At the top of the Australian Alps is a tiny
working gold mine.
The Red Robin's gold has kept Ken Harris going
for 34 years.
Hello Ken
G'day Clive. Just ah mind your head on those
stulls when you're coming up
there's not a lot of space here as you can
see.
I've come here to see the geological structure
that contains all the gold-bearing veins
it's called the Red Robin Fault.
You can see here that's quartz right away
across there.
And that's sort of one, that's the footwall
and there's the hangingwall and you just take the quartz out.
And then every now and again it will narrow
in.
I've come because I want to explore why faults
and veins so often go together
so I'll talk to Ken and one of the world's
top academics in the field.
Well originally the reef outcropped on the
surface here.
But it was quartz reef and basically I've
just sunk down on it more or less the whole distance.
But to make sure that's it got gold in it
you have to take samples to check for gold
but basically it comes down to following the
quartz.
This quartz lies along a steep fault, which
extends from the surface,
about 100 metres above us to at least 20 metres
below.
And like almost every gold mine, if there
was no fault here
there'd be no quartz, and no gold.
Ken struggles and squeezes to find the gold-bearing
quartz.
His work slowly reveals the path that gold-rich
fluids once followed.
But the fault is not as simple as it seems
- the veins pinch and swell
and their complex shapes show that fluids
invaded the fault repeatedly.
But even complex systems can make sense when
you understand the different elements at play.
One scientist who understands these elements
more than most is Professor Rick Sibson.
He realised a key to understanding the relationship
between faults and quartz veins - is earthquakes.
There's one thing we know now is that, you know, faults
mostly move during earthquakes.
It's actually quite rare around the world
for a fault to move steadily
and not in jerks during earthquakes.
Rick came to study earthquakes by chance.
In the 1970s he left New Zealand to study
for a PhD.
So I ended up in Scotland working in the Outer
Hebrides
where there was an ancient fault zone that
had been neglected
because it penetrated some rather uniform
rocks, the Lewisian Gneiss
and you couldn't make an ordinary sort of
geologic map.
But what I realised you could do was map bands
of different types of fault rock,
the products of fault movement, and a pattern
came out of that.
It was common back then to picture faults
as slow creeping structures,
but Rick found evidence that some faults had
moved very rapidly.
And that made me realise that the rocks held
clues as to what does go on during an earthquake.
But as a geologist I became aware that quite
a lot of faults that we routinely map
have some sort of hydrothermal vein material
along the faults
and sometimes it's calcite or quartz or gypsum.
But one of the characteristics is that there
is textural evidence
that this material was not all deposited in
one episode
but it came in a series of pulses of fluid
and the natural thing was to say
that well, perhaps this incremental deposition is
something to do with
incremental displacement on the faults,
it's tied in to earthquake slip episodes.
Rick realised, that to fully understand the
vein-earthquake relationship,
he had to factor in something else,
that earthquakes behave differently, at different
depths.
For much of the earth, earthquake activity
is in the top half of the crust,
it only goes down to about 15 kilometres,
maybe 20 kilometres and then it stops,
and so as a fault zone goes from the top to
the bottom of the crust
it changes from being brittle or frictional
to being more ductile and flowing with depth.
But one of the fascinations about Victorian
gold and the other orogenic gold deposits
is that they have formed probably mostly in
the range of sort of 7 to 15 kilometres,
something that like.
But at these depths the pressure of overlying
rock is enormous.
And horizontal compression, or shortening,
is often an additional force in active orogenic terrains.
So how can vein fluids enter fractures and
faults under these extreme conditions?
Here the rocks are being squashed and so its
counter-intuitive isn't it that,
It's totally counter-intuitive
that you would get veins.
Yes, in fact you see huge vein systems localised
on pretty steep reverse faults
in these zones of compression and it raises
a very interesting point. You say,
why on earth would you get hydrothermal deposition,
which involves the creation of space
on a fault almost perpendicular to the shortening
direction.
So what's the key to understanding that?
To create space at depth you have to counteract
all the overburden pressure
with abnormal fluid pressure.
The huge weight of overlying rock keeps fractures
tightly sealed
but if gold-bearing fluids become highly pressured
they can literally pump open fractures and
faults.
And the fluids will seek the fractures that
require the least pressure to open.
One of the attributes that's notable about
these orogenic gold deposits
is that you see quite a lot flat veins associated
with them,
some of which have beautiful fibre structures,
they've opened vertically
and in simple mechanical terms the only way
we can really understand this
is if we have a fluid pressure driving crack
opening
that is counter-acting the overburden pressure
and that requires very high fluid pressures.
The overburden pressure is a force to be reckoned
with.
The vein fluids had to counteract it, and
it's now the same for Ken.
He needs to keep the fault from collapsing
using closely-spaced timbers.
There's not enough room for rock bolts here,
hardly enough room to use, use a rock drill so
I just put in these timbers.
