Mass-balance analysis of mineralized skarn systems: Implications for replacement processes, carbonate mobility, and permeability evolution

In infiltrative skarn systems, especially those associated with sulphide and oxide mineralization, there are pronounced depletions in CO

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and Ca coincident with increasing abundance of Si, Al, Fe, etc. This is regardless of the mass-balance technique used to examine skarns. The assumption of constant volume (Lingren’s law) is not entirely justified. Net mass changes, however, are generally small except in the most proximal endoskarn and exoskarn, although this depends a lot on the nature of the infiltrative magmatic system. The outer distal reaction margin exhibits the greatest change. This reflects the infiltrative decarbonation front enhanced by the dissolution of excess carbonates by carbonic acid generated by the skarn reactions, as well as the acidity (HCl) of the magmatically-dominated hydrothermal fluid. Using infiltrative solubility constraints, it is possible to estimate the minimum fluid-rock ratio (F/R) associated with earch part of the skarn. The distal skarn has the lowest F/R and the proximal exoskarn having the highest F/R. Net mass increases are related to hydrofracturing processes coupled with F/R reaction. Enrichment of sulphides occurs just within the outer (distal) reaction margin, indicating that acid neutralization is key to sulphide saturation. Because the reaction front continually changes, resolution-precipitation reactions must typify sulfide-bearing exoskarns.