Unusual REE distribution patterns in fluorites from Sn-W deposits of the quartz-cassiterite and quartz-wolframite type
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Abstract:
Fluorite is well known as an excellent tracer for REE fractionation in ore-forming processes (Balashov, 1976; Bau & Dulski, 1992). We present REE data for fluorite from three granite-related Sn–W deposits of the quartz– cassiterite and quartz–wolframite type, located in different regions of Eurasia: Ehrenfriedersdorf (Erzgebirge), Urmi (Russian Far East) and Aqshatau (Central Kazakhstan). Fluorite is a common mineral in all stages of oreformation on these deposits. Normally, REE distribution patterns of fluorites are similar to those of the host granites for endocontact samples, i.e., they are characterized by enrichment in both LREE and HREE and a strong negative Eu anomaly. In exocontact, distinct positive Eu anomalies are observed together with some depletion in HREE (Goldstein et al., 1995; see also Fig. 1, left). In contrast, an unusual REE distribution is found for a rare pale rose or pale blue fluorite in mineral associations relatively uncommon for deposits of the quartz–cassiterite and quartz–wolframite type (Fig. 1, right). Pale rose fluorite occurs in early tourmaline veins with beryl and cassiterite from the Ehrenfriedersdorf deposit (1). The pale rose fluorite in massive quartz–topaz–cassiterite metasomatites from Urmi forms similar intergrowth with tourmaline needles as in Ehrenfriedersdorf (2). In Aqshatau, pale rose fluorite in wolframite-bearing quartz–mica greisen with sericite (3) and pale blue fluorite from quartz-topaz bodies with beryl (4) yield the same REE characteristics as the samples from Ehrenfriedersdorf and Urmi: enrichment in HREE and strong depletion in LREE, occasionally with a weak positive Eu anomaly. Such REE behaviour cannot be explained by the influence of wall rocks (granites, schists, gneisses, etc.), or fractionation between different minerals and fluids within veins or metasomatites. We assume that these REE distribution patterns represent the primary REE characteristics of the fluids.Keywords:
Cassiterite
Wolframite
Fluorite
Greisen
Tourmaline
Topaz
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The Bjorkdal quartz vein-hosted gold deposit is located ~25 km northwest of Skellefte in northern Sweden, within a Paleoproterozoic volcanosedimentary sequence at the margin of a quartz-monzodiorite granitoid. Northeast-trending (030°– 050°) quartz veins from the eastern open pit within the Bjorkdal deposit contain quartz, scheelite, tourmaline, calcite, and sulfides, with visible gold. Vein quartz shows undulatory extinction and sutured margins or is polycrystalline in form, features which suggest postcrystallization deformation. Coarse scheelite crystals (>5 mm) within the quartz veins are crosscut by thin veins of quartz, calcite, sulfides, and gold. The calcite in these crosscutting fractures is variably replaced by biotite or actinolite. Tourmaline from the quartz veins has low total REE contents (<1 × chondrite) and LREE-enriched patterns [(La/Sm)N = 2.8–4.5, (La/Yb)N = 1.8–5.1] with strong positive Eu anomalies (Eu/Eu* = 3.9–17.4). In contrast, the scheelite has a bell-shaped REE pattern, enriched in MREE, but also with positive Eu anomalies (Eu/Eu* = 1.4–2.4). The REE pattern of scheelite results from a strong crystallographic effect, largely due to the size of the Ca site and charge balance. Sm-Nd dating of scheelite from the Bjorkdal ore yields an age of 1893 ± 34 Ma, which coincides with a previously suggested age of the host intrusion. The eNd values of the scheelite (+1.8) and 87Sr/86Sr initial ratios of the tourmaline (0.7013–0.7014) are also consistent with derivation of REE and Sr in these minerals from the Jorn granitoids. Overall, the petrographic, geochemical, and isotopic data strongly suggest that Bjorkdal is an intrusion-related gold deposit, and that there is no requirement for involvement of external post-magmatic hydrothermal fluids.
