桂西田阳堆积型铝土矿矿物学及地球化学

[摘 要]桂西田阳堆积型铝土矿产于第四纪红土层中。本文通过对桂西田阳第四系堆积型铝土矿进行一系列矿物学与地球化学研究,探索铝土矿的物质来源。矿物学分析显示铝土矿石中主要矿物为硬水铝石、赤铁矿和锐钛矿,含有少量高岭石、鲕绿泥石、三水铝石、针铁矿、金红石、伊利石、蒙脱石和石英等。地球化学分析显示,组成铝土矿石的主要化学组分有Al₂O₃、Fe₂O₃、SiO₂、TiO₂、FeO 和H₂O⁺;微量元素分析显示,铝土矿石相对地壳和原始地幔富集B、Li、Ga、Zr、Hf、Nb、Ta、W、Th、U 等元素,而亏损Ni、Cr 等元素。矿石中明显富集稀土元素,稀土总量变化大,轻重稀土分异明显,轻稀土元素富集明显;矿石具有明显的铈异常和铕异常,且铕异常相对稳定,为一致的负异常。元素相关性分析显示常量元素间相关性较差;部分微量元素如Zr、Hf、Nb 和Ta 间呈现出较高的正相关性。通过Zr/ Hf、Nb/ Ta 图解、Eu/ Eu* - TiO₂/ Al₂O₃图解和Zr-Cr-Ga 图解综合分析显示铝土矿物质来源复杂,底板碳酸盐岩和周围火成岩都为铝土矿提供物源。     

滇东南上二叠统吴家坪阶下部铝土矿 地球化学特征及物源分析

[摘 要] 滇东南地区上二叠统吴家坪阶下部铝土矿不整合于峨眉山玄武岩或灰岩之上,查明其物质来源对铝土矿勘探具有重要意义。对铝土矿及峨眉山大火成岩省玄武岩、花岗岩及下伏灰岩的地球化学指标进行研究,结果表明:滇东南铝土矿常量元素主要由SiO₂、Fe₂O₃、Al₂O₃、TiO₂ 和FeO 组成;铝土矿中富集Zr、Hf、Nb、Ta 元素,表明Zr、Hf、Nb、Ta 等高场强元素在滇东南地区铝土矿矿化过程中较为稳定,且Zr、Hf、Nb、Ta 表现出良好的相关性;铝土矿球粒陨石标准化曲线与峨眉山玄武岩配分曲线趋势一致,均富集轻稀土元素,而与下伏灰岩差异较大,且Zr-Hf、Nb-Ta 图解与峨眉山玄武岩呈线性关系,而与矮郎河过铝质花岗岩的相关性不强。据此推断滇东南地区上二叠统吴家坪阶下部铝土矿主要物质来源为峨眉山玄武岩。     

贵州省凯里市大田铝土矿矿床地质特征及成矿规律分析

凯里市大田铝土矿是贵州省重要铝土矿成矿带凯里─黄平矿集区(带)内的重要矿床。通过工作发现:①二叠系梁山组是其含矿层位,矿体呈似层状、透镜状/漏斗状产出,总体走向NE,平均厚2.59 m,w(Al₂O₃)平均值为64.25%,铝硅比平均为5.25,铝土矿(332+333)矿石资源量978.38×10⁴ t;②矿石矿物以一水硬铝石为主,矿石结构一般为自形、半自形或它形粒状结构,构造主要为块状构造、孔隙状构造、层纹状构造,矿石类型以低硫低铁铝土矿为主;③矿床受断裂构造影响较大,在该区西北部的泥质岩相带中铝土矿成矿较好,矿体厚度与含矿岩系厚度成正比,矿石的w(Al₂O₃)值与w(SiO₂)值成反比;④高坡场组古岩溶洼地、槽沟是该区铝土矿找矿的有利部位。     

