102  CONTRIBUTIONS   TO   ECONOMIC    GEOLOGY,  1905. 
The  copper  deposits  of  Morenci  and  Clifton  have  been  investigated  in  detail  by  Lindgren, 
from  whose  report  a  the  following  notes  are  extracted : 
The  geographical  distribution  of  the  copper  deposits  is  practically  coextensive  with  a 
great  porphyry  stock  and  its  dike  systems.  The  deposits  occur  either  in  the  porphyry 
or  close  to  its  contacts,  or  along  dikes  of  porphyry  in  some  other  rock.  Areas  without 
intrusives  are  practically  barren.  This  intimate  connection  between  porphyry  and  ore 
is  as  important  here  as  it  is  at  Cananea.  The  two  important  mining  centers,  Morenci  andi 
Metcalf,  are  situated  on  the  main  contact  between  a  porphyry  stock  and  Paleozoic  lime- 
stones; elsewhere  the  porphyry  adjoins  granite  or  Cretaceous  beds. 
The  ores  consist  mainly  of  chalcocite,  malachite,  azurite,  chrysocolla,  brochantite, 
cuprite,  and  native  copper.  Covellite  and  bornite  are  practically  absent.  Brochantite, 
the  basic  copper  sulphate,  is  an  important  ore  mineral  present  especially  in  oxidized  veins 
in  porphyry.     It  occurs  intimately  intergrown  with  malachite. 
These  deposits  carry  copper  ores,  gold  and  silver  occurring  only  in  minute  quantities! 
The  great  bodies  of  oxidized  ores  are  in  limestone,  and  occur  along  bedding  planes  and 
dike  walls  as  irregular  or  rudely  tabular  masses.  The  ore  bodies  of  this  character  which 
are  derived  by  oxidation  from  low-grade  contact-metamorphic  limestones  are  now  nearly 
worked  out. 
Both  the  porphyry  and  the  contact  zone  about  it  are  traversed  by  fissure  veins,  which 
carry  pyrite  with  small  amounts  of  chalcopyrite,  zinc  blende,  and  molybdenite.  The 
principal  ore  mineral,  however,  is  chalcocite,  derived  by  secondary  sulphide  enrichment 
from  the  original  pyritic  ore. 
The  main  output  of  the  camp  is  now  derived  from  these  veins  and  chiefly  from  dissem- 
inated chalcocite  occurring  in  the  crushed  and  altered  rock  alongside  of  the  veins,  forming 
the  so-called  disseminated  ore.  An  oxidized  and  almost  barren  zone  extends  usually 
from  the  surface  to  a  depth  of  100  to  2(X)  feet.  Below  this  occurs  a  zone  of  copper  glance, 
which  descends  to  a  maximum  depth  of  500  or  (KK)  feet  below  the  surface.  Underneath 
this  enriched  zone  the  original  pyritic  low-grade  ore  is  found.  The  Humboldt  is  the  most 
important  vein,  running  northeast  and  southwest  through  Copper  Mountain. 
The  chalcocite  ores  of  these  veins,  though  of  low  grade,  are  extensively  worked  by  both 
tin-  Arizona  and  the  Detroit  companies.  The  Coronado  vein  differs  from  the  others,  being 
a  fault  fissure  with  a  1,000-foot  throw,  between  granite  and  quartzite,  followed  for  part 
of  the  distance  by  a  later  diabase  dike.  There  is  little  or  no  gangue  mineral.  The 
pyritic  ores  are  contained  in  altered  porphyry,  and  the  upper  few  hundred  feet  of  the  vein 
contain  an  enriched  zone  with  chalcocite  ore. 
These  deposits  are  thus  of  economic  importance  only  when  secondary  enrichment  has 
taken  place  by  the  action  of  descending  waters,  and  this  enrichment  zone  is  from  200  to 
400  feet  deep. 
The  Detroit  Copper  Company  and  the  Arizona  Copper  Company  are  working  the  low- 
grade  bodies  underlying  Copper  Mountain,  the  ores  carrying  from  3  to  5  per  cent  copper 
and  needing  concentration  before  smelting.  The  mines  of  this  character  include  the  Hum- 
boldt, Yavapai,  Arizona  Central,  Copper  Mountain,  and  West  Yankie.  The  mining  costs 
range  from  $1.50  to  $2  per  ton.  The  ores  are  of  the  character  mentioned  above,  be>ng 
decomposed  porphyry,  with  finely  disseminated  pyrite  and  glance. 
Mohave  County  yields  a  small  production  of  copper,  but  contains  no  large  mines. 
LESSER   DISTRICTS. 
Pima  County  has  one  large  producer— the  Imperial  Copper  Company,  operating  the 
Mammoth  mine,  formerly  known  as  the  "  Old  Boot."  This  mine  is  in  the  Silver  Bell  Moun- 
tains, an  isolated  range  northwest  of  Tucson,  having  an  elevation  of  about  3,000  feet  above 
sea  level.  The  ore  bodies  are  said  to  be  irregular  lenses  running  northwestward  in  parallel 
shear  planes.     There  are  prominent  gossan  cappings,  and  the  granite-porphyry  is  impreg- 
aProf.  Paper  U.  S.  Geol.  Survey  No.  43,  1905. 
