岩石学报  2020, Vol. 36 Issue (4): 967-994, doi: 10.18654/1000-0569/2020.04.01   PDF    
中国还原性斑岩矿床研究进展及判别标志
申萍1,2,3, 潘鸿迪4     
1. 中国科学院矿产资源研究重点实验室, 中国科学院地质与地球物理研究所, 北京 100029;
2. 中国科学院地球科学研究院, 北京 100029;
3. 中国科学院大学, 北京 100049;
4. 长安大学地球科学与资源学院, 西安 710054
摘要: 世界上大多数斑岩矿床的成矿流体为氧化流体(CO2>>CH4)。然而,Rowins(2000)提出一些斑岩Cu-Au矿床的成矿流体为富含CH4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CH4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CH4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CH4、C2H6等还原性气体的来源,多数学者认为CH4、C2H6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CH4还原气体密切相关,因此,富含CH4还原流体是还原性斑岩矿床形成的关键。
关键词: 还原性斑岩矿床    还原流体    CH4来源    钛铁矿    判别标志    
Advances and its diagnostic criteria in the study of the reduced porphyry ore deposits in China
SHEN Ping1,2,3, PAN HongDi4     
1. Key Laboratory of Mineral Resources, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China;
2. Innovation Academy of Earth Science, Chinese Academy of Sciences, Beijing 100029, China;
3. University of Chinese Academy of Sciences, Beijing 100049, China;
4. College of Earth Sciences, Chang'an University, Xi'an 710054, China
Abstract: Most porphyry ore deposits worldwide have fluid inclusions with CO2>>CH4 that are indicative of high oxygen fugacity (fO2). 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 (fO2). 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.
Key words: Reduced porphyry ore deposit    Reduced ore-forming fluid    CH4 source    Ilmenite    Diagnostic criteria    

斑岩矿床是世界上铜、钼和铼的主要来源和金的重要来源,是矿床研究和勘查的重要目标(Hou et al., 2003, 2015; Sillitoe, 2010Sun et al., 2013, 2015; Mao et al., 2014Chen et al., 2017a, b毛景文等,2018)。世界上大多数斑岩铜矿床的一个重要特征是成矿早阶段流体为氧化性流体,其氧逸度(log fO2)>NNO(NNO是镍-氧化镍缓冲剂;Hemley et al., 1992; Williams et al., 1995),随着磁铁矿大量结晶,流体氧逸度降低,金属以硫化物形式发生沉淀(Sun et al., 2013, 2015),成矿流体中CO2> >CH4(Mungall, 2002Rusk and Reed, 2002);矿床发育表征高氧逸度的热液矿物(如磁铁矿、赤铁矿和硬石膏等)以及黄铜矿、斑铜矿等铜-铁硫化物;矿床的形成与磁铁矿系列的氧化性Ⅰ型花岗岩类有关(Cooke et al., 2005Sillitoe, 2010; Sun et al., 2013, 2015),岩浆氧逸度log fO2> FMQ+2(FMQ是橄榄石-磁铁矿-石英缓冲剂;Mungall, 2002)。斑岩-矽卡岩铜矿床也具有类似的特点,且矽卡岩以富Fe3+的钙铁榴石和富Mg的透辉石为特征,铜-铁硫化物矿物以斑铜矿和辉铜矿为主(Meinert et al., 2005毛景文等, 2013, 2018)。斑岩钼矿床包括Climax-型、Endako-型和大别型(Chen et al., 2017a, b; Sinclair, 2007),这些矿床均发育磁铁矿、赤铁矿、硬石膏等矿物,成矿流体中几乎不出现CH4,成矿岩浆的氧逸度不一定很高,但也应满足log fO2>FMQ+0.5的条件(Candela and Holland, 1984Tacker and Candela, 1987)。为了便于研究,我们将上述矿床称之为经典的斑岩矿床。

尽管如此,Rowins(2000)发现位于环太平洋成矿域的一些斑岩Cu-Au矿床具还原性成矿特点,比如,矿床缺乏磁铁矿、赤铁矿、硬石膏等表征高氧逸度的矿物,而发育大量的磁黄铁矿;成矿流体为富含CH4的还原流体(log fO2 < FMQ);矿床形成与含钛铁矿的、还原性的Ⅰ型花岗岩类(log fO2 < FMQ)有关;矿床规模小,Rowins(2000)将其称之为还原性斑岩Cu-Au矿床,以加拿大西部的North Fork、Madeleine和Rosslan、墨西哥的Minãs de San Anton以及西澳大利亚的17 Mile Hill和Boddington等矿床为代表(Ague and Brimhall, 1988Randall et al., 1994Rowins, 1999, 2000)。随后,Rowins研究团队还发现加拿大西部的Catface斑岩Cu-Mo-Au矿床也具有还原性的成矿特点,将还原性斑岩Cu-Au矿床的还原性定名,扩大到了还原性斑岩Cu-Mo-Au矿床的范畴(Smith et al., 2012),此后,还原性斑岩铜矿床的定名被普遍应用。

近年来,经我国学者研究提出了还原性斑岩-矽卡岩铜矿床(张伟等, 2017)的认识,我们研究还发现一些斑岩钼矿床也具有上述还原性成矿特征,并将其命名为还原性斑岩钼矿床(曹冲,2018Cao et al., 2020)。可见,世界上既发育还原性斑岩铜矿床,也发育还原性斑岩-矽卡岩铜矿床和还原性斑岩钼矿床。由于这些矿床具有共同的还原性成矿特点(比如,发育大量的磁黄铁矿,成矿流体富含CH4),为了便于同经典的斑岩矿床相区别,也便于进一步研究,我们将这三者统称为还原性斑岩矿床。

基于本课题组近十年来对新疆包古图、苏云河、石屋、宏远、吐克吐克、喇嘛苏以及内蒙古白乃庙等还原性斑岩矿床研究工作(Shen et al., 2010a, b; Shen and Pan, 2013, 2015; 潘鸿迪和申萍,2014鄢瑜宏等,2015钟世华等,2015Cao et al., 2017, 2020Li et al., 2017; 李昌昊等,2017申萍等,2017李文广等,2018李文广,2019马阁等,2019),并结合前人研究所取得的重要进展(Rowins, 1999, 2000李文博等,2007石海岗,2011Smith et al., 2012郎兴海,2012何鹏等,2013Cao et al., 2014a, bRen and Ma, 2015Tang et al., 2015王辉等,2015王银宏等,2015吴楚等,2015张方方,2016周云等,2017周振华等,2017Xie et al., 2018Zhu et al., 2012Wei et al., 2019),在简要介绍中国典型还原性斑岩矿床实例的基础上,本文重点讨论了中国还原性斑岩矿床成矿特点、成矿流体中的CH4来源及其有关的成矿作用、成矿岩浆氧化还原状态及其成因、围岩特点、成矿构造背景等方面的研究进展与存在问题,并补充了还原性斑岩矿床的判别标志,期望对世界斑岩矿床的成矿理论进行一些补充。

1 中国还原性斑岩矿床的厘定

关于还原性斑岩矿床的命名,虽然Rowins(2000)提出还原性斑岩Cu-Au矿床的形成与含钛铁矿的、还原性的Ⅰ型花岗岩类有关,但是,在他列举的11个矿床实例中,有6个矿床(如San Anton、Copper Canyon、Shotgun、Boddington、Clark Lake、Lac Trollos)尚未确定岩体为含钛铁矿的、还原性的Ⅰ型花岗岩类,有1个矿床(Madeleine)岩体确定为含钛铁矿和磁铁矿的花岗岩类,甚至有1个矿床(Rossland)岩体确定为含磁铁矿的花岗岩类,只有3个矿床(17 Mile Hill、Liberty Bell、Fort Knox)明确岩体为含钛铁矿的、还原性的Ⅰ型花岗岩类,然而,这3个矿床中有2个矿床(Liberty Bell、Fort Knox)实际上是金矿床。尽管如此,Rowins(2000)仍然依据其矿床具有明显的还原性成矿特点而将这11个矿床一并称之为还原性斑岩Cu-Au矿床,可见,Rowins(2000)命名还原性斑岩Cu-Au矿床的依据主要是矿床明显具有还原性成矿特点,并不一定需要限定成矿岩体的氧化还原性质。

实际上,矿床学中这种命名方法并不少见,譬如,相对于经典的还原性矽卡岩型金矿床(矽卡岩以钙铁辉石为主,硫化物大量发育,且硫化物以磁黄铁矿和毒砂为主,成矿岩体为还原性的闪长岩-花岗闪长岩),氧化性矽卡岩型金矿床的厘定仅仅考虑了矿床具有明显的氧化性成矿特点(比如,矽卡岩以透辉石为主,硫化物不发育,且硫化物以黄铁矿为主),并不涉及岩体的氧化还原性质(Brooks et al., 1991Meinert et al., 2005)。因此,可以认为,还原性斑岩矿床定名的关键是矿床是否具有明显的还原性成矿特点,辅以岩体的氧化还原性质。当然,随着研究的深入,这一认识将被补充或修订。

我国学者已经提出西藏的雄村Ⅰ号铜金矿体(郎兴海,2012)、内蒙古的布敦化(武新丽等, 2012)和太平川(黄世武等, 2010)等为还原性斑岩铜金矿床,也提出云南的浪都(Ren and Ma, 2015)和新疆的色勒特果勒(张伟等,2017)等为还原性斑岩-矽卡岩铜矿床(图 1)。我们将中国发育的斑岩矿床中符合Rowins(2000)提出的还原性斑岩Cu-Au矿床特点的其它一些大、中型矿床一并呈现在图 1中,并显示在表 1中,这些矿床包括新疆的包古图(申萍等,2010aShen et al., 2010a; Shen and Pan, 2013, 2015; Cao et al., 2014a, b潘鸿迪和申萍,2014; 李文广等,2018)和达巴特(刘畅,2018)、内蒙古的白乃庙(李文博等,2007周振华等,2017)等斑岩铜矿床,新疆的喇嘛苏(石海岗,2011Zhu et al., 2012李文广,2019)、青海的赛什塘(王辉等,2015Lu et al., 2016)等斑岩-矽卡岩铜矿床、新疆的苏云河(钟世华等,2015Cao et al., 2017, 2020申萍等,2017曹冲,2018)、宏远(鄢瑜宏等, 2014, 2015申萍等,2017)以及白山(王银宏等,2015张方方,2016)等斑岩钼矿床。

