Abstract:

The Dehelongwa copper (gold) deposit, located in the northeast part of QinghaiTibet Plateau, is a magmatic hydrothermal deposit. Three types of inclusions hosted in quartz are found in the orebodies, i.e., liquidrich twophased aqueous inclusions (type Ⅰ), vaporrich aqueous inclusions (type Ⅱ), and aqueous inclusions with daughter mineral(s) (type Ⅲ). Inclusions of type Ⅰ homogenize to liquid at temperatures of 187413 ℃ and have salinities from 2.0% to 19.0% (NaClequiv.). Inclusions of type Ⅱ homogenize to vapor at temperatures from 350 ℃ to 413 ℃ and the salinities are 5.3% to 9.8% (NaClequiv.). Inclusions of type Ⅲ can be further classified into two subtypes (Ⅲa,Ⅲb) depending on dissolution of daughter minerals before or after the disappearance of the gas bubbles during the heating processes. Inclusions of subtype Ⅲa have liquidvapor homogenization temperatures of 276 ℃ to 520 ℃，with daughter minerals melting temperatures from 192 ℃ to 491 ℃ and corresponding salinities from 31.5% to 57.1% NaClequiv.. Inclusions of subtype Ⅲb have homogenization temperatures (daughter minerals melting temperatures) from 250 ℃ to 500 ℃, with corresponding salinities from 33.0% to 59.8% (NaClequiv.), and liquidvapor homogenization temperatures of 195325 ℃. Based on the fluid inclusion data, three types of oreforming fluids are deduced, they are strongly overpressured and high salinity magmatic hydrothermal fluid A, mediumhigh salinity boiling hydrothermal fluid B, and lowmedium salinity fluid C. These fluids were all originated from different evolutional stages of the primitive fluids derived from the magmatic intrusion. Based on the highest boiling ceasing temperature of fluid B (350 ℃) and calculated pressure (160 bar), the mineralization depth is interpreted to be 1.6 km. It is suggested that oreforming elements were transported by high salinity magmatic fluids, with Cu mainly as chlorocomplex such as CuCl2(aq) and CuCl- and Au mainly as sulfurarsenic complex. The oreforming components were precipitated due to boiling and fluid mixing as well as the decrease in salinity, temperature, and pressure of the fluids.

Key words: fluid inclusion, oreforming fluid, coppergold deposit, Dehelongwa