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1.
石英(滑石)菱镁岩是基性岩、超基性岩与富CO2流体反应形成的一套硅化-碳酸盐化蚀变岩,常伴生金、汞、菱镁矿、碱金属矿化而在国外备受关注,但国内相关研究较薄弱。巴尔蛇绿岩位于西藏阿里地区,石英菱镁岩主要出露在蛇绿岩体东北边界,出露厚度约20 m,近北西西向延伸数千米。本文系统研究了雅鲁藏布缝合带西段巴尔蛇绿岩边部石英菱镁岩矿物学及地球化学特征。根据尖晶石镜下特征,划分为两个阶段:一阶段石英菱镁岩中尖晶石稳定存在,二阶段石英菱镁岩中尖晶石几乎全部蚀变(代表更强程度蚀变作用)。石英菱镁岩主要地球化学组成为Si O2、Mg O及CO2,8件样品主量元素含量变化较大,Si O2/Mg O比值波动较大,反映石英菱镁岩形成为非等化学过程。蛇纹石化橄榄岩与石英菱镁岩有着相似微量元素分布规律,表明同源性,但两者仍有较明显差异:两阶段石英菱镁岩LREE都较地幔橄榄岩略富集,从地幔橄榄岩到两阶段石英菱镁岩,HREE先亏损再略富集。微量元素标准化图解中,Sr在橄榄岩与石英菱镁岩中都为正异常,其含量随蚀变强度增强而增加。此外,两阶段石英菱镁岩均无Au、Hg矿化。  相似文献   

2.
The Xinqiao deposit is one of several polymetallic deposits in the Tongling ore district. There are two types of mineralization in the Xinqiao: skarn-type and stratiform-type. The skarn-type mineralization is characterized by iron oxides such as magnetite and hematite, whereas stratiform-type mineralization is characterized by massive sulfides with small amounts of magnetite and hematite. We defined three types of ores within the stratiform-type mineralization by the mineral assemblages and ore structures. Type I ore is represented by magnetite crosscut by minor calcite veins. Type II is a network ore composed of magnetite and crosscutting pyrite. Type III is a massive ore containing calcite and hematite. Type I magnetite is characterized by highly variable trace element content, whereas Type II magnetite has consistently higher Si, Ti, V, and Nb. Type III magnetite contains more In, Sn, and As than the other two types. Fluid–rock interaction, oxygen fugacity (fO2), and temperature (T) are the main factors controlling element variation between the different magnetite types. Type I magnetite was formed by more extensive fluid–rock interaction than the other two types at moderate fO2 and T conditions. Type II magnetite is thought to have formed in relatively low fO2 and high-T environments, and Type III in relatively high fO2 and moderate-T environments. Ca?+?Al?+?Mn and Ti?+?V discrimination diagrams show that magnetite in the Xinqiao deposit is hydrothermal in origin and is possibly linked with skarn.  相似文献   

3.
The distribution of platinum-group elements (PGEs), together with spinel composition, of podiform chromitites and serpentinized peridotites were examined to elucidate the nature of the upper mantle of the Neoproterozoic Bou Azzer ophiolite, Anti-Atlas, Morocco. The mantle section is dominated by harzburgite with less abundant dunite. Chromitite pods are also found as small lenses not exceeding a few meters in size. Almost all primary silicates have been altered, and chromian spinel is the only primary mineral that survived alteration. Chromian spinel of chromitites is less affected by hydrothermal alteration than that of mantle peridotites. All chromitite samples of the Bou Azzer ophiolite display a steep negative slope of PGE spidergrams, being enriched in Os, Ir and Ru, and extremely depleted in Pt and Pd. Harzburgites and dunites usually have intermediate to low PGE contents showing more or less unfractionated PGE patterns with conspicuous positive anomalies of Ru and Rh. Two types of magnetite veins in serpentinized peridotite, type I (fibrous) and type II (octahedral), have relatively low PGE contents, displaying a generally positive slope from Os to Pd in the former type, and positive slope from Os to Rh then negative from Rh to Pd in the latter type. These magnetite patterns demonstrate their early and late hydrothermal origin, respectively. Chromian spinel composition of chromitites, dunites and harzburgites reflects their highly depleted nature with little variations; the Cr# is, on average, 0.71, 0.68 and 0.71, respectively. The TiO2 content is extremely low in chromian spinels, <0.10, of all rock types. The strong PGE fractionation of podiform chromitites and the high-Cr, low-Ti character of spinel of all rock types imply that the chromitites of the Bou Azzer ophiolite were formed either from a high-degree partial melting of primitive mantle, or from melting of already depleted mantle peridotites. This kind of melting is most easily accomplished in the supra-subduction zone environment, indicating a genetic link with supra-subduction zone magma, such as high-Mg andesite or arc tholeiite. This is a general feature in the Neoproterozoic upper mantle.  相似文献   

