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What mineral resources lie beneath the French soil?

Pierre-Alexandre Reninger
Geophysicist at the French Geological and Mining Research Bureau (BRGM)
Mathieu Chevaillard
Matthieu Chevillard
metallogenic geologist at the French National Geological Survey (BRGM)
Key takeaways
  • BRGM is to carry out an inventory of mineral resource potential, in order to characterise the resources naturally present in France.
  • With this inventory, the scientists will be compiling a set of uniform data that is unprecedented, since the last inventory did not cover 30% of the areas with mining potential, and many metals that are now considered critical were not analysed.
  • The technique used, airborne geophysics, is unprecedented for this purpose and on this scale in France.
  • By combining three methods (magnetism, gamma spectrometry and electromagnetism), it will be possible to obtain accurate data quickly.
  • In a second phase, targeted areas of interest will be analysed in greater detail using geochemical and geophysical techniques on the ground. Other innovative techniques such as hydrogeochemistry, biogeochemistry and geophysical acquisition by drone will also be tested.

The eco­log­i­cal plan pre­sent­ed by the French gov­ern­ment in Sep­tem­ber 2023 has sev­er­al axes, one of which con­cerns secur­ing access to raw mate­ri­als. Lithi­um, cobalt and nick­el are essen­tial to the eco­log­i­cal tran­si­tion, but a small num­ber of coun­tries have the monop­oly. But what resources are avail­able in France? To answer this ques­tion, a new inven­to­ry of min­er­al resource poten­tial will be car­ried out by BRGM. It fol­lows on from the last inven­to­ry car­ried out between 1975 and 1992.

What are the challenges of this new mining inventory?

Matthieu Chevil­lard. Dur­ing the pre­vi­ous inven­to­ry, around 30% of prospec­tive areas (with min­ing poten­tial) were not cov­ered. The new inven­to­ry aims to cov­er the entire met­ro­pol­i­tan area of inter­est. This includes ancient moun­tain ranges (Mas­sif Armor­i­can, Mas­sif Cen­tral, Vos­ges, Mau­res, Ardennes, Cor­si­ca) and recent moun­tain ranges (the Alps and Pyre­nees). The chal­lenge is to cov­er areas that have nev­er been stud­ied, but also to re-eval­u­ate those already cov­ered dur­ing the pre­vi­ous inven­to­ry: in this way, we will be build­ing up an unprece­dent­ed uni­form data set.

Anoth­er impor­tant issue con­cerns the met­als we are look­ing for. The list of sub­stances con­sid­ered ‘crit­i­cal’ (on which Europe is large­ly depen­dent) is grow­ing every year. At the time, many of these sub­stances – used in renew­able ener­gies, elec­tron­ic devices and so on – had not been analysed. The aim was to assess the region’s poten­tial. Only 22 ele­ments were researched, com­pared with around fifty today. Most of the crit­i­cal met­als, such as lithi­um, rare earths, gal­li­um and ger­ma­ni­um, will be analysed this time.

Will you be using any new tools compared with the last survey?

Pierre-Alexan­dre Reninger. Yes, we will be using air­borne geo­physics, a tech­nique that has nev­er before been used for this pur­pose and on this scale in France. It pro­vides infor­ma­tion on the nature of the rocks present – pos­si­bly at a depth of sev­er­al kilo­me­tres – with­out any impact on the envi­ron­ment, unlike drilling, for exam­ple. It’s a bit like an MRI scan.

We will be using three dif­fer­ent geo­phys­i­cal meth­ods: mag­net­ism, gam­ma spec­trom­e­try and elec­tro­mag­net­ism. Each mea­sures dif­fer­ent prop­er­ties of the sub­soil, pro­vid­ing geol­o­gists with infor­ma­tion about its geo­log­i­cal struc­ture. The instru­ments are tak­en on board a plane for flat or hilly areas, or a heli­copter above moun­tain­ous ter­rain. We are using a unique high-res­o­lu­tion elec­tro­mag­net­ic sys­tem devel­oped by the Uni­ver­si­ty of Aarhus in Den­mark and oper­at­ed by SkyTEM. It takes the form of a large 300 m2 loop that we fly 50 metres above the ground. We have already suc­cess­ful­ly test­ed its use in a num­ber of projects in France and around the world.

Geo­physics pro­vides clues about geo­log­i­cal struc­tures at depth, but it needs to be sup­ple­ment­ed by sur­face measurements.

What tools will be used for surface analysis?

M. C. Ini­tial­ly, a region­al geo­chem­istry pro­gramme will be car­ried out on stream sed­i­ments. The method involves tak­ing sam­ples of sed­i­ment – par­ti­cles derived from the weath­er­ing of rocks – from small streams. Their chem­i­cal analy­sis in the lab­o­ra­to­ry is designed to mea­sure their met­al con­tent: 49 chem­i­cal ele­ments will be mea­sured simul­ta­ne­ous­ly. This data will give us an indi­ca­tion of the pres­ence of pos­si­ble min­er­al deposits, which will have to be stud­ied in greater detail lat­er on. We will be tak­ing new sam­ples from areas that have nev­er been sur­veyed, as well as re-analysing sam­ples from the first min­ing inven­to­ry, some of which are care­ful­ly pre­served at BRGM.

