甘肃合作早子沟金矿微量元素、同位素特征与成矿物质来源

Characteristics of trace elements and isotopes of Zaozigou gold deposit in Hezuo County,Gansu Province and their indication for the source of ore-forming materials

甘肃合作早子沟金矿是西秦岭夏河—合作金矿富集区最著名的特大型金矿床,经过近20年持续勘查,已探明资源量超过150 t。笔者等通过对2018~2021年实施的深部科研钻探SDZK8314岩芯进行系统编录和测试分析,为研究深部流体和成矿物质来源提供新的资料。研究表明,早子沟金矿中不同类型的岩石与矿石的稀土微量元素具有相似配分曲线特征,含矿岩石相对围岩具有明显更低ΣREE含量和更高的K、Sr含量,同时矿石中的金品位与K_(2)O含量呈现明显的正相关关系。早子沟金矿中的石英闪长玢岩n(^(208)Pb)/n(^(204)Pb)=38.33~38.48、黄铁矿—毒砂n(^(208)Pb)/n(^(204)Pb)=38.23~39.19、辉锑矿n(^(208)Pb)/n(^(204)Pb)=37.75~38.63,具有多源、混合铅的特征,从闪长玢岩黄铁矿—毒砂辉锑矿具有从上地壳到造山带甚至地幔的演化趋势。H—O同位素显示,成矿流体主要来源于岩浆水,后期有大气降水和围岩循环水的混入。矿石中δ^(34)S_(V-CDT)值的范围为-14.7‰~-7.0‰,平均值-10.3‰,硫主要来源于岩浆热液,与围岩发生强烈的水岩反应而带入一部分地层硫。早子沟金矿的形成与区域较晚期岩浆—构造活动直接相关,成矿物质主要来源于深部石英闪长岩。

The Zaozigou gold deposit,the most famous super large gold deposit in the Xiahe-Hezuo gold district witch in the West Qinling gold province,Gansu Province,has undergone continuous exploration and mining during the late two decades,with proved resources of more than 150 t.Although several investigations have conducted on this deposit,the ore-forming material sources remain disputed.Thus,we performed systematic trace elemental and H-O-Pb-S isotopic studies on fresh samples from drill hole SDZK8314,with main aims to reveal the sources of deep fluids and ore-forming materials in the Zaozigou gold deposit.Trace element data of various ores show similar partition curves,characterized by that the ore-bearing rocks have significantly lowerΣREE and higher K and Sr contents than surrounding rocks.In addition,the gold grade in various ores displays an obviously positive correlation with their contents of K_(2)O.Lead isotopes show n(^(208)Pb)/n(^(204)Pb)ratios varying from 38.23~39.19 for quartz diorite porphyrite,38.23~39.19 for pyrite and arsenopyrite,and 37.75~38.63 for stibnite,suggesting multi-sources and mixed lead characteristics,with corresponding variation trend from upper crust to orogenic belt(even mantle).H-O isotopes show that the ore-forming fluids of the Zaozigou gold deposit derive mainly from magmatic water,as well as the mixture of atmospheric precipitation during the later metallogenic stage.Sulfur isotopes showδ^(34)S range from 14.7 to-7.0,with an average value of-10.3.Through comparison with previous studies,we argue that the sulfur in ores is from magmatic and hydrothermal fluids,with partial low value sulfur from contaminated surrounding rocks.Accordingly,we propose that the formation of the Zaozigou gold deposit is closely correlated with the late magmatic-tectonic activity,and the ore-forming materials are originated from deep quartz diorites.