Dear Christophe,
As this is the ETSAP TIMES Forum, it would be more helpful for the other Forum members if you would refer to the TIMES attribute names, instead of using names that are specific to a certain user shell. It seems to me that you are using the VEDA user shell, and some model that has a scenario called "Scen_B_Extraction"?
"Mining technologies" are just exogenous trade processes in TIMES. As far as I can see, there is no reason whatsoever why they could not be specified with capital, operational and flow-related costs (NCAP_COST, NCAP_FOM, ACT_COST, FLO_COST, FLO_DELIV). I don't even remember what the "COST" parameter means in VEDA, and so I would really recommend to use the much more transparent TIMES cost attributes instead. So, I fully agree with you that it would make sense to use the NCAP_COST, NCAP_FOM, ACT_COST, FLO_COST, FLO_DELIV attributes instead of just "COST" (whatever that means).