This method is not new; it was used for the first inven­to­ry. But thanks to improve­ments in ana­lyt­i­cal tech­niques, we are now able to detect chem­i­cal ele­ments in much low­er con­cen­tra­tions than at the time of the first inventory.

What are the advantages of the exploration methods that will be used?

M.C. In geo­chem­istry, detec­tion lim­its are 100 to 1,000 times low­er since the last min­ing inven­to­ry. For exam­ple, we are now able to detect con­cen­tra­tions of 0.2 ppm of cop­per or nick­el, com­pared with 10 ppm at the time. This tech­no­log­i­cal improve­ment also enables us to analyse new metal­lic ele­ments of major inter­est today.

P‑A R. The three geo­phys­i­cal meth­ods used pro­vide infor­ma­tion at increas­ing­ly greater depths: we will be able to obtain pre­cise data rang­ing from very close to the sur­face right down to the first kilo­me­tre or so. The elec­tro­mag­net­ic method offers the enor­mous advan­tage of pro­vid­ing 3D data on the struc­ture of the sub­soil. Above all, the acqui­si­tion times are unpar­al­leled by con­ven­tion­al geo­phys­i­cal tech­niques, which involve deploy­ing instru­ments on the ground. We can cov­er thou­sands of kilo­me­tres in a week!

M.C. These meth­ods also offer a par­tic­u­lar­ly advan­ta­geous cost/survey area/value ratio. This first scan of the whole of Met­ro­pol­i­tan France requires a sub­stan­tial invest­ment, but one that is rel­a­tive­ly rea­son­able giv­en the use­ful­ness of the data acquired. This data will con­tribute to a bet­ter under­stand­ing of the sub­soil and will be use­ful in oth­er sec­tors: envi­ron­men­tal and hydro­ge­o­log­i­cal stud­ies, nat­ur­al risk assess­ment, and infra­struc­ture stud­ies for region­al planning.

Why weren’t these tools used for the first mining survey?

P‑A R. Air­borne geo­physics is a method that has been around since the mid­dle of the 20th Cen­tu­ry. In France, the oil indus­try used it for sev­er­al sur­veys in the 60s and 80s over the Paris and Aquitaine Basins. Then France slowed down its oil and min­ing activ­i­ties con­sid­er­ably, and the use of this method came to a halt, unlike in major min­ing coun­tries such as Aus­tralia, Cana­da and Fin­land. Air­borne geo­physics was then rede­ployed as part of a pro­gramme to acquire sub­soil infra­struc­ture data. French Guiana was cov­ered in 1996 and the Armor­i­can Mas­sif in 1998. Sev­er­al more sur­veys have been car­ried out since 2010.

Since the first airborne geophysical surveys, computerised methods have made huge strides forward. Are these developments useful for the mining industry?

M.C. At the time of the first min­ing inven­to­ry, geol­o­gists some­times analysed geo­phys­i­cal and chem­i­cal data sep­a­rate­ly. Now they are inter­pret­ed joint­ly, thanks for exam­ple to the inno­v­a­tive pre­dic­tive map­ping tools devel­oped by BRGM. An arti­fi­cial intel­li­gence algo­rithm com­bines all the infor­ma­tion – geo­chem­istry, geo­physics, but also geol­o­gy and known deposits – to pro­duce min­er­al poten­tial maps for the var­i­ous sub­stances of inter­est. We are look­ing at the pos­si­bil­i­ty of going beyond 2D car­to­graph­ic analy­sis and mak­ing 3D pre­dic­tions, using elec­tro­mag­net­ic geo­phys­i­cal data.

Ultimately, more than 30 years after the last inventory, the new mining inventory does not seem to be based on any breakthrough innovations…

M.C. Air­borne geo­physics and geo­chem­istry are the only meth­ods that can cov­er large areas quick­ly, which is an essen­tial first step in the new inven­to­ry. They pro­vide an enor­mous amount of infor­ma­tion! In a sec­ond phase, tar­get­ed areas of inter­est will be analysed in greater detail using oth­er tech­niques. In addi­tion to the tra­di­tion­al geo­chem­i­cal meth­ods, inno­v­a­tive tools – some of which are still exper­i­men­tal – will be used. I’m think­ing, for exam­ple, of bio­geo­chem­istry using plants, or hydrogeochemistry. 

P‑A R. This more local scale is tra­di­tion­al­ly cov­ered by tools on the ground, deployed on foot. Many teams are cur­rent­ly work­ing on devel­op­ing reli­able geo­phys­i­cal tools on board drones. This is a fast-grow­ing field, with some tools already ful­ly devel­oped (such as mag­net­ism) and oth­ers still in the pro­to­type stage. Progress is so rapid that some will prob­a­bly be oper­a­tional by the time the 2nd stage of the inven­to­ry is implemented.

Anaïs Maréchal

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