The
REGCHEM (Regional Exploration Geochemistry) database of open file mining company
data, managed by GNS Science, has recently been upgraded with the compilation of
additional data, especially for the Otago region (see
http://maps.gns.cri.nz/website/minmap). In June 2007, data compiled for the
Otago region included more than 2000 stream sediment samples from 19 surveys,
more than 800 BLEG (bulk leach extractible gold) samples from 12 surveys and a
few pan concentrate samples. The stream sediment samples were typically analysed
for Au and As, and in many cases for other elements including Cu, Pb, Zn, Sb, W
and Mo. The BLEG samples were analysed for Au and
in some cases also for As. The main exploration target was mesothermal gold
deposits in the Otago Schist, although one survey of the Otago Peninsula targeted potential gold deposits associated
with the Dunedin
volcanic complex. Geochemically, the mesothermal gold deposits are characterised
by anomalous As and Au, with some also characterised by anomalous W and Sb, and
these four elements are the best pathfinders in stream sediment geochemical
surveys. The introduction of lower detection limits in Au analyses in the more
recent stream sediment surveys and in the BLEG surveys enables the use of this
element as the primary pathfinder for mesothermal gold
deposits.