滇西北中甸岛弧印支期斑岩铜矿床的保存与剥蚀程度研究：低温年代学制约

The Zhongdian Arc is located in the southern part of the Yidun Arc in the Sanjiang Tethyan orogenic belt, SW China. This arc formed during westward subduction of the Ganzi-Litang paleo-oceanic crust during the Late Triassic. There are two belts hosting Indosinian acidic-intermediate porphyries that are closely associated with Cu (Au) mineralization in the form of moderate to superlarge porphyry-type or skarn-type copper polymetallic deposits. In this project, selected deposits from the two belts including Pulang, Chundu, Lannitang and Langdu will be examined in order to evaluate their exhumation histories and the broader implications for regional exhumation of the two porphyry belts in the Zhongdian arc. Constructive evaluation of the potential for further mineralization in the Zhongdian area will result, with important implications for exploration. A broad spectrum of low-temperature thermochronology methods including (U-Th)/He, fission track, and 40Ar/39Ar dating, in combination with fluid inclusion microthermometry, mineralogical thermometry and barometry will be employed. In addition, detailed studies of mineral association, paragenesis and textures, and the characteristics of mineralization and hydrothermal alteration will be undertaken to provide the proper context for interpretation. Finally, we will reveal the uplift and erosion history of the eastern Tibetan Plateau through comparative studies and further discuss how to evaluate the exhumation of those deposits, which formed in different epochs to the Sanjiang Tethyan orogenic belt.

中甸岛弧位于"三江"特提斯构造域之义敦岛弧的最南端，形成于晚三叠世甘孜-理塘古洋壳的向西俯冲。区内发育着东、西两条与铜（金）矿化关系密切的印支期中酸性斑（玢）岩带，并产出众多斑岩型、矽卡岩型大-中型铜多金属矿床。本项目将以其中新发现的普朗、春都、烂泥塘和浪都等代表性矿床为重点研究对象，拟通过系统的磷灰石和锆石的裂变径迹和(U-Th)/He、黑云母和钾长石的40Ar/39Ar等低温热年代学研究，并适当结合流体包裹体显微测温学、矿物化学以及矿床的蚀变和矿化特征等方面的综合研究，定量评价这些矿床的剥蚀程度，并试图揭示中甸岛弧东、西两个斑岩带的地表剥露情况,进而探讨该地区的找矿潜力与找矿方向。同时，通过对比研究，揭示义敦岛弧及青藏高原东缘的隆升与剥蚀历史，并进一步探索如何科学评价"三江"特提斯成矿域不同时代不同类型矿床的保存和剥蚀情况。

斑岩型矿床是世界上最重要的Cu、Mo、Re、Au和Ag等金属的来源，具有十分重要的研究价值。前人研究表明，这类矿床主要分布在新生代，其次为中生代，寒武纪及之前的斑岩矿床极少。这种时代分布特征通常归因于成矿后的抬升与剥蚀作用。因此，定量约束斑岩矿床的保存与剥蚀程度成为资源评价研究中的重要科学问题。云南中甸地区位于构造异常活跃的青藏高原东南缘，其中产有众多大型-超大型印支期斑岩Cu-Au矿床，从而成为我们研究构造活动带中古老斑岩矿床剥蚀过程与保存机制的理想对象。.本项目以普朗、雪鸡坪和地苏嘎3个矿床（点）为典型实例，采用垂直剖面取样法，结合锆石U-Pb、锆石(U-Th)/He和磷灰石(U-Th)/He的三重定年（triple-dating）、数值模拟以及矿物温压计等方面的综合研究，精细刻画了这3个矿床（尤其是普朗）的冷却与剥露历史。结果表明：(1) 它们的侵位深度约为5~7 km，相对较深的就位深度不仅导致了矿区岩浆-热液系统的缓慢冷却（~100℃/Myr，持续几个百万年），而且有利于矿床在后期剥露过程中得以保存。(2) 普朗矿区经历了两阶段的剥蚀历史，早阶段从早侏罗世持续至晚白垩世，剥蚀速率为 33–45 m/Myr，剥蚀量约为4.5–6.6 km；晚阶段从晚白垩世持续至今，矿体遭受侵蚀破坏，速率为 5–17 m/Myr，剥蚀深度为0.6–1.1 km，剥蚀量约为30 %，属于中度剥蚀。(3)雪鸡坪矿区剥蚀深度约为1395 m，剥蚀量为40%左右，属于中-重度剥蚀；而地苏嘎矿区剥蚀深度超过4000m，剥蚀量达80%，属于重度剥蚀，因此这两个矿区的深部找矿潜力相对有限。(4) 普朗矿区均经历了十分漫长（> 50 Ma）、且极端缓慢（<20 m/Myr）的剥露历史，这主要归因于青藏高原东南缘准平原的形成。因此，残留准平原可能是构造活动带中古老斑岩矿床得以保存的先决条件，在找矿勘探过程中应予以重视。