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近年来,我国黄金生产和黄金需求均稳居世界第一(魏春霞, 2018),但随着国家战略资源需求加强和国际经济形势的下滑,黄金价格逐年上涨,有关金资源的开发利用技术已成为全球各国重点攻关的研究方向之一。从矿床类型来看,我国金矿床主要包括造山型、卡林型、斑岩型、浅成低温型和砂岩型等(薛建玲等, 2017)。其中,造山型金矿形成时代广,赋存深度宽,品位高并规模大,是全球金矿勘查的重要类型,其资源量占到全球金资源量的40%以上(Goldfarb et al., 2019)。如胶东造山型金成矿区形成了近20个大型、超大型金矿床,金储量占据了中国黄金资源总量的近1/4(Fan Hongrui et al., 2003)。按成矿作用过程矿化特点的不同可将造山型金矿划分为两类:即石英脉型金矿床和蚀变岩型金矿床(毛景文等, 2005)。我国造山带金矿主要分布在秦岭地区、胶东地区和江南造山带。其中江南造山带经历了多次金矿化事件,已成为华南地区最重要的金成矿省,总计金资源量超过970t(Deng Jun et al., 2016; Xu Deru et al., 2017),成矿潜力巨大。
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黔东南地区位于江南造山带雪峰古陆西南端,是江南造山带多金属成矿省的重要组成部分(图1a)。大量的造山型石英脉金矿(点)产于新元古界下江群浅变质碎屑岩系中,构成了贵州省第二大金矿集区,成矿前景广阔(Wang Jiasheng et al., 2015, 2019; Liu Anlu et al., 2017, 2018)。从成矿地质背景来看,黔东南金矿床与胶东、西秦岭以及桂北地区的蚀变岩型造山型金矿床类似,推测黔东南石英脉型金矿深部可能存在蚀变岩型矿体(陶平, 2012)。按照已知金矿床(点)产出规律统计分析,区域内含矿围岩均发生不同程度的低—中温热液蚀变作用,金的富集成矿通常与蚀变作用过程中形成石英、毒砂、黄铁矿等矿物关系密切,尤其是蚀变复杂及多种蚀变作用的叠加处,可能是富矿体的产出部位(刘坤等, 2015)。成矿年代学研究显示,含金毒砂的Re-Os等时线年龄将成矿时代限制在425~400Ma左右(Liu Anlu et al., 2017; Wang Jiasheng et al., 2019),揭示成矿与加里东期造山运动有关。然而,王加昇等(2011)采用毒砂Re-Os同位素定年方法将金井金矿床的成矿时代确定为174±15Ma,显示区域金成矿可能受到了燕山期成矿的叠加。最近的找矿勘探成果显示,在黔东南坑头金矿床在深部找矿中,发现剪切破碎带中除石英脉型矿体外,存在蚀变岩型矿体(张泽等, 2019)。蚀变岩型矿体也是胶东金成矿区的主要金矿化类型,常形成于成矿流体与围岩发生强烈黄铁绢英岩化区域,并形成大量含Au金属硫化物(范宏瑞等, 2021)。然而,对于黔东南坑头金矿床,这种蚀变岩型矿体的矿体特征(构造形态、蚀变类型,与石英脉型矿体之间关系)和金的富集过程尚不清楚,限制了该成矿带内金成矿规律的全面认识,这也阻碍了进一步的找矿勘探工作。为此,本文以坑头金矿床为研究目标,紧密与当前的勘查工作结合,围绕石英脉型矿体和新发现的蚀变岩型矿体为研究切入点,借助微区分析技术(扫描电镜和电子探针)进行系统的“流体-蚀变-成矿”研究。目的在于查明石英脉型和蚀变岩矿体的时空关系,揭示金的赋存状态,为进一步的找矿勘查提供理论依据。
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图1 黔东南金矿区大地构造位置(a)和金成矿区地质略图(b)(据陶平等, 2009略修改)
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Fig.1 The geotectonic location (a) and simplified geological map (b) of the gold belt in southeastern Guizhou (modified after Tao Ping et al., 2009)
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1 区域背景及矿床地质
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黔东南地区位于江南造山带西南段——雪峰构造带的西南端,经历了自武陵运动之后的多期构造活动(Liu Anlu et al., 2017)。在多期复杂构造运动的叠加作用下,黔东南地区地壳经历了早期大洋地壳-过渡性地壳-稳定地台的演化过程,不同时期构造相互叠加改造,形成以北东、北北东向为主要架格的复杂构造组合(戴传固等, 2010;图1b)。其中加里东及燕山运动对区域构造形态影响最为显著,奠定了研究区主要地质构造格架(张泽等, 2019)。
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黔东南地区的金矿床的主要赋矿地层为新元古界下江群的番召组、清水江组、平略组和隆里组,岩性以变余砂岩、粉砂质板岩、粉砂岩、变余凝灰岩、条带状凝灰质板岩、千枚岩等组成的浅变质碎屑岩系复理石建造为主(刘坤等,2015)。研究区金矿床(点)成矿年代主要为加里东期(425~400Ma; Liu Anlu et al., 2017; Wang Jiasheng et al., 2019),与邻近的湘西金成矿区成矿时代一致,因而将江南造山带西段称之为“湘西-黔东加里东成矿带”(朱笑青等, 2006; 戴传固等, 2010)。在黔东南金成矿区东南侧,分布有大量的新元古代花岗岩体,这些岩体的侵位时代在825~780Ma之间(Liu Anlu et al., 2017)。从金成矿时代与花岗岩体的侵位时代来看,二者之间无直接联系。
