中硫型浅成低温热液金多金属矿床基本特征、研究进展与展望

摘要

继20世纪80年代以来低硫型和高硫型浅成低温金矿床概念提出及成矿模型建立之后,相继发现一些浅成低温热液矿床不具上述两类矿床端元的成矿特点,相反兼具过渡性质;很多学者将其作为单一矿床类型,定义为中硫型浅成低温热液矿床.作为一个新的矿床类型,中硫型矿床是否有单独划分的必要？该类矿床具有什么样的地质特征？长期以来这些问题令人困惑.本文从大量文献中,在全球范围内甄别出24个比较明确的中硫型浅成低温热液金(多金属)矿床,基于其基本特征和研究进展的系统梳理与分析,从中硫型矿床的时空展布、地质特征、矿物组合、金属源区特征、中硫型与高硫型金(铜)矿和低硫型金矿的主要区别,以及目前国际研究进展及难点等方面进行总结阐述.中硫型金多金属矿床具有如下六大特征:(1)发育富碳酸盐-贱金属硫化物成矿体系,碳酸盐矿物可见于各成矿阶段热液脉系中,尤其在热液晚阶段以碳酸盐矿物为主;贱金属硫化物主要为Cu、Pb、Zn、Fe等的硫化物;(2)发育中硫化态矿物组合,如贱金属硫化物黄铜矿、闪锌矿、方铅矿、黄铁矿、黝铜矿等;可少量发育明矾石和冰长石;(3)含矿脉系中富硫化物(总量大于5％),且在斑岩铜矿系统中较富黄铜矿;(4)普遍发育浅色贫铁闪锌矿(有待进一步证实);(5)普遍赋存在挤压岛弧背景下斑岩Cu-Au-Mo矿的外围;(6)空间上可与高硫型和低硫型金多金属矿床共存.普遍发育斑岩型Cu-Au-(Mo)矿床和浅成低温热液型矿床的世界著名三大成矿域(滨太平洋成矿域、古亚洲洋成矿域和特提斯-喜马拉雅成矿域),同样具有形成中硫型矿床的有利成矿条件.未来关于中硫型矿床的研究亟需解决以下几个关键问题:(1)目前尚未有文献对“富碳酸盐-贱金属(Cu、Pb、Zn、Fe等)”进行详细报道,这种成矿体系是如何形成的？流体中CO2、H2S及贱金属元素对Au的运移和沉淀有何影响？此问题是认识中硫型金多金属矿床成矿机制的关键所在.(2)中硫化态矿物的矿物(黄铁矿、闪锌矿、方铅矿、黝铜矿、砷黝铜矿、黄铜矿等)的沉淀环境？与高硫化态、低硫化态矿物有何区别？(3)从成矿系统、成矿过程和矿物形成的复杂性来考虑,显然以闪锌矿中FeS的含量多少作为区别IS型、HS型、LS型矿床的特征地化标志过于简单,且与已有关于闪锌矿的矿物学研究成果相矛盾,因此仍需进一步工作.(4)早期形成的作为赋矿围岩的火山岩地层或者次火山岩体是否提供了成矿物质？是否充当了浅成低温热液矿物沉淀的地球化学屏障？其具体过程是怎样的？以上问题的解决可辅助揭示IS型矿床的成因机制和形成过程,并为同类型矿床的勘查工作提供支持.%Following the concept of epithermal gold deposits of high sulfidation (HS) and low sulfidation (LS) types deposits were suggested and ore-forming models were proposed since the 1980s,many epithermal deposits have been found.These deposits do not have the ore-forming characteristics of HS and LS type deposits,but have a transitional geological nature.Many researchers take them as a single type of epithermal deposit,which is defined as intermediate sulfidation (IS) type epithermal deposits.As a new type,is there a necessary to separate it from HS and LS epithermal deposits? What kind of geological feature does it has? These questions have puzzled most of the researchers,who engaged in the research work on epithermal Au and porphyry Cu deposits for a long time.This paper sums up the basic features and research progresses based on twenty-four IS-type epithermal deposits indentified so far in the world,and mainly focus on the temporal and spatial distribution,geological features,mineral assemblage characteristics,metal source,main differences to HS and LS type deposits.The IS-type epithermal deposits have six essential characteristics.Firstly,the development of a kind of ore system which is rich in carbonate and base metals.Carbonate mineral can be found in every stage of veins,and play a leading role in the last hydrothermal event.Base metals are in the form of sulfides of Cu,Pb,Zn,and Fe.Secondly,the development of intermediate sulfidation mineral assemblage:chalcopyrite,pyrite,galena,sphalerite,and tetrahedrite.A few alunite and adularia can also be found.Thirdly,most of the total content of suifides in ore-bearing veins is over 5％,especially rich in chalcopyrite.Fourthly,light color sphalerite which is poor in iron is well developed.Fifthly,this type of deposit is welldeveloped around the porphyry type Cu-Au-Mo deposits under the background of island arc.Lastly,IS type deposits could coexist with HS and LS type deposits in the same ore system? Three world-renowned metallogenic province (Circum-Pacific metallogenic province,Paleoasian metallogenic province and Tethys-Himalayan metallogenic province),which were well developed with porphyry Cu-Au-(Mo) deposits and epithermal-type deposits,have the favorable conditions for the formation of intermediate sulfidation type deposits.The future research needed to solve several key problems as below:(1) there is no detailed research work on the "rich carbonate-base metals (Cu,Pb,Zn,Fe,etc.)" ore-forming system.How does this kind of system form? Do the CO2,H2S and base metal elements in the fluid play an important role for migration and precipitation of Au? This problem is the key to know the formation mechanism of intermediate sulfidation type deposits.(2) what kinds of precipitating environment do the intermediate sulfidation minerals need? And are there differences of precipitating environment between the intermediate sulfidation,high sulfidation and low sulfidation minerals? (3) considering from the complexity of metallogenic system,mineralization process and mineral formation,it is too simple to take FeS (mol％) content in sphalerite as geological mark to identify intermediate sulfidation,high sulfidation and low sulfidation deposits.This opinion is inconsistent with the results of prior mineralogical studies on sphalerite,and therefore needs further work to confirm it.(4) Do the volcanic or subvolcanic rocks,which are taken as the host rocks of intermediate sulfidation deposits in shallow crust,provide the ore-forming? Do they play the role of geochemical barrier for epithermal deposits? What is the real process? The solving of these problems may help to reveal the genetic mechanism of IS type deposits and their formation processes,and provide supports for the exploration of the same type of deposits.