桂西念寅铝土矿稀土元素地球化学特征及物质来源分析

[摘要]桂西靖西念寅铝土矿不整合于早二叠茅口灰岩之上,分析其稀土元素地球化学特征对研究其物质来源具有重要意义。对念寅原生铝土矿、堆积型铝土矿及其围岩的11个样品进行稀土元素含量测试,并同靖西、平果等地各类铝土矿、茅口灰岩等样品对比分析,结果表明: （1）念寅铝土矿主量元素主要SiO₂、Fe₂O₃、Al₂O₃、TiO₂组成,Al₂O₃与Fe₂O₃和SiO₂呈负相关;（2）念寅铝土矿球粒陨石标准化曲线与茅口灰岩配分曲线趋势一致,具有ΣREE富集、LREE与HREE分异且LREE相对富集、Eu负异常等显著特征;（3）念寅铝土矿显示出被动大陆边缘地带海相沉积的典型特征,原生铝土矿成矿物质来源主要为下伏的茅口灰岩。     

内蒙古固阳绿岩带三合明BIF型铁矿的形成时代、地球化学特征及地质意义

三合明BIF型铁矿位于华北克拉通西部陆块北缘,产出于固阳绿岩带。矿体赋存于新太古界色尔腾山群斜长角闪岩中。矿石主要呈粒状变晶结构、粒状-针柱状变晶结构,条带-条纹状构造;组成矿石的金属矿物主要为磁铁矿,非金属矿物主要为石英,次为角闪石等。对选自斜长角闪岩中的锆石进行SIMS U-Pb定年,具有核边结构、Th/U比大于0.4的锆石其核部给出了2562±14Ma的上交点年龄,可大致作为三合明BIF的形成时代。原岩恢复显示斜长角闪岩为正变质岩,Zr/Ti-Nb/Y图解显示为亚碱性玄武岩系列,Fe₂O₃^T+TiO₂-Al₂O₃-MgO图解落入高铁拉斑玄武岩区;斜长角闪岩主量元素特征与MORB相近,REE配分曲线和蛛网图都近于平坦,且介于N-MORB和E-MORB之间,LREE略微富集,Th、U相对亏损,Nb、Ta、Zr和Hf无明显异常;(La/Sm)_N和Nb/U分别为0.76和50,由此推断原岩可能为T-MORB。Ti-V、Th-Hf-Ta构造环境判别图解中,分别落入MORB和弧后盆地的重叠区、N-MORB区。结合T-MORB形成的构造环境以及前人提出的岛弧叠加地幔柱模式,本文认为三合明BIF形成于弧后盆地并有地幔柱叠加的构造环境。铁矿石化学组分主要为SiO₂、Fe₂O₃和FeO,较低的Al₂O₃(0.68%)、极低的TiO₂(0.04%)和HFSE表明只有极少量陆源碎屑物质的加入。铁矿石的球粒陨石标准化REE配分模式与固阳绿岩带底部的科马提岩极为相似,PAAS标准化的铁矿石REY配分模式与高温热液海水混合物相似,即LREE亏损,HREE富集((La/Yb)_SN=0.34),具有明显的正Eu异常(δEu=2.33)和微弱的正Y异常(δY=1.13),Y/Ho重量比29,摩尔比53。根据铁矿石兼具有与科马提岩和高温热液海水混合物相似的地球化学特征,本文推断海底高温热液淋滤科马提岩为三合明BIF型铁矿提供了大量的Fe和Si。     

华南埃迪卡拉系底部钾质斑脱岩的岩石地球化学特征及其地质意义

我国华南扬子地台不同地区（湖北宜昌、安徽休宁）埃迪卡拉系底部的盖帽碳酸盐岩中均保存有浅色粘土岩层。岩石薄片和扫描电镜观察,以及X射线衍射分析表明,这些粘土岩层以伊利石和伊/蒙混层矿物为主,同时含有港湾状熔蚀石英、高温透长石及岩浆锆石等斑晶矿物。X荧光光谱仪和ICP-MS等离子质谱仪等地球化学元素分析表明,这些粘土岩层具有相似的元素组成和配分模式,以高K₂O、低TiO₂,富含Nb、Ta、Zr、Hf、Th、Y等亲石元素为特征,Al₂O₃/TiO₂、Zr/Hf 和Ti/Th 值均指示了酸性火山灰的性质,同时稀土元素配分模式与花岗岩较为相似。岩石学和地球化学特征表明,这些浅色粘土岩层是火山喷发事件形成的钾质斑脱岩。通过两处剖面的对比,可以将安徽休宁地区的钾质斑脱岩年龄限制在大约635Ma左右。岩浆判别图指示了这些钾质斑脱岩的原始岩浆为高钾亚碱性流纹英安岩;依据微量元素特征和构造环境判别图,初步认为原始岩浆形成于大陆弧环境。这些钾质斑脱岩在区域上的分布,表明我国扬子地台埃迪卡拉纪早期发生了广泛的火山活动,“雪球地球”事件的结束可能是导致这期以钾质斑脱岩为代表的火山喷发事件的诱因。     

