金川超大型Ni-Cu-(PGE)矿床橄榄石微量元素特征及地质意义

我国金川超大型铜镍硫化物矿床是世界上第三大在采岩浆硫化物矿床,Ni开采量仅次于俄罗斯Noril'sk-Talnakh和加拿大Sudbury矿床,其成因研究备受关注。利用激光—等离子体质谱(LA-ICP-MS)原位分析了金川岩体中橄榄石微量元素含量,并探讨了影响元素含量变化的因素,进而阐述成岩及成矿过程。分析结果显示橄榄石中元素Ni,Cr与Fo呈负相关,Mn/Fe与Fo呈正相关,而Mn/Zn,Zn/Fe与Fo无相关性。在原始地幔橄榄石多元素标准化图中,金川Ⅰ号和Ⅱ号岩体橄榄石具相同的配分模式,均显示Cr,V,Ni,Co和Ti的亏损,富集不相容元素Zr,Y,Ti,Sc和Ca的特征。元素变化特征暗示Ⅰ号和Ⅱ号岩体具相同的母岩浆成分;与铬尖晶石的共结使橄榄石亏损Cr,V和Ti元素,而熔离的硫化物及其与橄榄石的相互反应共同影响着橄榄石中Ni和Co元素的含量。Ⅱ号岩体橄榄石较Ⅰ号岩体具较低的Cr和V含量,暗示Ⅱ号岩体母岩浆较Ⅰ号岩体经历了更高程度的演化。橄榄石高的Mn/Zn值(>13)和低的Zn/Fe值(<11)指示金川岩体岩浆可能起源于橄榄岩地幔的部分熔融,而非辉石岩地幔源区。     

金川Ⅰ号岩体橄榄石Ni-MgO相互关系及其地质意义

金川超镁铁质岩体赋存着仅次于加拿大Sudbury和俄罗斯Noril'sk-Talnakh的世界第三大在采铜镍硫化物矿床,岩体以一系列的NE向断层为界分成四个小岩体,由西至东依次为:Ⅲ、Ⅰ、Ⅱ、Ⅳ岩体.Ⅰ号岩体主要由二辉橄榄岩、含辉橄榄岩和橄榄二辉岩构成,橄榄石在各岩相中均为保存较好的主要造岩矿物.本文研究得到金川Ⅰ号岩体上部二辉橄榄岩和含辉橄榄岩中橄榄石的镁橄榄石F_o值介于83.9～85.7,而其Ni含量为1396×10~(-6)～2043×10~(-6),大多低于从S不饱和玄武岩浆中结晶出来的橄榄石的Ni含量.模拟计算结果表明橄榄石较低的Ni含量是因为橄榄石结晶的同时,发生了强烈的硫化物熔离;橄榄石的Fo-Ni关系还因与晶间硅酸盐熔浆的物质交换而发生改变.模拟计算还证明大约有30%的晶间硅酸盐岩浆与橄榄石发生Fe-Mg物质交换反应,导致早结晶橄榄石的Fo值减少了1～1.5.同时,橄榄石较小的Fo值变化表明,在橄榄石结晶和硫化物熔离过程中,不断有新的岩浆贯入和补充.     

橄榄石微量元素原位分析的现状及其应用

随着高精度EMPA和LA-ICP-MS分析技术的发展和矿物微量元素测试精度的提高,利用橄榄石中的微量元素示踪地幔部分熔融、地幔交代作用、岩浆早期结晶过程等地质问题成为近年来一个新兴的研究方向。一系列开拓性的研究发现也被陆续的发表,主要涉及橄榄石中Ni、Co、Al、Cr、Zn、Ti、Li、V、Sc、Mn、Ca和P等元素的示踪使用。一些卓有成效的示踪方法为:Ca、Al、Ti、Ni及Mn能够很好的用于区分橄榄石捕掳晶和斑晶;橄榄石-尖晶石地幔演化趋势线(OSMA:olivine-spinel mantle array)图解可以用于表征岩浆源区的亏损程度;玄武岩中橄榄石斑晶的Li同位素及Li含量可以很有效地指示岩浆源区是否存在地壳物质再循环及地幔交代作用;橄榄石斑晶中Ni、Ca、Mn、Cr和Al协变关系图解可以识别岩浆的辉石岩源区;利用橄榄石捕掳晶中Zr和Sc的含量差异特征可将橄榄岩中三种最主要的类型(尖晶石橄榄岩、石榴石橄榄岩以及尖晶石-石榴石橄榄岩)区分开来;一些元素的比值或组合(例如Ni/Co、Fe/Mn、V/Sc、Zr和Sc、Ca和Ti)可以指示源区交代作用、岩浆作用过程及氧化状态;基于橄榄石中Al、Cr及Ca的地质温度计可以为推算地幔热状态提供新方法;基于橄榄石分离结晶Fo-NiO演化线的原始岩浆计算模型可以较好的推算原始岩浆成分;利用橄榄石的环带及微量元素的扩散机制可以判别更多岩石成因信息,如识别交代介质、熔体类型以及地质构造背景等。基于上述最新研究的相关资料和已有成果,本文对橄榄石微量元素的地球化学示踪方法做系统性的归纳整理,并对橄榄石微量元素赋存状况、橄榄石微量元素测试方法、橄榄石微量元素的使用条件及需注意的问题等进行讨论,为读者在做相关研究时提供参考。     

