西南三江兰坪盆地大规模成矿的流体动力学过程——流体包裹体和盆地流体模拟证据

中国西南部云南兰坪盆地因金顶Zn-Pn矿床和新发现的白秧坪超大型Cu-Co-Ag矿床而驰名。金顶矿床以白垩系和第三系陆相碎屑岩为主岩,拥有2亿吨矿石,平均品位Zn6.08%、Pb1.29%(1500万吨金属),是目前中国最大的Zn-Pb矿床,也是世界上形成时代最新且唯一产于陆相沉积岩容矿的超大型Zn-Pb矿床。不同于世界上人们公认的沉积岩容矿基本类型,即SST、MVT和Sedex型,金顶矿床也许代表了Zn-Pb矿床的一个新类型。通常认为兰坪盆地大规模成矿流体起源于盆地卤水,流体流动以重力驱动为主,压力体系接近静水压力。但基于矿田内水压破裂观察、流体包裹体研究和盆地流体动力学模拟,我们认为深部超压流体的注入对整个成矿系统起着重要作用。闪锌矿及相关脉石矿物(石英、天青石、方解石、石膏)中流体包裹体观测的均一温度主体在110～150℃,盐度(质量分数)在1.6%～18.0%NaCl;在时间上,大规模成矿主要阶段伴随着流体温度的不断升高和盐度的逐渐降低;在空间上,金顶矿区空间上从东到西,成矿流体温度明显降低,盐度系统性升高。富CO2流体包裹体揭示成矿流体曾高达(513～1364)×105Pa,大大高于静水压力。数值模拟表明,盆地沉积和压实产生的流体超压可以忽略,区域构造推覆也不足以产生如此高的流体压力。我们认为成矿流体超压很可能是幔源流体注入引起的;幔源含矿的相对高温低盐度流体沿导矿构造注入金顶穹隆构造-岩性圈闭并与其中富H2S的相对低温高盐度卤水混合是兰坪盆地大规模成矿的关键动力学过程。这个特殊的流体动力学过程和成矿系统,使兰坪盆地的成矿有别于世界其他沉积盆地已知的成矿作用。

Fluid dynamic processes of large-scale mineralization in the Lanping Basin, Yunnan, SW-China: evidence from fluid inclusions and basin fluid modeling

The Lanping Basin,Yunnan Province,South-west China,contains the giant Jinding Zn-Pb deposit and the newly discovered Baiyangping Cu-Co-Ag super-large deposit.The Jinding deposit,with reserves of ～200 Mt ore,grading 6.08% Zn and 1.29% Pb(i.e.metal resources of ～15 Mt)is the largest Zn-Pb deposit in China.The ore is hosted by Cretaceous and Tertiary terrigenous rocks.The Jinding is also the youngest sedimentary rock-hosted and the only continental super-large Zn-Pb deposit in the world.Differing from other major types of sediment-hosted Zn-Pb deposits in the world,including SST,MVT and SEDEX,the Jinding deposit represents a new type of sedimentary rock-hosted Zn-Pb mineralization.Most previous studies assumed that the mineralizing fluids were derived from within the basin and that the fluid flow was largely driven by topographic relief under a hydrostatic regime.However,the observations of hydraulic fractures and fluid inclusion data in this study indicate that the mineralizing fluid system was strongly over-pressured.The study of fluid inclusions in sphalerites and associated gangue minerals(quartz,celestite,calcite and gypsum)shows that homogenization temperatures cluster around 110-150℃,with salinities of 1.6-18.0 wt% NaCl equivalent.The fluid temperature increased with the decrease in salinities during the main ore stages,and there is also a systematic westward decrease in temperature and increase in salinity in the Jinding ore district.Fluid pressures as high as(513-1 364)×105 Pa are indicated by CO2-rich fluid inclusions.The results of basin fluid dynamic modeling indicate that the overpressures could not have been produced by normal sediment compaction,and the overpressure related to the thrusting may be inadequate to explain the high fluid pressures indicated by fluid inclusions.The injection of mantle-derived fluids is thought to be a viable mechanism for the build-up of high overpressures.The mixing of two types of fluids in a structural-lithologic trap may have been the key dynamic process for the large-scale mineralization in the Lanping Basin:one was a mantle-derived fluid enriched in metals and CO2 with higher temperatures and lower salinities,and the other was H2S-rich saline formation water with lower temperatures and higher salinities.The special hydrodynamic regime and potential contribution of mantle-derived fluids to the mineralizing system distinguish Jinding from other known sedimentary basin-associated Pb-Zn deposits.