贵州福泉高洞铝土矿床成矿地质地球化学特征

贵州福泉高洞铝土矿床是近年在贵州新的铝土矿成矿区探明的中型矿床之一,其含矿岩系为下石炭统九架炉组,铝土矿呈层状、似层状产于上泥盆统高坡场组碳酸盐岩古侵蚀面之上,与上覆中二叠统梁山组和下伏上泥盆统高坡场组均呈假整合接触,矿床成因类型为古风化壳沉积型。通过对含矿岩系中敏感元素Ga、Ba、Sr、Th含量及B/Ga、Rb/K、V/Zr比值特征分析,认为铝土矿主要形成于陆相沉积环境;δCe、Ni/Co、Th/U、Sr/Cu、V/Cr特征揭示铝土矿成岩成矿过程主要为氧化环境。含矿岩系中Al与Zr、Hf、Nb、Ta、Th、U等呈正相关性反映出铝土矿成矿过程中具同迁移、显著富集特征;含矿岩系中微量元素含量与下伏高坡场组碳酸盐岩相比,总体成强烈富集态势。与黎彤值对比,含矿岩系中高温成矿元素(W、Sn、Mo、Bi)、高场强元素(Nb、Ta、Th、U、Hf、Zr)、粘土矿物易吸附元素(Li、V)及细碎屑岩中高地球化学背景元素(Sb)富集强度高。LREE与HREE分馏作用明显,推测为次生风化淋滤过程中离子半径较小的HREE向下渗滤迁移的速率大于离子半径较大的LREE所致。     

江西雅山花岗岩岩浆演化及其Ta-Nb富集机制

雅山岩体是华南地区著名的富含钽铌矿的稀有金属花岗岩。从早阶段到晚阶段花岗岩中的云母的Li、F和Rb₂O含量逐渐升高,其类型变化为“黑鳞云母→Li-云母→锂云母”。锆石的Zr元素被Hf、U、Th、Y和P等元素的置换比例随着岩浆演化程度升高而增大。云母和锆石矿物成分变化特征与全岩体系的Zr/Hf、Nb/Ta比值不断下降而F、Li和P₂O₅含量逐渐升高的趋势一致,将可以用于指示岩浆演化程度。在岩浆演化过程中不断富集的P、F、Li元素增加了熔体中非桥氧数（NBO）,促使钽-铌元素在岩浆中的溶解度加大而逐渐富集,在最晚阶段的黄玉锂云母花岗岩具有最高的Ta、Nb元素含量。因此,雅山花岗岩具有较高的F、Li、P₂O₅含量是其岩浆演化及其Ta-Nb富集的重要机制。西华山花岗岩中的云母与雅山花岗岩中的锂云母相比,具有明显较低的F、Li、Rb₂O含量,表明西华山花岗岩的岩浆演化程度相对低于雅山花岗岩。西华山花岗岩中的钨富集与流体作用密切相关,体系氧逸度的降低促使了钨成矿。因此,岩浆演化程度的不同可能是造成华南稀有金属花岗岩发生不同成矿作用（如Ta-Nb矿和W矿）的重要原因。     