图 1 中国主要还原性斑岩矿床及其分布 还原性斑岩铜矿床:新疆的包古图(Shen et al., 2010a)和达巴特(刘畅,2018),内蒙古的白乃庙(李文博等,2007)、布敦化(武新丽等, 2012)和太平川(黄世武等, 2010),西藏的雄村Ⅰ号矿体(郎兴海, 2012);还原性斑岩-矽卡岩铜矿床:新疆的喇嘛苏(Zhu et al., 2012)和色勒特果勒(张伟等,2017),青海的赛什塘(何鹏等,2013Lu et al., 2016),云南的浪都(Ren and Ma, 2015);还原性斑岩钼矿床:新疆的苏云河(钟世华等,2015曹冲,2018)、宏远(鄢瑜宏等,2015)、莱历斯高尔(Zhu et al., 2012)和白山(张方方,2016) Fig. 1 Distribution map of the major reduced porphyry ore deposits in China Reduced porphyry Cu deposits: Baogutu (Shen et al., 2010a) and Dabate (Liu, 2015) in Xinjiang; Bainaimiao (Li et al., 2007), Budunhua (Wu et al., 2012) and Taipingchuan (Huang, 2010) in Inner Mongolia; Xiongcun No. I orebody in Tibet (Lang, 2012). Reduced porphyry-skarn Cu deposits: Lamas (Zhu et al., 2012) and Selateguole (Zhang et al., 2017) in Xinjiang; Saishitang in Qinhai (He et al., 2013; Lu et al., 2016); Langdu in Yunnan (Ren and Ma, 2015). Reduced porphyry Mo deposits: Suyunhe (Zhong et al., 2015; Cao, 2018), Hongyuan (Yan et al., 2015), Lailisigaer (Zhu et al., 2012) and Baishan (Zhang, 2016) in Xinjiang

表 1 中国主要还原性斑岩矿床的基本特征 Table 1 The basic characteristics of major reduced porphyry deposits in China

① 申萍等. 2010a.国家科技支撑计划“大型斑岩型铜(钼、金)矿床预测和靶区评价技术与应用研究”(2006BAB07B01)课题报告.162-182

Rowins(2000)提出的还原性斑岩Cu-Au矿床规模小不同,中国发育许多大型还原性斑岩矿床(图 2表 1),比如雄村Ⅰ号、包古图和白乃庙等斑岩铜矿床(Cu金属量>50万吨)、苏云河和白山等斑岩钼矿床(Mo金属量>50万吨)、喇嘛苏和赛什塘等斑岩-矽卡岩铜矿床(Cu金属量>40万吨)。

图 2 中国大型还原性斑岩矿床成岩和成矿年龄以及铜、钼储量柱状图(资料来源见表 1) Fig. 2 Plot of the tonnage of contained copper and molybdenum metal versus age based on the large-size porphyry deposits in China (Data listed in Table 1)
2 中国还原性斑岩矿床典型实例 2.1 还原性斑岩铜矿床 2.1.1 包古图铜矿床

包古图斑岩铜矿床位于新疆西准噶尔南部,是新疆第二大斑岩铜矿床,铜、钼和金的金属量分别为63万吨、1.8万吨和14吨,其平均品位分别为0.28%、0.011%和0.1g/t(表 1)。在前人研究(张锐等,2006宋会侠等,2007刘玉琳等,2009张志欣等,2010)的基础上,我们研究认为,包古图晚石炭世含矿岩体是一个由早期闪长岩体和晚期闪长玢岩岩脉组成的中偏基性的复式岩体,并叠加有隐爆角砾岩(潘鸿迪和申萍,2014)。该复式岩体的直接围岩是早石炭世包古图组凝灰质砂岩、沉凝灰岩和少量安山岩以及希贝库拉斯组凝灰质砂岩和砾岩(沈远超和金成伟,1993)。复式岩体的早期闪长岩体包括闪长岩、似斑状闪长岩和似斑状石英闪长岩以及少量的辉长岩;晚期闪长玢岩岩脉包括闪长玢岩和石英闪长玢岩(潘鸿迪和申萍,2014);矿化以浸染状矿化为主,有少量细脉状矿化,主要赋存于早期闪长岩体中(Shen et al., 2009, 2010a, b申萍等,2009潘鸿迪和申萍,2014)。矿床热液蚀变具有明显的分带性(图 3),在岩体及其与围岩的接触带发育复杂的钾硅酸盐化带,在岩体边部和围岩中发育青盘岩化带,在二者之上叠加了晚期的绢英岩化带,矿体主要赋存于钾化带中,富矿体出现在绢英岩化带中(申萍等,2010bShen et al., 2010a, b)。

图 3 包古图还原性斑岩铜矿床成矿阶段及蚀变矿化组合示意图(据Shen et al., 2010a, b修改) Fig. 3 Schematic plot of mineralization stages and assosicated alteration and mineralization assemblages in the Baogutu reduced porphyry Cu deposit (modified after Shen et al., 2010a, b)

前人研究认为,包古图斑岩铜矿床发育两期成矿作用,早期为斑岩矿化,形成了Cu-Mo-Au组合;后期叠加了Cu-Au-Ag-Te-Bi矿化,规模小,但对矿石起到了加富作用(宋会侠等,2007)。我们研究认为,包古图斑岩铜矿床包括两个成矿期6个成矿阶段(图 3);主成矿期与闪长岩体有关,又分为4个成矿阶段;阶段1A为钙钠硅酸盐阶段,形成阳起石+钠长石+钛铁矿+绿帘石蚀变组合,发育特殊的钛铁矿化及磁黄铁矿化,一般不含矿(图 3a);阶段1B为钾硅酸盐化阶段,为主要成矿阶段,形成浸染状和脉状矿化(图 3b),又分为两个亚阶段,亚阶段1B-1发育黑云母化及特殊的钛铁矿化和磁黄铁矿化,具有黑云母+石英+钛铁矿±金红石±钾长石蚀变组合和黄铜矿+黄铁矿+磁黄铁矿硫化物组合,出现石英-磁黄铁矿细脉及独立的磁黄铁矿脉;亚阶段1B-2发育黑云母化及特殊的磁黄铁矿和毒砂,具有黑云母+石英+榍石蚀变组合和黄铜矿+黄铁矿+磁黄铁矿+闪锌矿+毒砂±辉钼矿硫化物组合,出现独立的毒砂脉;阶段1C为绢英岩化阶段(图 3c),也是主要成矿阶段,形成浸染状和脉状矿化,发育绢云母、毒砂和碲铋矿物,具有辉钼矿+黄铜矿+黄铁矿+毒砂+闪锌矿±碲铋矿物组合,出现与毒砂共结晶的碲化铋以及独立的毒砂细脉,钼矿化主要形成于此阶段,叠加在铜矿化之上,独立的钼矿体位于矿床深部;阶段1D为泥化阶段(图 3d),发育高岭石、石膏和磁铁矿及金矿化,独立的金矿体主要位于矿床的东北部。晚成矿期与闪长玢岩岩脉有关,又分为2个成矿阶段,阶段2A为钾硅酸盐化阶段(图 3e),发育黑云母化,形成少量浸染状和脉状矿化,具有黑云母+石英+磁铁矿蚀变组合和黄铜矿+黄铁矿硫化物组合;阶段2B为碳酸盐化阶段(图 3f),发育隐爆角砾岩,矿化弱(申萍等, 2009, 2010bShen et al., 2010a, bShen and Pan, 2013, 2015; 潘鸿迪和申萍,2014李文广等,2018)。

不同成矿阶段的石英流体包裹体均缺乏磁铁矿、赤铁矿、硬石膏等子矿物,但富含CH4(Shen et al., 2010aShen and Pan, 2013, 2015Cao et al., 2014),激光拉曼光谱分析表明,阶段1B的石英流体包裹体成分为H2O和CH4,阶段IC石英流体包裹体成分为H2O和CH4,出现CO2;四极质谱分析表明,阶段1B和1C的石英流体包裹体中气相组分均为H2O、CH4、CO2、N2、C2H6和H2S,但阶段1B的CH4含量普遍高于阶段1C,CO2含量普遍低于阶段1C。这些特点表明,主成矿阶段成矿流体为富含CH4的还原流体,并且流体经历了从还原向氧化的转变(Shen et al., 2010aShen and Pan, 2013, 2015),在阶段1D,出现热液石膏和磁铁矿(李文广等,2018),表明成矿流体已经完全演化为氧化性流体;在阶段2A,也出现磁铁矿,成矿流体为氧化性流体(Shen et al., 2010a)。

上述研究可见,包古图矿床主成矿阶段缺乏磁铁矿、赤铁矿、硬石膏等热液矿物,而发育大量的磁黄铁矿和毒砂;石英流体包裹体也未见磁铁矿、赤铁矿、硬石膏等子矿物,但富含CH4;成矿特点与Rowins(2000)提出的还原性斑岩铜矿床的特点一致,基于此,我们于2010年首次将包古图矿床定名为还原性斑岩铜矿床(申萍等,2010a),随后,Cao et al.(2014)也得出了相同的认识。

2.1.2 白乃庙铜矿床

白乃庙铜矿床位于内蒙古乌兰察布盟四子王旗,是内蒙古地区最早开发的大型铜矿床之一(李文博等,2007),铜金属量为51万吨,平均品位为0.8%,金的金属量为19.1吨,品位为1~6g/t,伴生有钼和银等(表 1)。在成因上虽有斑岩-火山热液复合型(聂凤军等, 1994)、海相火山沉积(变质)-热液叠加复合型(李进文等, 2007)和造山型(李文博等, 2007)等不同的认识,但目前普遍认为矿床是斑岩型铜矿床(孟良义, 1992Li et al., 2012毛景文等,2013)或斑岩-叠加改造型铜矿床(周振华等,2017马阁等,2019)。

矿区出露的地层主要为白乃庙群一套变质的绿片岩和长英片岩,与矿化关系密切的晚奥陶世花岗闪长斑岩(445.0±6Ma;Li et al., 2012)分布于矿区中北部,在矿区南部深处亦见有早志留世含矿的花岗闪长斑岩岩枝(438.6±3.4Ma;赵利刚等,2016)。我们获得的矿区北部和南部含矿斑岩的锆石LA-ICP-MS U-Pb年龄分别为444.8±3.4Ma和437.3±2.9Ma(马阁等,2019),与前人认识一致。矿床包括南、北两个矿带,南矿带的矿体以似层状、扁豆状、透镜状和脉状产在绿片岩内,北矿带的矿体零星分布在花岗闪长斑岩体内。金属矿物主要为黄铜矿、黄铁矿、辉钼矿,次要矿物为磁黄铁矿、闪锌矿、方铅矿、斑铜矿、辉钴矿和自然金。围岩蚀变主要有钾长石化、黑云母化、硅化-绢云母化和绿泥石-绿帘石化等,其中南矿段以黑云母化和硅化为主,北矿段以石英-钾长石化、石英-绢云母化为主;钾长石化和硅化与成矿关系密切(李文博等,2007周振华等,2017)。成矿作用包括三个阶段,阶段Ⅰ为石英-黄铁矿阶段,金属矿物以黄铁矿为主,有少量黄铜矿,矿化弱;阶段Ⅱ为石英-硫化物阶段,是主成矿阶段,金属矿物以黄铜矿和辉钼矿为主,黄铁矿、方铅矿和闪锌矿次之;阶段Ⅲ为石英-碳酸盐岩阶段,发育石英-方解石脉,矿化弱(李文博等,2007周振华等,2017)。

