陆-陆碰撞造山环境下含铜斑岩岩石成因——以藏东玉龙斑岩铜矿带为例

大型斑岩铜矿不仅可以形成于陆缘弧和岛弧构造环境,而且还可以形成于陆-陆碰撞造山环境.然而,对陆-陆碰撞造山环境下含矿斑岩的成因仍存在很大的争论.本文以藏东玉龙斑岩铜矿带为例,从岩相学、元素地球化学以及Sr-Nd-Pb-Hf同位素地球化学等方面较系统地研究了含矿斑岩岩石成因.结果表明,含矿斑岩属于钾玄质岩石,同时具有埃达克岩某些地球化学特征.岩石是由至少100km深处的二辉橄榄质岩石圈地幔中交代成因的金云母-石榴石单斜辉石岩脉发生低程度部分熔融而形成的.印度-亚洲大陆碰撞形成了金沙江区域性走滑断裂系统,并导致软流圈地幔上涌,最终诱发交代岩石圈地幔的部分熔融而形成含矿斑岩.它对于进一步认识陆-陆碰撞造山环境下含矿斑岩的起源以及斑岩型铜矿的成因具有较为重要的意义.     

大型斑岩铜矿成矿的深部构造岩浆活动背景

笔者以深部地球物理探测结果为基础，将地球物理与地质、地球化学资料结合起来分析了南美稳态的洋陆俯冲体制下安第斯斑岩铜矿带、藏南不稳态的陆陆碰撞体制下冈底斯斑岩铜矿带和不稳态的陆岛陆碰撞体制下三江地区的玉龙斑岩铜矿带渐新世以来深部成矿的构造岩浆活动背景条件，总结归纳出6条成矿的基本条件：（1）要有上地幔来源的高温岩浆热液的大量供给；（2）要有区域性铜的萃取来源；（3）要有上地壳的部分熔融层或岩浆房。以提供含铜热液的循环沉淀条件；（4）地壳上层构造体系中的张性断裂。以供岩浆热液的就位；（5）围岩的化学性质对矿质沉淀是重要的；（6）构造的相对稳态期。为矿质大量沉淀积累创造条件。在不稳态体制下要寻找相对稳态的时段。陆陆碰撞体制与洋陆俯冲体制下成矿作用的主要不同之处是构造的不稳态期长、陆壳加厚与构造复杂化．岩石层碎裂发育机制等。提出了冈底斯带的新的找矿远景地带。     

碰撞造山带斑岩型矿床的深部约束机制

在印度-亚洲大陆碰撞过程中,俯冲板片断离触发了幔源岩浆底侵作用、下地壳部分熔融和冈底斯岩基带以及同岩基斑岩的产生.在此过程中,幔源岩浆分离结晶的产物、下地壳岩石部分熔融残余和地壳分异过程中下沉的镁铁质块体,构成了加厚下地壳.随着造山岩石圈的冷却和加厚下地壳重力不稳定性的增加,岩石圈拆沉作用触发了后碰撞斑岩型岩浆活动.与此相应,碰撞造山带斑岩型矿床可以形成于同碰撞和后碰撞两个不同的构造阶段.同碰撞成矿作用发生于岩基带形成时期,成矿物质主要来自于底侵幔源岩浆及更深部的含矿流体,其触发机制是俯冲板片的断离.后碰撞成矿作用发生于加厚下地壳冷却之后,成矿物质主要来自于新生矿源层和更深部的含矿流体,其触发机制为岩石圈拆沉作用.在同碰撞构造阶段,伴随着幔源岩浆的底侵作用,深部流体和幔源岩浆所含的成矿物质被注入到岩基岩浆中,与从岩基岩浆源区萃取的成矿物质汇聚在一起,一部分受岩基热的驱使上升成矿.由于流体中成矿元素的浓度强烈依赖于压力,另一部分成矿元素则滞留在难熔残余中形成新的矿源层.当发生岩石圈拆沉作用时,由此矿源层部分熔融形成的斑岩岩浆将相对富含成矿物质,导致碰撞造山带第二次成矿作用大爆发.     

新疆尼勒克县圆头山后碰撞花岗斑岩的同位素年代学及地球化学特征

圆头山黑云母花岗斑岩位于尼勒克县城南,岩体普遍发育的铜矿化,以中部碎裂带中矿化最强。岩石具有典型的斑状结构,斑晶由碱性长石和黑云母组成,基质以长石、云母和石英为主,并含有少量的硫化物。圆头山黑云母花岗斑岩的轻重稀土分馏明显,并且明显富集大离子亲石元素(LILE) (除Sr外)和Pb,同时亏损高场强元素(HFSE)和Nb、Ta、P、Ti,显示出弧火山岩的地球化学特征。圆头山黑云母花岗形成年龄为269±3Ma,其源区以具有弧火山岩特征的下地壳为主,同时含有少量的幔源物质,而且在岩体的侵位过程中有高(⁸⁷Sr/⁸⁶Sr)_i (0.706054~0.709228)和高Pb含量(5.05×10^-6~32.5×10^-6)的陆壳物质混染。在下地壳物质熔融过程中,位于下地壳底部的富水和成矿元素的MASH带物质也被圈入,使原始岩浆富水和成矿物质。圆头山黑云母花岗斑形成于碰撞后阶段,岩石的形成主要受岩石圈地幔拆沉作用的控制,而岩石圈地幔的拆沉可能与中亚造山带的垮塌或塔里木地幔柱活动有关。     

西藏雄村矿集区侏罗纪斑岩磷灰石矿物化学特征对新特提斯洋俯冲成矿作用的指示

雄村矿集区位于西藏冈底斯成矿带南缘,区内成矿作用与新特提斯洋俯冲作用有关,现已勘查评价了一个大型还原性斑岩铜金矿床(I号矿体)和一个大型氧化性斑岩型铜金矿床(Ⅱ号矿体)。为了查明引起I、Ⅱ号矿体成矿特征差异的原因和含矿斑岩的区别标志,本文选取I、Ⅱ号矿体含矿斑岩和非含矿斑岩中磷灰石为研究对象,通过对三套斑岩磷灰石单矿物的挑选、阴极发光照相(CL)和电子探针分析(EMPA),提出了磷灰石中Cl含量受控于共存硅酸盐熔体组分,I、Ⅱ号矿体含矿斑岩比非含矿斑岩具有更高的Cl/F(分别为0.76、0.26、0.07),高的Cl含量对促进Cu、Au的溶解和迁移具有重要作用的新认识;含矿斑岩磷灰石高Cl低S指示金属元素Cu、Au可能以Cl的络合物为主进行迁移([CuCl_2]~–、[CuCl]~0、[AuCl_2]~–);I、Ⅱ号矿体含矿斑岩氧逸度低于非含矿斑岩,含矿斑岩较低的氧逸度可能指示成矿过程可能发生了结晶硫化物相的预富集;雄村矿集区I、Ⅱ号矿体含矿斑岩磷灰石与非含矿斑岩相比具有更高的Mn、Ca、Cl含量,更低的Si、S、F含量,可以作为含矿斑岩的判别标志,对指导区域与新特提斯洋俯冲有关的斑岩型铜金矿床找矿突破有重要义。     

西藏玉龙斑岩铜矿带南段日曲花岗闪长斑岩成因:锆石U-Pb年代学和地球化学约束

为探讨西藏玉龙斑岩铜矿带南段斑岩的成因及其动力学机制,对该铜矿带南段日曲岩体开展了岩石学、同位素年代学和地球化学研究。结果显示,两件花岗闪长斑岩锆石U-Pb年龄分别为(38.2±0.2)Ma、(38.8±0.2)Ma,为喜马拉雅早期;花岗闪长斑岩富集大离子亲石元素,亏损高场强元素,属弱过铝质高钾钙碱性花岗岩;斑岩具有C型埃达克岩的地球化学特征,为加厚下地壳部分熔融的产物,未经明显的地壳混染;源区残留相主要由角闪石、石榴子石、金红石组成,并经历了较弱的壳幔混合作用。研究表明,日曲岩体的形成与印度板块与欧亚板块碰撞诱发妥坝-芒康左行走滑断裂活动有关,下地壳部分熔融及少量幔源物质的混入使斑岩具有壳幔混合的特征。     

Geochronology and Geochemistry of the Mamupu Cu-Au Polymetallic Deposit, Eastern Tibet: Implications for Eocene Cu Metallogenesis in the Yulong Porphyry Copper Belt

