滇西北中甸岛弧及金沙江带印支期成矿和不成矿斑岩（侵入体）的对比研究

Compared with the common occurrence for porphyry intrusions, the economic porphyry copper deposits are rare. It is a big challenge for economic geologists to figure out whether some porphyry has the potential to form copper deposit or not. Many researchers attempt to evaluate it by the oxygen fugacity of magma. Actually, the combination of high oxidation state and sulfur content, relatively hydrous but shallow conditions and a high Cl/H2O ratio could lead to loss of a large proportion of magmatic Cu to the aqueous phase, all of which are critical for ore formation. Relatively opposite conclusions may be attained if basing on only single magma condition. There occur a lot of porphyry copper deposits within the Tethyan Metallogenic Belt in the Sanjiang Region, SW China, most of which are distributed in the Indosinian Zhongdian Arc porphyry Cu Belt, such as the Pulang and Xuejiping porphyry Cu deposits. The Jinshajiang Belt is also characterized by the emplacement of many Indosinian porphryries or plutons, where only limited porphyry Cu deposits (i.e., Yangla) have been found. The reason for the huge distinction on Cu mineralization between the two areas is still uncertain. In this study, we propose to focus on the ore-related and barren granitic intrusions between the two regions. On the basis of detailed field investigation and previous studies, the oxidation state, water and sulfur content, Cl/H2O ratio, and emplacement depth will be estimated by in situ micro-analysis data on appropriate minerals, and the key factors controlling those magma conditions will also be discussed. This work may provide a good example for quantitatively revealing the difference of magma conditions between ore-related and barren porphyries, which will be meaningful to perfect the theoretical system and also mineral exploration on porphyry Cu deposit.

世界上斑岩很多，但能形成斑岩铜矿的则较少。如何区分成矿与不成矿斑岩，是国际关注的重要科学问题。以往多用氧逸度高低等单一指标对斑岩能否成矿进行评价。实际上，岩浆具有高氧逸度、富水、富硫、较高的Cl/H2O比值及较浅的侵位深度，都能使铜尽可能多地自岩浆分异到热液进而利于斑岩成矿。因此，若仅研究上述某个条件，很可能只会得到相对片面的结论。西南“三江”地区印支期的斑岩铜矿集中分布在中甸岛弧带。虽然在与之相邻的金沙江带印支期中酸性侵入体（含斑岩）也广泛分布，但相关斑岩铜矿仅零星产出。本项目拟选择中甸岛弧印支期成矿斑岩以及金沙江带印支期成矿和不成矿斑岩作为研究对象，系统研究对比这两个区域斑岩体系的岩浆物理化学条件（氧逸度、水含量、硫逸度、Cl/H2O比值及侵位深度等）及其制约机制，为定量揭示成矿和不成矿斑岩成岩物理化学条件的异同提供范例。这些研究对辨析成矿与不成矿（或贫矿）斑岩具有重要理论和实际意义。

相对于斑岩的广泛分布，斑岩铜矿相对很少，因而如何区分成矿与不成矿斑岩是国际关注的重要科学问题。西南“三江”地区印支期的斑岩铜矿集中分布在中甸岛弧带。虽然在与之相邻的金沙江带印支期中酸性侵入体（含斑岩）也广泛分布，但相关斑岩铜矿产出非常少。本项目选择中甸岛弧印支期成矿斑岩以及金沙江带印支期成矿和不成矿斑岩开展了对比研究，估算了它们的岩浆含水量、Cl含量、氧逸度、硫逸度、侵位压力等物理化学参数，并依据其岩石学和地球化学特征建立了相应的成岩-成矿模型。经过4年努力，我们圆满实现了研究目标，并获得了如下四点认识：（1）金沙江带与古特提斯洋俯冲-闭合有关的斑岩岩浆相对还原，不利于斑岩铜矿的行成，并初步判断该带寻找印支期斑岩铜矿的潜力较小；中甸地区斑岩岩浆普遍较氧化并富水，因而有利于斑岩铜矿的形成；（2）金沙江带斑岩主要来自于俯冲流体交代地幔楔的部分熔融，并受到了地壳物质不同程度的混染，而中甸地区的斑岩可能形成于俯冲后背景，其岩浆可能来自于扬子西缘新元古代弧型下地壳的部分熔融，并继承了新元古代弧岩浆的高氧逸度、富水的特征；（3）普朗斑岩铜矿发育多期斑岩体，与成矿前和成矿后斑岩相比，成矿期斑岩相对富S和Cl，结合该期斑岩含有大量暗色微粒包体，推测基性岩浆注入可能触发了斑岩成矿；（4）系统的岩浆物理化学条件的估算可以较好地识别成矿和不成矿岩体。我们的工作不仅为定量辨析成矿和不成矿岩体提供了范例，同时有关成矿斑岩来自于新元古代弧型下地壳重熔的论述更正了前人的认识，明确新元古带弧型下地壳分布区具有巨大的找矿前景。