The gold from Guelmim-Oued Noun (Morocco)

The recent launching to the mineral specimen market of the new, interesting, native gold samples from the Guelmim province in Morocco, caused keen reactions, but also mixed enthusiasm with some doubts about its natural origin.

The good quality of specimens and the lack of specific information about the deposit of origin raised doubts, so Jordi from Fabre Minerals sent to the lab a couple of samples for evaluation.

Of course, the samples are not representative of the general quality of the specimens on sale; instead, are more «sacrificial» samples for the study. But, still, are beautiful. The samples form frequent filiform structures, with textures from spongy to roughly crystalline. Occasionally it forms beautiful aggregates of distorted octahedra. The crystal habits and elongated, filiform shapes made some people question whether the samples were fabricated or not, and if the specimens are forgeries. The examination under stereomicroscope of the samples shows that are very likely natural (native) gold.

The gold are accompanied by iron oxides (a common association). The analysis showed both hematite and goethite.

The goethite covers the gold forming botryoidal textures, and also are intergrown. Interestingly, the goethite and hematite appears in different stages of the mineral formation.

Goethite on gold
Detail of the gold sample

The goethite and hematites were identified by Raman spectroscopy. It is interesting that the Raman signal of the iron oxide minerals are unusually intense. This could be explained by intimate mixing with gold particles that could promote the Raman signal (SERS).

Goethite (rightmost side) and hematite (middle and left side of the sample) on gold.

The analysis of the gold shows an intermediate composition between pure gold and electrum (Au with >20% Ag). Formally is silver-bearing gold, with a range between 5 and 6.5% Ag. The metal also contains traces of copper (<0.2%) and lead (<1%).

Goethite (upper side) and hematite (lower side) on gold
Element composition of the bulk sample

The detailed study of the sample by SEM-EDS showed a more complex mineralogy and composition, which can be summarized by the following findings:

  • Au-Ag composition is not uniform in the samples, showing zonation and even segregation of silver-rich alloy and silver grains.
Complexity of the sample composition. The lead in sample is associated to iron oxides, probably in the form of sulfate grains included in the oxide. Calcium carbonate is also associated to the iron oxides, and the gold-silver alloy shows a very variable composition and even segregation of silver grains.
  • Lead is associated to the iron oxides in the form of small grains of lead sulfates and lead carbonates. Lead-gold alloy grains (anyulite) are not discarded and are under study.
  • Chlorargyrite was found associated to calcium carbonates and iron oxides, in the form of microscopic grains.
  • metallic copper grains, Cu-Zn alloy and copper-zinc carbonates were also found and are under study.
SEM image of the gold sample
Spectrum of AgCl grains found associated to CaCO3

Overall, the studied samples are very likely natural, and the diagnostic is Ag-bearing gold with goethite and hematite. There are no evidences of manipulation or forgery, at least in the studied samples.

Gold composition inhomogeneity

Gold-silver composition of the samples are not homogeneous, showing strong variations in the composition, and even boundaries between zones with different siver content. The richer silver zones also show variable copper content.

Identified associated minerals

Goethite and hematite are the main associated minerals to gold, and this is a relatively common association in native gold. The colloform goethite layers with gold grains and other minerals is expected. Other gangue mineral associated to the gold is calcite, forming layers intermixed with iron oxides. The hematite is probably formed by dehydration and aging of the goethite.

The chlorargyrite (AgCl) is a very interesting mineral found in the gold, particularly observed associated to the goethite layers. The presence of chlorargyrite is explainable due to the arid conditions of the deposit environment. Gold and silver are mobilized in the form of halide complexes; the silver, due to its higher reactivity, dissolves first, so the silver chloride could be consistent with the slow mobilization of the metals due to fluids enriched in chloride because the arid environment.

A peculiarity of the deposit is the presence of lead minerals associated to gold. This could be related also to the arid environment of the deposit. In the samples we found a lead chloride mineral (possibly laurionite) and phosphohedyphane (calcium-lead chlorophosphate) in the form of small crystals attached to the gold surface and associated to the iron oxides.

Small laurionite acicular crystals on gold.
Phosphohedyphane crystal attached to gold surface

Finally, the most interesting find: inclusions, coatings and exolutions of copper-zinc alloy (danbaite, according with the composition).

https://www.fabreminerals.com/include/Moroccan_Gold-intro/gold_study-1.pdf

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