The weathering profile developed on the Birimian rocks of West Africa is, or looks, relatively simple. But its genesis is not. A few years ago, within a research team of IRD, I worked in the Tenkoto area (Senegal), already studied by P. Michel in the 60's and 70's and very adapted to examine the filiation of the ferricrete.
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With the recent easing of the lockdown in France, and before being able to return in the field overseas, three days ago I did a small field trip in my native geological backyard, around Villefranche-de-Rouergue, in Aveyron.
The Villefranche granitic massif and its periphery are rich in quartz veins with Pb-Zn-(Cu) mineralization, particularly along the famous Villefranche fault that separates the Variscan basement from the Aquitaine basin. La Capelle Bleys (LCB) leucogranite massif which stands out to the east, is located in a favorable position at the intersection of several major faults, including the Sillon Houiller, giving more diverse mineralization. This small zone was explored for uranium, tungsten, and fluorite in the 1970s the early 80s.
About 10 years ago with a group of prospector friends, we found there and described a suite of rare mineral species. Fractures and thin oxidized quartz-pyrite veins are filled not only by secondary uranium phosphates but also by species such as natrodufrénite, leucophosphite, meurigite, cyrilovite and kapundaite. This time I was lucky enough to find nice autunite and maybe libethenite, a new phosphate for the site to be confirmed by analysis.
In the early 2000s, during an exploration drilling program on the rim of the Goro deposit, our earthworks uncovered an old adit. It was dug by ‘cobaleurs’, those cobalt miners who were active in New Caledonia at the beginning of the 20th century. At that time, the Caillou was almost the only world producer of this rare ore, then the discovery of the Copper Belt and its rich sulfides and oxides Co deposits will rapidly reduce New Caledonian production.
The Co bearing phases are composed of minerals such as heterogenite and asbolane. These manganese-cobalt-(nickel) hydroxides are mainly found in the oxide ore (yellow laterite), just above the contact with silicate ore (saprolite). Their indurated forms were the target of the cobaleurs. They form encrustations along root systems that are particularly common when the oxide ore is close to the surface and would suggest that deposition of these minerals occurs particularly at the level of fluctuating water table.
Today, New Caledonia is a small producer, with cobalt being a by-product that is used to valorize its low-grade nickel ore processed on site at the Goro HPAL plant or exported. This ore is supposed to be 'clean ore' compared the one coming from some operations in the DRC.
In 2005, during a nickel exploration program in the vicinity of Kalgoorlie, I found the opportunity to visit the 132 North nickel mine. The small deposit located near Widgiemooltha, away from the famous Kambalda dome, is modest in terms of resources, but special from a mineralogical point of view.
Classically in a nickel deposit the secondary species are not varied and limited in number; arsenates such as Annabergite in sulphide-type deposits and silicates such as Népouite in laterite-type deposits.
Ernest H Nickel who studied the mineralogy of the mine exploited by WMC in the 90s identified more than fifteen secondary nickel species. This makes 132 N the most prolific deposit in mineral species for this metal.
In the supergene profile, the primary sulfides pyrrhotite-pentlandite-chalcopyrite are altered to violarite-pyrite, and then to nickel carbonates, topped by an Fe-oxides and silica gossan layer. Veins are filled by carbonates (Gaspéite, and rare Kambaldaite, Widgiemoolthaite), but also sulfates (like Hydrohonessite), silicates (Népouite) and chlorides. Few years ago, what was labeled Paratacamite has been identified as a new species, Gillardite - nothing to do with the former PM.
During a field trip in South Africa, I visited the famous Blue Mine in Springbok. This old mine is part of the O’Okiep copper district located in the Namaqualand Metamorphic Complex.
The Blue Mine is historically significant as it is considered as the first ‘mining operation’ in South Africa. The mineralised outcrops have been mined by the indigenous people of Namaqualand long before the arrival of European, but industrial mining started only in 1852 with this very mine.
Copper mineralisation is found in isolated bodies of mafic intrusive rocks like orthopyroxenite. This magmatic sulfide deposits are still difficult to classify with high Cu/Ni and Cu/S ratios and possible similarities with IOCG deposits. Economic sulfides are mainly chalcopyrite and relatively abundant bornite. On many deposits metamorphic and meteoric fluids have remobilized Cu to form sulfides such as chalcocite and covellite.
The Blue Mine name founds its origin in the secondary minerals formed in the oxidation zone. But not much carbonates such as azurite in this context, instead supergene species are mainly phosphates like cornetite and libethenite, with also silicates (chrysocolla).
Author - C. Gineste
Field notes, photos and memories of outstanding outcrops and rocks observed during my geological wanderings.
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