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黔东南金矿床矿体类型主要以顺层产出的层间整合脉为主,分布于褶皱背斜的核部及翼部层间破碎带,形态及规模受构造作用控制明显,矿体与围岩分界线突出(图2、图3)。另一种矿体类型为不整合剪切带破碎脉,该类型脉产于层间剪切破碎带,为成矿流体沿剪切层破碎带上升过程中沉淀出的含金石英脉,多成透镜状或不规则状产出,稳定性差,常与蚀变断层泥、蚀变角砾共同构成剪切破碎矿化带(图3)。
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本次研究的坑头金矿床位于黔东南金成矿区的东北部(图1b)。矿区出露新元古界下江群清水江组第二段地层,该地层厚度较大,区域上一般厚近千米,岩性以浅变质的板岩与凝灰岩为主。矿区地表主要发育NE向F1断层、近EW向断层F2断层与北东向坑头背斜,另有一系列北东向的成矿后期小断层,地表构造格架清晰、简单(图2)。近年在钻孔中新发现了F10与F20两条NE向隐伏断层,F10断层破碎带内见构造蚀变岩型矿体,F20内仅显示构造蚀变岩型金矿化(图3)。
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坑头金矿床矿体类型主要包括石英脉型和蚀变岩型。石英脉型金矿体是整个黔东南金成矿区常见的矿体类型。在坑头金矿床中,金矿体的产出明显受构造剪切带的控制,热液角砾岩和石英脉体呈不同形态贯穿在这些剪切带中(图4a)。如在F20剪切破碎带内,石英脉宽约7m(图4b),由于受后期构造的叠加破坏,整个石英脉呈破碎状。石英脉的产出形态显示(图4c、d),这些石英脉呈多期型(如图4c可以观察到3期),而含金的石英脉通常为最晚期形成(图4d)。另一方面,在热液活动影响下,成矿流体交代围岩,并使其发生硫化形成蚀变岩矿体(图4e~g)。蚀变岩型矿体是近些年在找矿勘探工作中新发现的一种金矿体类型(张泽等, 2019)。蚀变岩型矿体主要表现为流体与围岩的相互作用,导致围岩发生硅化、绿泥石化和硫化等蚀变。坑头金矿床蚀变岩型金矿体主要发育在清水江组第一段和第二段,金品位主要介于1~4g/t之间,含大量浸染状毒砂和黄铁矿细脉(图4f、g),围岩主要表现为硅化和绿泥石化(图4f)。
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图2 黔东南坑头金矿床平面地质图(内部资料)
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Fig.2 Geological map of the Kengtou gold deposit in southeastern Guizhou (unpublished data)
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2 样品采集和分析方法
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本研究的样品主要采自钻孔样品(ZK7-10、ZK7-14和ZK9-8,钻孔位置见图2),重点对钻孔所揭示的隐伏控矿断裂F10和F20破碎带和蚀变带进行采样。根据钻孔编录情况,本次采集样品最高品位为8.9g/t,主要集中在2~4g/t之间。本次研究室内工作包括光学显微观察、扫面电镜(SEM)和电子探针(EPMA)分析,旨在查明蚀变矿物类型和金的赋存状态。上述实验均在中国科学院地球化学研究所矿床地球化学国家重点实验室完成。扫面电镜分析仪器为SM-6700F场发射扫描电镜,由二次电子和背散射电子检测器及能谱仪组成,其分析精度可达10nm,可用来观察矿物在纳米尺度的形态特征。电子探针(JXA8530Fplus型)使用条件:电压10kV;电流10nA;束斑1~10 μm。硫化物(黄铁矿和毒砂)分析元素主要为Fe、S、As、Cu、Sb、Ag、Au、Co、Ni、Pb、Bi、Zn、Se。金红石分析元素为Sb2O5、FeO、V2O3、TiO2、Al2O3、WO3、ZrO2、Nb2O5、ThO2、UO2。所有分析元素的检测限约为300×10-6。
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3 结果和讨论
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3.1 蚀变类型、矿物组成及成矿期次
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通过对钻孔样品进行仔细的矿物学分析,结果显示蚀变型矿体围岩蚀变主要为硅化、绢云母化和绿泥石化,硫化物(毒砂和黄铁矿)主要呈浸染状(图5a)。毒砂常与黄铁矿共生,呈自形细粒状(粒径＜20 μm;图5a),二者含量在蚀变岩型矿体中约占40%。坑头金矿床石英脉呈多期特征,其中与金成矿相关的石英脉常呈乳白色,并包含围岩角砾(图4a)。在构造破碎带中,由于受流体交代的影响,在石英脉与围岩接触带形成具环带结构的黄铁矿(图5b)。另外,石英脉型矿体中常见有自然金(图5c),由于自然金的高识别性,因此该类型矿体一直是黔东南金矿床的主要开采对象。
