Lndgren.]    PRODUCTION    OF    GOLD    IN    UNITED    STATES    TN    1904.  37 
The  most  productive  of  the  late  Tertiary  veins  are  those  of  the 
fiuorite  class,  characterized  by  the  presence  of  fiuorite  and,  normally, 
also  of  tellurides.  These  dry  ores  yielded  at  least  $16,000,000  in 
Colorado;  $2,270,000  in  South  Dakota,  and  $1,200,000  in  eastern 
Montana,  a  total  of  $19,470,000.  They  often  occur  in  connection  with 
phonolitic  rocks. 
GOLD  DERIVED  FROM  COPPER  ORES. 
Auriferous  copper  ores  occur  throughout  the  United  States  in  de- 
posits of  widely  differing-  age  and  form.  The  copper  ores  of  Arizona, 
except  those  of  United  Verde,  contain  very  little  gold;  the  Michigan 
ores  are  practically  free  from  it.  A  few  copper  deposits  are  con- 
nected with  very  old  metamorphic  rocks,  and  are  probably  of  pre- 
Cambrian  age.  Among  these  are  the  Ducktown  deposits  of  Tennessee 
and  the  Encampment  mines  in  Wyoming;  but  these  contain  little 
gold.  The  California  deposits  of  the  Mountain  Copper  Company 
and  Bully  Hill  are  essentially  sheared  zones  or  replacement  veins  in 
altered  volcanic  rocks,  and  are  probably,  like  the  quartz  veins,  of  late 
Mesozoic  age.  The  gold  from  this  source  is  estimated  at  $272,000, 
and  has  remained  about  constant.  The  deposits  of  Montana  and 
Utah,  to  which  the  largest  part  of  the  remaining  production  from  this 
source  should  be  credited,  are,  as  a  rule,  replacement  veins  of  various 
forms  occurring  in  granite  or  early  Tertiary  igneous  rocks.  Their 
age  is  most  probably  early  Tertiary,  in  part  possibly  late  Tertiary. 
Montana's  yield  from  copper  ores  is  approximately  $1,100,000,  and 
this  amount  does  not  differ  greatly  from  that  of  previous  years.  The 
mining  of  auriferous  copper  ores  in  Utah  has  received  a  great  impetus 
during  recent  years,  and  the  production  of  gold  from  this  source  has 
steadily  increased.  It  is  estimated  at  $2,100,000  for  1904,  but  this 
includes  some  of  the  smelting  ores  from  Tintic,  which  are  apt  to  con- 
tain both  copper  and  lead.  The  gold  from  copper  ores  in  Colorado 
is  difficult  to  estimate,  as  no  exact  statistics  are  available,  but  it  prob- 
ably does  not  amount  to  $500,000.  Part  of  it  come  from  concentrates 
of  Tertiary  veins  in  San  Juan  County;  another  part  from  the  pyrite 
jores  of  Leadville,  which  form  irregular  bodies  in  sedimentary  rocks 
and  are  connected  with  intrusive  porphyries.  The  age  of  these  latter 
deposits  is  believed  to  be  late  Mesozoic.  Scattering  amounts  are 
j received  from  almost  every  mountain  county  in  the  State.  Idaho 
and  Arizona  derive  small  amounts  of  gold  from  copper  ores. 
The  amount  of  gold  derived  from  copper  ores  in  1904  should  be 
about  $4,300,000. 
