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1.
Combined research in geochemistry and mineral chemistry of the hydrothermally altered W-Sn specialized granite of Regoufe and its derivatives in Portugal was undertaken to gain insight in the mineralogical changes associated with hydrothermal processes within a single granite cupola. Over 1000 unpolished rock sections were analyzed by automated X-ray fluorescence spectrometry (XRF). On the basis of the XRF data, a small number of these same sections was selected for investigation by electron probe microanalysis. The study focuses on fourteen elements of interest that are measurable with the chosen techniques. Major pervasive alteration within the Regoufe granite is virtually contemporaneous with mineralization in the form of Sn- or W-bearing quartz veins. Two phases of hydrothermal activity are discerned, characterized by different element associations. Fluids of the first phase were especially rich in Sn, Cs and F, whereas the second phase was marked by a W-Ta-Nb-Rb association and presumably carried less F. Phosphorus probably was an important fluid component in both phases. The fluids are inferred to have fractionated from a related granitic magma at depth.Tin, W, Nb and Ta are mainly found as substitutions or inclusions in biotite in the least altered part of the Regoufe granite. Tantalo-niobian rutile is an important control for the distribution of Nb. Tin occurs in rutile and rarely as cassiterite. Muscovitization caused leaching of Ti and Zr from the granite. Tin, supplied or mobilized by the hydrothermal fluids, behaves differently from W, Nb and Ta. In the most altered rocks, muscovite hosts significant amounts of Sn. Whereas Sn is still related to Cs and Ti, elements that probably represent altered biotite, W, Ta and Nb are related to newly formed Rb-rich muscovite. Columbitetantalite was detected embedded in late muscovite. In contrast to Sn, the fluid-supplied W was predominantly deposited as wolframite in quartz veins and the altered granite is not enriched in W compared to the relatively unaltered rocks. Strontium is preferentially hosted by K-feldspar in the least altered granite; Ca in this rock is still partly contained in albite. With increasing albitization and muscovitization, Sr and Ca were released and are partly bound in newly formed apatite. The primary magmatic apatites have near-ideal formula compositions, but mobilization of P during hydrothermal activity resulted in the formation of Mn-rich apatite in all parts of the granite, and Sr- and probably Li-rich varieties in the most altered rocks. Eosphorite, and scorodite as the oxidation product of arsenopyrite, were also formed as a result of P mobilization. In the least altered rocks, Cs is mainly contained in biotite. In the most altered granite and aplitic rocks, enrichment of Cs and Rb is evenly distributed over K-feldspar and micas. The processes that lead to increased Rb are partly independent of Cs enrichment, and apparently related to the W-Nb-Ta mineralization event, separate from the preceding Sn mineralization. 相似文献
2.
Summary The strongly peraluminous, P- and F-rich granitic system at Podlesí in the Krušné Hory Mountains, Czech Republic, resembles
the zonation of rare element pegmatites in its magmatic evolution (biotite → protolithionite → zinnwaldite granites). All
granite types contain disseminated Nb-Ta-Ti-W-Sn minerals that crystallized in the following succession: rutile + cassiterite
(in biotite granite), rutile + cassiterite → ferrocolumbite (in protolithionite granite) and ferrocolumbite → ixiolite →
ferberite (in zinnwaldite granite). Textural features of Nb-Ta-Ti-W minerals indicate a pre-dominantly magmatic origin with
only minor post-magmatic replacement phenomena. HFSE remained in the residual melt during the fractionation of the biotite
granite. An effective separation of Nb + Ta into the melt and Sn into fluid took place during subsequent fractionation of
the protolithionite granite, and the tin-bearing fluid escaped into the exocontact. To the contrast, W contents are similar
in both protolithionite and zinnwaldite granites.
Although the system was F-rich, only limited Mn-Fe and Ta-Nb fractionation appeared. Enrichment of Mn and Ta was suppressed
due to foregoing crystallization of Mn-rich apatite and relatively low Li content, respectively. The content of W in columbite
increases during fractionation and enrichment in P and F in the melt. Ixiolite (up to 1 apfu W) instead of columbite crystallized
from the most fluxes-enriched portions of the melt (unidirectional solidification textures, late breccia). 相似文献
3.
