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