What mineral resources lie beneath the French soil?

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.

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.

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.

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.

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.

P‑A R. Air­borne geo­physics is a method that has been around since the mid­dle of the 20^th 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.

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.

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 2^nd stage of the inven­to­ry is implemented.

Anaïs Maréchal