造山带铜镍硫化物矿床的岩浆起源:以东天山黄山南铜镍矿床为例

位于中亚造山带南缘的新疆东天山地区因其出露大量的二叠纪镁铁质-超镁铁质岩体并产出一系列铜镍硫化物矿床而成为近年来地质学界关注的焦点.选择新疆东天山地区黄山南含铜镍矿镁铁质-超镁铁质岩体为研究对象,对其开展了系统的岩石学、矿物学和地球化学研究,以探讨造山带铜镍硫化物矿床的岩浆起源与性质.黄山南岩体主要由方辉橄榄岩、二辉橄榄岩、橄榄二辉岩、(橄榄)辉长苏长岩和闪长岩组成.各岩相显示富集大离子亲石元素和轻稀土元素、强烈亏损Nb-Ta、Ti,类似于典型岛弧火山岩特征.黄山南镁铁质-超镁铁质岩具有较大变化范围的ε_Nd(t=282.5 Ma)值(-1.31～4.22)和(⁸⁷Sr/⁸⁶Sr)i比值(0.703 2～0.706 9)以及高的(²⁰⁶Pb/²⁰⁴Pb)i比值(17.67～18.90),暗示其来源于一个适度富集的亏损地幔并经历了5%～20%新生地壳物质混染和～5%上地壳物质混染.根据橄榄石最高Fo牌号(摩尔含量为86.6%)计算的黄山南母岩浆为苦橄质岩浆(MgO=12.11%、F...     

峨眉山大火成岩省平川苦橄岩地幔源区性质——来自橄榄石微量元素的制约

橄榄石是基性岩浆中最早期结晶的硅酸盐矿物之一,其主量、微量元素特征可以反映出岩浆演化环境、岩浆源区岩性和再循环组分性质等重要信息.本次研究通过对峨眉山大火成岩省平川苦橄岩中橄榄石主量和微量元素分析,以及橄榄石内尖晶石包裹体分析,并与大理苦橄岩中橄榄石和尖晶石成分进行对比,来探讨不同苦橄岩母岩浆氧逸度及源区性质的异同.橄...     

Distribution of major and trace elements in La Luna Formation, Southwestern Venezuelan Basin

The La Luna Formation (Maraca section), Maracaibo Basin, was studied by means of V and Ni analysis of the bitumen, total organic carbon (TOC), total sulfur (St), major elements (Si, Al, Fe, Mg, Mn, Ca, Ti, Na, K, P), trace elements (V, Ni, Co, Cr, Cu, and Zn), and electron microprobe analysis (EPMA) of the whole rock, and St, major elements (Si, Al, Fe, Mg, Mn, Ca, Ti, Na, K, P), trace elements (V, Ni, Co, Cr, Cu, Zn, Mo, Ba, U, Th) and rare earth elements (La, Ce, Nd, Sn, Eu, Th, Yb, Lu) of the carbonate-free fraction. The results are discussed based on the organic and inorganic association of trace elements and their use as paleoenvironmental indicators of sedimentation. An association between V and organic matter is suggested by means of correlation between V and Ni vs. TOC, the use of EPMA (whole rock) and V and Ni concentrations (carbonate-free fraction), whereas Ni is found in the organic matter and the sulfide phase. Fe is present as massive and framboidal pyrite, whereas Zn precipitates into a separate phase (sphalerite), and Ni, Cu and, in some cases, Zn, can be found as sulfides associated with pyrite. Concentrations of V and Ni (bitumen), TOC, St, V, Ni, Cr, Cu and Zn (whole rock), U, Th, Mo (carbonate-free fraction) are indicative of changes in the dysoxic sedimentation conditions in the chert layers (TOC, St, V, Ni, Cu and low Zn and V/Cr <4) to euxinic anoxic conditions in the argillaceous limestone (TOC, St, V, Ni, Cu and high Zn and V/Cr >4). In the sequence corresponding to the argillaceous limestone, variations in the concentrations of TOC, St, V, Ni, Zn, Cu and Cr (whole rock) can be observed, also suggesting variable sedimentation conditions. The following is proposed: (i) sedimentation intervals under euxinic conditions associated with high contribution and/or preservation of organic matter, allowing a high concentration level of V and Ni in the organic phase and the accumulation of Cu, Zn and Ni (in a smaller proportion) in the sulfide phase; (ii) sedimentation intervals under anoxic conditions and in the presence of relatively lower H₂S, which allowed lower concentrations of V and Ni in the organic phase and higher concentrations of Cu, Zn, and Ni in the sulfide phase. Rare earth elements (REE) concentrations exhibit a marked increase in Ce, Nd, Sm, Eu, Y and Lu for the QM-3 interval, relative to Post-Archean Average Shale (PAAS). REE enrichment in shales has been related to the presence of phosphate minerals such as monazite or apatite. However, these minerals were not detected through XRD or EPMA in the whole rock or in the carbonate-free fraction. The association of REE with organic matter is suggested due to the absence of phosphate minerals, although assessment of these elements require further analysis.     

