Abstract:

This paper presents new petrographic observations and microprobe analyses for the dioritic porphyrite from the Jiaochong gold deposit. Petrographic observations indicate the existence of unique quartz, plagioclase and hornblend phenocrysts in the rock. The quartz phenocryst is composed of resorbed core and growth rim, with melt inclusions along the border between core and rim. The plagioclase phenocrysts consist of resorbed core, resorbed growth mantle, and growth rim, with melt inclusions along resorption surfaces in the mantle. Hornblende poikilotopes are observed within core and mantle of the plagioclase phenocrysts. Hornblend phenocrysts are elongated in shape and contain acicular apatite poikilotopes. Microprobe analyses show that the An content of plagioclase from core to rim presents a reduced tendency and appears some An peaks at mantle, the aluminum content of hornblende from core of plagioclase to the matrix is decreasing. Using AmpPl geothermobarometry and wholeAl content of hornblende geobarometry the crystallization temperature and pressure of plagioclase phenocryst core and hornblende inclusions within it, the mantle of plagioclase and its hornblende inclusions, and coexisting HblPl micolites in the matrix are calculated respectively. The results show that the temperatures are 806.84808.75, 791.00797.86 and 660.3683.9  ℃, respectively. The pressures are 6.757.06 kb, 4.634.87 kb, and 2.062.12 kb respectively. The corresponding formation depths are 25.5226.70, 17.5018.40, and 7.798.02 km, respectively. Besides, the crystallization conditions of hornblende phenocryst are similar to those of embedded hornblende in the mantle of plagioclase phenocryst, the pressure of crystallization and corresponding depths are 4.484.83 kb and 16.9318.25 km, respectively. On the basis of the petrographic and geochemical analytical results, the present paper proposes the model of multiple magma chambers: the deep mantlederived transmagmatic fluids activate differentlevel magma chambers, as a result, forming intermediatebasic and granodioritic magmas,and the magma mixing between them has occurred. Finally, the mixing intermediate meltfluid flow driven by fluid pressure emplace rapidly near the surface, meanwhile, large amounts of volatile (orebearing fluid) are released. The exhalation process caused by decompression is probably the main mineralization mechanism.

Key words: resorption, multiple magma chamber, dioritic porphyrite, Jiaochong gold deposit, Tongling