The Central Maratoto Valley in the Coromandel Peninsula contains a sequence of veins in
pervasively altered dacite + andesites of the Waipupu Formation. We recognise at
least eight different vein types:
(1) pyrite stringers,
(2) quartz + carbonate veins,
(3) crystalline quartz veins,
(4) carbonate veins,
(5) chlorite + carbonate veins,
(6) siderite veinlets,
(7) siderite + carbonate veins and
(8) carbonate + pyrite veins.
The overall temporal trend of the veins is from early
pyrite to quartz and then to late carbonate veins. Carbonate veins and pyrite
stringers both form planar and irregular veins, whereas some carbonate veins
have diffuse contacts with the wall rock. Gold and base metal sulphides are
associated with some of the quartz + carbonate veins. Textures within the veins
reflect open space filling and multi-phase reopening. A partial record of
hydrothermal activity has been preserved by vein crosscutting relationships,
hydrothermal alteration minerals and fluid inclusions.
Hydrothermal alteration minerals display zonation from
south to north. The alteration increases from unaltered to weak propylitic
alteration of andesite in the south (MVRCDC1), to sericitic alteration of
andesite and dacite in the north (MVRCDC3). The alteration assemblage is
dominantly sericitic, with no adularia present, drawing parallels to the
Thames region where there is no
adularia.