The
REGCHEM (Regional Exploration Geochemistry) stream sediment geochemistry
database of open file mining company data, managed by GNS Science, was recently
made accessible via the internet (http://maps.gns.cri.nz/website/minmap/).
REGCHEM provides coverage of selected elements in mineral rich areas of
New
Zealand. Analysis of REGCHEM data highlights
problems with inconsistencies between different surveys in terms of sampling
methods, number and types of elements analysed, analytical techniques and
detection limits. New, consistent multi-element geochemical surveys with
national coverage are required. To test sampling, analytical and data treatment
techniques for the new surveys, a pilot stream sediment geochemical survey was
carried out in the Thames area, Hauraki
Goldfield. Seventy samples were collected in an area of 220 km2 representing
about one sample every 3 km2. The samples were analysed for 50 elements, mostly
by XRF.

Regional patterns are
apparent in the concentrations of some elements. Several elements, such as As,
Cd, Cu, K, Pb, Sb, Se, and Zn exhibit anomalous concentrations mainly in the
western part of the study area, related to Au-Ag-Zn-Pb-Cu epithermal quartz
veins and porphyry Cu occurrences in hydrothermally altered andesitic
terrane.

Mercury is anomalous in
southern samples, reflecting cinnabar occurrences in the Kauaeranga valley (eg
Mangakirikiri), and the occurrence of cinnabar as a late stage mineral in some
of the Thames epithermal veins. Ba and U are
anomalous in the eastern samples, and these anomalies are probably related to
the presence of rhyolitic rocks in the eastern part of the project
area.