在栗木矿田沟挂垒钻孔深部发现花岗岩体下部有厚度大于190m的二云母花岗岩,通过对二云母花岗岩以及上部白云母花岗岩的岩相学研究和分析对比,认为该区第二阶段的花岗岩上部为白云母花岗岩,在深部相变为二云母花岗岩,岩体中、上部的白云母花岗岩相主要是二云母花岗岩不同程度蚀变交代形成的,而不是岩浆就地重力分离结晶形成的,其蚀变程度相当于该区含矿花岗岩体矿化蚀变带下部的钾长石化带。无斑的白云母花岗岩相在钾长石化带中蚀变最强,最靠近主矿化蚀变带,有重要的找矿意义。各岩相的化学分析表明,W、Sn、Nb、Ta的矿化富集可能与岩浆气液分离作用或深部岩浆分异有关。 相似文献
4.
云南个旧锡矿是全球最大的锡多金属矿床之一,但矿区内同时代花岗岩成锡矿潜力差异显著,其控制因素仍不清楚。本文选取贫矿的龙岔河似斑状花岗岩和成锡矿的老厂-卡房(后文简称老-卡)花岗岩为研究对象,通过全岩地球化学成分和黑云母成分分析,系统研究个旧矿区不同花岗岩成锡矿潜力差异的控制因素。测试结果表明,龙岔河花岗岩和老-卡花岗岩具有相似的、以表壳物质为主的岩浆源区以及较高的初始熔融温度,表明岩浆源区和熔融条件不是控制二者成矿潜力差异的主要原因。黑云母成分显示老-卡花岗岩和龙岔河花岗岩均具有较低的氧逸度,岩浆演化过程中锡为不相容元素,有利于锡在残余熔体中富集,表明氧逸度条件也不是导致成矿潜力差异的关键因素。龙岔河花岗岩发育角闪石、榍石、黑云母,而老-卡花岗岩发育岩浆白云母,指示后者分异程度更高。此外,与龙岔河花岗岩相比,老-卡花岗岩具有富硅,贫钛、铁、镁、钙和稀土元素特征,稀土元素呈现“海鸥式”配分模式,并且具有较低的Nb/Ta、Zr/Hf、K/Rb和较高的Rb/Sr比值,同样指示老-卡花岗岩具有更高的结晶分异程度。并且相比于龙岔河花岗岩为准铝质的特征,老-卡花岗岩的过铝质特征有利于锡分配进入岩浆出溶的流体相中富集成矿。因此,岩浆性质和演化程度是导致个旧地区不同花岗岩成矿潜力差异的主要原因,龙岔河花岗岩形成锡矿化的潜力较小。 相似文献
5.
New trace element data were obtained by ICP-MS for 58 samples representing eight intrusive phases of the Raumid granite Pluton. All of the rocks, except for one sample that was deliberately taken from a greisenized zone, were not affected by postmagmatic fluid alteration. The sequential accumulation of incompatible trace elements (Rb, Ta, Nb, Pb, U, and others) in the Raumid Pluton from the early to late phases coupled with a decrease in incompatible element contents (Sr, Eu, Ba, and others) indicates a genetic link between the granites of all phases via fractional crystallization of a granite melt. The REE distribution patterns of final granite phases are typical of rare-metal granites. The Ta content in the granites of phase 8 is only slightly lower than that of typical rare-metal granites. Greisenization disturbed the systematic variations in trace element distribution formed during the magmatic stage. The ranges of trace element contents (Rb, Sr, Ta, Nb, and others) and ratios (Rb/Sr, La/Lu, Eu/Eu*, and others) in the Raumid granite overlap almost entirely the ranges of granitic rocks of various compositions, from the least differentiated with ordinary trace element contents to rare-metal granites. This indicates that the geochemical signature of rare-metal granites can develop at the magmatic stage owing to fractional crystallization of melts, which is the case for the melt of the Raumid granite. 相似文献
6.
