The Metamorphic Gold Model – The New Zealand Story

The Metamorphic Gold Model – The New Zealand Story

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This old mine was abandoned over a century
ago. At that time researchers already knew that
gold and quartz had precipitated from fluids. But the question was. How had this fluid
formed in the first place? Some geologists believed that granite was
involved the magmatic theory. But we now know that the veins and the quartz are 50 to 80 million years older than the
granite. So it can’t be the granites. These days most researchers have come around
to the orogenic or metamorphic model. In this theory, the gold-bearing fluids were
formed from hot rocks at great depths. When oceanic sedimentary rocks were deeply
buried and metamorphosed to about 600 degrees Celsius, in conditions known as amphibolite facies,
some minerals released water. The immense amount of fluid given off – fuelled
a natural gold factory. Tiny concentrations of gold that naturally
occurred in the rocks, were dissolved and carried upwards in a gold-laden
fluid. When it reached the greenschist facies, where
temperatures were between 300 and 400 degrees the fluid precipitated quartz and gold, in
faults and fractures. If this theory is correct we should find there
is a large area of high-grade metamorphic rock beneath the goldfield, that is depleted in
gold. So the irony is, that to prove the theory,
we’re now looking for an absence of gold. And the expert in this is Dr Iain Pitcairn. Iain travels around the world collecting samples
of both high and low grade metamorphic rocks looking for levels of depletion or concentration
of gold. And in this he has found evidence to support
the metamorphic model. I met up with Iain in Victoria, at the old
Wattle Gully Gold Mine. Well the first thing I would talk about is
that in some orogenic gold terranes you don’t actually see granites, however we
do see the gold deposits, and that’s been one of the first things particularly
in Otago in New Zealand which has led people to suggest “well if you don’t see any granites then how
can the gold deposits be formed by these fluids”. But the metamorphic model has come into vogue. So why do you think there has been a shift
in ideas. What’s the evidence? There’s certainly a set of characteristics, which are observable in a number of different
orogenic gold terrains, which point towards a metamorphic fluid, being
the fluid, which forms the, these gold deposits. And firstly the composition of a fluid that’s
released during dehydration reactions in metamorphism that’s quite well known. We quite, understand that rather well from
experimental work. Now by dehydration you mean, liberation of
water. Yeau exactly, the liberation of water during
the metamorphic process. And we see that the composition of fluids
that we observe in veins such as this, they fit very very well with what we know
about the composition of a metamorphic fluid and how it’s, and what it should be composed
of. Basically water, a little bit of CO2 and some
dissolved sulphur, H2S species. So the experimental work and the observations
of what we see in the veins they fit together very well. The fluids that formed veins were derived
from deep level metamorphism but vein deposition actually occurred at a
much higher level, where temperatures were lower. Almost every orogenic gold belt be it whatever
age from the Archaean, 2.7 billion years old or something that’s formed in the last few
million years. All of the gold deposits occur, or almost
all of them occur, at what we would call greenschist facies. This is a grade of metamorphic conditions,
probably around 300 to 400 degrees Celsius. That would have been the temperature the rocks
were heated to. That’s the grade of conditions where we see
all these deposits. Greenschist rocks are easily exposed by erosion
– so we know they have elevated gold. But a problem for Iain is that the assumed
source of gold, the amphibolite facies, often remains deeply
buried. Fortunately he found a place where a massive
portion of the lower crust had conveniently made itself accessible in
a most spectacular fashion. The rocks are very well exposed in New Zealand. On the west coast you have a, there’s a large
fault line called the Alpine Fault, and we’ve had uplift on the Alpine Fault which
has caused the southern Alps, these big mountain range on the west coast
of New Zealand. They’re beautiful, it’s a fantastic area to
do field work, really great, but what it meant geologically was that the
deep level metamorphic rocks from around six hundred degrees temperature, representing very deep levels in the crust. They’ve been uplifted by this uplift on the
fault and so these deep level rocks are exposed
so we can actually sample what would have been at 25 kilometres depth
in the crust. So on the flanks of this mountain range you
see the lower grade rocks? The further away from the fault you actually
get the lower metamorphic grades are the rocks, and eventually when you get over towards the
east coast of New Zealand you’re in rocks that haven’t been metamorphosed
at all, they’re just shales and greywackes. So it’s perfect, we see full exposure of this
complete crustal section from surface levels to around 25 kilometres depth. From rocks that were unmetamorphosed to ones
that were metamorphosed to 600 degrees Celsius, so we can look at the rock at each metamorphic
grade and compare the chemistry to try and see if there’s been any systematic
changes which might have caused formation of these
deposits. So by collecting rocks from each metamorphic
grade, Iain hoped to find some systematic changes
in chemistry that would give a clue to the source area
of the gold. I collected about 950 kg of rock – had it
shipped back to Southampton where I was doing my PhD and analysed the
rock for 60 different elements, major elements, trace elements but specifically
focussing on gold and silver and other elements which are assoc.., which
are often enriched in these gold deposits in these gold deposits such as arsenic, antimony
and mercury. And what I was looking for was rocks that
might be depleted in gold where the gold had been removed from the rock
by a fluid. So that which would represent the source area. Iain’s target area, the high-grade metamorphic
rocks, were derived from oceanic sediments, which
naturally contain low levels of gold. an average piece of rock would have for example
one parts per billion, maybe two parts per billion gold in it. And so if we were looking for depletions on
an element like gold which is already low concentration I needed a method that was maybe ten parts
per trillion.. That’s remarkable. ..detection limits, so very low level. Iain used his ultra sensitive method to analyse
rocks from the whole range of metamorphic grades. looking for those rocks that were depleted
in gold. And we found that the rocks from high metamorphic
grades so something around five hundred to six hundred
degrees was the temperature that they’d been metamorphosed
at. They were systematically depleted in gold
and silver and arsenic and antimony. The same suite of elements that we see enriched
in the deposits. So these rocks, the high metamorphic grade
rocks, represented the, what we think represented the source area
for the deposits in New Zealand. And that’s what the metamorphic model suggests
is that you have this leaching of this large area
of source rock. The metamorphic model can equally explain
the origin of Victoria’s gold deposits, whether large or small. Natures own gold factory leached the precious
metal from high-grade metamorphic rocks at depth. The gold bearing fluids rose to higher levels
to form the deposits we see today. This is the essence of the metamorphic model,
and the basis of the Orogenic deposit type.

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