Compositional variations of olivine from the Jinchuan Ni–Cu sulfide deposit,western China: implications for ore genesis

The Jinchuan Ni–Cu sulfide deposit is hosted by an elongated, olivine-rich ultramafic body that is divided by subvertical strike-slip faults into three segments (central, eastern, and western). The central segment is characterized by concentric enrichments of cumulus olivine crystals and interstitial sulfides (pyrrhotite–pentlandite–chalcopyrite intergrowth), whereas the eastern and western segments are characterized by an increase of sulfides toward the lower contacts. In all segments sulfides are concentrated at the expense of intercumulus silicates. Olivine re-crystallization is found to be associated with actinolite alteration in some samples. The compositional variations of primary olivine from the sulfide-poor samples can be explained by a small degree of olivine crystallization (<5%) from a basaltic magma followed by local re-equilibration of the olivine with up to 30% trapped silicate liquid. In the sulfide-bearing samples the compositions of primary olivine record the results of olivine-sulfide Fe–Ni exchange that occurred after the trapped silicate liquid crystallized. Our olivine data indicate that Ni in the original sulfide liquids increased inward in the central segment and laterally away from the lower contact in the eastern segment. Variations in the compositions of sulfide liquids are thought to result from fractional segregation of immiscible sulfide liquid from a basaltic magma in a staging chamber instead of in situ differentiation. High concentrations of olivine crystals (mostly >50 modal%) and sulfide (averaging ~5 wt%) in the rocks are consistent with the interpretation that the Jinchuan deposit was formed by olivine- and sulfide-laden magma successively ascending through a conduit to a higher, now-eroded, level. Sulfide enrichment toward the center in the central segment and toward the lower contact in the eastern and western segments may have, in part, resulted from flow differentiation and gravitational settling during magma ascent, respectively.Editorial handling: P. Lightfoot     

Modeling of trace-element composition of sulfide liquid in a crystallizing basalt magma: Development of the <Emphasis Type="Italic">R</Emphasis>-factor concept

This paper presents principles of the method of modeling metal concentrations in sulfide at the occurrence of silicate–sulfide liquid immiscibility during middle and late stages of the crystallization of mafic–ultramafic magmas. The proposed approach combines the preliminary modeling of the crystallization of heterogeneous (melt + crystals) and initially sulfide-undersaturated systems with the calculation of the trace-element composition of sulfide liquid after its appearance in the system. The first part of the problem is solved using the sulfide version of the COMAGMAT-5 program, and the second part employs the modified Campbell–Naldrett equation. It was shown that a delay in silicate–sulfide liquid immiscibility and the low proportion of separated sulfide to major minerals are important factors of the geochemical evolution of sulfide liquid. By the example of the behavior of Cu, Pd, and Re, we discussed the reasons for differences between calculations for crystallizing multiphase cumulates and simplified (sulfide–silicate melt) systems.     

Origin of anomalously Ni-rich parental magmas and genesis of the Huangshannan Ni–Cu sulfide deposit,Central Asian Orogenic Belt,Northwestern China

