Ruby Mineralization in Southwest Madagascar

Gem-variety of red corundum (i.e. ruby) is produced in the Ejeda-Fotadrevo area, in southwestern Madagascar. The primary ruby deposits are closely associated with basic/ultrabasic complexes within the high grade metamorphic terranes of the Precambrian Vohibory unit. Ruby is recovered from amphibolite and anorthosite veins within these complexes. Petrographic data and P-T estimates indicate that the ruby-bearing rocks crystallized under granulites facies conditions of 750–850°C and 9–11.5kbar, in accordance with the conditions recorded from the surrounding granulites. The Malagasy ruby deposits present numerous similarities with East African deposits, especially Tanzanian, indicating similar geological context of ruby mineralization and suggesting that ruby formation in both these areas resulted from a same mineralizing event when Madagascar was still adjacent to East Africa (Kenya, Tanzania) in the Gondwanaland assembly at the end of Proterozoic times.     

Ruby–sapphire–spinel mineralization in marble of the middle and southern Urals: Geology,mineralogy, and genesis

Ruby and spinel occurrences hosted in marble on the eastern slope of the Urals are considered. Ruby- and spinel-bearing marble is a specific rock in granite-gneiss complexes of the East Ural Megazone, which formed at the Late Paleozoic collision stage of the evolution of the Urals. Organogenic marine limestone is the protolith of the marble. No relict sedimentary bedding has been retained in the marble. The observed banding is a secondary phenomenon related to crystallization and is controlled by flow cleavage. Magnesian metasomatism of limestone with the formation of fine-grained dolomite enriched in Cr, V, Ti, Mn, Cu, Zn, Ga, and REE took place at the prograde stage of metamorphism. Dedolomitization of rocks with the formation of background calcite marble also developed at the prograde stage. Mg-calcite marble with spinel and ruby of the first type formed in the metamorphic fluid circulation zone. Magnesian metasomatism with the formation of bicarbonate marble with ruby, pink sapphire, and spinel of the second type developed at the early retrograde stage. The formation of mica-bearing mineralized zones with corundum and spinel of the third type controlled by cleavage fractures is related to the pneumatolytic–hydrothermal stage. The data on ruby-bearing marble in the Urals may be used for forecasting and prospecting of ruby and sapphire deposits hosted in marble worldwide.     

Gem corundum deposits of Madagascar: A review

Madagascar is one of the most important gem-producing countries in the world, including ruby and sapphires. Gem corundum deposits formed at different stages in the geological evolution of the island and in contrasting environments. Four main settings are identified: (1) Gem corundum formed in the Precambrian basement within the Neoproterozoic terranes of southern Madagascar, and in the volcano-sedimentary series of Beforona, north of Antananarivo. In the south, high-temperature (700 to 800 °C) and low-pressure (4 to 5 kbar) granulites contain deposits formed during the Pan-African orogenesis between 565 and 490 Ma. They accompany mafic and ultramafic complexes (ruby deposits of the Vohibory group), skarns at the contact between Anosyan granites and the Proterozoic Tranomaro group (sapphire deposits of the Tranomaro–Andranondambo district), and shear-zone corridors cross-cutting feldspathic gneisses, cordieritites and clinopyroxenites in the Tranomaro, Vohimena and Androyan metamorphic series (biotite schist deposits of Sahambano and Zazafotsy, cordieritites of Iankaroka and Ambatomena). The circulation of fluids, especially along discontinuities, allowed in-situ alkaline metasomatism, forming corundum host rocks related to desilicified granites, biotitites, “sakenites” and “corundumites”. (2) Gem corundum also occurs in the Triassic detrital formations of the Isalo group, as giant palaeoplacers in the Ilakaka–Sakaraha area. Here, sapphires and rubies may come from the metamorphic granulitic terranes of southern Madagascar. (3) Gem corundum deposits occur within the Neogene-Quaternary alkali basalts from Ankaratra (Antsirabe–Antanifotsy area) and in the Ambohitra Province (Nosy Be, Ambato and Ambondromifehy districts). Primary deposits are rare, except at Soamiakatra where ruby in gabbroic and clinopyroxenite xenoliths within alkali-basalts probably derive from mantle garnet peridotites. The blue-green-yellow sapphires typical of basaltic fields are always recovered in palaeoplacer (in karst formed upon Jurassic limestones from the Montagne d'Ambre, Antsiranana Province) and alluvial and soil placers (Ankaratra volcanic massif). (4) Deposits occur within Quaternary eluvial, colluvial and alluvial concentrations, such as high-quality rubies from the Andilamena and Vatomandry deposits.     

