Why is Victoria Nugget rich?
Conventional thinking insists that gold nuggets originate from quartz reefs where they were formed by the reaction of gold-bearing fluids with chemically reactive (usually carbon-rich) indicator beds adjacent to the reefs. Such thinking cannot explain why the Golden Triangle accounts for more than 90 percent of the nuggets exceeding 500 oz found in the world. Rocks very similar to the sandstones and slates which host the central Victorian gold deposits occur in many goldfields (e.g. Meguma, Canada) and hard-rock masses of gold larger than any reported in Victoria have been found in reefs in New South Wales (e.g. Holtermann Nugget, Hill End- 7,500 oz) and California. Why then are large near-surface nuggets such as occur in the Golden Triangle not found with the same abundance in other goldfields where large masses of gold occur in reefs?The only place in Victoria where a link between reef and near-surface nuggets can be argued with any credibility is Ballarat which produced three-quarters of the state's reef nuggets. The link remains tenuous, however, because the largest mass of gold found in a reef (Lady Don- 600 oz) was only about one-quarter the size of the largest "alluvial" nugget (Welcome- 2,200 oz). In addition, the reef nuggets invariably contained more than 4 percent silver whilst the "alluvial" ones were very pure (e.g. Welcome- 99.2 percent gold).
If conventional thinking cannot explain the facts, old ideas must be abandoned and new hypotheses considered. The possibility that nuggets can grow in situ from groundwater solutions deserves serious consideration. There is ample evidence that gold can be transported in solution and deposited elsewhere under the low-temperature, low-pressure conditions which prevail at the Earth's surface. Such evidence includes a brass cartridge lying in mine water being completely replaced by gold (Tasmania), a $20 gold coin being recovered from a stream after several decades coated in gold crystals (Alaska), gold impregnations in coal and fossilised wood (Ballarat), gold inside a fossil shell (Croydon, Queensland) and iron concretions in a nugget identical to those in the surrounding soil (Coolgardie, W.A.).
What would be required to grow a large nugget in situ? The basic requirements appear to be:
1. a source of gold
2. a means of dissolving the gold and holding it in a stable form in the groundwater for transportation (probably over considerable distances)
3. steady delivery of dissolved gold to a precipitation site for a long time
4. some nucleating material and a precipitation mechanism.
How could these requirements have been met within the Golden Triangle? A source of gold is no problem given the number of reefs present. The most likely experimentally-proven reaction whereby gold in the reefs could have been dissolved involves chloride and hydrogen ions and oxygen: salt, acid and air in layperson's terms. Salt could have been concentrated in groundwater by evaporation when an arid climate prevailed. Acid could have been produced by the weathering of pyrite (iron sulphide) which is abundant in Victorian reefs. Air would have been present within the pores of the weathered rock above the watertable (the surface below which all the spaces in the rock are filled with water).
Steady delivery of gold to a precipitation site would require a very stable watertable (i.e. one not moving up and down very much). This would entail a low rainfall, a flat land surface and a lack of earth movements. The fact that within a particular area (e.g. Rheola) many nuggets occur at the same depth (and not always on the bottom) could reflect their growth at the same old watertable level. Agents which could cause gold to precipitate include copper, gold, organic matter (such as buried logs) and iron (in the ferrous state).
It can be argued that the Golden Triangle was unique in its ability to grow nuggets for the following reasons:
1. the reefs contained pyrite which weathered to form sulphuric acid
2. the acid was not neutralised because there was little carbonate present in either the reefs or their host rocks
3. at some stage during the Tertiary period (perhaps 5 million years ago) central Victoria had a climate sufficiently arid to generate saline groundwater (it is interesting to note that the distribution of nuggets within Victoria mirrors the distribution of saline groundwater even today)
4. at the same time central Victoria was very flat and free of significant earth movements (the Kosciusko Uplift had not begun) so that the watertable was very stable
5. the region contained iron-rich rocks (e.g. basalt at Ballarat, gabbro at Rheola) to aid precipitation of gold.
If nuggets did grow in situ at old watertables there is a possibility that predictive methods can be used in their location. It should be possible to plot old watertables on cross sections using reported nugget depths. Such surfaces are probably fairly regular (similar to the land surface at the time) but may have been tilted from the horizontal or disrupted by faulting during the Kosciusko Uplift. The trick is to find the intersection of an auriferous old watertable with the present surface in an area where nobody has taken a metal detector yet. The other advice for prospectors is to pay particular attention to areas where iron-rich rocks occur.
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