Gold originates from where you find it, at least in proximity to its primary deposit. So the gold did not "travel" to Ballarat or Creswick but rather originally came from primary reef deposits in Ballarat and Creswick themselves.
The gold we detect today however may have mixed sources either from the original primary reef system or from a secondary source so that could put a little further distance into the equation. The weight of gold ensures that it is very difficult to move far at all by most natural measures.
We refer to alluvial gold as a secondary deposit either in deep leads covered by later alluvium or lava flows, shallow gullies that the early diggers exploited or even gold in the soil. Some refer to the latter as elluvial gold, but it is all part of the same process whereby gold sheds from a primary reefing source working its way by gravity to lower and lower levels into a secondary deposit.
Some secondary deposits are now elevated protected from erosion by a hard capping of cemented river gravels called conglomerate. These also can shed gold where their edges are exposed on hillsides but the gold is usually smooth and worn indicating it has come from a secondary deposit whereas gold shedding from a primary deposit close to its source will often show sharper characteristics and sometimes it is still attached to its quartz and is then known as a specimen.
As to where the gold in the primary source (quartz reefs) originated,, my understanding
is that it was was deposited by superheated and super pressurised metal and mineral rich hydrothermal fluids which were squeezed from their deep source associated with magma chambers.
These fluids flowed into and enlarged naturally occurring cracks, fissures and faults in the overlying rocks or forced passages along loosely bound strata. Tectonic forces have squeezed the rocks of Central Victoria into an almost vertical concertina like fold system, fracturing those rocks and made them particularly suitable for intrusion by those gold bearing hydrothermal fluids.
Once these superheated and super pressurised fluids reach a cooler zone where the temperature and pressures are just right (the goldilocks zone) deposition of the gold and other minerals takes place. An analogy would be the experiment we did in our school days growing crystals from a warm saturated solution of copper sulphate suddenly crystallised when the solution cooled enough.
One study in WA involved slicing through a quite large nugget to examine its internal crystallinity. That indicated a temperature of about 300 Celsius for its formation. Using the known temperature gradient of the earths crust of about 30 degrees rise per Km of depth that also gives an estimate of the depth at which the gold deposited (approx 10Km).
Looking at the goldfields today one may think that this process would take an almost immeasurable time to erode away the overlying rocks and expose the reefs, but the climate was much different then to what it is now for much of Australia's history.
65 millions years ago Australia was part of Antarctica and has moved through many climatic zones since to its present position providing opportunity for much wetter climates to increase erosional rates. Other factors are evidence of mountain uplift increasing erosion rates and also that the hard metamorphic slates we now see were at the bottom of a large sequence of sediment deposited in a deep ocean trough that would no doubt have been much softer in their uppermost unmetamorphised layers.
If we go back even earlier, before 370 million years ago but still after the rocks were deposited, there would not have even been trees or grasses on land to slow erosion by rains of unknown intensity. There would have been plenty of opportunity to erode the kilometres of rock necessary to expose the reefs on the surface.
As to whether the nuggets originated in the reefs or grew in situ in secondary deposits, whilst I have personally seen evidence that suggests that gold can grow in situ, I think that evidence does not favour the growth of large nuggets or even the smallest nuggets we now detect. There is good evidence that rich nuggetty gold forms in the quartz veins themselves and their growth is influenced by what are called indicator strata.
An interesting read is Bulletin no 9 of the Geological Survey of Victoria by W Bradford on the Dunolly Wedderburn goldfields. He had the advantage of inspecting many of the small rich nuggetty reefs when they were still open. His theory was that nuggetty gold forms in quartz veins when the movement of the gold bearing fluids was hindered or slowed (or possibly chemically influenced - a later theory) by intersection with thin iron rich strata allowing gold masses to grow in size. These thin lines were known as indicators and he noted that these were to be found right through the nuggetty goldfields he inspected. You can download the GSV memoirs and bulletins from Earth Resources Vic for free. It is however a large file nearing 1GB containing nearly 100 individual reports
If only I could identify an indicator at the surface that is where I would be looking, but I have never been able to do that.
