新疆东戈壁钼矿床地质特征及成矿热液作用阶段划分、矿物生成顺序研究

新疆东戈壁斑岩型钼矿床是西北地区发现的第一个特大型钼矿床,矿床产于下石炭统浅变质碎屑岩夹火山岩中,位于雅满苏大断裂附近.文章通过分析含矿脉体之间的交切关系及生成先后顺序,将东戈壁钼矿成矿期热液作用划分为4个阶段;根据金属矿物之间的穿插、交代、共边关系,确定了矿物生成的先后顺序.     

Fast Variscan anticlockwise P-T-t path in the Eastern Alps: evidence from metasedimentary rocks and metamorphic veins of the Silvretta thrust sheet

Abstract

In the South-Western part of the Silvretta thrust sheet (Pischa area), metasedimentary rocks and metaso-matie veins show evidence of a Variscan anticlockwise P-T-t path, which differs from previously proposed P-T-t paths in the pre-Mesozoic basements of the Alps. The prograde path corresponds to a HT-LP metamorphism illustrated by sillimanite blastesis after andalusite. Maximum temperature (ca. 550 °C) is constrained by the stability of staurolite and muscovite. After temperature climax, pressure increased quasi-isothermally to P max of ca. 0.5–0.6 GPa. P max is limited by phase relations and fluid inclusion data. The prograde path is characterized by the development of an S2 schistosity and mineral lineation L2 oriented E-W, and by the formation of a first generation of quartz-andalusite veins (type 2 quartz veins) parallel to the S2. The decompression path ends in the stability field of andalusite and is characterized by extensional structures such as symmetric and asymmetric foliation boudinage showing a stretching direction still oriented E-W. The extensional structures also result in the formation of quartz-andalusite-muscovite aggregates. Shear bands affecting both metapelites and type 1 and 2 quartz veins indicate a top to the W-SW sense of shear. The last generation of quartz-andalusite veins (type 3) strikes N-S and vertically cross cuts the foliation. Phase relations suggest that the Silvretta thrust sheet underwent an anticlockwise P-T path, probably a result of magmatism and/or nappe emplacement. The prograde part of this P-T loop developed between 320 and 300 Ma, and cooling already occurred at ca. 300 Ma. Therefore, the Variscan cycle was fast in this part of the Alps.     

青藏高原南部拉萨地块中新世超钾质岩石中的锆石记录

幔源岩浆上升的过程中捕获的锆石为揭示深部地壳"隐藏"的岩浆作用事件提供了宝贵机会。本文对采自南部拉萨地块学那地区的超钾质脉岩中的锆石进行了U-Pb年代学、微量元素和Hf同位素研究。研究结果表明,学那超钾质岩石中的锆石主要展示出4个主要的年龄峰值,分别是:<100Ma、300~400Ma、450~500Ma以及700~850Ma。这些锆石高U/Yb比值、低Y含量的特征暗示起源于大陆地壳。而新生代-中生代(<100Ma)和晚古生代(300~400Ma)的岩浆活动在南部拉萨地块上广泛发育,这表明南部拉萨地块新生地壳物质对学那超钾质岩浆活动的贡献。但是超钾质脉岩中早古生代和元古代(450~500Ma和 700~850Ma)锆石捕掳晶的存在则证实印度大陆地壳物质的加入。此外,从大约55Ma左右开始,锆石颗粒的(Dy/Yb)_N比值开始逐渐增高,ε_Hf(t)值则从+10~+5迅速下降至-10~-25。考虑到南部拉萨地块新生地壳的同位素组成特征,超钾质脉岩中的这些锆石颗粒可能记录了印度-亚洲陆陆汇聚过程中地壳的显著加厚以及俯冲的印度大陆地壳物质对南部拉萨地块后碰撞岩浆作用的贡献。     

Lateral variations in the lithology and organic chemistry of a black shale sequence on the Mesoarchean seafloor affected by hydrothermal processes: The Dixon Island Formation of the coastal Pilbara Terrane,Western Australia

