与赤铁矿、高岭石、地开石共生。三类明矾石形成于不同环境下:ⅠA和ⅠB型明矾石形成于岩浆热液环境下,是大矾山明矾石矿区的主要产物;Ⅱ型细粒明矾石分布在矾山酸性蚀变岩帽的非明矾石矿区,是表生环境下的产物。ⅠA型明矾石的~(40)Ar-~(39)Ar定年的坪年龄为131±6Ma,代表了矾山酸性蚀变岩帽的形成时代。与Ⅱ型明矾石密切共生的金红石U-Pb定年结果为32. 7±4Ma,在该期间,整个盆地内无岩浆活动发生,该年龄反映了矾山酸性蚀变岩帽经历表生氧化作用的时间。明矾石和金红石定年结果分别对应岩浆热液和表生明矾石的形成时代。在利用明矾石进行找矿工作时需先明确明矾石成因,矾山酸性蚀变岩帽中深成明矾石是下一阶段的找矿研究的基础。     

Assessment of groundwater recharge processes through karst vadose zone by cave percolation monitoring

Recharge processes of karst aquifers are difficult to assess given their strong heterogeneity and the poorly known effect of vadose zone on infiltration. However, recharge assessment is crucial for the evaluation of groundwater resources. Moreover, the vulnerability of karst aquifers depends on vadose zone behaviour because it is the place where most contamination takes place. In this work, an in situ experimental approach was performed to identify and quantify flow and storage processes occurring in karst vadose zone. Cave percolation monitoring and dye tracing were used to investigate unsaturated zone hydrological processes. Two flow components (diffuse and quick) were identified and, respectively, account for 66% and 34% of the recharge. Quickflow was found to be the result of bypass phenomenon in vadose zone related to water saturation. We identify the role of epikarst as a shunting area, most of the storage in the vadose zone occurring via the diffuse flow component in low permeability zones. Relationship between rainfall intensity and transit velocity was demonstrated, with 5 times higher velocities for the quick recharge mode than the diffuse mode. Modelling approach with KarstMod software allowed to simulate the hybrid recharge through vadose zone and shows promising chances to properly assess the recharge processes in karst aquifer based on simple physical models.     

The palaeomagnetism and magnetic fabric of cave sediments from Grønligrotta and Jordbrugrotta, Norway

Summary. Twenty-four oriented specimens of laminated clay were obtained from vertical sections in two caves in northern Norway. Studies of the magnetic remanence and susceptibility anisotropy show that the magnetization is depositional in origin and due to magnetite. The palaeomagnetic record in one section is correlated with results from a Swiss lake sediment core to suggest an age of 9600–6800 yr bp for the cave clay. It is shown that the susceptibility lineation produced on gently sloping surfaces was predominantly controlled by gravity and cannot therefore be used directly to estimate the palaeoflow or palaeofield directions.     

The MAR-Vema Fracture Zone intersection surveyed by deep submersible Nautile

One of the two objectives of the Vemanaute cruise of the French deep submersible Nautile, was the geological study of the eastern intersection area between the Mid-Atlantic Ridge (MAR) and the Vema Fracture Zone in the equatorial Atlantic. Fourteen dives were conducted that allowed detailed geological survey and sampling of the main morphostructural units of this area: the northern and southern walls of the fracture zone, the median ridge, the northern and southern troughs and the nodal basin. In situ observations of recent tectonic features such as furrows, ridges and circular depressions, concentrated within the southern trough, allowed us to establish the location and the size of the present-day displacement zone. Geological investigations have shown that the nodal basin is entirely floored by basalts thus contrasting with other equivalent areas such as the Kane and Oceanographer fracture zone-MAR eastern intersections. Finally, this study stresses the great opposition between the relatively old and tectonically inactive northern part of the fracture, and the southern part which shows active tectonics and recent volcanic activity.     

Submarine reworking of exhumed subcontinental mantle rocks: field evidence from the Lherz peridotites,French Pyrenees

In the Pyrenees, the lherzolites nowhere occur as continuous units. Rather, they always outcrop as restricted bodies, never more than 3 km wide, scattered across Mesozoic sedimentary units along the North Pyrenean Fault. We report the results of a detailed analysis of the geological setting of the Lherz massif (central Pyrenees), the type‐locality of lherzolites and one of the most studied occurrences of mantle rocks worldwide. The Lherz body is only 1.5 km long and belongs to a series of ultramafic bodies of restricted size (a few metres to some hundreds of metres), occurring within sedimentary formations composed mostly of carbonate breccias originating from the reworking of Mesozoic platform limestones and dolomites. The clastic formations also include numerous layers of polymictic breccias reworking lherzolitic clasts. These layers are found far from any lherzolitic body, implying that lherzolitic clasts cannot derive from the in situ fragmentation of an ultramafic body alone, but might also have been transported far away from their sources by sedimentary processes. A detailed analysis of the contacts between the Lherz ultramafic body and the surrounding limestones confirms that there is no fault contact and that sediments composed of ultramafic material have been emplaced into fissures within the brecciated carapace of the peridotites. These observations bear important constraints for the mode of emplacement of the lherzolite bodies. We infer that mantle exhumation may have occurred during Albian strike‐slip deformation linked to the rotation of Iberia along the proto‐North Pyrenean Fault.