GRANITE-BIMETALLIC    AND    CABLE    MINES,   MONTANA.  49 
northwest  end  of  the  Lake  stope;  it  is  about  3  feet  in  diameter  and 
is  completely  surrounded  by  coarsely  crystalline  calcite  containing 
quartz,  pyrite,  and  chalcopyrite  and  is  cut  by  veins  of  the  same 
material  from  1  to  3  inches  wide.  Similar  veins  cut  the  limestone  and 
more  sparingly  the  magnetite.  The  coarsely  crystalline  calcite 
grades  into  the  limestone  and  commonly  the  contact  is  frozen  so 
tightly  that  in  milling  it  is  necessary  to  treat  a  considerable  amount  of 
barren  limestone  with  the  calcite  ore. 
Fissures. — Fissures,  later  than  the  ore,  cross  the  ore  zone  at  several 
places.  On  the  tunnel  level,  250  feet  southeast  of  the  engine  station, 
a  fault  striking  N.  70°  E.  dips  to  the  east  about  70°.  East  of  the  fault 
the  southern  contact  of  granite  and  limestone  has  moved  about  18  feet 
to  the  north.  Other  fissures  cut  across  the  ore  zone  at  various  angles, 
but  the  displacement  along  them  is  not  great.  Most  of  these  fissures 
carry  a  few  inches  of  gouge,  but  are  not  mineralized.  There  are 
also  faults  of  earlier  age  than  these,  but  later  than  the  granite  intru- 
sion. It  is  not  possible  to  follow  these  earlier  fissures  in  the  coarsely 
crystalline  calcite  on  account  of  complete  recementati  m,  and  they  are 
probably  more  numerous  than  they  appear  to  be.  The  brecciation  of 
the  tongues  of  granite  is  of  the  same  age  as  this  Assuring,  which  prob- 
ably occurred  soon  after  the  granite  became  hard  but  before  it  became 
cold,  since  there  was  much  recrystallization  afterwards.  These  fis- 
sures are  healed  by  coarsely  crystalline  calcite  and  quartz;  the  princi- 
pal ore  bodies  do  not  appear  to  be  related  to  them,  however,  but  are 
altogether  irregular  in  outline. 
ORE    DEPOSITS. 
General  character. — The  ore  deposits  are  large,  irregular  bodies  of 
calcite  and  quartz,  containing  a  large  amount  of  mixed  iron  and  cop- 
per sulphides  and  iron  oxides.  These  are  confined  to  a  long,  narrow 
zone  of  limestone  and  calcareous  shale  that  trends  southeastward  and 
is  bounded  by  two  granite  intrusives,  the  walls  of  which  are  steeply 
inclined,  nearly  parallel,  and  from  80  to  360  feet  apart.  The  nonmet- 
allic  minerals  are  calcite,  quartz,  barite,  and  dolomite,  of  which  cal- 
cite is  most  abundant.  The  original  metallic  minerals  are  pyrite, 
chalcopyrite,  pyrrhotite,  arsenopyrite,  magnetite,  specularite,  and 
gold.  Marcasite  should  perhaps  be  placed  in  this  class,  also.  The 
secondary  metallic  minerals  are  hematite,  limonite,  manganese  oxide, 
siderite,  malachite,  azurite,  chrysocolla,  bornite,  chalcocite,  gold,  and 
copper. 
In  most  of  the  ore  of  the  lower  levels  calcite,  quartz,  pyrite,  and 
chalcopyrite    are    interlocking,    irregular    bodies,    which    apparently 
were  deposited  at  one  time.     When  a  magnet  is  passed  over  the 
crushed  ore,  there  is  nearly  always  a  separation  of  magnetite  and 
Bull.  315—07 1 