4.
熊燕云  李兵  陈静  周涛发 《岩石学报》2019,35(12):3811-3824
斑岩型矿床具有十分广泛和明显的热液蚀变带,青磐岩化通常位于中心钾化带外侧,可分为三个亚带,绿帘石是青磐岩化带最重要的蚀变矿物之一,但目前对青磐岩化带内各亚带之中的绿帘石的特征研究较为薄弱。近年来,短波红外光谱技术(SWIR)已经广泛应用于热液蚀变矿物的识别,但由于短波红外光谱通过识别特定的基团进而识别含此基团的矿物,难以识别斑岩中部分关键蚀变矿物,如钾长石、钠长石和硬石膏。X射线衍射技术(XRD)能识别大部分蚀变矿物,可对红外光谱技术在斑岩矿床中的应用进行补充。本文以江西省德兴地区富家坞矿区为研究对象,提出应用SWIR和XRD分析辅助蚀变填图。富家坞矿区发育了十分广泛的绿帘石化,根据其共生矿物组合及空间分布,划分了三种绿帘石类型。第Ⅰ类:矿物组合为绿帘石-钠长石-石英-方解石,绿帘石和钠长石以脉状形式产出于钾化花岗闪长斑岩体内,穿切钾长石斑晶;第Ⅱ类:矿物组合为绿帘石±绿泥石-石英-方解石,绿帘石和绿泥石以集合体形式交代早期岩体内黑云母、斜长石的方式产出,并保留有原生矿物晶型,伴有方解石、石英等矿物;第Ⅲ类绿帘石共生矿物主要为石英、方解石、沸石等,含少量高岭石,主要呈不规则脉状,分布于蚀变花岗闪长斑岩外侧。通过对以上三类绿帘石进行系统SWIR和XRD分析,发现类型Ⅰ绿帘石大部分表现出较大的Fe-OH吸收峰位值(Pos2252 2255),类型Ⅱ和类型Ⅲ绿帘石大部分表现为较小的Fe-OH吸收峰位值(Pos2252 2255)。XRD结果显示绿帘石晶体特征更为明显,特征衍射峰位值{■13}晶面表现出相对集中的特征,但是次峰{022}晶面表现出一定的差异性,且绿帘石{022}晶面2. 40?衍射峰半高宽(FWHM-2)与绿帘石Dep2334/Dep2252值呈现负相关的关系。故本文认为绿帘石的次峰{022}晶面2. 40?衍射峰值及其半高宽(FWHM-2)可以作为讨论斑岩矿床围岩蚀变矿物绿帘石的结晶度的主要参数。三种绿帘石结晶指数的差异可能是热液流体演化过程中温度降低和距离岩体中心的远近造成的。  相似文献   