Corundum formation by metasomatic reactions in Archean metapelite,SW Greenland:Exploration vectors for ruby deposits within high-grade greenstone belts

Corundum (ruby-sapphire) is known to have formed in situ within Archean metamorphic rocks at several localities in the North Atlantic Craton of Greenland. Here we present two case studies for such occurrences: (1) Maniitsoq region (Kangerdluarssuk), where kyanite paragneiss hosts ruby corundum, and (2) Nuuk region (Storø), where sillimanite gneiss hosts ruby corundum. At both occurrences, ultramafic rocks (amphibole-peridotite) are in direct contact with the ruby-bearing zones, which have been transformed to mica schist by metasomatic reactions. The bulk-rock geochemistry of the ruby-bearing rocks is consistent with significant depletion of SiO₂ in combination with addition of Al₂O₃, MgO, K₂O, Th and Sr relative to an assumed aluminous precursor metapelite. Phase equilibria modelling supports ruby genesis from the breakdown of sillimanite and kyanite at elevated temperatures due to the removal of SiO₂. The juxtaposition of relatively silica- and aluminum-rich metasedimentary rocks with low silica ultramafic rocks established a chemical potential gradient that leached/mobilized SiO₂ allowing corundum to stabilize in the former rocks. Furthermore, addition of Al₂O₃ via a metasomatic reaction is required, because Al/Ti is fractionated between the aluminous precursor metapelites and the resulting corundum-bearing mica schist. We propose that Al was mobilized either by complexation with hydroxide at alkaline conditions, or that Al was transported as K-Al-Si-O polymers at deep crustal levels. The three main exploration vectors for corundum within Archean greenstone belts are: (1) amphibolite- to granulite-facies metamorphic conditions, (2) the juxtaposition of ultramafic rocks and aluminous metapelite, and (3) mica-rich reactions zones at their interface.     

大理岩型红宝石矿床成因研究

大理岩型红宝石矿床是红蓝宝石矿床的重要类型。对此类型矿床与找矿相关的成因问题，长期存在模糊认识。本文在实际研究和文献资料整理的基础上，得出以下结论：大理岩型红宝石矿床产于有深大断裂构造活动的深成造山变质带；含矿岩石是钙质结晶大理岩，而非鲜质大理岩或镁质夕卡岩；含矿岩石中的角闪石为富铝贫硅含铬的钙质闪石，如含铬的镁砂川闪石，而非绿色透闪石；矿床成因类型属区域热动力变质型，而不是“气成－热认型”或“夕卡岩型”。     

Oxygen isotope systematics of gem corundum deposits in Madagascar: relevance for their geological origin

The oxygen isotopic composition of gem corundum was measured from 22 deposits and occurrences in Madagascar to provide a gemstone geological identification and characterization. Primary corundum deposits in Madagascar are hosted in magmatic (syenite and alkali basalt) and metamorphic rocks (gneiss, cordieritite, mafic and ultramafic rocks, marble, and calc-silicate rocks). In both domains the circulation of fluids, especially along shear zones for metamorphic deposits, provoked in situ transformation of the corundum host rocks with the formation of metasomatites such as phlogopite, sakenite, and corundumite. Secondary deposits (placers) are the most important economically and are contained in detrital basins and karsts. The oxygen isotopic ratios (¹⁸O/¹⁶O) of ruby and sapphire from primary deposits are a good indicator of their geological origin and reveal a wide range of δ¹⁸O (Vienna Standard Mean Ocean Water) between 1.3 and 15.6‰. Metamorphic rubies are defined by two groups of δ¹⁸O values in the range of 1.7 to 2.9‰ (cordieritite) and 3.8 to 6.1‰ (amphibolite). “Magmatic” rubies from pyroxenitic xenoliths contained in the alkali basalt of Soamiakatra have δ¹⁸O values ranging between 1.3 and 4.7‰. Sapphires are classified into two main groups with δ¹⁸O in the range of 4.7 to 9.0‰ (pyroxenite and feldspathic gneiss) and 10.7 to 15.6‰ (skarn in marble from Andranondambo). The δ¹⁸O values for gem corundum from secondary deposits have a wide spread between −0.3 and 16.5‰. The ruby and sapphire found in placers linked to alkali basalt environments in the northern and central regions of Madagascar have consistent δ¹⁸O values between 3.5 and 6.9‰. Ruby from the placers of Vatomandry and Andilamena has δ¹⁸O values of 5.9‰, and between 0.5 and 4.0‰, respectively. The placers of the Ilakaka area are characterized by a huge variety of colored sapphires and rubies, with δ¹⁸O values between −0.3 and 16.5‰, and their origin is debated. A comparison with oxygen isotope data obtained on gem corundum from Eastern Africa, India, and Sri Lanka is presented. Giant placer deposits from Sri Lanka, Madagascar, and Tanzania have a large variety of colored sapphires and rubies with a large variation in δ¹⁸O due to mingling of corundum of different origin: mafic and ultramafic rocks for ruby, desilicated pegmatites for blue sapphire, syenite for yellow, green, and blue sapphire, and skarn in marbles for blue sapphire.     