盘石山幔源透辉石微量元素地球化学

用质子探针和电子探针分别测定盘石山幔源橄揽岩包体中透辉石、橄榄石的微量元素和主量元素丰度;用X-射线荧光光谱法测定包体全岩的主量和微量元素丰度。定量讨论微量元素的分布和分配。其中绝大部分Sr、Zr、Ti、Na赋存在透辉石中,绝大部分Mn、Zn、Ni赋存在橄揽石中,在T=1194-1435K范围,分配系数变化如下:InD_Ol/Cpx^Zn=1.44-2.19;InD_Ol/Cpx^Mn=0.21-0.40,InD_Ol/Cpx^Ti=4.76-5.61,InD_Ol/Cpx^Ni(平均值)=2.20.透辉石中Ti/Zr=42-103,低于原始地幔值,由地幔隐性交代作用造成。     

甘肃金川铜镍(铂)硫化物矿床中高镁辉绿岩脉的发现及其意义

位于华北板块西缘赋存于超镁铁质岩中的金川矿床,是目前世界第三大镍硫化物矿床.金川铜镍硫化物矿床的原生岩浆问题一直存在着较大的争议,前人通过研究金川铜镍硫化物矿体中的堆晶橄榄岩中橄榄石的成分,从而推导原生岩浆的成分.而作者通过对金川铜镍硫化物矿体内部的基性岩脉深入研究,从另一个角度探究金川铜镍硫化物矿床的原生岩浆成分.通过对岩脉岩相学、主量元素的研究表明金川铜镍硫化物矿体中的岩脉主要是辉绿岩,因其MgO的含量的不同可以划分为高镁辉绿岩和低镁辉绿岩.辉绿岩脉的主量元素和微量元素显示这两类岩脉发生过分离结晶作用.PGE元素特征显示辉绿岩脉和金川矿床是同一期次产物,Pmelts的模拟演化得出本文中辉绿岩脉的液相线矿物橄榄石的牌号为Fo86,与金川矿床发现最高牌号Fo86一致.同时Ol-CATS-Q相图表明JC100925-5样品形成的源区在3GPa以上.多种因素显示这种高镁的岩浆是金川矿区的原生岩浆.     