Bristen granite is a body of fine-grained leucogranite occurring in the Gotthard rail base tunnel in the Central Alps. During construction of the tunnel, Bristen granite (Brgr) has been drilled along a 600 m long section. The aplite-granite belongs to the suite of Variscan granitoid intrusions of the Aar massif and contains a variety of accessory minerals typical of highly differentiated granites. Rock forming fluorite, partly enriched in yttrium (Y) and rare earth elements (REE), is intergrown with the late Y- and REE-bearing carbonate mineral synchysite. The granite contains a variety of Ti- and Y-REE-niobates, thorite, and zircon. Compared with the calc-alkaline central Aar granite (cAgr), Bristen granite is strongly depleted in Ti, P, Mg, Sr, and Ba and shows a remarkable enrichment in incompatible elements such as Rb, Th, U, Nb, Y, HREE and F. Bristen granite is the most evolved granitoid rock of the Aar massif. The composition of Brgr is typical of post-collisional reduced (ferroan) A-type granites. The Brgr melt formed in the lower crust and crystallized from a highly differentiated melt at the cotectic point in the quartz-feldspar system close to 100 MPa and 700 °C. The Brgr intruded as a small isolated stock pre-Variscan gneisses with sharply discordant contacts. The primary igneous host of Nb, Ta, Y, U, Th and REE is biotite in addition to minor amounts of allanite, and zircon. The presence of Y-REE-fluorite, synchysite, parisite and Y- and Ti-niobates and other REE-minerals can be related to reaction of igneous biotite and primary fluorite with hydrothermal fluids. The reaction is associated with alpine metamorphism, because Y-bearing fluorite and synchysite have been reported from Alpine fissures. The transformation of primary biotite to chlorite and muscovite released the heavy metal oxides under lower greenschist facies conditions that formed the Alpine diagnostic mineral stilpnomelane at about 300 °C. 相似文献
7.
D. Küster 《Mineralium Deposita》1990,25(1):25-33
he Sn-(Nb, Ta) mineralization of the Wamba field (central Nigeria) occurs in muscovite-quartz-microcline pegmatites, which are related to the late-orogenic Pan-African (f 550 Ma) "Older Granites". The emplacement of granites and pegmatites was controlled by late Pan-African shear tectonics. The granitoid magmatism was multiphase and has produced peraluminous biotite granite, biotite-muscovite granite, and muscovite granite plutons. Sodic metasomatism has altered highly evolved granite cupolas and many of the pegmatite dikes. The pegmatitic mineralization of predominantly cassiterite is closely associated with albitization. Chemical data of granites and granitic and pegmatitic muscovites show that Rb, Cs, Sn, Nb, and Ta are enriched during both magmatic and postmagmatic evolution, with highest contents of these elements in early muscovites of the albitized and mineralized pegmatites. Trace-element chemistry of the pegmatitic muscovites reveals a chemical zonation of the pegmatite field related to the late-orogenic shear system. 相似文献
8.
The Jiepailing mining district in the Nanling range in South China is well-known for its granite-related Sn–Be–F-mineralization. Recently, drill holes have exposed an Nb–Ta–W–Sn mineralized granitic porphyry and topaz-bearing granite–greisen at depth, which we have studied here, using mineral (columbite, rutile, wolframite, cassiterite, zircon, and mica) major- and trace-element compositional data, mineral textures, and zircon and columbite U–Pb geochronology. Our age data shows that the porphyry and the granite and their mineralization formed at ~ 91–89 ± 1 Ma in the late-Cretaceous, and thus subsequent to the main ore-forming events of the region. Continuous mineral compositional trends indicate that the studied granitoids are related by progressive fractionation. We propose that: (1) subhedral–euhedral, low-Ta columbite crystallized from melt; (2) euhedral–subhedral rutile and wolframite and subhedral and subhedral cassiterite up to ~ 30 μm in size formed at the magmatic–hydrothermal transition of the system; and (3) high-Ta columbite and subhedral cassiterite up to ~ 10 μm in size formed from subsolidus hydrothermal fluids. In combination with the Nb, Ta, W, and Sn compositions of zircon and mica, their textures and compositional variation allow us to track the magmatic to hydrothermal rare-metal fractionation (concentration, mobilization, and deposition) of the system in detail, despite our limited access to it through only two exploration drill cores. Using the Nb, Ta, W, and Sn concentrations in zircon (refractory, early-crystallized) and in micas (late equilibrated), respectively, was particularly useful for tracing the partial loss of Sn and W ore components from the intrusion, and to constrain the information which is crucial for any rigorous ore exploration. 相似文献
9.
