In situ elemental and isotopic analysis of fluorapatite from the Taocun magnetite-apatite deposit, Eastern China: Constraints on fluid metasomatism
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Metasomatic alteration of fluorapatite has been reported in several iron-oxide apatite (IOA) deposits, but its effect on elemental and isotopic variations has not been well understood. In this study, we present integrated elemental, U-Pb, Sr, and O isotopic microanalytical data on fresh and altered domains in fluorapatite from the Taocun IOA deposit, Eastern China, to evaluate the timing and nature of the metasomatism and its effects on the ore-forming event. Orebodies of the Taocun deposit are spatially associated with a subvolcanic, intermediate intrusion, which displays zonal alteration patterns with albite in the center and increasing actinolite, chlorite, epidote, and carbonate toward the margin. Both disseminated and vein-type ores are present in the Taocun deposit, and fluorapatite commonly occurs with magnetite and actinolite in most ores.Keywords:
Fluorapatite
Metasomatism
Actinolite
Ore genesis
Andradite
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Pyroxene
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The axinite-bearing Gukjeon Pb–Zn deposit is hosted by the limestone, a member of the Jeonggaksan Formation, which, in turn, forms the part of the Jusasan subgroup of the Yucheon Group in the Gyeongsang Basin in the southeastern part of the Korean Peninsula. In this study, we attempted to interpret the spatial and temporal relationships among geologic events, including the mineralization of this deposit. We constructed a new 3D orebody model and suggested a relationship between skarn alteration and related mineralization. Mineralization timing was constrained using SHRIMP zircon age dating results combined with boron geochemistry on coeval intrusive rocks. Skarn alterations are restrictively found in several horizons of the limestone formation. The major skarn minerals are garnet (grossular), pyroxene (hedenbergite), amphibole (actinolite and ferro-actinolite), axinite (tizenite and ferro-axinite), and epidote (clinozoisite and epidote). The three stages of pre-skarn, syn-skarn, and post-skarn alteration are recognized within the deposit. The syn-skarn alteration is characterized by prograde metasomatic pyroxene and garnet, and the retrograde metasomatic amphibole, axinite, and epidote. Major skarn sulfide minerals are sphalerite, chalcopyrite, galena, and pyrite, which were predominantly precipitated during the retrograde stage and formed amphibole and axinite skarns. The skarn orebodies seem to be disc- or flat-shaped with a convex form at the central part of the orebodies. The vertical ascending and horizontal infiltration of boron-rich hydrothermal fluid probably controlled the geometry of the orebodies. Considering the whole-rock major, trace, and boron geochemical and geochronological results, the timing of Pb–Zn mineralization can be tightly constrained between the emplacement of boron-poor intrusion (fine-grained granodiorite, 82.8 Ma) and boron-rich intrusion (porphyritic andesite in Beomdori andesitic rocks, 83.8 Ma) in a back-arc basin setting. The boron for mineralization was sourced from late Cretaceous (Campanian), subduction-related magmatic rocks along the margin of the Pacific plate.