塔里木东北缘坡十Ni矿化侵入体中橄榄石和铬尖晶石对成岩成矿及源区特征的约束

坡十Ni矿化超镁铁侵入体的矿化岩相主要为第二侵入期次的（斜长）单辉橄榄岩、（斜长）二辉橄榄岩、 纯橄岩等岩相。坡十超镁铁岩的橄榄石成分变化范围较大, 橄榄石的Fo值在76.8～89.6之间, Ni含量为767×10^-6～4 580×10^-6。铬尖晶石的Mg^#值和Cr^#值变化范围分别为19.4～41.9和49.8～64.8, 原生铬尖晶石中Cr₂O₃和Al₂O₃表现为负相关, 蚀变改造的铬尖晶石则表现为正相关。橄榄石成分剖面显示坡十母岩浆处于一个动态的岩浆系统中, 成分稳定的新鲜岩浆的补给、 持续向上的动力及浅部橄榄石快速分离结晶,造成了不同深度橄榄石成分的不同变化。坡十侵入体母岩浆估算结果为MgO=14.49％, FeO=10.01％,模拟结果显示橄榄石中Ni含量的变化主要受橄榄石结晶分异和硫化物不混溶作用共同控制,其中橄榄石与硫化物熔体发生明显的Fe-Ni交换反应。坡十母岩浆中橄榄石分离结晶造成的硫饱和,是坡十硫化物熔离的重要因素。橄榄石高Fo值、母岩浆高MgO、超镁铁岩中斜长石发育、矿物高结晶温度和铬尖晶石成分的弧岩浆特征显示,塔里木东北缘坡十侵入体是俯冲交代的岩石圈地幔部分熔融形成的母岩浆的产物,表现出低压高温的演化特征,其中源区熔融机制可能与塔里木二叠纪地幔柱提供的热源或该区大规模拆沉作用造成的软流圈上涌有关。     

Decoupling of cobalt and nickel in high-temperature magmatic processes and its metallogenic indicationSCIEI北大核心CSCD

Cobalt ( Co) and nickel (Ni) have similar geochemical behaviors and commonly co -vary in nature, but they may undergo decoupling in certain geological processes, especially in ore -forming processes, and the decoupling mechanism is not yet clear. This paper analyzes the distribution characteristics and migration behaviors of Co and Ni in major rock -forming minerals in different types of mafic-ultramafic rocks. The Ni (2500 x 10(-6) 3400 x 10(-6)) and Co (110 x 10(-6) 160 x 10(-6)) contents of mantle olivine are both higher than those of the primitive mantle. The Ni content shows no obvious correlation with the Fo value, while the Co content has a strong negative correlation with both the Fo value and the Ni/Co ratio. This indicates that partial melting does not cause significant changes in Ni content of olivine, but can control the Co content and the Ni/Co ratio, mainly due to the differences in compatibility between the two elements in olivine. Melt-rock reactions can cause significant Co-Ni decoupling between minerals and within minerals, but the mechanisms differ. In the process of reacting with basaltic magma, olivine mainly absorbs Co and loses Ni, while being affected by coexisting spinet minerals. When reacting with carbonatite melt, olivine loses Co, but Ni remains almost unchanged. As mantle derived basaltic magma intrudes into the lower crust, the migration of Co and Ni in pyroxenes is controlled by concentration gradients and sulfide crystallization. The local enrichment of Co provides an important material basis for the formation of Co -rich melts in deep crustal melting. During the differentiation and evolution of mafic-ultramafic magma, the chromite fractional crystallization and sulfide segregation can cause Co-Ni decoupling at both rock and/or mineral scales. Meanwhile, the enriched hydrous fluids released during chromite aggregation and sulfide melt solidification can significantly promote the efficiency of element exchange in sub-solidus state, especially strengthening the activity of Co, thus causing negative correlations between Co and Ni in olivine and even in chromite. Comparative studies show that the trends of Co and Ni content changes in mineral assemblages are evidently different, or even opposite, between mineralized and barren bodies and between ore -bearing and ore -free rocks, which provides a basis for identifying mineralization in rock bodies and guiding prospecting.     

新疆东天山黄山东岩体橄榄石成因意义探讨

黄山东岩体位于新疆东天山造山带中段,由四次岩浆侵入形成:第一次侵入形成了岩体上部的橄榄辉长岩、角闪辉长岩和闪长岩,构成岩体的主体;第二次侵入形成辉长苏长岩,呈岩墙状分布于岩体西端和西北部;第三次侵入岩石为斜长二辉橄榄岩,位于岩体下部,为主要的赋矿岩石;第四次侵入岩石为底部角闪辉长岩。橄榄石为第三次侵入的斜长二辉橄榄岩和第一次侵入的橄榄辉长岩主要造岩矿物之一,橄榄石的镁橄榄石牌号(Fo)值介于68.5～82.5之间。其中含硫化物斜长二辉橄榄岩中的橄榄石具有较高的Fo值(79.7～82.5);斜长二辉橄榄岩中橄榄石的Fo值为78.3～79.9;而基性程度较低的橄榄辉长岩中橄榄石具有较低的Fo值(68.5～72.2)。利用橄榄石矿物成分计算得出黄山东岩体母岩浆Mg#(Mg2+/(Mg2++Fe2+))为0.59,为原生玄武质岩浆经历结晶分异作用形成。模拟计算结果显示黄山东岩体不含矿岩石中橄榄石是母岩浆经过2%的橄榄石结晶分异且硫达到饱和后,在硫化物熔离的同时岩浆发生橄榄石结晶而形成,并且橄榄石︰硫化物≈50︰1,部分橄榄石成分投点在橄榄石结晶和硫化物熔离的模拟曲线右下侧,指示它们可能受到晶间硅酸盐熔浆作用的影响。含硫化物斜长二辉橄榄岩中Fo值与Ni含量呈负相关关系,说明橄榄石与硫化物熔体之间发生了Fe-Ni交换反应。     