江达-维西火山岩浆弧中段德钦岩体年代学、地球化学及岩石成因

本文对江达-维西火山岩浆弧德钦岩体的寄主岩石——花岗闪长岩及其镁铁质微粒包体(MME)——闪长岩进行了详细研究。二者LA-ICPMS锆石U-Pb年龄分别为254.6±1.8Ma和253.5±1.6Ma,二者形成时代一致。地球化学研究结果表明,花岗闪长岩富K₂O和Na₂O,且K₂O>Na₂O,富Al₂O₃,A/CNK平均为0.96;闪长岩富K₂O和Na₂O,但K₂O2O,富Al₂O₃和MgO,A/CNK平均为0.72;花岗闪长岩的稀土总量低于闪长岩,二者轻重稀土分馏明显,配分曲线为右倾的轻稀土富集型;二者均富集大离子亲石元素而亏损高场强元素Nb、Ta等;二者均具有相对较高的Mg^#(58.8~65.8),并具有相对较高的相容元素Cr、Ni含量(花岗闪长岩平均值分别为115×10^-6和31.6×10^-6,闪长岩平均值分别为398×10^-6和98.2×10^-6;花岗闪长岩和闪长岩¹⁷⁶Hf/¹⁷⁷Hf的平均值分别为0.282383和0.282287,二者ε_Hf(t)平均值分别为-8.3和-11.8,反映了二者属于I型花岗岩,可能为岩浆混合作用的产物。地球化学特征及Hf同位素组成一致显示岩浆来源于地壳的部分熔融,伴有不同比例的地幔物质加入,形成于弧陆碰撞-后碰撞的构造背景,暗示金沙江结合带在~255Ma已经进入了弧陆碰撞-后碰撞的地质时期。     

江西宜丰县大港超大型含锂瓷石矿床地质特征及成因机制探讨

大港含锂瓷石矿床产于宜丰钽铌锂稀有金属成矿带甘坊花岗岩岩体内,属典型的含锂云母碱长花岗岩型矿床。含锂瓷石矿体厚度21.62~148.88 m,其中w（Al₂O₃）为15.38%~23.97%,w（Li₂O）为0.30%~1.06%,同时伴生Rb、Cs、Ta、Nb等稀有金属。矿石矿物组成为钠长石、锂云母或锂白云母,白度达80%以上。化学组成上,表现出富硅,贫铁、镁、钛,高磷的特征;铝饱和指数A/CNK值大于1.20,Zr/Hf比值（< 12）与Nb/Ta比值（< 2）极低,表明具有高分异的过铝质S型花岗岩特性。由构造环境判别图解可知,大港含锂云母碱长花岗岩应形成于同碰撞或后碰撞早期阶段的伸展构造背景下,极低的CaO/Na₂O比值（< 0.2）,指示岩体的源区应为富黏土矿物的变质泥质岩。     

Mineralogical and geochemical investigations of the Dajia Salento-type bauxite deposits,western Guangxi,China

The Dajia Salento-type bauxite deposit in western Guangxi is hosted within the Quaternary ferrallitic soil profile, and it formed via breaking up, weathering and oxidizing of Permian bauxite orebodies occurring as a semi-continuous layer in the upper Permian. Mineralogical analyses reveal that diaspore, hematite and kaolinite are the major minerals in bauxite ores with small amounts of anatase, chamosite, gibbsite, goethite, illite, zircon, quartz and pyrite. The ore texture and mineral assemblage reveal that the depositional/diagenetic environment of the Dajia bauxite was much close to phreatic environment. Both the ore texture and the morphology of zircon grains also indicate that most of the bauxitic soils were transported a short distance. Diaspore is suggested to be non-metamorphic in origin and mainly formed in a reducing condition of diagenetic environment, while kaolinite is the product of the in situ epigenetic replacement of alumina in diaspore by dissolved silica. Geochemical analyses indicate that Al₂O₃, Fe₂O₃, SiO₂ and TiO₂ are the main components of the bauxite ores and trace elements such as Zr, Hf, Nb, Ta, Th and U were enriched during the bauxitization process. Simultaneously, Zr vs. Hf and Nb vs. Ta show a high correlation. Geochemical indices such as Zr/Hf, Nb/Ta and Eu/Eu* (among others) denote that the magmatic rocks related to the Emeishan plume in western Guangxi and the carbonates in the underlying Maokou Formation provided the main sources of material for the bauxite ores.     

