黑龙江省四平山金矿床成矿作用及矿床成因——来自矿床地质、地球化学、锆石U-Pb年代学及H-O-S同位素的制约

Mineralization and Genesis of Sipingshan Gold Deposit,Heilongjiang,China:Evidence from Ore Deposit Geology,Geochemistry,Zircon U-Pb Ages,and H-O-S Isotopes

四平山金矿床位于我国黑龙江省东部的完达山陆缘增生带内,矿体主要赋存于下白垩统四平山组,表现出明显的两阶段成矿作用特征。为进一步确定四平山金矿床的成矿作用及矿床成因,本次研究对其成矿地质特征、地球化学、成矿年代学及成矿物质来源进行了分析。地球化学分析结果表明:成矿花岗斑岩为具有高SiO_(2)(70.07%~72.93%)、(K_(2)O+Na_(2)O)(7.17%~8.42%)质量分数和右倾“海鸥型”稀土配分型式特征的A型花岗岩;含矿层状硅质岩中,Al/(Fe+Al+Mn)<0.32、Fe/Ti>20、(Fe+Mn)/Ti>20,并具有较高Fe_(2)O_(3)(0.18%~1.79%)、FeO(0.17%~0.96%)、MnO(0.03%~0.12%)、TiO_(2)(0.03%~0.04%)、Al_(2)O_(3)(0.30%~2.79%)和(K_(2)O+Na_(2)O)(0.07%~1.76%)质量分数,指示四平山金矿床形成于活动大陆边缘构造背景下与岩浆活动有关的热水沉积环境。四平山金矿床成矿花岗斑岩锆石U-Pb年龄为(113.5±0.7)Ma。H、O、S同位素分析结果(δD=-182.3‰~-84.0‰、δ^(18)O_(V-SMOW)=6.5‰~14.3‰、δ^(34)S=-12.4‰~29.7‰)显示,成矿流体主要为大气降水,成矿物质具有以地层为主的多来源特征。综上,四平山金矿床为形成于燕山晚期活动大陆边缘构造背景下的热水沉积型矿床,太平洋板块持续俯冲造成了区域上大规模的拆沉作用,导致区域构造背景由挤压向伸展发生转换,此时形成的花岗斑岩岩浆运移至地壳浅部岩浆房加热上覆地层,在地壳浅部形成了温度与压力梯度带,促使地表的大气降水沿裂隙进入地层后形成流体循环系统,并不断萃取地层中的Au元素形成含矿流体。在成矿早阶段,当压力驱动含矿流体喷出地表后与地表水相遇快速冷却,成矿物质快速沉淀,形成含矿层状硅质岩和硅质胶结砾岩,并通过交代下伏流纹斑岩形成层状、似层状矿体;在成矿晚阶段,随着岩浆房提供的热动力减弱,含矿流体滞留在裂隙通道中或穿切进入早期固结的含矿层裂隙中,沉淀形成脉状-网脉状矿体。

Sipingshan gold deposit is located in the Wandashan epicontinental accretional terrane in eastern Heilongjiang,China.The ore bodies are mainly hosted in the Lower Cretaceous Sipingshan Formation,showing obvious two-stage mineralization.To further determine the mineralization and genesis of the deposit,this study analyzes its metallogenic geological characteristics,metallogenic chronology and source of metallogenic materials.The samples from the ore-related granite porphyry have relatively high SiO_(2)(70.07%-72.93%)and(K_(2)O+Na_(2)O)(7.17%-8.42%),which belong to A-type granite.The ore-bearing siliceous rocks are characterized by Al/(Fe+Al+Mn)<0.32,Fe/Ti>20,(Fe+Mn)/Ti>20,Fe_(2)O_(3)(0.18%-1.79%),FeO(0.17%-0.96%),MnO(0.03%-0.12%),TiO_(2)(0.03%-0.04%),Al_(2)O_(3)(0.30%-2.79%),(K_(2)O+Na_(2)O)(0.07%-1.76%),indicating that Sipingshan gold deposit is formed in a hydrothermal depositional environment related to magmatic activity in the tectonic setting of active continental margins.The granite porphyry is identified as the parent rock of Sipingshan gold deposit,with a U-Pb zircon age of(113.5±0.7)Ma.The H(δD=-182.3‰--84.0‰),O(δ^(18)O_(V-SMOW)=6.5‰-14.3‰),and S(δ^(34)S=-12.4‰-29.7‰)isotopic results indicate that the ore-forming fluid is mainly atmospheric precipitation,and the ore-forming materials have stratigraphically dominated multi-source features.To sum up,Sipingshan gold deposit is a hydrothermal sedimentary deposit formed in the Late Yanshanian active continental margin tectonic background.The continuous subduction of the Pacific plate caused large-scale delamination,resulting in a transition from extrusion to extension.This granite porphyry magma formed at this period migrated to the shallow crustal magma chamber to heat the overlying stratum,and formed a temperature and pressure gradient zone in the shallow crust,which prompted the atmospheric precipitation on the surface to enter the stratum along the fissures to form a fluid circulation system,and continuously extract Au elements in the formation to form ore-bearing fluids.In the early stage,when the pressure drived the ore-bearing fluids to be ejected from the surface,it encountered the surface water and cooled rapidly,and the ore-forming materials precipitated rapidly,forming the layered ore-bearing siliceous rocks and siliceous cemented conglomerates,which formed layered orebodies by metasomatism of the underlying rhyolite porphyry.This process was weakened with the thermal power provided by the magma chamber dying down.In the later stage,the ore-bearing fluids stayed in the fissure channels or penetrated into the fissures of the early consolidated ore-bearing layer,then precipitated to form vein-like and reticulated-vein-like ore bodies.