塔里木克拉通东北缘二叠纪镁铁质岩浆氧化物与硫化物成矿条件对比

塔里木克拉通东北缘坡北、磁海等地二叠纪幔源岩浆活动形成了镍钴硫化物矿床和铁钴氧化物矿床,两者赋矿镁铁-超镁铁岩体的年龄相近(290～260 Ma),主、微量元素和Sr-Nd-Hf同位素组成相似,分配系数接近的微量元素比值分布于相同趋势线,揭示两者岩浆源区相同,可能为俯冲板片流体交代的亏损地幔或软流圈地幔。两类矿床镁铁-超镁铁质岩中Co与Ni含量正相关,Co主要富集在基性程度高的岩石中;块状硫化物与磁铁矿矿石中Co与Ni相关性差,Co和Ni具有不同的富集机制,Co热液富集作用明显。北山镁铁-超镁铁杂岩体是地幔柱相关软流圈上涌,诱发俯冲板片交代的亏损岩石圈地幔发生部分熔融,形成的高镁母岩浆演化过程中经历壳源混染、硫化物饱和富集镍钴形成铜镍钴硫化物矿床,富铁母岩浆氧逸度高、富水,岩浆分离结晶磁铁矿、叠加热液作用富集钴,形成铁钴氧化物矿床。     

苏鲁超高压变质带胡家林超镁铁质岩成因及构造意义

胡家林超镁铁质杂岩体产于苏鲁超高压变质带中部,纯橄岩和(石榴)单斜辉石岩呈不连续透镜体产于蛇纹石化橄榄岩中。纯橄岩遭受了部分蛇纹石化(烧失量=6.6%～13.2%),全岩富集强相容元素(Ni、Cr、Co)和Ir族PGE(IPGE;Ir、Os、Ru)及高IPGE/PPGE值,亏损Al、Ti、V,具高Mg~#橄榄石(Fo=91.7～92.4)和高Cr~#(0.68～0.76)尖晶石。纯橄岩高耐熔地球化学及矿物化学特征和古老的大陆岩石圈地幔相一致,表明其原岩来源于弧前地幔,代表了华北克拉通古老的大陆岩石圈地幔残留。(石榴)单斜辉石岩全岩呈相对低含量的强相容元素(Cr、Ni、Co)和IPGE,高含量的Al、Ti、V和流体迁移元素(Sr、Pb和Ba),球粒陨石标准化REE配分图呈明显"上凸"型,具低Mg~#橄榄石(Fo=76.6～76.8)和低Al_2O_3(2.76%)和高SiO_2(54.56%～56.87%)的单斜辉石。全岩组成和矿物化学表明其原岩为俯冲带内超镁铁质火成堆晶岩,最初岩浆由地幔岩高程度部分熔融的熔体和俯冲带中富H_2O流体和/或熔体构成。(石榴)单斜辉石岩原岩曾被地幔流带入扬子大陆俯冲板片和上覆地幔楔之间的俯冲通道,经历了超高压变质作用和生成大量石榴石。(石榴)单斜辉石岩在折返过程中,与大陆岩石圈地幔楔剥离的蛇纹石化橄榄岩及纯橄岩相结合,形成超镁铁质杂岩体,整体被低密度的俯冲板片(主要由花岗质片麻岩和变质沉积岩组成)裹挟,折返至地壳浅部。     

Geochemical Characteristics and Metallogenesis of the Qingkuangshan Ni‐Cu‐PGE Mineralized Mafic‐Ultramafic Intrusion in Huili County,Sichuan Province,SW China