The Dixon Island Formation of the coastal Pilbara Terrane, Western Australia is a 3.2 Ga volcanic–sedimentary sequence influenced by syndepositional hydrothermal activity formed in an island‐arc setting. We documented lateral variations in stratigraphy, hydrothermal alteration, and biological activity recorded in the sedimentary rocks (over several kilometers), with the aim of identifying areas of biological activity and related small‐scale structures. The Dixon Island Formation comprises volcaniclastics, black chert, and iron‐rich chert within seven tectonic blocks. Based on detailed geological mapping, stratigraphic columns, carbon isotope composition, and organic carbon (C_org) content, we found lateral (>5 km) variations in stratigraphy and carbon isotope compositions in a black chert sequence above the Mesoarchean seafloor with hydrothermal activity. Two felsic tuff layers are used as stratigraphic marker beds within a black chert sequence, which was deposited on altered volcanic rocks. The black chert sequence in each tectonic block is 10–20 m thick. Thickness variations reflect topographical undulations in the paleo‐ocean floor due to faulting. Early‐stage normal faults indicate extensional conditions after hydrothermal activity. Black chert beds in the topographically subsided area contain higher C_org contents (about 0.4 wt%) than in areas around the depression (<0.1 wt%). Carbon isotope compositions for the black chert vary from ?40 to ?25‰, which are similar to values obtained for a black chert vein within the komatiite–rhyolite tuff sequence (underlying the black chert sequence). Those for other rock types in the Dixon Island Formation are ?33 to ?15‰. Results indicate that deformation occurred soon after the final stages of hydrothermal activity. After this early‐stage deformation, organic‐rich sediments were deposited over an area several kilometers across. The organic‐rich sediments indicate stagnant anoxic conditions that resulted in the deposition of siliceous and organic matter from hydrothermal vein systems. When hydrothermal activity terminated, normal faulting occurred and organic matter was deposited from the sea surface and silica from the seafloor.     

深海热液区化能自养菌Caminibacter profundus氢酶对环境因子的响应特点

以深海热液区化能自养菌Caminibacter profundus为对象,采用扫描电镜和化学能谱分析,研究了菌株表面形态和矿物元素沉积;同时研究了菌株生长、甲基紫精(MV)还原的氢酶活性及膜结合的类型I NiFe氢酶基因(hynL)表达对盐度、pH和温度几种环境因子变化的响应特点。结果表明,菌株表面被Si、Ca、S和Fe等多种元素组成的矿物质层所包裹。不同条件下,菌株生长、MV还原的氢酶活性和hynL的表达趋势相一致,其最适条件为盐度30、pH5.5和55℃培养温度。研究结果表明,在环境改变时,C.profundus通过调控hynL的表达,以调整菌株的能量代谢,维系菌株的生长繁殖。     

黑色岩系矿床成因及其海水、热水与生物有机成矿作用

黑色岩系型矿床是矿床学研究的一个重要对象,其成因由于成矿作用复杂而存在争议。为深入理解这种复杂性并为解决争议提供参考信息,通过深入剖析国内外这类矿床的典型实例,述评了矿床成因研究的现状与进展,并进一步提出了值得加强研究的方向。全球黑色岩系矿床分布广泛,矿床形成过程中受多种地质作用影响,并可主要归纳为3种：海水、热水与生物有机成矿作用。其中,海水和热水为成矿提供元素及有利的成矿条件(如还原的沉积环境)。相比而言,生物有机质的成矿作用则主要体现在对海水和/或热水提供的元素进行富集,并在合适条件下成矿。可见,这3种成矿作用的多元复合作用使得成矿过程极其复杂,这是导致很多矿床成因至今未完全确定的一个重要原因。今后可从两方面开展深化研究：一是揭示多期复杂成矿演化过程;二是剖析生物有机成矿作用。这些认识对沉积岩型矿床的研究同样具有普遍参考意义。     

陕南地区硝璜洞金矿地质特征及成因

陕南秦巴山区是我国重要的金矿产区,尤其是安康西北部的羊坪湾-流芳金成矿带更是秦巴山区的典型代表,硝璜洞金矿就位于这一成矿带上.硝璜洞金矿产于志留纪大贵坪组第一岩性段的含炭质云英片岩夹薄层炭质石英岩内,显示岩性对金矿化的制约.矿体严格受韧性剪切带控制,主要赋存于与主剪切带呈“入”字形相交的次级韧性断裂带中.矿体呈似层状、透镜状、脉状产出.金矿物以自然金形式赋存,自然金粒度呈粗粒、巨粒.野外可识别出3期硅化蚀变,金矿化与第1期蚀变关系密切.矿床成因为韧性剪切带控制的变质热液型金矿床.