中国还原性斑岩矿床研究进展及判别标志

世界上大多数斑岩矿床的成矿流体为氧化流体(CO_2 CH_4)。然而,Rowins(2000)提出一些斑岩Cu-Au矿床的成矿流体为富含CH_4的还原流体,矿床缺乏磁铁矿、赤铁矿和硬石膏等表征高氧逸度的矿物,而发育大量的磁黄铁矿,矿床规模小,矿床形成与含钛铁矿的还原性的Ⅰ型花岗岩类有关,并将其称之为还原性斑岩Cu-Au矿床。我国学者研究发现,中国不但发育还原性斑岩铜矿床,还发育还原性斑岩-矽卡岩铜矿床和还原性斑岩钼矿床,我们建议将这三种矿床统称为还原性斑岩矿床。本文基于课题组近十年来的研究工作,并结合前人的研究成果,综合分析了中国发育的大中型还原性斑岩矿床的典型实例,在此基础上,重点阐明中国大型还原性斑岩矿床的特点、流体中CH_4来源及其有关的成矿作用、容矿围岩特点、成矿岩浆氧化还原状态及其成因、矿床形成的构造背景等。与Rowins(2000)提出的还原性斑岩铜矿床规模小的特点不同,中国发育的一些还原性斑岩矿床规模大;我们研究还识别出该类矿床发育独特的热液矿物和矿石矿物,比如,还原性斑岩铜矿发育热液钛铁矿,矿石矿物以黄铜矿为主,罕见斑铜矿、辉铜矿等矿物;还原性斑岩钼矿床出现热液钛铁矿,矿石矿物以辉钼矿为主,罕见黑钨矿和锡石等矿物;还原性斑岩-矽卡岩铜矿床的矽卡岩期发育钙铝榴石、钙铁辉石等还原性矽卡岩矿物和大量的磁黄铁矿,热液期以发育黄铜矿而非斑铜矿和辉铜矿等矿石矿物为特征。因此,还原性斑岩矿床除了Rowins(2000)提出的发育富CH_4还原流体和磁黄铁矿等识别标志之外,还可辅以独特的脉石矿物(如钛铁矿、钙铝榴石、钙铁辉石等)和简单的矿石矿物(如黄铜矿、辉钼矿等)这两个标志进行识别。中国还原性斑岩矿床含矿岩体的围岩中普遍发育还原性岩石(如含碳质沉积岩或火山沉积岩、含亚铁的火山岩或火山沉积岩等);对于成矿流体中CH_4、C_2H_6等还原性气体的来源,多数学者认为CH_4、C_2H_6等还原性气体主要源于还原性围岩,部分源于岩浆。关于还原性斑岩矿床的成矿岩体是否为含钛铁矿的、还原性的花岗岩类,目前研究较少且存在争议,多数学者认为成矿原始岩浆为氧化性岩浆,但其氧逸度偏低,少数学者认为成矿岩浆始终为还原岩浆。还原性斑岩矿床与经典的斑岩矿床的成矿构造背景类似,二者没有明显区别。还原性斑岩矿床显示的还原性热液蚀变和成矿特点均与成矿流体富含CH_4还原气体密切相关,因此,富含CH_4还原流体是还原性斑岩矿床形成的关键。

Advances and its diagnostic criteria in the study of the reduced porphyry ore deposits in China

Most porphyry ore deposits worldwide have fluid inclusions with CO₂>>CH₄ that are indicative of high oxygen fugacity (f_O2). However, Rowins (2000) proposed that some porphyry Cu-Au deposits lack primary hematite, magnetite and anhydrite, but contain abundant hypogene pyrrhotite, which commonly have carbonic-rich ore fluids with substantial CH₄. These porphyry Cu-Au deposits, associated with ilmenite-bearing, reduced I-type granitoids, are small in size, and are called the reduced porphyry Cu-Au deposits. Recently, Chinese scholars recognized the reduced porphyry-skarn Cu deposits and the reduced porphyry Mo deposits besides the reduced porphyry Cu deposits in China. We named all of them as the reduced porphyry ore deposits. Based on lots of investigations and studies in recent ten years done by the authors, combined with the previous researches, this paper tentatively makes a comprehensive elucidation of typical reduced porphyry deposits in China and focuses on the characteristics of large reduced porphyry deposits and their wall rocks, the source of CH₄ in the ore-forming fluid and related mineralization, the redox state of the ore-forming magmas and its genesis, and the metallogenic tectonic setting. Unlike the small-size reduced porphyry Cu deposits proposed by Rowins (2000), many reduced porphyry deposits in China are large-size. Our study also identified some unusual hydrothermal alteration and metallogenic characteristics. For example, the hydrothermal alteration of the reduced porphyry Cu deposit is characterized by predominant ilmenization rather than magnetization and its ore minerals are chalcopyrite with rare bornite, chalcocite; the hydrothermal alteration of the reduced porphyry Mo deposit occurs ilmenization and its ore minerals is molybdenite, without wolframite and cassiterite. The reduced porphyry-skarn Cu deposits are marked by a reduced skarn mineralogy (e.g. hessonite and hedenbergite) and abundant pyrrhotite in the skarn stage and by major chalcopyrite rather than bornite and chalcocite in the hydrothermal stage. The diagnostic criteria of reduced porphyry deposits, therefore, also includes the criteria of unique gangue minerals (e.g. ilmenite, hessonite and hedenbergite) and simple ore minerals (e.g. chalcopyrite, molybdenite) besides the marks (e.g. CH₄-rich fluid and abundant pyrrhotite) proposed by Rrowins (2000). Generally, the porphyry ore deposits exposes reductive strata (e.g. carbon-bearing sedimentray rocks, ferrous-bearing mafic volcanic rocks) where the mineralized porphyry intruded. There are different opinions about the origin of CH₄ in ore-forming fluid. Most scholars believe that major CH₄ is derived from reductive wall rocks and minor from magma. The oxidation state of the most mineralized intrusions has not been identified or is controversial. Most scholars propose that the primary magma was oxidized magma but with relatively low oxygen fugacity (f_O2). Minor suggests that the magma was always reduced magma. As far as the tectonic setting was concerned, it is similar between the reduced porphyry deposits in China and typical porphyry deposit in the world. The reductive characteristics of the reduced porphyry deposits in China are closely related to the CH₄-bearing ore-forming fluid, which is the key to the formation of reduced porphyry deposits.