白乃庙矿床南、北矿带成矿流体特征基本相似,成矿流体含有H2O、CO2、CH4、N2等气体,阶段Ⅰ以H2O+CH4为主,阶段Ⅱ出现大量的CO2,有时可见CH4+CO2共生,阶段Ⅲ与阶段Ⅰ类似,主要为H2O+CH4(周振华等,2017)。

由此可见,白乃庙矿床缺乏磁铁矿、赤铁矿、硬石膏等,而发育大量的磁黄铁矿;石英流体包裹体也缺乏磁铁矿、赤铁矿、硬石膏等子矿物,含有H2O、CO2、CH4、N2等气体;其成矿特点与Rowins(2000)提出的还原性斑岩铜矿床的特点一致,基于上述资料,我们认为,白乃庙矿床应归属为还原性斑岩铜矿床。

2.1.3 雄村Ⅰ号铜-金矿体

雄村斑岩铜-金矿床位于西藏日喀则地区谢通门县,是冈底斯成矿带中发现的第一个具有铜-金组合的斑岩型矿床(Tang et al., 2015; 唐菊兴等, 2017)。成矿作用包括与中侏罗世早期角闪石英闪长玢岩有关的早期成矿作用和与中侏罗世晚期含眼球状石英斑晶的角闪石英闪长玢岩有关的晚期成矿作用;前者形成Ⅱ号矿体,以发育大量的磁铁矿和石膏为特征,属经典的(氧化性)斑岩铜矿床;后者形成雄村Ⅰ号矿体等,以形成大量的磁黄铁矿为特征,属还原性斑岩铜金矿床(郎兴海, 2012)。

雄村Ⅰ号矿体包含铜金属量103.6万吨,平均品位0.43%,金为143.3吨,平均品位0.51g/t(表 1),矿区出露的地层主要为中-下侏罗统雄村组铁镁质凝灰岩,中侏罗世晚期含眼球状石英斑晶的角闪石英闪长玢岩侵入其中。矿区热液蚀变包括钾硅酸盐化、绢英岩化和青磐岩化等蚀变,矿化与钾硅酸盐化蚀变密切相关。成矿包括三个阶段,阶段Ⅰ与钾硅酸盐化蚀变有关,发育脉状和细脉浸染状黄铜矿、黄铁矿、磁黄铁矿,形成高品位的Cu-Au矿体;阶段Ⅱ与绢英岩化蚀变有关,发育细脉浸染状黄铁矿和少量的黄铜矿,形成低品位的Cu-Au矿体;阶段Ⅲ发育脉状闪锌矿、黄铁矿和少量的黄铜矿、方铅矿和磁黄铁矿,局部形成高品位的Zn (Pb)矿体(郎兴海, 2012; Tang et al., 2015)。金属矿物包括黄铜矿、黄铁矿、磁黄铁矿、辉铜矿、辉钼矿、毒砂、辉砷铜矿、蓝辉铜矿、赤铜矿、蓝铜矿、蓝辉铜矿、自然金、自然铜(Tang et al., 2015唐菊兴等,2017)。石英流体包裹体拉曼光谱分析结果显示,矿化石英脉中流体包裹体发育H2O、CO2、CH4、N2(周云等,2017)。

总之,雄村Ⅰ号矿体以形成大量的磁黄铁矿为特征,缺乏热液赤铁矿、硬石膏,罕见热液磁铁矿;石英流体包裹体也缺乏磁铁矿、赤铁矿、硬石膏等子矿物,发育CO2、CH4、N2郎兴海(2012)认为雄村Ⅰ号矿体属还原性斑岩铜金矿床。

2.2 还原性斑岩-矽卡岩铜矿床 2.2.1 喇嘛苏铜矿床

喇嘛苏铜矿床位于新疆伊犁赛里木湖西北部3km处,是新疆最大的斑岩-矽卡岩铜矿床,矿床铜储量66.9万吨,品位为0.3%~3.68%(表 1)。矿区出露中元古界蓟县系库松木契克群下亚群的大理岩、大理岩化灰岩、灰岩、含炭质板岩等,与成矿关系密切的中-晚泥盆世英云闪长斑岩(380.9±3.9Ma,Zhu et al., 2012;390±7.7Ma,Zhang et al., 2008)和晚泥盆世花岗闪长斑岩(366.3±1.9Ma,唐功建等,2008)在矿区成群分布,晚期可见少量辉绿玢岩和闪长岩脉穿插早期岩体(Zhang et al., 2008Zhu et al., 2012)。与成矿相关的蚀变主要有斑岩蚀变和矽卡岩蚀变,斑岩蚀变局部发育,主要为黑云母-钾长石化、石英-方解石-钾长石化、水云母-伊利石化(王永新,1994);矽卡岩蚀变在矿区广泛发育,80%以上的铜矿体赋存于矽卡岩中,矿体形态主要为层状、似层状和脉状(石海岗,2011)。

石海岗(2011)提出成矿过程包括矽卡岩期和热液期,矽卡岩期又包括干矽卡岩和湿矽卡岩阶段,热液期包括氧化物、早期石英-硫化物和晚期石英-硫化物等阶段;干矽卡岩阶段主要形成石榴子石、透辉石、硅灰石等无水矽卡岩矿物;湿矽卡岩阶段主要形成绿帘石、透闪石等含水矽卡岩矿物;氧化物阶段为磁铁矿形成的主要阶段;石英-硫化物早阶段以黄铜矿、磁黄铁矿、闪锌矿、石英等的大量出现为特点;石英-硫化物晚阶段金属矿物主要为黄铁矿、闪锌矿,含少量黄铜矿。我们研究表明,干矽卡岩阶段形成的石榴子石包括钙铝榴石和钙铁榴石,湿矽卡岩阶段发育铁次透辉石、铁角闪石和大量的磁黄铁矿,氧化物阶段并不明显,且常见磁黄铁矿和磁铁矿共生,缺乏赤铁矿,石英-硫化物阶段发育黄铜矿和黄铁矿,缺乏斑铜矿和辉铜矿(李文广,2019)。

激光拉曼光谱分析显示,石榴石流体包裹体含CH4(Zhu et al., 2012);石英-硫化物早阶段成矿流体为H2O-NaCl-CaCl2体系,含少量的CH4,石英硫化物晚阶段成矿流体逐渐向H2O-NaCl体系转变,含有大量的CH4(石海岗,2011),随着成矿流体演化,富CH4的包裹体数量逐渐增加。

可见,喇嘛苏铜矿床流体包裹体气相成分含有CH4,矿区发育大量的磁黄铁矿,矽卡岩期以发育相对还原的钙铝榴石和钙铁辉石为特点,热液期发育黄铜矿和黄铁矿。基于前人的资料和我们近期的工作认识,我们将喇嘛苏铜矿床定名为还原性的斑岩-矽卡岩铜矿床(李文广,2019)。

2.2.2 赛什塘铜矿床

赛什塘铜矿床位于东昆仑多金属成矿带最东端的鄂拉山地区,是青海省重要的矽卡岩铜矿床之一(王辉等,2015),矿床的铜资源量为44.48万吨(表 1)。矿区出露地层为中-下三叠统变碎屑岩-变碳酸盐岩岩石组合,侵入岩以石英闪长岩岩体最大,与成矿关系也最密切。主矿体产于石英闪长岩与大理岩及钙质变粉砂岩接触带的矽卡岩中,矿体形态主要为层状、似层状。

赛什塘铜矿床发育硅化、绿泥石化、绢云母化和透辉石化等,成矿阶段包括矽卡岩阶段、退化蚀变阶段、石英-硫化物阶段及石英-碳酸盐阶段;矽卡岩阶段主要形成石榴子石、辉石及少量硅灰石等;退化蚀变阶段发育绿帘石、角闪石及磁铁矿,有少量辉钼矿、磁黄铁矿等;石榴子石主要属于钙铁-钙铝榴石固溶体系列,辉石为钙镁辉石与钙铁辉石;石英-硫化物阶段发育磁黄铁矿、黄铁矿、黄铜矿;石英-碳酸盐阶段发育石英-碳酸盐脉,伴生少量的硫化物(何鹏等,2013; 王辉等,2015Lu et al., 2016路英川,2016)。金属矿物主要为黄铜矿、磁黄铁矿、黄铁矿及磁铁矿,其次为闪锌矿、方铅矿、黝锡矿、毒砂及少量的斑铜矿、辉钼矿、辉铜矿等(何鹏等,2013Lu et al., 2016; 路英川,2016)。

激光拉曼光谱分析表明,石榴子石包裹体气相成分主要为CH4,部分为H2O,透辉石气相成分为CH4和H2O,成矿流体属于NaCl-H2O-CH4体系;石英包裹体气相成分为CO2与CH4共存,部分只有CH4,成矿流体属于NaCl-H2O-CO2-CH4体系;从早期的石榴子石、透辉石到晚期的石英中,包裹体中H2O的含量增多(何鹏等,2013Lu et al., 2016; 路英川,2016)。

可见,赛什塘矽卡岩铜矿床流体包裹体气相成分大部分为CH4,石榴子石具有钙铝榴石和钙铁榴石组合,辉石具有透辉石与钙铁辉石组合,金属硫化物以黄铜矿、磁黄铁矿、黄铁矿为主,因此,该矿床具有还原性成矿流体和还原性矿物组合,是一个还原性的矽卡岩铜矿床。

2.3 还原性斑岩钼矿床 2.3.1 苏云河钼矿床

苏云河斑岩钼矿床位于新疆西准噶尔巴尔鲁克山,目前已查明钼金属量57万吨,平均品位0.05%~0.09%(表 1)。矿区出露中泥盆统巴尔鲁克组火山-沉积地层(新疆地质矿产勘查开发局第一区域地质调查大队,2014),早二叠世侵入岩发育,由深部的花岗岩体(中粒花岗岩和细粒花岗岩)和浅部的酸性斑岩岩株(花岗闪长斑岩、花岗斑岩、二长花岗斑岩以及英云闪长斑岩)以及大量切穿地层的岩脉组成,以深部的花岗岩体为主(Shen et al., 2017; 申萍等,2017)。