Source rocks of gem minerals

Field and laboratory studies on the geology and mineralogy of gem sediments and associated rocks reveal that gem minerals occur both in garnetiferous gneisses and granulites. It has been observed that PT conditions characteristic of granulite facies had favoured the formation of gem minerals such as topaz, corundum, beryl, spinel, tourmaline and zircon.

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Resume Les travaux sur le terrain ainsi qu'au laboratoire portant sur les sédiments et les roches associes aux pierres precieuses montrent que les minéraux précieux se trouvent dans les gneiss à grenat et les granulites. Il a été observé que les conditions de pression et de température qui caractérisent le faciès des granulites ont favorise la formation des minéraux precieux tels que des topazes, corindons, béryls, spinelles, tourmalines et zircons.

     

Corundum inclusions in diamonds—discriminatory criteria and a corundum compositional dataset

Mineral inclusions of corundum are reported from diamonds from alluvial deposits of tributaries of the Rio Aripuanã, Juina, Brazil. We present the first recorded occurrence of sapphire as an inclusion in diamond and expand on the database of ruby and white corundum inclusions. Ruby inclusions are found to occur both as isolated and touching grains with aluminous pyroxene and associated with ferropericlase. Mineral chemistry and phase relations place the origin of such ruby-bearing diamonds within the lower mantle at 770 km. Mineral associations indaving other corundum inclusions were not observed; hence, their depth of origin is less certain.

Compositions of corundum samples were characterised by electron and ion microprobe. Given the scarcity of literature data, corundum samples from a variety of other geological settings were also analysed. Samples comprised corundums associated with granitic emplacement, metasomatism, amphibolite-facies and granulite-facies rocks, gem and industrial synthetic origins and carmine-coloured corundums recovered from kimberlite drill cores.

In addition to variable amounts of Cr, Fe, Ti, Mg and Si, measurable quantities of other transition elements and high field strength elements were also detected. Corundums from similar geological settings show very similar compositions and are easily distinguishable from other settings. Irrespective of locality, rubies from Norwegian, Tanzanian and Kenyan amphibolite-facies rocks are compositionally indistinguishable. Additionally, corundums from metasomatised zones associated with contact metamorphism from Arizona and Japan were very similar, particularly characterised by unusually high abundance of mobile Zr and Nb (tens of ppm). All Juina inclusions are particularly distinguishable from other corundums by high concentrations of Ni (18–171 ppm weight), typically at least an order of magnitude enriched over the same corundum varietal types from elsewhere. Furthermore, the sapphire inclusion exhibited much larger ratios of Ga and Ge to HFSE elements compared to otherwise similar samples, and ruby inclusions are distinguished by high Mg/Fe ratios (0.27–1.56 by weight). Compositional differences between inclusions in diamonds and corundums from other settings in addition to corundum's physical and chemical durability suggest that with the employment of rapid identification tools such as energy dispersive spectrometry (EDS) and laser-ICPMS, corundum has promise as an indicator of diamond prospectivity.     

