38  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  1905. 
of  parallel  faults  with  offsets  of  about  6  inches.  A  pegmatite  dike  1  foot  in  width  in  the 
Silver  Horn  mine  is  similarly  step  faulted.  As  these  transverse  faults  seldom  cause  diffi- 
culties in  mining  they  are  of  little  economic  importance  except  when,  as  occasionally  hap- 
pens, they  are  themselves  mineralized  and  their  intersections  with  the  main  vein  produce 
pay  shoots. 
Very  well-marked  and  comparatively  recent  movements  have  taken  place  along  some 
of  the  main  veins  and  in  some  cases  have  crushed  and  mixed  the  ore  with  so  much  tritu-i 
rated  rock  and  gouge  that  the  resulting  mass  is  of  little  value,  although  the  uncrushed  ore 
streak  might  have  been  worked  with  profit. 
MINERALOGICAL  CHARACTER   OF   THE  ORES. 
The  chief  gangue  minerals  accompanying  the  ores  are  quartz,  siderite,  barite,  calcite, 
rhodochrosite,  and  magnesite.  Willi  the  exception  of  quartz,  which  is  the  conspicuous 
gangue  mineral  of  the  district,  these  minerals  are  found  only  in  rare  instances  and  in  small 
amounts. 
The  principal  on*  minerals  found  in  the  mines  to  the  south  and  west  of  Idaho  Springs 
are  pyrite,  galena,  sphalerite,  and  chalcopyrite,  and  they  are  pit  sent  in  greatly  varying 
proportions.  The  values  are  principally  in  gold,  silver,  lead,  zinc,  and  copper.  The  gold, 
although  sometimes  free,  is  usually  combined  with  pyrite  and  chalcopyrite.  In  the  area 
to  the  north  of  Idaho  Springs  the  ore  is  mainly  cupriferous  pyrite  and  quartz,  while  to 
the  south  and  west  of  the  town  galena  and  some  sphalerite  usually  accompany  the  pyrite 
and  quartz  in  the  gold-bearing  ores.  As  in  the  ores  of  the  Georgetown-Silver  Plume  area, 
the  silver  generally  accompanies  galena  and  sphalerite,  and  when  in  quantity  is  usually 
associated  with  tetrahedrite  ("gray  copper "«)  poly basite,  tennantite,  or  argentite.  The 
galena  is  both  argentiferous  and  auriferous,  as  is  also  the  sphalerite  to  a  much  less  degree. 
In  the  vicinity  of  the  Lamartine  mine,  which  lies  about  halfway  between  Idaho  Springs 
and  Georgetown,  the  or<  s  are  chiefly  silver  bearing,  and  galena  and  sphalerite  predominate. 
For  example,  in  the  Lamartine  mine  itself  the  ratio  of  silver  to  gold  has  averaged  68  to  1 
by  weight.  East  and  northeast  from  the  Lamartine  the  proportion  of  gold  increases  until 
some  of  the  mines,  which  have  cupriferous  pyrite  and  quartz  veins,  such  as  the  Centurion, 
produce  practically  no  silver. 
FORM  OF  OCCURRENCE  OF  THE  ORE  DEPOSITS. 
Nearly  all  the  ores  occur  along  fissures  which  are  younger  than  the  porphyry,  as  well  as 
the  older  rocks.  Most  of  the  prominent  veins  in  the  northeastern  portion  of  the  quad- 
rangle are  parallel  to  the  porphyry  dikes,  either  along  the  wall  or  a  short  distance  away 
from  the  dike.  This  close  relation  of  the  vein  fissures  to  the  dikes  may  be  due  to  the  fact 
that  the  contact  of  the  porphyry  with  the  other  rocks  constituted  planes  of  easy  move- 
ment, and  the  intrusion  of  the  dikes  may  also  have  made  the  adjacent  rocks  more  suscep- 
tible to  fracturing;  or  the  veins  may  have  followed  the  same  planes  of  weakness,  or  planes 
of  the  same  system,  along  which  the  porphyry  dikes  were  earlier  intruded.  In  a  few  cases, 
however,  for  example,  in  the  Gomer  and  Centurion  mines,  the  veins  cut  transversely  across 
the  porphyry  dikes.  The  general  trend  of  the  main  veins,  as  well  as  of  the  principal  porphyrj 
dikes,  is  from  southwest  to  northeast. 
The  fact  that  the  ore-bearing  zone  is  coextensive  with  the  belt  of  porphyry  intrusion  is 
significant. 
TYPES   OF    VEINS. 
The  veins  in  the  vicinity  of  Idaho  Springs  may  be  separated  into  several  types  depend- 
ent on  the  grouping  of  the  separate  fracture  zones.  One  type  consists  of  a  single  or  several 
parallel  well-defined  veins.  These  usually  occur  along  or  near  and  parallel  to  the  contact 
of  porphyry  and  the  older  rocks.  Another  type  has  a  main  or  master  vein  from  which 
smaller  veins  and  veinlets  branch.     These  smaller  veins  may  in  turn  successively  subdivide 
"The  polybasite  and  tennantite  are  also  called  "gray  copper"  by  the  miners. 
