Mineralogy and chemical distribution study of placer cassiterite and some associated new recorded minerals, east Rosetta, Egypt

Highly purified picked minerals of cassiterite and associated new recorded minerals were chemically and mineralogically investigated. Most of the investigated cassiterite exhibits homogeneous grains without obvious zoning. The analyzed cassiterites have more than 98 wt.% SnO₂, which reveal clearly their considerable purity. Minor gold with traces of ferrotapiolite, cinnabar, native lead, chromite, and chevkinite are well detectable within the obtained cassiterite concentrate. The origin of the present cassiterite and the associated minerals is also discussed. The variation in color and grain size of cassiterite may be attributed to the various lithology and/or areas drained by the River Nile. The color of cassiterite is appeared to be intensified with increased Nb and Fe contents. Three categories of cassiterites are identified, (a) Ta₂O₅-rich (0.46–2.65 wt.%); (b) TiO₂-rich (0.42–1.41 wt.%), and (c) Ta₂O₅-Nb₂O₅-Fe₂O₃ rich one (Ta₂O₅:0.42–3.58 wt.%, Nb₂O₅: 0.7–1.98 wt.% and Fe₂O₃: 0.56–1.02 wt.%). Sn is usually substituted by Ta, Nb, and Fe. Minor gold with traces of new recorded ferrotapiolite, cinnabar, native lead, chromite, and chevkinite are well detectable within the obtained cassiterite concentrate. Ferrotapiolite is composed mainly of Ta, Fe, and Nb with minor Ti, Sn, and Mn, which similar to that derived from pegmatites and quartz veins. Chevkinite is generally enriched in Ti, Fe, and LREEs and depleted in P, Th, and U which analogous to that crystallized from felsic igneous rock suites.     

Contrasting evolution of low-P rare metal granites from two different terranes in the Hoggar area, Algeria

Two mineralogically different rare metal granites located in two distinct terranes from the Tuareg area are compared: the Tin-Amzi granite in the north of the Laouni Terrane and the Ebelekan granite in the Assodé–Issalane Terrane.The Tin-Amzi granite is enclosed within Eburnean granulitic gneisses, and consists of albite, quartz, protolithionite, K-feldspar and topaz granite (PG). The accessory minerals include columbite tantalite, U- and Hf-rich zircon, Th-uraninite, wolframoixiolite and wolframite. This facies is characterised by a mineralogical evolution from the bottom to the top underlined by a strong resorption of K-feldspar and albite and the crystalliK-feldspar of more abundant topaz and protolithionite II which is further altered in muscovite and Mn-siderite. It is underlain by an albite, K-feldspar, F-rich topaz, quartz and muscovite granite (MG), with W–Nb–Ta oxides, wolframite, Nb-rutile, zircon and scarce uranothorite as accessories.The Ebelekan granite intrudes into a coarse-grained biotite granite enclosed within upper amphibolite-facies metasediments. It comprises a zinnwaldite, albite, topaz porphyritic granite (ZG) with “snow ball” quartz and K-feldspar. The accessories are zircon, monazite, uranothorite, Ta bearing cassiterite, columbite tantalite and wodginite. It is capped by a banded aplite-pegmatite (AP).The geochemistry of Tin-Amzi and Ebelekan granites is nearly comparable. Both are peraluminous (A/CNK=1.10–1.29; ASI=1.17–1.31), sodolithic and fluorine rich with high SiO₂, Al₂O₃, Na₂O+K₂O, Rb, Ga, Li, Ta, Nb, Sn and low FeO, MgO, TiO₂, Ba, Sr, Y, Zr and REE contents. These rare metal Ta bearing granites belong to the P-poor subclass, relating to their P₂O₅ content ( 0.03–0.15 wt.%). Nevertheless, they are distinguished by their concentration of W, Sn and Ta. The Tin-Amzi granite is W–Ta bearing with high W/Sn ratio whereas the Ebelekan granite is Ta–Sn bearing with insignificant W content.At Tin-Amzi the W–Nb–Ta minerals define a sequence formed by W-columbite tantalite followed by wolframoixiolite and finally wolframite showing the effect of hydrothermal overprinting with an extreme W enrichment of the fluids. At Ebelekan, the Sn–Nb–Ta oxides follow a Mn sequence: manganocolumbite→manganotantalite→wodginite+titanowodginite→cassiterite that represents a trend of primary crystallisation resulting from progressive substitution Fe→Mn and Nb→Ta during the magmatic fractionation.     

