Characteristics Of Fluid Inclusions And Genesis Of Dongan Antimony Deposit, Hunan Province

Abstract:The Dongan antimony deposit is located in the south of antimony ore belt in central of Hunan province, where is favor for nonferrous metals deposit forming. Structures are very developed in Xianjiangchong mine area and are the most important condition controlling mineralization. The orebodies occur as veins, with strongly silicified alteration which is the most significant prospecting mark. In this paper, based on geologic characteristics of the ore body, the characteristics of fluid inclusions in quartz of different types of ore and isotope and REE of monomineral studied and the genesis is discussed.According to geological characteristic of stibnite vein, two metallogenic stages have been divided. In the early stage of mineralization named pyrite-stibnite-quartz stage, wall rock is strongly silicified, with deeper mineralization position. In the late stage of mineralization named quartz-stibnite stage, wall rock is weak silicified, with more shallow mineralization position.Fluid inclusions of quartz crystals can be classified into two-phase gas-liquid inclusions (A type), pure liquid inclusions (B type) and pure CO2inclusions(C type). The homogenization temperature of ore-forming fluid, which has low density ranging from0.818to0.987g/cm3, ranges from130to280℃and were positively correlated with salinity of2.632to9.188wt.%equivalent of Nacl. The ore-forming fluid belongs to low-salinity and low-density fluid system. The average capture pressure of fluid inclusions is9.34MPa and the average depth of metallogenesis is0.93km from surface. It is,therefore, a low pressure epithermal deposit. The value of pH is5.53to5.89, indicating that the metallogenic environment is weak acid environment.The positive ions of fluid inclusions are mainly Ca2+and K+, the main negative ions are SO42-and Cl-, and the gas component of fluid inclusions is dominated by H2O and CO2, which means the ore-forming fluid belongs to weak alkaline solution rich in silicon, and also means the metallogenic environment is reducing environment. While the ore-forming fluid has high content of SO42-, the ratio of F-/Cl-, Na+(Ca2++Mg2+) and Na+/K+are small, showing a multiple sources of the ore-forming fluid, which may contain meteoric water, magmatic hydrothermal and metamorphic fluid.The homogeneous temperature distribution of fluid inclusions, combined with the location and occurrences of ore bodies, indicates that at least two transport centers of ore-forming fluid exist in this area. One transport center should be between8-12lines, the other should go through elevation of550, lines44.The research of H-O isotope of the fluid inclusion in quartz displays that the ore-forming fluid originated from the mixing of meteoric water through deep circulation, magmatic water and metamorphic water. And with the reduction in depth, the proportion of magmatic water were decreased, the proportion of meteoric water increased. The proportion of meteoric water in early ore-forming fluid is more then the late.The study of REE confirms the existence of two metallogenic stages. And combined with geological characteristic, it can be found that the total REE of ore-forming fluid is less and the fractionation between LREE and HREE is weak in the early stage of mineralization, while the total REE is more than early and LREE concentrate in the late stage. The study of REE also shows that the ore matter mainly derived from crust, mixed deeper source.It is inferred that the sulfur of deposits mainly derived from magma, and mixed the sulfur in the surrounding rock in the process of mineralization, with the characteristics of multi-source, in terms of S isotope characteristics of sulfide.