IRON    ORES    OF    THE    WEST.  199 
DETAILED   MAPPING  IN   UTAH. 
During  the  summer  of  1905  detailed  mapping  of  the  iron  ores  of  Iron  County,  Utah,  was 
begun  by  C.  K.  Leith,  with  the  assistance  of  E.  C.  Harder  and  F.  J.  Katz,  who  continued 
the  work  independently  during  the  fall.  The  iron  ores,  extending  in  narrow  and  discon- 
tinuous belts  for  a  distance  of  about  18  miles  northeast  and  southwest,  were  mapped  with 
plane  table  on  the  scale  of  250  feet  to  the  inch,  all  pits  and  exposures  (numbering  700  to 
800)  being  shown.  The  total  area  so  mapped  was  5|  square  miles.  Pits  and  tunnels 
were  measured  and  an  attempt  made  to  estimate  tonnage.  On  the  completion  of  the 
topographic  base,  the  geology  of  a  somewhat  wider  area  will  be  mapped  during  the  present 
winter  and  the  report  and  maps  will  be  submitted  at  an  early  date  thereafter. 
The  ores  appear  on  the  erosion  surface  at  or  near  the  contact  of  intrusive  andesite  lac- 
colites  with  limestone  and  sandstone  and  in  one  instance  conglomerate;  also  in  veins  in  the 
andesite  itself.  The  ore  is  clearly  a  replacement  of  the  limestone  or  an  infiltration  in  veins 
in  the  other  rocks.  Evidences  of  two  and  perhaps  three  periods  of  concentration  were 
observed.  Under  the  influence  of  intrusion  of  the  andesite  laccolites  iron  carbonate,  iron 
sulphide,  and  magnetite  developed  locally  along  contacts.  During  the  subsequent  erosion 
there  seems  to  have  been  a  further  concentration,  consisting  in  alteration  of  the  carbonate 
and  sulphide  to  hydrated  oxide  and  perhaps  further  replacement  of  limestone  by  the 
hydrated  oxide.  These  two  concentrations  have  not  yet  been  certainly  separated  and 
there  is  a  possibility  that  they  are  really  but  one.  The  last  and  most  conspicuous  concen- 
tration occurred  under  the  influence  of  later  lavas  which  were  poured  out  upon  the  eroded 
edges  of  the  series,  resulting  in  the  coarse  recrystalizalion  of  the  ores,  the  development  of 
magnetite  and  siderite,  and  the  deposition  of  apatite  and  quartz,  leaving  the  ores  substan- 
tially as  we  now  find  them,  though  in  the  subsequent  weathering  they  had  been  broken 
down  and  covered  with  desert  varnish. 
The  ore  at  the  surface  is  thus,  because  of  the  last  concentration,  predominantly  mag- 
netite and  hematite  containing  chalcedonic  quartz  and  a  considerable  amount  of  apatite  in 
crystals  sometimes  reaching  a  length  of  2  or  3  inches.  Beneath  the  surface  the  ore  becomes 
a  soft  blue  and  red  hematite  with  subordinate  quantities  of  magnetite.  The  average  per- 
centage of  iron  varies  widely  from  45  to  66.  A  large  tonnage  will  average  above  50  per 
cent.     The  phosphorus,  for  the  most  part,  averages  above  0.1  per  cent. 
The  depth  to  which  the  ores  extend  along  the  inclined  (30°)  contact  of  the  andesite  and 
limestone  is  not  yet  known.  Pits  have  shown  it  to  extend  for  150  feet  in  depth.  Along 
the  strike  the  ores  pinch  out  in  lens  fashion.  The  longest  lens  in  the  district  reaches  2,000 
feet. 
The  relations  of  the  ores  to  the  adjacent  rocks  are  complicated  by  faulting  of  block  type, 
characteristic  of  the  basin  region.  The  main  directions  of  the  fault  planes  are  a  little  west 
of  north  and  north  of  east.  The  planes  are  vertical.  The  district  lies  about  15  miles  west 
of  the  great  Hurricane  fault,  marking  off  the  high  plateau  region  from  the  basin  region  to 
the  west,  where  the  displacement  is  down  on  the  west  side. 
CONCLUSION. 
The  ores  of  the  Hartville  district  of  Wyoming  seem  to  have  certain  features  in  common 
with  Lake  Superior  ores,  but  much  work  remains  to  be  done  on  them  before  their  origin 
can  be  definitely  stated.  With  this  exception  the  iron  ores  of  the  western  United  States 
seen  by  the  writer  and  other  deposits  of  which  he  has  seen  reports  occur  along  or  near  the 
contact  of  limestones  with  intrusive  igneous  masses,  both  acid  and  basic,  often  laccolitic 
in  form,  and  are  frequently  associated  also  with  surface  eruptives.  The  ores  are  clearly 
replacements  of  limestone  and  to  a  minor  extent  vein  fillings  in  the  limestone  or  igneous 
rock.  Their  association  with  igneous  rocks  and  frequent  content  of  met  amorphic  min- 
erals point  clearly  to  their  development  and  concentration  under  the  influence  of  igneous 
rocks  and  in  some  cases  to  more  than  one  concentration.     The  precise  nature  of  the  influence 