The Huangshannan Ni–Cu sulfide deposit at the southern margin of the Central Asian Orogenic Belt (CAOB) is an important recent discovery in the Eastern Tianshan Region, Northwestern China. The Huangshannan Intrusion is composed of mafic and ultramafic rocks, and its websterite and lherzolite sequences host the sulfide orebodies. Olivine is the dominant mineral in the Huangshannan Intrusion, occurring as olivine inclusions hosted by pyroxene oikocrysts, as olivine crystals in magmatic sulfides, and as poikilitic crystals in the lherzolite. Small olivine inclusions always coexist with large poikilitic olivine crystals in the same sample, resulting in a heterogeneous texture on the scale of the oikocrysts. The Ni abundance ranges from 1540 to 3772 ppm in poikilitic olivine grains, from 2114 to 3740 ppm in olivine grains hosted by sulfide minerals, and from 2043 to 4023 ppm in olivine inclusions hosted by pyroxene oikocrysts. For the three types of olivine, the ranges in forsterite (Fo) content are 78.97–84.92 mol.%, 81.57–84.79 mol.%, and 80.33–84.68 mol.%, respectively. The Ni content of olivine in the lherzolite is anomalously high relative to the range found in most within plate olivine-bearing mafic-ultramafic rocks. The composition of olivine is controlled mainly by that of the parental magma, fractional crystallization and reactions with interstitial silicate and sulfide melts. Both fractional crystallization and reaction with interstitial silicate may cause a decrease in the Ni content of olivine. The possibility that Ni–Fe exchange causes the anomalously high Ni contents in olivine can be excluded because the olivine grains contained in sulfide have similar or lower Ni content than the olivine grains hosted in the silicate rock. Most of the olivine grains are unzoned, and they have anomalously high Ni contents throughout the crystal. Assuming a partition coefficient of Ni between olivine and silicate magma to be 7, the measured Ni content of olivine in the lherzolite (1540–4023 ppm with a mean of 2907 ppm) indicates that the parental magma contains 220–575 ppm (average of 415 ppm) Ni. This value is higher than that found in basaltic magmas that crystallized olivine with similar Fo contents compared to the Huangshannan Intrusion. As mentioned above, the symmetric and reproducible variations in both Fo and Ni contents from core to margin in most of the olivine grains cannot be explained by fractional crystallization and reactions with interstitial silicate or sulfide melts but may reflect the equilibration of the olivine with new fluxes of magma as the chamber was replenished. The anomalously Ni-rich composition of the parental magmas of the Huangshannan Intrusion, relative to those of many other mineralized olivine-bearing mafic-ultramafic intrusions, may be produced by upgrading and scavenging of metals from a previously formed sulfide melts by a moderately Ni-rich magma. The mass-balance calculations of PGE data indicate that the parental magma that formed lherzolite contains 0.04 ppb Os, 0.02 ppb Ir and 0.4 ppb Pd, whereas the parental magma that formed websterite has 0.02 ppb Os, 0.009 ppb Ir and 0.75 ppb Pd. Rayleigh modeling using PGE tenors indicates that the massive sulfides may be produced by monosulfide solid solution (MSS)-sulfide liquid fractionation from the magma that formed the websterite. Rayleigh modeling of Fo and Ni contents of olivine shows that the parental magma that formed the lherzolite has experienced previous sulfide segregation and olivine crystallization.     

A laser ablation inductively coupled plasma mass spectrometry study of the distribution of chalcophile elements among sulfide phases in sedimentary and magmatic rocks of the Duluth Complex,Minnesota, USA

Nickel-copper sulfide deposits occur in the basal unit of the Partridge River Intrusion, Duluth Complex (Minnesota, USA). Many lines of evidence suggest that these sulfides are formed after assimilation of the proterozoic S-rich black shales, known as the Bedded Pyrrhotite Unit. In addition to S, black shales are enriched in Te, As, Bi, Sb and Sn (TABS) and the basaltic magma of the intrusion is contaminated by the partial melt of the black shales. The TABS are chalcophile and together with the platinum-group elements, Ni and Cu partitioned into the magmatic sulfide liquid that segregated from the Duluth magma. The TABS are important for the formation of platinum-group minerals (PGM) thus their role during crystallization of the base metal sulfide minerals could affect the distribution of the PGE. However, the concentrations of TABS in magmatic Ni-Cu-PGE deposits and their distribution among base metal sulfide minerals are poorly documented. In order to investigate whether the base metal sulfide minerals host TABS in magmatic Ni-Cu-PGE deposits, a petrographic and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) study has been carried out on base metal sulfide and silicate phases of the Partridge River Intrusion, Duluth Complex.Petrographic observations showed that the proportions of the base metal sulfide minerals vary with rock type. The sulfide assemblage of the least metamorphosed Bedded Pyrrhotite Unit from outside the contact metamorphic aureole consists of pyrite with minor pyrrhotite plus chalcopyrite (<5%), whereas within the contact aureole the sulfide assemblage of the Bedded Pyrrhotite Unit rocks consists dominantly of pyrrhotite (>95%) with small amount of chalcopyrite (<2%). The sulfide mineral assemblage in the xenoliths of the Bedded Pyrrhotite Unit and in the mafic rocks of the basal unit contains two additional sulfides, pentlandite and cubanite.Our LA-ICP-MS study shows that sulfides of the Bedded Pyrrhotite Unit are rich in TABS; consistent with these S-rich black shales being the source of TABS that contaminated the mafic magma. Most of the TABS are associated with sulfides and platinum-group minerals in the rocks of the Bedded Pyrrhotite Unit from the contact aureole, the Bedded Pyrrhotite Unit xenoliths and the mafic rocks of the Duluth Complex. In addition to these phases the laser maps show that silicate phases, i.e., orthopyroxene and plagioclase contain Sn and Pb respectively. In contrast, in the least metamorphosed samples of the Bedded Pyrrhotite Unit from outside the contact aureole although the pyrite contains some TABS mass balance calculations indicates that most the TABS are contained in other phases. In these rocks, galena hosts significant amounts of Te, Bi, Sb, Sn and Ag and few very small grains of Sb-rich phases were also observed. The host phases for As were not established but possibly organic compounds may have contributed.     