云南会泽大黑山铝土矿地球化学特征及其物源分析

大黑山铝土矿赋存于上二叠统宣威组底部,下伏地层为峨眉山玄武岩。矿石结构主要以泥晶结构为主,具有少量鲕状、粒屑结构。矿石构造以致密块状构造为主,局部呈层状、似层状。铝土矿石中Al2O3与Fe2O3、Si O2呈现较好的负相关关系,Ti O2与Zr、Hf、Nb、Ta相关性较好,Zr-Hf、Nb-Ta的相关性拟合度很高。铝土矿与峨眉山玄武岩样品具有相似的稀土元素配分模式。综合稳定元素相关性、微量元素及稀土元素标准化图解、lg Ni和lg Cr二元图解等分析方法对大黑山铝土矿的成矿物质来源进行探讨,研究结果表明铝土矿的成矿物质来源主要来自峨眉山玄武岩。     

Cryptic sediment-hosted critical element mineralization from eastern Yunnan Province,southwestern China: Mineralogy,geochemistry, relationship to Emeishan alkaline magmatism and possible origin

A previous study briefly described the occurrence of a new type of Nb(Ta)-Zr(Hf)-REY-Ga (REY: rare earth elements and yttrium) polymetallic mineralization in eastern Yunnan, southwest China. In this paper, the mineralogical and geochemical features have been further advanced through a study of two regionally extensive and relatively flat-lying mineralized layers from No. XW drill core. The layers are clay-altered volcanic ash and tuffaceous clay, and are dominated by clay minerals (mixed layer illite/smectite, kaolinite, berthierine, and chamosite); with lesser amounts of quartz and variable amounts of anatase, siderite and calcite; along with trace pyrite, barite, zircon, ilmenite, galena, chalcopyrite, and REE-bearing minerals. The mineralized samples have higher Al₂O₃/TiO₂ values (13.7–41.4) and abundant rare metal elements (Nb, Ta, Zr, Hf, REE, Ga, Th, and U) whereas less mineralized samples are rich in V, Cr, Co, and Ni and have lower Al₂O₃/TiO₂ values (2.32–7.67). The mineralized samples also have strong negative δEu in chondrite-normalized REE patterns. Two processes are most likely responsible for the geochemical and mineralogical anomalies of the mineralized samples: airborne volcanic ash and multi-stage injection of low-temperature hydrothermal fluids. Based on paragenetic analysis, this polymetallic mineralization is derived from the interaction between alkaline volcanic ashes and subsequent percolation of low-temperature fluids. The intense and extensive alkaline volcanism of the early Late Permian inferred from this study possibly originated from the coeval Emeishan large igneous province (ELIP). This unique Nb(Ta)-Zr(Hf)-REE-Ga mineralization style has significant economic and geological potential for the study of mineralization of the lowest Xuanwei Formation.     

Importance of hydrogeological conditions during formation of the karstic bauxite deposits,Central Guizhou Province,Southwest China: A case study at Lindai deposit

Karstic bauxite deposits are widespread in Central Guizhou Province, SW China, and high-grade ores are frequently sandwiched with overlying coal and underlying iron-rich layers and form a special “coal–bauxite–iron” structure. The Lindai deposit, which is one of the most representative karstic bauxite deposits in Central Guizhou Province, was selected as a case study. Based on textural features and iron abundances, bauxite ores in the Lindai deposit are divided into three types of ores, i.e., clastic, compact, and high-iron. The bauxite ores primarily comprise diaspore, boehmite, kaolinite, illite, and hematite with minor quartz, smectite, pyrite, zircon, rutile, anatase, and feldspar. The Al₂O₃ (53–76.8 wt.%) is the main chemical contents of the bauxite ore samples in the Lindai district, followed by SiO₂, Fe₂O₃, TiO₂, CaO, MgO, S, and P etc. Our geological data on the Lindai deposit indicated that the ore-bearing rock series and its underlying stratum have similar rare earth elements distribution pattern and similar Y/Ho, Zr/Hf, and Eu/Eu¹ values; additionally, all ore-bearing rock samples are rich in MgO (range from 0.16 wt.% to 0.68 wt.%), and the plots of the dolomites and laterites lie almost on or close to the weathering line fit by the Al-bearing rocks in Zr vs. Hf and Nb vs. Ta diagrams; suggesting that the underlying Middle Cambrian Shilengshui Formation dolomite is the parent rock of bauxite resources in the Lindai district.Simulated weathering experiments on the modern laterite from the Shilengshui Formation dolomite in the Lindai bauxite deposit show that hydrogeological conditions are important for karstic bauxite formation: Si is most likely to migrate, its migration rate is several magnitudes higher than those of Al and Fe under natural conditions; the reducing inorganic acid condition is the most conducive to Al enrichment and Si removal; Fe does not migrate easily in groundwater, Al enrichment and Fe removal can occur only in acidic and reducing conditions with the presence of organic matter.The geological and experimental studies show that “coal–bauxite–iron” structure in Lindai deposit is formed under certain hydrogeological conditions, i.e., since lateritic bauxite or Al-rich laterite deposited upon the semi-closed karst depressions, Si can be continuously removed out under neutral/acidic groundwater conditions; the coal/carbonaceous rock overlying the bauxitic materials were easily oxidized to produce acidic (H₂S, H₂SO₄, etc.) and reductant groundwater with organic materials that percolated downward, resulting in enrichment of Al in underlying bauxite; it also reduced Fe³⁺ to its easily migrating form Fe²⁺, moving downward to near the basal carbonate culminated in precipitating of ferruginous (FeS₂, FeCO₃, etc.) strata of the “coal–bauxite–iron” structure. Thus, the bauxitic materials experienced Al enrichment and Si and Fe removal under above certain hydrogeological conditions forming the high-quality bauxite.     

