Wall-Rock Alteration in the Goldfield District, Nevada

Hydrothermal alteration of early Tertiary dacite, andesite, and latite has produced the following miner-alogical zones which flank a complex system of gold-bearing veins: (1) propylitic zone farthest from the vein; (2) argillic zone, divided into a montmorillonite subzone in the outermost part, and an illite-kaolinite sub-zone in the innermost part; (3) alunite-quartz zone, immediately adjacent to the vein. The boundary between the alunite-quartz zone and the relatively thin illite-kaolinite subzone is sharp. The latter subzone grades into the relatively wide montmorillonite subzone. The degree of crystallinity of kaolinite increases and the polymorphic form of the 9.9 Å mica varies from lMd (illite) to 2M (K-mica) as the alunite-quartz zone is approached. The propylitic zone is characterized by chlorite (14 Å) and calcite. The gradational transition to the argillic zone is marked by the occurrence of a 1:1 mixed-layer chlorite-vermiculite. Alunite is believed to have formed by reaction of sulfuric acid and K-mica and the chemical reaction is believed to be {aastex} {amsbsy} {amsfonts} {amssymb} {bm} {mathrsfs} {pifont} {stmaryrd} {textcomp} {portland,xspace} {amsmath,amsxtra} {wasysym} {empty} {10}{9}{7}{6} {document}KAl₃Si₁₀(OH)₂₍ₛ₎ + 4H⁺(aq) + 2SO⁼4(aq) = KAl₃(SO₄)₂(OH)₆₍ₛ₎ + 3SiO₂₍ₛ₎,{document} where (s) and (aq) indicate the solid and aqueous state, respectively. The theoretical standard free energy of this reaction is approximately -42.1, -54, and -72 kcal/mole alunite at 25° C, 200° C, and 300° C, respectively. The uncertainty of the free energies is 3 kcal. at 25° C. and of the order of 10 kcal. at the higher temperatures. Consideration of the chemical kinetics of the decomposition of the illite is very important in the reaction.