New textural and mineralogical constraints on the origin of the Hongge Fe-Ti-V oxide deposit, SW China

The Hongge magmatic Fe-Ti-V oxide deposit in the Panxi region, SW China, is hosted in a layered mafic–ultramafic intrusion. This 2.7-km-thick, lopolith-like intrusion consists of the lower, middle, and upper zones, which are composed of olivine clinopyroxenite, clinopyroxenite, and gabbro, respectively. Abundant Fe-Ti oxide layers mainly occur in the middle zone and the lower part of the upper zone. Fe-Ti oxides include Cr-rich and Cr-poor titanomagnetite and granular ilmenite. Cr-rich titanomagnetite is commonly disseminated in the olivine clinopyroxenite of the lower parts of the lower and middle zones and contains 1.89 to 14.9 wt% Cr₂O₃ and 3.20 to 16.2 wt% TiO₂, whereas Cr-poor titanomagnetite typically occurs as net-textured and massive ores in the upper middle and upper zones and contains much lower Cr₂O₃ (<0.4 wt%) but more variable TiO₂ (0.11 to 18.2 wt%). Disseminated Cr-rich titanomagnetite in the ultramafic rocks is commonly enclosed in either olivine or clinopyroxene, whereas Cr-poor titanomangetite of the net-textured and massive ores is mainly interstitial to clinopyroxene and plagioclase. The lithology of the Hongge intrusion is consistent with multiple injections of magmas, the lower zone being derived from a single pulse of less differentiated ferrobasaltic magma and the middle and upper zones from multiple pulses of more differentiated magmas. Cr-rich titanomagnetite in the disseminated ores of the lower and middle zones is interpreted to represent an early crystallization phase whereas clusters of Cr-poor titanomagnetite, granular ilmenite, and apatite in the net-textured ores of the middle and upper zones are thought to have formed from an Fe-Ti-(P)-rich melt segregated from a differentiated ferrobasaltic magma as a result of liquid immiscibility. The dense Fe-Ti-(P)-rich melt percolated downward through the underlying silicate crystal mush to form net-textured and massive Fe-Ti oxide ores, whereas the coexisting Si-rich melt formed the overlying plagioclase-rich rocks in the intrusion.     

新疆东天山土墩基性-超基性杂岩体橄榄石成分特征及其成因意义

橄榄石通常是玄武质岩浆最早结晶出的矿物之一,其化学成分可以很好地反演母岩浆成分、岩浆结晶分异、硫化物熔离等成岩及成矿信息。本文以土墩镁铁质-超镁铁质杂岩体为研究对象,采用电子探针对岩体中的橄榄石矿物颗粒进行化学成分测试。利用橄榄石的Fo值和其中Ni含量,计算得到土墩杂岩体母岩浆中Mg O含量约为12.95%,是一种富镁的玄武质岩浆。同时,定量模拟结果表明,土墩杂岩体母岩浆中硫化物熔离几乎与橄榄石结晶作用同时进行,早阶段由橄榄石结晶(分离结晶程度约2%)而导致硫化物的熔离程度为0.2%。随后,橄榄石分离结晶程度在6%~7%时,硫化物熔体的熔离程度仅为0.01%。这些表明土墩杂岩体发生过一定程度的硫化物熔离,但成矿前景不是很好。此外,部分数据显示出Ni-Fo的负相关性,表明少许富铁橄榄石和晶间硫化物熔浆发生了Fe-Ni物质交换反应,这对橄榄石的成分有重要影响。     

