Geology
The Northparkes deposits occur within the Ordovician Goonumbla
Volcanics, part of a volcanic belt in the Central Lachlan Orogen of
NSW. The ore deposits are typical copper-gold porphyry systems; the
highest grades associated with the most intense stockwork veining.
Sulphide species in the systems are zoned from bornite-dominant
cores, through a chalcopyrite-dominant zone to minor distal
pyrite.
The porphyry copper deposits at Northparkes are typically narrow
but extend to great depths. The E26 and E48 deposits range from 200
to 400m in diameter (>0.5% copper) and extend vertically for
more than 1,000m.
Exploration History
Geopeko (now Rio Tinto) recognised the potential for
Volcanogenic Massive Sulphide (VMS) hosted Pb-Zn deposits in the
Goonumbla Volcanic Complex and commenced exploration in 1971.
Regional mapping and rock geochemical sampling were undertaken to
elucidate the regional geology and to define favourable
environments for VMS mineralisation. Early encouragement was
provided in 1973 when outcropping skarn mineralisation was
identified at several prospects. By 1975, regional geological
mapping of outcropping areas was complete and had identified
several more prospects. However, large prospective areas under
cover remained unexplored.
Regional traverses of auger-core drill holes to collect samples of
bedrock under cover for geological and geochemical assessment was
undertaken. This technique was applied by drilling 1 km-spaced
holes across geological strike of the prospective volcanic belt,
commonly using public road reserves for access. In 1976 auger-core
hole 697-21 was drilled in a traverse along Adavale Lane on the
edge of what is now known as the E22 porphyry copper deposit.
'Patches and disseminations of bornite and chalcocite were reported
in an 'odd fragmental rock' that assayed 0.30% Cu. Follow-up
drilling ensued and intersected 203m grading 0.66% Cu and 0.77 g/t
Au from 65m depth and was the discovery of the first porphyry
copper deposit at Northparkes. Follow-up of other anomalous bedrock
samples from 1km-spaced auger-core holes led to the discovery of
the E27 deposit in 1977 and 1978.
In 1978 quartz-malachite veined monzonite was mapped 2 km
south-east of E22 and follow-up drilling ensued. Extension of the
bedrock geochemical sampling grid to the south in 1980 identified a
major bedrock copper anomaly. The first hole to test the anomaly,
DDH26, returned a mineralised interval of 391m grading 0.48% Cu -
this prospect became the E26 deposit! The 'regional' auger-core
drilling programme continued, although the hole spacing was reduced
to 400m (the size of the anomaly over E22), and in 1981 led to the
discovery of the E37 and E37W porphyry systems some 5km west of
E22.
In 1992 the discovery of the last orebody from the Geopeko era,
E48, resulted from testing a magnetic anomaly described as 'an E26
look-a-like' mid-way between E22 and E26. This discovery proved
positive as the E48 mine was commissioned in late-2010 and will
allow Northparkes Mines to continue production until 2024.
Current Exploration
Rio Tinto has continued to invest in the highly-prospective
Northparkes district and currently holds ~1,000 km2 of
Exploration Leases around the Northparkes Mines. The Exploration
Team onsite is focused toward increasing existing mineral resources
at E22, E26 and E48 as well as locating and evaluating additional
prospects within the district.
One example of a recent discovery was the stock-hosted GRP314
orebody located 500m northeast of E48. This orebody is unlike the
other traditional Cu-Au porphyry deposits historically mined at
Northparkes. GRP314 is a low-grade-high-tonnage deposit with the
mineralisation hosted in monzonite stock. This new mineralisation
style opens up a great deal of potential across the district as
historic exploration was highly focused on "pencil" porphyry style
ore deposits.
The Exploration Team utilises the latest technologies including
trace geochemistry studies, regional geophysical surveys, multiple
drilling techniques and surface outcrop mapping to test the 200+
targets that have been identified over Northparkes exploration
leases. Exploration is dedicated to provide Northparkes Mines the
ability to extend the mine life past the current E48 life-of-mine
and develop additional prospects into operating mines of the
future.
Download Exploration
booklet (PDF 1.57MB)