Mass transfer during gold precipitation within a vertically extensive vein network (Sigma deposit Abitibi greenstone belt Canada). Part II. Mass transfer calculations

摘要

A companion work (Garofalo, this issue) shows that along the Archean, Au-mineralized quartz-tourmaline veins of the Sigma deposit of Canada, fluid-rock interaction generated two types of hydrothermal alteration during gold deposition, one tourmaline-rich and another albite-rich. This work combines petrographic and mass transfer studies to monitor the chemical exchange occurring between the Au-bearing hydrothermal fluid within the veins and a porphyritic diorite wall rock in which two alteration envelopes of the different types developed. Tourmaline-rich alteration consists of replacement of the host rock by tournialine-calcite-quai-tz-pyrite-pyrrhotite at the vein walls. In these haloes, components like SiO2, B, Al2O3, Na2O, CO2, CaO, S, and Au were transferred from the vein fluid to the host rock, while other components like K2O and FeO were transferred from the rock to the fluid. Albite-rich alteration consists of progressive replacement of biotite, chlorite, epidote, and quartz of the host rock by the assemblage albite-pyrite-pyrrhotite, and the trends of gains and losses are opposite of those calculated for the tourmaline-rich haloes. Only B, S. and Au are transfer-red from the fluid to the rock in both alteration types. Progressive mineralogical and chemical changes of the wall rock during, alteration indicate that diffusion metasomatism was important for generating the two alteration types.These results underscore some key characteristics of the hydrothermal alteration at Sigma. First, the distinct alteration types formed most probably from chemically distinct hydrothermal fluids, in agreement with independent fluid inclusion data (Garofalo et al., 2002b). These two fluids reacted with the wall rocks, producing the contrasting mineralogical and chemical modifications recorded within the albite-rich and tourmaline-rich haloes. The trends of mass transfers are peculiar because the components given by the albite-rich haloes to the vein fluid are those given by the vein fluid to the tourmaline-rich haloes. Hence, albite-rich alteration was functional to the (generation of tourmaline-rich haloes in the wall rocks and to tourmaline precipitation with the veins. Second, the combined transfers into and out of the veins caused modifications in the concentration of major components in the fluid, like SiO2 and Al2O3. The concentration ratios of other major fluid components like Na and K remained unchanged because of buffering from the vein mineral assemblage. while that of some minor and trace components like REE, Rb, Sr, and high field strength elements varied significantly. A comparison between the mass transfer data presented here and that of other vein-hosted Au deposits with alteration characteristics similar to Sigma shows that, with the exception of Au, S, and CO2, inconsistent mass transfer trends are typical in these deposits and include the so-called "immobile" components (i.e. Al2O3, TiO2, and Zr), This shows that mass transfer data cannot be used for formulating genetic models of vein hosted Au deposits at the global scale, but mainly for constraining models for single deposits, where the background geological and geochemical information are in general well defined.