Magnetite/Ilmenite–series Classification and Magnetic Susceptibility of the Mesozoic-Cenozoic Batholiths in Peru

Abstract: Plutonic rocks of the Coastal Batholith of Peru were evaluated in terms of the granitoid-series classification using the bulk ferric/ferrous ratio from the literature and new measurements of magnetic susceptibility. The batholith is largely composed of magnetite-series plutonic rocks; the magnetite series make up 85% by number of chemical analyses (n=130) and 80% by measurement of magnetic susceptibility (n=210). The ilmenite-series rocks are mostly found in the felsic facies of the batholith. Asymmetrical distribution of magnetic susceptibility is not clear as in the Japanese Islands and Peninsular Range Batholith, but the magnetic susceptibility may decreases continentward (i. e., Peninsular Range type).
The Cordillera Blanca Batholith and stocks are also composed of mainly magnetite series plutonic rocks, but ilmenite-series rocks may be more predominant than in the Coastal Batholith, which is also indicated by the presence of Sn and W mineralizations.     

Microtextures of opaque inclusions: their use as indicators for hiatuses during garnet porphyroblast growth

Within an analysed garnet porphyroblast, opaque inclusions imaged with the backscatter facility of a scanning electron microscope show different microtextures depending on their position within the porphyroblast. Three different zones can been distinguished: Zone 1 contains a Ti‐rich magnetite that has decomposed to a symplectite of fine and narrowly spaced exsolution lamellae of ilmenite and magnetite. Zone 2 shows a Ti‐rich magnetite symplectite with thicker and more widely spaced exsolution lamellae of ilmenite and magnetite. Within zone 3, Ti‐rich magnetite symplectite has totally been replaced by recrystallized magnetite crystals bordered by a thin ilmenite rim. Similar microtextures within ulvöspinel‐rich magnetite have elsewhere shown to be the result of an increase in oxidation and rate of diffusion. During metamorphism of metapelites, such an increase can be reasonably envisaged because of dehydration reactions progressing during rising temperatures, and this has occurred during the overgrowth of the three different microtextures by the garnet porphyroblast. Because the microtextures are homogeneous within the three different zones, it is deduced that the oxidation reaction rate of the opaque inclusions was substantially lower than the garnet growth rate. As a consequence, hiatuses in the garnet growth history must have occurred between the evolution from one microtexture to the next. A comparison between the inclusion trail geometry and the microtextural zone boundaries shows a perfect coincidence between these and the sites where inclusion trails become strongly deflected and truncated. This correlation confirms that, in the studied case, sharp microstructural boundaries (as truncation zones or deflection zones) coincide with growth hiatuses. The study therefore highlights the potential use of opaque inclusions to confirm or reject the occurrence of growth hiatuses within garnet porphyroblasts, especially in cases where discontinuities in the inclusion trail patterns are otherwise arbitrarily associated with growth hiatuses.     

广西海岸带有用重矿物粒级分布

从广西海岸带自西向东依次选取了13个具有代表性的矿物样品进行了有用矿物粒级分析,研究了广西海岸沉积物钛铁矿、电气石、锆石在不同粒级中含量的分布变化,结果表明:钛铁矿和电气石粒级分布相对较宽,在3.5φ～4.0φ粒度区间内相对百分含量最高;锆石粒级分布窄,高度集中在3.5φ～4.5φ,优势粒级是3.5φ～4.0φ,有个别样品在4.0φ～4.5φ之间锆石颗粒分数较高。除2.0φ～4.0φ粒级之外,大部分样品的有用矿物在1.5φ～2φ和4.0φ～4.5φ之间存在一定量的分布,在这两个粒级中合计,钛铁矿颗粒分数为7.15%、电气石颗粒分数为5.34%、锆石则颗粒分数为17.58%,这一结果意味着重矿物分析选取2.0φ～4.0φ粒级是合理的,但具体到矿种的资源评价上是有一定偏差的,而且有的情况下这种偏差还不小。在广西海岸带重矿物资源评价中,要具体考虑到不同重矿物在1.5φ～4.5φ不同分布有利于减少偏差。     

ICPAES法同时测定钛铁矿中钾钠钙镁钡锶锌

采用HClHNO3HF HClO4,HClHNO3HF H2SO4敞口和HClHNO3HFHClO4,HClHNO3HF H2SO4微波消解4种方法溶矿,利用电感耦合等离子体发射光谱（ICPAES）测定,建立了钛铁矿中钾、钠、钙、镁、钡、锶、锌等元素的同时测定方法。对电感耦合等离子体发射光谱仪测定的最佳仪器条件及分析谱线进行了选择,并对钛、铁基体的影响、溶矿提取酸度以及溶矿残渣进行了研究。实验结果表明：采用HClHNO3HF HClO4敞口溶矿,具有检出限低、灵敏度高,操作简便、快速等突出优点。该方法测定值与推荐值基本吻合,分析方法的精密度满足《地质矿产实验室测试质量管理规范》的要求。     

Sulfur isotopic characteristics of late Cenozoic ore deposits at the arc junction of Hokkaido, Japan

Abstract Sulfide minerals of late Cenozoic vein-type deposits of southwest Hokkaido and Kuril Islands yielded δ³⁴_CDT S values of 2 to 8 permil, which are typical green-tuff values of magnetite-series igneous terrane. Sulfides of the Kitami district of northeast Hokkaido, on the other hand, are characterized by negative δ³⁴S_CDT values, ranging from 0 to - 7 permil. This unique value among ore deposits in the late Cenozoic back-arc terranes in the Japanese Islands is considered to have resulted from extraction of ³²S enriched sulfur from the basement rocks, because of well-developed N-S fracturing in the basement, which is characteristic of the axial belt and Kitami district of Hokkaido.     

