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IDS RUG 24'94 <br />Precambrian "pink gneiss" 3,2g5,200 ions <br />(end suhjacenl clay zone where presenQ <br />wur[lrred/wunincralized hangingwall rnc~ls <br />n/tercd/rnilteralizedJuolwall rnda <br />1=nntwall Gneiss Waste Rock 2,907,492 tons <br />742 No .001 P.06 <br />The numbers about indictAC [hat a higher relative percentage of unaltered and unmine~alizul <br />hangingw:;Il rocks (Santa Pe. "pink gneiss", and Clay) is proposed to be reutrned to t.l a "Ucep Wcst 1'il" <br />as hackfill than was originally removed from the pit (i.e.altered and mineralized finnWall gneiss}. 't'his <br />is Wrote clearly indicated in the ratios of the lotwages of these materials as follows: <br />[b'iginnl rnlio rJ'ttnnlterec!/wtmineralized <br />bnngingwull rock to fuuhvnll gneiss in <br />r~~nserl [.nr rll..-(lrca uj Ihc' "ntcp West Pit" <br />.b7 l0 1 <br />Ratio ujurtahered/wnnincra/iz'd hanRingsarrll <br />ruck to jonrwall gneiss in pr rased hackfill <br />materials jrorn the "Gneis !'il A~en" <br />1.38 to 1 <br />These ratios reveal that approximately twice as much unaltered and uttmincralized (i.~. nun-sulfide <br />hc:u'ing) rock is proposal to he returned to the "llccp Wcst Pil" as hackfill than wax Originally <br />amlaincd within the same area. <br />(Tcologic evaluation of proposed hackfill materials <br />Knowledge of Orc composition of proposed hackfill materials is uC importance due to ~hydn+lol;ic swdies <br />which indicate that a portion of the backfilled materials will saturate with water. In nsidcring <br />pulemial environmental intpau, lilhologies are best discussed in terms of the three pr viously discussed <br />categories: <br />(1) Quaternary alluvium/arlluviunt, Satt[a Pe Formation, Precambrian "I'ittk Gneiss", and Paull <br />~191ayronc combintxl. <br />foolwall gneiss non-mineralized or' mineralizul lu sub-ecrnt~mic grades. <br />~" (3) 1'rcu+mhrian firutwall gneiss minaralizul to ore gold grades (i.c. >.01R dunces Au per limy. <br />kc,_n"~irJ~o As previously discussed, rocks in Ca[egory 1 were not Significantly afft•,taed by Un: I drotherntal <br />mineralizing processes which formed the San Luis gold deposit. Sulfide concenu'atic ns arc nil to a ace <br />~~o C~ and inu•oduced mends a+necnnations are thus very low (data from original drill hole logs and from this <br />~'r study). Carbon:+tc in the lomt of secondary calcite is commonly present within thesq rucks which may <br />servo as a neuu'alizing buffer to potentially introduced acidic solutions. Acidification/ neuu'alirdion <br />tests anxluclod during original permitting on two samples of both the "pink gneiss" ¢ud the Santa Irc. <br />Ponuation indicate "levels of sulfur below the detection lhnits .. , no potential for acid production, rout <br />a moderate capacity fm acid neu[ralizing" (512K report). These resuhs are cnnsisten) with gwlufic <br />obsuvalions and indicate drat Category 1 rocks are not :+ potential environmental ha~arrl in [cans of <br />their metals content and acrid gctu;rating potential. Accordingly, chest rocks were nc(t xtudied iu detail <br />during this evaluation. <br />U. M. lunr.c Page 3 C: ulugr' nJ l'rnpocccf <br />Mny 1994 ~nckfill Wn.+7c /torA <br />