云南白马寨铜镍硫化物矿床形成过程的定量模拟

岩浆的分离结晶作用和地壳同化混染作用是造成硫饱和的重要因素。本文以金平-Song Da地区二叠纪低钛苦橄岩为原生岩浆,使用MELTS程序模拟了岩浆在分离结晶和围岩同化混染作用的控制下达到硫饱和,发生硫化物熔体的熔离。模拟结果表明,低钛苦橄质岩浆从源区上升到浅部岩浆房的过程中发生了约10%的橄榄石的分离结晶,形成高镁的玄武质岩浆。高镁玄武质岩浆在浅部岩浆房内同化混染>18%的围岩,并经历约27%硅酸盐矿物的分离结晶后达到硫饱和。熔离的硫化物熔体在岩浆通道内聚集形成了白马寨铜镍硫化物矿。经历硫化物熔体熔离后的残余岩浆喷出地表形成了金平地区亏损Ni和Cu并具有强烈地壳混染特征的低钛玄武岩。     

A Model of Magmatic Crystallization

A computer model simulating fractional crystallization at oneatmosphere pressure incorporates nine broadly-defined minerals—magnetite,olivine, hypersthene, augite, quartz, plagioclase, orthoclase,leucite, and nepheline. The crystallization temperature of eachmineral is considered to be a smooth function of the compositionof the magmatic liquid. These mineral temperature equationsare obtained by multiple linear regression analysis of informationfrom published silicate systems and rock melting experiments.The nine equations are solved for any primary liquid, withinthe broad range of common magma types, to select the crystallizingmineral or minerals. Partition ratios from published experimentsand analyses of lavas and phenocrysts permit calculation ofthe composition of the crystallizing mineral assemblage. Subtractionof a small amount of that composition from the primary liquidyields a new liquid, which may be recycled to yield a sequenceof liquids during fractional crystallization. The crystallizationmodel handles assemblages of co-precipitating minerals, andcan trace progressive saturation in new minerals, substitutionof a new mineral for an old mineral, and cessation of crystallizationof a mineral. The sequences of minerals and liquids derivedfrom a broad set of primary liquids are geologically realistic,so the model is useful in predicting phenocrysts in volcanicrocks and events during crystallization of shallow intrusions.     

硫化物熔离对岩浆硫化物含矿岩体中橄榄石Ni含量的影响——以金川岩体为例

为探讨硫化物熔离对含矿岩体中橄榄石Ni含量的影响,在前人橄榄石结晶硫化物熔离模型的基础上,定量模似计算了分离结晶过程中橄榄石Ni含量,并将其应用于金川橄榄石成园研究。研究表明,部分橄榄石落于无硫化物熔离橄榄石结晶趋势线下方,暗示其母岩浆为S饱和。根据模拟计算S饱和母岩浆橄榄石分离结晶趋势线,指出金川深部岩浆房中母岩浆橄榄石的分离结晶程度小于或等于3％,而由橄榄石结晶所导致熔离的硫化物熔体与橄榄石之间质量比约为40。     

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.     

