THE  IDAHO  SPRINGS  MINING  DISTRICT,  COLORADO. 
By  J.  E.  Spurr  and  George  H.  Garrey. 
INTRODUCTION. 
A  preliminary  report  on  the  geology  and  ore  deposits  of  the  Georgetown  quadrangle, 
based  upon  work  done  in  the  vicinity  of  the  towns  of  Georgetown,  Silver  Plume,  and 
Empire,  appeared  in  1905. «  Last  summer  (1905)  Mr.  Spurr  spent  ten  days  and  Mr.  Garrey 
was  engaged  for  six  weeks  in  studying  mines  south  and  west  of  Idaho  Springs — that  is,  in 
the  northeastern,  previously  unstudied  portion  of  the  Georgetown  quadrangle.  The  follow- 
ing brief  report  presents  the  main  results  of  this  second  season's  work. 
Idaho  Springs,  which  is  on  a  branch  of  the  Colorado  and  Southern  Railroad,  lies  about 
40  miles  due  west  of  Denver,  in  the  valley  of  Clear  Creek,  near  the  junction  with  Chicago 
Creek.  It  is  the  largest  town  in  Clear  Creek  County  and  has  a  population  of  about  2,500.  b 
There  are  at  least  12  concentrating  mills  operating  at  Idaho  Springs,  which  is  the  milling 
center  for  the  district. 
TOPOGRAPHY. 
Idaho  Springs  has  an  elevation  of  about  7,525  feetc  above  sea  level.  From  this  altitude 
in  Clear  Creek  Valley  the  mountains  rise  by  gentle  slopes  to  elevations  as  great  as  11,000 
feet  a  few  miles  distant.  In  their  rounded  forms,  however,  the  mountains  here  contrast 
strikingly  with  the  rugged  peaks  near  Silver  Plume  and  Georgetown. 
The  topographic  map  of  the  Georgetown  quadrangle  on  the  scale  of  1  mile  to  the  inch, 
made  by  Frank  Tweedy  during  the  summer  of  1903,  is  now  in  print. d  This  map  and  the 
Idaho  Springs  special  map,  on  a  scale  of  1,000  feet  to  the  inch,  made  by  Pearson  Chapman 
during  the  summer  of  1904,  were  used  as  a  basis  for  the  geological  work. 
GEOLOGY. 
General  description. — The  rocks  near  Idaho  Springs  are  almost  identical  with  (hose  in 
the  vicinity  of  Georgetown.  The  oldest  formations  of  the  Rocky  Mountains,  presumably 
pre-Cambrian,  form  a  " complex"  comprising  the  main  mass  of  the  mountains.  These; 
rocks  include  schists,  gneisses,  basic  igneous  rocks  of  the  nature  of  horn  blend  it  es  and 
diorites,  granites,  and  pegmatites. 
All  these  rocks  are  cut  by  later  intrusive  bodies  of  porphyry fi  The  dikes  of  porphyry, 
which  vary  considerably  in  color  and  texture,  often  contain  angular  fragments  of  the  wall 
rocks  and  of  fine-grained  contact  phases  of  the  porphyry  itself.  They  are  much  more 
numerous  in  this  part  of  the  quadrangle  than  in  the  vicinity  of  Georgetown  and  Silver 
Plume.     That  the  porphyry  as  a  rule  is  earlier  than  the  veins  is  shown  by  the   fact  that 
aSpurr,  J.  E.,  and  Garrey,  G.  H.,  Preliminary  report  on  the  ore  deposits  in  the  Georgetown,  Colo., 
mining  district:  Bull.  U.  S.  Geol.  Survey  No.  200,  1905,  pp.  99-120. 
b  Census  report  for  1900. 
c Idaho  Springs  special  topographic  map,  by  Pearson  Chapman. 
d  Copies  of  tin*  map  can  be  obtained  from  the  Director,  U.S.  Geological  Survey.  Washington,  D.C. 
«  For  a  fuller  description  of  these  rocks,  see  Bull.  U.  S.  Geol.  Survey  No.  260,  1905,  pp.  101-106. 
35 