The Qingkuangshan Ni-Cu-PGE deposit, located in the Xiaoguanhe region of Huili County, Sichuan Province, is one of several Ni-Cu-PGE deposits in the Emeishan Large Igneous Province (ELIP). The ore-bearing intrusion is a mafic-ultramafic body. This paper reports major elements, trace elements and platinum-group elements in different types of rocks and sulfide-mineralized samples in the intrusion. These data are used to evaluate the source mantle characteristics, the degree of mantle partial melting, the composition of parental magma and the ore-forming processes. The results show that Qingkuangshan intrusion is part of the ELIP. The rocks have trace element ratios similar to the coeval Emeishan basalts. The primitive mantle-normalized patterns of Ni-Cu-PGE have positive slopes, and the ratios of Pd/Ir are lower than 22. The PGE compositions of sulfide ores and associated rocks are characterized by Ru depletion. The PGE contents in bulk sulfides are slightly depleted relative to Ni and Cu, which is similar to the Yangliuping Ni-Cu-PGE deposit. The composition of the parental magma for the intrusion is estimated to contain about 14.65 wt% MgO, 48.66 wt% SiO2 and 15.48 wt% FeOt, and the degree of mantle partial melting is estimated to be about 20%. In comparison with other typical Ni-Cu-PGE deposits in the ELIP, the Qingkuangshan Ni-Cu-PGE deposit has lower PGE contents than the Jinbaoshan PGE deposit, but has higher PGE contents than the Limahe and Baimazhai Ni-Cu deposit, and has similar PGE contents to the Yangliuping Ni-Cu-PGE deposit. The moderate PGE depletions in the bulk sulfide of the Qingkuanghan deposit suggest that the parental magma of the host intrusion may have undergone minor sulfide segregation at depth. The mixing calculations suggests that an average of 10% crustal contamination in the magma, which may have been the main cause of sulfide saturation in the magma. We propose that sulfide segregation from a moderately PGE depleted magma took place prior to magma emplacement at Qingkuangshan, that small amounts of immiscible sulfide droplets and olivine and chromite crystals were suspended in the ascending magma, and that the suspended materials settled down when the magma passed trough the Qingkuangshan conduit. The Qingkuangshan sulfide-bearing intrusion is interpreted to a feeder of Emeishan flood basalts in the region.     

Trace Element Geochemistry of Magnetite from the Fe(-Cu) Deposits in the Hami Region, Eastern Tianshan Orogenic Belt, NW China

Laser ablation–inductively coupled plasma–mass spectrometry(LA–ICP–MS) was used to determine the trace element concentrations of magnetite from the Heifengshan, Shuangfengshan, and Shaquanzi Fe(–Cu) deposits in the Eastern Tianshan Orogenic Belt. The magnetite from these deposits typically contains detectable Mg, Al, Ti, V, Cr, Mn, Co, Ni, Zn and Ga. The trace element contents in magnetite generally vary less than one order of magnitude. The subtle variations of trace element concentrations within a magnetite grain and between the magnetite grains in the same sample probably indicate local inhomogeneity of ore–forming fluids. The variations of Co in magnetite between samples are probably due to the mineral proportion of magnetite and pyrite. Factor analysis has discriminated three types of magnetite: Ni–Mn–V–Ti(Factor 1), Mg–Al–Zn(Factor 2), and Ga– Co(Factor 3) magnetite. Magnetite from the Heifengshan and Shuangfengshan Fe deposits has similar normalized trace element spider patterns and cannot be discriminated according to these factors. However, magnetite from the Shaquanzi Fe–Cu deposit has affinity to Factor 2 with lower Mg and Al but higher Zn concentrations, indicating that the ore–forming fluids responsible for the Fe–Cu deposit are different from those for Fe deposits. Chemical composition of magnetite indicates that magnetite from these Fe(–Cu) deposits was formed by hydrothermal processes rather than magmatic differentiation. The formation of these Fe(–Cu) deposits may be related to felsic magmatism.     

In-situ LA-ICP-MS trace elemental analyses of magnetite: Fe–Ti–(V) oxide-bearing mafic–ultramafic layered intrusions of the Emeishan Large Igneous Province,SW China