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충주 지역은 소위 옥천층군에 해당되는 계명산층이 분포하며, 계명산층내에는 변성화산암 모암과 페그마타이트 모암의 희토류 광체가 배태한다. 변성화산암 모암의 희토류 광체를 구성하는 희토류광물은 갈렴석, 저어콘, 인회석, 스핀이 산출하나 갈렴석이 가장 우세하게 산출한다. 페그마타이트 모암의 희토류 광체를 구성하는 희토류 광물은 퍼구소나이트, 카르나수르타이트, 저어콘, 토륨석이 산출하나 퍼구소나이트가 가장 우세하다. 변성화산암에서 산출하는 갈렴석의 화학식은 (Ca, Y, REE, Th)$_{2.095}$ (Mg, Al, Ti, Mn, $Fe^{3+})_{2.770}(SiO_4)_{2.975}(OH)$ 로서 TREO 23.89-29.12 wt%, $La_2O_3$ 4.71-9.92 wt%, $Ce_2O_3$ 11.30-14.33 wt%, $Y_2O_3$ 0.11-0.29 wt%, $ThO_2$ 0.12-0.94 wt% 이다. A(2) 사이트 에서 $Ca^{2+}$ 와 $REE^{3+}$ , M(2) 사이트에서 $Al^{3+}$ 과 $Fe^{2+}$ 의 치환이 일어나는데 이는 갈렴석의 화학조성에 밀접한 관련을 갖는 특징이고, Fe의 함량이 일반 갈렴석보다 높은 Ce 계열의 Ferriallanite에 해당된다. 이는 모암인 계명산층을 주로 구성하는 변성화산암(변성조면암)의 원암이 Fe이 풍부한 함철층이기 때문인 것으로 판단된다. 페그마타이트 모암에서 가장 우세하게 산출하는 퍼구소나이트의 화학조성은 A 사이트에서 Y-REE, Y-Th 치환이 우세하게 일어났으며, B 사이트에서는 Nb-Ta-Ti의 치환이 주로 초래되었으며, 계산된 화학식은 $YNbO_4$ 이다. 또한 $Y_2O_3$ 와 $Nb_2O_5$ 만의 비율로 상관도를 확인 한 결과 연구지역에서 산출되는 퍼구소나이트는 Y과 Nb의 이상적인 비율인 1:1 비율과 달리 1:1.5의 비율을 나타내고 있으며, Nb의 함량이 Y 함량보다 높으며, Y 사이트 즉, A 사이트에서 희토류 원소의 치환이 활발하게 초래되었다. 페그마타이트에서 산출하는 카르나수르타이트는 REE 및 Th를 치환하는 조성은 각각 $Ce_2O_3$ 9.16-22.88 wt%, $La_2O_3$ 2.15-9.16 wt%, $ThO_2$ 0.44-10.8 wt%, 화학조성으로 계산된 구조식은 (Y, REE, Th, K, Na, Ca)$_{1.478}$ (Ti, Nb)$_{1.304}$ (Mg, Al, Mn, $Fe^{+3})_{0.988}$ (Si, P)$_{1.431}O_7(OH)_4{\cdot}3H_2O$ 이다. 870-860 Ma 인 초기 원생대에 로디니아 대륙의 분열기로서 한반도에서 A-1형 화산활동이 초래되어 철, 희토류원소 및 고장력원소(Nb, Zr, Y 등)가 풍부한 변성화산암으로 주로 구성되는 계명산층을 형성 시켰다면 갈렴석은 모암이 형성될 당시 알카리 화산암에서 정출되었거나 변성작용이 초래된 고생대 말(300-280 Ma, Cho et al., 2002) 광역변성작용에 의해 형성 되었을 가능성이 높다. 희토류를 함유하는 페그마타이트에서 산출하는 저어콘 연대가 190 Ma 인 것은 쥬라기에 충주지역에서 광범위하게 초래된 화강암 정치활동과 관련된 가능성이 크다. 따라서 충주지역 계명산층 내 배태된 희토류 광체는 인접한 지역에 배태되어 있지만 매우 차별적 희토류 광화작용이 초래 되어 희토류광물조성과 광체의 산상이 차별적으로 나타나는 것으로 해석된다. The Chungju Fe-REE deposit is located in the Kyemyeongsan Formation of the Ogcheon Group. The Kyemyeongsan Formation includes meta-volcanic rocks and pegmatite hosted REE deposit which show different kind of REE-containing minerals. The meta-volcanic rocks hosted REE deposits' main REE minerals are allanite, zircon, apatite, and sphene, whereas the pegmatite hosted REE deposits is mainly composed of fergusonite, and karnasurtite, zircon, thorite. The meta-volcanic rock hosted major REE mineral is allanite as the form of aggregation and contains 23.89-29.19 wt% TREO (Total Rare Earth Oxide), 4.71-9.92 wt% $La_2O_3$ , 11.30-14.33 wt% $Ce_2O_3$ , 0.11-0.29 wt% $Y_2O_3$ , 0.15-0.94 wt% $ThO_2$ , as a formula of (Ca, Y, REE, Th)$_{2.095}$ (Mg, Al, Ti, Mn, $Fe^{3+})_{2.770}(SiO_4)_{2.975}(OH)$ . Accompanying REE in a coupled substitution for $Ca^{2+}$ (M1 site) and $Al^{3+}-Fe^{2+}$ (M2 site) leads to a large chemical variety. Due to the allanite's high contents of Fe, it belongs to Ferrialanite. The pegmatite hosted deposit's domi-nant REE mineral is fergusonite as prismatic or subhedral grains associated with zircon, fluorite and karnasurtite. Geochemical composition of the fergusonite($YNbO_4$ ) suggests substitution of Y-REE and Y-Th in A-site, and Nb-Ta-Ti in B-site, furthermore the proportion of $Y_2O_3$ and $Nb_2O_5$ is oddly 1:1.5 comparing to the ideal ratio 1:1 and Nb is higher than Y, also A-site Y actively substitutes with REE. Karnasurtite in pegmatite variously ranges 9.16-22.88 wt% $Ce_2O_3$ , 2.15-9.16 wt% and $La_2O_3$ , 0.44-10.8 wt% $ThO_2$ , as a calculated formula (Y, REE, Th, K, Na, Ca)$_{1.478}(Ti, Nb)_{1.304}$ (Mg, Al, Mn, $Fe^{3+})_{0.988}$ (Si, P)$_{1.431}O_7(OH)_4{\cdot}3H_2O$ . Firstly the 870-860 Ma is the initial age of the supercontinent Rhodinia dispersal and subsequent A-1 type volcanism, which contains Fe, REE, and HFS(High Field Strength elements; Nb, Zr, Y etc.) elements in Fe-rich meta-volcanic rocks dominant Kyemyeongsan Formation, might mineralized allanite. Another synthesis is that regional metamorphism at late Paleozoic 300-280 Ma(Cho et al., 2002) might cause allanite mineralization. Also pegmatite REE mineralization highly related to the granite intrusion over the Chungju area in Jurassic(190 Ma; Koh et al., 2012). Otherwise above all, A-1 type volcanism at the same time of the Kyemyeongsan Formation development, regional metamorphism and pegmatite, might have caused REE mineralization. Although REE ore bodies display a close spatial association, each ore bodies display temporal distinction, different mineral assemblage and environment of ore formation.