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图3 黔东南坑头金矿床勘探线剖面图(内部资料)
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Fig.3 Cross section of the Kengtou deposit showing the distribution of Au-carrying quartz veins and alteration-type ores in southeastern Guizhou (unpublished data)
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蚀变矿物对于识别矿床成因具有重要的指示意义。在造山型金矿床中,大多数学者认为成矿流体为变质热液流体(Goldfarb et al., 2005; Large et al., 2007, 2011; 王庆飞等, 2019)。绿片岩相岩石发生变质脱水,并从源岩中提取Au形成变质成矿流体是造山型金矿的流体和物质来源(王庆飞等, 2019)。当这些成矿流体与围岩发生进一步的交代蚀变时,形成独居石、磷灰石、金红石等典型蚀变副矿物(Goldfarb et al., 2005, 2019)。坑头金矿床不论是在蚀变岩型还是石英脉型矿体,均发育大量的磷灰石、金红石和独居石(图6)。磷灰石常位于石英脉与围岩的接触带附近(图6a、b),呈他形粗粒状,并与含金黄铁矿共生。另外,这些磷灰石也与金红石共生(图6a、b、e、f),金红石在背散射(BSE)下呈现出成分变化,暗示受后期流体的再交代,这也与典型造山型金矿的流体周期性脉冲一致 (Sibson et al., 1988)。独居石作为含稀土矿物,常作为造山型金矿理想的定年时钟(Qiu Kunfeng et al., 2020),在坑头金矿床在蚀变岩型矿体中,与黄铁矿共生(图6c、d)。
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根据围岩蚀变和矿物组合关系,本文对坑头金矿床成矿过程进行了识别,在此基础上厘定了蚀变岩型和石英脉型矿体之间的关系。
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坑头金矿床赋矿地层新元古代下江群为一套浅变质碎屑岩,在赋矿围岩成岩过程中,主要形成了草莓状黄铁矿和含铁白云石(图7a)。在成矿期,金的富集主要经历了3个阶段。① 绢云母+毒砂+黄铁矿+石英(Ser+Apy+Py+Qtz)阶段(图7b):受造山作用的影响,早期成矿流体进入剪切破碎带,致使围岩发生石英绢云母化和绿泥石化,同时成岩期形成的铁白云石等碳酸盐矿物溶解,去Fe与流体中的硫氢根离子形成大量硫化物,因此成矿早期主要表现为浸染状黄铁矿和毒砂的大量出现(也有少量方铅矿),或在构造裂隙中形成了细脉状黄铁矿;② 黄铁矿+毒砂+石英(Py+Apy+Qtz)阶段(图7c):围岩进一步遭受热液蚀变,成矿早期的硫化物发生重结晶,形成大量半自形的黄铁矿和毒砂,由于流体的过饱和,他形自然金沿着硫化物裂隙和空隙发生沉淀;③ 自然金+石英(Au0+Qtz)阶段(图7d):主要表现为石英大脉的形成,之前形成的硫化物被再次溶解,再沉淀的过程中形成自形粒状的自然金。通过以上矿物学分析,坑头金矿床蚀变岩型矿体主要形成于Py+Apy+Qtz阶段,而石英脉型矿体形成于成矿晚期Au0+Qtz阶段。
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图4 黔东南坑头金矿床石英脉型和蚀变岩型矿化以及多期构造(热液)特征
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Fig.4 Photographs of quartz-vein-type, alteration-type ores and multi-stages structures in southeastern Guizhou (ore fluids)
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(a)—剪切带中的热液角砾岩和石英脉;(b)—钻孔(ZK7-14)所揭露的断层(F20)带中的石英大脉(脉宽约7m);(c)—多期石英脉相互穿插;(d)—含金石英脉切穿蚀变岩型矿体;(e)—断层F20上盘的蚀变岩型矿石,浸染状硫化物主要呈层状分布,围岩硅化、绿泥石化,见后期石英细脉;(f)—层状硫化物与绿泥石化围岩;(g)—沿着流体交代方向,硫化物含量逐渐减少
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(a)—Hydrothermal breccias and quartz veins in the shear zones; (b)—the quartz veins in fault zone (F20) are explosed by drill hole (ZK7-14); (c)—multistage quartz veins are interspersed; (d)—gold-bearing quartz veins crosscut the alteration-type ores; (e)—the characteristic of alteration-type ores, showing disseminated sulfides are stratiform-like and wall rocks are silicization and chloritization; (f)—stratiform-like sulfides within chloritization wall rock; (g)—the path of ore-forming fluids reacting with wall rock