The Taihe, Baima, Hongge, Panzhihua and Anyi intrusions of the Emeishan Large Igneous Province (ELIP), SW China, contain large magmatic Fe–Ti–(V) oxide ore deposits. Magnetites from these intrusions have extensive trellis or sandwich exsolution lamellae of ilmenite and spinel. Regular electron microprobe analyses are insufficient to obtain the primary compositions of such magnetites. Instead, laser ablation ICP-MS uses large spot sizes (~ 40 μm) and can produce reliable data for magnetites with exsolution lamellae. Although magnetites from these deposits have variable trace element contents, they have similar multi-element variation patterns. Primary controls of trace element variations of magnetite in these deposits include crystallography in terms of the affinity of the ionic radius and the overall charge balance, oxygen fugacity, magma composition and coexisting minerals. Early deposition of chromite or Cr-magnetite can greatly deplete magmas in Cr and thus Cr-poor magnetite crystallized from such magmas. Co-crystallizing minerals, olivine, pyroxenes, plagioclase and apatite, have little influence on trace element contents of magnetite because elements compatible in magnetite are incompatible in these silicate and phosphate minerals. Low contents and bi-modal distribution of the highly compatible trace elements such as V and Cr in magnetite from Fe–Ti oxide ores of the ELIP suggest that magnetite may not form from fractional crystallization, but from relatively homogeneous Fe-rich melts. QUILF equilibrium modeling further indicates that the parental magmas of the Panzhihua and Baima intrusions had high oxygen fugacities and thus crystallized massive and/or net-textured Fe–Ti oxide ores at the bottom of the intrusive bodies. Magnetite of the Taihe, Hongge and Anyi intrusions, on the other hand, crystallized under relatively low oxygen fugacities and, therefore, formed net-textured and/or disseminated Fe–Ti oxides after a lengthy period of silicate fractionation. Plots of Ge vs. Ga + Co can be used as a discrimination diagram to differentiate magnetite of Fe–Ti–(V) oxide-bearing layered intrusions in the ELIP from that of massif anorthosites and magmatic Cu–Ni sulfide deposits. Variable amounts of trace elements of magmatic magnetites from Fe–Ti–(P) oxide ores of the Damiao anorthosite massif (North China) and from Cu–Ni sulfide deposits of Sudbury (Canada) and Huangshandong (northwest China) demonstrate the primary control of magma compositions on major and trace element contents of magnetite.     

滇东南都龙锡锌铟多金属矿床磁铁矿矿物化学组成特征及其对成矿作用的约束

都龙锡锌铟多金属矿床位于著名的滇东南钨锡多金属成矿区之老君山矿集区,成矿与白垩纪大规模花岗岩活动关系密切,沿隐伏花岗岩接触带周边发育石榴子石、透辉-透闪石等矽卡岩蚀变和条带状(似层状)、脉状(囊状)的锡石、闪锌矿及磁铁矿、辉钼矿、黄铁黄铜矿等矿化,形成超大规模的岩浆热液-矽卡岩成矿系统。野外观测及研究发现,早期(矽卡岩期)高温阶段形成的磁铁矿可分为I阶段交代型磁铁矿(I-Mag)和II阶段充填型磁铁矿(II-Mag)两类:前者多呈囊状、条带状,与矽卡岩矿物共生;后者为脉状,与金属硫化矿物共生。利用ICP-AES、ICP-MS对两类磁铁矿进行主、微量元素测试,从I-Mag到II-Mag,Si、Ca、Mn及ΣREE、Pb、Zn、Ti含量增加,Mg及Sn、W、In、V、Cr、Ga含量减少,REE配分型式也由平缓向右倾的逐渐变化。TiO 2-Al 2 O 3-(MgO+MnO)、(Ti+V)-(Ca+Al+Mn)、Ni/Cr-Ti、(Ti+V)-Ni/(Cr+Mn)成因判别图解表明,磁铁矿属岩浆热液-矽卡岩成因类型;Ti、V与Zr、Hf、Nb、Ta,以及Y/Ho(24~3414)、Ni/Co(<2→>2)、Ti/V(<25→>25)、Hf/Zr(003~006→004~005)存在着线性关系和规律变化特征,指示两类型磁铁矿具有相同的物质来源,为同一成矿过程不同阶段的产物。而代表成矿流体REE组成的II-Mag的REE组成继承了老君山花岗岩REE配分趋势和Eu负异常特征,表明磁铁矿与白垩纪老君山花岗岩具有一致的物质来源。Cr-V、(Ti+V)-(Al+Mn)、Ga-Mg及Ga-Sn图解显示相同的成因类型和一致的线性关系,指示磁铁矿主体形成于较高氧逸度和温度(约300℃)的成矿环境下,并且从I-Mag到II-Mag,存在着氧逸度逐渐升高、温度逐渐降低的演化趋势。     

Petrogenesis of the Pt–Pd mineralized Jinbaoshan ultramafic intrusion in the Permian Emeishan Large Igneous Province,SW China

The Jinbaoshan ultramafic intrusion is a sheet-like body with a thick wehrlite unit in the center and thin pyroxenite units at the margins. PGE are enriched in several disseminated sulfide zones in the intrusion. Olivine from the intrusion has low Fo and depleted Ni contents compared to olivine from coeval Emeishan picrites. Whole rock major and trace element concentrations suggest that the Jinbaoshan wehrlites originally contained <30% trapped liquid. The total amount of sulfide in the rocks exceeds that which could have been dissolved in the trapped liquid. The Jinbaoshan wehrlites are interpreted to represent residual assemblages formed by dissolution of plagioclase by passing magma. No clear evidence of crustal contamination is indicated by S, Nd and Os isotopes. We envision that sulfide saturation occurred at depth due to olivine and chromite crystallization. Immiscible sulfide droplets were transported to the Jinbaoshan conduit where they accumulated and reacted with magma successively passing through the conduit to achieve high PGE concentrations.     

