Timing of mineralization and source of fluids in a slate-belt auriferous vein system, Hill End goldfield, NSW, Australia: Evidence from [formula omitted] dating and O- and H-isotopes

Abstract

40Ar 39Ar dating of metamorphic biotite and alteration muscovite from the auriferous veins and host rocks at the Hill End goldfield, N.S.W., Australia, has distinguished four major geological events, including the timing of gold mineralization. The earliest hydrothermal event occurred during the Middle Devonian Tabberabberan Orogeny (370–380 Ma) and resulted in the formation of quartz veins barren of Au. A second and major episode of vein emplacement occurred in the Early Carboniferous during the principal phase of metamorphism and deformation at 359–363 Ma. This was followed by Au accumulation in two stages: (1) after the major phases of quartz deposition, and (2) during and after the development of conspicuous internal vein laminations (~ 357 Ma and ~ 343 Ma, respectively). Two sources of fluid are proposed for vein and ore formation. The first is a local metamorphic fluid characterized by δ 18O H 2O values of 8.9 to 12.5 per mil and δD H 2O values of −87 to −90 per mil. The second is a mixed ore fluid with δ 18O and δD values in the range of δ 18O H 2O 8.4 to 11 per mil and δD H 2O of −49 to −36 per mil. Progressive entry of this second fluid, sourced from trough-fill or deeper crustal rocks, is linked closely to cycles of gold precipitation at Hill End.