云南都龙锡锌矿区同位素年代学研究

摘要

The Laojunshan pluton is composed of muscovite granite,granite porphyry and two-mica granite.The Dulong skarn-type tin-zinc deposit occurs in the Middle Cambrian Tianpeng Formation along the exocontact zone of the Laojunshan pluton.In the periphery of the deposit,a few small cassiterite-wolframite quartz veins occur in the two-mica granite.Based on field investigation and analysis of major elements,trace elements and rare earth elements as well as dating results,the authors studied the ages of tin-zinc mineralization and petrogenesis,the relationship between the granites and the deposit,and the tectonic environment of the Dulong deposit and the Laojunshan granite in Yunnan Province.The Laojunshan granites,characterized generally by the existence of muscovite,A/CNK larger than 1.1,K2O/Na2O larger than 1,high alkali,low calcium,low magnesium and titanium,and enrichment of thorium and uranium.The major elements and REE and the distribution patterns of REE are very similar to those of the gneiss near the ore deposit.Geological and geochemical features indicate that these rocks belong to S-type granite.All kinds of granites are considered to have been formed by consanguineous magma because they have similar distribution patterns.The LA-ICP-MS U-Pb dating of zircons yielded ages between (87.6± 1.4) Ma and (91.7 ± 1.8) Ma for the muscovite granite,an age of (87.3 ± 2.1) Ma for the granitic porphyry and ages between (84.3 ± 2.2) Ma and (85.0 ± 1.3) Ma for the two-mica granite.40Ar-39Ar isotopic dating of phlogopite from the skarn yielded a weighted average age of (93.6 ± 1.1) Ma from 1100℃ to 1400℃.Re-Os isotopic dating of molybdenite from the altered rock surrounding the cassiterite-wolframite quartz veins yielded model ages between (75.04 ± 1.78) Ma and (79.16 ± 2.43) Ma.According to the modes of occurrence of granites and previous research results,the magmatism could be divided into two phases:Muscovite granite and granite porphyry emplaced between (87.3 ± 2.1) Ma and (92.9 ± 1.9) Ma and later two-mica granite emplaced from (83.3 ± 1.5) Ma to (85.0 ± 1.3) Ma.The corresponding ore-forming process could also be divided into two phases:the skarn type tin-zinc deposit related to muscovite granite and the granite porphyry formed between (79.8 ± 3.2) Ma and (93.6 ± 1.1) Ma,and later the tin-and tungsten-bearing quartz veins related to two-mica granite were formed between (75.04 ± 1.78) Ma and (79.16 ± 2.43) Ma.The Laojunshan granites and associated tin-zinc and tungsten ore bodies in the Dulong ore deposit resulted from the large-scale granitic magmatic mineralization in an extensional setting in Late Cretaceous.%笔者对云南都龙锡锌矿床以及与之相关的老君山花岗岩进行了系统的野外地质调查、成岩成矿年龄测试和岩石化学研究,探讨了成岩成矿时代、花岗岩与矿床的成因联系以及成岩成矿的构造环境.地球化学研究表明,老君山花岗岩普遍含有白云母,A/CNK＞ 1.1,K2O/Na2O＞1,碱质含量较高,镁、钙、钛等含量偏低,富集Th和U等,其主量和稀土元素含量以及稀土元素配分型式与矿区附近的片麻岩十分接近,属S型花岗岩.各类花岗岩的稀土元素和微量元素配分型式相似,暗示其为同源岩浆所形成.运用LA-ICP-MS锆石U-Pb法测得白云母花岗岩的年龄为(87.6±1.4) Ma～ (91.7±1.8) Ma,花岗斑岩为(87.3±2.1) Ma,二云母花岗岩为(84.3±2.2) Ma～(85.0±1.3) Ma;应用40Ar/39Ar法和Re-Os法分别测得矽卡岩中金云母1100～1400℃的40Ar/39Ar同位素加权平均年龄(WMPA)为(93.6±1.1) Ma,含锡钨石英脉云英岩化围岩中辉钼矿的模式年龄为(75.04±1.78) Ma～(79.16±2.43) Ma.根据花岗岩的产状和前人研究成果,将该区的岩浆侵入活动分为2期:早期(87.3±2.1) Ma～ (92.9±1.9) Ma,形成了白云母花岗岩及花岗斑岩；晚期(83.3±1.5) Ma～(85.0±1.3) Ma,形成了二云母花岗岩.与岩浆作用对应的成矿作用亦分为2期:早期(79.8±3.2) Ma～ (93.6±1.1)Ma,形成了与白云母花岗岩及花岗斑岩有关的矽卡岩型锡锌矿；晚期(75.04±1.78) Ma～(79.16±2.43) Ma,形成了与二云母花岗岩关系更密切的含锡钨石英脉.老君山花岗岩以及与之相关的锡锌钨矿是晚白垩世伸展构造环境下大规模花岗质岩浆-成矿作用的产物.