东昆仑夏日哈木超大型岩浆镍钴硫化物矿床成矿特征

超大型岩浆镍钴硫化物矿床的形成多与大陆裂谷或地幔柱活动密切相关,是何种机制促使深部幔源金属元素超常富集于东昆仑造山带而形成超大型岩浆镍钴硫化物矿床呢?文章通过剖析夏日哈木岩体及矿体基本特征,发现赋矿岩体整体呈东宽西窄的楔状体,以富集轻稀土元素、贫重稀土元素以及Nb、Ta、Zr、Hf等高场强元素的亏损为特征,镍钴矿体主要富集于辉石岩和橄榄岩中,西段矿石富,东段矿石贫,矿石品位与橄榄石含量呈正相关关系.精细矿物学研究发现,寄主矿物"巨型"斜方辉石所包裹的橄榄石Fo值及Ni含量从核部到边部均增高,揭示含矿岩相侵入过程中可能存在至少2次岩浆活动.由橄榄石和铬尖晶石成分分析,推测夏日哈木母岩浆为低Ti、高Mg的拉斑玄武质岩浆,可能起源于软流圈地幔.结合区域构造演化和岩体形成时代,认为东昆仑夏日哈木镁铁-超镁铁质岩体形成于伸展环境,深部部分熔融形成的岩浆多次脉冲上涌叠加,地壳混染对硫化物达到饱和具有关键作用,于柴达木盆地南缘东昆仑造山带中构造薄弱部位成岩成矿,可能是伴随古特提斯洋演化开始大陆裂解岩浆镍钴成矿响应的表现.该认识对丰富造山带中的岩浆镍钴硫化物矿床成矿理论研究、拓宽岩浆镍钴矿床找矿空间与潜力、支撑并指导区域找矿新突破,具有重要作用.     

扬子地块北缘周庵超镁铁质岩体矿物学特征及其对铜镍矿化的启示

扬子地块北缘~635 Ma周庵超镁铁质岩体是一个新发现的含铜镍硫化物矿化的隐伏岩体,主要由二辉橄榄岩组成。文章通过研究周庵岩体中橄榄石、铬铁矿和辉石的矿物成分变化探讨了岩浆演化过程和含矿岩体成因。根据岩石的矿物组合和蚀变程度,岩体从上到下分为3个部分:上部绿泥石-蛇纹石化二辉橄榄岩相带、中部二辉橄榄岩相带和下部绿泥石-角闪石化二辉橄榄岩相带。根据岩体中部带橄榄石和铬铁矿的成分,计算得到母岩浆的Mg^#值为0.63,MgO/FeO摩尔比值为1.72,w(Al₂O₃)为10.2%~11.7%,w(Ni)为476×10^-6,说明其为高镁玄武质岩浆;岩体中部带原生铬铁矿和粒间相铬铁矿核部的Cr₂O₃和Al₂O₃呈正相关关系,说明铬铁矿与粒间硅酸盐熔体发生了平衡交换,铬铁矿的高w(TiO₂) 和Cr^#值与拉张环境中层状岩体的铬铁矿特征一致;根据辉石温压计得到岩体中部单斜辉石和斜方辉石的共结温度为1 017~1 077℃,压力为(3.6~4.5)×10⁸ Pa,暗示形成岩体的浅部岩浆房深度约为12 km。岩体上部和中部带的橄榄石Fo值大部分集中在80 mol%~85 mol%,w(Ni)介于2 255×10^-6~4 455×10^-6,说明这些橄榄石是从没有经过强烈分离结晶和硫化物熔离的岩浆中结晶出来的。岩体下部带橄榄石的Fo值(67 mol%~68 mol%)和w(Ni) (1 500×10^-6~2 000×10^-6)都低于岩体上部和中部带的橄榄石相应值,说明岩体下部带的橄榄石可能形成于演化程度较高、并经历了硫化物熔离的岩浆。因此,笔者认为周庵岩体是由相对原始的和演化了的高镁玄武质岩浆两期侵位形成的。