① 新疆地质矿产勘查开发局第一区域地质调查大队.2014.新疆裕民县苏云河钼矿详查报告

矿区热液蚀变主要出现在侵入体附近的围岩中,包括钾硅酸盐化、绿泥石-白云母化和绢英岩化蚀变等,后二者与矿化密切有关。成矿包括以下四个阶段:阶段Ⅰ为钾硅酸盐化阶段,具有钾长石+石英+磁铁矿组合,矿化弱;阶段Ⅱ是成矿主要阶段,为绿泥石-白云母化蚀变阶段,具有绿泥石+白云母+钠长石组合,发育石英+辉钼矿±钠长石±黄铁矿脉、石英-辉钼矿-绿泥石-钛铁矿脉;阶段Ⅲ也是成矿主要阶段,为绢英岩化蚀变阶段,具有绢云母+石英±绿泥石组合,发育石英+多金属硫化物脉,黄铁矿、辉钼矿与黄铜矿呈浸染状分布于石英脉内;阶段Ⅳ为方解石化蚀变阶段,无矿化(钟世华等,2015; Cao et al., 2017; 曹冲,2018)。

激光拉曼光谱分析显示,在阶段Ⅱ、Ⅲ、Ⅳ的石英脉中的流体包裹体中有明显的CO2峰值和CH4峰值,与阶段Ⅱ、Ⅲ相比,阶段Ⅳ的石英脉中CO2与CH4的峰强度弱。四极质谱群体包裹体中气相组分以H2O(>90%)为主,其次是CO2(1.049%~8.968%),有少量的CH4和C2H6,二者之和变化于0.046%~1.954%之间(钟世华等,2015Cao et al., 2017)。

总之,苏云河钼矿床主成矿阶段缺乏磁铁矿、赤铁矿、硬石膏等热液矿物,发育钛铁矿,石英流体包裹体也缺乏磁铁矿、赤铁矿、硬石膏等子矿物,发育CO2、CH4和C2H6;矿床的成矿特点与Rowins(2000)提出的还原性斑岩铜矿床的特点类似,我们将其称之为还原性斑岩钼矿床(曹冲,2018)。

2.3.2 白山钼矿床

白山斑岩钼矿床位于新疆哈密市东南约140km处,东天山康古尔-黄山韧性剪切带东段,北距康古尔断裂2km,是新疆最大的斑岩钼矿床,目前,已查明钼金属资源量70.08万吨,平均品位0.06%,铼平均品位1.4g/t(表 1)。

矿区赋矿地层为下石炭统干墩组第二岩性段黑云母长英质角岩和阳起绿帘片岩,岩浆岩主要为黑云母二长花岗岩(181±3Ma,李华芹等,2005)及花岗斑岩(226.8±3.2Ma,王银宏等,2015),黑云母二长花岗岩呈岩株状侵位于下石炭统干墩组下部地层中,花岗斑岩呈脉状出露于地表,或呈岩株状隐伏于矿区深部,侵入干墩组地层中,在岩体与围岩接触带附近发育强烈的钾化(卢鸿飞等,2013),岩体边部可见浸染状辉钼矿,该花岗斑岩体与钼矿化关系最为密切。

热液蚀变包括硅化、钾化、绢英岩化、青磐岩化和萤石化等蚀变,钼矿化与硅化、钾化和绢云母化蚀变关系最为密切(Zhang et al., 2016)。热液蚀变自岩体向围岩可划分为石英-钾长石化带(石英-钾长石-黑云母组合)、石英-绢云母化带(绢云母-石英组合)和青磐岩化带(绿帘石-钠长石组合)(路魏魏等,2013),其中,石英-钾长石化带是矿化-蚀变活动的中心,石英-绢云母化带多与石英-钾化带叠加,叠加部位为钼矿体的主要赋存部位,青磐岩岩化带分布于矿区的最外侧(邓刚等,2004项楠等,2013)。

成矿阶段包括四个阶段:阶段Ⅰ为钾长石-石英阶段,发育钾长石和石英矿物及少量的黄铁矿;阶段Ⅱ为石英-辉钼矿阶段,是白山钼矿床重要的Mo成矿阶段,发育石英和辉钼矿及少量的钾长石和黄铁矿,呈石英-辉钼矿±黄铁矿细脉;阶段Ⅲ为石英-多金属硫化物阶段,出现辉钼矿、黄铁矿、黄铜矿、斑铜矿、方铅矿及闪锌矿等;阶段Ⅳ为石英-方解石脉阶段,基本不含硫化物。矿石矿物主要为辉钼矿,次为黄铜矿、斑铜矿、黄铁矿、磁黄铁矿及少量闪锌矿和方铅矿等(Zhang et al., 2016张方方,2016)。

石英流体包裹体激光拉曼光谱分析表明,气相成分主要为H2O,其次为CH4和少量的CO2。其中,阶段Ⅰ含有H2O和CH4气体,阶段Ⅱ含有CH4和H2O气体,阶段Ⅲ含有H2O及少量的CH4和罕见的CO2;阶段Ⅳ含有H2O。因此,前两个阶段的流体属于H2O-NaCl-CH4流体系统,Ⅲ阶段的流体属于H2O-NaCl±CH4±CO2流体系统(Zhang et al., 2016张方方,2016)。

可见,白山钼矿床缺乏磁铁矿、赤铁矿、硬石膏等热液矿物,发育磁黄铁矿;石英流体包裹体也缺乏磁铁矿、赤铁矿、硬石膏等子矿物,发育CH4;基于此,我们将其归属为还原性斑岩钼矿床。

3 成矿流体的来源 3.1 CH4还原流体的来源

还原流体是形成还原性斑岩矿床的关键(Shen and Pan, 2013, 2015),而CH4是还原流体的最重要的组成部分(表 2),因此,查明CH4的来源对研究还原性斑岩矿床成因尤为重要。目前,我国学者仅对还原性斑岩铜矿床和钼矿床的成矿流体CH4的来源进行了研究。

表 2 中国还原性斑岩矿床成矿流体包裹体成分及碳同位素组成 Table 2 Gaseous composition and C isotopic data of CH4 and CO2 of fluid inclusions from the reduced porphyry deposits in China
3.1.1 自然界中CH4的来源

自然界中甲烷的形成包括有机和无机成因,其中,有机成因进一步可分为微生物成因和热分解成因(Whiticar,1999Ueno et al., 2006);微生物成因的甲烷是由一些微生物的代谢活动产生的,其形成温度低于120℃,CH4δ13C CH4值普遍小于-50‰(Whiticar,1999Ueno et al., 2006)或介于-69.2‰~-66.1‰之间(Schoell, 1988; Avery et al., 1999)以及介于-110‰~-50‰之间(Whiticar,1999);热分解成因是由有机质热分解产生的烷类气体,所形成的还原性气体比值CH4/C2H6<100(Fiebig et al., 2009),CH4δ13CCH4值集中于-30‰~-20‰范围(Panichi et al. 1977; Giggenbach, 1995)或介于-50‰~-20‰之间(Whiticar,1999),且明显小于其有机来源物的δ13Corg,即△δ13C(Corg-CH4)>0,主要为0‰~30‰(Whiticar,1999)。此外,含石墨或碳质的岩石加热或变质也能产生大量的甲烷(Kenney et al., 2002McCollom and Seewald, 2007)。

无机成因也是CH4形成的一个重要方式,它既可以出现在大洋中脊环境中(Liu and Fei, 2006; McCollom and Seewald, 2007),也可以出现在俯冲带环境中(Fiebig et al., 2007),其来源包括以下四种:(1)直接来自地幔(Liu and Fei, 2006);(2)俯冲洋壳与下地壳的脱水过程中,俯冲板片含碳沉积物中的碳与H2O反应形成CH4和CO2 (Ballhaus, 1993),通过去挥发分作用直接释放CH4 (Song et al., 2009); (3)橄榄岩的蛇纹石化过程驱动热液系统并产生CH4和H2(Mccollom et al., 2010; Dias et al., 2010);(4)菲舍尔-托型(Fischer-Tropsch type, FTT)和菲舍尔-托(FT)反应也是非生物作用形成CH4的重要反应方式(Horita and Berndt, 1999Fiebig et al., 2009),其反应机理为CO2或CO在催化剂表面形成活性炭物质,该物质和氢气反应进而聚合,最终形成烷烃和烯烃类物质(Holloway, 1984李昌昊等,2017),FTT和FT反应式如下:

(1)
(2)

FTT反应可分为高温(300~350℃)和低温(220~270℃)反应,这一反应可以在热液条件下进行(Berndt et al., 1996)。

对于无机成因的甲烷而言,源于地幔CH4δ13CCH4值大于-25‰(Jenden et al., 1993);但是,对于FTT反应形成CH4的判断标准目前是不明确的(Horita and Berndt, 1999),一些人建议δ13CCH4值大于-25‰(Welhan, 1988)或变化于-26‰~-9‰之间(Ueno et al., 2006),另外一些人建议δ13CCH4值小于生物成因的δ13CCH4值(Horita and Berndt, 1999)。一般,单独的δ13CCH4不能作为判识FTT反应合成的甲烷的有效指标,还应考虑温度及天然矿物催化剂等因素(Horita and Berndt, 1999; McCollom and Seewald, 2007)。

3.1.2 还原性斑岩铜矿床中CH4的来源

Rowins(2000)对于还原性斑岩铜矿床CH4的来源尚未涉及,Smith et al.(2012)提出CH4源于深部超铁镁质熔体的认识,但未提供相应的证据。我国学者对还原性斑岩铜矿床成矿流体中CH4来源的研究较少,且存在争议,一部分学者认为CH4主要为围岩中有机质热分解(Shen and Pan, 2013, 2015Cao et al., 2017)或外部还原性物质加入(Ren and Ma, 2015)所致;另一部分学者认为CH4是FTT反应产物(Cao et al., 2014b)。本文以包古图铜矿床为例,说明还原性斑岩铜矿床成矿流体中CH4的来源。

包古图斑岩铜矿床成矿流体中含有CH4、C2H6和N2等还原性气体,以CH4为主(表 2)。成矿流体中CH4δ13CCH4值为-22.6‰~-36.0‰(Shen and Pan, 2013, 2015Cao et al., 2014a),这些数据主要落在有机质热分解成因CH4δ13CCH4值变化范围(图 4)。此外,成矿流体中CH4δ13CCH4值和还原气体的组成也可以示踪CH4的来源,前已述及,热分解所形成的还原性气体CH4/C2H6<100(Fiebig et al., 2009),包古图铜矿床CH4/C2H6比值集中在0.8~70,在图 5中,主要落在了“热分解”和“高焓地热气体”范围内及其附近,所有的数据点均远离无机成因范围。

