Linking mineral deposits to crustal necking: insights from the Western Alps

Although many studies link mineral deposit formation to rifting and hydrothermal processes, we present a study that focuses on the relationship between crustal necking and mineral deposit formation. Necking corresponds to the timing, location, and process of rift localization and abrupt crustal and lithospheric thinning. Although necking is well identified and described from present-day rifted margins and has been modeled numerically, little is known about the necking process and its possible link to ore deposit formation. We present observations from the Mont-Blanc fossil detachment system, one of the few exposed examples of a necking detachment fault. We show that fluids flowed along the fault zone and leached metals (mainly Pb and Zn). This process was associated with the hydrothermal breakdown of feldspar and biotite at temperatures of 200 °C and salinities ranging from 5 to 20 eq. wt% with a H₂O-NaCl (-KCl) composition. The resulting metal-rich fluids reacted with mainly carbonate-rich units to form Pb-Zn ore deposits in basement and sedimentary cover rocks. A direct link can, therefore, be demonstrated between fluid and reaction-assisted breakdown of silicates, metal transfer and trapping along detachment faults, and the overlying sedimentary rocks during necking. Similar ore deposits can be found throughout the inner External Crystalline Massif of the Western Alps, interpreted as the former necking domain of the Alpine Tethys. This leads to the suggestion that necking and Pb-Zn deposit formation may be closely linked, a hypothesis, if correct, that has the potential to predict additional Pb-Zn-Ba-F resources in rifts, rifted margins, and reactivated fossil rifted margins forming collisional mountain belts.