To me it’s an amazing thought to think that
gold, and these quartz veins, formed from a fluid
In a previous film I interviewed Professor
Stephen Cox
who explained the origin of these gold-bearing fluids.
If you would like to see that interview just
follow the links.
Now in this film, which is Part 2 of that
interview, Stephen explains the relationship
between the quartz veins, faults and earthquakes.
And he starts off by explaining that some
quartz veins grow in layers
each layer representing a separate growth episode.
I asked Stephen, what’s the best evidence for this.
The really compelling evidence is in the gold
deposits themselves.
So if we look at the fault-fill veins, all
that quartz and gold which occurs within the
fault zones themselves,
often you see textural evidence for episodicity.
You see bands of differently textured quartz,
bands with sulphides in them,
there’s clearly a sort of cyclic pulsing
going on, it’s not just one event
which has produced the gold.
We have evidence at the large scale in the
mine and down at the small scale in the microscope
that these fractures of episodic have opened up
and then they’ve been sealed by mineral deposition and they’ve opened up again.
How many opening episodes do you think?
It’s hard to estimate but some of the vein
textures suggest that many deposits may have
up to thousands of cycles of failure and mineral sealing.
So you can actually see the layers in the
veins and count them.
Yes, particularly in the extension veins adjacent
to the fault zones
there are very fine delicate structures we call crack seal bands.
And in any individual vein which may be perhaps
10cm thick you might find a thousand
or more of these crack-seal events
and we think, but we can’t prove it, that
each crack-seal event probably relates to
a failure event on the fault zone which hosts
that gold deposit.
So is this at the microscopic scale?
This is down at the sort of hundred micron
type scale.
And yet it’s quite common especially in
the bedding parallel veins in Bendigo to see
quite thick multiple layers of a centimeter
each.
Yes, that’s right. Bendigo you see this
crack sealing at different scales
so you might have these centimeter scale veins
separated by little screens of wall rock.
But in some of those centimeter-wide so-called
laminated veins if you go under the microscope
you’ll see hundreds and thousands of these
little crack-seal bands
spaced maybe 50 to 100 microns apart.
To explain how layered veins formed we need
to go back a few steps and understand the
source of the gold-bearing fluids.
Well the source of the fluid is devolatilisation
reactions as rocks deep in the crust are metamorphosed,
that is cooked up.
As the fluid comes out of the crystal structure
of the various mineral grains it will carry
gold from those minerals, in the solution.
Plus as the fluid migrates up through the
crust it also has the capacity to scavenge
gold from the rocks through which it’s passing.
But there’s a change in the way the fluid
moves as it rises above the more ductile crust.
Ore deposit geologists for may years have
had a concept of fluid flow being very continuous,
a steady state sort of process.
In fact that’s probably not the way the
entire crust behaves.
In the deep crust, the rocks are very hot
and that actually makes them quite plastic
although at the earth’s surface they are
strong and brittle.
When you heat rocks up to five or six hundred
degrees they are very plastic and they flow
much like plasticine does on time scales of
millions of years.
In that regime, the rocks deform very plastically
without really breaking and producing earthquakes.
This plastic flow is steady and continuous.
So we think in the deep crust, the regime
where earthquakes aren’t produced,
which we call the aseismic regime, we think
the fluid flow is continuous or quasi-continuous.
But there’s an interesting change occurs
at shallower levels in the crust.
At about 15 or so kms deep in the crust the
rocks are cool enough and strong enough
but that rather than flowing plastically, they
tend to break episodically.
And this process is the process which generates earthquakes.
The plates are colliding and as they’re colliding the stresses are built up
and when the stress imposed on the rock, or
the force imposed on the rock
exceeds the strength of the rock it suddenly breaks
and that produces an earthquake
so we think this process may actually generate  episodic flow in the higher crust.
With each earthquake fluids can move into
a fault zone allowing quartz and gold to precipitate as veins.
But the formation of new veins has an interesting effect on the later fluid flow.
This deposition of quartz is very important
because it destroys the permeability
and if you think of fluids coming up through
a plastically deforming shear zone into a
cooler zone where quartz is being deposited
and destroying the permeability,
what happens is you’re basically putting
a seal on that transport zone,
it’s like turning the tap off, and if you
keep supplying fluid to this, towards this
area where the tap’s turned off, what happens?
The fluid pressure builds up.
After an earthquake the new veins seal the
fault zone so that fluid flow is again restricted.
This allows fluid pressure to build up.
It’s exactly the same as the tyre analogy
on a car.
When the tyre’s flat the car is low lower down on the ground than it normally is.
You apply air pressure to the tyre, the air
pressure is actually lifting the car up.
It’s the same within a fault zone or a shear zone.
Eventually the fluid pressure in that actively
deforming shear zone
can exceed the confining pressure, the rock pressure acting on it
and that will actually nucleate a failure
event which will actually fail or breach that sealed zone.
That will allow the highly pressurized fluid,
which was trapped beneath the seal, to suddenly
escape up the earthquake rupture zone.
There it then percolates into various structures,
which give rise to the gold deposits.
Why are earthquakes important to this process?
Well it’s very interesting. We could produce
the gold deposit with continuous flow
but when we had this episodic failure and
sudden fluid flow events
and then tail of a flow and quiescence,
that actually creates better opportunities
for creating gold deposits.
Simply because if you have, fluid pressure
builds up prior to the earthquake
as soon as you have a failure event,
the fluid rushes up the fault zone, as it
does so,
the pressure in that fluid drops dramatically.
And we know that the solubility of quartz
is very pressure dependant,
so that helps dump the quartz out.
But there are also pressure dependencies in the solubility of gold and the species that transport it
which means if we have a very sudden and large fluid-pressure drop in that fluid as it courses
through the fracture zone, that will help
dump the gold out.
