The geology of the Taparko gold deposit, Birimian greenstone belt, Burkina Faso, West Africa

The Taparko gold deposit, located in the eastern branch of the Proterozoic Birimian Bouroum-Yalogo greenstone belt (Burkina Faso) consists of a network of quartz veins developed in a N 170° trending shear zone (250 m wide, 4 km long) superimposed on the regional Birimian structural pattern. The quartz vein network is composed of: (a) a dominant array of quartz veins (type 1), parallel to the shear zone and comprising strongly deformed dark quartz exhibiting foliation, layering, ribbon, tension gashes, etc.; (b) oblique and subparallel related veins (type 2) of gray to white weakly deformed quartz crosscutting the dominant quartz veins resulting in breccia structures; and (c) shallow dipping veins (type 3), cross-cutting veins types 1 and 2 and filled by undeformed white buck structure quartz. Cross-cutting relationships and different quartz types in different veins and within individual veins imply a concomitant filling of the veins during the progressive deformation. Initial sinistral transcurrent shearing evolved with time to sinistral reverse shearing. Metallic minerals occur only in type 1 and 2 veins and were deposited in two stages, with native gold being related to second stage sulfides. Gold (and chalcopyrite) precipitated preferentially upon the surfaces of fractured pyrite grains in low-pressure sites (pressure shadow zones) around and/or within the sulfide grains (along subsequently annealed fractures). The formation of the South Taparko deposit can be divided into a succession of events: (a) during the first event, N 170°-directed sinistral transcurrent shearing resulted in a N 20° mylonitic foliation and fractured rock which allowed H₂O-, CO₂- and SiO₂-rich fluids to circulate and deposit quartz with buck texture; (b) during the second event, type 1 quartz was strongly deformed and type 2 veins formed with sigmoidal shapes as viewed on a horizontal plane; and (c) during the third event, the sinistral transcurrent shearing evolved to sinistral reverse shearing and the deformation style evolved correspondingly from ductile to brittle-ductile. During the last phase of deformation gold nucleated and deposited in low-pressure zones. Received: 9 July 1997 / Accepted: 23 March 1998     

湘赣边区NNE向走滑断裂-流体-铀成矿动力学分析

以湘赣边区ＮＮＥ向走滑断裂系统构造解析及其演化和动力学分析为主线,结合含矿流体的运动学过程研究,进行热液铀成矿动力学分析,以探讨成矿作用的动力来源、作用机制和演化过程。     

用套筒致裂法测试岩体应力反弹性模量与讨论

在改进套筒致裂装置性能的基础上,使压力-体积曲线的记录与处理得到改善,对潘三矿东采区的测试实践表明,得出的压力-体积曲线较好地反映了钻孔围岩随内压增加所表现出的力学性质。还探讨了如何通过改进套筒致裂技术来实现由围岩应力测试发展到原岩应力测试以及围岩损伤区的测定。     

储层裂缝特征测井解释方法综述

为了更好地研究裂缝性砂砾岩储层和其它裂缝性储层裂缝的测井解释问题,综述了目前在裂缝性储层裂缝测井解释中主要采用的常规评价、人工神经网络和斯通利波３类方法在识别裂缝带和定量计算储层裂缝参数两个方面的应用原理和研究现状,并指出了今后的一些研究方向。     

裂缝储层岩芯检测技术及其应用

针对目前裂缝岩芯检测的技术缺陷,建立了裂缝面、在层层面及钻井井身等产状要素之间的空间几何学联系,利用这种联系可以确定非定向取芯井裂缝的真实产状,研制了裂缝岩芯检测资料处理系统软件,可提高代作效率和精确度;应用该技术对江汉盆地王场地区潜江组非砂岩进行了研究,结果揭示了目的层裂缝成因类型、空间分布及对油气的控制作用。据此所提出的靶区经钻探已获得油流,说明该技术具有实用价值。     

