柴北缘赛坝沟金矿床成因探讨：来自流体包裹体及H-O-S-Pb同位素证据

何书跃; 张勇; 林贵; 舒树兰; 刘永乐; 张大明; 刘智刚; 赵芙蓉

doi:10.12401/j.nwg.2024008

柴北缘赛坝沟金矿床成因探讨：来自流体包裹体及H-O-S-Pb同位素证据

Genesis of the Saibagou gold deposit in the northern margin of Qaidam: evidence from fluid inclusions and hydrogen, oxygen, sulfur and lead isotopes

摘要

摘要:
目的赛坝沟金矿床位于柴北缘成矿带东端乌兰地区，是柴北缘代表性金矿床之一，近年来在其深部及外围找矿取得了新的突破，但该矿床的成因机制仍有争议。
方法本文针对含金石英脉开展了流体包裹体和H-O-S-Pb同位素的研究。
结果研究显示赛坝沟金矿流体包裹体主要是含CO₂三相水溶液与气液两相包裹体，包裹体均一温度主要介于200 ℃～320 ℃，盐度范围为0.18%NaCl_eqv～14.67%NaCl_eqv，流体密度介于0.67 g/cm³～1.13 g/cm³，具中温、中–低盐度、低密度的特征，属H₂O-NaCl-CO₂体系。H-O同位素分析显示δD_V-SMOW值介于−75.1‰～−65.2‰，δ¹⁸O_H2O值介于2.3‰～3.3，暗示岩浆水是成矿流体的主要来源，伴有少量变质水的加入。S同位素研究显示，黄铁矿δ³⁴S介于−2.52‰～2.61‰，在陨石S的组成范围之内，反映出矿床的硫主要来源于幔源流体，与岩浆活动有关。黄铁矿²⁰⁶Pb/²⁰⁴Pb为18.0314～18.2503，²⁰⁷Pb/²⁰⁴Pb为15.5907～15.7308，²⁰⁸Pb/²⁰⁴Pb为38.1726～38.4326，说明Pb主要来源于岩浆，可能有部分寒武-奥陶系地层Pb的加入。
结论根据矿床地质特征、流体包裹体及H-O-S-Pb同位素研究成果，结合区域构造演化背景认为，赛坝沟金矿床为柴达木地块与欧龙布鲁克地块陆陆碰撞形成的造山型金矿，与矿区脉岩活动密切相关。

Abstract:
Objective‌ The Saibagou gold deposit, located in the Wulan area, eastern part of the northern Qaidam metallogenic belt, is one of the representative gold deposits in the northern Qaidam metallogenic belt. Recently, new breakthroughs have been made in prospecting at the deep and peripheral areas, but the genetic mechanism of the deposit is still controversial.
Methods Fluid inclusions and H-O-S-Pb isotopes of gold-bearing quartz veins from the Saibagou deposit are examined in this study.
Results The results show that the fluid inclusions in the Saibagou Gold deposit are mainly characterized by gas-liquid two-phase inclusions and CO₂-bearing three-phase inclusions. The homogenization temperature of the inclusions ranges from 200 ℃ to 320 ℃, the salinity ranges from 0.18%NaCleqv to 14.67%NaCleqv, and the fluid density ranges from 0.67 g/cm³ to 1.13g/cm³. These lines of evidence support an ore-forming fluid characterized by medium temperature, medium-low salinity and low density, and belonging to the H₂O-NaCl-CO₂ system. The δD_V-SMOW values are between −75.1‰ and −65.2‰, and δ18O_H2O values are between 2.3‰ and 3.3‰, suggesting that the ore-forming fluid is mainly derived from magmatic water with a small amount of metamorphic fluid. S isotope studies show that δ³⁴S of pyrite is between −2.52‰ and 2.61‰, which is within the composition range of meteorite S, reflecting that the sulfur in the deposit mainly comes from deep mantle, which may be related to magmatic activity. The pyrite ²⁰⁶Pb/²⁰⁴Pb ranges from 18.0314to 18.2503, the ²⁰⁷Pb/²⁰⁴Pb ranges from 15.5907 to 15.7308, and the ²⁰⁸Pb/²⁰⁴Pb ranges from 38.1726 to 38.4326, indicating that Pb is mainly derived from magmas, and to a lesser extent, the Cambrian-Ordovician strata.
Conclusion Combined with the geological characteristics, fluid inclusions and H-O-S-Pb isotope research results, as well as the regional tectonic evolution background, it can be concluded that the Saibagou gold deposit is an orogenic gold deposit formed by continental collision between the Qaidam Block and Olongbuk Block and its genesis is closely related to the dike activities in the mining area.

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