Age and origin of gem corundum and zircon megacrysts from the Mercaderes–Rio Mayo area, South-west Colombia, South America

Potential sources for alluvial gem corundum and zircon from the Rio Mayo area, near Mercaderes, Colombia are reviewed, based on U–Pb dating of syngenetic and protogenetic mineral inclusions in corundum samples and on a zircon megacryst. Corundum recovered from the region (approx. 99% sapphire, 1% ruby) commonly shows growth banding, includes colour change stones and exhibits overlaps in colour ranges and inclusion characteristics. This suggests a contiguous genetic suite. The U–Pb dating used laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) techniques. Because of the young ages and low-U contents of the dated zircons, the acquired data required a special statistical treatment. The results from zircon, fluorapatite and allanite-(Ce) inclusions provide a corundum crystallization age of 8 to 11 Ma, in relation to northern Andean Miocene uplift and magmatism. The zircon megacryst gave a younger crystallization age of c. 0.6 Ma, unrelated to the corundum genesis. Geochemical parameters (trace element and O isotope ranges) for corundum samples suggest a metamorphic/metasomatic origin. The age data rules out corundum genesis during the Late Cretaceous ophiolitic generation, but leave open possible later metasomatic interactions with this substrate. The Cr/Ga and Ga/Mg ratios and O isotope range for the corundum fall within the known limits for metasomatic, desilicated felsic/ultramafic ‘plumasitic’ associations, suggesting a possible parental source. Allanite, extremely rare as an inclusion in corundum elsewhere, may prove a characteristic inclusion for Rio Mayo corundum.     

大理岩型红宝石矿床的若干问题

大理岩型红宝石矿床是红蓝宝石矿床的最重要类型之一。对此类型矿床与找矿相关的若干问题,长期存在模糊认识。本文经过实际研究和相关文献资料的搜集整理,得出了以下重要结论：大理岩型红宝石矿床产于有深大断裂构造活动的深成造山变质带;矿床成因类型属区域热动力变质型;含矿岩石是钙质结晶大理岩,而非镁质大理岩或镁质矽卡岩;含矿岩石中的角门石为富铝贫硅含铬的钙质问石,而非绿色透闪石。     

变质矿床成因分类的讨论

在探讨了变质矿床的定义并结合前人研究的基础上,对变质矿床初步提出了一个新的成因分类方案。该方案共分为五个大类和十三个亚类,并对每个大类和亚类都提出了一些代表性矿床实例。新的变质矿床成因分类方案为：（1）受变质矿床类,分为受变质硅铁质建造沉积矿床、受变质硅铁质建造火山–沉积矿床、受变质其它建造沉积–火山沉积矿床、受变质火成岩改造矿床和受变质细碧角斑质火山喷发沉积矿床五个亚类;（2）区域变质作用变成矿床类,分为变质重结晶型、变质化学反应重组型和变质热液型三个亚类;（3）局部变质作用变成矿床类,分为接触交代夕卡岩矿床、局部接触热变质变成矿床和局部动力变质变成矿床三个亚类;（4）受变质沉积–火山沉积变质热液叠加改造矿床类;（5）混合岩化作用形成矿床类,又分为混合岩化交代型和后期混合岩化热液型两个亚类。     

Gem-corundum megacrysts from east Australian basalt fields: trace elements,oxygen isotopes and origins∗

Gem corundum, a minor but persistent megacryst in east Australian basalt fields, is mined from some placer concentrations. Laser ablation, inductively coupled plasma mass spectrometry analyses and O isotope determinations on a colour range of corundum from different fields, show that chromophore (Fe, Cr, Ti, V) and genetic indicator (Ga, Mg, δ¹⁸O) values can distinguish corundum sources (magmatic, metamorphic and metasomatic) before basalt incorporation. They also characterise corundum groups from different fields. This identified two metamorphic groups, one carrying ruby at Barrington Tops, and a magmatic group distinct from those from other gem fields (lower Fe, northeast Tasmania; higher Fe, Yarrowitch). Ruby-bearing groups show clear provincial characteristics and include lower temperature spinel-facies groups (Barrington, Yarrowitch) and higher temperature garnet-facies groups (Cudgegong–Macquarie River). High Mg/Fe and Ni values in the latter approach those for corundum in diamond, and are a possible diamond indicator. The corundum derived from diverse fold-belt and felsic sources in underlying lithosphere forms a dataset for comparing corundum from other basalt fields.     