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图5 黔东南坑头金矿床主要蚀变特征及矿物组成
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Fig.5 The mineral assemblage and alteration type of the Kengtou gold deposit in southeastern Guizhou
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(a1~a3)—蚀变岩型矿体浸染状硫化物,围岩硅化和绢云母化;(b1~b4)—晚期石英脉和蚀变围岩,围岩中发育浸染状环带状黄铁矿;(c1~c4)—石英脉型矿体,石英脉包含有围岩角砾,见半自形自然金;Py—黄铁矿;Apy—毒砂;Qtz—石英;Ser—绢云母化;Au0—自然金
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(a1~a3)—Disseminated sulfides occur in alteration-type ores, showing that these sulfides are dominated by pyrite and minor arsenopyrite, and wall rocks are silicization and sericitization; (b1~b4)—quartz veins and associated wall rock, and pyrites are zoned texture; (c1~c4)—native gold grains occur in quartz veins; Py—pyrite; Apy—arsenopyirte; Qtz—quartz; Ser—sericitization; Au0—native gold
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3.2 金红石成因及矿物化学特征
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金红石是花岗岩、伟晶岩等长英质火成岩中常见的副矿物,同时也是变质岩和相关矿床的一种常见成分,包括矽卡岩、榴辉岩相到绿片岩相变质岩(Agangi et al., 2019)。由于金红石富集高场强(HFSEs)元素,因而常利用HFSEs特征来指示金红石成因和演化过程(Porter et al., 2020)。另外,利用锑(Sb)和钨(W)在非矿化与矿化金红石中含量的不同,被用于绿岩带金矿床的勘查示踪(Agangi et al., 2019)。本文对坑头金矿床的金红石进行矿物学分析显示,金红石具有复杂了结构和元素分布(图6f,图8b)。另外,金红石中存在含Zr-Th等矿物包体(图8b),指示形成于一个较高温的变质环境(Shulaker et al., 2015),与西澳Capricorn造山带和南非Barberton绿岩带的金矿床金红石特征类似(Agangi et al., 2019)。坑头金矿床金红石EPMA分析显示(表1,分析点位如图8a),金红石中微量元素主要为Zr(ZrO2＜0.27%,平均0.06%)、W(WO3高达4.05%,平均0.75%)、Nb(Nb2O5介于0.09%~1.31%之间,平均0.35%)、Fe(FeO介于0.03%~1.70%之间,平均0.61%)和Al(Al2O3介于0.03%~1.01%之间,平均0.10%)以及微量的Sb(Sb2O5＜0.04%)和V(V2O3＜0.15%)。其中W含量在金红石中含量变化较大,少部分低于检测限 (~0.03%),最高可达4.05%,这种W含量的不同导致金红石在背散射(BSE)照片下出现明暗变化(图8b),反映金红石形成于热液交代过程。根据W-Sb的相关型图(图8c),显示坑头金矿床金红石不是变质成因,也不是岩浆成因(花岗岩),而是与金矿化有关的热液成因,其投点也与Obuasi绿岩带金矿床金红石类似(Agangi et al., 2019)。另外,坑头金矿床100×(Fe+Cr+V)-W×1000-Ti相关性图显示(图8d),这些W含量较高的金红石对造山型金矿床和绿岩带石英脉型金矿特征(Agangi et al., 2019)。因此,坑头金矿床与成矿相关热液金红石矿物化学揭示了典型的造山型金矿床的蚀变特征。
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图6 黔东南坑头金矿床主要蚀变矿物特征(背散射照片)
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Fig.6 The BSE photographs of alteration minerals of Kengtou gold deposit in southeastern Guizhou
