CO2-rich inclusions in vein gold–copper mineralization of the Sarekoubu–Qiaxia District, southern Altaides, China: Implication for ore genesis

Abstract

The Sarekoubu–Qiaxia area of the southern Altaides hosts a number of vein gold and copper deposits, in which a great number of carbonic fluid inclusions were found. For many years, the ore genesis of these deposits and the origin of the carbonic inclusions are still unclear. This study attempts to explain where the carbonic inclusions came from and their contribution to gold mineralization. The Sarekoubu gold deposit (SR) and the Qiaxia copper–gold deposit (QX) occur in the lower Devonian Kangbutiebao Formation (D1k22). Two types of quartz veins in these deposits can be distinguished: (1) lentoid or banded quartz veins (QI) parallel to the foliation of metamorphic rocks; and (2) chalcopyrite-bearing quartz veins (QII) cutting across the foliation of metamorphic rocks and banded sulfides.Three main types of fluid inclusions can be found in the quartz veins: CO2–H2O fluid inclusions (Type CW), carbonic inclusions (Type C), and salt-aqueous inclusions (Type W). The CO2-rich fluid inclusions (type CW and type C) occur both in the QI and QII veins. The fluid inclusion assemblage (FIA) method was applied in petrography study and thermometry data analysis. The results show that CO2-rich fluid inclusions have been trapped with the minimum temperatures ranging from 243 to 395°C for the early vein quartz stage (QI), and 230 to 328°C for late polysulfide quartz veins (QII) of the Sarekoubu deposit. For the Qiaxia deposit, the minimum trapping temperatures are from 201 to 382°C for QI, and from 207 to 365°C for QII respectively. The estimated minimum trapping pressures are from 110 to 330MPa which are in agreement with the P–T conditions of the regional metamorphism.The CO2-rich fluids in the vein gold mineralization at the Sarekoubu–Qiaxia area might be produced after peak regional metamorphism during the early Carboniferous to the Permian due to decomposition of marble and calcareous siltstone in the Lower Devonian Kangbutiebao Formation. High density carbonic inclusions in SR are much more abundant than in QX, indicating that regional tectonic stress in SR might be higher than in QX. The hydrothermal alterations are also much more intensive in SR than in QX, which may illustrate loss of water in CO2–H2O fluids in SR during the process of wall rock alterations and leaving CO2-rich fluids behind. The CO2-rich fluids may have played an important role in gold mineralization and modifying the VMS deposits.