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