冀东-辽西太古宙火成岩岩石组合和动力学意义

冀东-辽西地区是华北克拉通北部出露面积最大的太古宙变质基底区.经过岩石组合填图和综合研究,将其太古宙变质火成岩分为2.64~2.60 Ga MORB型拉斑玄武质火山岩、2.61~2.52 Ga拉斑玄武质-钙碱性变质火山岩、2.52~2.50 Ga浅变质钙碱性火山岩组合和2.54~2.50 Ga英云闪长质-奥长花岗质-花岗闪长质片麻岩、2.54~2.51 Ga闪长质-石英闪长质-英云闪长质-奥长花岗质-花岗闪长质片麻岩、2.54~2.51 Ga紫苏花岗闪长质-紫苏花岗质岩石、2.57~2.52 Ga闪长质-石英二长闪长质-花岗闪长质-二长花岗质片麻岩和2.53~2.51 Ga弱片麻状到块状二长花岗质-正长花岗质深成侵入体岩石组合.这些岩石组合从东北部的辽西阜新到西南部的遵化马兰峪地区呈现出条带状时-空分布特征.变质作用研究揭示了青龙-上营-洒河桥-马兰峪为高压麻粒岩带,记录了ITD型PTt轨迹,与NNW-NW向SSE-SE方向逆冲推覆构造相伴生;而三屯营-太平寨高温麻粒岩带,记录了IBC型PTt轨迹,与锦州-兴城-安子岭-界岭口-太平寨-三屯营伸展-底劈构造带相伴生.综合分析表明冀东-辽西太古宙晚期的构造-岩浆活动形成于热造山带型俯冲-弧后伸展到碰撞隆升的侧向增生动力学过程.      

Rare-earth element geochemistry of Madras granulites

Archaean granulites from the type charnockite locality of Pallavaram, Madras City, Tamil Nadu, southern India consist of a bimodal suite of basic and silicic orthogneisses, associated with metasedimentary gneisses (khondalites). Charnockite is the dominant rock type. Basic granulites display a tholeiitic trend of strong iron enrichment accompanied by an increase in the concentration of the rare earth elements (REE), and the development of appreciable negative europium anomalies. These trends are considered to reflect low pressure fractional crystallisation of pyroxene-plagioclase assemblages under conditions of lowf _{O 2}. Ultramafic granulites may represent the cumulus material of such a process. The silicic granulites (charnockites) are generally K₂O rich and in marked contrast to the low K₂O (tonalitic) silicic gneisses which dominate most granulite facies terrains. Their REE abundances, however, embrace the complete range of patterns observed in both K-rich and K-poor Archaean gneisses. The presence of a large number of pre-granulite facies potassic pegmatites in the area suggests metasomatism of an originally less potassic suite of rocks. Trace element considerations lead to a model whereby metasomatism and partial fusion of silicic gneisses in the terrain preceded the granulite facies metamorphic event. This sequence of events is best related to fluctuations in the composition of metamorphic fluids in the lower crust.     

中国新生代碱性玄武岩型宝石矿床

中国东部分布着大量的新生代玄武岩,其中第三纪的中新世和上新世的碱性玄武岩是蓝宝石、红宝石、红色锆石、橄榄石、镁铝榴石、月光石和辉石类宝石的寄主岩石,已在其内发现了多种宝石的原生矿床和矿化点。今后应重视碱性玄武岩型宝石矿床的研究和找矿评价工作。     

Evidence for Post-Metamorphic Metasomatism of High-Grade Orthogneisses from Sri Lanka

