Geology of New Zealand Field Trip Guidebook - ResearchGate

West Coast Gold- Significant quantities of gold were produced in the area from Ross north
to Hokitika and Greymouth in the 19th century. Virtually all of this gold recovered from the
region occurred as placer deposits. Gold typically occurs as a nearly pure metal (a “native”
element) in hydrothermal veins. Because gold is both very dense (specific gravity 19.3 vs.
11.4 for Pb) and chemically inert, it persists through the weathering cycle and can be
concentrated as a lag deposit in sands and gravels. Most of the gold in the Ross to
Greymouth region was eroded by the vigorous west coast glaciers and deposited in glacial
outwash, or outwash deposits reworked by stream or beach processes [Beanland, 1987 #3].
These alluvial deposits are called placers. Between Greymouth and Hokitika there are a
series of marine terraces that were prime gold prospecting areas. The terraces consist of a
seaward facing cliff with a marine beach deposit at its base. Many of these marine terrace
deposits are “blacksand leads”—lag deposits of heavy minerals washed out of the material
that originally made the adjacent cliff (Suggate 1978). You are familiar with blacksand leads
from Stradbroke Island. Blacksand leads on the west coast of the South Island indicated to
prospectors that heavy material, including gold, had been concentrated. Prospectors used a
variety of techniques to find and concentrate placer gold, including one you’re probably
familiar with – panning. Mechanized dredges are used for commercial placer mining today;
however, there are currently no active gold mines in this area. The terraces would be worth
noting even if they did not contain gold. Between Hokitika and Greymouth there are
successions of marine cliffs/terraces extending more than 11 km inland with elevations up to
300 m above modern sea level (Suggate 1978). These raised terraces result from a
combination of tectonic uplift and glacial/interglacial and eustatic sea level fluctuations.
Time, weather and equipment permitting, we’ll try our hands at a little gold panning.
West Coast Jade—The term jade applies to two different hydrothermal minerals (really to
two different monomineralic rocks). The most precious form of jade, sometimes called
“Imperial Jade,” is composed of the sodium aluminum silicate mineral called Jadeite, a
single-chain silicate. The other more common form of jade is Nephrite jade, which is
composed of a calcium, iron and magnesium double chain silicate, typically ferroactinolite.
Jadeite is an example of a pyroxene, nephrite an amphibole. Both forms produce dense,
green, translucent masses that are valued both for their beauty and, in the past, their utility.
The latter property comes from the structure of the material. Both the pyroxene and the
amphibole minerals occur in elongate crystals. In jades, these crystals are a few microns
across and many microns long. They are intergrown in complex, interlocking patterns to
produce a very tough and durable material. Most materials that we think of as tough are very
hard, but also brittle. Jade is different. Jade is soft enough to be relatively easily worked, but
is nearly unbreakable. Because of this, Maori jade was a prized material both for carvings
and for superior stone tools. In fact, the Maori had a major collection and trade network for
pounamu (jade) up and down the West Coast, across Arthur’s Pass and across the Canterbury
Plain to ports that took the material to settlements on the North Island. In New Zealand,
nephrite is found as boulders in streams draining the Southern Alps. The location of major
pounamu boulders were closely guarded secrets among Maori traders. Many of these
nephrite boulders were discovered by Kiwis of European descent during the gold rush days,
but in recent years supplies have dwindled and prices have risen. Hokitika remains the center
of New Zealand’s jade carving industry.
Stop 1: Hokitika
We will stop for lunch and some free time in Hokitika. It was a booming port during the gold
rush years, but now is a little tired and struggling to maintain an economy from the few
26