分形理论在断裂工程活动性评价中的应用——以鄂西恩施、建始断裂为例

运用分形理论对鄂西恩施、建始断裂的几何分形结构特征进行了研究,并选取有代表性的碎裂岩作了粒径—频度分析,表明其同样具有分形结构,分维值可以定量地反映恩施、建始断裂同为地壳上部浅层构造,几何结构较为简单,构造活动性弱,分别趋于构造衰亡和发育成熟阶段。在这两条断裂附近通过水系的分形统计结果分析,断裂几何分维值的意义与其最后一次活动表现出的水系切错和同步弯曲现象相吻合。分维值可以作为断裂工程活动性评价的一个定量指标。     

Ray equation migration of wide-angle reflections in Dabie orogenic zone

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Burst strain disturbance—A new test study on fracture precursor

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K–Ar ages from seamount chains in the back-arc region of the Izu–Ogasawara arc

The Izu–Ogasawara arc contains, from east to west, a volcanic front, a back-arc extensional zone (back-arc knolls zone), and a series of across-arc seamount chains that cross the extensional zone in an east-northeast and west-southwest direction and extend into the Shikoku Basin. K–Ar ages of dredged volcanic rocks from these across-arc seamount chains and extension-related edifices in the back-arc region of the Izu–Ogasawara arc were measured to constrain the volcanic and tectonic history of the arc since the termination of spreading in the Shikoku Basin. K–Ar ages range between 12.5 and 1 Ma. Andesitic to dacitic rocks of 12.5–2.9 Ma occur mainly on the western part of the chains. The western part of the chains are the locus of volcanism behind the front which erupted mainly calc-alkaline andesitic lavas. The youngest rocks (< 2.8 Ma), characterized by cpx-ol basalt, occur along the western margin of the back-arc knolls zone. Basaltic rocks of 12.5–2.9 Ma have relatively high concentrations of Na₂O (> 2.0 wt%), Zr (> 50 p.p.m.) and Y (> 20 p.p.m.) and low CaO (< 12 wt%). On the other hand, basalts of 2.8–1 Ma have lower Na₂O (< 1.8 wt%), Zr (< 50 p.p.m.) and Y (< 20 p.p.m.), but significantly higher CaO (> 12 wt%). The age inferred for the initiation of back-arc rifting (∼ 2.35–2.9 Ma: Taylor 1992 ) behind the current volcanic arc coincides with the time that basalt chemistry changed drastically (eruption of the low-Na₂O and high-CaO basalt). This implies that post-2.8 Ma volcanism in the back-arc knolls zone is associated with rifting. Similarly, the change in chemical composition might be explained by a different type of source mantle following rift initiation. Volcanism in the western seamounts ceased after the onset of rifting at ∼ 2.8 Ma.     

P–T–t paths and post-metamorphic exhumation of the Day Nui Con Voi shear zone in Vietnam

The Day Nui Con Voi belt in Vietnam is the southeasternmost part of the Red River shear zone in Asia. It is a narrow high-grade metamorphic core complex consisting of garnet–sillimanite–biotite gneisses, mylonite bands, amphibolite layers and migmatites. Geothermobarometric study of the complex revealed that the peak metamorphism took place under amphibolite-facies conditions of 690₋₆₀⁺³⁰°C and 0.65±0.15 GPa and the subsequent mylonitization occurred under greenschist-facies conditions of 480°C and under 0.3 GPa. Fifteen synkinematic hornblende and biotite separates from gneisses, amphibolites and mylonites were dated with the K/Ar method. Hornblende separates from the Day Nui Con Voi give K–Ar ages of 26.4–28.5 Ma, and the biotite separates do give 24.5–24.7 Ma. Combination of thermobarometric and geochronological data yields the cooling history of 500°C at 28 Ma and 300°C at 24 Ma with a cooling rate of 70–110°C Ma⁻¹, and 23 km post-metamorphic exhumation of the core complex. The first 16 km exhumation from the peak of metamorphism (at probably 31 Ma) to 28 Ma was triggered by the left-lateral strike-slip displacement of the Red River shear zone.