Our Geologist Says: “We have ‘Free Milling’ Gold

March 19, 2011

Gold Mining

OK all you geologists, and research hounds, what the heck is “free milling” gold?

Mike Kande, our geologist, sent me this picture and comment: “A stamp mill was actually installed in the lease from 1963 to 1972.  It was used to crush rocks and the crushed rocks fed into rod mills and button pans to retrieve gold from the ore.  See picture of the stamp mill, now collapsed and lying in the creek gully.


Broken Historic Stamp Mill


I guess that is why the equipment we need to put on site include  jaw crushers and a rod mill.

I wanted to understand more and I found this article on the internet.  It seems to have been meant for geologists so I did some research and added pieces to hopefully make this more understandable.

I also added comments that our geologist gave to us, to show why we can fit into this description pretty nicely.

My comments I put in purple as I am not a geologist, Mike’s descriptions are in blue.

Just trying to help you understand how we are getting at the gold we know exists abundantly on these leases we are going to mine.


From the internet:

“Free milling ores are defined as those from which Cyanidation can extract approximately 95% of the gold  when the ore is ground to size 80% < 75 µm, (divide this by 1000 to get mm so .0075 mm or VERY TINY) as commonly applied in industrial practice, without incurring prohibitively high reagent consumptions. We are told we do not even have to resort to cyanidation, which adds some sort of cyanide mixture to a pile of very crushed rocks to free the gold.  Good for us and good for the environment.

Frequently some of the gold is recovered by gravity concentration
and amalgamation and gangue minerals composition does not significantly affect the processing requirements.

The two main classes of free milling ores are palaeo-placers and quartz vein gold ores.

Some epithermal deposits may be free milling but more commonly contain significant concentrations of sulphides and are considered in other classes.

Palaeo-placers are literally fossilized placers, the most famous being the Witwatersrand lake bed reefs in South Africa.  Other includes Jacobina (Brazil), Blind River,  Elliot Lake (Canada) and Tarkwa (Ghana).

This kind of deposits consists of lithified (the formation of massive rock from loose sediment) conglomerates which contain small rounded pebbles of quartz in a matrix of pyrite, fine quartz, micaceous material and small quantities of heavy resistant materials such as magnetite, uraninite, platinum group metals, titanium minerals and gold.

From mineral processing point of view palaeo-placers differ from young alluvial placers as the gold is unliberated and the ore is consolidated.

Crushing and grinding is required to liberate the gold to an extent which allows efficient gold extraction.

Palaeo-placer gold deposits have been mined at depths of up to 3 km and therefore both mining and mineral processing costs are generally
more than an order of magnitude greater than those for young placer deposits.

Various non-placer gold ore types can be classified as free milling. These are usually formed as a result of deposition from hydrothermal
solutions. Epithermal deposits may fall in this category, but quite often have some refractory components.

Quartz-gold veins or lode comprise a variety of deposits which are essentially hydrothermal veins of quartz and gold that either replace wall rock or fill open spaces along fracture ones. Most are pre-Cambrian or tertiary in age and can occur to depth in excess of 1 km. the main categories are as follows:
* · Auriferous vein, lodes, sheeted zones and saddle reefs in faulted or folded sedimentary rocks.
* · Gold/silver veins, lodes or stock works and irregular silicified bodies in fractures, faults, shear, breccia or sheeted zones in volcanic
* · Gold/ silver occurrence as above, though in a complex geological environment comprising sedimentary, volcanic and various igneous intrusive and granitized rocks.”

Here is what our geologist says about the area where he has gathered samples for assaying:

From a brief initial geological mapping I’ve done over exposed areas within the lease, a few potential gold bearing zones have been identified and sampled.  A total of 14 samples that were collected have been submitted for gold assay.


The zones include an approximate 7m wide clay-pyrite-quartz altered zone with peripheral areas strongly bleached and clay altered (Sample No. RC429-06 & 07).  This rock type looks similar to rock fragments that were sampled about 100m east of this location (Sample No.F429-05).  The rock fragments sampled were said to be from a high grade zone less than 0.5m wide, but now covered by small land slip from above.


Sample numbers RC429-13 & 14 were taken within an approximate 3 meter wide altered zone with strong manganese oxide and weak quartz veining clay mineralisation.  The zone is vuggy and strongly oxidised.  The areas have actually been tested by locals and have panned gold out of it but because it is along the road side and because of lack of water, no further work was done.

The general area where samples RC429-01 to RC429-04 were taken also show mineralisation with stringers of manganese oxide and quartz veinlets as stockworks within a strongly weathered and fractured andesite and phyllites of the Kaindi metamorphics.  Depending on the results, these would be a bulk mining area.

Samples RC429-09 to 11 were taken from a strongly fractured and oxidised zone with several thin zones of clay with occasional quartz veinlets, manganese oxide associated and generally silicified zone.  The location was actually dug up by an excavator and it was worked by locals but was stopped due to its closeness to the existing vehicle track .

This explanation helped me understand what the geologist was saying, as we are fortunate to be able to extract gold from these leases just by crushing up the rock and running water through it.

Let me know what questions you have or feel free to leave comments.

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    4 Responses to “Our Geologist Says: “We have ‘Free Milling’ Gold”

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    1. Releasing Gold From The Rocks and Minerals | PNG Gold Ltd Partnership - March 27, 2011

      [...] Since PNG is so close to Australia, it is not surprising that we saw they had been using the old Stamp mill which we saw here, in our previous post. [...]

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