The travel of Alluvial goldl

Ok folks lets hear your opinions.
The scenario!

There's a reef at the top of a slope.

Part way down the old timers have dug and found gold.

The question:
So which stopped first, the big gold as it hung up on its travel down the hill or the small gold as it dropped out along the way.

Really interested to hear your opinions. Thanks.

Too wider subject but this might help.
http://www.goldgold.com/gold-prospectingplacer-geology.html

Thanks Jaros but I was thinking specifically of only the elluvial element to the process. That article actually covers the whole of the journey of gold to its final resting place in a pocket in a stream.
I was thinking more of the process stopping in the elluvial stage where the majority of the gold "hangs" at a position but some continues, either the small stuff or the big stuff and I can't work out which would continue on down the slope, or travel further.

Tassie Daz said:

Thanks Jaros but I was thinking specifically of only the elluvial element to the process. That article actually covers the whole of the journey of gold to its final resting place in a pocket in a stream.
I was thinking more of the process stopping in the elluvial stage where the majority of the gold "hangs" at a position but some continues, either the small stuff or the big stuff and I can't work out which would continue on down the slope, or travel further.

Click to expand...

Im guessing it would all travel downwards or be stopped by natural impediments such as clay barriers etc but just at different rates.

I'm thinking gravity may help the heavier nuggets travel down further unless it rained heavily and carried the lighter stuff off down the hillside. All depends on obstacles along the way that may have been there at one stage.
Either way if I was to find gold at the base of a hill I'd be looking up the hill further anyway.

Good to see your still around Taz

The tendency is for most large nuggets to stay in soil just below their source. Here is a map showing the nuggets greater than 15 kg - all are barely moved from their sources in the Ballarat West, Ballarat East and Nerrina (Little Bendigo) fields. All the shafts shown are on quartz veins (which define the three parallel fields). Similar on other fields (eg the Welcome Stranger was found just downhill from its probable source). Few nuggets ever make it further than the gully at foot of slope (if that far), much less a major stream.



There is a pattern - eluvial placers tend to involve removal of lighter material and leaving nuggets at source - enriching the gold by removing lighter rock etc. The finer gold that gets into the stream and gets carried away, it then gets dropped out in places where the velocity of the stream decreases, So two types of placer, winnowing of light material at source, dumping of fine gold when streams slow.

If you want coarse gold, stay close to source.

Awesome explanation goldierocks!

Great post goldierocks :Y:

Here is a map that shows how the fine gold travels until a drop in stream velocity dumps it out:



The gold originates in the reefs around Beechworth and gets carried into the stream to the north on the Mt Pilot Batholith, then westward to Eldorado where it has been extensively mined as a deep lead and dredged at shallow depth. The Mt Pilot Batholith is a granite and contains no gold veins at all, so all the gold at Eldorado (and in the Woolshed etc along the way but mostly closer to Beechworth) is derived from south of the granite at Beechworth, 26 km upstream. The reason the last of the gold dumped out at Eldorado is that the stream ran west off the granite into the metamorphic (originally sedimentary) rocks cooked up by the granite on its western margin. This hard rock dammed the stream a bit and caused it to slow, dropping the fine gold out of suspension at Eldorado.

You only have to look at a slope or a hill with a reef or outcrop on or towards the top. The rocks are bigger closer to the outcrop and smaller down slope.

madtuna said:

You only have to look at a slope or a hill with a reef or outcrop on or towards the top. The rocks are bigger closer to the outcrop and smaller down slope.

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Then add the fact that gold is about 7 times as heavy as rock the same size.....

Thanks for the explanations fellas. All this got me thinking about the stuff I have and thinking about the Berlin goldfield at Rheola I remembered there were big nuggets which hung in Griffiths gully but there were even bigger ones that made it into the creek which is today Cattos Paddock. And I read somewhere that the gold originated from the granite......I guess like a rock bar in a creek when the area was covered in water.
So it all really depends on the obstructions along the way, whether at or underground level that causes the large nuggets to settle I guess.
For a great map of the area check out the Geological survey map of the Berlin Gf.

Tassie Daz said:

Thanks for the explanations fellas. All this got me thinking about the stuff I have and thinking about the Berlin goldfield at Rheola I remembered there were big nuggets which hung in Griffiths gully but there were even bigger ones that made it into the creek which is today Cattos Paddock. And I read somewhere that the gold originated from the granite......I guess like a rock bar in a creek when the area was covered in water.
So it all really depends on the obstructions along the way, whether at or underground level that causes the large nuggets to settle I guess.
For a great map of the area check out the Geological survey map of the Berlin Gf.

Click to expand...

I had a look at Berlin and found little accessible ground (private). No, the granite has little to do with the gold, most of the granite (eg Mt Moliagul range to the north) postdates the gold. The "granite" referred to in old reports is actually diorite in just a few tiny intrusions - it pre-dates the gold and is cut by some gold veins, but is probably no better than the surrounding rocks for prospecting, the gold was not brought in by it, it is simply older and already there, so the gold cut it as with all the rocks already there. Catos =- the potato paddock (potatos being nuggets) was noted for nuggets at surface obtained by simply turning the ground over with picks and shovels without sluicing. Private land and thoroughly gone over. Others may know the area better than me, but I always thought poor access limited its interest.

You seem to be confusing different ideas - hard rock gold and alluvial gold. It affects where you detect.....