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(a、b)—石英脉中发育金成矿相关的磷灰石和金红石,细粒黄铁矿裂隙中含有自然金;(c、d)—蚀变岩型矿体中的细粒黄铁矿,见有独居石与黄铁矿共生;(e、f)—粗粒状毒砂包含有细粒黄铁矿,并与金红石和磷灰石等矿物共生,金红石背散射照片呈现出成分不均一;Ap—磷灰石;Rt—金红石;Py—黄铁矿;Qtz—石英;Ser—绢云母;Moz—独居石;Au0—自然金
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(a, b)—The apatite and rutile occur in quartz veins and native gold occur in interstitial of pyrites; (c, d)—monazites coexisting with pyrite in alteration-type ores; (e, f)—coarse-grained arsenopyrotes include the pyrite, which coexisting with apatite and rutile; Ap—apatite; Rt—rutile; Py—pyrite; Qtz—quartz; Ser—sericite; Moz—monazite; Au0—native gold
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3.3 金的赋存状态
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国内外大量研究证实,造山型金矿床Au的赋存状态主要以自然金以及黄铁矿和毒砂中的不可见金为主要赋存特征(Liu Anlu et al., 2017, 2018; Qiu Kunfeng et al., 2020)。对于黔东南地区的金矿床,前人研究显示自然金是主要的赋存状态(刘坤等, 2015; 张泽等, 2019)。本文以坑头金矿床的蚀变岩型和石英脉型矿体为主要目标,开展了系统的硫化矿物化学(EPMA)分析。其结果显示(表2),成岩期黄铁矿(Py-I:图7a)Au、Sb、Ag含量大多低于检测限(bdl:~0.03%),As含量介于bdl~0.21%之间,平均0.04%。在成矿期,Ser+Apy+Py+Qtz阶段黄铁矿(Py-II:图7b)Au含量介于bdl~0.08%之间,集中在0.03%~0.06%之间,平均0.03%;As含量介于bdl~2.31%之间,集中在0.66%~1.66%之间,平均0.77%;另外Sb、Ag含量大多低于检测限,但在样品10-26中,Sb含量高达0.11%。Py+Apy+Qtz阶段黄铁矿(Py-III:图7c)Au含量介于bdl~0.07%之间,平均0.03%;As含量介于0.12%~3.07%之间,集中在0.90%~1.42%之间,平均1.05%;Ag、Sb含量大多低于检测限。从黄铁矿的成矿元素含量可以看出,随着成矿作用的进行,As、Au含量逐渐增加,而Sb在Py-II中含量最高。对不同成矿阶段的毒砂进行含金性分析表明,相对于含金黄铁矿,毒砂中Au含量介于bdl~0.07%(Apy-I)、bdl~0.13%(Apy-II)和bdl~0.08%(Apy-III)之间,可见在Py+Apy+Qtz阶段毒砂中Au含量最高。整体来看,毒砂的Au含金性优于同时期黄铁矿(图9b)。另外,毒砂的Sb含量也明显优于黄铁矿,其范围主要介于0.05%~0.33%之间,平均0.13%。在黄铁矿中,As与S存在线性负相关关系(图9a),指示As替代S进入黄铁矿晶格。
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图7 黔东南坑头金矿床成矿阶段划分和不同成矿阶段矿物组合特征
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Fig.7 The mineral associations in diffrent ore-stages for the Kengtou deposit in southeastern Guizhou
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(a1~a3)—成岩期:主要发育细粒草莓状黄铁矿(Py-I)和含铁白云石(Dol-I),见有后期脉状白云石(Dol-II)切穿Dol-I;(b1~b3)—成矿期Ser+Apy+Py+Qtz阶段,硅化和绢云母化矿化围岩(金品位一般小于1g/t)中发育星点状细粒他形-半自形黄铁矿(Py-II)和少量毒砂(Apy-I),同时见有细脉状黄铁矿;(c1~c3)—Py+Apy+Qtz阶段,构造破碎带中的半自形-自形黄铁矿-毒砂(Py-III和Apy-II)集合体,其中黄铁矿呈环带结构(核部为草莓状黄铁矿Py-I),见有他形自然金(Au0);(d1~d3)—Au0+Qtz阶段,含毒砂(Apy-III)-自然金石英脉切穿早期白云石化围岩,在石英脉中发育自形自然金;Dol—白云石;Py—黄铁矿;Apy—毒砂;Qtz—石英;Ser—绢云母;Au0—自然金
