We report 40Ar/39Ar ages for adularia and illite from veins and wall rocks
at the Waihi and Favona deposits, in the Waihi district. One sample of vein
adularia from the Favona deposit yielded a plateau age of 6.04 0.06 (la) Ma,
and two samples of vein adularia from the Waihi deposit yielded plateau ages of
6.17 0.03 and 6.17 0.09 Ma. Two samples of wall rock adularia from the Waihi
deposit yielded plateau ages of 6.07 0.06 and 6.08 0.03 Ma. Adularia ages
overlap at the 2s level, and we do not consider them to be significantly
different. Two samples of illite from wall rocks at Favona have total gas ages
of 5.66 0.04 and 5.65 0.04 Ma, and retention ages of 6.57 0.04 and 6.61
0.04 Ma, whereas one sample of vein illite yielded a total gas age of 5.29
0.17 Ma, and a retention age of 6.26 0.19 Ma. The total gas ages of illite are
minimum ages that are equivalent to conventional K/Ar analyses, whereas
retention ages of illite are maximum ages. Although interpretation of the illite
data is complicated, the adularia results strongly suggest that the Waihi and
Favona deposits formed contemporaneously in the same fossil geothermal system at
6.0 to 6.1 Ma.