64  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  1905. 
the  Gold  Mountain  mining  district,  organized  January  25, 1868.  Mines  have  been  worked  in 
the  district  intermittently  since  that  time  and  several  mills  have  been  built,  none  of  which 
are  now  in  operation.  Old  residents  estimate  the  total  product  of  the  district,  including 
Old  Camp,  Tokop,  and  Lime  Point,  as  $500,000. 
Biotite-granite,  probably  of  post-Jurassic  age,  is  the  predominant  rock  in  the  vicinity  of 
Old  Camp.  It  has  many  facies,  the  prevailing  type  being  coarse  grained  and  pink.  Areas 
of  altered  Cambrian  limestones  and  schists  are  less  common  than  in  the  vicinity  of  Tokop 
and  Lime  Point.  Rhyolitic  and  basaltic  flows  of  Tertiary  age  cap  some  of  the  neighboring 
hills,  and  such  buttes  are  in  strong  contrast  to  the  rugged  granite  hills. 
Abandoned  mines  and  prospects  aro  numerous  in  the  vicinity  and  a  number  of  prospects: 
are  being  developed  2  or  3  miles  south  of  the  old  town.     The  Central   mine,  which  sup-  j 
plied  the  ore  used  in  the  mill  at  Old  Camp,  may  be  taken  as  a  type  of  the  ore  deposits  in] 
the  granite  of  this  region.     This  mine  is  situated  on  the  side  of  a  deep  gulch  1  \  miles  north 
of  east  of  the  town.     Five  tunnels  with  an  average  length  of  300  feet  pierce  the  granite,  and 
all  are  situated  on  a  single  vein  or  system  of  connecting  veins. 
The  feldspar  of  the  granite  within  20  feet  of  the  quartz  zone  is  considerably  kaolinized 
and  the  biotite  is  bleached  or  altered  to  a  sericitic  mineral.  The  quartz  vein,  or  rather  the 
zone  within  which  the  innumerable  connecting  quartz  veinlets  and  stringers  occur,  is  from  1^ 
to  6  feet  wide.  The  proportional  amount  of  quartz  increases  with  the  narrowing  of  the  zone.] 
The  ordinary  veinlets  are  from  2  to  5  inches  wide.  The  silicified  zone  also  contains  many 
ellipsoidal  and  globular  areas  of  quartz  which,  at  least  in  the  plane  of  observation,  are  inde- 
pendent masses. 
The  quartz  zone  in  places  changes  its  direction  90°  within  100  feet.  While  minor  post- 
mineral  faults  are  common  and  fault  the  zone  from  6  inches  to  7  feet,  the  major  changes 
in  strike  and  dip  are  evidently  original  structures.  The  form  of  this  zone  is  probably  due 
to  original  lines  of  weakness  similar  to  those  to  which  the  pitches  and  flats  of  the  south- 
western Wisconsin  lead  and  zinc  mines  owe  their  origin. 
The  quartz  is  white  and  translucent  or  slightly  smoky,  with  a  strong  vitreous  luster.  It 
contains  occasional  vugs  with  crystals  up  to  an  inch  in  length.  In  places  the  clear  quartz 
seems  to  grade  into  a  gray  chalcedonic  form,  the  deposition  of  which  by  water  can  scarcely  be 
doubted.  Isolated  crystals  of  pyrite,  with  very  rare  crystals  of  chalcopyrite  and  galena,  are 
embedded  in  the  quartz.  The  former  abundance  of  the  pyrite  in  particular  is  shown  by 
numerous  iron-stained  cavities  of  cubical  form.  Both  hematite  and  limonite  occur,  and 
where  these  are  abundant  the  gold  values  rise.  An  occasional  malachite  stain  and  aJ 
cerussite  coating  are  present.  Manganese-dioxide  dendrites  occur  in  the  granite  and  are 
probably  derived  from  the  alteration  of  the  granite  rather  than  from  a  decomposing  vein 
mineral.  Horn  silver  is  reported  to  be  present  in  small  amounts,  but  was  not  seen.  The 
ore  is  free-milling,  the  arrastres,  now  abandoned,  having  saved  about  75  per  cent  of  the 
assay  value. 
Films  of  a  bluish-white  chalcedony  were  noted  at  a  number  of  places.  Some  chalcedony 
was  deposited  prior  to  the  oxidation  of  the  pyrite,  while  some  was  deposited  after  partial  or 
complete  oxidation. 
While  the  quartz  is  rather  similar  to  quartz  of  pegmatitic  origin  in  the  district  and  the 
form  of  the  ore  deposited  rather  suggests  that  of  a  pegmatitic  dike,  the  apparent  gradation 
into  chalcedony  is  strongly  against  such  a  view.  Water  appears  to  have  rilled  a  most  com- 
plex zone  of  fracture  with  quartz  and  sulphides,  the  gold  probably  originally  being  con- 
tained in  pyrite.  .Later  oxidizing  waters  attacked  the  sulphides  and  altered  them  to  oxides 
and  carbonates,  setting  the  gold  free.  These  waters  apparently  carried  some  silica  in  solu- 
tion, which  was  deposited  as  chalcedony. 
The  mines  in  the  vicinity  of  Old  Camp  are  situated  near  water  sufficient  for  mining  and 
milling  purposes.  Wood  for  fuel  and  mining  timber  is  standing  within  2  or  3  miles.  The 
railroad  terminus  at  Goldfield  is  35  miles  distant. 
Oriental  Wash. — Several  prospects,  now  abandoned,  are  located  16  miles  south  of  Lidal 
on  the  western  border  of  the  area  surveyed.     The  hills  in  which  these  prospects  occur  are 
