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Ironstone is something that still fascinates me every time I come across it. I dont just mean the occasional little pea-sized piece you see kicking around on the surface at times, but the stuff that exists in more serious quantities. As a relative newbie to the hobby (Ive been swinging the GPX 5000 for just over a year now), it still quite often stops me in my tracks in wonder when I come across it.
I should have taken some photos of examples that have fascinated me from time to time, but as Im already loaded up with a pick, pinpointer, scoop, sP-01 enhancer, drink bottle, Snake Bandages & Camelbak as well as my detector with harness, theres really no room for a decent camera, and once Ive put my phone away safely in a back pocket to keep it away from my 25 coil, Ironstone seems hardly worth the effort of stopping and getting it out for a snap or two.
In some places as Im sure many members have seen, broken pieces of Ironstone can be found in huge chunks, and in one particular detecting session at Kingower some months ago I came across a grapefruit sized chunk that had a side where it hung down in festoons. I had assumed at the time that this was the result of some molten upsurge from within the earth, until only recently when learning that Ironstone is in fact sedimentary. Now in the absence of any Geological training, this revelation completely turned my understanding of things upside down. Upon further reading I came across something that member Goldierocks had written..
In Post # 57 under the Thread heading Gold Beyond The Workings in the Metal Detecting for Gold forum https://www.prospectingaustralia.com/forum/viewtopic.php?pid=518855#p518855
Goldierocks states the following-
(i) the nuggets found in the laterites are usually secondary, not primary. Primary nuggets (that were deposited with quartz veins etc) usually have at least 3% silver (up to more than 50% silver). Secondary nuggets that grow in laterites usually have no detectable silver (e.g. they are 99.7% gold, the 0.3% commonly being copper).
(ii) any nuggets found within the laterite are usually above or only offset horizontally at most tens of metres from the source at depth (but that can be as much as 30 m below, often with white clay lacking gold in the intervening interval). The nuggets actually grow in the iron-rich ferricrete at the top of the laterite (the gold gets dissolved from the primary ore at depth and re-deposited where there is iron near surface - they actually require the iron for the gold to be re-deposited). For those chemically inclined the gold is dissolved in cold, oxygen-rich, highly salty water at depth as a gold chloride ion, and has to be reduced again to metallic gold by encountering iron (oxidation state +2) that reduces it to metallic gold. Nuggets of this type sometimes have ironstone inside them, completely enclosed by gold. Since we know the ironstone formed by weathering, the gold must also have deposited in the weathering zone because it includes ironstone inside it. When the gold dissolves from silver-rich gold at depth, the silver and gold separate from each other and re-deposit separately (sometimes the silver simply stays in the groundwater and washes away), giving the high purity of secondary gold nuggets.
So my question is this- Im assuming that here in Victoria we dont have the laterite soils of Western Australia? But is it possible that a similar process still takes place in some Victorian goldfields whereby primary gold, or particles of gold in the soil in general, gets dissolved by surface water mixing with the high salt content in the soil of areas affected by naturally occurring Dryland Salinity, and the readily available oxygen of the atmosphere at the surface? Im intrigued by the whole thing because a number of times now I have seen photos of, or read about a decent sized gold nugget found here in Victoria that is associated with Ironstone that is nowhere near a reef or other obvious primary gold origin. And my other question is this- When such nuggets are found, if formed by a similar process as mentioned above by Goldierocks, would they exhibit a different grain/crystalline structure inside when cut in half compared to gold that is primary gold? Or, wouldnt their purity also be much higher than any reef gold nearby that might be suspected of being the primary source?
I love reading the contributions of Goldierocks, Swright and other Geologists on the forum and Id appreciate if someone could enlighten me further please on gold when it is associated with Ironstone.
Thanks in advance
D.S.
I should have taken some photos of examples that have fascinated me from time to time, but as Im already loaded up with a pick, pinpointer, scoop, sP-01 enhancer, drink bottle, Snake Bandages & Camelbak as well as my detector with harness, theres really no room for a decent camera, and once Ive put my phone away safely in a back pocket to keep it away from my 25 coil, Ironstone seems hardly worth the effort of stopping and getting it out for a snap or two.
In some places as Im sure many members have seen, broken pieces of Ironstone can be found in huge chunks, and in one particular detecting session at Kingower some months ago I came across a grapefruit sized chunk that had a side where it hung down in festoons. I had assumed at the time that this was the result of some molten upsurge from within the earth, until only recently when learning that Ironstone is in fact sedimentary. Now in the absence of any Geological training, this revelation completely turned my understanding of things upside down. Upon further reading I came across something that member Goldierocks had written..
In Post # 57 under the Thread heading Gold Beyond The Workings in the Metal Detecting for Gold forum https://www.prospectingaustralia.com/forum/viewtopic.php?pid=518855#p518855
Goldierocks states the following-
(i) the nuggets found in the laterites are usually secondary, not primary. Primary nuggets (that were deposited with quartz veins etc) usually have at least 3% silver (up to more than 50% silver). Secondary nuggets that grow in laterites usually have no detectable silver (e.g. they are 99.7% gold, the 0.3% commonly being copper).
(ii) any nuggets found within the laterite are usually above or only offset horizontally at most tens of metres from the source at depth (but that can be as much as 30 m below, often with white clay lacking gold in the intervening interval). The nuggets actually grow in the iron-rich ferricrete at the top of the laterite (the gold gets dissolved from the primary ore at depth and re-deposited where there is iron near surface - they actually require the iron for the gold to be re-deposited). For those chemically inclined the gold is dissolved in cold, oxygen-rich, highly salty water at depth as a gold chloride ion, and has to be reduced again to metallic gold by encountering iron (oxidation state +2) that reduces it to metallic gold. Nuggets of this type sometimes have ironstone inside them, completely enclosed by gold. Since we know the ironstone formed by weathering, the gold must also have deposited in the weathering zone because it includes ironstone inside it. When the gold dissolves from silver-rich gold at depth, the silver and gold separate from each other and re-deposit separately (sometimes the silver simply stays in the groundwater and washes away), giving the high purity of secondary gold nuggets.
So my question is this- Im assuming that here in Victoria we dont have the laterite soils of Western Australia? But is it possible that a similar process still takes place in some Victorian goldfields whereby primary gold, or particles of gold in the soil in general, gets dissolved by surface water mixing with the high salt content in the soil of areas affected by naturally occurring Dryland Salinity, and the readily available oxygen of the atmosphere at the surface? Im intrigued by the whole thing because a number of times now I have seen photos of, or read about a decent sized gold nugget found here in Victoria that is associated with Ironstone that is nowhere near a reef or other obvious primary gold origin. And my other question is this- When such nuggets are found, if formed by a similar process as mentioned above by Goldierocks, would they exhibit a different grain/crystalline structure inside when cut in half compared to gold that is primary gold? Or, wouldnt their purity also be much higher than any reef gold nearby that might be suspected of being the primary source?
I love reading the contributions of Goldierocks, Swright and other Geologists on the forum and Id appreciate if someone could enlighten me further please on gold when it is associated with Ironstone.
Thanks in advance
D.S.