Sulfide-associated mineral assemblages in the Bushveld Complex, South Africa: platinum-group element enrichment by vapor refining by chloride–carbonate fluids

The petrology of base metal sulfides and associated accessory minerals in rocks away from economically significant ore zones such as the Merensky Reef of the Bushveld Complex has previously received only scant attention, yet this information is critical in the evaluation of models for the formation of Bushveld-type platinum-group element (PGE) deposits. Trace sulfide minerals, primarily pyrite, pyrrhotite, pentlandite, and chalcopyrite are generally less than 100 microns in size, and occur as disseminated interstitial individual grains, as polyphase assemblages, and less commonly as inclusions in pyroxene, plagioclase, and olivine. Pyrite after pyrrhotite is commonly associated with low temperature greenschist alteration haloes around sulfide grains. Pyrrhotite hosted by Cr- and Ti-poor magnetite (Fe₃O₄) occurs in several samples from the Marginal to Lower Critical Zones below the platiniferous Merensky Reef. These grains occur with calcite that is in textural equilibrium with the igneous silicate minerals, occur with Cl-rich apatite, and are interpreted as resulting from high temperature sulfur loss during degassing of interstitial liquid. A quantitative model demonstrates how many of the first-order features of the Bushveld ore metal distribution could have developed by vapor refining of the crystal pile by chloride–carbonate-rich fluids during which sulfur and sulfide are continuously recycled, with sulfur moving from the interior of the crystal pile to the top during vapor degassing.     

甘肃北山黑山岩浆铜镍硫化物矿床橄榄石特征及成因意义

黑山铜镍硫化物矿床是近年在甘肃北山发现的大型岩浆铜镍硫化物矿床,含矿岩体主要由含矿橄榄岩相和南部边缘的角闪辉长岩相构成。研究发现含矿岩体中的橄榄石属贵橄榄石(Fo值为81.54~86.87),其w(Ni)介于(801.53~2 703.19)×10-6;利用橄榄辉长岩中最高Fo值和主量元素反演,表明原始岩浆属高镁玄武质岩浆,w(MgO)=11.65%,w(FeO)=10.12%;橄榄石分离结晶模拟计算结果表明,橄榄石结晶过程中伴随有0.12%~0.17%硫化物熔离,深部岩浆房中橄榄石分离结晶程度小于3%,橄榄石与硫化物最小质量比约14∶1;隙间硅酸盐熔浆和硫化物熔浆作用明显,是造成早期结晶橄榄石成分变化的重要原因。     

云南牟定安益铂钯钛磁铁矿床铂族矿物特征及成因初探

云南牟定安益矿床为一处铂族金属与钛磁铁矿共同产出的大型钛磁铁矿铂族金属矿床。目前对该矿床中铂族元素的赋存状态研究甚少。结合野外宏观地质特征和室内岩矿鉴定,笔者利用TIMA和LA-ICP-MS-Mapping分析方法,对安益矿床中铂族金属矿物学特征进行研究,发现安益矿床中的铂族元素（PGEs）主要以独立矿物的形式存在。铂族矿物（PGMs）多为铂和钯的砷化物、碲化物,如砷铂矿、砷钯矿、黄碲钯矿、碲钯矿等;主要分布于硅酸盐矿物中,其次为硫化物边缘,部分分布于磁铁矿边缘;铂族矿物成因主要有岩浆成因和热液成因2种。岩浆作用形成的铂族矿物分布于硅酸盐矿物中或硫化物边缘,硅酸盐中的铂族矿物是早期PGE与半金属元素形成的纳米团簇颗粒随岩浆演化形成矿物颗粒,被结晶的硅酸盐矿物包裹;分布于硫化物边缘的铂族矿物是残余熔浆结晶的结果。热液作用将PGE以类质同象的形式富集于钛磁铁矿单辉岩的部分矿物中,如热液蚀变较强烈的黄铜矿中含有较高的Rh,这也与铂族矿物集中分布在钛磁铁矿单辉岩中一致。     

Evidence for the trapped liquid shift effect in the Mount Ayliff Intrusion,South Africa

Postcumulus trapped liquid shift in layered complexes produces cumulate minerals with more fractionated compositions than the original primary phases. This effect is shown by olivine compositions from the base of the Mount Ayliff Intrusion, where varying proportions of olivine to interstitial liquid produce a suite of rocks which define a tight linear trend on a binary whole-rock plot of MgO versus FeO. Extrapolation of this trend constrains the composition of the primary cumulus olivine to the range Fo_84–86, whereas olivine actually present have compositions Fo_77–83. The magnitude of the discrepancy between the theoretical and observed olivine compositions correlates directly with the weight fraction of interstitial liquid. These observations are quantitatively predicted by the trapped liquid shift model. They also argue against significant migration of residual liquid. Trapped liquid shift is documented over a vertical interval of 60 m. It occurred in rocks lying only 1 m above the basal contact of the intrusion and hence must be a comparatively rapid process.     