Tin-carrier minerals in metaluminous granites of the western Nanling Range (southern China): Constraints on processes of tin mineralization in oxidized granites

Huashan, Guposhan and Qitianling are three similar and representative metaluminous A-type tin granites in the western Nanling Range, China. They all have a high oxidization state with magnetite as the dominant Fe–Ti oxide. This study presents an understanding of systematic mineralogy of Sn-bearing minerals (biotite, titanite, magnetite and cassiterite) in the three granites. Biotite has an annite composition and both electron-microprobe and LA-ICP-MS analyses indicate trace amounts of tin in biotite (approximately 100–20 ppm). Chloritization of biotite is accompanied by formation of Sn-rich rutile and cassiterite. Titanite has a long history of crystallization from the early-magmatic stage through the late-magmatic stage to the hydrothermal stage. Owing to its solid-solution relationship with malayaite (CaSnSiO₅), titanite always contains tin to various extents. Early-magmatic titanite contains about 0.5 wt.% SnO₂, while the late-magmatic titanite is markedly enriched in tin (on average 14.8 and 3.4 SnO₂ in titanite from the Qitianling and Huashan granites, respectively). Magnetite grains typically display a trellis structure with ilmenite lamellae, where microinclusions of cassiterite (<1 μm in size) are present. This is likely consistent with features of the “oxy-exsolution” process of Sn-bearing titanomagnetite precursor. Cassiterite may be observed as late-magmatic phase, but most commonly appears as an alteration product of other primary minerals. All tin-bearing minerals in the three granites record a complete process of tin mineralization in granite. The features of tin in primary biotite, titanite and magnetite reflect an initial enrichment during the early stage of magmatic crystallization of the Huashan, Guposhan and Qitianling granites. Association of interstitial Sn-titanite and cassiterite suggests further tin enrichment related to fractional crystallization of granitic magmas. Fluids and alteration of primary minerals play an important role in the leaching, concentration and transportation of Sn during hydrothermal processes, which favors vein-type Sn mineralization.     

黑龙江省漠河市782高地铌多稀有金属矿床的地质特征及找矿标志

黑龙江省漠河市782高地铌多稀有金属矿床位于漠河市北偏西38 km处,是近年来在大兴安岭北部发现的一个铌资源量达到中型规模的矿床,也是至今在黑龙江省内发现的唯一的铌多稀有金属矿床。矿床的大地构造位置处于额尔古纳地块,漠河前陆盆地与富克山-兴华变质基底杂岩的交接部位。区域地球化学分析显示与稀有金属有关的元素组合为U、Th、Nb、Be;区域地球物理特征表明该区处于航空放射性高场中。勘查结果表明,稀有金属矿化产于晚寒武世—早奥陶世碱长花岗岩顶部,呈似层状、板状产出,矿床Nb2O5、Ta2O5、Y2O3、ZrO2平均品位分别为0.12%、0.0087%、0.108%、0.80%,最高品位分别为0.529%、0.0354%、0.309%、1.18%。主要围岩蚀变为钠长石化,铌等稀有金属矿化与其呈正相关关系。初步研究表明,该矿床形成于早奥陶世拉张伸展背景之下,与该时期板块裂解有关,为一处岩浆晚期自交代矿床。文章旨在分析和讨论该矿床的产出地质环境、矿床地质特征和找矿标志,为额尔古纳地块相似类型的矿床的勘查提供理论参考。     