You call them stulls, so you have to cut a
hole in the footwall,
so a toe for the stull, measure it accurately
and just slam it in
with a seven kilo sledge hammer and um put
in the wooden wedges as extra support.
But it's got to be tight otherwise the whole
thing can fall in.
You're always checking it, so you'll see.
This is reasonable the way I've got it at the moment
but it's pretty loose overhead so it's sort
of proceed with caution.
I'm due to put in another stull just there
actually.
Ken has only 100 metres of rock above him,
but when the veins formed there may have been
kilometres of overburden.
This weight is called lithostatic pressure.
This is an immense force but gold-bearing
fluids released during metamorphism,
can counteract it - and form veins.
Nearer the surface it's more likely that hydrostatic
pressure prevails.
This is basically just the weight of the
water itself,
and is much weaker than lithostatic pressure
at depth.
Rick realised this was a very significant
difference and from there,
developed his fault-valve model.
So the notion is that if you can make a link
between these two zones,
suddenly, a fracture-permeable pathway, associated
with a rupturing of a fault
then you're likely to get discharge along
the fault from the area of abnormal over-pressure
back towards the hydrostatic condition.
So faults can act like fluid valves.
Fluid pressure rises below a zone of impermeable
rocks near the base of the seismogenic zone.
If a fault ruptures the impermeable seal,
overpressured fluid can escape
into the hydrostatic zone above.
This drop in pressure creates ideal conditions
for vein deposition.
And what we would suggest is that there are
endless cycles of fluid-pressure accumulation
somewhere around the bottom of the seismogenic
zone with episodic discharge through the faults
every time they fail during an earthquake.
Each earthquake is a potential vein-forming
event.
So vein textures are good evidence for this
cyclic behaviour.
You can see that when you look at a vein like
this that it's composite
and I can see probably at least a dozen episodes
in there,
but microscopically you could probably find
even more episodes of fracturing,
hydrothermal deposition, re-fracturing and
so on.
By the late 1980s Rick's interest in veins
led to a serendipitous meeting
with Canadian gold experts, Howard Poulsen
and Francois Robert,
which led him to fully develop the fault-valve
model.
And then we were sitting in a bar in a place
called Fort Frances,
watching ice hockey and discussing mesozonal
vein systems,
and Francois was drawing sketches of Sigma
Mine on beer mats.
It was a wonderful evening, it was one of
those evenings
you remember as a scientist all your life
because everything suddenly clicked.
And um, you had the flat veins, you had steep
reverse faults.
And I said hang on, what's the angle, between
the two,
and it turned out to be 70 degrees.
And I said that means it can only move if
it's wildly over-pressured.
And ah, that was when valving really took
off
because we sort of saw that you need this
rather exceptional combination of circumstances.
I mean I look back to an undergraduate lecture.
The lecturer said the thing to remember is,
it takes a lot of coincidences to make a mineral
deposit, it's never one thing,
and trying to identify those coincidences
is very interesting.
You've alluded to the fact that you've got
high fluid pressure in a steeply-dipping fault.
But exactly what is the significance of that
relationship.
Well it comes out of some pretty basic mechanics.
And you can actually do the algebra on an
envelope.
But faults are easiest to move
when they are aligned at about 30 degrees
to the compressive stress axis.
And in a compressional regime that's horizontal.
So that's an angle something like that.
Yes, and we see an awful lot of thrust faults
that are active at dips of about 30 degrees.
But if you look at the global distribution
of active reverse faults
we see that there is in fact a range that
goes from sort of 10 degrees up to about 60
and we don't see any reverse faults active,
at steeper than 60 degrees
in terms of modern earthquakes.
And the significant thing about that is that
it turns out from the algebra
that if the easiest orientation to make a
fault move is 30 degrees
then the frictional lock-up angle beyond which
it's extraordinarily difficult
to make a fault move, is 60 degrees.
And ah, there is a condition if you have fluid
pressures a little bit more than lithostatic
that you can actually make a fault move when
it's 60 degrees or more.
And that is what you seem to be seeing in
a lot of these deposits.
So the key relationship is that if you raise
fluid pressure enough,
presumably with some seal, capping the fluids
that even a steep fault that normally could
not move, will move.
Provided that there aren't any better oriented
faults in the neighbourhood,
OK
and that's a key issue because if there's
still a favourably oriented fault
that will go in preference.
Rick has shown that if gold-bearing fluids
exert enough pressure,
they can force their way into faults and fractures
- even very steeply dipping faults.
Most of all, Rick's work shows that veins
formed in very active fault systems.
I think that's one of the exciting things
that we're seeing,
is that the structures weren't dead when the
fluids were moving around.
And that's something the old timers knew very
well,
they comment on it again and again in the
old mining literature of the 1890s, 1920s.
They say look, its clear that these structures
were active
when the hydrothermal fluids were moving through
them
and it's true, and that's exciting.