5.
The Heijianshan Fe–Cu (–Au) deposit, located in the Aqishan-Yamansu belt of the Eastern Tianshan (NW China), is hosted in the mafic–intermediate volcanic and mafic–felsic volcaniclastic rocks of the Upper Carboniferous Matoutan Formation. Based on the pervasive alteration, mineral assemblages and crosscutting relationships of veins, six magmatic–hydrothermal stages have been established, including epidote alteration (Stage I), magnetite mineralization (Stage II), pyrite alteration (Stage III), Cu (–Au) mineralization (Stage IV), late veins (Stage V) and supergene alteration (Stage VI). The Stage I epidote–calcite–tourmaline–sericite alteration assemblage indicates a pre-mineralization Ca–Mg alteration event. Stage II Fe and Stage IV Cu (–Au) mineralization stages at Heijianshan can be clearly distinguished from alteration, mineral assemblages, and nature and sources of ore-forming fluids.Homogenization temperatures of primary fluid inclusions in quartz and calcite from Stage I (189–370 °C), II (301–536 °C), III (119–262 °C) and V (46–198 °C) suggest that fluid incursion and mixing probably occurred during Stage I to II and Stage V, respectively. The Stage II magmatic–hydrothermal-derived Fe mineralization fluids were characterized by high temperature (>300 °C), medium–high salinity (21.2–56.0 wt% NaCl equiv.) and being Na–Ca–Mg–Fe-dominated. These fluids were overprinted by the external low temperature (<300 °C), medium–high salinity (19.0–34.7 wt% NaCl equiv.) and Ca–Mg-dominated basinal brines that were responsible for the subsequent pyrite alteration and Cu (–Au) mineralization, as supported by quartz CL images and H–O isotopes. Furthermore, in-situ sulfur isotopes also indicate that the sulfur sources vary in different stages, viz., Stage II (magmatic–hydrothermal), III (basinal brine-related) and IV (magmatic–hydrothermal). Stage II disseminated pyrite has δ34Sfluid values of 1.7–4.3‰, comparable with sulfur from magmatic reservoirs. δ34Sfluid values (24.3–29.3‰) of Stage III Type A pyrite (coexists with hematite) probably indicate external basinal brine involvement, consistent with the analytical results of fluid inclusions. With the basinal brines further interacting with volcanic/volcaniclastic rocks of the Carboniferous Matoutan Formation, Stage III Type B pyrite–chalcopyrite–pyrrhotite assemblage (with low δ34Sfluid values of 4.6–10.0‰) may have formed at low fO2 and temperature (119–262 °C). The continuous basinal brine–volcanic/volcaniclastic rock interactions during the basin inversion (∼325–300 Ma) may have leached sulfur and copper from the rocks, yielding magmatic-like δ34Sfluid values (1.5–4.1‰). Such fluids may have altered pyrite and precipitated chalcopyrite with minor Au in Stage IV. Eventually, the Stage V low temperature (∼160 °C) and low salinity meteoric water may have percolated into the ore-forming fluid system and formed late-hydrothermal veins.The similar alteration and mineralization paragenetic sequences, ore-forming fluid sources and evolution, and tectonic settings of the Heijianshan deposit to the Mesozoic Central Andean IOCG deposits indicate that the former is probably the first identified Paleozoic IOCG-like deposit in the Central Asian Orogenic Belt.  相似文献   

6.
陕西略阳煎茶岭金矿矿集区中的张家山金矿主要由破碎蚀变岩型、角砾岩型和含金石英黄铁矿脉型矿石组成。含金石英-黄铁矿脉型矿石产于断层下盘的石英菱镁岩中。黄铁矿发育富As黄铁矿边,环边受As含量的变化呈现一定的韵律变化,自然金赋存在富As黄铁矿中。在断裂发育形成断层角砾岩的过程中,流体充填破碎石英菱镁岩的裂隙中形成热液矿物,包括硫化物、硒化物以及自然金。石英菱镁岩发生破碎形成的网状裂隙被含金石英-方解石-黄铁矿脉充填。破碎蚀变岩型矿石中,自然金主要分布在含金石英-黄铁矿脉的石英之中或靠近热液脉的菱镁矿或石英间隙。随着大量方解石脉沿裂隙贯入,进一步促进石英菱镁岩的破碎及岩石角砾的分离,形成由石英菱镁岩碎屑、石英和褐铁矿组成的复成分角砾岩,自然金和硒化物呈浸染状分布在角砾岩中。笔者在角砾岩型矿石中发现了灰硒汞矿、直硒镍矿、硒铅矿等硒化物,这些硒化物往往与自然金密切共生。结合矿物组合以及相关化学反应关系,通过热力学计算,构建了该矿床在不同温度条件下的热力学相平衡关系图,限定了硒化物与其他相关矿物稳定存在的物理化学条件。硒化物一般与自然金和石英共生,高的f(Se2)值和f(Se2)/f(S2)比值是控制硒化物形成的关键因素。  相似文献   

7.
Pelitic schists of the Sanbagawa metamorphic belt contain several types of polymineralic veins that formed during the late stages of exhumation. The vein mineral assemblages are quartz + albite + K-feldspar + chlorite ± calcite (Type I, II) and quartz + albite + calcite (Type III). Type I and II veins contain quartz and albite with stretched-crystal and elongate-blocky textures, respectively. The mineral species within Type I veins vary with compositional bands within the host rocks. Type III veins are characterized by euhedral to subhedral quartz grains with concentric zoning and a homogeneous distribution along the vein length. The vein textures vary depending on the crack aperture during multiple crack-seal events: <0.08 mm for Type I, and 0.5–10 mm for Type III. Type II veins show intermediate features between Type I and III veins in terms of mineral distribution (weak dependence on the host rock composition) and apparent crack aperture (less than 1–15 mm). These observations suggest a transition in the dominant transport mechanism of vein components with increasing crack aperture, from diffusion from host rocks to fluid advection along cracks.  相似文献   