Features of tin mineralization in carbonate deposits,as in Eastern Siberia

The paper shows differences in character of tin mineralization in skarns which develop after dolomites (magnesial) and limestones (limy). Cassiterite is not deposited in the former, since because of the relatively high alkalinity and the high activity of boron (B₂0₃), tin preferentially enters the crystal structures of Sn-borates such as hulseite, ludwigite, and nordenskioldine rather than cassiterite. In limy skarns, tin is largely dissipated in garnets (up to 0.62%), axinites (up to 0.2%), and other minerals. It is only at the end of the appropriate stage, at relatively high acidity, that tin is deposited as cassiterite in magnetitic and amphibole-axinitic skarns. The latter are very favorable for development of economic ore bodies. Conclusions are made to the effect that a) tin is present in the postmagmatic solutions during each of the three stages, and also b) concerning optimum conditions for deposition of tin minerals in any stage (depending on magnitudes of Eh and pH environments and B₂O₃ and S^2- activities. -- Author.     

Sn-polymetallic greisen-type deposits associated with late-stage rapakivi granites, Brazil: fluid inclusion and stable isotope characteristics

Tin-polymetallic greisen-type deposits in the Itu Rapakivi Province and Rondônia Tin Province, Brazil are associated with late-stage rapakivi fluorine-rich peraluminous alkali-feldspar granites. These granites contain topaz and/or muscovite or zinnwaldite and have geochemical characteristics comparable to the low-P sub-type topaz-bearing granites. Stockworks and veins are common in Oriente Novo (Rondônia Tin Province) and Correas (Itu Rapakivi Province) deposits, but in the Santa Bárbara deposit (Rondônia Tin Province) a preserved cupola with associated bed-like greisen is predominant. The contrasting mineralization styles reflect different depths of formation, spatial relationship to tin granites, and different wall rock/fluid proportions. The deposits contain a similar rare-metal suite that includes Sn (±W, ±Ta, ±Nb), and base-metal suite (Zn–Cu–Pb) is present only in Correas deposit. The early fluid inclusions of the Correas and Oriente Novo deposits are (1) low to moderate-salinity (0–19 wt.% NaCl eq.) CO₂-bearing aqueous fluids homogenizing at 245–450 °C, and (2) aqueous solutions with low CO₂, low to moderate salinity (0–14 wt.% NaCl eq.), which homogenize between 100 and 340 °C. In the Santa Bárbara deposit, the early inclusions are represented by (1) low-salinity (5–12 wt.% NaCl eq.) aqueous fluids with variable CO₂ contents, homogenizing at 340 to 390 °C, and (2) low-salinity (0–3 wt.% NaCl eq.) aqueous fluid inclusions, which homogenize at 320–380 °C. Cassiterite, wolframite, columbite–tantalite, scheelite, and sulfide assemblages accompany these fluids. The late fluid in the Oriente Novo and Correas deposit was a low-salinity (0–6 wt.% NaCl eq.) CO₂-free aqueous solution, which homogenizes at (100–260 °C) and characterizes the sulfide–fluorite–sericite association in the Correas deposit. The late fluid in the Santa Bárbara deposit has lower salinity (0–3 wt.% NaCl eq.) and characterizes the late-barren-quartz, muscovite and kaolinite veins. Oxygen isotope thermometry coupled with fluid inclusion data suggest hydrothermal activity at 240–450 °C, and 1.0–2.6 kbar fluid pressure at Correas and Oriente Novo. The hydrogen isotope composition of breccia-greisen, stockwork, and vein fluids (δ¹⁸O_quartz from 9.9‰ to 10.9‰, δD_H2O from 4.13‰ to 6.95‰) is consistent with a fluid that was in equilibrium with granite at temperatures from 450 to 240 °C. In the Santa Bárbara deposit, the inferred temperatures for quartz-pods and bed-like greisens are much higher (570 and 500 °C, respectively), and that for the cassiterite-quartz-veins is 415 °C. The oxygen and hydrogen isotope composition of greisen and quartz-pods fluids (δ¹⁸O_qtz-H2O=5.5–6.1‰) indicate that the fluid equilibrated with the albite granite, consistent with a magmatic origin. The values for mica (δ¹⁸O_mica-H2O=3.3–9.8‰) suggest mixing with meteoric water. Late muscovite veins (δ¹⁸O_qtz-H2O=−6.4‰) and late quartz (δ¹⁸O_mica-H2O=−3.8‰) indicate involvement of a meteoric fluid. Overall, the stable isotope and fluid inclusion data imply three fluid types: (1) an early orthomagmatic fluid, which equilibrated with granite; (2) a mixed orthomagmatic-meteoric fluid; and (3) a late hydrothermal meteoric fluid. The first two were responsible for cassiterite, wolframite, and minor columbite–tantalite precipitation. Change in the redox conditions related to mixing of magmatic and meteoric fluids favored important sulfide mineralization in the Correas deposit.     