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(a1~a3)—Pre-ore (diagenetic) stage is characterized by framboidal pyrite (Py-I) and Fe-dolomite (Dol-I), and Dol-II in late-ore stage commonly crosscut the Dol-II; (b1~b3)—Ser+Apy+Py+Qtz stage, scattered fine-grained pyrite (Py-II), arsenopyrite (Apy-I) and veined pyrite in silicified and sericitizated wall rock (＜1g/t); (c1~c3)—Py+Apy+Qtz stage: subhedral-euhedral arsenopyrite (Apy-II) and core-rim pyrite (Py-III) in fault zone, and anhedral native grains (Au0) filling the fracture in sulfides; (d1~d3)—Au0+Qtz stage: arsenopyrite (Apy-III)-gold-carrying quartz crosscut the dolomitized wall rock, and euhedral native gold grains occur in quartz; Dol—dolomite; Py—pyrite; Apy—arsenopyrit; Qtz—quartz; Ser—sericite; Au0—native gold
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上述测试表明,虽然成岩期黄铁矿含有一定的Au,但大多数测点低于检测限,相反Ser+Apy+Py+Qtz和Py+Apy+Qtz阶段的黄铁矿和毒砂含金性最优,是主要的载金矿物。根据Au-As相关性图解和Au的溶解度极限(Au/As=0.02; Reich et al., 2005),当Au含量超过在黄铁矿中的溶解度极限时,形成显微-次显微纳米金,反之以化学结合态(Au+1)的方式赋存在黄铁矿或者毒砂中。坑头金矿床Au-As关系图表明(图9b),成岩期黄铁矿高于检测限(~0.03%)测点和成矿早期As含量小于0.5%的测点位于Au的溶解度极限曲线上方,暗示Au以显微-次显微纳米金的方式赋存在黄铁矿中。成矿Ser+Apy+Py+Qtz阶段As含量大于0.5%的测点以及主成矿期黄铁矿的大部分测点落在Au的溶解度极限曲线下方,暗示Au以化学结合态(Au+1)的方式赋存在该类型黄铁矿或者毒砂中。值得一提的是,在Py+Apy+Qtz阶段,黄铁矿和毒砂中伴生有大量的自然金(图7c),揭示该成矿期流体为过饱和状态,Au的含量超过了在黄铁矿中的溶解度极限(Reich et al., 2005)。由此可见,坑头金矿床金的赋存状态多样,在成矿Ser+Apy+Py+Qtz阶段蚀变岩矿体中Au以显微-次显微纳米金赋存在黄铁矿中,而在成矿Py+Apy+Qtz阶段则以化学结合态方式赋存在黄铁矿和毒砂中。他形自然金主要赋存在蚀变岩硫化物裂隙中,自形自然金主要赋存在石英脉中。
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图8 黔东南坑头金矿床金红石矿物特征(a、b)及W-Sb(c)和100×(Fe+Cr+V)-W×1000-Ti图解(d) (底图引自Agangi et al., 2019)
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Fig.8 The BSE photographs (a, b) and diagram of W-Sb (c) and 100×(Fe+Cr+V)-W×1000-Ti (d) (modified from Agangi et al., 2019) of rutile from Kengtou deposit in southeastern Guizhou
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图9 黔东南坑头金矿床黄铁矿S-As关系图(a)和As-Au关系图(b)(Au溶解度极限曲线引自Reich et al., 2005)
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Fig.9 Correlation of S-As values (a) and As-Au values (b) in pyrite and arsenopyrite from the Kengtou deposit in southeastern Guizhou (Au solubility limit curve modified from Reich et al., 2005)
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注:Py-I、Py-II、Py-III、Apy-I、Apy-II、Apy-III矿物类型为图7中所示;bdl代表低于检测限;“/”代表无数值或无计算结果。
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3.4 金的累积富集机制以及对找矿勘查的指示