Petrological and geochemical evolution of the Kymi stock, a topaz granite cupola within the Wiborg rapakivi batholith, Finland 总被引：5，自引：0，他引：5
The 6×3 km Kymi monzogranite stock represents the apical part of an epizonal late-stage pluton that was emplaced within the 1.65 to 1.63 Ga Wiborg rapakivi batholith. The stock has a well-developed zonal structure, from the rim to the center: stockscheider pegmatite, equigranular topaz granite, porphyritic topaz granite. The contact between the two granites is usually gradational within a few centimeters, but local inclusions of the porphyritic granite in the equigranular granite indicate that the latter solidified later. Hydrothermal greisen and quartz veins, some of which contain genthelvite, beryl, wolframite, cassiterite, and sulfides, cut the granites of the stock and the surrounding country rocks. The equigranular granite contains 1 to 4 vol.% topaz, and its biotite is lithian siderophyllite; the porphyritic granite has 0 to 3 vol.% topaz, and the mica is siderophyllite. The equigranular granite is geochemically highly evolved with elevated Li, Rb, Ga, Ta, and F, and very low Ba, Sr, Ti, and Zr. The REE patterns show deep negative Eu anomalies and tetrad effects indicating extreme magmatic fractionation and aqueous fluid–rock interaction. The zonal structure of the stock is interpreted as a result of differentiation within the magma chamber. Internal convection in the crystallizing magma chamber and upward flow of residual melt as a boundary layer along sloping contacts resulted in accumulation of a layer of highly evolved, volatile-rich magma in the apical part of the chamber. Crystallization of this apical magma produced the stockscheider pegmatite and the equigranular granite; the underlying crystal mush solidified as the porphyritic granite. Much of the crystallization took place from volatile-saturated melt, and episodic voluminous degassing expelled fluids into opened fractures where they or their derivatives reacted with country rocks and caused alteration and mineralization. 相似文献
10.
The Nuweibi albite granite is one of 14 known Sn-Ta-Nb bearing granitoids in the Eastern Desert region of Egypt. The granite
is a highly leucocratic, albite-rich rock with accessory columbite-tantalite, cassiterite, microlite and ixiolite as well
as topaz, garnet and white mica. Ages of 450–600 Ma were obtained from zircons by the 207Pb/206Pb evaporation method. Great uncertainty is caused by the small size and poor quality of the grains, but the precision is
sufficient to indicate that the granite is late- or postorogenic with respect to the Panafrican orogeny. The Nuweibi granite
is divided into a western and an eastern part by a regional fault. Both parts of the granite are compositionally similar but
there are important differences and a clear compositional gap between them, so they are considered separate facies of an intrusive
complex. The eastern part of the granite is more highly mineralized, has higher modal albite contents and higher Ta/Nb ratios,
both in the whole rock and in the ore minerals. It is suggested that the two parts of the granite evolved from a common source
and were emplaced sequentially, the eastern part representing a later, more fractionated magma. Textural evidence strongly
suggests that the granite has a magmatic origin overall, but disturbance of geochemical trends at the whole-rock scale and
at the scale of zoning profiles in individual grains of columbite-tantalite indicate post-magmatic overprinting. By analogy
with other Ta-bearing albite granites, the sodic bulk composition of the Nuweibi granite can be explained by fluorine enrichment
in the magma. Fluorine contents in the magma were high enough to stabilize topaz, and muscovites contain 2–4 wt.%. F. However,
whole-rock F contents are low. We speculate that the low Ca, Al and P contents of the magma prevented abundant F-bearing minerals
to form and led to loss of fluorine to now-eroded roof rocks.
Received: 8 November 1995 / Accepted: 10 June 1996 相似文献
11.
12.