图 4 中国还原性斑岩矿床成矿流体甲烷δ13CCH4值与自然界甲烷δ13CCH4值比较图 生物成因的数据来自Schoell (1988);有机质热分解的数据来自Giggenbach (1995)Whiticar (1999);非生物成因的数据来自Jenden et al. (1993);FTT反应的范围来自Welhan (1988)Ueno et al. (2006)Horita and Berndt (1999)Whiticar (1999);包古图数据来自Shen et al.(2010b)Shen and Pan(2013, 2015)和Cao et al. (2014a);苏云河数据来自钟世华等(2015)Cao et al. (2017); 宏远数据来自鄢瑜宏等(2015) Fig. 4 Comparison of the δ13CCH4 values of the ore-forming fluids in the reduced porphyry deposits in China with the nature's δ13CCH4 values Data of biogenic methane from Schoell (1988); data of thermogenic methane from Giggenbach (1995) and Whiticar (1999); data of abiogenic methane from Jenden et al. (1993); data of FTT reaction methane from Welhan (1988), Ueno et al. (2006), Horita and Berndt (1999) and Whiticar (1999). δ13CCH4 values of the Baogutu from Shen et al.(2010b), Shen and Pan(2013, 2015) and Cao et al. (2014a); δ13CCH4 values of the Suyunhe from Zhong et al. (2015) and Cao et al. (2017); δ13CCH4 values of the Hongyuan from Yan et al. (2015)

图 5 中国还原性斑岩矿床成矿流体气相组分CH4/(C2H6+C3H8)对δ13CCH4图解(据Cinti et al., 2011; 方框范围来自Hunt, 1996) Fig. 5 CH4/(C2H6+C3H8) vs. δ13CCH4 diagrams for the gases from the porphyry deposits in China (after Cinti et al., 2011; boxes are from Hunt, 1996)

利用CO2的碳同位素和CO2的含量可以很好的判别CO2的成因,当δ13CCO2>-8‰,且CO2的含量>60 %时,CO2为无机成因;而当δ13CCO2 < -10‰,且CO2的含量 < 20%时,CO2为有机成因;当δ13CCO2介于-8‰~-10‰之间时,CO2则可能是混合成因(陈传平等,2004戴金星,2005何家雄等,2005米敬奎等,2008)。包古图矿床成矿流体中CO2气体含量少(< 20%,表 2),δ13CCO2值变化于-20‰~-6.8‰之间(Shen and Pan, 2013, 2015Cao et al., 2014),主要落在有机质热分解成因范围(图 6),表明成矿流体中CO2气体主要为有机成因,少量为无机成因。

图 6 中国还原性斑岩矿床成矿流体CO2δ13CCO2值与自然界CO2δ13CCO2值比较图 有机成因、无机成因和混合成因的数据来自米敬奎等(2008)戴金星(2005)何家雄等(2005)陈传平等(2004);包古图和苏云河数据来源同图 4 Fig. 6 Comparison of the δ13CCO2 values of the ore-forming fluids in the reduced porphyry deposits in China with the nature's δ13CCO2 values Nature's δ13CCO2 values abiogenic CO2 are from Mi et al. (2008), Dai (2005), He et al. (2005), and Chen et al. (2015); δ13CCO2 values of the Baogutu and the Suyunhe are the same as Fig. 4

我们研究发现包古图成矿岩浆在浅部就位时发生了明显的同化混染作用(潘鸿迪和申萍,2014),矿区发育的早石炭世火山-沉积岩围岩中发育有机碳,其平均含量为0.45%(Shen and Pan, 2015),在岩浆侵入及同化混染过程中,含碳质围岩中的有机质受热发生分解,产生烷类还原性流体(Shen 20132015 Shen and Pan, 20132015潘鸿迪和申萍,2014)。此外,有机质热分解除了产生甲烷之外,还可产生乙烷和丙烷等其它碳氢化合物(Whiticar,1999Fiebig et al., 2009),包古图矿床成矿流体中有乙烷出现(表 2),表明在岩浆侵入过程中加热或同化混染含碳质围岩,使其发生水-岩反应形成碳氢化合物(Shen 20132015 Shen and Pan, 20132015潘鸿迪和申萍,2014)。

前已述及,通过有机物热分解形成的CH4δ13CCH4低于有机物自身的δ13Corg(Des Marais et al., 1988Whiticar,1999),包古图铜矿床地层中有机碳的碳同位素(δ13Corg)为-25.8‰~-23.1‰(Shen 20132015 Shen and Pan, 20132015),成矿流体中CH4δ13CCH4值为-22.6‰~-36.0‰(Shen and Pan, 2013, 2015Cao et al., 2014),绝大多数样品的Δδ13Corg-CH4值大于0(Shen 20132015 Shen and Pan, 20132015),也表明成矿流体中CH4主要是有机物热分解的产物。

我们也考虑了FTT反应,认为包古图成矿流体中CH4不可能大量源于FTT反应,依据如下:(1)前已述及,单独的δ13CCH4不能作为判识FTT反应合成的甲烷的有效指标,还应考虑温度及天然矿物催化剂等因素,即便如此,成矿流体中CH4δ13C(-22.6‰~-36.0‰)主要落在有机质热分解范围内,与FTT反应形成甲烷的范围不同(图 4);(2)FTT反应在热液条件下需要合适的催化剂,以Fe-Ni矿物(镍铁矿)为催化剂只能形成CH4,以Fe-Cr矿物(铬铁矿)为催化剂可形成CH4、C2H6和C3H8(Horita and Berndt, 1999; Foustoukos and Seyfried, 2004季福武等,2007);然而,包古图矿床未出现镍铁矿和铬铁矿等天然矿物催化剂(李昌昊等,2017),因此,其成矿流体中含有的CH4、C2H6等还原性气体(表 2)不可能主要来源于FTT反应;(3)FTT反应需要有大量H2(H2/CO2=4ã1,如橄榄石蛇纹石化作用)的存在,才能保证反应式(1)和反应式(2)的顺利进行(季福武等,2007; Mccollom et al., 2010),并且在实验生成物中H2的含量仍然很高(Berndt et al., 1996),然而,包古图铜矿床中虽然可能有可以释放出H2的蚀变作用,但是在流体包裹体气相成分分析结果未发现成矿流体中含有H2,相反却发现有CO2的存在(表 2),因此,包古图铜矿床流体中不含H2或含量很少,并且H2/CO2比值远远低于FTT反应所需要的4︰1的条件,包古图铜矿床的流体条件非常不利于CH4的形成;(4)FTT反应在温度低于300℃时形成的CH4的Δδ13CCO2-CH4=20‰~30‰,在温度低于200℃时形成的CH4的Δδ13CCO2-CH4=~50‰(Horita and Berndt, 1999);包古图斑岩铜矿床成矿温度集中在200~350℃(Shen et al., 2010a),Δδ13CCO2-CH4变化于8.2‰~25‰之间,且大多数样品Δδ13CCO2-CH4<20‰(Shen 20132015 Shen and Pan, 20132015Cao et al., 2014),这与高温和低温FTT反应下形成的CH4性质均不同;(5)通过FTT反应还原CO2形成的CH4,其CO2和CH4气体可达到碳同位素平衡(Giggenbach,1995Horita and Berndt, 1999Fiebig et al., 2007, 2009),然而,在log(XCH4/XCO2)-T图中(图 7),所有数据点远离气液相平衡曲线,表明在温度低于500℃热液系统中,包古图矿床成矿流体中CO2和CH4气体的化学和同位素交换未能达到化学平衡。

图 7 中国还原性斑岩矿床成矿流体中气相组分log(XCH4/XCO2)值与碳同位素平衡温度的关系图(底图据Fiebig et al., 2004) Fig. 7 Correlation between log(XCH4/XCO2) values and apparent isotopic temperatures of the gases (base map after Fiebig et al., 2004)

综上所述,我们提出包古图斑岩铜矿床成矿流体中CH4和C2H6等还原气体主要是含碳质围岩中的有机质热分解形成的,少量是无机成因的,FTT反应不可能作为包古图铜矿床成矿流体中CH4和C2H6等还原气体的主要来源(Shen 20132015 Shen and Pan, 20132015)。

3.1.3 还原性斑岩钼矿床中CH4的来源

目前,国内外对还原性斑岩钼矿床成矿流体中CH4来源的研究均较少,本文以新疆苏云河钼矿床为例说明还原性斑岩钼矿床成矿流体中CH4的来源。苏云河斑岩钼矿床流体包裹体气相成分主要为H2O、CO2,有少量的N2、CH4、C2H6(表 2),样品的δ13CCH4值的变化范围于-23.2‰和-28‰之间,平均值是-25.3‰(钟世华等,2015Cao et al., 2017),在图 4中,大多数样品落在有机质热分解成因的δ13CCH4值变化范围,少量样品有更高的δ13CCH4值(-8.5‰~-14.6‰),落在非生物成因的δ13CCH4值变化范围;成矿流体中CH4/C2H6比值很低(< 10),与有机质热分解成因的CH4类似(图 5)。

苏云河成矿流体中CO2δ13CCO2值变化于-14.0‰和0.0‰之间(表 2),在图 6中,样品横跨有机与无机成因的范围,显示两种成因CO2的混合,即岩浆成因和含碳质围岩混染成因(曹冲,2018)。在log(XCH4/XCO2)-T图(图 7)中,所有数据点偏离气液相平衡曲线,表明苏云河矿床成矿流体中CO2与CH4并没有达到碳同位素平衡,也说明无机成因不是CH4的主要来源(Cao et al., 2017, 2020; 曹冲,2018)。

苏云河矿区发育中泥盆统巴尔鲁克组火山-沉积岩,其中发育杂砂岩等含碳质地层,当在岩浆侵位提供热量的情况下,这些地层中的有机物发生分解,除了产生CH4,还产生C2H6等气体。因此,苏云河矿床中成矿流体中的CH4主要来自于原位有机物热分解,部分源于岩浆(Cao et al., 2017, 2020; 曹冲,2018)。