Heavy Mineral Placer Sand Deposits of Kontiagarh Area,Ganjam District,Orissa, India

The Kontiagarh placer deposit in the Ganjam district, Orissa, India extends in northeast direction having a width of 700–1000 m. A total of 187 samples were collected meterwise from 55 bore holes in a grid pattern from beach, frontal, intermediate and back dunes covering an area of approximately 1 km². Light minerals decrease in size from the beach to the back dunes, whereas the size distribution of heavy minerals in the beach and dunes is more or less uniform. The average heavy mineral content in the beach and dunes vary from 9.38% to 24.20%. The heavy minerals are ilmenite, garnet, sillimanite, rutile, monazite, and zircon with trace amounts of magnetite, hornblende, diopside, sphene, tourmaline, and epidote. Heavy minerals are mostly less than 350 µm in size, with a peak distribution in the range between 180 and 125 µm. Ilmenite shows exsolution intergrowth with hematite. Mineral chemistry of ilmenite, hematite, leucoxene, magnetite, monazite and sillimanite are examined by EPMA. Leucoxene is lower in Fe and higher in Ti, Al, Cr and V than ilmenite. The litho‐units of the Precambrian Eastern Ghats Mobile Belt, comprising primarily khondalite, charnockite, calc‐silicate granulite and gneiss, are the source of heavy minerals for this deposit. The bulk sample has 7.30% ilmenite, 5.24% sillimanite, 9.16% garnet, 0.18% rutile, 0.14% monazite, 0.06% zircon and 0.52% other heavy minerals. The deposit has good potential for economic exploitation of ilmenite, rutile, sillimanite, monazite, zircon and garnet.     

Mineralogical Characteristics of the Separated Magnetic Rutile of the Egyptian Black Sands

The Egyptian black sands contain several economic minerals, such as ilmenite, magnetite, garnet, zircon, rutile and monazite. During the concentration and separation of a high-grade rutile concentrate a bulk magnetic fraction is obtained. This fraction is composed mainly of opaques, titanhematite, ilmenite–titanhematite exsolved intergrown grains, magnetic leucoxene in addition to chromite, and magnetic rutile. The magnetic rutile occupies 6 wt.% of the bulk magnetic fraction or approx. 4 wt.% of the original rutile content in the raw sands. Most of magnetic rutile crystals are contaminated with opaque inclusions, staining-coating and/or composite locked grains. This magnetic rutile has a magnetic range from strongly paramagnetic to very weak paramagnetic. Electron microprobe analysis for twenty-three magnetic rutile grains identified mineral components of rutile, titanhematite, pseudorutile, leached pseudorutile and ilmenite in decreasing order of abundance. Some other inclusions are also detected in the different magnetic rutile grains. They are most probably garnet, silica, amphibole, ilmenite, feldspar, mica and zircon. The presence of these inclusions reflect the derivation of magnetic rutile of various crystalline igneous and metamorphic rocks. The magnetic susceptibility of magnetic rutile depends on the associated mineral components and their relative volumes in comparison to the rutile mineral component. Magnetic susceptibility of magnetic rutile is also related to both type and size of the associated mineral inclusions. The average chemical composition of the magnetic rutile is 66.34 wt.% TiO₂, 21.71 wt.% Fe₂O₃, 6.39 wt.% SiO₂, 1.80 wt.% Al₂O₃, 1.19 wt.% CaO and 0.10 wt.% Cr₂O₃. Thus, the contamination of magnetic rutile in the non-magnetic rutile concentrate would decrease the market value of the rutile concentrate. Alternatively these magnetic rutile grains are recommended to be blended with magnetic leucoxene or some types of ilmenite concentrate to improve the overall marketable specifications especially for both of Ti, Fe and Cr contents.     

雷州半岛英峰岭新生代玄武岩中的捕虏巨晶

广东雷州半岛英峰岭火山岩中的巨晶包括石榴子石、单斜辉、斜长石和钛铁矿。石榴子石和单斜辉石以富铁低镁为特征,Ｍｇ＾＃分别为０．２８～０．４６和０．４７～０．５８。同时,石榴子石富含ＣａＯ（７．０２％和低Ｃａ为特征,属三元长石类的含钾更长石（Ａｎ３０．３～１５．２Ａｂ６４．７～７４．８Ｏｒ５．０～１０．０）。石榴子石、单斜辉石、斜长石和钛铁矿巨晶各自具有明显的成分变化,而且相互间具有相关性,它们是由一     

中—酸性侵入岩中钛铁矿的锰含量及其岩石成因意义

美国西部和中国东部15个中性侵入岩和I型花岗岩中的钛铁矿绝大部分属氧化出溶型,主要形成温度为755°—450℃。氧化出溶型钛铁矿的锰含量与样品中石英加碱性长石含量呈正相关,与铁镁硅酸盐矿物含量和该含量与磁铁矿含量之比值呈负相关。原始岩浆成分、结晶方式及结晶过程的氧逸度/水逸度比值对锰的晶体/熔体分配系数产生直接影响,是制约钛铁矿中锰含量的主要因素。钛铁矿中锰含量有助于估测主要造岩矿物丰度,判别某些酸性火山岩和次火山岩,推测分离结晶方式及结晶过程的物理化学条件。     

钛铁矿中二氧化硅的硅钼蓝分光光度法测定

用氢氧化钠-过氧化钠混合熔剂分解样品，酸提取后加入亚硫酸钠水浴加热消除过氧化氢和大量钛的干扰，硅钼蓝分光光度法快速测定钛铁矿中二氧化硅的含量。经国家一级标准物质分析验证，结果与标准值相符，相对标准偏差（RSD，n=7）为0．34％～2．24％。