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선캄브리아기 경기육괴의 북서부에 위치한 포천 스카른 광상은 명성산 화강암과 선캄브리아기 변성퇴적암류에 협재된 탄산염암의 접촉대를 중심으로 산출되며, N-S방향 전단대를 따라 배태되고 있다. 포천 스카른대 분포와 함께 광물학적 특성은 구조 규제와 암층 규제에 따라 유도되었다. 포천 스카른은 스카른 광물조합에 따라 백운암을 교대한 Na-Ca계열과 Mg계열 스카른 및 석회암을 교대한 Ca계열 스카른으로 구분된다. 철광화작용은 주로 Na-Ca계열 스카른대를 따라 배태되고 있으며, 후퇴 스카른 단계에 일부 동 광화작용이 중첩된다. Na-Ca계열 스카른은 주로 추휘석, 투휘석, 조장석, 석류석, 자철석, 매그헤마이트, 경석고, 인회석, 스핀의 공생관계를 보이며, 후퇴 스카른에서는 투각섬석, 금운모, 녹렴석, 견운모, 석고, 녹니석, 석영, 방해석, 황화광물로 구성된다. 한편 Mg계열 스카른은 주로 감람석과 투휘석의 단순한 광물조합을 보이며, 투휘석과 감람석은 투각섬석과 함께 소량 금운모, 사문석, 녹니석으로 교대된다. 반면에 Ca계열 스카른은 전진 스카른 단계에서 주로 단사휘석, 석류석, 규회석이 정출되며, 후퇴 스카른에서 녹렴석, 베수비아나이트, 각섬석, 흑운모, 녹니석, 자철석, 석영, 방해석, 황화광물이 수반된다. 전자현미분석 결과에 의하면 포천 스카른광물은 대부분 Na-Mg 성분이 부화되었으며, 높은 $Fe^{3+}/Fe^{2+}$ 비, $Mg^{2+}/Fe^{2+}$ 비, $Al^{3+}/Fe^{2+}$ 비의 조성 특징을 보인다. 즉 단사휘석은 추휘석과 투휘석 조성이 부화되어 있는 반면, 석류석은 상대적으로 그로슐라 조성이 부화된 경향을 보인다. 또한 각섬석은 투각섬석, 파가사이트, Mg-헤이스팅사이트로 조성변화를 보인다. 한편 전진 스카른 단계의 주요 광물조합은 약 0.5 kbar와 $X(CO_2)=0.10$ 조건에서 $400^{\circ}{\sim}500^{\circ}C$ 온도와 높은 산화 환경( $fO_2=10^{-23}{\sim}10^{-26}$ )을 지시하고 있다. 후퇴 스카른 단계에서는 물-암석반응이 증가됨에 따라 녹렴석, 베수비아나이트, 각섬석, 녹니석, 석영, 방해석은 $X(CO_2)=0.10$ 조건에서 $250^{\circ}{\sim}400^{\circ}C$ 온도 범위에서 정출되었다. 【The Pocheon skarn deposit, located at the northwestern part of the Precambrian Gyeonggi massif in South Korea, occurs at the contact between the Cretaceous Myeongseongsan granite and the Precambrian carbonate rocks, and is also controlled by N-S-trending shear zone. The skarn distribution and mineralogy reflects both structural and lithological controls. Three types of skarn formations based on mineral assemblages in the Pocheon skarn exist; a sodiccalcic skarn and a magnesian skarn mainly developed in the dolostone, and a calcic skarn developed in the limestone. Iron mineralization occurs in the sodic-calcic and magnesian skarn zone, locally superimposed by copper mineralization during retrograde skarn stage. The sodic-calcic skarn is composed of acmite, diopside, albite, garnet, magnetite, maghemite, anhydrite, apatite, and sphene. Retrograde alteration consists of tremolite, phlogopite, epidote, sericite, gypum, chlorite, quartz, calcite, and sulfides. Magnesian skarn mainly consists of diopside and forsterite. Pyroxene and olivine are mainly altered to tremolite, with minor phlogopite, talc, and serpentine. The calcic skarn during prograde stage mainly consists of garnet, pyroxene and wollastonite. Retrograde alteration consists of epidote, vesuvianite, amphibole, biotite, magnetite, chlorite, quartz, calcite, and sulfides. Microprobe analyses indicate that the majority of the Pocheon skarn minerals are enriched by Na-Mg composition and have high $Fe^{3+}/Fe^{2+}$ , $Mg^{2+}/Fe^{2+}$ , and $Al^{3+}/Fe^{2+}$ ratios. Clinopyroxene is acmitic and diopsidic composition, whereas garnet is relatively grossular-rich. Amphiboles are largely of tremolite, pargasite, and magnesian hastingsite composition. The prograde anhydrous skarn assemblages formed at about $400^{\circ}{\sim}500^{\circ}C$ in a highly oxidized environment ( $fO_2=10^{-23}{\sim}10^{-26}$ ) under a condition of about 0.5 kbar pressure and $X(CO_2)=0.10$ . With increasing fluid/rock interaction during retrograde skarn, epidote, amphibole, sulfides and calcite formed as temperature decreased to approximately $250^{\circ}{\sim}400^{\circ}C$ at $X(CO_2)=0.10$ .】