PS. Above is my simplified view of a complex issue but always stand ready to be corrected by evidence pointing in another direction.
The gold we detect today however may have mixed sources either from the original primary reef system or from a secondary source so that could put a little further distance into the equation. The weight of gold ensures that it is very difficult to move far at all by most natural measures.
We refer to alluvial gold as a secondary deposit either in deep leads covered by later alluvium or lava flows, shallow gullies that the early diggers exploited or even gold in the soil. Some refer to the latter as elluvial gold, but it is all part of the same process whereby gold sheds from a primary reefing source working its way by gravity to lower and lower levels into a secondary deposit.
Some secondary deposits are now elevated protected from erosion by a hard capping of cemented river gravels called conglomerate. These also can shed gold where their edges are exposed on hillsides but the gold is usually smooth and worn indicating it has come from a secondary deposit whereas gold shedding from a primary deposit close to its source will often show sharper characteristics and sometimes it is still attached to its quartz and is then known as a specimen.
As to where the gold in the primary source (quartz reefs) originated,, my understanding
is that it was was deposited by superheated and super pressurised metal and mineral rich hydrothermal fluids which were squeezed from their deep source associated with magma chambers.
These fluids flowed into and enlarged naturally occurring cracks, fissures and faults in the overlying rocks or forced passages along loosely bound strata. Tectonic forces have squeezed the rocks of Central Victoria into an almost vertical concertina like fold system, fracturing those rocks and made them particularly suitable for intrusion by those gold bearing hydrothermal fluids.
Once these superheated and super pressurised fluids reach a cooler zone where the temperature and pressures are just right (the goldilocks zone) deposition of the gold and other minerals takes place. An analogy would be the experiment we did in our school days growing crystals from a warm saturated solution of copper sulphate suddenly crystallised when the solution cooled enough.
One study in WA involved slicing through a quite large nugget to examine its internal crystallinity. That indicated a temperature of about 300 Celsius for its formation. Using the known temperature gradient of the earths crust of about 30 degrees rise per Km of depth that also gives an estimate of the depth at which the gold deposited (approx 10Km).
Looking at the goldfields today one may think that this process would take an almost immeasurable time to erode away the overlying rocks and expose the reefs, but the climate was much different then to what it is now for much of Australia's history.
65 millions years ago Australia was part of Antarctica and has moved through many climatic zones since to its present position providing opportunity for much wetter climates to increase erosional rates. Other factors are evidence of mountain uplift increasing erosion rates and also that the hard metamorphic slates we now see were at the bottom of a large sequence of sediment deposited in a deep ocean trough that would no doubt have been much softer in their uppermost unmetamorphised layers.
If we go back even earlier, before 370 million years ago but still after the rocks were deposited, there would not have even been trees or grasses on land to slow erosion by rains of unknown intensity. There would have been plenty of opportunity to erode the kilometres of rock necessary to expose the reefs on the surface.
As to whether the nuggets originated in the reefs or grew in situ in secondary deposits, whilst I have personally seen evidence that suggests that gold can grow in situ, I think that evidence does not favour the growth of large nuggets or even the smallest nuggets we now detect. There is good evidence that rich nuggetty gold forms in the quartz veins themselves and their growth is influenced by what are called indicator strata.
An interesting read is Bulletin no 9 of the Geological Survey of Victoria by W Bradford on the Dunolly Wedderburn goldfields. He had the advantage of inspecting many of the small rich nuggetty reefs when they were still open. His theory was that nuggetty gold forms in quartz veins when the movement of the gold bearing fluids was hindered or slowed (or possibly chemically influenced - a later theory) by intersection with thin iron rich strata allowing gold masses to grow in size. These thin lines were known as indicators and he noted that these were to be found right through the nuggetty goldfields he inspected. You can download the GSV memoirs and bulletins from Earth Resources Vic for free. It is however a large file nearing 1GB containing nearly 100 individual reports
If only I could identify an indicator at the surface that is where I would be looking, but I have never been able to do that.
PS. Above is my simplified view of a complex issue but always stand ready to be corrected by evidence pointing in another direction.
Last edited:
