大兴安岭岔路口巨型斑岩钼矿床角砾岩相的划分、特征及成因

Petrofacies classification,characteristics and formation mechanism of breccias in Chalukou giant molybdenum deposit of Da Hinggan Mountains

岔路口矿床是大兴安岭北段新发现的巨型斑岩钼矿床(Mo金属量246万t,工业品位0.087%),其形成与晚侏罗世晚期侵入的细晶斑岩和花岗斑岩关系最为密切。岔路口矿床中发育多种类型的角砾岩,包括岩浆角砾岩和热液角砾岩。通过对这些角砾岩详细的填图和鉴定,并根据其角砾类型、基质、胶结物和结构差异,又将热液角砾岩分为A相、B1相、B2相和E相4个亚相。其中,A相角砾岩形成最早,是细晶斑岩流体释放的产物,以无矿石英胶结物为特征。随后,花岗斑岩侵位形成了以长英质岩浆(含石英、长石斑晶)为胶结物的岩浆角砾岩,花岗斑岩流体的释放造成的超压作用和流体演化,形成了胶结物组合分别为石英+辉钼矿+黄铁矿、绢云母+伊利石+黄铁矿+萤石和绿泥石+碳酸盐+黄铁矿+闪锌矿+方铅矿+萤石的B1相、B2相和E相角砾岩。在角砾岩形成过程中,流体化作用造成的角砾混合和磨损是B1相和B2相角砾岩中复杂成分角砾和大量岩粉基质产生的原因。富基质的角砾岩虽然由于渗透性的降低,造成自身钼品位较低,但它代表了流体聚集的位置;在角砾岩形成过程中,它是高渗透性带,可以作为流体运移的通道,在成矿过程中起重要作用。略深的岩体侵位深度、单次较小释放量流体的多次注入、富氟的岩浆-热液系统及围岩先存薄弱构造是岔路口斑岩钼矿床内角砾岩主要呈脉状产出的原因。

The Chalukou porphyry molybdenum deposit, a newly discovered giant deposit in north Da Hinggan Mountains, is closely associated with late Late Jurassic aplite porphyry and granite porphyry. Various breccias were observed in the Chalukou deposit, comprising magmatic breccias and hydrothermal breccias. Based on the clast form and composition, matrix/cement constituents and textures, the authors classified hydrothermal breccias into four facies： Facies A, Facies B1, Facies B2 and Facies E. Facies A breccias, cemented by barren quartz, was formed by the fluid exsolution from aplite porphyry and seemed to be the earliest breccias. The emplacement of granite porphyry caused the development of magmatic breccias cemented by quartz- and feldspar-bearing felsic magma. Subsequently, overpressure caused by fluid exsolution from granite porphyry and fluid evolution constituted main factors for the formation of Facies B1, B2 and E breccias, which were characterized by cement assemblages of quartz ＋ molybdenite ＋ pyrite, sericite ＋ illite ＋ pyrite ＋ fluorite, and chlorite ＋ carbonate ＋ pyrite ＋ sphalerite ＋ galena ＋ fluorite, respectively. During the formation of breccias, fragment mixing and abrasion caused by fluidization resulted in the complex clasts and abundant matrix dominated by rock flour. Although the high matrix breccias have low Mo grade due to reduction of permeability, they represent the location of fluid accumulation during the formation of breccias and have high permeability. Thus, they might have acted as the channel to transport Mo-bearing fluid. The factors responsible for the existence of breccias in the veined form in the Chalukou Mo deposit may include large depth of intrusive emplacement, mutil-input of smaller volume of fluid exsolution at each stage, fluorine-rich magmatic-hydrothermal system and pre-existing weak structure of wall rocks.