The Songshugang granite, hidden in the Sinian metasedimentary stratum, is a highly evolved rare-element granite in northeastern Jiangxi province, South China. The samples were systematically taken from the CK-102 drill hole at the depth of 171–423 m. Four types of rocks were divided from the bottom upwards: topaz albite granite as the main body, greisen nodules, topaz K-feldspar granite and pegmatite layer. Electron-microprobe study reveals that the rare-element minerals of the Songshugang granite are very different from those of other rare-element granites. Mn# [Mn/(Fe + Mn)] and Ta# [Ta/(Nb + Ta)] of columbite-group minerals and Hf# [Hf/(Zr + Hf)] of zircon are nearly constant within each type of rocks. However, back-scattered electron imaging revealed that Nb–Ta oxides and zircon of the Songshugang granite, especially those of topaz albite granite, topaz K-feldspar granite and greisen, are commonly characterized by a specific two-stage texture on the crystal scale. The early-stage Nb–Ta oxide is simply subhedral-shaped columbite-(Fe) (CGM-I) with low Mn# (0.16–0.37) and Ta# (0.05–0.29). Columbite-(Fe) is penetrated by the later-stage tantalite veinlets (CGM-II) or surrounded by complex Nb–Ta–Sn–W mineral assemblages, including tantalite-(Fe), wodginite (sl), cassiterite, and ferberite. Tantalite has wide range of Mn# values (0.15–0.88) from Fe-dominance to Mn-dominance. Wodginite with Ta>Nb has large variable concentrations of W, Sn and Ti. Cassiterite and ferberite are all enriched in Nb and Ta (Nb2O5 + Ta2O5 up to 20.12 wt.% and 31.42 wt.%, respectively), with high Ta# (>0.5). Similar to Nb–Ta oxides and Nb–Ta–Sn–W mineral assemblages, the early-stage zircon is commonly included by the later-stage zircon with sharply boundary. They have contrasting Hf contents, and HfO2 of the later-stage zircon is up to 28.13 wt.%. Petrographic features indicate that the early-stage of columbite and zircon were formed in magmatic environment. However, the later-stage of rare-element minerals were influenced by fluxes-enriched fluids. Tantalite, together with wodginite, cassiterite, and ferberite implies a Ta-dominant media. An interstitial fluid-rich melt enriched in Ta and flux at the magmatic–hydrothermal transitional stage is currently a favored model for explaining the later-stage of rare-element mineralization. 相似文献
13.
福建永定大坪铌钽矿化花岗斑岩体位于永定县城南部的大石凹-蓝地火山喷发盆地,对斑岩型铌钽矿床的产出具有重要的指示意义。本文通过岩相学、显微测温和激光拉曼等实验对大坪岩体ZK10001和ZK10401钻孔不同深度岩石样品中的流体和熔体包裹体进行了研究,试图揭示岩体的熔体-流体演化过程,分析铌钽等成矿元素的富集机制。观测结果表明,大坪岩体主要发育气液两相盐水溶液包裹体和硅酸盐熔体包裹体。流体包裹体均一温度集中在175～225℃,盐度集中在3%～7%NaCleq,密度集中在0.75～0.95g/cm~3,成矿流体主要为中低温、低盐度和低密度的流体,总体属于H_2O-NaCl体系。熔体包裹体主要分布于石英斑晶雪球结构的环带中,含有钠长石、石英和钽铁矿等子矿物。熔体包裹体完全均一温度较高,能够代表早期原始岩浆的组成。研究表明,大坪岩体的原始岩浆富铌钽等成矿元素和碱性组分,大坪岩体的铌钽矿化是岩浆高度分异的产物,铌钽的富集过程经历了斑晶阶段和基质阶段等两阶段结晶分异过程:在早期斑晶结晶阶段,少量铌钽矿物与斑晶一起结晶,并被斑晶包裹;岩浆演化晚期发生流体出溶现象,但未分异出大量流体,F等挥发分促进了铌钽在结晶残余熔体中富集,并在基质间隙中沉淀。大坪矿化岩体的存在指示出斑岩型铌钽矿床存在的可能性。 相似文献
14.
