黔东南坑头金矿床热液蚀变特征及成矿过程研究

The study of hydrothermal alterations and ore-forming process of the Kengtougold deposit,southeastern Guizhou,China

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(Au~0+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;Au~0+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Au~0+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。

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 Au~0+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 Au~1+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.