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通过对坑头金矿床成矿过程的梳理,发现该矿床的形成过程受多期构造(流体)的影响。在多数造山型和卡林型金矿床的成矿过程中,多期流体引发的持续水/岩作用对金的富集起着重要作用(Kerr et al., 2018; Goldfarb et al., 2019; Holley et al., 2019; Wu Yafei et al., 2019; Xing Yanlu et al., 2019)。Kerr et al.(2018) 对加拿大Hope Bay绿岩带造山型金矿床进行研究,发现不同阶段流体的叠加对金的富集具有累积效应,晚期岩浆-热液叠加在已存在的含金造山金矿化体上对金的富集作出了巨大贡献;Xing Yanlu et al.(2019) 通过水/岩反应模拟,认为多期热液事件引发的持续性水/岩作用可对低品位金进行累计富集,形成高品位的大型矿床。坑头金矿床黄铁矿形貌研究显示,随着成矿作用的进行,黄铁矿形貌依次为草莓状(图7a)→他形细粒状(集合体星点状;图7b)→半自形粗粒状(具有环带结构;图7c)。另外,自然金在蚀变岩型矿体中呈他形粒状嵌在硫化物裂隙或者空隙中(图7c),而在成矿Au0+Qtz阶段石英脉中,自然金呈自形粒状(图7c)。这种矿物由他形向自形的转变,揭示了矿物(成矿元素)的溶解-再沉积机制(Wu Yafei et al., 2019)。另一方面,从蚀变岩型矿体和石英脉型矿体之间的成矿联系来看,蚀变型矿体主要与成矿早期流体的交代蚀变有关,在蚀变围岩中形成大量含金的黄铁矿和毒砂(图4f)。而对于石英脉型矿体,主要形成在空间相对开放的剪切带,快速降压或者是外来流体的混入导致的流体不混溶是石英脉型金矿化的主要机制(范宏瑞等, 2021)。对于造山型金矿床石英型中的高品位可见金,主要有两种解释,其一为可见金是从成矿流体中过饱和沉淀(如流体总硫含量减少导致金过饱和:Feng Kai et al., 2018),其二为后期流体活化早期黄铁矿中的不可见金并累计富集形成可见金(Cook et al., 2013)。在坑头金矿床中,石英脉中的可见金主要分布在石英脉体与围岩(蚀变岩)的接触部位(图5c),揭示晚期石英脉在形成过程中,流体与围岩(蚀变岩)发生了再交代。因此,推断早期的流体蚀变(硫化作用)形成了蚀变岩型矿体,而晚期石英脉在形成过程中促使流体与这些蚀变岩发生再交代,导致金累计富集。根据本文详细的岩相学研究,综合认为坑头金矿床的形成主要由三阶段组成:① 在围岩沉积和绿片岩化过程中,形成了大量的草莓状黄铁矿和含铁白云石;② 成矿流体与围岩反应,形成蚀变岩型矿体;③ 受后期构造(热液)事件的影响,蚀变岩型矿体被叠加改造导致矿物(元素)发生溶解-再富集,有可能形成高品位的石英脉型矿体。由此可见,这种多期构造(流体)的交代富集可能是坑头金矿床形成的主要原因之一。
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4 结论
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(1)岩相学和矿物学研究显示,金的富集过程主要经历了三个阶段: Ser+Apy+Py+Qtz阶段主要表现为含矿流体与围岩的初级交代,形成大量浸染状黄铁矿+毒砂;Py+Apy+Qtz阶段为流体与围岩发生强烈的硫化作用,形成蚀变岩型矿体;Au0+Qtz阶段主要形成含金石英大脉。
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(2)金的赋存状态研究显示,成矿Ser+Apy+Py+Qtz阶段蚀变岩矿体中Au以显微-次显微纳米金赋存在黄铁矿中;而在成矿Py+Apy+Qtz阶段主要以化学结合态方式赋存在黄铁矿和毒砂中,同时由于流体过饱和在硫化物裂隙中形成他形自然金。
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(3)综合研究认为,成矿早期流体与围岩的硫化作用形成了蚀变岩型矿体,而高品位石英脉型矿体的形成可能与多期构造(流体)引发的溶解-再沉淀过程有关。
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摘要
黔东南金成矿区位于江南造山带金成矿省的西南端,成矿条件优越。坑头金矿床是黔东南金成矿区的一个中型矿床,在其深部找矿中,发现除石英脉型矿体外,还存在蚀变岩型矿体。然而,这种蚀变岩型矿体的构造形态、蚀变类型、与石英脉型矿体之间关系和金的赋存状态尚不清楚。本研究与当前的勘查工作紧密结合,围绕石英脉型矿体和新发现的蚀变岩型矿体为研究切入点,借助微区分析技术(扫描电镜和电子探针)进行系统的“流体-蚀变-成矿”研究。蚀变矿物金红石矿物化学显示为热液成因,具有典型造山型金矿床的金红石标型特征。围岩的沉积-成岩过程(包括低级变质作用过程),主要形成了草莓状黄铁矿和含铁碳酸盐岩,为后期含金硫化物(黄铁矿和毒砂)的形成提供物质基础(如Fe)。金的成矿富集过程主要经历了绢云母+毒砂+黄铁矿+石英(Ser+Apy+Py+Qtz)阶段、黄铁矿+毒砂+石英(Py+Apy+Qtz)阶段和自然金+石英(Au0+Qtz)阶段。在Ser+Apy+Py+Qtz阶段,主要表现为含矿流体与围岩的初级交代,形成大量浸染状黄铁矿+毒砂的硫化带;Py+Apy+Qtz阶段主要为流体沿着剪切带再交代,形成蚀变岩型矿体;Au0+Qtz阶段主要表现为含金石英大脉的形成。金的赋存状态研究显示,蚀变岩矿体中Au以他形显微-次显微自然金赋存在蚀变岩硫化物裂隙中,或以化学结合态方式赋存在黄铁矿和毒砂中(后者占主导)。在晚期Au0+Qtz阶段,自然金呈自形、粗粒(~0.5 mm)赋存在石英脉中。综合研究认为,多期构造(流体)交代导致的溶解-再沉淀可能是坑头金富集成矿主要原因之一。
