FMF - Friends of Minerals Forum, discussion and message board

[Cesar M. Salvan]

Recently, my dear friend Jordi Fabre sent me a couple of samples from a new, really interesting find: native gold from a unknown deposit (as far as I know) in the region of Guelmim-Oued Noun in Morocco. The samples are small and the quality does not represent the overall quality of the specimens I've shown, but were selected as 'sacrificial' for analytical purposes. So, do not consider the pictures here as representative of the quality and beauty specimens recovered, which in most cases, shows well developed crystals.

The crystal habits and elongated, filiform shapes made some people question whether the samples were fabricated or not, and if the specimens are forgeries. Of course, with just pictures is not possible to determine any valid diagnostics. So, I was very happy when I received the samples for first hand study. They show textures from spongy to roughly crystalline, filiform and are accompanied by abundant iron oxides.

The identification of iron oxides shows both hematite and goethite, the last forming characteristic botryoidal textures. The iron oxides and gold have grown intimately and it is interesting that they are usually not mixed. Instead, they appear to be formed in two stages, one in which pulverulent hematite was deposited followed by growth of goethite layers.
Occasional quartz grains appear included in the material. The mixing of iron oxides with gold particles is so intimate that the samples showed an unusual enhancement of Raman signals of the former for the species (probably a solid state SERS effect, I will investigate that more)

The analysis of metal showed gold in the range 88.5 to 93%, with silver (3-6.4%) and traces of copper and lead. The composition is slightly variable depending on the iron oxide presence. Overall, in the studied samples there is no evidence of forgeries. No "wires", around which an unusually skilled scam artist grew gold crystals. The composition and mineralogy point clearly to native gold, at least in the analyzed samples.

Caveat: I would like to remind that inductive thinking is one of the weaker in Science. What does that mean? It means that if I analyze a sample from a deposit, I can speak only about that analyzed sample; I cannot make a general statement about all material purportedly mined in that deposit, especially considering the complete lack of geological context of the samples. That is why the concept of "representative sample" is important. In this case, the samples are only representative of a determined lot with similar characteristics and mineralogy.

[Tomasz Praszkier]

Hi,
Thanks for the data, They confirm microscopic observations that we did.
We never sent them for more analyses as there was never doubts that they are fake for several reasons. One reason is that they are overgrown by thick Mn-Fe oxides which additionally are overgrown by calcite. What's more, quartz crystals are frequently found growing on the gold, but also some gold is growing on the quartz (multistage quartz growth). I went through about 500 crystals under the microscope and there is nothing suspicious in them, besides the quality and enormous size for gold.
There are several habits of twins from the find - from very thin (pseudo-wire habit - sic! no wires!) to quite thick. Some of them are without visible crystal faces, and others are very sharp with great luster. Bigger crystals on the edges of the twins are more or less hoppered.
The specimen in the photo (by Mark Mauthner) is 4.5 cm tall, and shows quartz growing on a cluster of spinel law twinned golds.

[David K. Joyce]

Hello All, I have never had any doubts about the natural nature of the gold crystals from this locality. They are natural and I am glad to see the above detailed analytical work. However, I have seen many faked specimens. The makers make by taking loose natural crystals and affixing them to the surface of barren matrices. It appears that they coat or soak the iron oxide in a glue and sprinkle gold crystals and more iron oxides on the surface of the matrix. The specimens look very convincing at first glance but the gold crystals are only on the surface of the matrix, laying fairly flat but not protruding out of the matrix. I have also purchased perfectly natural gold specimens from this locality with gold crystals naturally protruding from the matrix. Caveat emptor!

David K Joyce

[Tomasz Praszkier]

Dave, you have seen faked specimens (glued to matrix) from this particular find? That would be interesting as I did not see any, and I saw the majority of the material. There are a lot of very small crystals and grains without crystal forms, so I guess this is "perfect" material to fake some specimens ...

[David K. Joyce]

Hi Tomek et al, I guess I’ll never know if it is the exact find but it looks the same. I examined all the specimens closely. All were specimens for sale from Moroccans, during Tucson, 2020. As I said, I did find bona fide specimens as well, but many were manufactured.

[Tomasz Praszkier]

If that was Tucson 2020 it can not be from this find. Most probably you are talking about golds from Bleida, They are also partly in gossan-type rocks associated with copper secondaries.

[David K. Joyce]

I bow to your knowledge of Moroccan minerals and localities. Still those were very similar specimens (no copper secondaries that I noticed) and people should be aware of it and the possibility that such specimens could be attributed to this newer find.-
David K Joyce

[Jordi Fabre]

The time helps to clarify things. Advancing of the right locality for these Golds, we already know the name of the closer village: Aouint Ighoman, and that some people use the name 'Aouint Ighoman Guelmim' for the labours where these wonderful Golds are collected.
There are a lot histories about it currently, telling that these labours are on an old mine. Will see in the future what real (or not ;-) are these histories.
Very few pieces with 'scorpion' crystals are on matrix there but some more on matrix with the scoriaceous type.
As a curiosity some piece have the Gold crystals on Calcite matrix.

[Cesar M. Salvan]

We keep studying the gold from Guelmim. The associated minerals tell us a history that, I hope, will be consistent with the geochemical context.
The gold is associated to a series of minerals, possibly related to the arid environment where the deposit is located:

-goethite/hematite. The association of gold with goethite is very common. The colloform goethite observed in the samples, mixed with gold layers is common in supergenic gold deposits. The iron oxides are also associated to aluminum hydroxides, silica and quartz grains.

- Chlorargyrite: Grains of AgCl are frequent in the sample. This could be surprising at first sight, but it is not at all. The Au is not alone in the Guelmim samples. Instead, the samples are composed with inhomogeneous gold-silver alloys, ranging from 1-5% silver to 28% silver. The richer silver regions of the samples also contains variable copper, up to 2%.
Silver, gold and lead are mostly mobilized in the form of halide complexes. as well as in the form of organic complexes. But geochemically, the halide complexes are really important. In an arid environment, fluids are very enriched in chloride. The formation of AgCl grains in the samples evidence the mobilization of silver (more reactive than gold) by the chloride-rich water.

- lead minerals: one peculiarity I found in the samples is the association of the gold with lead minerals: lead chloride (probably laurionite) and phosphohedyphane. Both are forming very small crystals associated to the goethite and the gold surfaces. Probably the origin of these minerals are related also to the arid environment, as lead is mobilized in the form of chloride complexes.

- copper-zinc alloy: In the gold surface are present exolutions, small inclusions and coatings of copper-zinc intermetallic compounds. The composition match with danbaite (CuZn2). It is interesting. I never found something similar in gold samples.