So do you think, is it more important to have
a lot of small episodes of release of fluid,
a lot of small earthquakes, is that more favorable
to form a gold deposit.
I don’t think we really understand that
issue yet.
What we do know is that a lot of the gold
deposits we see in Victoria and similar deposits
around the world
is that each deposit is located or associated
with quite small displacement fault zones.
For example the Wattle Gully deposit, there
the maximum slip on that fault zone is about 50m
that’s probably accumulated through maybe
one or two thousand magnitude 4 earthquakes
and the other interesting thing is that often
these clustered gold deposits are spatially
associated with bigger faults
and maybe these are the faults, which are
the main shock structures.
And the view we’re starting to develop through
studying many of these deposits and doing
some modelling work is that the big faults
may act as the deep level fluid conduits
and you have a main shock on that, the fluid
rushes up that main shock rupture which has
high permeability, high porosity.
It then gets distributed out into smaller
faults like the branches on a river again but
in maybe the delta type environment where
you’ve got distributaries.
Is it more favourable to have the fluid dispersed
into the smaller fracture networks rather
than stay in the big shear zone.
It very much depends on the sort of reaction
you want
or what is your favourite reaction to scavenge the gold out of the deposit.
If you just want to get the gold out through
purely a fluid pressure drop
you would like that fluid pressure drop to
be really large and you then want to be on
the main shock structures, the big faults.
But if you need some chemical-fluid-rock interaction,
which for the Victorian deposits we seem to need
you want to be in a small structure where
the fluid can actually discharge into the
carbon-bearing wall rocks and react with them,
and produce species such as methane which
actually helps scavenge the gold.
So it’s not enough just to have a gold-bearing fluid you need some chemical process
to draw the gold out of solution.
That’s absolutely right. There’s two processes
which you need to make a gold deposit,
one is the physical process to generate fracture
permeability to get the fluid to the site
then at the site where the fluid is flowing
you need the chemistry to be right.
And often that will involve wallrocks which
are appropriately reactive
which set up the reactions which help scavenge the gold from that fluid.
Well of course there’s miner’s folklore
going back 150 years in Victoria
that some black shales are very favourable
for gold deposition.
Yes these black shales seem to be very important
and they’re black because they’re quite rich in carbon.
So there’s some truth to this then.
I think there’s a lot truth in it, and those
guys knew what they were doing
they were very good observers and they saw the spatial association.
We think know we understand some of that association
and one model we have is that the fluids that
are coming up these fault zones
discharge from the faults will react with
those carbon-rich fluids, produce methane
and then we’d have a major rupture event.
Some of those fluids are drawn back into the
fracture system and mix and it’s a,
there’s a redox reaction or oxidation-reduction
reaction goes on
which helps that fluid deposit its gold.
In many mines it’s common to see a vertical
structure, a main lode or reef is nearly vertical
and yet, the veins to one side are usually
horizontal, why is this so.
That’s an interesting question and a very
important one, the fault-fill lodes are filling
steeply inclined faults which have
slipped parallel to the fault surface like that.
The extension veins off to the side have a
different mechanism of formation
they form parallel to the plane of the maximum stress
or force
and perpendicular to the plane of the minimum
force acting on the rock
and in central Victoria when these deposits
formed that minimum force or stress was oriented
vertically so they’ve formed perpendicular
to that.
They actually tells us a lot about the orientation
of the stress field during these deposits.
So the individual crystals in the vein, in
the horizontal vein
can grow most easily in the vertical direction.
That’s right, because the fracture is actually
opening in a vertical direction like that
and crystals tend to grow at a high angle
to the fracture wall.
But there’s a very important concept here
as well and that is it says, that those steeply
inclined faults are not particularly favourably
oriented relative to the regional stress field
and we know from our rock mechanics studies
and experiments that to actually get one of
these steeply inclined faults to slip you
need extremely high fluid pressures
in fact the fluid pressures have to exceed
the vertical stress, and that again tells
us we’ve got not just high fluid pressures,
but to achieve that we need very high fluid flows.
So if there no hydrothermal fluids interacting
with the fault, the fault may not have much displacement. Or any?
It would never be activated at all.
So that’s a really important and interesting
point isn’t it. That there’s this dynamic
interplay between deformation and fluid pressure.
Yeah, well it’s actually more amazing than that.
There’s this coupling between deformation,
fluid pressure and the chemical processes going,
they're all fully coupled and we need to produce,
to produce a gold deposit we actually need
all these things to work together to produce
the right scenario,
and if they don’t work together we don’t
get the gold deposit.
Often when we see these horizontal extension
veins
say in the footwall of the main steeply-dipping lode
they appear to be actually isolated from the
main reef.
So is that implying that there’s some inherent
porosity of the wallrock allowing the fluids
to permeate through them.
Yeah the fact that see the quartz, and the
gold and various other minerals
in those isolated veins does really tell us the fluid has percolated through the rock
and what we find in high fluid-pressure environments
although the rock intrinsically
has low porosity, low permeability
The high fluid pressure can actually act on
that rock to actually crack the grains and
allow the fluids to work its way out there.
And the veins actually grow where the fluid
pressure
exceeds that minimum vertical stress  acting on the rock
and wherever that situation occurs, bang we’ll
start to forming these veins.
Often say at Bendigo you’ll find these isolated
veins in big sandstone layers.
So it’s a very opportunistic fluid isn’t it.
Absolutely. The important concept is
most of the breaking or failure process is fluid driven.
The stress controls the orientation of the
structures,
but it’s the fluid which generates it’s own permeability.
Basically munches its way through the crust to go where it needs to go, to drop its pressure.