The Pemali tin deposit,Bangka, Indonesia

The Pemali tin deposit is located in a Triassic granite pluton the magmatic evolution of which is characterized by a decrease of compatible Ca, Mg, Ti, P and Zr in the sequence: medium- to coarse-grained biotite granite, megacrystic medium-grained biotite granite, two-mica granite/muscovite granite. The tin mineralization is confined to the two-mica granite and consists of disseminated cassiterite as well as greisen-bordered veins. The highly evolved muscovite granite is tin-barren and is distinguished from the two-mica granite by its low mica content and low loss-on-ignition values. The fluid inclusions in quartz and fluorite of the two-mica granite and of the greisen homogenize in the 115–410 °C temperature range; the salinities are in the range of 0.4–23 equiv wt% NaCl and the CO₂ concentrations are < 2 mole%.     

Petrogenesis of the granite related to the Baishaziling Sn deposit,Dayishan ore field,Southern China

The Baishaziling greisen-type tin deposit is located in the Dayishan ore field, Nanling Range, Southern China. In this study, for the first time, we present both zircon and cassiterite UPb dating, whole-rock elements, zircon LuHf and apatite Nd isotopic compositions to better constrain the petrogenesis of granite and its genetic link with Sn mineralization. Zircon UPb ages of fine-grained granite and coarse-grained granite are 154 ± 1.8 Ma and 153 ± 2.1 Ma, respectively, which are consistent with the cassiterite UPb dating of 154 ± 5.4 Ma, implying genetic relationship between the Baishaziling granite and tin metallogenesis. The Baishaziling granites exhibit high SiO₂, K₂O + Na₂O, Zr + Nb + Ce + Y contents, low P₂O₅ and Sr contents, and high ratios of Ga/Al, ^TFeO/(^TFeO + MgO), implying A-type granite affinity with characteristics of high-K calc-alkaline and weakly peraluminous. The zircon ε_Hf(t) values and apatite ε_Nd(t) values of the granite vary from ?4.46 to ?1.81 and ?8.37 to ?7.10, with two-stage Hf and Nd model ages of 1.40 to 1.50 Ga and 1.52 to 1.64 Ga, indicating that they were generated by the partial melt of the Proterozoic basement with the involvement of mantle magma. In addition, formation of Dayishan granite was likely associated with an intraplate extensional setting caused by the subduction of the Palaeo-Pacific plate. The Baishaziling reduced granites have high stannum and boron contents, which are in favor of the tin mineralization.     

The combined effects of ƒO2 and melt composition on SnO2 solubility and tin diffusivity in haplogranitic melts