滇东南晚二叠世铝土矿地球化学特征及物源分析

滇东南地区晚二叠世铝土矿属典型的喀斯特型铝土矿,矿体赋存于上二叠统吴家坪组(龙潭组),分析其地球化学特征对研究其物质来源具有重要意义。全岩分析显示铝土矿石成分以Al₂O₃、Fe₂O₃、SiO₂和TiO₂为主,其中Al₂O₃与Fe₂O₃ 、SiO₂具有较好的负相关关系。微量元素Cr、Zr、 Hf、Ta、Th、U和稀土元素在铝土矿矿化过程中不断富集,元素Zr-Hf、Nb-Ta之间具有明显的正相关关系。lgCr-lgNi图解、稳定元素比值(Zr/Hf)及稀土元素配分模式等地球化学特征说明峨眉山玄武岩为铝土矿的形成提供了主要的物质来源,同时下伏碳酸盐岩也提供了部分成矿物质。     

Mineralogical and geochemical investigations of the Mombi bauxite deposit,Zagros Mountains,Iran

The Mombi bauxite deposit is located in 165 km northwest of Dehdasht city, southwestern Iran. The deposit is situated in the Zagros Simply Fold Belt and developed as discontinuous stratified layers in Upper Cretaceous carbonates (Sarvak Formation). Outcrops of the bauxitic horizons occur in NW-SE trending Bangestan anticline and are situated between the marine neritic limestones of the Ilam and Sarvak Formations. From the bottom to top, the deposit is generally consisting of brown, gray, pink, pisolitic, red, and yellow bauxite horizons. Boehmite, diaspore, kaolinite, and hematite are the major mineral components, while gibbsite, goethite, anatase, rutile, pyrite, chlorite, quartz, as well as feldspar occur to a lesser extent. The Eh–pH conditions during bauxitization in the Mombi bauxite deposit show oxidizing to reducing conditions during the Upper Cretaceous. This feature seems to be general and had a significant effect on the mineral composition of Cretaceous bauxite deposits in the Zagros fold belt. Geochemical data show that Al₂O₃, SiO₂, Fe₂O₃ and TiO₂ are the main components in the bauxite ores at Mombi and immobile elements like Al, Ti, Nb, Zr, Hf, Cr, Ta, Y, and Th were enriched while Rb, Ba, K, Sr, and P were depleted during the bauxitization process. Chondrite-normalized REE pattern in the bauxite ores indicate REE enrichment (ΣREE = 162.8–755.28 ppm, ave. ∼399.36 ppm) relative to argillic limestone (ΣREE = 76.26–84.03 ppm, ave. ∼80.145 ppm) and Sarvak Formation (ΣREE = 40.15 ppm). The REE patterns also reflect enrichment in LREE relative to HREE. Both positive and negative Ce anomalies (0.48–2.0) are observed in the Mombi bauxite horizons. These anomalies are related to the change of oxidation state of Ce (from Ce³⁺ to Ce⁴⁺), ionic potential, and complexation of Ce⁴⁺ with carbonate compounds in the studied horizons. It seems that the variations in the chemistry of ore-forming solutions (e.g., Eh and pH), function of carbonate host rock as a geochemical barrier, and leaching degree of lanthanide-bearing minerals are the most important controlling factors in the distribution and concentration of REEs. Several lines of evidences such as Zr/Hf and Nb/Ta ratios as well as similarity in REE patterns indicate that the underlying marly limestone (Sarvak Formation) could be considered as the source of bauxite horizons. Based on mineralogical and geochemical data, it could be inferred that the Mombi deposit has been formed in a karstic environment during karstification and weathering of the Sarvak limy Formation.     