Here we discuss the post-metamorphic metasomatism of high-grade orthogneisses by studying granite-looking, pink-coloured microcline-bearing rocks exposed around Ambagaspitiya, Sri Lanka. These rocks are medium- to coarse-grained, and are more or less homogeneous, and isotropic. Textural, and petrographic analyses clearly show that these special rocks are neither deformed nor metamorphosed, and that they are not any kind of intrusive rocks. The present study shows that these rocks have formed through K-metasomatism of once intensely deformed, and metamorphosed granodiorite, tonalite, monzodiorite, and quartz monzodiorite. The modal compositions of most of these metasomatic rocks of Ambagaspitiya are very similar to those of syenite, quartz syenite, monzonite, quartz monzonite, and quartz monzodiorite.All the original metamorphic rocks — namely granitic gneiss, metagranite, metagranodiorite, metatonalite, metamonzodiorite, metaquartz monzodiorite, metadiorite, basic dikes, and metapelites — had undergone at least five ductile deformations, D₁ to D₅, and had been metamorphosed under upper amphibolite to granulite facies conditions prior to the metasomatism. Almost all the parent metamorphic rocks had acquired a well-developed gneissic foliation (S2), and had suffered at least two intense folding events (F₃, and F₅) before the metasomatism occurred. All the metamorphic, and deformational fabrics of affected metamorphic rocks have been completely or partially obliterated by the metasomatism. This indicates that the metasomatic process post-dates all ductile deformations (D₁ — D₅), and the regional metamorphism. Of the parent metamorphic rocks, metagranodiorite, metatonalite, metamonzodiorite, and metaquartz monzodiorite have undergone intense metasomatism. It is shown that the metasomatism has nucleated along late-stage, post-D₅ shear zones, which may form an interconnected network. Potassium-bearing metasomatic fluids, derived from a deep-seated K-rich source, may have migrated along these shear zones. The fluids which entered the shear zones have pervaded the orthogneisses through foliation planes, and along grain boundaries, and microcracks in minerals, transforming the host gneisses to metasomatic rocks. The main metasomatic transformation has taken place through the replacement of metamorphic plagioclase, and plagioclase-quartz by microcline, and through formation of myrmekite. Further studies are necessary to unravel the nature, composition, and the source of these late-stage K-rich fluids in the lower crust.     

青海省矿产资源发育特征概述

通过对青海省矿产资源发育特征的系统研究,发现青海省矿产资源丰富,种类齐全。截止2007年底,共发现各类矿产132种,其中探明有资源储量的矿产为107种;发现各类矿床、矿（化）点3921处,其中重要矿产大型以上有矿床96处,中型123处,小型211处;矿床类型复杂多样,主要有岩浆型、斑岩型、伟晶岩型、接触交代（矽卡岩）型、热液型、海相火山岩型、变质型、沉积型（喷气-沉积型）、砂矿型、风化壳型、盐湖型、火山沉积型、构造-蚀变岩（造山）型、陆相火山岩型等14种类型;资源查明率低,其中,煤13.07%、铁16.65%、铜14.24%、铅15.35%、锌17.87%,岩金3.64%。由此可知,青海省主要优势与战略性矿产资源的进一步找矿空间和潜力是十分巨大的。     

中国典型钴矿床地质特征及重点地区矿产资源预测

我国既是钴矿资源消费大国,又是钴矿资源进口大国,受新能源电动汽车工业的影响,近年来钴矿资源得到越来越多的关注.为了科学地评估我国钴矿资源潜力,指导找矿勘查部署工作,立足国内,提高我国钴矿资源的保障能力,笔者２０１９—２０２１年开展了９个重点省(自治区)的钴矿资源潜力评价工作.本文从我国钴矿床分布和地质特征角度,先以成矿地质作用为主线,厘定我国找矿预测矿床类型,包括与沉积地质作用有关的风化型、化学沉积型、砂岩型和海底喷流沉积型,与火山地质作用有关的海相火山岩型、陆相火山岩型,与侵入岩浆地质作用有关的岩浆型、矽卡岩型(接触交代型)、热液脉型和斑岩型,与变质地质作用有关的沉积变质型,以及少量与大型变形地质作用有关的变质核杂岩型;进而梳理并总结了各类型钴矿有关的成矿地质体.通过分析我国钴矿的矿床地质特征和时空分布规律,综合地质、物探、化探和遥感信息等预测要素,建立找矿预测模型,并在此基础上圈定找矿远景区,估算资源量.结果表明:我国钴矿包括风化型镍钴矿、海底喷流沉积型铜钴矿、海相火山岩型铁铜钴矿、海相火山岩型块状硫化物铜锌钴矿、岩浆型铜镍钴矿和热液脉型钴矿６种主要找矿预测类型;圈定416个钴矿最小预测区,圈定64个找矿远景区,优选99个找矿靶区,提出下一步勘查建议;９个重点省(自治区)累计查明钴资源储量45.3万t,预测钴资源量约420万t.     

