滇西北红牛-红山矽卡岩型铜矿床石榴子石原位LA-SF-ICP-MS U-Pb定年及地球化学特征

红牛-红山铜矿床是义敦岛弧南段已发现最大的(远端)矽卡岩型矿床，目前其赋矿矽卡岩成岩时代缺乏直接的年代学证据。矽卡岩成岩年龄的精准测定对于该矿床在义敦岛弧南段中生代2期斑岩成矿事件中的定位具有重要意义。红牛-红山矿床矽卡岩中石榴子石可分为2期，早期石榴子石多见于贫矿矽卡岩，粗粒结构，为均质体；晚期石榴子石多见于富矿矽卡岩，细—中粒结构，发育振荡环带，具非均质光性异常。对2期石榴子石开展了原位LA-SF-ICP-MS U-Pb定年和微区成分分析，获得早期石榴子石和晚期石榴子石的U-Pb年龄分别为84.2±3.0 Ma和81.7±3.5 Ma。主量和微量元素含量表明，2期石榴子石属钙铁榴石-钙铝榴石系列，亏损Rb、Ba、K、Sr等，富集Th、U、P；其中，早期石榴子石相对富钙铁榴石组分、LREE/HREE值高，Eu呈高正异常，晚期石榴子石相对富钙铝榴石、LREE/HREE值低、Eu无异常。结合本区燕山晚期碰撞型斑岩成矿事件，认为约80 Ma为本矿床远端矽卡岩成岩成矿时期，成矿作用与同期酸性斑岩关系密切。石榴子石地球化学特征揭示，干矽卡岩阶段水/岩比值高，以渗滤交代作用为主，早期流体为氧化、高温、富铁、弱碱性的相对封闭体系，晚期为相对还原、相对低温、富铝、酸性的开放体系。

In-situ LA-SF-ICP-MS U-Pb dating and geochemistry of garnets from Hongniu-Hongshan skarn-type copper deposit in northwestern Yunnan

Hongniu-Hongshan copper deposit is the largest(distal)skarn type deposit discovered in the south section of Yidun Arc.However, there is no direct chronological evidence for the diagenetic age of ore-bearing skarn.The accurate determination of the diagenetic age of skarn is of great significance to the location of this deposit in the two stages of Mesozoic metallogenic events of porphyry in the south section of Yidun Arc.The garnets in skarn of the deposit can be divided into two stages, where the early garnets(Grt Ⅰ)are mostly found in skarn of lean ore with coarse grain structure and isotopic body; and the late garnets(GrtⅡ)are mostly found in skarn of rich ore with fine-medium grain structure, oscillatory zoning and heterogeneous and optical anomaly.Based on in-situ LA-SF-ICP-MS U-Pb dating and electron microprobe in-situ concentrations analysis of two stages of garnets in this paper, the U-Pb ages of GrtⅠ and GrtⅡ are 84.2±3.0 Ma and 81.7±3.5 Ma, respectively.The contents of major and trace elements suggest that the garnets of the two stages are classified in the andradite and grossularite series, lack of Rb, Ba, K, Sr, etc.and rich in elements Th, U and P, where GrtⅠare characterized by relatively rich in andradite, high LREE/HREE ratio and high positive anomaly of Eu, while GrtⅡ features relatively rich in grossularite, low LREE/HREE ratio and no abnormal Eu.In combination of the Late Yanshanian mountain metallogenic events of regional collisional porphyry, it is deemed that the diagenetic and metallogenic period of distal skarn in this deposit is at about 80 Ma, when that mineralization is closely related to the acidic porphyry over the same period.The geochemical characteristics of garnets reveal that the dry skarn stage is characterized by a high water/rock ratio and dominated by infiltration metasomatism.The early fluid developed into a relatively closed system of oxidation, high temperature, rich iron and weak alkalinity, and the late stage formed an open system of a relative reduction, relatively low temperature, rich aluminum and acid.