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Gangue
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Carbonatite
Fluorite
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Allanite
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This study focuses on concentrations and fractionation of rare earth elements (REE) in a variety of minerals and bulk materials of hydrothermal greisen and vein mineralization in Paleoproterozoic monzodiorite to granodiorite related to the intrusion of Mesoproterozoic alkali- and fluorine-rich granite. The greisen consists of coarse-grained quartz, muscovite, and fluorite, whereas the veins mainly contain quartz, calcite, epidote, chlorite, and fluorite in order of abundance. A temporal and thus genetic link between the granite and the greisen/veins is established via high spatial resolution in situ Rb-Sr dating, supported by several other isotopic signatures ( δ 34 S, 87 Sr/ 86 Sr, δ 18 O, and δ 13 C). Fluid-inclusion microthermometry reveals that multiple pulses of moderately to highly saline aqueous to carbonic solutions caused greisenization and vein formation at temperatures above 200–250°C and up to 430°C at the early hydrothermal stage in the veins. Low calculated ∑REE concentration for bulk vein (15 ppm) compared to greisen (75 ppm), country rocks (173–224 ppm), and the intruding granite (320 ppm) points to overall low REE levels in the hydrothermal fluids emanating from the granite. This is explained by efficient REE retention in the granite via incorporation in accessory phosphates, zircon, and fluorite and unfavorable conditions for REE partitioning in fluids at the magmatic and early hydrothermal stages. A noteworthy feature is substantial heavy REE (HREE) enrichment of calcite in the vein system, in contrast to the relatively flat patterns of greisen calcite. The REE fractionation of the vein calcite is explained mainly by fractional crystallization, where the initially precipitated epidote in the veins preferentially incorporates most of the light REE (LREE) pool, leaving a residual fluid enriched in the HREE from which calcite precipitated. Fluorite occurs throughout the system and displays decreasing REE concentrations from granite towards greisen and veins and different fractionation patterns among all these three materials. Taken together, these features confirm efficient REE retention in the early stages of the system and minor control of the REE uptake by mineral-specific partitioning. REE-fractionation patterns and fluid-inclusion data suggest that chloride complexation dominated REE transport during greisenization, whereas carbonate complexation contributed to the HREE enrichment in vein calcite.
Greisen
Fluorite
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Tourmaline
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Wolframite
Scheelite
Trace element
Rare-earth element
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Rare earth element (REE) contents and distribution patterns in the major rock-forming minerals (vesuvianite, wollastonite, garnet, pyroxene, hornblende, epidote and fluorite) from the Shizhuyuan W, Sn, Mo and Bi-containing skarn deposit of Hunan Province of South China have been determined. Three types of REE distribution patterns were found: 1) vesuvianite and wollastonite, formed at the expense of carbonates and shales of the Upper Devonian Shetianqiao Formation during thermal metamorphism stage, show linear light REE enrichment and small Eu anomalies; 2) garnet, the most abundant mineral in the skarn deposited at metasomatic stage, exhibits high REE enrichment and considerable negative Eu anomalies; 3) epidote and fluorite, precipitated during retrograde alteration stage, display very strong negative Eu anomalies. Comparison of relative REE abundances in these minerals with those in the carbonates and shales as well as in the Qianlishan Yanshanian granite suggests that REE compositions of the vesuvianite and wollastonite appear to have been inherited from precursor carbonates and shales, whereas REE contents and patterns in the metasomatic and the retrograde alteration products, such as garnet and epidote, seem to have been determined by the interactions between the reacting minerals and hydrothermal solutions originated from Qianlishan granite.
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Wollastonite
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