The diffusion profile method has been employed to measure tin diffusion coefficients and SnO₂ solubility in water-saturated, peralkaline to peraluminous haplogranitic melts at 850°C, 2 kbar, and log ƒ_O2 conditions ranging from FMQ - 0.57 to FMQ + 3.49. At reduced conditions cassiterite is highly soluble and tin is present dominantly as a Sn²⁺ species, whereas at oxidized conditions SnO₂ is much less soluble, and tin is present dominantly as a Sn⁴⁺ species. There is a strong melt composition control on SnO₂ solubility; solubilities are at a minimum at the subaluminous composition, increase strongly with alkali content in peralkaline compositions and weakly with Al content in peraluminous compositions. In the case of the latter, this increase can only be distinguished at reduced conditions, e.g., at a log ƒ_O2 of FMQ - 0.57 cassiterite solubility increases from 2.78 to 4.11 wt% SnO₂ for melt with Al/(Na + K)compositions (A.S.I.) of 1.0 and 1.2, respectively. At oxidized conditions SnO₂ solubility is 500 ppm for both the A.S.I. 1.0 and 1.2 compositions. By comparison Sn0₂ solubilities in the most peralkaline composition investigated range from 3.94 wt% to -10 wt% Sn02, for the most oxidized to the most reduced conditions, respectively. Thermodynamic modelling of the data indicates that the Sn⁴⁺/ΣSn ratio in the melt is also at a minimum at the subaluminous composition, ranging from 0.4 at log ƒ_O2 of FMQ + 3.49 to 0.01 at FMQ - 0.57. Over the same log foZ range the Sn⁴⁺/ΣSn ratio for the A.S.I. 0.6 composition ranges from 0.98 to 0.4 and for the A.S.I. 1.25 composition, from 0.8 to 0.02.Tin diffusivity is dependent on both f_O2 and melt composition. The effective binary diffusion coefficient of tin at reduced conditions is approximately 10^−7.5 cm²/sec for the peraluminous compositions and 10^−8.2 cm²/sec for the peralkaline compositions. At oxidized conditions these values decrease to approximately 10^−8.2 and 10^−9.0 cm²/sec, respectively. These are interpreted to reflect relatively fast diffusion where Sn²⁺ is the dominant valence and tin in this case behaves similar to a network modifier and relatively slow diffusion where Sn⁴⁺ is dominant and tin likely has a lower coordination number. Alternatively, the coordination of Sn²⁺ and Sn⁴⁺ is the same, but the bond strengths are different. At fixed f_O2 the faster diffusivity in the peraluminous compositions reflects the lower Sn⁴⁺/Sn²⁺ ratio. The fact the Sn⁴⁺/Sn²⁺ ratio in melts varies greatly with ƒ_O2 at redox conditions near FMQ suggests that the partitioning behaviour of tin possibly changes during the evolution of an igneous suite in general and of a peraluminous granite suite in particular.     

包古图斑岩铜矿床的钛矿物特征及其成因意义

在大量薄片鉴定的基础上,本文对包古图斑岩铜矿的钛矿物组合特征及其形成机制进行了研究。主要钛矿物为榍石、钛铁矿和金红石。其中榍石在成岩期和成矿期均有形成,但主要见于成矿期的钾化阶段,与钾长石、黑云母共生;在后期的青盘岩化和沸石化阶段也有出现,但含量相对较少。成岩期的钛铁矿很少见,成矿期各蚀变阶段均有分布,但最常见于钾化阶段。金红石仅见于成矿期,钾化阶段早期可与钾长石、黑云母、石英等共生,还见有被榍石包裹的细粒金红石,青盘岩化阶段亦有形成,与绿泥石共生。这些钛矿物最常出现于黑云母颗粒内部或其附近,这可能是斑岩型矿床的特征之一。榍石SiO₂和CaO含量与理论值接近,TiO₂偏低;钛铁矿均含锰,MnO含量1.97%~4.49%,还见有锰钛铁矿;金红石含有一定量的SiO₂和FeO^T,个别颗粒还含有少量Al₂O₃、MgO和P₂O₅。钛矿物组合特征表明包古图含矿斑岩为I型,形成于较高氧逸度环境,侵位深度不大。     

The origin of the Tongkeng-Changpo tin deposit,Dachang metal district,Guangxi, China: clues from fluid inclusions and He isotope systematics