华南是我国重要的战略性矿产资源基地,以花岗岩相关的稀有和稀土金属成矿作用而举世瞩目。其中,铌的成矿作用一般与过铝质高分异花岗岩有关,稀土元素则随岩浆演化程度增强而富集程度降低,而江西铁木里含黑云母碱长花岗岩体同时富集铌和稀土元素,矿化组合极具特色。本文在详细的矿物岩相学研究基础上,利用电子探针、飞秒激光电感耦合等离子质谱对铌和稀土矿物进行了矿物地球化学分析,借此对铁木里碱长花岗岩中铌和稀土元素的富集机制进行探讨。铁木里岩体由肉红色含黑云母碱长花岗岩(r-G)和灰白色含黑云母碱长花岗岩(g-G)组成,发育暗色包体。r-G中的铌矿物主要为岩浆期形成的铌铁金红石,稀土矿物包括岩浆期形成的硅钛铈矿、独居石、磷灰石和热液期形成的独居石和氟碳(钙)铈矿。g-G中的铌矿物包括岩浆期形成的铌铁金红石和热液期形成的铌铁金红石、易解石、铌铁矿,稀土矿物包括岩浆期磷灰石和热液期磷灰石、独居石、氟碳(钙)铈矿。暗色包体为岩浆混合成因,内含磷灰石、独居石和零星的硅钛铈矿、金红石。矿物组合特征显示,铁木里碱长花岗岩中的铌和稀土元素经过了岩浆和热液两个时期的富集。应用金红石、磷灰石、绿泥石等矿物成分特征约束了岩浆-... 相似文献
15.
Mohamed Abdel-Moneim Mohamed 《Arabian Journal of Geosciences》2013,6(10):4021-4033
Rare-metal granites of Nuweibi and Abu Dabbab, central Eastern Desert of Egypt, have mineralogical and geochemical specialization. These granites are acidic, slightly peraluminous to metaaluminous, Li–F–Na-rich, and Sn–Nb–Ta-mineralized. Snowball textures, homogenous distribution of rock-forming accessory minerals, disseminated mineralization, and melt inclusions in quartz phenocrysts are typical features indicative of their petrographic specialization. Geochemical characterizations are consistent with low-P-rare metal granite derived from highly evolved I-type magma in the late stage of crystallization. Melt and fluid inclusions were studied in granites, mineralized veins, and greisen. The study revealed that at least two stages of liquid immiscibility played an important role in the evolution of magma–hydrothermal transition as well as mineral deposition. The early stage is melt/fluid case. This stage is represented by the coexistence of type-B melt and aqueous-CO2 inclusions in association with topaz, columbite–tantalite, as well as cassiterite mineral inclusions. This stage seems to have taken place at the late magmatic stage at temperatures between 450 °C and 550 °C. The late magmatic to early hydrothermal stage is represented by vapor-rich H2O and CO2 inclusions, sometimes with small crystallized silicic melt in greisen and the outer margins of the mineralized veins. These inclusions are associated with beryl, topaz, and cassiterite mineralization and probably trapped at 400 °C. The last stage of immiscibility is fluid–fluid and represented by the coexisting H2O-rich and CO2-rich inclusions. Cassiterite, wolframite ± chalcopyrite, and fluorite are the main mineral assemblage in this stage. The trapping temperature was estimated between 200 °C and 350 °C. The latest phase of fluid is low-saline, low-temperature (100–180 °C), and liquid-rich aqueous fluid. 相似文献
16.
《Chemie der Erde / Geochemistry》2023,83(1):125924
Two-mica granite is the most common magmatic rock type in the Himalayan leucogranite belt, which has close relationship with rare metal mineralization. Its genesis is generally attributed to magmatic differentiation. In recent years, the mineral geochemical compositions are increasingly used to study magmatic differentiation, which are significant for deciphering the melt evolution and element migration processes. In this study, in-situ major and trace element and boron isotope compositions for tourmalines from two-mica granites in the Cuona and Cuonadong leucogranites in the Cuona area are conducted to determine microscopic changes in mineral assemblages and geochemical compositions. Analytical results show that the tourmalines in the Cuonadong leucogranite were crystallized earlier relative to the tourmalines in the Cuona leucogranite during magmatic differentiation. The volatile contents have a genetic relationship with incompatible elements in tourmaline, which is possibly responsible for the formation of tourmaline zonation and the enrichment of Sr, Zn, and Pb during magmatic differentiation. The B isotopic composition of tourmaline in the Cuona area suggests that the granitic magma was dominantly derived from the partial melting of the metasedimentary source rocks. Their B isotope variations likely resulted from fluid exsolution during B-rich melt evolution. High rare metal contents in tourmalines indicate that the two-mica granites in the Cuona area may have great mineralization potential. 相似文献
17.
