Phase relations in albite granite-H2O-HF system and their petrogenetic applications.
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Phase relations were experimentally determined in albite granite-H2O-HF system with 2, 4 and 6 wt.% fluorine at 100 MPa pressure. With the increase of fluorine content from 2 to 6 wt.% in the system, liquidus temperature increased from 768 to 790°C and the solidus temperature decreased from 540 to 490°C. Maximum temperature of quartz and topaz stability field increased with F content, whereas that of alkali-feldspar decreased and its stability field disappeared at 6 wt.% fluorine. The mineral assemblage of topaz granite (quartz + albite + K-feldspar + mica + topaz) was stable above solidus at F ≤ 4 wt.%, and that of topaz greisen (quartz + topaz + mica) was stable above solidus at F = 6 wt.%. The experiments showed that the residual melt with high F content can be formed by fractional crystallization of a leucogranitic melt with relatively low F content. These experimental results provide reliable evidences for the formation of topaz granite and topaz greisen at low temperature magmatic conditions.Keywords:
Topaz
Greisen
Solidus
Liquidus
Alkali feldspar
Phase relations were experimentally determined in albite granite-H2O-HF system with 2, 4 and 6 wt.% fluorine at 100 MPa pressure. With the increase of fluorine content from 2 to 6 wt.% in the system, liquidus temperature increased from 768 to 790°C and the solidus temperature decreased from 540 to 490°C. Maximum temperature of quartz and topaz stability field increased with F content, whereas that of alkali-feldspar decreased and its stability field disappeared at 6 wt.% fluorine. The mineral assemblage of topaz granite (quartz + albite + K-feldspar + mica + topaz) was stable above solidus at F ≤ 4 wt.%, and that of topaz greisen (quartz + topaz + mica) was stable above solidus at F = 6 wt.%. The experiments showed that the residual melt with high F content can be formed by fractional crystallization of a leucogranitic melt with relatively low F content. These experimental results provide reliable evidences for the formation of topaz granite and topaz greisen at low temperature magmatic conditions.
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Topaz was previously considered as a typical hydrothermal mineral, and topaz greisen a typically altered rock. However, the geological and geochemical evidence in recent years has made it clear that the layer-like topaz greisen at the top of Ta- and Nb-bearing albite
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Topaz, so common an accessory in tin lodes in all parts of the world, has hitherto been looked upon as comparatively rare in Cornwall. When massive, or even in the form of small, well-developed crystals, it is a mineral easily overlooked, or apt cursorily to be mistakcn for quartz. The presence of topaz as a microscopic accessory in granite, greisen, and rarely elvans, from most of the Cornish masses, has been determined by Dr. J. S. Flett in the course of the work of the Geological Survey; while the late Mr. J. H. Collins was responsible for the record of other interesting localities. In the present notes I have collected all the available information respecting its mode of occurrence and localities ; and have added several new ones, at some of which the mineral occurs in considerable mass, thus affording evidence that topaz is in Cornwall, as in the case of other countries, a comparatively common associate of cassiterite and wolframite, both in granite and sedimentary rocks.
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Cassiterite
Wolframite
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Greisen
Topaz
Tourmaline
Muscovite
Fluorite
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The topaz-alkali-feldspar-granite, the most evolved facies of the Água Boa pluton, was affected by hydrothermal alteration, represented by greisens and quartz veins, the main host for Sn- and subordinated Zn mineralization. The greisens are classified as quartz-topaz-siderophyllite-greisen, topaz-siderophyllite-greisen and quartz-topaz-quartz-greisen. They are composed essentially of quartz, topaz and siderophyllite, accompanied by variable amounts of fluorite, zinnwaldite, sphalerite, cassiterite, zircon and anatase and locally Ce-monazite, galena, pyrite, chalcopyrite and native bismuth. EMPA studies allowed identifying three types of micas: (1) brown siderophyllite from topaz-granite; (2) the green siderophyllite of greisens and (3) zinnwaldite, weakly colored, found as thin and discontinuous rims around green siderophyllite, and quartz vein. The siderophyllite composition of the granite towards greisens shows an evolution of siderophyllite to Li-siderophyllite with increase of VI Al, Li and Si contents. On the other hand, the siderophyllite of the greisen was partially replaced by zinnwaldite, with increase of VI Al, Li and Si contents. The cassiterite in the greisens forms euhedral to subeuhedral, twin and zoned crystals, with strong pleochroism. It occurs as aggregates together with topaz, quart and fluorite. The pure composition and the low content of the Nb and Ta of cassiterite indicate hydrothermal conditions.
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Greisen
Cassiterite
Tourmaline
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Wolframite
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Abstract The composition of topaz from different granites and greisen in the Krušné Hory/Erzgebirge area was investigated using electron microprobe analysis (EMPA) and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS). All topaz grains are rich in F (17.9 19.8 wt.%, 1.73 1.90 a.p.f.u.) and the most important minor/trace elements are P, Ge and Ga. Contents of P up to 1 wt.% P2O 5 (0.025 a.p.f.u.) were found in topaz from the strongly peraluminous P-rich magmatic systems at Podlesí. Regardless of genetic type, topaz from granites typically contains 50 100 ppm Ge. The greatest amounts (up to 204 ppm Ge) were found in topaz from quartz-topaz-apatite greisen in Krásno. In fractionated granites and greisens, topaz is calculated to contain 23 87% of the bulk Ge content in the rock. In contrast, topaz does not concentrate Ga. The Ga content of topaz (typically 5 35 ppm in S-type granites, <10 ppm Ga in A-type granites) is usually smaller than the bulk Ga content of the rock. In addition, up to 16 ppm Sc, 23 ppm Sn and >400 ppm Fe may be present.
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The Ahvenisto rapakivi complex is a satellite northwest of the large Wiborg rapakivi batholith in Southeastern Finland.The rapakivi rocks are surrounded by rapakivi-age gabbros, anorthosites, monzodiorites and diabases.The complex consists of a highly evolved suite of several texturally, mineralogically and geochemically different rapakivi types.They are anorogenic and their age is 1640-1630 Ma.The youngest phases of the rapakivi suite are even-grained, topaz-bearing and geochemically specialized tin granites.They also show the characteristics of A-type granites.Earlier known rapakivi granites in Finland of the same type have been described from Eurajoki and Kymi.These specialized Kuusisuo granites are highly evolved microcline-albite granites, which are modified by many (auto)metasomatic and late/postmagmatic alteration reactions.They have high Si, F, Li, Rb, Sn, Nb, Th and U, and low/extremely low Ti, Fe, Mg, Ca, P, Zr, Ba, Sr and La.They intrude earlier phases of rapakivi granites as a small cupola, which in its upper parts is altered to topaz-bearing rapakivi and mineralized topaz-greisen.Mineralized greisen veins occur in the surrounding rapakivi granites.The uneconomic mineralizations contain SnO and sulphides of Cu, Zn and As.
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Topaz was previously considered as a typical hydrothermal mineral, and topaz greisen a typically altered rock. However, the geological and geochemical evidence in recent years has made it clear that the layer-like topaz greisen at the top of Ta- and Nb-bearing albite granite has magmatic characteristics.
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