Tongkeng-Changpo is the largest tin deposit within the giant Dachang polymetallic tin ore field in Guangxi, southern China, which is part of a large skarn system associated with Cretaceous granitoids. The Tongkeng-Changpo mineralization consists of veins and stockworks in the upper levels and replacement stratiform orebodies (mantos) at lower levels. Based on textural relationships, three major mineralizing stages can be recognized: stage I with cassiterite, sulphides, stannite, tourmaline, and quartz; stage II with cassiterite, sulphides, sulphosalts, quartz, and calcite; and stage III with calcite as the main phase. The study of fluid inclusions has shown that there are two main fluid types: CO₂ and NaCl-H₂O. Homogenization temperatures are 270 to 365°C, 210 to 240°C, and 140 to 190°C for stages I, II, and III, respectively. Salinities range from 1 to 7 wt.% NaCl equiv. in the early ore stage and 3 to 10 wt.% NaCl equiv. in the late stages. Laser Raman Spectroscopy indicates that the inclusion fluids in stages I and II were of carbono-aqueous composition, with minor amounts of CH₄ and H₂S, whereas those in stage III were aqueous. Helium isotopic analyses of inclusion fluids indicate that the ³He/⁴He ratios in the ore veins are in between 1.2 to 2.9 Ra (Ra = 1.4 × 10⁻⁶, modern atmospheric ratio), and range from 1.6 to 2.5 Ra in the stratiform orebodies. This range of ³He/⁴He ratios is significantly higher than that of crustal fluids (0.01–0.05 Ra). The similar characteristics of fluid inclusions and their He isotopic composition, as well as age constraints, indicate that the ore veins and stratiform orebodies of the Tongkeng-Changpo deposit formed from the same hydrothermal system, likely related to granite intrusions of the Mesozoic Yanshanian tectono-thermal event. In addition, the high R/Ra ratios indicate a mantle contribution in the ore fluids.     

Malayaite and tin‐bearing silicates from a skarn at Doradilla via Bourke,New South Wales

An extensive complex zoned skarn is developed at the contact of a leucoadamellite intrusive at Doradilla, NW New South Wales. The skarn is a disequilibrium assemblage resulting from a progressive sequence of replacement of a carbonate precursor. Early grossular‐clinopyroxene rocks are replaced by andradite with 0.5–3.5 wt.% SnO₂ clinopyroxene and quartz. Later alteration along fractures and bedding planes of the garnet‐clinopyroxene quartz assemblage has produced calcite‐malayaite (CaSn0.95Ti0.05SiO₅) veins. The final replacement stage was the overprinting of the silicate phases by assemblages containing sulphides, cassiterite, magnetite, titanite, fluorite, biotite and chlorite. The tin content of garent increases with increasing andradite component suggesting replacement of Fe³⁺ by Sn⁴⁺. Associated clinopyroxenes contain 0.1% SnO₂. The coexistence of titanite and its tin isomorph malayaite with extremely limited solid solution indicates late stage skarn temperatures of less than 400°C.     

内蒙古赤峰维拉斯托大型锡多金属矿的地质地球化学特征

2014年发现的维拉斯托锡锌矿是继20世纪末该矿区铜锌矿之后的重要找矿进展,已控制Sn金属资源量10万t。成矿作用与隐伏花岗岩体有关,该岩体侵入于前寒武纪变质岩中。矿化类型包括岩体顶部的花岗岩型锡锌矿、岩体外侧的石英脉型锡锌矿以及外围的铜锌矿。针对花岗岩、各类矿体开展了岩石学、矿床学、主微量元素地球化学、年代学等研究,初步查明岩浆演化机制、矿床成因及三类矿化的关系。细粒斑状碱长花岗岩La-ICPMS锆石UPb年龄(139.5±1.2)Ma(MSWD=3.3)。岩石中发育多级斑晶,结晶(沉淀)顺序为钠长石→石英→钾长石→钠长石→石英、黄玉、锡石、闪锌矿。花岗岩富Si O2贫Al2O3、Ti O2、TFe2O3、Ca O等,高Rb、Cs、Nb、Ta及W、Mo、Bi、Cu、Zn、In等元素,低Sr、Ba等,钠长石An0.3,与锡钨多金属矿成矿花岗岩性质相似。岩浆晚期经历了岩浆-热液过渡阶段(浆液过渡态流体),自硅酸盐相中分离出富Si、富F和富S的流体相,分别形成花岗岩型矿石中的石英、黄玉、锡石-闪锌矿囊状体(珠滴),伴随熔融包裹体和熔流包裹体,晚期逐渐、连续地向热液阶段过渡。岩浆-热液过渡阶段在岩体顶部形成花岗岩型锡锌矿石,热液阶段在岩体外侧和外围形成石英脉型锡锌矿及铜锌矿、铅锌银矿。这些矿体连同成矿花岗岩共同构成岩浆-热液型锡多金属矿床成矿系统。锡林郭勒—赤峰地区,很多脉状铅锌银矿的成矿作用与酸性侵入岩有关,深部可能存在大规模岩浆-热液型锡(钨)多金属矿。     

