Abstract

To increase our knowledge of the factors that control the mineralization potential of silicic magma systems, the magmatic-hydrothermal evolution of the ~125 Ma Huangshan granite in eastern China was reconstructed. This granite shows geochemical similarities to Climax-type porphyry Mo mineralizing magmas but is evidently barren. The chemical evolution of the Huangshan pluton and the fluids exsolving thereof were reconstructed based on laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analysis of melt inclusions and fluid inclusions hosted in quartz phenocrysts, matrix quartz, pegmatitic quartz, and euhedral quartz crystals in miarolitic cavities. The melt inclusions have rhyolitic compositions (72.6–77.7 wt % SiO2) and show a wide range of Cs contents, from 10 to up to 2,000 ppm, implying melt entrapment at crystallinities up to >99%. The compositions of the least evolved melt inclusions (<30 ppm Cs) overlap with those of whole rocks, whereas highly evolved melts (>100 ppm Cs) followed an unusual evolution trend that appears to be related to high F contents (1.0–2.5 wt % F). The Huangshan magma was saturated in a large number of accessory minerals including zircon, apatite, fluorite, monazite, columbite, thorite, and molybdenite. The Mo content of the bulk magma is estimated at 3 to 6 ppm.

The fluid evolution of the Huangshan granite was dominated by intermediate-density fluids with salinities of 4.5 to 5.0 wt % NaCl equiv. No evidence for fluid immiscibility was found, which is indicative of relatively high confining pressures. This conclusion is supported by reconstructed near-solidus crystallization conditions of ~700°C and ~1.8 kbar based on coexisting fluid and melt inclusions in miarolitic cavities. Due to the enrichment of fluorine during fractional crystallization, some very late stage, residual silicate melts remained liquid down to 600°C. The intermediate-density, magmatic fluids contained 60 to 95 ppm Mo. Therefore, both the fluids and the melts of the Huangshan pluton contained similar amounts of Mo as those present in porphyry Mo mineralized systems. The barren nature of the Huangshan granite is thus not a consequence of low Mo abundances. In addition, the reconstructed melt viscosity (log η = 3.8–5.2 Pa s), the magma volume (230 km3), and the emplacement depth (~7 km) are within the ranges displayed by mineralized systems. The only parameter that seems to have been different at Huangshan is a lack of focused fluid flow, as indicated by the common presence of miarolitic cavities. Most likely, the lack of focused fluid flow resulted from its flat roof geometry.

You do not currently have access to this article.