8.
徐向珍  杨经绥  郭国林  李金阳 《岩石学报》2011,27(11):3179-3196
西藏雅鲁藏布江缝合带西段普兰蛇绿岩以出现面积约600余平方千米的特大型地幔橄榄岩体而引人注目.该地幔橄榄岩以方辉橄榄岩为主体,含有少量的二辉橄榄岩和纯橄榄岩,岩体中另有一些橄榄单斜辉石岩、辉长岩和辉绿岩等侵入体.地幔橄榄岩的主要造岩矿物橄榄石的Fo 90～93,其中呈包裹体的橄榄石的Fo略高,斜方辉石为顽火辉石(En 88～90),单斜辉石主要为顽透辉石和透辉石,以低铝(0.48％～3.96％)和高Mg#(91～96)为特征,铬尖晶石的Cr#值为18～69,其中方辉橄榄岩和二辉橄榄岩中的铬尖晶石属富铝型尖晶石,而纯橄岩中为富铬型尖晶石.橄榄单斜辉石岩的橄榄石Fo值一致较低,平均为88.4,斜方辉石En平均87,单斜辉石以透辉石为主,铬尖晶石的Cr#值为45～69.普兰地幔橄榄岩及橄榄单斜辉石岩都具有相似的稀土元素和微量元素配分模式,表现为LREE相对富集,Eu亏损不明显,微量元素中大离子亲石元素含量较低,部分样品高场强元素亏损,另一些则相对富集,显示地幔橄榄岩具有亏损地幔源区特征,但也具有俯冲带流体的交代特征,表明普兰岩体可能经历了MOR和SSZ两种构造环境,该特征与雅鲁藏布江缝合带东段的罗布莎地幔橄榄岩的特征可以对比.  相似文献   

9.
The Sar-Cheshmeh porphyry Cu–Mo deposit is located in Southwestern Iran (65 km southwest of Kerman City) and is associated with a composite Miocene stock, ranging in composition from diorite through granodiorite to quartz-monzonite. Field observations and petrographic studies demonstrate that the emplacement of the Sar-Cheshmeh stock took place in several pulses, each with associated hydrothermal activity. Molybdenum was concentrated at a very early stage in the evolution of the hydrothermal system and copper was concentrated later. Four main vein Groups have been identified: (I) quartz+molybdenite+anhydrite±K-feldspar with minor pyrite, chalcopyrite and bornite; (II) quartz+chalcopyrite+pyrite±molybdenite±calcite; (III) quartz+pyrite+calcite±chalcopyrite±anhydrite (gypsum)±molybdenite; (IV) quartz±calcite±gypsum±pyrite±dolomite. Early hydrothermal alteration produced a potassic assemblage (orthoclase-biotite) in the central part of the stock, propylitic alteration occurred in the peripheral parts of the stock, contemporaneously with potassic alteration, and phyllic alteration occurred later, overprinting earlier alteration. The early hydrothermal fluids are represented by high temperature (350–520 °C), high salinity (up to 61 wt% NaCl equivalent) liquid-rich fluid inclusions, and high temperature (340–570 °C), low-salinity, vapor-rich inclusions. These fluids are interpreted to represent an orthomagmatic fluid, which cooled episodically; the brines are interpreted to have caused potassic alteration and deposition of Group I and II quartz veins containing molybdenite and chalcopyrite. Propylitic alteration is attributed to a liquid-rich, lower temperature (220–310 °C), Ca-rich, evolved meteoric fluid. Influx of meteoric water into the central part of the system and mixing with magmatic fluid produced albitization at depth and shallow phyllic alteration. This influx also caused the dissolution of early-formed copper sulphides and the remobilization of Cu into the sericitic zone, the main zone of the copper deposition in Sar-Cheshmeh, where it was redeposited in response to a decrease in temperature.  相似文献   