Cassiterite exsolution with ilmenite lamellae in magnetite from the Huashan metaluminous tin granite in southern China

Sn⁴⁺ is generally the dominant form of tin in magnetite-series granites as shown by the presence of cassiterite or its incorporation into Ti-bearing minerals such as biotite and titanite. Little is known about the behavior of tin in magnetite. The Huashan granite is an oxidized tin granite in the Nanling Range, southern China, where it contains magnetite as the dominant Fe oxide mineral. It is included in biotite as an early phase and also as interstitial grains spatially associated with ilmenite, cassiterite, Sn-rich titanite (SnO₂ up to 5.9?wt.%), fluorite and apatite. This association indicates that tin enrichment occurred during the late stage of magma crystallization. Ilmenite lamellae display a trellis structure consistent with features of the “oxy-exsolution” process of Sn-bearing titanomagnetite precursor. Micro-inclusions of cassiterite (<1?μm in size) are found only within ilmenite lamellae. This suggests that magnetite with cassiterite inclusions is likely an indicator mineral of oxidized tin granites. Although rare in nature, Sn-bearing magnetite from weathered granites where concentrated in stream sediments, may serve as a strategic tracer for tin exploration in granite districts and in placer deposits, in general.     

Partitioning of ferric and ferrous iron between plagioclase and silicate melt

Using the thermodynamic algorithm of Sugawara (Contributions to Mineralogy and Petrology 141, 2001, p. 659–686), FeO and Fe₂O₃ concentrations in plagioclase were computed for 420 published experiments on tholeiitic, FeTi-tholeiite, calc-alkaline, and alkaline magma compositions. Estimates of the partition coefficient between plagioclase and liquid range from 0.19 to 0.92 for Fe₂O₃ and from 0.008 to 0.050 for FeO, i.e. ca. twenty times greater for Fe₂O₃ than for FeO. Partitioning of Fe₂O₃ and FeO is independent of both oxygen fugacity and plagioclase composition, contradicting the common assumption that partitioning of Fe₂O₃ correlates positively with the amount of aluminium in plagioclase. In contrast, the SiO₂-content of the magma correlates positively with the partition coefficients for Fe₂O₃ and FeO. This is ascribed to increasing activity of iron in polymerised SiO₂-rich magma. Advances of micro-beam Fe-XANES techniques allow the determination of Fe³⁺/Fe in plagioclase. Using such plagioclase data and the partition coefficients for Fe₂O₃ and FeO, the Fe₂O₃/FeO and oxygen fugacity of equilibrium magma may be estimated. As petrological examples, we estimate that the oxygen fugacity of the Palisades sill ranged from the QFM buffer to 0.5 log unit below it (QFM to QFM –0.5), the Lake County basalt from QFM to QFM –2, and Upper Zone a of the Skaergaard intrusion from QFM –1 to QFM –1.5.Editorial responsibility: I. Parsons     

Infrared reflectivity spectra of single crystal cassiterites

The infrared reflectivity spectra of two natural cassiterite single crystals from Portugal and Vietnam have been studied and analysed in the frequency range of 15–4000 cm^–1 at room temperature. The optical mode parameters are deduced by simulation of the experimental spectra using the factorised form of the dielectric function. The difference with synthetic SnO₂ spectra is a dip near 500 cm^–1 understood as the activation of an infrared forbidden E _g mode due to the amount of Fe³⁺ and Ti⁴⁺ impurities. Another result of this work is the derivation of the correct values of the static dielectric constant of cassiterite.     

