Using fluid inclusions to trace formative fluid evolution at the Verde and Pancho porphyry Au deposits of the Refugio District, Chile
Creators: name: Aillaud, Brian Joshua
Publisher: name: University of Nevada, Las Vegas
Year: 2010

description: Evolution of magmatically-derived formative fluids, and the processes that control the ratio of ore metals in porphyry and high-sulfidation epithermal deposits remain enigmatic. The Refugio district in the Maricunga Belt of Northern Chile hosts the temporally and spatially related Pancho and Verde porphyry gold deposits. Vein types at Pancho include A-veins, transitional veins, banded veins, D-veins and quartz-alunite ledges that formed by replacement. Verde East and West lack A-veins, and show an earliest vein type with transitional characteristics of A- to banded veins, banded veins and D-veins. Fluid inclusions and quartz textures were characterized by using optical microscopy and cathodoluminesence (CL). Microthermometry, and analyses using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) were used to constrain fluid chemistries at discrete stages in time and space, documenting the chemical evolution of the Pancho and Verde deposits. Fluid inclusions were grouped by vein type and inclusion characteristics. A-vein liquid+vapor+daughter mineral(s) (LVD) inclusions had vapor bubbles that comprised 10-40% with a mode of 20% of the inclusion volume by visual estimation, a halite daughter, and possible small (<5%) opaques. Eight A-vein LVD inclusions were analyzed microthermometrically, and 12 by LA-ICP-MS. This inclusion type had salinities ranging from 34-38 wt.% NaCl eq. and homogenization temperatures of 430-510ºC. A-vein, banded vein and post-flashing banded vein liquid+vapor (LV) inclusions typically had a vapor bubble comprising 15-30% of the inclusion volume. Microthermometrically, 43 inclusions were analyzed, and 12 inclusions were analyzed by using LA-ICP-MS. LV inclusions in A-veins, banded veins and post-flashing banded veins have salinities of 3-6, 8-13 and 3-6 wt.% NaCl eq., respectively, and homogenization temperatures of 290±3ºC, 177-362ºC and 285ºC (for all inclusions except one with a Th of 278ºC), respectively. Cathodoluminesence and petrographic observations revealed no textural differences between quartz that hosted LV and LVD inclusions in A-veins. Single phase vapor inclusions (V) analyzed in banded veins have an average Cu/Au ratio of 7E3. A-vein LVD and LV inclusions have average Cu/Au ratios of 4E4 and 6E3, respectively. This is low when compared to the Cu/Au ratio of 1.5E5 for the bulk ore of porphyry Cu deposits. Fluid inclusion data suggest that the low Cu/Au ratio in fluid inclusions is the result of a low Cu/Au ratio in the formative fluids, although the data are not inconsistent with Cu being retained in the vapor phase. This interpretation implies that the fluid chemistry was controlled by processes in the magma plumbing system prior to degassing, while vein evolution was controlled by the pressure and temperature conditions of the country rock. Fractionation of a Cu-Fe-S phase(s) during magmatic differentiation may have preferentially sequestered Cu from the melt, resulting in a low Cu/Au ratio of the melt prior to volatile saturation.