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
Broken Hill ore deposit hornblende schists. The ore consists of massive, recrystallised sphalerite-rich, galena-sphalerite and galena-rich sulfide lenses in folded horizons, known as No. 2 Lens Formation and No.3 Lens Formation. Both have distinctive metal ratios and gangue minerals. The ore boundaries are parallel to the bedding planes. According to Gustafson, "There is evidence that the ore has been introduced into an old, already folded, rock and has replaced certain favourable layers of that rock, particle by particle, with large tonnages of garnet, rhodonite, fluorite, marmatite (and wurtzite), galena, and other minerals that constitute the orebodies." The Potosi-Footwall Gneiss is found on both sides