3.2 成矿流体中水的来源

前人对还原性斑岩矿床成矿流体中水的来源进行了H-O同位素的研究,结果表明(表 2图 8),包古图铜矿床的矿脉中石英δ18OH2O=3.2‰~9.3‰,δD=-74‰~-107‰(张志欣等,2010Shen et al., 2012);白乃庙铜矿床的石英δ18OH2O=-3.2‰~5.5‰,δD值为-69‰~-94.2‰(赵云等,2014);雄村矿区Ⅰ号矿体的石英δ18OH2O=-6.1‰~6.9‰,δD值为-82‰~-109‰(郎兴海,2012Tang et al., 2015)。喇嘛苏铜矿床石英-硫化物阶段的矿脉中石英δ18OH2O=1.2‰~5.1‰,δD=-75.3‰~-88.7‰(石海岗, 2011);赛什塘铜矿床石英闪长岩和石英脉中的石英δ18OH2O=2.5‰~10.4‰,δD=-89‰~-125.9‰(Lu et al., 2016)。苏云河钼矿床的石英δ18OH2O=0.1‰~8.9‰,δD=-60.6‰~-95.9‰(钟世华等,2015Cao et al., 2017);白山钼矿床的石英δ18OH2O=1.6‰~5.2‰,δD=-81‰~-116‰(Zhang et al., 2016)。

图 8 中国还原性斑岩矿床δD-δ18OH2O图解(据Taylor, 1974;数据来源见表 2) Fig. 8 δD vs. δ18OH2O diagram of fluid inclusions in the reduced porphyry deposits in China (after Taylor, 1974; data listed in Table 2)

与经典的斑岩矿床相比,上述还原性斑岩矿床石英流体包裹体的δD值偏低(图 8),热液矿床成矿流体中低的δD值(< 80‰)可能是深源流体与容矿围岩中低的δD值有机物相互作用的结果,也可能是成矿流体中的HS-或H2S的H+被金属离子替换的结果(陈衍景和张莉, 2008),也可能与岩浆阶段的脱气过程有关(Hedenquist and Lowenstern, 1994)。中国的还原性斑岩矿床围岩通常都具有还原特点(表 1),成矿流体较低的δD值可能主要是流体与还原性围岩发生水岩反应所致。

4 还原性流体成矿作用

无论成矿岩浆的氧化-还原性质及成矿流体的来源如何,还原性斑岩矿床的一个重要特征是成矿流体为富含CH4的还原性流体,其氧逸度较低(log fO2≤FMQ),在这种还原条件下,流体中的S直接以还原硫S2-(H2S/HS-/S2-)或S22-、S3-形式存在,几乎没有S6+(图 9),Fe主要以低价态(如Fe2+)的形式存在,因此,在富含CH4的还原流体作用下,可形成斑岩矿床的许多还原性的成矿特点。

图 9 依据XANES(实线)、EPMA(虚线)估计的S6+/ΣS(据Jugo, 2009; Jugo et al., 2005, 2010)玄武岩玻璃的硫曲线与fO2图解 图还显示了不同的构造环境(MORB、OIB和地幔楔以及岩浆弧). MORB-洋中脊玄武岩;IAB-岛弧玄武岩;BABB-弧后盆地玄武岩;OIB-洋岛玄武岩 Fig. 9 Sulfur speciation curve vs. fO2 for basaltic glasses based on the S6+/ΣS estimates of XANES (in bold), EPMA (dashed line) (after Jugo, 2009; Jugo et al., 2005, 2010) Also shown are fields of different tectonic settings, MORB, OIB and mantle wedge, and arc. MORB-mid-ocean ridge basalt; IAB-Island arc basalt; BABB-Back arc basin basalt; OIB-Oceanic island basalt
4.1 还原性斑岩铜矿床

以包古图还原性斑岩铜矿床为例,我们研究发现,钾硅酸盐化阶段(黑云母化)发育大量的热液钛铁矿而非磁铁矿(图 10a),这是由于在含CH4的还原性流体中,伴随着黑云母化,原生黑云母中的Fe3+被还原成Fe2+,形成热液黑云母,同时析出热液钛铁矿(方程式3):

图 10 包古图还原性斑岩铜矿床显微照片(据Shen and Pan, 2013, 2015) (a)钾化带发育的黑云母-钛铁矿-金红石-黄铜矿组合,反射光;(b)绢英岩化阶段黑云母发生绿泥石化,正交偏光.矿物缩写:Pl-斜长石;Bt-黑云母;Cpy-黄铜矿;Ilm-钛铁矿; Rt-金红石;Chl-绿泥石 Fig. 10 Photomicrographs of the Baogutu porphyry copper deposit (after Shen and Pan, 2013, 2015) (a) alteration assemblage of Bt+Ilm+Rt+Cpy in potassic zone under reflected light; (b) alteration assemblage of Ser+Chl in phyllic zone under crossed polarizers. Abbreviations: Bt-biotite; Pl-plagioclase; Ilm-ilmenite; Rt-rutile; Cpy-chalcopyrite; Chl-chlorite
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钾硅酸盐化阶段发育热液钛铁矿,这是区别于经典的斑岩铜矿床发育热液磁铁矿的重要蚀变特征(Shen and Pan, 2013, 2015)。与此同时,钾硅酸盐化阶段成矿流体发育大量的Fe2+,在还原流体中发生沉淀,形成磁黄铁矿(方程式4;Heinrich, 1990)和黄铜矿(方程式5;Heinrich, 1990):

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此外,我们研究发现,钾硅酸盐化阶段硫逸度(log fS2)较高,log fS2为-1.5~-9.5(李文广等,2018),利于磁黄铁矿沉淀,该阶段常见黄铜矿在磁黄铁矿解理间隙中充填,也见有黑云母、黄铁矿和黄铜矿共生,发育黄铜矿+黄铁矿+磁黄铁矿金属矿物组合(Shen and Pan, 2013, 2015李文广等,2018)。

在绢英岩化阶段,一些黑云母在CH4流体作用下,被蚀变成绿泥石(方程式6,图 10b),该阶段硫逸度(log fS2)降低,log fS2为-9.7~-15.8(李文广等,2018),已经不利于磁黄铁矿的沉淀而利于黄铁矿(方程式7)和毒砂(方程式8)的大量沉淀,同时发生黄铜矿(方程式5)的集中沉淀,因此,绢英岩化阶段发育黄铜矿+黄铁矿+辉钼矿+毒砂+闪锌矿±斑铜矿±碲铋矿物组合(Shen and Pan, 2013, 2015李文广等,2018),其中,闪锌矿为相对还原的富铁闪锌矿(李文广等,2018)。

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一般,经典的斑岩铜矿床中的硫化物基本不出现磁黄铁矿和毒砂(Sillitoe, 2010),而还原性斑岩铜矿床中却发育大量的磁黄铁矿和毒砂,闪锌矿为富铁闪锌矿,这些均是由于金属元素在强还原流体中发生沉淀所致。

需要指出的是,经典的斑岩铜矿床在温度小于400℃液相中可存在SO2的歧化反应(Ohmoto and Goldhaber, 1997),导致硬石膏的形成和金属硫化物沉淀(方程式9):

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最近,Henley et al.(2015)通过高温实验证明SO2气体和富钙长石可以发生化学吸附反应,从而快速有效地形成硬石膏和氢硫化物气体,进而导致金属硫化物的沉淀(方程式10)。

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然而,在还原流体作用下,CH4作为还原剂,抑制了SO2的歧化反应和化学吸附作用。因此,与经典的斑岩铜矿床成矿特点不同,在还原流体作用下,包古图还原性斑岩铜矿床钾硅酸盐化阶段发育钛铁矿化及磁黄铁矿、黄铜矿、黄铁矿等硫化物沉淀;随后流体温度和硫逸度(log fS2)降低,氧逸度(log fO2)轻微升高,发生绢英岩化,伴随着毒砂、黄铁矿和富铁闪锌矿的沉淀,形成黄铜矿和辉钼矿等硫化物(Shen et al., 2010aShen and Pan, 2013, 2015; 李文广等,2018)。

4.2 还原性斑岩钼矿床

以苏云河矿床为例,我们研究表明,钾硅酸盐化阶段的成矿流体为NaCl-H2O-CO2±CH4,为氧化性流体体系,仅有少量辉钼矿沉淀(钟世华等,2015Cao et al., 2017曹冲,2018),与经典的斑岩钼矿床一致,苏云河钼矿床在这种相对氧化的条件下,亲铜元素以不相容元素的形式保留在流体中而发生迁移,形成石英-磁铁矿-钾长石脉及有关的围岩蚀变(方程式11):

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然而,在绿泥石-白云母化蚀变阶段,成矿流体为富含CH4的还原流体,流体中的S直接以还原硫S2-形式存在,Fe主要以Fe2+的形式存在,在还原性流体作用下,发生绿泥石化和钛铁矿化(图 11a, b;方程式12;曹冲,2018),同时发生石英和辉钼矿的集中沉淀(方程式13)。

图 11 苏云河斑岩钼矿床和喇嘛苏斑岩-矽卡岩铜矿床显微照片 苏云河斑岩钼矿床:(a)花岗岩发生绿泥石化;(b)花岗岩发生绿泥石化和钛铁矿化(Cao et al., 2020).喇嘛苏斑岩-矽卡岩铜矿床:(c)矽卡岩中发育的钙铝榴石;(d)磁铁矿与磁黄铁矿共生.矿物缩写:Pl-斜长石;Bt-黑云母;Kfs-钾长石;Abl-钠长石;Sph-榍石;Chl-绿泥石;Qtz-石英;Hes-钙铝榴石;Cc-方解石; Ilm-钛铁矿; Pyr-磁黄铁矿;Mt-磁铁矿 Fig. 11 Photomicrographs of the Suyunhe reduced porphyry Mo deposit and Lamasu reduced porphyry-skarn copper deposit Suyunhe porphyry Mo deposit: (a) chlorite in the granite; (b) chlorite and ilmenite in the granite (Cao et al., 2020). Lamasu porphyry-skarn copper deposit: (c) hessonite in the skarn; (d) pyrrhotite and magnetite paragenesis. Abbreviations: Bt-biotite; Pl-plagioclase; Ilm-ilmenite; Chl-chlorite; Qtz-quartz; Kfs-K-feldspar; Abl-albite; Sph-sphene; Hes-hessonite; Cc-calcite; Pyr-pyrrhotite; Mt-magnetite
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可见,苏云河钼矿床钾硅酸盐化阶段的氧化性流体作用下,发育磁铁矿沉淀,金属元素发生迁移,没有明显的矿化;随后还原性流体加入,流体温度和氧逸度降低,发生绿泥石化、白云母化和钛铁矿化,同时发生辉钼矿大量沉淀。

4.3 还原性斑岩-矽卡岩铜矿床

以喇嘛苏斑岩-矽卡岩铜矿床为例。Zhu et al.(2012)石海岗(2011)对喇嘛苏斑岩-矽卡岩铜矿床进行了激光拉曼光谱分析,结果显示,矽卡岩期和热液期成矿流体均含有CH4包裹体。已有的研究表明,在矽卡岩矿物中,流体的氧化还原状态影响石榴石Fe、Al的含量及辉石Fe、Mg的含量,在还原性流体中结晶的石榴石和辉石,由于Al、Mn、Fe2+易进入石榴石而形成钙铝榴石,石榴石具有更多的钙铝榴石端元组分,Fe、Mn易进入辉石形成钙铁辉石,辉石具有更多的钙铁辉石端元组分(Meinert et al., 2005)。

