Pressure Of Fluid Inclusions Research Articles

Genesis of the Saishitang skarn type copper deposit, West Qinling, Qinghai Province: Evidence from fluid inclusions and stable isotopes

The Saishitang skarn type copper deposit, located in the southeast part of the Dulan–Ela Mountain Triassic volcanic–magmatic arc and forearc accretionary wedge, belongs to the Tongyugou–Saishitang tin–copper polymetallic ore field in West Qinling, Qinghai province. Based on the contact/crosscutting relationships, mineral associations and mineralization characteristics, hydrothermal fluid evolution can be divided into three stages: skarn (I), quartz sulfide (II) and polymetallic sulfide-bearing quartz–calcite vein (III). The quartz sulfide stage (II) can be further divided into a massive sulfide stage (II-1) and a layered sulfide stage (II-2). This paper presents detailed analysis of fluid inclusions, H–O, S and Pb isotope compositions of rock samples from each of the above three stages as well as analysis of fluid inclusions from quartz diorite. The homogenization temperature, salinity, density and pressure of fluid inclusions in quartz diorite and typical transparent minerals showed a tendency of gradual decline in these evolutionary stages. The ore-forming fluid can be classified as a Na+–Ca2+–SO42−Cl− system with a minor proportion of a Na+–Ca2+–NO3−SO42− system, which likely resulted from mixing of magmatic and formation water. The H–O isotope composition indicates that the proportion of formation water increased during the ore-forming process, and meteoric water was mixed in the late quartz–calcite vein stage. The δ34S (CDT) values (−6.45–5.57‰) and Pb isotope compositions show that the ore-forming materials were mainly derived from magmatic fluid. Ore-forming fluid was boiling during the main ore-forming stage (II-1) due to pressure decrease. Consequently, the physical and chemical conditions (i.e., pH, Eh, fO2, fS2) changed, and metallic elements (including Cu) in the fluid could no longer exist in the form of complexes and precipitated from the fluid. According to the integrated analysis of ore features, mineral associations, alteration characteristics, ore-forming environment and fluid evolutionary process, it is concluded that the Saishitang deposit is a typical skarn deposit.

Determination of the Internal Pressure of Fluid Inclusions by Using Raman Spectroscopy

In situ Raman spectroscopic measurements of H2O-NaCl systems with three different salinities (0, 5.0, and 10.0 wt% NaCl) in the region of O-H stretching vibration were obtained at pressures up to 1800 MPa and temperatures from 298 to 453 K, with a hydrothermal diamond-anvil cell. The peak position was determined by fitting the obtained O-H stretching band with one Gaussian component. At a given temperature, the shift of the band decreased systematically with increasing pressure, and the data show a good linear relationship. For systems of different salinity, the slopes of the isotherms seem to be independent of temperature under the conditions investigated. With increasing salinity, the slope of the isotherm gradually increases. The relationships measured for the shift of the O-H stretching band with temperature, salinity, and pressure can be used to determine the internal pressure and isochore of fluid inclusions as well as the formation temperature and pressure of host minerals.