New
40Ar/39Ar dates of vein and
wall rock adularia from seven epithermal deposits and one porphyry copper
prospect help constrain the ages of mineralisation in the Hauraki Goldfield.
Vein adularia from the Paritu region yielded a single 40Ar/39Ar plateau date of
16.315 0.066 Ma. Five 40Ar/39Ar plateau dates of
three samples of vein adularia from the Opitonui deposit overlap within error to
yield a preferred age of 13.149 0.016 Ma. Two samples of vein adularia from
the Night Reef at the Broken Hills deposit yield exceptionally flat 40Ar/39Ar plateaus, and all
dates overlap within error to provide an age of 7.121 0.010 Ma. Two 40Ar/39Ar
plateau
dates of one sample of
vein adularia from the Wharekirauponga deposit overlap within error to provide
an age of 6.318 0.061 Ma. Four 40Ar/39Ar plateau dates of
three samples of vein adularia from the Maratoto deposit overlap within error to
provide an age of 6.411 0.022 Ma. Two samples of adularia from altered wall
rock at the Sovereign deposit yield three 40Ar/39Ar plateau dates that
range from 6.547 0.021 to 6.806 0.028 Ma. This suggests that mineralisation
formed around 6.6 Ma, but the results are complicated because the adularia
contains minor illite that post-dates adularia formation. Results from the
Karangahake deposit are complex. Two
samples of vein
adularia yield three 40Ar/39Ar plateau dates that
range from 6.116 0.116 to 6.901 0.101 Ma, whereas one sample of wall rock
adularia that contains minor late illite yields two 40Ar/39Ar plateau dates that
overlap within error to provide a date of 5.734 0.041. The range of dates
indicates that more work is necessary to understand the history of
mineralisation at this deposit. Two samples of vein adularia from the Tui
deposit yield four 40Ar/39Ar plateau dates that
overlap within error to provide an age of 5.714 0.017 Ma. These results
confirm previous work that indicates that the ages of epithermal deposits young
southward in the Hauraki Goldfield. These results are also consistent with
previously postulated
different ages for
deposits hosted in Kuaotunu Subgroup rocks in the northern goldfield (>10
Ma), and deposits hosted in Waiwawa Subgroup rocks in the southern goldfield
(7.1 to 5.7 Ma).