我们对喇嘛苏斑岩-矽卡岩铜矿床研究表明,干矽卡岩阶段发育钙铝榴石(图 11c)和少量的钙铁榴石,电子探针数据表明(李文广,2019),湿矽卡岩阶段发育次透辉石、铁次透辉石、铁角闪石(方程式14, 15):

(14)
(15)

表明矽卡岩期含CH4的流体的氧逸度低,在还原流体作用,形成相对还原的钙铝榴石和次透辉石、铁次透辉石、铁角闪石等,与经典的斑岩-矽卡岩铜矿床发育相对氧化的钙铁榴石和透辉石矿物组合明显不同。此外,喇嘛苏铜矿床含CH4的还原性流体致使矿床的氧化物阶段并不发育,然而,磁黄铁矿大量发育,赤铁矿较少,且常见磁黄铁矿和磁铁矿共生(图 11d)。在热液期的石英-硫化物阶段,在含CH4的还原性流体作用下,发生相对还原的黄铁矿和黄铜矿的大量沉淀,与经典的斑岩-矽卡岩铜矿床发生相对氧化的斑铜矿和辉铜矿大量沉淀不同。

可见,在含CH4的还原性流体作用下,还原性斑岩-矽卡岩铜矿床的矽卡岩期形成相对还原的矽卡岩矿物(钙铝榴石、钙铁辉石)及磁黄铁矿大量沉淀,在热液期的石英-硫化物阶段形成黄铁矿和黄铜矿的大量沉淀。

5 还原性斑岩矿床围岩特点及含矿岩浆成因 5.1 还原性斑岩矿床围岩特点及其作用

经典的斑岩铜矿床和斑岩钼矿床成矿岩体的直接围岩一般为中酸性火山岩和花岗岩类,经典的斑岩-矽卡岩铜矿床的围岩一般是钙质砂岩、灰岩和大理岩等。然而,还原性斑岩矿床成矿岩体的直接围岩一般发育有还原性岩石,包括含碳质的沉积岩、火山-沉积岩和含亚铁的镁铁质火山岩等(表 1),比如,包古图矿区发育早石炭世包古图组凝灰质砂岩、层凝灰岩以及希贝库拉斯组凝灰质砂岩、砾岩等(沈远超和金成伟,1993Shen et al., 2013);雄村矿区发育中下侏罗统雄村组铁镁质凝灰岩(郎兴海,2012Tang et al., 2015);白乃庙矿区发育寒武系白乃庙群一套中浅变质的绿片岩、长英片岩(李文博等,2007周振华等,2017);苏云河矿区发育中泥盆统巴尔鲁克组一套陆缘细碎屑沉积建造,岩性为凝灰质细砂岩、凝灰质粉砂岩、凝灰岩夹生物碎屑灰岩、碧玉岩、中粒砂岩、杂砂岩(新疆地质矿产勘查开发局第一区域地质调查大队,2014);白山矿区出露下石炭统干墩组变质岩,包括黑云母长英质角岩、阳起绿帘片岩、含碳质黑云母片岩等(Zhang et al., 2016)。喇嘛苏矿区发育中元古界蓟县系库松木切克群一套浅海相碳酸盐岩夹硅质岩和碎屑岩沉积建造,包括含碳质砂板岩、碳泥质灰岩等(Zhu et al., 2012),赛什塘矿区发育中下三叠统变碎屑岩-变碳酸盐岩组合(路英川,2016)。

鉴于矿区地层中普遍含有还原物质,岩浆在侵位时不可避免地会发生同化混染作用,在此过程中,围岩中的还原物质可进入岩体,从而改变岩体的氧化还原状态,同时,还原物质也可进入成矿流体中,致使成矿流体成为还原流体。

5.2 成矿岩浆氧化-还原性质及其成因 5.2.1 还原性斑岩铜矿床

对还原性斑岩铜矿床成矿岩浆的氧化-还原性质,目前存在不同的认识,国外学者和我国部分学者认为成矿岩浆为还原性岩浆(Rowins,2000;Smithson,2000;Smith et al., 2012Cao et al., 2014),我国大部分学者则认为成矿原始岩浆为氧化性岩浆(Shen and Pan, 2013, 2015潘鸿迪和申萍,2014Li et al., 2017; Xie et al., 2018Zhu et al., 2018Wei et al., 2019)。

Rowins(2000)认为成矿岩体为还原性的、钛铁矿系列Ⅰ型花岗岩类,该岩体可能由氧化性岩浆在上地幔局部熔融带受到含碳变质沉积岩同化混染形成,但并未提供明确的证据;他也提出S型花岗岩是还原性的、钛铁矿系列花岗岩,但是S型花岗岩不能提供足量的S和Cu、Au等金属元素,因此,不能形成斑岩Cu-Au矿床。近年来确定的加拿大Catface还原性斑岩铜-钼-金矿床(Smith et al., 2012),根据含矿石英闪长岩的钾长石-黑云母-钛铁矿-石英组合及磷灰石具有较低的SO3含量(< 0.12%)的特点,Smith et al.(2012)认为成矿岩浆为还原性岩浆,然而,该矿床石英闪长岩除了钛铁矿之外,还发育有磁铁矿,角闪石大部分为镁角闪石(Smith et al., 2012),因此,Catface矿床的成矿岩浆可能并不是典型的还原性岩浆。

对于雄村Ⅰ号矿体,含矿斑岩体为含眼球状石英斑晶的角闪石英闪长玢岩,郎兴海(2012)提出含矿岩浆起源于弧底部的新生玄武质岩层,推测源区矿物组合为石榴石+斜长石+角闪石+辉石+含钛矿物组合,其部分熔融形成的长英质岩浆氧逸度较低。最新的研究表明,雄村矿区Ⅰ号矿体还原性矿床和Ⅱ号矿体氧化性矿床的成矿岩体锆石微量元素具有类似的Ce4+/Ce3+比值,前者为12~579,平均值为175,后者为11~489,平均值为216(Xie et al., 2018)。因此,雄村矿区Ⅰ号和Ⅱ号矿体的成矿岩浆均为氧化性岩浆。

对于新疆包古图斑岩铜矿床,基于成矿闪长岩体中含有大量的钛铁矿及磷灰石SO3含量较低且变化大等特点,部分学者认为成矿岩浆始终为还原性岩浆(Cao et al., 2014)。我们研究认为成矿的原始岩浆为氧化性的中基性岩浆,在侵位时受到含碳质围岩的混染作用而被还原(Shen and Pan, 2013潘鸿迪和申萍,2014),虽然成矿闪长岩体中发育钛铁矿,但是这些钛铁矿包括三种不同成因:(1)原始岩浆结晶形成的钛铁矿;(2)岩浆同化混然还原性围岩形成的钛铁矿,即包古图中基性岩浆在侵位时受到含碳质围岩的混染作用,导致原始的氧化岩浆被还原,结晶出混染成因的钛铁矿;(3)含CH4还原流体热液蚀变形成的钛铁矿,即,在钾硅酸盐化阶段发育钛铁矿化(图 10a)所致(Shen and Pan, 2013, 2015; 潘鸿迪和申萍,2014)。包古图岩体中发育这三种成因的钛铁矿,使得岩体因含钛铁矿较多显示出钛铁矿系列岩石的特点,实际上,包古图岩体是氧化性岩体(Shen and Pan, 2013, 2015潘鸿迪和申萍,2014)。

包古图岩体中磷灰石也有三种成因:(1)原生磷灰石,是闪长岩中发育的磷灰石,其SO3含量高(图 12);(2)混染成因磷灰石,实际上,包古图闪长岩体中的一些英云闪长斑岩是原始的闪长岩经强烈的围岩混染所致,这种混然成因的英云闪长斑岩中发育的磷灰石为混染成因的磷灰石,其SO3含量很低(图 12);(3)热液磷灰石,在成矿早阶段发育A型磷灰石脉(Shen et al., 2010a)。因此,包古图岩体中部分磷灰石的SO3含量较低是混染成因磷灰石的特点,并不能代表原始岩浆的特点(Shen and Pan, 2013, 2015)。

图 12 包古图矿床闪长岩中磷灰石SO3-Cl含量图 数据源于Shen and Pan(2013, 2015) Fig. 12 SO3 vs. Cl diagram for the apatite in the diorite at Baogutu Data from Shen and Pan(2013, 2015)

包古图铜矿床原始岩浆为氧化性的中基性岩浆(Shen and Pan, 2013潘鸿迪和申萍,2014),依据如下:(1)辉长岩和闪长岩中的角闪石均为高镁角闪石(Mg#=0.73~0.81),属于富镁系列,闪长岩中的黑云母也属于富镁系列,在氧逸度-温度图解(图 13)中,角闪石位于NNO之上;此外,通过对磁铁矿-钛铁矿矿物对分析所获得的数据也落在NNO之上;(2)原生的磷灰石具有较高的SO3(>0.17%;图 12);(3)锆石微量元素具有Ce正异常,Ce4+/Ce3+平均值为54(Shen et al., 2015);这些特点表明,成矿原始岩浆为氧化性岩浆,但是岩浆氧逸度较低(>NNO)。Wei et al. (2019)通过对矿区不同岩石中的角闪石、锆石和磁铁矿-钛铁矿矿物对等分析也得出了相同的认识。

图 13 包古图铜矿床氧逸度-温度图解 HM是赤铁矿-磁铁矿缓冲剂;NNO是镍-氧化镍缓冲剂;QFM是橄榄石-磁铁矿-石英缓冲剂(Chou, 1987)和CO2-CH4 (Candela, 1989).包古图数据来自Shen and Pan(2013, 2015)和Wei et al.(2019) Fig. 13 Oxygen fugacity and temperature diagram of the Baogutu Cu deposit HM: hematite-magnetite, NNO: nickel-nickel oxide, QFM: quartz-fayalite-magnetite (Chou, 1987) and CO2-CH4 (Candela, 1989). Data the Baogutu are from Shen and Pan(2013, 2015) and Wei et al.(2019)
5.2.2 还原性斑岩钼矿床