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Tremolite
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Abstract The Qozlou Fe skarn deposit is located at the Abhar–Mahneshan belt of the Central Iranian Zone. It is associated with Upper Eocene porphyritic granite that intruded into the Upper Cretaceous impure carbonaceous rocks. The Qozlou granite has high-K calc-alkaline affinity and is classified as subduction-related metaluminous I-type granitoids. Skarn aureole in the Qozlou is composed of endoskarn and exoskarn zones, with the exoskarn zone being the main skarn and mineralized zone. It includes garnet skarn, garnet-pyroxene skarn, pyroxene skarn, epidote skarn, and pyroxene-bearing marble sub-zones. The Qozlou Fe deposit is 300 m long and 5–30 m wide. Magnetite is the main ore mineral associated to pyrite, chalcopyrite, and pyrrhotite. Garnet, clinopyroxene, actinolite, epidote, calcite, and quartz occur as gangue minerals. Covellite, hematite, and goethite were formed during the supergene processes. The ore and gangue minerals have massive, banded, disseminated, brecciated, vein–veinlets, replacement, and relict textures. EPMA data indicate that garnets have andradite–grossularite compositions (Ad 39.97–100 –Gr 0–49.62 ) and clinopyroxenes have diopsidic composition (En 29.43–42.5 –Fs 14.31–20.99 –Wo 43.08–50.17 ). Based on mineralogical and textural criteria, skarnification processes in the Qozlou skarn can be categorized into three discrete stages: (1) isochemical (metamorphic–bimetasomatic), (2) metasomatic prograde, and (3) metasomatic retrograde. Anhydrous calc-silicate minerals (garnet and clinopyroxene) were formed during the prograde metasomatic stage, while ore minerals and hydrous calc-silicate minerals were formed during the retrograde ore-forming sub-stage. Temperature and ƒO 2 conditions range between 430 and 550 °C and 10 −26 and 10 −23 , respectively, for the metasomatic prograde stage. The retrograde metasomatizing fluids had likely ƒS 2 = 10 −6.5 and temperatures < 430 °C at the beginning of the ore-forming sub-stage.
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Actinolite
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선캄브리아기 경기육괴의 북서부에 위치한 포천 스카른 광상은 명성산 화강암과 선캄브리아기 변성퇴적암류에 협재된 탄산염암의 접촉대를 중심으로 산출되며, N-S방향 전단대를 따라 배태되고 있다. 포천 스카른대 분포와 함께 광물학적 특성은 구조 규제와 암층 규제에 따라 유도되었다. 포천 스카른은 스카른 광물조합에 따라 백운암을 교대한 Na-Ca계열과 Mg계열 스카른 및 석회암을 교대한 Ca계열 스카른으로 구분된다. 철광화작용은 주로 Na-Ca계열 스카른대를 따라 배태되고 있으며, 후퇴 스카른 단계에 일부 동 광화작용이 중첩된다. Na-Ca계열 스카른은 주로 추휘석, 투휘석, 조장석, 석류석, 자철석, 매그헤마이트, 경석고, 인회석, 스핀의 공생관계를 보이며, 후퇴 스카른에서는 투각섬석, 금운모, 녹렴석, 견운모, 석고, 녹니석, 석영, 방해석, 황화광물로 구성된다. 