Abstract
The gold deposits in southeastern Guizhou are located within the southwestern Jiangnan orogen, and have significant exploration potential. The Kengtou gold deposit is one of the numerous lode gold deposits in the southeastern Guizhou district. In recent years, in addition to the lode-type ores proven by the prospectors, alteration-type ores have also been discovered. However, the ore-related structures, alteration type, relationships between lode-type and alteration-type ores are poorly constrained and remain unclear. This study combines current exploration with scanning electron microscope (SEM) and electron probe microanalysis (EPMA) data from ore sulfides and altered minerals to present a new ore-forming process (fluids-alteration-mineralization) for the Kengtou deposit. The mineralogical chemistry of rutile indicates a typical orogenic origin. Mineralogy studies show that framboidal pyrites and Fe-dolomite were formed in the diagenetic process of wall rock (low-grade metamorphism), which could provide metals (e.g., Fe) for ore-stage sulfides. The three gold mineralization stages identified in this study include Ser+Apy+Py+Qtz, Py+Apy+Qtz, and Au0+Qtz stages, based on minerals textures and crosscutting relationships. Our study shows that Ser+Apy+Py+Qtz stage is dominated by ore fluids interacting with the wall rock that caused disseminated pyrite and arsenopyrite deposition. The Py+Apy+Qtz stage formed the alteration-type ores and is dominated by gold-bearing pyrite and arsenopyrite, as well as anhedral native gold grains. In contrast, the late-ore stage is characterized by lode-type ores, and contains the quartz veins and euhedral native gold grains. The study of gold occurrences shows that gold in pyrite at early-ore stage is present as micro-submicroscopic gold nanoparticles, whilst the main-ore stage present as Au1+in the pyrite and arsenopyrite. The native gold grains have been found in fractures of sulfides at the main-ore stage and within quartz veins at the late-ore stage. Overall, we propose that multi-episodic mineralization processes lead to gold enrichment, which generated high-grade gold resources.
Keywords
gold occurrences ； alteration-type ； lode-type ； Kengtou gold deposit ； southeastern Guizhou