10.
Early Archean (3.46 Ga) hydrothermally altered basaltic rocks exposed near Marble Bar, eastern Pilbara Craton, have been studied in order to reveal geological and geochemical natures of seafloor hydrothermal carbonatization and to estimate the CO2 flux sunk into the altered oceanic crust by the carbonatization. The basaltic rocks are divided into basalt and dolerite, and the basalt is further subdivided into type I, having original igneous rock textures, and type II, lacking these textures due to strong hydrothermal alteration. Primary clinopyroxene phenocrysts are preserved in some part of the dolerite samples, and the alteration mineral assemblage of dolerite (chlorite + epidote + albite + quartz ± actinolite) indicates that the alteration condition was typical greenschist facies. In other samples, all primary minerals were completely replaced by secondary minerals, and the alteration mineral assemblage of the type I and type II basalts (chlorite + K-mica + quartz + carbonate minerals ± albite) is characterized by the presence of K-mica and carbonate minerals and the absence of Ca-Al silicate minerals such as epidote and actinolite, suggesting the alteration condition of high CO2 fugacity. The difference of the alteration mineral assemblages between basalt and dolerite is probably attributed to the difference of water/rock ratio that, in turn, depends on their porosity.Carbonate minerals in the carbonatized basalt include calcite, ankerite, and siderite, but calcite is quite dominant. The δ13C values of the carbonate minerals are −0.3 ± 1.2‰ and mostly within the range of marine carbonate, indicating that the carbonate minerals were formed by seafloor hydrothermal alteration and that carbonate carbon in the altered basalt was derived from seawater. Whole-rock chemical composition of the basaltic rocks is essentially similar to that of modern mid-ocean ridge basalt (MORB) except for highly mobile elements such as K2O, Rb, Sr, and Ba. Compared to the least altered dolerite, all altered basalt samples are enriched in K2O, Rb, and Ba, and are depleted in Na2O, reflecting the presence of K-mica replacing primary plagioclase. In addition, noticeable CO2 enrichment is recognized in the basalt due to the ubiquitous presence of carbonate minerals, but there was essentially neither gain nor loss of CaO. This suggests that the CO2 in the hydrothermal fluid (seawater) was trapped by using Ca originally contained in the basalt. The CaO/CO2 ratios of the basalt are generally the same as that of pure calcite, indicating that Ca in the basalt was almost completely converted to calcite during the carbonatization, although Mg and Fe were mainly redistributed into noncarbonate minerals such as chlorite.The carbon flux into the Early Archean oceanic crust by the seafloor hydrothermal carbonatization is estimated to be 3.8 × 1013 mol/yr, based on the average carbon content of altered oceanic crust of 1.4 × 10-3 mol/g, the alteration depth of 500 m, and the spreading rate of 1.8 × 1011 cm2/yr. This flux is equivalent to or greater than the present-day total carbon flux. It is most likely that the seafloor hydrothermal carbonatization played an important role as a sink of atmospheric and oceanic CO2 in the Early Archean.  相似文献   

11.
A corundum-bearing Type II mafic rock, within the Horoman peridotite,Japan, was petrologically examined in detail to obtain the P–Tpaths of the mafic rock as well as of the host peridotite. Ofall the mafic rocks documented from the Horoman complex, onlythe corundum-bearing mafic rock has preserved, at least partly,its high-pressure mineralogy; all of the others have been completelyrecrystallized at low pressures. The Type II mafic rocks wereinitially formed at <1·0 GPa as cumulates of olivine,plagioclase and clinopyroxene. Corundum was then formed by metamorphismand/or partial melting of the Type II protolith at higher pressures(>1·5 GPa) than the initial condition of formation.Corundum reacted with clinopyroxene during exhumation of theHoroman peridotite down to the plagioclase stability field.The field and petrographical observations of the Type II maficrocks (± corundum) coupled with published isotopic datasuggest a complicated spiral-like P–T trajectory for theHoroman peridotite. The Type II protolith was formed at lowpressure within the peridotite at the time of initial formationof the Horoman peridotite as a residue from primitive mantleat  相似文献   

12.
The ultramafic sequence and associated chromitites of the Nan-Uttaradit ophiolite in the northeastern part of Thailand have been studied in the field and by applying petrography and geochemistry to whole rock samples and minerals. The ultramafic rocks comprise irregulary shaped bodies of dunite, harzburgite, orthopyroxene-rich lherzolite and orthopyroxene-rich harzburgite, clinopyroxene-rich dunite and intrusive clinopyroxenite-websterite bodies. Three types of chromitite were distinguished. Type I chromitite lenses and type II layers which are hosted in orthopyroxenite in the northern part and in dunite in the central part of the ophiolite. Type III chromitite forms lenses or layers in clinopyroxenites in the central and southern parts of the belt. According to the modal and chemical composition the peridotites and orthopyroxenites are strongly refractory. They originated during different stages of interaction between percolating melts and peridotite. The chromitites of types I and II, which are very rich in Cr (up to 68 wt.% Cr203), crystallized from a boninitic parental magma under highly reducing conditions in the northern part and moderate oxygen fugacities (FMQ) in the central part of the ophiolite. The chromitite of type III which are characterized by the highest Fe3+/(Fe3+ + Cr + Al) -ratios, and hosted in intrusive clinopyroxenite-websterite-rocks, cumulated from a CaO-rich transitional boninitic melt under fO2 conditions around FMQ.  相似文献   