云南个旧芦塘坝矿段层间矿成因及构造-岩相控矿规律

云南个旧锡矿是世界级超大型锡多金属矿床,层间矿床(锡石-硫化物)是其中重要的矿床类型之一,其成因也一直存在争议,为了进一步探讨其成因并对该类型矿床的深部勘查提供理论依据,重点对个旧锡矿高松矿田芦塘坝矿段10号矿群不同中段层间矿体的岩石矿物组构和地球化学特征进行了研究.结果表明层间矿床中的原生矿体多为块状硫化矿石,金属硫...     

Studies on tapiolite from Arehalli pegmatite, Hassan District, Karnataka

Tapiolite hosted in lithium-caesium-tantalum (LCT)-type granitic pegmatite, near Arehalli, of Holenarsipur schist belt (HSB), occurs as euhedral crystals within the muscovite books or in association with columbite-tantalite. In this study, compositional and crystallographic characteristics of the tapiolite are presented. The tapiolite is Ta-rich (79.8% Ta₂O₅) and also Fe-rich (13.1% FeO) with minor amounts of Nb (3.80% Nb₂O₅) and Mn (0.30% MnO₂), and is a Fe-end member of the FeTa₂O₆-MnTa₂O₆ series, ferrotapiolite. The tapiolite has very restricted compositional range. Calculated structural formula of the investigated tapiolite is [(Fe_0.27 Mn_0.66)_0.93 (Nb_0.45 Ta_1.57 Ti_0.004 Sn_0.03)_2.027O₆].     

湘东桐木山云英岩型锡矿床锡石及共生石英中流体包裹体研究

桐木山云英岩型锡矿床是湘东锡田锡多金属矿田中一个典型矿床,在详尽的野外考察、矿石结构观察以及流体包裹体岩相学研究的基础上,采用流体包裹体组合的研究方法,利用冷热台、激光拉曼等测试手段,对矿床中锡石中流体包裹体进行直接测定,同时开展与锡石共生的石英及切割矿体的后期石英脉石英中流体包裹体对比研究。结果显示,锡石中流体包裹体的组分、均一温度、盐度与共生的石英存在明显差异,表明锡石与石英形成的P-T-X条件不同。切割矿体的石英脉为成岩成矿后流体作用的产物。与锡石形成相关的流体为中高温、中低盐度的Na Cl-H_2O流体体系,与石英形成相关的流体为复杂的含碳流体。流体体系的冷却作用及流体与围岩的反应可能是导致锡石沉淀的主要因素,与石英形成相关的流体在演化过程中则经历了明显的流体不混溶作用。     

Partial molar volumes of oxide components in silicate liquids

Densities of 21 silicate liquids have been determined from 1,000 ° to 1,600 ° C. The compositions studied contain from two to eight oxide components and have the following ranges in composition (mole %): SiO₂, 35–79%; TiO₂, 4–36%; Al₂O₃, 5–25%; FeO, 11–41%; MgO, 7–28%; CaO, 7–35%; Na₂O, 5–50%; and K₂O, 4–20%. The compositions thus cover the upper range observed in magmas for each oxide. Precision for each determination of liquid density is always better than ±1%.Volumes/gfw (gram formula weight) calculated from the density measurements and the chemical compositions of the analyzed liquids have been combined with data on 96 silicate liquids reported in the literature. From this data set we derive, by using multiple linear regression, partial molar volumes of the components SiO₂, TiO₂, A1₂O₃, FeO, MgO, CaO, Na₂O, and K₂O at five temperatures. The standard deviation of the multiple regression is 1.8% of the molar volumes, which is considered about equal to the total errors due to compositional and instrumental uncertainties.These derived partial molar volumes have been used to calculate volumes/gfw of natural silicate liquids which are found to agree within 1% of the measured values. No compositional dependence of the partial molar volumes can be detected within the error considered to be typical of the measurements. This is further supported by the close agreement between the calculated volumes of CaMgSi₂O₆ and Fe₂SiO₂ liquids derived from the initial slopes of their fusion curves and their heats of fusion, and the volumes obtained by summing the respective partial molar volumes. The experimental data indicate that silicate liquids mix ideally with respect to volume, over the temperature and composition range of this data set.