新疆东昆仑达拉库岸镁铁-超镁铁质岩体年代学、地球化学及成矿条件

达拉库岸镁铁-超镁铁质岩体位于东昆仑造山带南带之喀拉米兰晚古生代沟弧系,主要由二辉橄榄岩、单辉橄榄岩、橄榄二辉岩、单辉辉石岩和辉长岩组成。辉长岩的锆石U-Pb谐和年龄为244.4±1.5Ma,属于中三叠世。岩石普遍弱富集稀土元素和大离子亲石元素(Rb、Ba、Sr),亏损不相容元素(Nb、Ta)。岩体原生岩浆为高镁拉斑玄武质岩浆(Mg O 11.4%,Fe O10.8%)。元素地球化学和Nd、Sr同位素组成特征表明岩浆源区为富集岩石圈地幔,岩浆运移和侵位过程中遭受不同程度地壳物质的同化混染作用。依据橄榄石组分模拟获得橄榄石结晶过程中母岩浆达到硫饱和,并发生硫化物的熔离作用。从岩体特征、矿石结构、原生岩浆性质、深部硫化物熔离和物探信息等方面综合分析,岩体具有形成岩浆型铜镍硫化物矿床的良好条件。     

阿拉斯加型岩体的基本特征、成岩过程及成矿作用

阿拉斯加型岩体是一类具有独特的岩性环带状结构的镁铁-超镁铁质侵入体,常呈链状分布于汇聚板块边缘。其形成时代跨度较大,从元古代到新生代均有分布,以中生代最为发育。大部分阿拉斯加型岩体规模较小,出露面积约12～14 km~2或更小,平面上呈近似同心环状结构,垂直剖面上呈管道状。岩体中心为纯橄岩,向外依次包括异剥橄榄岩、橄榄单斜辉石岩、单斜辉石岩、角闪单斜辉石岩、角闪石岩和辉长岩。造岩矿物为橄榄石、单斜辉石、角闪石等,副矿物为铬铁矿、磁铁矿、钛铁矿等,超镁铁质岩石中少或无斜方辉石,斜长石仅出现在边缘的辉长质岩石中。磁铁矿在单斜辉石岩和角闪石岩中为常见矿物,含量最高达15%～20%。阿拉斯加型岩体的主量元素成分揭示所有岩石均为与拉斑玄武质岩浆分异有关的亚碱性堆晶岩。微量元素成分上显示平坦的稀土元素配分型式和较低的微量元素含量,且富集大离子亲石元素,亏损高场强元素。矿物化学特征上,橄榄石富镁且Fo值变化较大;单斜辉石主要为富Ca的透辉石,其成分变化具有弧堆晶趋势;角闪石主要是镁角闪石和韭角闪石;铬铁矿富集Fe-Al,贫Cr。这些特征揭示,该类岩体成因明显不同于层状岩体和阿尔卑斯型岩体。综合岩石学、矿物学和地球化学分析表明,阿拉斯加型岩体形成于与板块俯冲作用有关的岛弧或者活动大陆边缘背景下,其母岩浆为受到熔/流体交代的地幔楔部分熔融产生的含水玄武质岩浆。各岩相为未受明显地壳混染的同源母岩浆在地壳深度结晶分异的产物。阿拉斯加型岩体的岩浆体系具有含水且高氧逸度的特征,其通常为铂族元素和铬铁矿矿床的重要载体,无或少铜镍矿化。     

Gough Island: Evaluation of a fractional crystallization model

Gough Island is composed of an alkaline olivine basalt-trachyte series. A fractional crystallization model for the development of these rocks has been evaluated by correlating the geochemical trends of major and trace elements. Beginning with an alkali olivine basalt parent the major element abundances were used to determine the varying proportions of crystallizing minerals required to generate the various residual liquids. A least-squares computer model was used for this calculation. The modal proportions of cumulative minerals and trace element distribution coefficients were used to predict the trace element abundances in each rock type.Three significant trace element trends are observed in Gough Island rocks: (1) increasing rare earth (RE) abundance and relative light RE enrichment with increasing major element differentiation, (2) marked Eu, Sr, and Ba depletions in late stage trachytes, (3) Cr and M enrichment in picrite basalt.The trace element abundances predicted by the fractional crystallization model are in good agreement with these observed trends. A fractional crystallization process involving olivine, pyroxene, feldspar, and apatite accounts for all the significant major and trace element trends observed in Gough Island rocks.     

