290  PRE-CAMBRIAN    GEOLOGY   OF    NORTH    AMERICA. 
amounts  of  silica,  and  gruenerite  schist.  All  are  found  well  developed 
at  the  Helen  mine,  and  all  but  the  gruenerite  schist  have  been  found 
in  the  Lake  Eleanor  iron  range  also,  while  granular  silica  and  sider- 
ite  occur  in  large  quantities  in  every  important  part  of  the  range, 
though  small  outcrops  sometimes  show  the  silica  alone.  All  the 
rocks  of  the  iron  formation  contain  considerable  amounts  of  iron 
pyrites.  The  grained  silica  and  the  granular  silica  is  similar  in  cer- 
tain respects  to  the  jaspers  and  ferruginous  cherts  of  the  United 
States,  and  their  origin  is  believed  to  be  the  same.  They  differ  in 
being  often  soft,  pulverulent,  and  brecciated.  The  black,  graphitic 
slate,  forming  a  thin  sheet  just  under  the  iron  range  proper  west  of 
the  Helen  mine  and  at  other  points  in  the  region,  seems  closely  related 
•to  the  granular  silica,  being  composed  of  the  same  material  with  a 
large  admixture  of  carbon,  which  smears  the  fingers. 
Iron  ore  is  mined  in  the  Helen  mine,  and  this  mine  is  described  in 
detail.  The  ore  body  is  located  at  the  east  end  of  the  deep  Sayers  Lake 
basin,  partly  above  and  partly  below  the  old  water  level.  The  lake 
has  now  been  drained,  and  the  ores  appear  in  a  great  amphitheater 
opening  out  to  the  west.  The  rocks  immediately  associated  with  the 
hematite  are  siliceous  ore,  ferruginous  cherts,  or  grained  silica  rocks. 
These  are  mapped  as  immediately  surrounding  the  iron  ore  and  also 
as  forming  for  the  most  part  the  north  wall  of  the  amphitheater.  The 
east  wall  of  the  amphitheater  is  composed  of  iron  carbonate  which 
shows  gradations  into  siliceous  ore  and  into  hematite  ore.  The  south 
wall  is  composed  of  Wawa  tuffs. 
The  ores  are  believed  to  have  resulted  from  secondary  alteration 
of  an  original  iron  formation  consisting  mainly  of  iron  carbonate, 
grained  silica,  and  limestone,  in  part  interbedded  wTith  the  Wawa 
tuffs,  but  mainly  deposited  above  them.  The  iron  formation  and  the 
tuffs  were  folded  together,  with  the  result  that  the  tuffs  were  formed 
into  a  trough  underlying  the  iron  formation  and  the  iron  formation 
within  this  trough  was  folded  and  brecciated.  Percolating  waters 
then  altered  the  iron  carbonates.  Probably  the  chief  solvent  of  the 
carbonates  was  acidic  ferric  sulphate  or  sulphuric  acid  resulting  from 
the  oxidation  of  the  iron  pyrites,  which  are  found  in  considerable 
quantity  throughout  the  iron  formation.  The  ore  body  has  resulted 
directly  from  the  alteration  of  iron  carbonate,  the  oxidation  of  the 
iron  sulphide  having  yielded  but  little  ore.  The  oxidation  of  the  iron 
took  place  where  solutions  of  iron  carbonate  came  into  contact  with 
waters  bearing  oxygen. 
The  principal  areas  of  iron  formation  possibly  bearing  iron  ore  are 
at  Gros  Cap,  Sayers  and  Boyer  lakes,  just  east  of  the  Helen  mine, 
around  Brooks  Lake,  south  of  Long  Lake,  just  east  of  Goetz  Lake,  in 
Parks  Lake,  and  between  Parks  and  Kimball  lakes. 
