Chlorine and sulfur evolution in magmatic rocks: A record from amphibole and apatite in the Tonglvshan Cu-Fe (Au) skarn deposit in Hubei Province, south China

Abstract

In order to trace chlorine and sulfur evolution during magma evolution, detailed petrographic observation, major- and trace-element analysis of amphibole and apatite were carried out on the ore-related pluton in the Tonglvshan Cu-Fe (Au) skarn deposit, Hubei Province, South China. Several generations of apatite and amphibole are identified, from early to late including: (1) apatite in euhedral amphibole; (2) euhedral amphibole; (3) apatite in subhedral-anhedral amphibole/plagioclase; (4) subhedral-anhedral amphibole + plagioclase; (5) apatite in K-feldspar or quartz; and (6) K-feldspar + quartz. Most apatites show homogeneous textures, but a few apatite grains enclosed in amphibole and plagioclase (1 and 3) display oscillatory zoning. These zoned apatites crystallized in semi-closed environments created as growth of amphibole or plagioclase removed components of the melt. The magmatic Cl and S evolution can be divided into three stages: (I) In the deep magma chamber, at ∼ 5 km below the surface, magma underwent ∼ 39% fractional crystallization, and its Cl and S contents increased simultaneously; (II) In a shallow magma chamber at ∼ 2.5 km depth, fractional crystallization elevates Cl and S contents in the residual melt, but fluid exsolution depletes them; (III) At near-solidus stage, when fluid exsolution becomes weak, crystallization elevates Cl and S contents in the residual melt simultaneously. This study suggests that the silicate mineral (fractional) crystallization can elevate Cl content in residual melt, and fluid exsolution can efficiently remove Cl from the magma. Bulk sulfur in magma may be transported not only as dissolved sulfur, but also as sulfates like anhydrite. The sulfur content of apatite is controlled by the sulfur composition in melt from which the apatite crystallizes, but Cl in apatite may represent re-equilibration between the apatite and the last melt in contact with the apatite, because of rapid diffusion of Cl in apatite. These characteristics along with complex apatite generations imply that in-situ study is necessary when using apatite to trace volatile composition in magma.