PACROFI VI - Electronic Program


Study Of Compositions Of Single-Inclusion Analysed By Laser Raman Microprobe And Evolution Of Ore-Forming Fluid In Zijinshan Copper-Gold Deposit, China

WEI JIAXIU and HU XIAODIE

(Institute of Mineral Deposits, C.A.G.S., Beijing, CHINA, 100037)


The Zijinsham copper-gold deposit is situated in the southwest margin of the Shanghang Early Cretaceous volcanic sedimentary basin of southwest Fujian. The deposit is hosted by the Yanshanian intrusion which consists of coarse-grained porphyriticoid granite, medium-grained biotite granite and muscovite granite. The country rocks have suffered sericitization, dickitization, alunitization, silification and pyritization. The alteration is characterized by horizontal and vertical zonation. The gold mineralization occurs in the alunite-dickite-pyrite-quartz zone, the copper mineralization occurs in the sericite-dickite-quartz and alunite-dickite-quartz zones. Volcanic activity was intense in the area. The Zijinshan copper-gold deposit occurs near a caldera. The gold mineralization is closely related temporally and spatially to the volcanic-intrusive activity. The dacite, crypto explosion breccia and dacite-porphyrite are major host rocks.

Most of the ore bodies are evidently controlled by the NW fractures, parallel to the crypto explosion breccia rocks which also occur localized in a NW direction. Gold-bearing minerals are mainly ferrohydrite, quartz and pyrite. Copper-bearing minerals are enargite, tennantite, digenite, chalcosite, chalcopyrite, covellite and pyrite. Two stages of mineralization can be recognized: a volcanic-subvolcanic hydrothermal stage and a supergene-oxidation stage.

The salinities and densities of the ore-forming fluid and metallogenic pressure have been obtained through measurements of the temperature, pressure and salinites of the fluid inclusions in minerals. The temperatures of silification are determined to be 420-100oC. The ore-forming temperatures range in 300-100oC; salinities from 28.3 through 7.6 eq. wt. % NaCl and 3.3 to 0.1 eq. wt. % NaCl, densities range from 0.52-0.79 g/cc to 0.97-1.10 g/cc; pH values are approximately 5.47-4.13-3.64.

The Laser Raman microprobe is a developing nondestructive analytical method applicable to micro-areas. The Laser Raman microprobe has been used effectively to identify polyatomic gases, complex ions and daughter crystals in fluid inclusions (E. Roedder, 1979; Dhamelincourt, 1979). At present, it is also used in the identification of the compositions of gas-liquid-solid phases of inclusions in many ore-deposits. This quantitative spectral analytical technique identifies compositions of certain points in fluid inclusion through projecting a monochromatic Laser beam and focusing the beam on those points.

Nowadays, fluid inclusions in quartz, alunite, dickite (a little) etc. of the copper-gold ore-deposit of Zijinshan in Fujian have been studied by geochemistry, microthemometry and Raman spectroscopy. Laser Raman microprobe has been used to scan nearly twenty type compositions, such as CO2, CO, CH4, H2S, Cl-, HCO3, HS-, and SO4-2.

Fluid inclusions of various types in Zijinshan copper-gold deposit contain a lot of S. The single-inclusions have K+, Na+, Cl-, HS-, H2O, H2S and SO4-2. The ore-forming fluid at the early stage is in the KCl-NaCl-H2O-H2S system being enriched in K+, Na+, Cl- and H2S, and along with the decreasing ore-forming temperature, evolves to the CO2-NaCl-H2O-SO4-2 system at the late stage with increasing H2O, CO2 and SO4-2. As a result, acidity (pH values from 5.47 to 3.64), and oxygen fugacity increase, the forming temperature decrease from 300 to 100oC, resulting in the formation of copper-gold deposit. The compositions and features of ore-forming fluids reveal the formation and evolution of ore-forming processes.

As mentioned above, the Laser Raman nondestructive analyse of a micro-area is conducive to study the compositions of fluid inclusions of different generations, stages and origins in ore-deposits. This analytical method does not destroy fluid inclusions and may eliminate the compositional pollution of colony inclusions. As a result, it is able to obtain highly precise data and believable important physicochemistry and geochemistry information about ore-forming processes.

The physico-chemical conditions and geochemical environments determined by the fluid inclusions indicate that the Zijinshan copper-gold deposit is an epithermal deposit related to late Yanshanian volcanic-subvolcanic activities. Compared with other similar deposits, the Zijinshan copper-gold deposit belongs to quartz-alunite-dickite formation in China.