需要说明的是,就斑岩铜矿床而言,矿体主要位于岩体内部,成矿岩体发生全岩矿化和有关的热液蚀变,导致岩石的化学成分分析结果会出现一定的偏差,一般不能利用Fe2O3/FeO~SiO2图解进行岩浆氧化还原状态的研究。然而,我国还原性斑岩钼矿床,除了宏远钼矿床位于岩体中,其它矿床,如苏云河、白山矿床,绝大多数矿体位于岩体外接触带及围岩中,岩体几乎没有蚀变或发育很弱的蚀变,岩石化学成分分析表明,全岩烧失量均小于1%(王银宏等,2015张方方,2016Shen et al., 2017),也表明岩体未受到明显的热液蚀变,因此,我们可以利用Fe2O3/FeO-SiO2图解对苏云河和白山斑岩成矿岩浆氧化还原状态进行研究。在图 14中,苏云河和白山钼矿床成矿岩体大多数数据都投在了磁铁矿与钛铁矿系列花岗岩分界线附近,显示岩浆氧逸度较低,与部分Endako型斑岩钼矿床成矿岩体特点一致,而与Climax-型斑岩钼矿床的成矿岩体特点明显不同。前已述及,经典的斑岩钼矿床成矿岩浆的氧逸度不一定很高,但也应满足log fO2>FMQ+0.5的条件;还原性斑岩钼矿床的成矿岩浆是否为还原性岩浆,有待进一步研究。

图 14 斑岩矿床全岩Fe2O3/FeO-SiO2图解(据Sinclair,2007) 数据来源:Climax-型和Endako-型斑岩钼矿床成矿岩体来自Sinclair(2007);苏云数据来自Shen et al. (2017a); 白山数据来自王银宏等(2015)张方方(2016) Fig. 14 Fe2O3/FeO vs. SiO2 diagram for whole rock at porphyry ore deposits (after Sinclair, 2007) Data sources: Climax-type and Endako-type from Sinclair (2007); Suyunhe from Shen et al. (2017a); Baishan from Wang et al. (2015) and Zhang (2016)
5.2.3 还原性斑岩-矽卡岩铜矿床

还原性斑岩-矽卡岩铜矿床成矿岩浆是否为还原性花岗质岩浆岩,鲜有报道,目前仅报道了矽卡岩具有还原特点,比如,赛什塘和喇嘛苏铜矿床,矽卡岩期形成的石榴子石属于钙铝榴石-钙铁榴石系列、辉石属于透辉石-钙铁辉石系列、角闪石属于铁角闪石(王辉等,2015李文广,2019)。这些还原特点与围岩的性质密切相关,比如,这两个矿床的围岩均发育含碳质的岩石,花岗质岩浆侵入时与还原性的围岩发育反应,形成还原性矽卡岩。

6 还原性斑岩矿床的成矿构造背景

中国还原性斑岩矿床成岩和成矿时代范围较广,从晚奥陶世到晚三叠世均出现(表 1图 2),其形成受中亚造山带和特提斯造山带构造体制的影响。比如,内蒙古白乃庙铜矿床位于华北北缘中段白乃庙陆缘弧,其南侧为赤峰-白云鄂博断裂带,北侧为温都尔庙俯冲-增生杂岩带,白乃庙铜矿床(445Ma)形成于古亚洲洋沿着华北克拉通北缘向南俯冲形成的早古生代陆缘弧环境(Li et al., 2012);新疆喇嘛苏斑岩-矽卡岩型铜矿床(380~366Ma)位于西天山别珍套-科古琴陆缘弧,形成于北天山洋向南俯冲于伊犁-中天山陆块的晚古生代陆缘弧环境(Zhu et al., 2012);新疆包古图斑岩型铜矿床(313~312Ma)位于西准噶尔石炭纪不成熟洋内弧,矿床形成与晚石炭世准噶尔洋(Xiao et al., 2008; Shen et al., 2009, 2012, 2013)或洋脊俯冲有关(Zhang et al., 2011);西藏雄村斑岩铜金矿床(165~157Ma)位于冈底斯中段南缘雄村弧,形成于中侏罗世新特提斯洋向北俯冲的洋内弧(Tang et al., 2015)。青海赛什塘矽卡岩型铜矿床(223~221Ma)处于东昆仑和西秦岭造山带的衔接转换部位(何鹏等,2013王辉等,2015; 路英川,2016),成矿构造环境属于活动大陆边缘带(芮宗瑶等,2006)。

环太平洋成矿域斑岩(-矽卡岩)铜矿床主要形成于新生代陆缘弧或岛弧(Richards, 2003; Cooke et al., 2005; Meinert et al., 2005Sillitoe, 2010Mao et al., 2014),特提斯成矿域斑岩(-矽卡岩)铜矿床主要形成于新生代陆陆碰撞环境,少量形成于中生代的俯冲环境(Hou et al., 2003, 2015; Yang and Cooke, 2019)。我国还原性斑岩铜矿床和还原性斑岩-矽卡岩铜矿床的形成与板块俯冲有关,主要形成于陆缘弧,少量形成于洋内弧,与世界经典的斑岩(-矽卡岩)铜矿床的构造背景类似。

我国还原性斑岩钼矿床位于中亚造山带,矿床的形成与古亚洲洋闭合有关,形成于同碰撞和/或碰撞后的伸展环境,如苏云河(298~294Ma)和宏远(294Ma)钼矿床的形成与高分异的花岗岩有关,形成于早二叠世同碰撞环境(Shen et al., 2017申萍等,2017曹冲,2018);白山钼矿床(228~223Ma)的花岗岩体可能来源于增厚下地壳物质的部分熔融,其花岗岩成分具有更接近于A-型花岗岩的特点,形成于东天山碰撞造山作用之后的板内伸展阶段或后碰撞环境(王银宏等,2015张方方,2016)。

世界上Climax-型钼矿床的形成与裂谷环境有关,Endako-型钼矿床的形成与弧环境有关(Sillitoe, 2010),中国秦岭-大别地区斑岩钼矿床主要形成于同碰撞、后碰撞以及碰撞后伸展的背景下(Chen et al., 2017a, b)。我国还原性斑岩钼矿床的形成与中国秦岭-大别地区斑岩钼矿床的构造背景一致,而秦岭-大别地区斑岩钼矿床并不具有还原性成矿特点。

从上述讨论来看,还原性斑岩矿床与经典斑岩矿床相比较,其形成的成矿构造背景条件并无明显区别。

7 还原性斑岩矿床特点及判别标志

从上述研究可知,与经典的斑岩铜矿床发育磁铁矿、赤铁矿、硬石膏等及矿石矿物以黄铜矿和斑铜矿为主明显不同,还原性斑岩铜矿床发育大量的磁黄铁矿和毒砂等;热液蚀变发育钛铁矿化;矿石矿物以黄铜矿为主,罕见斑铜矿。与经典的斑岩钼矿床发育磁铁矿、赤铁矿、硬石膏等以及矿石矿物以辉钼矿、黑钨矿、白钨矿、锡石等为主不同,还原性斑岩钼矿床主成矿阶段发育磁黄铁矿;热液蚀变发育钛铁矿化;矿石矿物以辉钼矿为主,一般缺乏黑钨矿、锡石。与经典的斑岩-矽卡岩铜矿床矽卡岩期发育钙铁榴石和透辉石及大量赤铁矿和磁铁矿、热液期发育斑铜矿和辉铜矿不同,还原性斑岩-矽卡岩铜矿床矽卡岩期发育钙铝榴石和钙铁辉石及磁黄铁矿,出现磁铁矿-磁黄铁矿共生现象,罕见赤铁矿;热液期发育黄铜矿,罕见斑铜矿和辉铜矿(表 3)。

表 3 中国大型还原性斑岩矿床与世界经典的斑岩矿床成矿特点对比表 Table 3 Comparison of the main characteristic between the large reduced porphyry deposits in China and the classic porphyry deposits in the world

前已述及,Rowins(2000)已经提出了还原性斑岩铜矿床的判别标志,我们研究认为还可增加特殊的脉石矿物和简单的矿石矿物这两个判别标志,因此,中国还原性斑岩矿床具有以下的判别标志。

还原性斑岩铜矿床:①成矿流体为富含CH4的流体;②发育磁黄铁矿和毒砂等;③热液蚀变发育钛铁矿化;④矿石矿物以黄铜矿为主,罕见斑铜矿。还原性斑岩钼矿床:①主成矿阶段成矿流体为富含CH4的流体;②主成矿阶段有少量磁黄铁矿;③热液蚀变发育钛铁矿;④矿石矿物以辉钼矿为主,缺乏黑钨矿、锡石等。还原性斑岩-矽卡岩铜矿床:①成矿流体为富含CH4的流体;②矽卡岩期发育钙铝榴石和钙铁辉石及大量的磁黄铁矿,常见磁铁矿和磁黄铁矿共生,少见赤铁矿;③石英-硫化物期发育黄铜矿,罕见斑铜矿和辉铜矿。

目前对于还原性斑岩矿床的研究依然处于起步阶段,随着研究的深入,上述认识将被补充或修订。

8 主要结论

(1) 还原性斑岩矿床除了Rowins (2000)提出的发育富CH4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;

(2) 还原性斑岩铜矿床发育富CH4流体及磁黄铁矿和毒砂等,热液蚀变发育钛铁矿,矿石矿物以黄铜矿为主,罕见斑铜矿;还原性斑岩钼矿床主成矿阶段发育富CH4流体及少量磁黄铁矿,热液蚀变出现钛铁矿,矿石矿物以辉钼矿为主,缺乏黑钨矿、锡石等;还原性斑岩-矽卡岩铜矿床发育富CH4流体,矽卡岩期发育钙铝榴石和钙铁辉石及大量的磁黄铁矿,少见赤铁矿,石英-硫化物期发育黄铜矿,罕见斑铜矿和辉铜矿;

(3) 还原性斑岩矿床一般发育含碳质沉积岩和含亚铁的镁铁质火山岩等还原性围岩;在岩浆侵位时,围岩中的还原物质有可能进入岩体和流体中,改变其氧化还原状态。还原性流体中CH4主要源于还原性围岩,部分为无机来源或直接源于岩浆;FTT反应作为斑岩铜矿床成矿流体中CH4的主要来源的可能性较小;

(4) 还原性斑岩矿床形成的关键是矿床发育富含CH4的还原流体,在还原流体(log fO2 < FMQ)作用下,伴随着磁黄铁矿沉淀以及钛铁矿和还原性矽卡岩矿物的形成,发生黄铜矿和辉钼矿成矿作用;

(5) 大多数还原性斑岩矿床成矿岩体的原始岩浆为氧化性岩浆,但其氧逸度偏低;还原性斑岩矿床与经典的斑岩矿床的成矿构造背景没有明显区别。

致谢      野外工作期间得到新疆地矿局和新疆有色集团及其多个地质大队的领导和职工给予的支持和帮助,两位匿名审稿人提出了许多宝贵的意见和建议,本刊编辑也提出了很好的修改建议,在此谨致谢忱!

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