Ore genesis accumulation of hematite gravels called "pisolites" which form channel-iron deposits. These are preferred because they are cheap to mine. Lead-zinc deposits are generally accompanied by silver, hosted within the lead sulfide mineral galena or within the zinc sulfide mineral sphalerite. Lead and zinc deposits are formed by discharge of deep sedimentary brine onto the sea floor (termed "sedimentary exhalative" or SEDEX), or by replacement of limestone, in skarn deposits, some associated with submarine volcanoes (called volcanogenic massive sulfide ore deposits or VMS), or in the aureole of subvolcanic intrusions of granite. The vast majority of SEDEX lead and zinc deposits
Silver Valley (Idaho) the Coeur d’Alene District veins. Galena is present in veins throughout the district. Sphalerite ((Zn,Fe)S) is the second most important ore mineral, present in at least small amounts in most veins. The sphalerite is an important source of zinc throughout the ore belts. The third most abundant ore mineral is tetrahedrite (Cu,Fe)SbS. However, the tetrahedrite is responsible for the majority of the silver produced by the Silver Valley, as it’s the tetrahedrite interspersed in the galena of the district that makes it so argentiferous. There are two alpine ski areas in the Silver Valley, both easily accessible from I-90. Lookout