滇西南惠民超大型铁矿床：一个晚三叠世火山沉积型矿床

惠民超大型铁矿床处在西南三江复合造山带中南段的昌宁-孟连结合带东侧，具有火山-沉积岩容矿、磁铁矿-菱铁矿矿化组合及普遍伴生热液蚀变的突出特点。目前，因其赋矿地层时代存在分歧，致使其矿床成因认识不清。野外地质调查表明，矿区内原划分为澜沧岩群惠民岩组的岩石组合可分为两个构造地层单元，即：下部为中—晚奥陶世弧火山岩组合，受到古特提斯洋俯冲增生作用的改造，显示一定的构造混杂特点，局部见铁铜矿化；其上被一套火山-沉积岩组合不整合覆盖。上覆火山-沉积岩由玄武岩、安山岩、凝灰岩及硅质岩组成，属于弧前环境沉积组合，是已知铁矿体的主要赋矿层位，至少可分为3个完整的火山沉积旋回，其间多为整合接触。铁矿体呈层状、似层状多层次产于火山岩、硅质岩中，显示同生沉积成矿特点，普遍伴生陡倾斜(细)脉状铁矿化。容矿火山岩上部火山岩及火山碎屑岩中岩浆锆石U-Pb测年结果显示，其结晶年龄位于235～214 Ma之间，峰期年龄为230～225 Ma,指示铁矿床成矿时代属晚三叠世。由此提出，惠民超大型铁矿床是一个早中生代火山沉积型矿床，其成矿严格受到晚三叠世(可早至晚二叠世)古特提斯洋向东俯冲消减(现今地理方位)有关弧火山活动...

The supergiant Huimin iron deposit in southwestern Yunnan Province, SW China: A Late Triassic volcanic sedimentary type deposit

The supergiant Huimin iron deposit is located in the eastern part of the Changning- Menglian suture belt in the Sanjiang compound orogenic belt, which notably features volcanoclastics- hosted, magnetite- siderite association, as well as pervasive hydrothermal alteration. At present, the genesis of the deposit is still debated due to controversy in the age of the ore- bearing strata. Detailed field observations reveal two tectonostratigraphic units separated by an unconformity, both of which belong to the Huimin Formation of the Lancang Group. The lower unit is the Middle to Late Ordovician volcanic- sedimentary rocks with arc affinity, which has been intensely modified by the Paleo- Tethyan subduction, and thus exhibits features of tectonic mélanges. Locally, iron and copper mineralization occurred in the older unit. The upper unit consists of basalt, andesite, crystal- rich tuff and hydrothermal siliceous rocks, where numerous iron- layers occurred. These iron layers form the main orebodies. This volcanic succession contains at least three volcanoclastic cycles with mostly conformity contacts between them. The iron ore bodies are stratiform- like and/or lamellar style in shape, and few are steeply- dipped veins and veinlets. Obviously, the mineralization was syn- sedimentary. New zircon LA- ICP- MS U- Pb dating results show that the ore- hosted volcanic rocks were emplaced during 235 to 214 Ma with the volcanism age peaking at 230 to 225 Ma. The Late Triassic volcanic rocks show arc- signatures geochemically, and likely formed in a forearc basin due to the eastward subduction (at present geographic coordination) of the Paleo- Tethys Ocean. These data, combined with the previously published data, suggest that the supergiant Huimin iron deposit is a volcanic- sedimentary deposit formed owing to subduction- related volcanism during the Early Mesozoic. Our new data and ideas presented in this paper shed light on understanding the mineralization of similar iron deposits in the adjacent regions of the Huimin iron deposit, and are helpful in guiding further prospecting.