13.
ABSTRACT

The Bir Umq ophiolite is one of the most important ophiolitic successions in the Arabian Shield, and represents an excellent case for the study of the tectonomagmatic evolution of the earliest Precambrian events in the juvenile part of the Arabian-Nubian Shield (ANS). It is a dismembered ophiolite, which includes a serpentinized peridotite with small amounts of gabbro and mélange, and is overlain by the Sumayir formation. The mantle section of the Bir Umq ophiolite has been pervasively sheared and folded during its emplacement and is extensively serpentinized, carbonated and silicified, resulting in the common development of magnesite and listwaenite along the shear zones. Listwaenite occurs in the form of upstanding ridges due to its resistance to erosion. Antigorite is the main serpentine mineral, which, however, has low amounts of lizardite and chrysotile, indicating that the present serpentinites formed by prograde metamorphism. The ophiolitic rocks of Bir Umq have undergone regional metamorphism up to the greenschist to amphibolite facies. The presence of mesh and bastite textures indicates harzburgite and dunite protoliths. The serpentinized peridotite preserves rare relicts of primary minerals such as olivine, pyroxene and Cr-spinel. The serpentinized ultramafics of Bir Umq have high Mg# [molar Mg/(Mg+Fe2+); 0.90–0.93), low CaO, and Al2O3 contents similar to that of the environment of the suprasubduction zone. Additionally, they are characterized by the depletion of some compatible trace elements (e.g., Nb, Sr, Ta, Zr, Hf and REE), but show a wide variation in the Rb and Ba. Moreover, they are enriched in some elements that have affinities for Mg-rich minerals such as Ni, Cr, V, and Co. Fresh relics of olivine have high Fo (av. 0.91) and NiO (av. 0.42) contents, similar to those in the mantle olivine. The fresh Cr-spinel has high Cr# (0.68) and low TiO2 content (av. 0.11), similar to those in modern fore-arc peridotites. The composition of both orth- and clinopyroxenes confirms the fore-arc affinity of the studied ultramafics. The present study indicates that the protoliths of the serpentinized ultramafics of Bir Umq have high partial melt degrees, which is consistent with the characteristics of ultramafic rocks formed in a subarc environment (fore-arc) within a suprasubduction zone system.  相似文献   

14.
The use of ASTER data and fieldwork supported by mineralogical and geochemical analyses enabled exploration of new gold occurrences in the alteration zones in the ultramafic–mafic successions at the Barramiya district. Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) band ratios (4/8, 4/2 and 8/9 in R, G and B channels, respectively) helped in recognising of two listwaenite alteration zones (areas 1 and 2) promising for gold in the north-east and south-east of the Barramiya gold mine. Mineralogical studies and X-ray diffraction analysis revealed that areas 1 and 2 are characterized by variable concentrations of talc, ankerite, magnesite, quartz and calcite. Ore microscope studies revealed the sulphide minerals carry gold within these alteration zones; moreover, goethite and malachite are also observed. Fire assay results show Au contents in the range of 5.04 ppm in the graphite schist, 4.02 ppm in the quartz veins and 3.76 ppm in the listwaenite alteration. The atomic absorption analysis (AAS) of samples from area 1 yields an average Au content in the quartz-veins of 2.4 ppm, Ag content is 8.0 ppm and Cu content is 2.4 wt%. The listwaenite alteration gives an average Au content of 4.4 ppm and a Cu content of 2.8 wt%. In area 2, the AAS of the quartz-veins revealed an average Au content of 2.6 ppm, 6.2 ppm Ag and 1.9 wt% Cu. The listwaenite alterations of area 2 grade 3.5 ppm Au and 2.4 wt% of Cu. The Barramiya District is made up of ophiolitic ultramafic belts of serpentinites, talc carbonates and talc graphite schists, mainly thrust over the metavolcanic sequences. They include highly strained and tectonised parts enriched in sulphides, iron oxides and carbonates, with developed listwaenite alterations along the thrust contacts. Gabbro and granitic intrusions were intruded in the ultramafics and metavolcanic rocks. ASTER data are an accurate and helpful tool for detecting and mapping alteration zones for gold exploration.  相似文献   

