Fluid inclusion study on the Tsumo skarn-type deposit, Southwestern Japan.

Abstract

The Maruyama orebody of the Tsumo mine belongs to the skarn type copper-zinc-tungsten deposit. Skarns are mainly classified into the following three types judging from their modes of occurrence and the mineral assemblages; banded skarn, massive skarn (vesuvianite-garnet-clinopyroxene skarn, wollastonite-garnetclinopyroxene skarn, and garnet-clinopyroxene skarn) and vein skarn (hedenbergite skarn). A copper-zinc mineralization took place in wollastonite-garnet-clinopyroxene skarn and garnet-clinopyroxene skarn, while a tungsten (scheelite) one is recognized not only in hedenbergite vein skarn, but also in the part of wollastonite-garnetclinopyroxene skarn.
Most of fluid inclusions in garnet, clinopyroxene and wollastonite are homogenized into liquid phase at about 300°-350°C. When the temperature correction is made considering salinity of the inclusion fluid and the formationlpressure on the basis of the sphalerite geobarometer, it is concluded that the formation temperature of the skarn minerals was about 400°-350°C.
Skarn minerals contain fluid inclusions, some of which are evidently primary in origin. Many of fluid inclusions contains solid phases, such as halite, sylvite, carbonate and metallic minerals. The variable volume ratio of the solid phases to the whole vacuole may suggest that the skarn minerals of the deposit were formed from saline solution of various concentrations. On the contrary, fluid inclusions in scheelite do not contain any solid phase. This implies that the mineral precipitated from a solution whose salinity was lower than 26 wt.% NaCl. Small grains of chalcopyrite are included in a sphalerite crystal. The fact that they array along a paticular plane may suggest that some of them is not the exsolution product, but minerals trapped during the growth of host mineral.
In general, fluid inclusions in skarn minerals have no liquid CO₂ phase at room temperatures. This turns out to be true for those in the Tsumo deposits. This fact agrees well with the conclusion derived from the solubility data of CO₂ for NaC1 solution and the calculated data for the stability field of the skarn minerals. However, the presence of gaseous inclusion, though the abundance is few, may suggest that two phase locally appeared during the skarn mineralization.