There has been much debate about the origin/formation of gold
nuggets. For a general summary of the types of gold deposits go to this word
One school of thought is that all nuggets originated from a primary source (hypogene), ie from the reef formations and over time the reefs were eroded down releasing rocks containing gold or pockets of gold onto the surface to be further weathered into specimens and gold nuggets.
Another school of thought is that nuggets form near the surface of the ground due to a process of precipitation and accretion (supergene).
One problem with the supergene idea of formation is that nuggets formed in this way should be only of high purity - no more than 6% silver.
I was always a skeptic of the 'precipitated' theory for gold nuggets as in my early days of detecting I had found quartz gold reefs with their typical shed of specimens (rock with visible gold in it) and further down the hill would be nuggets with less and less rock in them until they had become clean nuggets. To me this was clear evidence that gold nuggets were shed out of the reefs. I had even found a quartz reef (no where near a salt lake by the way!) with a cavity filled with almost 50 ounces of massive spongy gold. This quartz reef would have over time eroded down and released this gold onto the surface of the ground to form large nuggets. In fact I did find a 10 ounce nugget at the bottom of the hill which had obviously been shed from the same reef.
Then, one day as I was detecting a patch of gold on a salt lake a prospector came up to me and told me that gold nuggets on salt lakes had precipitated out as someone had dissected a salt lake nugget and found growth 'rings' . I laughed at this idea at the time and I still have not seen any photo of a sliced nugget with 'growth rings'.
It was this particular patch of gold though that first got me thinking differently. After a rain storm the lake filled with water and I noticed to my surprise that the water lingered along a line marking the position of some large peculiar deep nuggets I had found. These large nuggets typically had a large amount of fine gold attached to their surface which would come off in heaps when I cleaned them with a toothbrush leaving a 'sparkly surface'. I was told that these sparkly salt lake nuggets were 'etched' nuggets ie the salt had 'etched' gold out of the surface of the nugget a bit like acid etching metals. These large salt lake nuggets are usually of strange angular or flat shapes different to the rounded normal nuggets found on 'dry land' and typically have no evidence of quartz in them.
I did not give the matter any more thought until I found a 'ball' of gold very deep actually within a wet hollow in the bedrock of the same salt lake. This ball (40 grams) was a fragile mass and being very naive at the time I believed it was only saleable as a solid nugget! - I proceeded to brush it vigorously with a toothbrush to remove the layers of fragile gold till I got down to a solid centre. Of course, I wish now that I had not touched it as it could have been the proof that some nuggets do grow by precipitation. Maybe the small solid centre was a nugget from a primary source which acted as the nucleus for the precipitation of the other layers.
The more I detected on salt lakes the more I began to think that some nuggets did indeed precipitate out, although I still believe that most are derived from a primary source ie reef gold but have been changed, moulded and rounded over many millions of years of erosion, transportation and burial again long after all evidence of the original primary reef has disappeared.
Some of the nuggets I found on salt lakes, typically were covered in a very hard outer layer of what I called 'calcrete' but I now know this is actually insoluble barium/strontium sulphate. The strange thing is that some of the smaller nuggets seem to be completely encrusted with this layer and inside it there is a crystalline blocky high purity nugget which does not show any signs of weathering. Some of these small nuggets found deep in the wet mud of salt lakes have been unaffected by any kind of weathering which would normally obliterate any external 'crystalline' features.
Then I found a patch of truly amazing salt lake nuggets which
changed my mind completely! There
was only the one patch but it was very rich. All the nuggets have a unique
fragile crystalline form and most also show a later layer of precipitated gold.
Many have gypsum crystals attached. All the nuggets found were in a small
area of deep salt lake a long way from shore and at the level of the water table
and most are flattish. (see photos next page).
I do believe that these nuggets could be supergene in origin and there could still be a large low grade ore body below somewhere giving rise to this precipitated supergene layer above.
The clear layer of later spongy looking precipitated gold over the other crystalline gold on these special salt lake nuggets proves that nuggets can 'grow' by precipitation.
Being close to the water table would explain the precipitation of gold at this level as it is here that the unstable complex chemical solutions containing dissolved gold could be precipitated out by changing conditions in ph due to such events as rainfall. The salt lakes of the Goldfields are actually ancient paleochannels where supergene enrichment is a well known phenomenon to mining companies.
There are many references to supergene gold on the internet - here is one document which refers to the salt lakes of the Goldfields written by C.R.M. Butt - Buttpdf
Here are some photos of supergene salt lake nuggets - NEXT