15.
Granite- and gabbrodiorite-associated skarn deposits of NW Iran   总被引：1，自引：0，他引：1  
Field and laboratory studies show that there are two types of skarn deposits in NW Iran: granite-associated (type I) and gabbrodiorite-associated (type II). Granite-associated deposits are accompanied by Cu and Fe mineralisation, whereas Mn and Fe are the main ore metals in gabbrodiorite-associated skarn deposits. There are some differences in the mineralogy of these skarn deposits. Bixbyite, piemontite and Cr-bearing garnet are found only in gabbrodiorite-associated skarns, whereas idocrase occurs only in granite-associated deposits. Type II skarns show exoskarn features, whereas some type I skarns have developed endoskarn as well. Evidence of boiling of hydrothermal fluid can be seen in both types and seems to be a common mechanism of mineral deposition. Gabbrodiorite-associated skarns show higher fO2 than granite-associated deposits. Based on mineralogical and textural evidence, mineralisation in both groups has started from about 550 °C. Early formed anhydrous minerals have begun to be replaced by hydrous minerals from about 400 °C.It seems that due to low fluid content in the gabbrodioritic magma, heated meteoritic water in the surrounding volcanoclastic and tuffaceous rocks was the main source of hydrothermal solution in the gabbrodiorite-associated skarn system.  相似文献   

16.
The Huai Kham On gold deposit is located in the central part of the Sukhothai Fold Belt, northern Thailand. The Sukhothai Fold Belt represents an accretionary complex formed by subduction and collision between the Indochina and Sibumasu Terranes. There are many small gold deposits in the Sukhothai Fold Belt; however, the styles and formation environments of those gold deposits are not clear. The geology of the Huai Kham On deposit consists of volcanic and volcanosedimentary rocks, limestone, and low‐grade metamorphic rocks of Carboniferous to Triassic age. Gold‐bearing quartz veins are hosted by volcanic and volcanosedimentary rocks. The quartz veins can be divided into four stages. The mineral assemblage of the gold‐bearing quartz veins of Stages I and II comprises quartz, calcite, illite, pyrite, native gold, galena, chalcopyrite, and sphalerite. Quartz veins of Stage III consist of microcrystalline quartz, dolomite, calcite, pyrite, native gold, and chalcopyrite. Veins of Stage IV consist of calcite, dolomite, chlorite, and quartz. Fluid inclusions in quartz veins are classified into liquid‐rich two‐phase (Types IA and IB), carbonic‐aqueous (Type II), and carbonic (Type III) fluid inclusions. The homogenization temperatures of Types IA and II fluid inclusions that are related to the gold‐bearing quartz veins from Stages I to III ranged from 240° to 280°C. The δ18O values of quartz veins of Stages I to III range from +12.9 to +13.4‰, suggesting the presence of a homogeneous hydrothermal solution without temperature variation such as a decrease of temperature during the formation of gold‐bearing quartz veins from Stages I to III in the Huai Kham On gold deposit. Based on the calculated formation temperature of 280°C, the δ18O values of the hydrothermal solution that formed the gold‐bearing quartz veins range from +3.2 to +3.7‰, which falls into the range of metamorphic waters. The gold‐bearing quartz veins of the Huai Kham On deposit are interpreted to be the products of metamorphic water.  相似文献   

17.
The Jiang Tso ophiolite,situated in the middle segment of the Bangong- Nujiang Suture Zone,is a part of the easternmost Qieli Lake ophiolite subzone and is close to the south of Pung Lake ophiolite. The rock association of Jiang Tso ophiolite is relatively complete and is mainly composed of metamorphic peridotite,gabbro and diabase. Comparing with N-MORB,the ophiolite is high in Mg and low in Ti,K,Na,P,and is depleted in Nb,Ta,Hf,Th and enriched in Rb,Sr and Ba. Geochemical characteristics of the Jiang Tso ophiolite indicate it is of a supra-subduction zone type formed in the spreading ridge of back arc basin. The SHRIMP U-Pb dating of zircons from the gabbro yielded a weighted average age of 188.1±4.1 Ma(MSWD=1.4),indicating the Jiang Tso ophiolite was formed in the late stage of early Jurassic. The Sr,Nd isotopic compositions show that the Tethyan mantle domain is the depleted mantle(DM),with enriched mantle domain II(EM II). They have the same Sr,Nd isotopic composition with the India Ocean MORB type.  相似文献   

