SPURR 
GARRET 
,*ND]       GEORGETOWN     MINING    DISTRICT,    COLORADO.  HI 
bhalcopyrite  ore  formed  the  original  vein,  and  that  the  vein  was  re- 
opened and  the  pyrite-siderite  filling  deposited  in  the  fissure  thus 
formed.  The  pyrite  has  been  deposited  in  such  abundance  that  it 
ordinarily  predominates  over  the  other  sulphides  in  the  present  veins. 
The  Centennial  ores,  which  come  under  this  general  type,  are  dis- 
tinguished from  the  above  in  being  chiefly  pyrite-chalcopyrite  ores, 
containing  less  amounts  of  galena,  zinc  blende,  and  siderite,  and  a 
greater  percentage  of  copper.  Bornite  is  a  characteristic  mineral  of 
the  ores  of  this  mine  at  times.     The  values  are  chiefly  in  gold. 
Fairly  high-grade  ores,  composed  of  chalcopyrite,  pyrite.  galena, 
and  zinc,  are  found  in  limited  quantities  in  the  Mint  mine  at  Empire, 
and  the  Ramsdale,  on  Lincoln  Mountain.  Here  also  the  values  are 
chiefly  in  gold. 
OCCURRENCE  OF  ORES   IN   THE  VEINS. 
Structure  of  the  lodes  and  ores. — Many  of  the  veins  of  the  district 
represent  the  filling  of  open  fissures  (the  Frostberg,  Terrible,  etc.). 
These  veins  vary  from  one-half  inch  to  6  or  7  feet  wide;    usually 
when   of  the  latter  width  they  consist  of  numerous  vein   branches 
looping   around   "horses"   of   country   rock.     In   places   the    fissure 
has  been  filled  completely  with  ore,  but  almost  invariably  a   layer 
of  comb  quartz  is  found  between  the  ore  and  the  wall  rock.     Many 
vugs  are  found  in  some  of  the  veins;    these  often  show  crustifica- 
tion.     Quartz,  barite,  siderite,  and  calcite  are  the  commonest  crys- 
tals  found   in   vugs,    although   galena    and    sphalerite    crystals   are 
not  uncommon.     In  many  cases  the  veins  have  the  nature  of  rubble- 
filled  fissures  in  which  angular  fragments  of  the  wall  rocks  have 
[been  cemented  together  by  ore,  thus  forming  a  breccia.     By  subse- 
quent movements  the  veins  have  sometimes  been  reopened  and  the 
pre  cement,  above  mentioned,  brecciated  in  turn  and  recemented  by 
ijpyrite,   siderite    (see   fig.    0),   or   quartz.     Many   good    examples   of 
brecciated  fragments  of  zinc  blende,  galena,  and  comb  quartz   in   a 
later  cementing  matrix  were  found  in  the  Terrible,  Frostberg,  Men- 
ipota,   Griffith,   Sceptre,   Sunburst,   and   other   mines.     Economically 
t|mportant  pyrite  acting  as  such  a  cement  is  found  in  some  of  the 
rold-bearing  veins,  as  the  Centennial.     In  the  Griffith- Annette  mines, 
t  par  the  Centennial,  intercrystallized  siderite  and  pyrite,  the  former 
predominant,  cement  the  antecedent  galena-blende  ores  and  make  up 
Jit  present  far  the  larger  part  of  the  vein.     Some  cases  show   that 
I  jater  than  this  the  siderite  itself  was  brecciated,  and  recemented  by 
i  |  dark-gray  jaspery  quartz,  as  in  the  Sceptre  and  Sunburst  mines. 
I(!  J  Many  veins  show  little  evidence  of  fissure  filling.     Others  show 
n  portions  crustification  and  the  cementing  of  angular  rock  frag- 
