隐伏斑岩成矿系统的远端热液产物研究：以澜沧裂谷大规模热液白云岩为例

The discovery of super-large concealed porphyry Mo deposit in Laochang, Lancang, Yunnan, has shed a new light on exploration of concealed Xishanian porphyry ore deposits in Lancang Rift. Compared with the typical porphyry mineralization alteration system, lithocap with prospecting indication does not develop at the shallow surface of Laochang mining area, but a set of breccia iron-manganese dolomite cutting through stratigraphy is developed in Lancang Rift. The preliminary researches by the applicant shows that the dolomite belongs to intermediate-acid magmatism controlled hydrothermal dolomite(HTD) with moderate to high temperature and Pb-Zn-Mo-Sn anomaly, suggests it is the distal hydrothermal product of the concealed Xishanian porphyry hydrothermal system in Laochang mining area. The formation mechanism of HTD and the typomophic characteristics indication to the concealed porphyry hydrothermal system are scientific problems needed to be solved. This project aim to establish the genetic model and typomophic characteristics of Lancang Rift HTD by researches focused with in-situ U-Pb dating of carbonate minerals, ore-forming fluid evolution in the vertical hydrothermal alteration system, C-O-Sr-Mg isotope evolution, combined with trace element geochemistry. The project is theoretical significant important not only helpful to improve the hydrothermal alteration model of the concealed porphyry mineralization system, but also to promote the theoretical study of hydrothermal dolomite in metal mineralization, more importantly, it can provide shallow indicator for geological exploration of concealed porphyry deposits related to Himalayan orogenic activities in Lancang rift.

云南澜沧老厂超大型隐伏斑岩钼矿的发现，为澜沧裂谷寻找喜山期隐伏斑岩型矿床提供了新的勘探方向。与典型斑岩成矿蚀变系统对比，老厂矿区浅表不发育具有找矿指示意义的岩帽，但裂谷内发育一套大规模分布、具穿层特征的角砾化铁锰白云岩。申请者的初步研究表明，这套白云岩具有中高温特征和Pb-Zn-Mo-Sn异常，属于中酸性岩浆活动有关的热液白云岩，是喜山期隐伏斑岩热液系统的远端热液产物，其成因机制和对隐伏斑岩热液系统的指示作用是急需解决的科学问题。本项目拟通过其中碳酸盐矿物的原位定年、垂向热液蚀变体系成矿流体演化、C-O-Sr-Mg同位素示踪，结合微量元素地球化学研究，建立澜沧裂谷热液白云岩的成因模型和指示标型。项目不仅有助于完善隐伏斑岩成矿体系的热液蚀变模型，推进热液白云岩在金属成矿作用中的理论研究，更可以为澜沧裂谷乃至滇西地区与复合造山作用有关的隐伏斑岩矿床的勘探提供地表标志，因而具有重要的理论意义。

澜沧老厂铅锌铜钼矿床是昌宁-孟连裂谷受喜山期花岗岩浆作用改造形成的典型矿床，斑岩基本未出露，浅表不发育硅酸盐岩帽但广泛发育一套热液白云岩。项目主要通过原位矿物定年、原位元素分析和流体包裹体等方法研究了该类型矿床的岩浆演化过程和热液垂向演化过程，论证了热液白云岩是隐伏斑岩成矿热液系统的远端产物，是隐伏斑岩矿床的浅表指示物，因而具有重要的应用价值。.以澜沧老厂和沧源金腊花岗（斑）岩中钾长石巨晶的捕虏矿物特征为主要线索，初步阐明了沧源-澜沧喜山期花岗岩浆的演化过程和成矿物质来源。金腊钾长石巨晶的捕虏矿物具有有序环边结构，巨晶捕虏的榍石原位LA年龄为44.5±4.2Ma（MSWD=1.9，n=22），捕虏的黑云母形成条件为564℃和97MPa（3.7公里）；基质中榍石的原位LA年龄为44.2±3.5Ma（MSWD=4.5，n=45），基质中黑云母形成条件从593℃和117MPa（4.4公里）向565℃和83MPa（3.1公里）演化。澜沧老厂钾长石巨晶的捕虏矿物无序，代表了巨晶在斑岩就位过程中的快速生长；斑晶黑云母的形成条件为577℃和93MPa（3.5公里）。原位元素分析表明，花岗（斑）岩中的长石是铅的成矿物源，黑云母是锌的成矿物源，钼可能来源于岩浆演化的晚期富集，铜可能来自循环热液对基性岩围岩的萃取。.澜沧老厂垂向蚀变体系包括斑岩的绢云母化、石炭纪玄武岩的矽卡岩化、C-P碳酸盐岩地层中的白云石化和方解石化。玄武岩是重要的地球化学障，斑岩和矽卡岩中热液硬石膏的沉淀和被溶解交代是成矿过程的重要环节，其中热液榍石的原位LA年龄为44.2±1.3Ma（MSWD =2.2，n=58）。矿区靠近斑岩的热液白云岩具有高温（集中在300-380℃）高盐度特征，而二叠纪灰岩中发育的粗晶方解石+钙锰矿晶洞则代表了热液演化最远端的低温氧化环境，其中方解石的原位LA年龄为38.6±5.0Ma（MSWD=2.9，n=245）。对热液碳酸盐矿物的元素分析表明，Mn和Mn/Fe比值的地球化学填图是确定深部隐伏斑岩热液体系的重要方向。