云南文山县天生桥铝土矿地球化学特征与物源分析

云南文山县天生桥地区铝土矿是典型的沉积型铝土矿,矿体赋存于上二叠统龙潭组,其下伏地层为上石炭统灰岩或与峨眉山大火成岩省有关的玄武岩。矿石以泥晶结构为主,含少量粒屑结构与鲕粒结构;矿石的结构特征表明风化物质可能经过短距离的搬运。常量元素分析结果显示铝土矿石中Al₂O₃分别与Fe₂O₃ 和TiO₂具有较好的负相关关系。在铝土矿矿化过程中,微量元素Zr、Hf、Nb、Ta和Bi相对于Li、Rb、Sr、Ba不断富集。元素Zr、Hf、Nb、Ta 分别与TiO2显示较高的相关性。运用稳定元素相关性(如Zr-Hf、Nb-Ta)、lgNi和lgCr二元图解以及稀土元素配分模式等方法探讨矿床成矿物质来源,研究结果表明玄武岩为铝土矿的形成提供了主要的物质来源,下伏碳酸盐岩可能也提供了少量的物质来源。     

叶蜡石化蚀变过程中的元素活动性与流体性质:以山西五台地区白云叶蜡石矿为例

热液蚀变过程中的元素活动性与流体性质对深入理解矿物稳定性和成矿作用具有重要的意义。本文以华北克拉通中北部山西五台地区的白云叶蜡石矿为例,研究了蚀变过程中元素迁移特征和流体性质。该矿体围岩以绿片岩相酸性火山岩为主,岩性为绢云钠长石英片岩并夹有少量的绿泥钠长片岩。矿区内蚀变分带明显,可分为早期的黄铁绢英岩化(绢云母-石英-黄铁矿)和晚期叠加的叶蜡石化(叶蜡石-伊利石-高岭石-石英),而金矿化则主要发育于黄铁绢英岩化带内。Log fo2-pH相图模拟结果显示,早期黄铁绢云岩化蚀变热液具有弱酸性至偏中性(pH=5.24~5.87)和较低氧逸度(位于黄铁矿+黄铜矿稳定相区内)特征;而引起叶蜡石化蚀变的热液具有强酸性(pH=2.07~2.20)和高氧逸度(位于HM缓冲线以上)特征。质量平衡迁移分析结果显示,随着叶蜡石化蚀变作用的增强,叶蜡石矿石中的Al2O3行为较稳定,SiO2、Na2O和K2O含量相对于围岩绢云钠长石英片岩呈不同程度的迁入,而其余氧化物大量活化迁出。微量元素Nb、Ta、Th、U、Rb和Ga含量相对升高,Th/U比值略有升高;Sr、Ba、Zr、Hf明显亏损,Zr/Hf比值从34~41下降到17~22。稀土元素均发生一定程度的活化迁移,且轻稀土迁出程度更高。Y/Ho比值(28~32)高于球粒陨石的Y/Ho(26~28),表明Y-Ho在叶蜡石化蚀变过程中表现出不同的地球化学行为。Eu负异常明显增大,这可能与长石的分解关系密切。围岩绢云钠长石英片岩中金属元素含量较高且Au与As含量之间呈明显正相关性,但在叶蜡石矿石中大部分金属元素含量均低于检出限,说明金属元素在叶蜡石化蚀变作用过程中发生了强烈的活化迁移,这与岩相学上叶蜡石矿石中可见港湾状细粒赤铁矿而缺乏黄铁矿的特征吻合。本文研究结果表明叶蜡石化过程中,大量的所谓不活动元素(如P、Ti、Zr、Hf、Y和Ho等)发生了显著迁移并导致Zr/Hf和Y/Ho比值的解耦,并伴随着大量金属元素的迁出,说明叶蜡石化不利于金矿化的形成。