Origin of dissolution vugs, caverns, and breccias in the Middle Devonian Presqu'ile barrier, host of Pine Point mississippi valley-type deposits
Origin of dissolution vugs, caverns, and breccias in the Middle Devonian Presqu'ile barrier, host of Pine Point mississippi valley-type deposits
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DOI：
10.2113/gsecongeo.89.4.858
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发表时间：
1994-07
期刊：
影响因子：
5.8
通讯作者：
H. Qing;E. Mountjoy
中科院分区：
文献类型：
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作者：
H. Qing;E. Mountjoy
位于Pine Point的密西西比河谷型铅锌矿床产于溶洞、洞穴和角砾岩中，这些矿床以前被解释为与中泥盆世次瓦特山暴露期间的大气岩溶作用有关。对Pine Point的露天矿坑和地下Presquile屏障34口威尔斯井的岩心样品进行的研究表明，与瓦特山下暴露有关的溶解作用相对较小，与密西西比河谷型矿床就位之前或同时发生的大规模热液溶解作用不同。与瓦特山下出露有关的溶洞、洞穴和角砾岩（1）通常充填有绿色页岩，局部有悬垂方解石胶结物，（2）保存在石灰岩和方解石中，（3）仅限于不整合面正下方的地层，（4）在缝合线石化、硅化和矿化之前形成。在Presqu 9 ile屏障中没有观察到诊断性的大气岩溶特征，如残溶洞、多利纳、钙质结砾岩、红土和洞穴积土。在瓦特山下暴露期间，有限的大气降水证据可能是由于干旱的气候和相对较短的暴露时间。相反，在Pine Point，容纳密西西比河谷型矿床的热液溶解洞、洞穴和角砾岩充满了与热液流体有关的层积方解石、鞍状方解石、硫化物矿物、晚期方解石和焦沥青;出现在瓦特山不整合面的上方和下方;遵循与屏障平行的区域管道系统;晚于样式，表明溶解必须在埋藏过程中发生;仅出现在被解释为由埋藏流体形成的碳酸盐岩中。流体包裹体测量表明，鞍状碳酸盐岩从92度和178度之间的流体中沉淀出来，具有11和29wt%NaCl当量之间的盐度。因此，大多数溶解溶洞，洞穴，和角砾岩主机密西西比河谷型矿床在松树点似乎是有关的热液卤水，而不是大气流体，并形成之前和/或后期阶段的石化和铅锌矿化。已经提出了几种机制来解释热液溶解，包括由热化学硫酸盐还原产生的酸性盆地流体，有机质的成岩作用，和/或富含CO2的盐水在其上倾运动期间的冷却，所有这些都可能在Pine Point活跃。
Mississippi Valley-type lead-zinc ore deposits at Pine Point occur in dissolution vugs, caverns, and breccias, which previously were interpreted to be related to meteoric karstification during Middle Devonian sub-Watt Mountain exposure. Studies of open pits at Pine Point and core samples from 34 wells from the subsurface Presqu9ile barrier revealed that dissolution related to the sub-Watt Mountain exposure was relatively minor and differed from the large-scale hydrothermal dissolution that occurred prior to or was contemporaneous with the emplacement of Mississippi Valley-type deposits. Dissolution vugs, caverns, and breccias related to the sub-Watt Mountain exposure are (1) commonly filled in with green shale, and locally, with pendant calcite cements, (2) preserved both in limestones and dolomites, (3) restricted to the strata immediately beneath the unconformity, and (4) formed prior to stylolitization, dolomitization, and mineralization. Diagnostic meteoric karst features, such as rundkarren, dolina, calcrete, terra rossa, and speleothems are not observed in the Presqu9ile barrier. The limited evidence of meteoric water during sub-Watt Mountain exposure is possibly due to an arid climate and relatively short periods of exposure.In contrast, hydrothermal dissolution vugs, caverns, and breccias that host Mississippi Valley-type ore deposits at Pine Point are filled with laminated dolomites, saddle dolomites, sulfide minerals, late-stage calcite, and pyrobitumen that are associated with hydrothermal fluids; occur both above and below the Watt Mountain unconformity; follow the regional conduit system that parallels the barrier; postdate stylolites, suggesting that dissolution must have occurred during burial; and occur only in dolomites interpreted to have been formed from burial fluids. Fluid inclusion measurements indicate that the saddle dolomites precipitated from fluids between 92 degrees and 178 degrees C having salinities between 11 and 29 wt percent NaCl equiv. Thus the majority of dissolution vugs, caverns, and breccias that host Mississippi Valley-type ore deposits at Pine Point appear to be related to hydrothermal brines, not meteoric fluids, and formed before and/or during late-stage dolomitization and Pb-Zn mineralization. Several mechanisms have been proposed to account for the hydrothermal dissolution, including acid basinal fluids generated by thermochemical sulfate reduction, diagenesis of organic matter, and/or cooling of CO 2 -rich brines during their updip movement, all of which may have been active at Pine Point.