한편 Mg계열 스카른은 주로 감람석과 투휘석의 단순한 광물조합을 보이며, 투휘석과 감람석은 투각섬석과 함께 소량 금운모, 사문석, 녹니석으로 교대된다. 반면에 Ca계열 스카른은 전진 스카른 단계에서 주로 단사휘석, 석류석, 규회석이 정출되며, 후퇴 스카른에서 녹렴석, 베수비아나이트, 각섬석, 흑운모, 녹니석, 자철석, 석영, 방해석, 황화광물이 수반된다. 전자현미분석 결과에 의하면 포천 스카른광물은 대부분 Na-Mg 성분이 부화되었으며, 높은 $Fe^{3+}/Fe^{2+}$ 비, $Mg^{2+}/Fe^{2+}$ 비, $Al^{3+}/Fe^{2+}$ 비의 조성 특징을 보인다. 즉 단사휘석은 추휘석과 투휘석 조성이 부화되어 있는 반면, 석류석은 상대적으로 그로슐라 조성이 부화된 경향을 보인다. 또한 각섬석은 투각섬석, 파가사이트, Mg-헤이스팅사이트로 조성변화를 보인다. 한편 전진 스카른 단계의 주요 광물조합은 약 0.5 kbar와 $X(CO_2)=0.10$ 조건에서 $400^{\circ}{\sim}500^{\circ}C$ 온도와 높은 산화 환경($fO_2=10^{-23}{\sim}10^{-26}$ )을 지시하고 있다. 후퇴 스카른 단계에서는 물-암석반응이 증가됨에 따라 녹렴석, 베수비아나이트, 각섬석, 녹니석, 석영, 방해석은 $X(CO_2)=0.10$ 조건에서 $250^{\circ}{\sim}400^{\circ}C$ 온도 범위에서 정출되었다. The Pocheon skarn deposit, located at the northwestern part of the Precambrian Gyeonggi massif in South Korea, occurs at the contact between the Cretaceous Myeongseongsan granite and the Precambrian carbonate rocks, and is also controlled by N-S-trending shear zone. The skarn distribution and mineralogy reflects both structural and lithological controls. Three types of skarn formations based on mineral assemblages in the Pocheon skarn exist; a sodiccalcic skarn and a magnesian skarn mainly developed in the dolostone, and a calcic skarn developed in the limestone. Iron mineralization occurs in the sodic-calcic and magnesian skarn zone, locally superimposed by copper mineralization during retrograde skarn stage. The sodic-calcic skarn is composed of acmite, diopside, albite, garnet, magnetite, maghemite, anhydrite, apatite, and sphene. Retrograde alteration consists of tremolite, phlogopite, epidote, sericite, gypum, chlorite, quartz, calcite, and sulfides. Magnesian skarn mainly consists of diopside and forsterite. Pyroxene and olivine are mainly altered to tremolite, with minor phlogopite, talc, and serpentine. The calcic skarn during prograde stage mainly consists of garnet, pyroxene and wollastonite. Retrograde alteration consists of epidote, vesuvianite, amphibole, biotite, magnetite, chlorite, quartz, calcite, and sulfides. Microprobe analyses indicate that the majority of the Pocheon skarn minerals are enriched by Na-Mg composition and have high $Fe^{3+}/Fe^{2+}$ , $Mg^{2+}/Fe^{2+}$ , and $Al^{3+}/Fe^{2+}$ ratios. Clinopyroxene is acmitic and diopsidic composition, whereas garnet is relatively grossular-rich. Amphiboles are largely of tremolite, pargasite, and magnesian hastingsite composition. The prograde anhydrous skarn assemblages formed at about $400^{\circ}{\sim}500^{\circ}C$ in a highly oxidized environment ($fO_2=10^{-23}{\sim}10^{-26}$ ) under a condition of about 0.5 kbar pressure and $X(CO_2)=0.10$ . With increasing fluid/rock interaction during retrograde skarn, epidote, amphibole, sulfides and calcite formed as temperature decreased to approximately $250^{\circ}{\sim}400^{\circ}C$ at $X(CO_2)=0.10$ .
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Scheelite
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Arsenopyrite
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