Textural development in sulfide-matrix ore breccias in the Voisey's Bay Ni-Cu-Co deposit,Labrador, Canada

Magmatic Ni-Cu sulfide ores at Voisey’s Bay contain complex assemblages of extremely heterogeneous rocks. These range from polymict breccias, with rock fragments in sulfide-rich and/or sulfide-poor matrices, to heterogeneous “vari-textured” gabbros with rapid short range variations in grain size and content of hydrous phases. Rock fragment populations in the breccias include endogenous olivine gabbros (cumulate and non-cumulate) and cumulate peridotites along with extensively depleted plagioclase-hercynite gneisses interpreted as restites from extensive partial melting of country rock quartzo-feldspathic paragneisses. Using a combination of desk-top microbeam XRF mapping at cm scale and 3D X-ray tomography, we show that both sulfide-poor and sulfide-rich breccias comprise heterolithic assemblages of clasts within a matrix of olivine gabbro. This matrix is characterised by an interconnected 3D framework of plagioclase crystals, highly variable in grain size at mm to cm scale, with interstitial olivine and poikilitic clinopyroxene, and is texturally indistinguishable from clast-free olivine gabbro. Sulfide forms interconnected networks at cm to dm scale and possibly larger. Much of the plagioclase developed by outgrowth from the margins of paragneiss xenoliths when the porosity was occupied by silicate melt. The observed range of textures is explained by a model of percolation of molten sulfide through variably crystalline inter-clast matrix, displacing the silicate melt to leave the refractory plagioclase-olivine or in some cases plagioclase-only component, now entirely within a sulfide matrix. The process is analogous to that believed to have formed interspinifex ore in komatiite-hosted deposits. Biotite rims on plagioclase enclosed in sulfide are interpreted as the result of reaction between plagioclase, olivine and a hydrous component derived from the sulfide melt itself, with a possible component of migrating residual silicate melt wicking along sulfide-silicate contacts. This sulfide infiltration model offers an alternative to the current model for upward emplacement of a slurry of silicate melt, sulfide melt and breccia fragments as a late stage injection into the dyke-sill complex. The preserved range of textures is interpreted as being due to gravity-driven percolation of sulfide liquid through a pre-existing partially molten intrusion breccia. In this model, the breccia serves as a physical trap site, accumulating downward migrating sulfide liquid. However, the invariable close mutual association of sulfide and rock fragments at Voisey’s Bay implies a common derivation.     

PGE geochemistry of the Eagle Ni–Cu–(PGE) deposit, Upper Michigan: constraints on ore genesis in a dynamic magma conduit

