安徽沙溪斑岩铜(金)矿床成岩成矿热历史探讨

Heat Evolution from Intrusion to Mineralization in Shaxi Porphyry Copper (Gold) Deposits, Anhui Province

采用Rb-Sr方法,测定沙溪斑岩铜(金)矿成矿岩体年龄为143.37±5.17 Ma;采用^(40)Ar/^(39)Ar快中子活化定年法,测定沙溪斑岩铜(金)矿成矿年龄为123.6±0.7 Ma,成矿岩体从固结成岩到成矿经历了20 Ma。根据Rb-Sr和K-Ar同位素体系封闭温度的不同,估算出沙溪岩浆热液成矿系统热衰减速率平均为20℃/Ma。单一的由成矿小岩体所提供的热能难以维持如此长时间热液成矿活动,矿区晚期的岩浆活动是沙溪斑岩铜矿成矿的重要能量来源,这是斑岩铜矿成矿岩体多属于多阶段侵入的复式岩体的原因。斑岩铜矿成矿系统能量耗散分析显示,斑岩铜矿成矿过程中有效能量耗散效率极低,如果没有巨大的能量来源维持长时间的热液活动,斑岩铜矿难以形成。

The Rb-Sr age of the mineralizing intrusive in the Shaxi porphyry copper (gold) deposits is 143. 37 ±5. 17 Ma. The age of copper mineralization determined by the 40Ar/39Ar fast-neutron activation technique is 123. 6±0. 7 Ma. The process from emplacement and solidification of the mineralizing intrusive to mineralization lasted about 20 Ma. The cooling rate 20℃/Ma in the Shaxi ore-forming system was estimated on the basis of the closure temperature difference between the Rb-Sr and K-Ar isotopic systems. The numerical modeling of heat evolution in the mineralizing intrusive indicates that the heat energy provided solely by the mineralizing intrusive could not support so long hydrothermal ore-forming activities and that late-stage magmatism should be the main energy source for mineralization of the Shaxi ore deposits. This is why porphyry copper deposits usually occur in complex polystage intrusives. Energy dissipation analyses in the ore-forming system show that the heat efficiency of the ore-forming processes was very low and that porphyry copper deposits could not form without a huge heat source to maintain long-continued hydrothermal activities.