甘肃干沙鄂博稀土矿床地质特征及矿床成因分析

GEOLOGY AND GENESIS OF THE GANSHANEBO REE DEPOSIT IN THE NORTHERN QILIAN FOLD BELT, GANSU PROVINCE

干沙鄂博稀土多金属矿床位于北祁连加里东褶皱带东段,成矿与干沙鄂博碱性斑岩体有关。干沙鄂博斑岩体为一小型岩株,地表出露面积约0.29km^2,岩性以霓辉正长斑岩为主,其次为霓辉碱长石英正长斑岩。岩体具全岩面型蚀变矿化特征,与稀土矿化关系密切的围岩蚀变主要为萤石化、碳酸盐化和硅化。矿化范围大,矿带长约800m,延深大于420m,5个主矿体长70-260m,平均厚度12-29m。矿石品位较低,稀土氧化物平均品位1.39%-1.65%,矿石中伴生金属元素有Cu、Pb、Mo。矿石矿物以氟碳钙铈矿为主,其他金属矿物有黄铜矿、方铅矿、斑铜矿及辉钼矿等。含矿斑岩体为一套准铝质和碱性岩石,总体具有高硅低钛、富钾低钠的特点。岩石富集K、Rb、Sr、Ba等不相容元素,亏损Ta、Nb、Zr、Ti等高场强元素;轻重稀土分馏强烈,显著富集轻稀土,具Eu负异常。以上地球化学特征表明干沙鄂博碱性斑岩体的成因可能与华力西晚期后造山伸展环境下富集地幔的部分熔融有关,并经历了显著的分离结晶作用。通过与国内外典型稀土矿床的对比分析认为,干沙鄂博稀土矿矿床成因类型为碱性斑岩岩浆期后热液型多金属稀土矿床,与冕宁牦牛坪和美国加里诺斯稀土矿的矿床成因类似。

The Ganshaebo REE （rare earth elements） deposit is located in the eastern portion of the Caledonian Qilian fold belt. The mineralization is hosted in and genetically related to the Ganshaebo alkaline porphyry, which has an exposure of ca. 0.29km^2 and is eompositionally dominated by aegirine augite syenite with lesser amount of aegirine augite quartz syenite. The intrusive rocks are characterized by extensive hydrothermal alteration, with fluoritization, carbonatization, and silieification being most closely related to REE mineralization. The deposit is manifested by a ca. 800-m-long mineralized zone and consists of five major ore bodies, individually 70-260 m long, 12-29 m thick on average and continuous for more than 420 m downdip. The ores have an tenor of 1.39-1.65 wt.% REO, with associated Cu, Pb, and Mo. Parasite is the most important ore mineral, which is variably associated with chalcopyrite, galena, bornite, and molybdenite. The mineralized porphyry is metaluminous and alkaline and has high SiO2 and K2O contents coupled with low TiO2 and Na2O abundances. Rocks are enriched large ion lithophile elements （LILE; e.g., Rb, Sr, Ba） but depleted in high field strength elements （HFSE; e.g., Ta, Nb, Zr, Ti）. In addition, they are characterized by strong fractionation between LREE and HREE with moderate to strong Eu anomalies. The geochemical signatures, when combined with available geological and geochronological data, indicate that the Ganshaebo alkaline porphyry was most likely derived from partial melting of an enriched lithosphere mantle source under an extensional regime related to the late Varisian orogenesis, followed with significant fractional crystallization. A comparative analysis of geological and mineralizing features between the Ganshaebo deposit and REE deposits worldwide suggests that the former resemble the Maniuping REE deposit in Mianning, Sichuan Province and the those in the Gallinas Mountain of American, and thus can be classified as a magmatic hydrothermal polymetallic REE deposits associated with alkaline porphyry.