The Eagle Ni–Cu–(PGE) deposit is hosted in mafic–ultramafic intrusive rocks associated with the Marquette–Baraga dike swarm in northern Michigan. Sulfide mineralization formed in association with picritic magmatism in a dynamic magma conduit during the early stages in the development of the ~1.1?Ga Midcontinent Rift System. Four main types of sulfide mineralization have been recognized within the Eagle deposit: (1) disseminated sulfides in olivine-rich rocks; (2) rocks with semi-massive sulfides located both above and below the massive sulfide zone; (3) massive sulfides; and (4) sulfide veins in sedimentary country rocks. The disseminated, massive and lower semi-massive sulfide zones are typically composed of pyrrhotite, pentlandite and chalcopyrite, whereas the upper semi-massive sulfide ore zone also contains pyrrhotite, pentlandite, and chalcopyrite, but has higher cubanite content. Three distinct types of sulfide mineralization are present in the massive sulfide zone: IPGE-rich, PPGE-rich, and PGE-unfractioned. The lower and upper semi-massive sulfide zones have different PGE compositions. Samples from the lower semi-massive sulfide zone are characterized by unfractionated PGE patterns, whereas those from the upper semi-massive sulfide zone show moderate depletion in IPGE and moderate enrichment in PPGE. The mantle-normalized PGE patterns of unfractionated massive and lower semi-massive sulfides are subparallel to those of the disseminated sulfides. The results of numerical modeling using PGE concentrations recalculated to 100% sulfide (i.e., PGE tenors) and partition coefficients of PGE between sulfide liquid and magma indicate that the disseminated and unfractionated massive sulfide mineralization formed by the accumulation of primary sulfide liquids with similar R factors between 200 and 300. In contrast, the modeled R factor for the lower semi-massive sulfide zone is <100. The fractionated sulfide zones such as those of the IPGE-rich and PPGE-rich massive sulfides and the upper semi-massive sulfide zone can be explained by fractional crystallization of monosulfide solid solution from sulfide liquids. The results of numerical modeling indicate that the sulfide minerals in the upper semi-massive sulfide zone are the products of crystallization of fractionated sulfide liquids derived from a primary sulfide liquid with an R factor similar to that for the disseminated sulfide mineralization. Interestingly, the modeled parental sulfide liquid for the IPGE-rich and PPGE-rich massive sulfide zones has a higher R factor (~400) than that for the unfractionated massive sulfide mineralization. Except one sample which has unusually high IPGE and PPGE contents, all other samples from the Cu-rich sulfide veins in the footwall of the intrusion are highly depleted in IPGE and enriched in PPGE. These signatures are generally consistent with highly fractionated sulfide liquids expelled from crystallizing sulfide liquid. Collectively, our data suggest that at least four primary sulfide liquids with different R factors (<100, 200–300, ~400) were involved in the formation of the Eagle magmatic sulfide deposit. We envision that the immiscible sulfide liquids were transported from depth by multiple pulses of magma passing through the Eagle conduit system. The sulfide liquids were deposited in the widened part of the conduit system due to the decreasing velocity of magma flow. The presence of abundant olivine in some of the sulfide ore zones indicates that the ascending magma also carried olivine crystals. Sulfide saturation was attained in the parental magma due in large part to the assimilation of country rock sulfur at depth.     

含硫化物镁铁质-超镁铁质侵入体中橄榄石Mg、Ni含量的制约:原理、模式及Voisey's Bay侵入体样品研究(英文)

镁铁质-超镁铁质岩浆结晶分离早期形成镁铁矿物,镁铁矿物中的Ni和Mg是相容元素。随着结晶分离作用的进行,Ni、Mg在硅酸盐岩浆及后形成的硅酸盐物质中的丰度下降。橄榄石中Ni含量及硅酸盐物质MgO/FeO比值都与母岩浆的相关值相关,据此可推断母岩浆的信息,它们之间可由实验测得的系数相联系。当岩浆饱和硫化物时,在结晶分离过程中硫化物珠滴会与镁铁硅酸盐物质一道析出,同时,与硫化物非饱和岩浆相比,过多的Ni会随之析出。这也反映在Ni、Mg含量比无硫化物分离时有更迅速的降低上。Ni、Mg含量变化值可以在VoiseysBay侵入体的模式曲线上反映出,加拿大Labrador的这一侵入体赋存了一个世界级的Ni-Cu-Co硫化物矿床。过去的作法是将侵入体中橄榄石的Ni、Mg含量与Simkin和Smith得出的各种火成岩中橄榄石的Ni、Mg含量相比较以确定Ni亏损,进而假定橄榄石来自硫化物饱和、有经济价值的岩浆。现在的研究显示这种简单的对比会导致错误。将样品数据与模式曲线对比并反映出侵入体矿物结晶堆积特征是重要的方法。使用这一方法,样品数据能很好地被模式曲线拟合。以在VoiseysBay的研究为例,当硫化物液相与硅酸盐矿物被去除后,硫化物非饱和的分离作用期就会显现出来,随后是硅酸盐结晶作用期。