18.
《Chemical Geology》2002,182(2-4):203-225
Accessory gangue scheelite (CaWO4) from the Archaean Mt. Charlotte lode Au deposit can be divided into two types with different rare earth element (REE) signatures. In some scheelite grains, specific REE signatures are reflected by different cathodoluminescence colours, which can be used to map their often complex oscillatory intergrowths. Domains with specific REE contents from two grains were sampled for Sm/Nd, Rb/Sr and Pb isotopic analyses using a micro-drilling technique.Type I scheelite is strongly enriched in middle REE (MREE) and Eu anomalies are either absent or slightly positive. Four fragments collected from Type I regions of two crystals have initial 87Sr/86Sr and εNd values ranging from 0.70141 to 0.70163 and +2.5 to +3.5, respectively, and Pb isotope ratios reflecting the composition of greenstone sequence. This may indicate that Nd and Pb have their source, either locally or regionally, in the greenstones. Basic greenstone lithologies have 87Sr/86Sr<0.7015, and the radiogenic Sr signatures indicate that part of the Sr originated from felsic lithologies located either within or beneath the host greenstone pile. Alternatively, the Sr signature may have evolved from preferential leaching of a Rb-rich mineral during hydrothermal alteration of the greenstone.The REE patterns of Type II scheelite are either flat or MREE-depleted and have strong positive Eu anomalies. Three fragments collected from Type II regions of the same two crystals have initial 87Sr/86Sr ratios and εNd values between 0.70130 and 0.70146, and +1.1 to +2.6, respectively, and Pb isotope signatures that are once again similar to that of the greenstone. This implies that 87Sr/86Sr ratios in Type II fluids were closer to those of the host dolerite (0.7008–0.7013), due to more extensive fluid interaction with the dolerite.A positive correlation between Na and REE suggests that REE3+ are accommodated by the coupled substitution REE3++Na+=2 Ca2+ into both Type I and Type II scheelite. This is consistent with a fractional crystallisation model to explain the change in REE patterns from Type I to Type II, but not with a model involving different coupled substitutions and fluids from different origins. We propose that the complex REE and isotopic signatures of scheelite at Mt. Charlotte are related to small (<m) to medium (<km) scale processes involving mixing between “fresh” batches of hydrothermal fluid with fluids that had already been involved in extensive wall-rock alteration.The very high-εNd values measured in some scheelites have been previously used to link gold mineralisation with komatiites containing unusually high Sm/Nd ratios. However, tiny (<20 μm) grains of secondary hydroxyl-bastnäsite were found within micro-fractures of one scheelite grain containing an extremely high-εNd signature. The hydroxyl-bastnäsite probably formed during recent REE redistribution within the scheelite as a result of meteoric fluid circulation. The scale of this cryptic low-temperature alteration is sufficient to explain the anomalously high-εNdi values observed in scheelite from Western Australia.  相似文献   

19.
以新疆西准噶尔玛依拉山-萨雷诺海蛇绿岩为重点进行讨论．该区出露的火成岩种类较多，表现出构造混杂特点．变质橄榄岩是具有一定亏损程度的地幔残体，LREE的富集主要是后期蚀变的结果．玄武岩的稀土分布型式是平坦型的或LREE略亏损型的．与MORB相比，REE的总量较高，高Fe和Ti，Mg值低，是一种演化的岩石，与东太平洋中隆地高Fe和Ti的铁质拉斑玄武岩成分相当，地球化学特征类似于MORB，可能形成于小洋盆环境．  相似文献   

20.
Leucogranitic lenses are found within the Xiwan ophiolitic mélange in northeastern Jiangxi Province, South China. The leucogranites occur exclusively within the serpentinized peridotite unit of the ophiolite suite. SHRIMP U–Pb zircon dating results indicate that these granites were formed at 880 ± 19 Ma, and were overprinted by an Indosinian tectono-thermal event at ~ 230 Ma. The leucogranites are peraluminous (A/CNK = 1.0–1.24), characterized by high Al2O3 (14–18.33%) and Na2O (6.5–10%) and clearly low εNd(T) values of 0.8 to − 3.9 compared with the other rock units of the ophiolite suite. On the basis of their REE characters, the leucogranites can be divided into three groups. Group I leucogranites show the most fractionated LREE-enrichment patterns (with LaN/YbN and LaN/SmN ratios of 30.1–75.0 and 2.3–3.9, respectively). Group II leucogranites have moderately fractionated LREE-enrichment patterns (with LaN/YbN and LaN/SmN ratios of 13.1–26.5 and 0.8–1.9, respectively). Group III leucogranites are characterized by obviously low total REE contents and flat REE patterns with significant positive Eu anomalies, probably due to small degrees of partial melting. All these leucogranites were likely formed by partial melting of sedimentary rocks from a marginal basin at the Yangtze side of the orogen, beneath a major thrust fault during the obduction of the ophiolite onto the continental crust. They are broadly similar to obduction-related granites within ophiolites identified in many places worldwide. Identification of the ca. 880 Ma obduction-type granites in the NE Jiangxi ophiolite provides a petrological constraint on the timing of the ophiolite obduction onto the continental crust. In combination with the termination of the Shuangxiwu arc magmatism at ca. 890 Ma, we interpret that the close of the Neoproterozoic back-arc basin and the termination of the continental amalgamation between the Yangtze and Cathaysia Blocks occurred at ca. 880 Ma.  相似文献   

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