Primary and secondary Gold

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Primary and secondary Gold Empty Primary and secondary Gold

Post  Guest on Fri Jul 17, 2009 10:51 pm

Hi All

The average prospector need be concerned with only two types of gold deposit:
1). Primary gold
2). Secondary gold


1). Primary gold
Formed 2.6 billion years ago from very hot (300-550oC) water dissolving low-level “background gold” out of a
large volume of rock & redeposits it in a number of small areas to form large tonnage, generally fine
grained, but low grade deposits, several kilometres below the then surface of the Earth in the Yilgarn Block
of W.A.
These deposits tend to form associated with vein quartz and sulphide minerals in particular rock types &
environments. Some specimen gold nuggets may form if this quartz vein gold physically erodes.
This is the gold that the big companies are exploring for & mining.
Primary gold deposits form deep underground but may produce “primary” gold nuggets on erosion.
PRIMARY GOLD DEPOSIT CHARACTERISTICS
• large tonnage (millions of tonnes)
• low grade (2-8 ppm) & fine grained gold (tens of microns)
• low purity gold (10-15% silver), jagged, sharp & spiky
Archaean (2.6 billion years old)
• formed from hot (300-550oC) water
• mineralized fractures associated with:
• faults
• quartz veins
• alteration zones
• sulphurous & carbonated waters
• doleritic sills & basaltic rocks
• sulphide minerals (pyrite & arsenopyrite)
• carbonates (calcite & ankerite)
• formed at great depth (10+ km)
• exposed through significant erosion over millions of years
Primary deposits are the main interest of corporate miners.

Primary and secondary Gold Primary-1

2). Secondary gold
This is gold formed relatively recently (100 million years ago right up to the present day).
It forms when highly saline groundwater redissolves primary gold in the near-surface weathering
environment & transports that gold in solution to the air-water interface (i.e. to the water table).
The gold re-deposits, often in nugget form, at or near the present ground surface as a result of rainwater
dilution of the saline groundwater (in an earlier wetter climatic regime, or associated with salt lakes), or as a
result of partial evaporation in the present arid climatic regime.
SECONDARY GOLD
Secondary gold (which is the main interest of most prospectors), needs all of the following in order to form:
1. a nearby “primary gold” source for the gold,
2. saline groundwater in which to dissolve the primary gold,
3. A means of bringing the dissolved gold to the surface, locally this is largely through rainfall drawing dissolved gold to the surface
4. a mechanism to re-precipitate the gold at or near the surface.
Secondary gold can form in laterite, calcrete or in salt lake environments, and although the gold enters solution the same way (oxidation of primary gold in saline groundwaters), it precipitates out of solution by differing mechanisms in each of these environments.
Secondary enrichment processes involving highly saline solutions, generally related to the groundwater environment, re-dissolving primary gold. Re-precipitation of the gold occurs at or near the watertable-air interface usually as a result a sharp decrease in metal solubility as a result of fresh water dilution of the saline waters via rainfall.

Primary and secondary Gold Secondary-1

The dissolved primary gold migrates towards the surface in a one way convenor belt - like process driven by solution diffusion mechanisms that attempt to even out the dissolved metal concentration in the groundwater
This can never be achieved however as fresh water is always being added to the surface through rainfall
.
SECONDARY GOLD DEPOSIT CHARACTERISTICS
• small tonnage
• high grade
• associated with a primary gold source at depth or up-slope
• coarse gold & nuggets (generally smooth & rounded shapes)
• high purity gold (low silver content)
• Tertiary (< 60 million years) to recent (still forming today)
• precipitated from saline groundwater
• precipitated through rain water dilution
• formed in laterites, calcretes, alluvial channels, & salt lakes
• associated with iron oxide (goethite) & calcrete
• formed at or near the water table
• formed at or near the ground surface
• exposed through minimal surface erosion & sheet wash
Secondary gold is the main interest of most prospectors.


Last edited by Jefgold on Sat Jul 18, 2009 2:27 pm; edited 1 time in total (Reason for editing : formatting)

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Primary and secondary Gold Empty Re: Primary and secondary Gold

Post  llochris2 on Sat Jul 18, 2009 5:24 pm

Goodone Gypsy very interesting printed it out so as to show some people.Hope hear from you soon Credo Lloyd
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Primary and secondary Gold Empty Re: Primary and secondary Gold

Post  gray.nomad on Sat Aug 01, 2009 7:44 pm

Hi Gypsy,
Thanks, that was very informative.

See you up them thar hills (or salt lakes) Very Happy
Frank
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Primary and secondary Gold Empty Re: Primary and secondary Gold

Post  Jonathan Porter on Tue Aug 18, 2009 9:11 am

Gypsy it might interest you to look into some of the latest CSIRO tests done on nuggets to determine if they are primary or secondary origin, fascinating stuff.

JP
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Primary and secondary Gold Empty Re: Primary and secondary Gold

Post  Guest on Tue Aug 18, 2009 10:34 pm

Hi,
yes it sure is fasinating stuff, I have alot of documents from CSIRO about primary and secondary gold
that i got when i did my courses, but unfortunatly a cannot put the white papers on a public forum,
one day we may crack the gold/rock code Very Happy
cheers Gypsy

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