东天山沙泉子铁铜矿床矿物学特征及其成矿意义

Mineralogy of the Shaquanzi Fe-Cu deposit, eastern Tianshan and its metallogenic implications

东天山沙泉子铁铜矿床赋存于沙泉子组火山岩地层之中,是阿齐山-雅满苏铁铜成矿带中重要的矿床之一。矿床的蚀变矿化期次可依次划分为早夕卡岩阶段、晚夕卡岩阶段、磁铁矿化阶段、黄铜矿化阶段和晚期绿帘石-碳酸盐化阶段,而磁铁矿化阶段又可细分为赤铁矿亚阶段、钾长石-绿帘石-磁铁矿亚阶段以及磁铁矿-黄铁矿亚阶段。电子探针分析表明,夕卡岩阶段主要发育钙铁榴石(And_(39~100)Gro_(0~50)Spe+Alm_(0.26~11.23))和阳起石,暗示早期流体具有高氧逸度和碱性的特征,并导致流体中的铁质不断富集。磁铁矿化阶段主要发育低钛磁铁矿(Ti O_2≤0.17%)、钾长石(Or_(97.09~97.43)Ab_(2.35~2.60)An_(0.22~0.31))、富铁绿帘石(TFe O=14.13%~16.32%)、黄铁矿、石英以及赤铁矿。随着钾长石和富铁绿帘石的开始形成,成矿流体中铁质大量沉淀形成富铁矿石;而赤铁矿和低钛磁铁矿的沉淀使得流体氧逸度逐渐降低,进而使黄铁矿发生沉淀。黄铜矿化阶段主要发育赤铁矿、富铁绿帘石(TFe O=11.95%~16.29%)、密绿泥石、黄铜矿、方解石等矿物,密绿泥石形成温度为147~190℃,平均为168℃,说明其为低温中性流体;该阶段赤铁矿和绿帘石先沉淀,随着氧逸度的降低,黄铜矿、密绿泥石等矿物形成。黄铁矿和方解石的硫、碳、氧同位素组成(δ^(34)S_(流体)=–1.7‰^+4.7‰以及+15.6‰和+17.5‰;δ^(13)C_(流体)=–6.6‰~–3.4‰;δ^(18)O_(流体)=–2.0‰^+0.7‰)表明磁铁矿化阶段流体主要为岩浆流体并伴随有海水的加入,而黄铜矿阶段为低温非岩浆热液流体,主要为盆地卤水或海水,可能伴随有大气降水的加入。结合沙泉子铁铜矿矿床地质、围岩蚀变特征以及矿床对比研究,我们认为沙泉子铁铜矿床的形成与矿区晚石炭纪闪长岩的侵入有关,不同于火山岩赋存的雅满苏铁矿,且与典型的夕卡岩型铁铜矿床不同,而具有许多与安第斯铁氧化物铜金(IOCG)矿床类似的特征。

The Shaquanzi Fe-Cu deposit is hosted in the volcanic rocks of the Shaquanzi Formation as one of the important deposits in the Aqishan-Yamansu Fe （-Cu） metallogenic belt. The alteration and mineralization sequence can be divided into the early skarn alteration stage, the late skarn alteration stage, the magnetite mineralization stage, the chalcopyrite mineralization stage and the late epidote-carbonatation stage. The magnetite mineralization stage can be subdivided into the hematite sub-stage, the K-feldspar-epidote-magnetite sub-stage and the magnetite-pyrite sub-stage. Electron microprobe analyses indicate that in the skarn stages are mainly developed andradite （And39-100Gro0-50Spe ＋ Alm0.26-11.23） and actinolite, with minor grossularite, indicating that the early fluid is characterized by high f（O2） and pH, resulting in the enrichment of Fe. The magnetite mineralization stage is characterized by low Ti magnetite （TiO2 ≤ 0.17%）, K-feldspar （Or97.09-97.43Ab2.35-2.60Arl0.22-0.31）, Fe-rich epidote （TFeO = 14.13%-16.32%）, pyrite, quartz and hematite. Accompanied by precipitating of K-feldspar and Fe-rich epidote, abundant high-grade iron ores were formed. After hematite and low-Ti magnetite were precipitated, the f（O2） of the ore-forming fluid tended to decrease, leading to the formation of pyrite. Dominated in the chalcopyrite mineralization stage are hematite, Fe-rich epidote （TFeO = 11.95%-16.29%）, pycnochlorite, chalcopyrite, calcite, etc. The chlorite geothermometer temperatures range from 147 ℃ to 190 ℃ with an average of 168 ℃, suggesting that the fluid developed in the chalcopyrite mineralization stage is characterized by low temperature （168 ℃） and medium pH. With the formation of hematite and epidote, thef（O2） of the ore-forming fluid tended to decrease, giving rise to the precipitation of chalcopyrite and pycnochlorite. The S, C, O isotope data of pyrite and calcite （δ^34Sfluid=-1.7‰-＋4.7‰, and ＋15.6‰, ＋17.5‰; δ^13Cfluid=-6.6‰-3.4‰; δ^18Ofluid=-2‰-＋0.7%0） indicate that the fluid developed in the magnetite mineralization stage is dominated by magmatic-hydrothermal water mixing with minor seawater, whereas that developed in the chalcopyrite mineralization stage is dominated by basin brine （seawater） with the mixture of some meteotic water. Ore geology of the Shaquanzi Fe-Cu deposit, together with wall rock alteration features and deposit comparison study, suggests that the Shaquanzi Fe-Cu deposit is genetically related to the diorite intrusion, which differs from the volcanic-hosted Yamansu Fe deposit and classic skarn type Fe （Cu） deposit, but shares similarities with the Central Andes IOCG deposits.