密西西比河谷型(MVT)铅锌矿床与蒸发岩的成因关系

Genetic relationship between Mississippi Valley-type(MVT)lead-zinc deposit and evaporite:A review

MVT铅锌矿床的形成与蒸发岩关系密切,但在蒸发盐矿物发生溶解或转变为其他矿物而“消失”时,人们常忽视其曾经存在。本文基于前人研究,对蒸发岩的转变、识别特征及其与MVT铅锌成矿流体来源、硫来源及储矿构造三个方面的关系进行综述。其中,MVT矿床迁移铅锌的盆地卤水主要来自蒸发海水,少量来自蒸发岩的溶解,区域上蒸发岩的发育可指示该区具有成矿流体的发育条件。铅锌金属硫化物中的硫均来自于硫酸盐还原作用,而石膏和硬石膏等蒸发盐矿物是硫酸盐的重要提供者。蒸发岩溶解垮塌利于形成角砾岩与垮塌空间,铅锌矿化可赋存于角砾岩内或其邻近岩层中;盐底辟构造会形成有利于流体聚集和矿质沉淀的构造/化学圈闭,铅锌矿化赋存于底辟体顶部冠岩、边部过渡层和围岩、直立的底辟角砾筒或侧向底辟形成的穹隆体顶部。此外,蒸发盐矿物的假晶、残留的蒸发盐矿物是“消失”蒸发岩存在的直接证据,钠长石和方柱石、正延性玉髓/燧石、富碱土元素双锥状石英等,可指示蒸发岩曾经存在;蒸发岩溶解垮塌构造具有顺地层延伸长等特点,盐底辟构造具有穿层及角砾来自下部层位等特点,是可以协助识别蒸发岩的相关构造。可以看出,区域地层记录有蒸发沉积环境是MVT铅锌成矿必要条件,蒸发岩溶解垮塌和底辟构造是重要找矿目标。

Mississippi Valley-type(MVT)lead-zinc deposits are closely related to evaporite rocks,however,the fact that evaporite minerals dissolve or transform into other minerals and"disappear"is often overlooked.Drawing on previous research results,we summarized the transformation and identification characteristics of evaporite,and its relationship with MVT Zn-Pb mineralization that is reflected in the source of mineralizing fluids,the source of sulfur,and the ore-bearing structures.The formation of MVT deposits involves basin brines,primarily derived from evaporated seawater,with a small portion originating from the dissolution of evaporite rocks.Meanwhile,the deve-lopment of regional evaporite rocks indicates the formation conditions for mineralizing fluids.The sulfur in lead-zinc metal sulfides totally comes from the reduction of sulfates,and evaporite minerals,such as gypsum,anhydrite,etc.,are important suppliers of sulfates.Moreover,dissolution-collapse evaporites form breccias and collapse spaces,and lead-zinc mineralization can occur within the breccias and/or adjacent strata.Salt diapiric structures create favorable structural/chemical traps for fluid accumulation and mineral precipitation,and lead-zinc mineralization can be found in the caprock,transition zones and peridiapiric rocks of salt diapirs,vertically oriented salt diapiric breccia pipes,or top of domes formed by lateral salt diapirism.Pseudomorphs after evaporite minerals and residual evaporite minerals are direct evidence of the existence of"disappeared"evaporites,while minerals such as albite and scapolite,length-slow chalcedony/quartz,alkali-enriched double-terminated quartz,etc.,may indicate the past existence of evaporites.The evaporite dissolution-collapse structures exhibit the characteristics of lateral extension along with the stratigraphic strike,and salt diapiric structures have features such as cross-cutting and breccias derived from lower strata,both of which can help identify evaporite-related structures.In exploration for MVT deposits,the regional stratigraphic record of evaporative sedimentary environments is necessary for MVT mineralization,and evaporite dissolution-collapse structures as well as salt diapiric structures are important prospecting targets.