泰国北部花岗岩黑云母成分特征及其对锡成矿作用的制约

The geochemical characteristics of biotites of granites in the northern Thailand and their constraints on the granite-related tin mineralization

本文通过电子探针分析对采自泰国北部成矿(沙蒙矿床)和贫矿(Tak岩体)花岗岩体中的黑云母开展了对比研究,以期厘清花岗岩制约锡成矿的控制因素。结果显示,成矿和贫矿花岗岩中的黑云母均属于岩浆成因,前者偏铁质,具有S型花岗岩黑云母成分特征;后者偏镁质,与I型花岗岩黑云母成分一致。利用机器学习拟合的黑云母温压计,估算成矿花岗岩早期岩浆结晶温度和压力分别为812~858℃(平均835℃)和702~882 MPa(平均781 MPa),而贫矿岩体对应的温度和压力分别为795~833℃(平均811℃)和188~373 MPa(平均226 MPa)。两类花岗岩具有明显不同的侵位深度、岩浆氧逸度和初始挥发分。研究表明,在伸展构造体制下,深部地壳的富F变沉积岩高温熔融形成的还原性岩浆,经长距离运移至较浅的深度侵位更有利于锡成矿。

A comparative study between compositions of biotites from the ore-forming(Samoeng deposit)and those of biotites from the tin-barren(Tak pluton)granites in the northern Thailand,has been carried out in this work to explore controlling factors of the granite-related tin mineralization.Our results show that biotites in the tin-bearing and tin-barren granites are all primary magmatic minerals.The former ones are Fe-rich biotites with compositional characteristics of biotites in the S-type granite,while the later ones are Mg-rich ones with consistent compositions of biotites in the I-type granite.By using the machine learning simulated biotite thermobarometry,the estimated early crystallization temperatures and pressures of the magma of ore-forming granite are 812~858℃(average 835℃)and 702~882 MPa(average 781 MPa),respectively,whereas those of the tin-barren granite are 795~-833℃(average 811℃)and 188~373 MPa(average 226 MPa),respectively.It is obtained that the tin-bearing and tin-barren granitic magmas had distinctly different emplacement depths,oxygen fugacities and initial volatile components.Our study supports that the granite-related Sn mineralization occurred more favorably in the granite of relative shollowly emplaced magma which was transported through long distance from the reduced magma derived from the high temperature partial melting of the F-rich metasedimentary protoliths in the deep crust under an extensional tectonic setting.