Terrestrial geochemistry of Cd,Bi, Tl,Pb, Zn and Rb

About 2000 common magmatic, metamorphic and sedimentary rocks and rockforming minerals contained in 465 individual samples have been analyzed for 6 trace metals and potassium with high precision, mainly by combined distillation and AAS methods. Estimates of average abundances in the continental crust are: 98 ppb Cd. 82 ppb Bi. 490 ppb Tl, 14.8 ppm Pb, 77 ppm Zn and 98 ppm Rb (K/Rb: 223). These averages are close to the mean concentrations of the 6 elements in sedimentary and in low to medium grade metamorphic rocks. In relation to the upper mantle the earth's crust has very effectively accumulated Rb, Pb, Tl (and Bi). Cd and Zn are equally distributed between the upper and lower crust. Bi, Tl, Rb, Pb and K are accumulated in the upper relative to the lower continental crust by factors between 3.5 and 1.4. This is mainly due to higher concentrations in granites and lower abundances in granulites relative to gneisses and schists. The five metals form large ions with bulk coefficients less than one for the partition between metamorphic rocks and anatectic granitic melts. The major hosts of Rb, Tl, Pb and Bi in rocks are minerals with 8- to 12-coordinated sites such as mica, K-feldspar, plagioclase etc. (except for some preference of Bi for sphene and apatite). As examples of significant correlations those of Pb with Tl, K, Bi and Rb in mafic rocks and of Bi with K, Rb, Tl and Pb in sedimentary rocks can be reported. In granites and gneisses hydroxyl containing Fe²⁺-Mg-silicates are major host minerals for Zn and Cd. Except in some carbonate rocks Cd has no preference for Ca minerals.     

Petrology of corundum-spinel-sapphirine-anorthite rocks (sakenites) from the type locality in southern Madagascar

‘Sakenites’ constitute a unique association of corundum‐, spinel‐ and sapphirine‐bearing anorthitic to phlogopitic rocks, first described in rocks from an exposure along the beds of the Sakena river to the NW of Ihosy, south Madagascar. The exposure has been revisited and subjected to a detailed petrological and geochemical study. The aluminous anorthitic rocks occur as boudinaged bands and lenses, closely associated with corundum‐, spinel‐ and sapphirine‐bearing phlogopitites, diverse calcsilicate rocks and marbles within a series of biotite‐sillimanite‐cordierite gneisses of the Ihosy granulite unit in the NW of the Pan‐African Bongolava‐Ranotsara shear zone. Bimineralic anorthite + corundum domains preserve the earliest record of a polyphasic evolutionary history that includes two distinct metasomatic episodes. Probable protoliths of these bimineralic rocks were kaolinite‐rich sediments or calcareous bauxites that were altered by Ca or Si infiltration‐metasomatism prior to or coeval with the development of the anorthite‐corundum assemblage. P–T pseudosection modelling of metapelitic gneisses suggests peak‐conditions around 800 °C and 6–7 kbar for the regional high‐grade metamorphism and deformation in the NW part of the Bongolava‐Ranotsara shear zone. The well‐annealed granoblastic‐polygonal textures indicate complete chemical and textural re‐equilibration of the foliated bimineralic rocks during this event. Subsequently, at somewhat lower P–T conditions (750–700 °C, 6 kbar), the influx of Mg‐, Si‐ and K‐bearing fluids into the anorthite‐corundum rocks caused significant metasomatic changes. In zones infiltrated by ‘primary’ potassic fluids, the bimineralic assemblage was completely replaced by phlogopite and Mg‐Al minerals, thereby producing corundum‐, spinel‐ and sapphirine‐bearing phlogopitites. Further advance of the resulting ‘residual’ Mg‐ and Si‐bearing fluids into anorthite‐corundum domains led to partial to complete replacement of corundum porphyroblasts by spinel, spinel + sapphirine or sapphirine, depending on the activities of the solutes. The static textures developed during this second metasomatic episode suggest fluid influx subsequent to intense ductile deformation in the Bongolava‐Ranotsara ductile shear zone c. 530–500 Ma ago.