The magmatic-hydrothermal evolution of two barren granites: a melt and fluid inclusion study of the Rito del Medio and Cañada Pinabete plutons in northern New Mexico (USA)

Abstract

The magmatic-hydrothermal evolution of two barren granites associated with the Tertiary Questa Caldera in northern New Mexico was reconstructed on the basis of microthermometric and laser-ablation inductively coupled plasma mass spectrometry analysis of fluid and melt inclusions in quartz of magmatic and hydrothermal origin. During progressive crystallization and fluid exsolution, the Cs content of the residual melt increased from 1 ppm to values as high as 5500 ppm, which requires an increase in crystallinity by at least 99.98%. In conjunction with a Rayleigh fractionation model simulating the melt evolution at fluid-saturated vs. fluid-undersaturated conditions, the melt inclusion data can be used to determine the crystallinity at which fluid saturation was reached. Our results suggest that both plutons attained fluid saturation before 30% crystallization and that evolved residual melts accumulated in their roof zones. At ∼90% crystallization, the exsolving fluids were of low salinity (∼5 wt.% NaCl equiv) and in the one-phase field, in accordance with phase relations at the reconstructed P and T conditions (∼1.1 to 1.3 kbar, 700 to 720°C). Fluid-melt partition coefficients for a range of metals determined on assemblages of coeval melt and fluid inclusions were generally too low to allow efficient metal extraction from the melt ( D X, fluid-melt < 22). As a result, the metal concentrations in both the residual melt and the coexisting fluid increased with progressive crystallization. Absolute metal contents in the fluids exsolving from barren systems appear low, however, when compared with mineralized systems. It is concluded that the absence of mineralization in the Rito del Medio and Cañada Pinabete plutons primarily stems from a low salinity of the exsolving fluids, resulting in a less efficient metal extraction from the melt.