The
Plumduff sinter occurs at Whenuaroa, near Puhipuhi, approximately 30 km north of
Whangarei. It is one of three residual sinters in the southern portion of a mid-
to late-Pliocene geothermal system. The deposit consists of many large sinter
breccia boulders, including rudrocks (coarse breccias), scattered around the
flanks and summit of the small conical hill of Plumduff. Three types of sinter
breccia occur: (i) rudrocks consisting of large, angular to sub-angular clasts
of layered sinter up to 0.8 m across, as well as smaller clasts of sinter (20 -
100 mm dia), and fragments of stromatolitic structures; all clasts are supported
in a matrix of microcrystalline, transparent to translucent to milky quartz;
(ii) fragments of layered angular to irregularly mottled sinter of white to grey
or bluish hue supported in a translucent to opaque silica matrix; (iii) smaller,
matrix-supported breccias with clasts both partially surrounded and/or
penetrated, by a clay-like weathering rind, and sinter conglomerates with
rounded to oblate clasts. Three principal types of fabrics constitute the bulk
of the sinter: (i) stromatolites, including columnar, palisade and filamentous
networks, showing progressive silicification, in fenestral, tufted
to
clotted fabrics; (ii)
peloids and pisoids; (iii) plant-rich facies in which various silicified plant
species occur in a silica matrix. All fabrics have counterparts in hot spring
deposits in the Taupo Volcanic Zone formed at temperatures from 75C to 35C.
Also present is steam-condensate-produced alteration: kaolinite and alunite as
well as boulders of quartz lattice texture. Cinnabar only occurs in sinter on
the north and south central slopes of the hillside. No quartz veins or geyserite
were recognised.