【作者】 蒋成竹；

【导师】 邓军；

【作者基本信息】 中国地质大学（北京） ， 矿物学、岩石学、矿床学， 2014， 博士

【摘要】 北衙金多金属矿床处于我国滇西地区扬子地块西缘，位于金沙江—哀牢山钾质斑岩带中段。截止2010年勘查成果显示已累计探明333类别以上黄金资源储量127.28吨。主要矿体类型为铁金矿体，为磁铁矿-赤铁矿-褐铁矿矿石。铁金矿体绝大部分沿新生代钾质斑岩边界分布，或分布于三叠纪灰岩接触带及断裂裂隙中。在北衙金矿万硐山矿段露天采场，大面积分布的石英正长斑岩基本无矿化蚀变，部分侵入其中的二长花岗斑岩具黄铁矿化，此外还有少量煌斑岩脉侵入其中。矿区石英正长斑岩与二长花岗斑岩表现出高钾富碱埃达克质岩石的地球化学特征，分析显示均具有较高K2O/Na2O比值（2.195~26.963）、Sr/Y比值（53.2~143.2）及(La/Yb)N比值（4.934~39.426）。通过锆石LA-ICPMS U-Pb年龄测定，这两类斑岩具有几乎一致的年龄，范围都在34~37Ma之间；并且二者岩浆锆石εHf(t)值也相似，范围在-6.82~+2.65之间。煌斑岩具很低Nb/U比值（1.6~4.8），显示形成矿区镁铁质侵入体的岩浆源区为交代的大陆岩石圈地幔。钾质斑岩与煌斑岩地球化学特征表现出一致的变化趋势，表明北衙矿区岩体是由富钾的幔源镁铁质岩浆与下地壳熔体混合上升演化形成。钾质斑岩中存在大量自形未遭受磨蚀的继承锆石，说明其来自原地及临近区域，其U-Pb年龄测试结果呈现三个峰值分别是840、500、280Ma。年龄集中于840Ma左右的继承锆石具有很大范围的εHf(t)值，从很低的负值到极高的正值均存在，如此大范围εHf(t)值暗示可能是经历了一次大洋板块俯冲作用而形成；500、280Ma继承锆石可能代表了另两次加厚下地壳与地幔的相互作用。以上说明可能是扬子地块西北缘在约840Ma遭受了交代大陆岩石圈地幔以及随后的两次壳幔相互作用。金属元素可能主要是在下地壳累积富集，然后随长英质岩浆运移到上地壳。通过研究认为矿区可能存在三幕成矿作用过程。第一幕在侵入岩与灰岩接触带形成矽卡岩及原生磁铁矿和硫化物。第二幕可能是形成二长花岗斑岩，且释放大量富含成矿金属元素的氧化性岩浆流体并形成大量黄铁矿。第三幕是铁金矿体遭受强烈表生风化作用影响而形成红土型矿体。这一连续多幕次的岩浆活动与成矿作用复合叠加使得北衙矿区金等成矿金属巨量富集。更多还原

【Abstract】 The Beiya gold deposit located in the western margin of the Yangtze block,Yunnan, SW China, and was situated in the middle segment of a belt of theJinshajiang-Ailaoshan potassic porphyry intrusion. It occupied an explored goldreserve about127.28t (the type of333and above) by the end of2010. Orebodiesmainly are massive magnetite-hematite-limonite ores, commonly called Fe-Auorebodies. The Fe-Au orebodies were mostly located along the boundary of theCenozoic potassic porphyry and within the faults in the Triassic limestone. In theopen-pit of the Wandongshan in Beiya gold deposit, quartz-K-feldspar porphyrylacking alteration and mineralization inside was intruded by the monzograniticporphyry with pyritization. In addition, some lamprophyre intruded into it.The two kinds of porphyries show potassic adakitic features in geochemistry, asexpressed by high K2O/Na2O (2.195~26.963), Sr/Y (53.2to143.2) and (La/Yb)N(4.934~39.426) ratios. They have identical zircon LA-ICPMS U-Pb ages from34Mato37Ma and similar εHf(t) ranging from-6.82to+2.65of the crystalline zircons. Thelamprophyres have extremely low Nb/U ratios of1.6~4.8, as is supportive that themafic intrusions were derived from a metasomatized subcontinental lithosphericmantle (SCLM). The geochemical features of the porphyries and the lamprophyre arecompatible with the explanation that the Beiya porphyries were formed via lowercrust melting and mixing with a K-rich mafic magma, which was derived frommetasomatized SCLM.Lots of inherited zircons entrained in the porphyries have euhedral shape,representing the parent rocks for the zircons were formed in the vicinity of the Beiyagold deposit. The inherited zircons have U-Pb ages clustered at peaks of800Ma,500Ma, and280Ma. The inherited zircons with ages around800Ma have εHf(t) rangingfrom largely negative to elevated positive values, which support the theory that thisgroup of zircons was formed via a slab subduction around800Ma. The detritalzircons with ages around500Ma and280Ma represent two other episodes of themantle interaction with thickened lower crust. These features can be explained by that the northwestern Yangtze experienced metasomatization of SCLM at~800Mafollowed by other episodic mantle-crust interactions. The barrenness of lamprophyresuggests that the metals might have accumulated in the lower crust, and then theywere carried into the upper crust via the felsic magma.The analysis via the Fe-Au orebody and porphyries suggested a three-episodeore-forming process. In the first episode, a small amount of skarn was developedlocally in the contact, and a few magnetite and sulfide. In the second episode, theoxidized elemental fluid migrated upwards, which perhaps relate to the alteredmonzogranite porphyry, and mostly pyrite were formed, as the upper crusts wasextensioning. In the third episode, the Fe-Au orebody suffered from intenseweathering and forming the laterite-type orebody.更多还原