Hamdy M. Abdalla 《Resource Geology》2009,59(2):121-139
Mineral chemistry and geochemical characteristics of beryl-bearing granitoids in Eastern Desert of Egypt, were examined in order to identify the metallogenetic processes of the host granitoids. The investigated Be-bearing granitoids and type occurrences are classified into two groups: (i) peraluminous, Ta ≥ Nb + Sn + Be ± W-enriched, Li-albite granite (e.g. Nuweibi and Abu Dabbab); and (ii) metasomatized, Nb >> Ta + Sn + Be ± W ± Mo-enriched alkali feldspar granite (i.e. apogranite; e.g. Homr Akarem, Homr Mikpid and Qash Amir). In these two groups, beryl occurs as stockwork greisen veins, greisen bodies, beryl-bearing cassiterite ± wolframite quartz veins, dissemination, and miarolitic pegmatites. Beryl of the Be-granitoids, particularly those of miarolitic pegmatites, contains appreciable contents of Fe, Na, and H2 O. An important feature of the Be-apogranites is the occurrence of white mica as the sole mafic mineral in the unaltered alkali feldspar granite in lower zones. Presence of white mica as volatile-rich pockets suggests that the melt underwent disequilibrium crystallization, rapid nucleation rates, and exsolving and expulsion of volatiles. 相似文献
19.
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. 相似文献
20.
Most rare-metal granites in South China host major W deposits with few or without Ta–Nb mineralization. However, the Yashan granitic pluton, located in the Yichun area of western Jiangxi province, South China, hosts a major Nb–Ta deposit with minor W mineralization. It is thus important for understanding the diversity of W and Nb–Ta mineralization associated with rare-metal granites. The Yashan pluton consists of multi-stage intrusive units, including the protolithionite (-muscovite) granite, Li-mica granite and topaz–lepidolite granite from the early to late stages. Bulk-rock REE contents and La/Yb ratios decrease from protolithionite granite to Li-mica granite to topaz–lepidolite granite, suggesting the dominant plagioclase fractionation. This variation, together with increasing Li, Rb, Cs and Ta but decreasing Nb/Ta and Zr/Hf ratios, is consistent with the magmatic evolution. In the Yashan pluton, micas are protolithionite, muscovite, Li-mica and lepidolite, and zircons show wide concentration ranges of ZrO2, HfO2, UO2, ThO2, Y2O3 and P2O5. Compositional variations of minerals, such as increasing F, Rb and Li in mica and increasing Hf, U and P in zircon are also in concert with the magmatic evolution from protolithionite granite to Li-mica granite to topaz–lepidolite granite. The most evolved topaz–lepidolite granite has the highest bulk-rock Li, Rb, Cs, F and P contents, consistent with the highest contents of these elements and the lowest Nb/Ta ratio in mica and the lowest Zr/Hf ratio in zircon. Ta–Nb enrichment was closely related to the enrichment of volatile elements (i.e. Li, F and P) in the melt during magmatic evolution, which raised the proportion of non-bridging oxygens (NBOs) in the melt. The rims of zoned micas in the Li-mica and topaz–lepidolite granites contain lower Rb, Cs, Nb and Ta and much lower F and W than the cores and/or mantles, indicating an exotic aqueous fluid during hydrothermal evolution. Some columbite-group minerals may have formed from exotic aqueous fluids which were originally depleted in F, Rb, Cs, Nb, Ta and W, but such fluids were not responsible for Ta–Nb enrichment in the Yashan granite. The interaction of hydrothermal fluids with previously existing micas may have played an important role in leaching, concentrating and transporting W, Fe and Ti. Ta–Nb enrichment was associated with highly evolved magmas, but W mineralization is closely related to hydrothermal fluid. Thus these magmatic and hydrothermal processes explain the diversity of W and Ta–Nb mineralizations in the rare-metal granites. 相似文献