The Mamupu skarn-type Cu-Au polymetallic deposit represents the first discovery of a medium deposit in the southern Yulong porphyry copper belt (YPCB), eastern Tibet. The Cu-Au mineralization mainly occurs as chalcopyrite in breccia, within the plate-like carbonate interlayer, being closely related to chloritization (e.g., chlorite, magnetite and epidote) and skarnization (e.g., diopside, tremolite and garnet). The ore-related quartz syenite porphyry (QSP) and granodiorite porphyry (GP) were emplaced at 40.1 ± 0.2 Ma and 39.9 ± 0.3 Ma, respectively. The QSP of Mamupu is an alkaline-rich intrusion, relatively enriched in LREE, LILE, depleted in HFSE, with no significant negative Eu and Ce anomalies, slightly high (87Sr/86Sr)i, low εNd(t), uniform (206Pb/204Pb)i and εHf(t) values, which indicates that the porphyry magma may be caused by both the mixing of metasomatized EM II enriched mantle and thickened juvenile lower crust. The QSP in the Mamupu deposit shares a similar genesis of petrology to other ore-related porphyries within the YPCB. High oxygen fugacity and water content of the magmas are essential for the formation of porphyry and skarn Cu deposits. The QSP has similar high magmatic oxidation states and water content to the Yulong deposit, which indicates that the Mamupu has a high prospecting potential. Differences in the geological characteristics and scale of mineralization between the Mamupu and other YPCB deposits may be due to the different emplacement depths of ore-related intrusions, as well as differences in the surrounding rocks.     

Geology and Hydrothermal Alteration of the Duobuza Gold‐Rich Porphyry Copper District in the Bangongco Metallogenetic Belt,Northwestern Tibet

The Duobuza gold‐rich porphyry copper district is located in the Bangongco metallogenetic belt in the Bangongco‐Nujiang suture zone south of the Qiangtang terrane. Two main gold‐rich porphyry copper deposits (Duobuza and Bolong) and an occurrence (135 Line) were discovered in the district. The porphyry‐type mineralization is associated with three Early Cretaceous ore‐bearing granodiorite porphyries at Duobuza, 135 Line and Bolong, and is hosted by volcanic and sedimentary rocks of the Middle Jurassic Yanshiping Formation and intermediate‐acidic volcanic rocks of the Early Cretaceous Meiriqie Group. Simultaneous emplacement and isometric distribution of three ore‐forming porphyries is explained as multi‐centered mineralization generated from the same magma chamber. Intense hydrothermal alteration occurs in the porphyries and at the contact zone with wall rocks. Four main hypogene alteration zones are distinguished at Duobuza. Early‐stage alteration is dominated by potassic alteration with extensive secondary biotite, K‐feldspar and magnetite. The alteration zone includes dense magnetite and quartz‐magnetite veinlets, in which Cu‐Fe‐bearing sulfides are present. Propylitic alteration occurs in the host basic volcanic rocks. Extensive chloritization‐silicification with quartz‐chalcopyrite or quartz‐molybdenite veinlets superimposes on the potassic alteration. Final‐stage argillic alteration overlaps on all the earlier alteration. This alteration stage is characterized by destruction of feldspar to form illite, dickite and kaolinite, with accompanying veinlets of quartz + chalcopyrite + pyrite and quartz + pyrite assemblages. Cu coexists with Au, which indicates their simultaneous precipitation. Mass balance calculations show that ore‐forming elements are strongly enriched during the above‐mentioned three alteration stages.     

基于蚀变信息提取的西藏班公湖-怒江成矿带中段斑岩铜矿找矿预测

本文选取西藏班公湖-怒江成矿带西段多不杂斑岩铜矿为已知矿床,通过对采样样品实测波谱特征的分析,基于 ASTER 遥感数据,采用目前主要的蚀变信息提取算法——彩色增强、比值算法及光谱角制图算法,在班公湖-怒江成矿带中段开展蚀变信息提取及找矿靶区圈定研究,预测了一处成矿前景区域.通过对此预测区域的野外查证,发现大量的孔雀石露头,室内电感耦合等离子质谱(ICP-MS)分析得出铜品位在0.18%~1.95%之间,平均为0.51%.对该区域进行了激电测量工作,测量结果和遥感结果相互印证,表明此区域为一处斑岩铜矿成矿前景区.