MINERAL    DEPOSITS    OF    NEW    MEXICO.  81 
Two  principal  ages  of  ore  formation  have  been  recognized.  The  older  is  pre-Cambrian, 
while  the  younger  or  post-Cretaceous  age  includes  two  periods — one  late  Cretaceous  or 
early  Tertiaiy,  and  another  extending  from  middle  Tertiary  practically  to  the  present  time. 
PRE-CAMBRIAN  DEPOSITS. 
It  is  generally  admitted  that  the  principal  period  of  ore  formation  in  the  Cordilleran 
region  falls  within  age  limits  ranging  from  Jurassic  to  late  Tertiary.  That  this  was  pre- 
ceded by  an  earlier  period  is  definitely  shown  in  the  Black  Hills  of  South  Dakota,  where 
Cambrian  conglomerates  with  detrital  gold  overlie  auriferous  stringer  leads  and  shear  zones 
in  pre-Cambrian  schists.  It  has  long  been  suspected  that  some  Wyoming  deposits'*  belong 
to  the  same  period  and  according  to  a  recent  report  by  Spencer^  some  of  the  copper  depos- 
its in  the  Encampment  region  in  Wyoming  are  surely  of  pre-Cambrian  age,  while  others, 
including  the  principal  productive  mine,  were  most  probably  also  formed  in  that  period. 
The  recognition  of  pre-Cambrian  ores  in  New  Mexico  justifies  the  inference  that  the  pre- 
Cambrian  complex  of  Colorado  may  contain  similar  ores,  and  it  is  probable  that  a  recon- 
naissance of  Colorado  would  soon  elucidate  this  question. 
As  elsewhere  in  the  Rocky  Mountain  region,  a  profound  unconformity  separates  the 
little-altered  series  of  Paleozoic  sediments  beginning  with  the  Cambrian  from  a  pre-Cambrian 
complex  of  granites,  gneisses,  and  various  crystalline  schists.  In  northern  New  Mexico 
the  Carboniferous  rests  directly  upon  these  oldest  rocks,  but  the  Cambrian  is  present  in 
the  southern  part  of  the  Territory,  as  it  is  in  Colorado.  There  can  be  no  error  or  misun- 
derstanding in  referring  to  the  deposits  in  question  as  pre-Cambrian. 
The  first  definite  proof  of  the  existence  of  these  old  deposits  was  found  at  the  Hamilton 
mine  on  the  upper  Pecos,  20  miles  east  of  Santa  Fe.  The  deposit  contains  chalcopyrite 
and  zinc  blende  and  is  inclosed  in  a  somewhat  schistose  amphibolite.  Its  weathered  out- 
crop is  directly  and  unconformable  overlain  by  nearly  horizontal  Carboniferous  sandstones 
and  limestones.  The  weathering,  which  does  not  extend  to  a  great  depth,  was  certainly 
effected  before  the  deposition  of  the  covering  of  the  sedimentary  rocks. 
Many  similar  deposits  occur  in  the  pre-Cambrian  of  the  Santa  Fe  Range  all  along  up 
toward  the  Colorado  line.  At  one  of  them,  situated  north  of  Taos,  development  work 
on  a  large  scale  has  been  attempted  by  the  Frazer  Mountain  Copper  Company,  and  during 
1903  a  considerable  amount  of  copper,  containing  some  gold  and  silver,  was  produced. 
The  evidence  of  age  is  not  as  conclusive  in  the  latter  case  as  it  is  at  the  Hamilton  mine, 
but  it  is  very  strong. 
In  the  Hopewell  and  Bromide  districts,  in  Rio  Arriba  County,  narrow  stringer  veins  in 
altered  amphibolite  carry  copper,  gold,  and  in  some  places  silver,  both  in  the  quartz  gangue 
and  in  the  adjoining  wall  rock.  The  proof  that  these  deposits  are  pre-Cambrian  is  not 
wholly  conclusive. 
The  pre-Cambrian  deposits  contain  chiefly  pyrite,  chalcopyrite,  and  zinc  blende;  the 
country  rock  is  usually  an  amphibolitic  or  chloritic  schist  derived  from  old  basic  intrusions 
into  the  prevailing  granites  and  gneisses;  the  tenor  of  gold  and  silver  is  ordinarily  low- 
say  $3  in  gold  and  3  ounces  of  silver.  Several  types  may  be  distinguished  among  them: 
In  one  the  sulphides  occur  intergrown  with  the  hornblende  or  other  minerals  of  the  contain- 
ing amphibolite  or  gneiss  and  appear  to  have  been  formed  during  the  metamorphism  of  the 
jrocks.  In  another  the  ores  are  contained  in  more  or  less  irregular  or  lenticular  veins  in  the 
(schists;  quartz  is  the  gangue,  sometimes  accompanied  by  tourmaline  or  garnet.  In  still 
another  type  the  copper  ores,  accompanied  by  lenticular  streaks  of  quartz  and  siderite, 
appear  along  chloritic  shear  zones  in  amphibolitic  rocks. 
These  types  strongly  recall  the  description  of  the  Encampment,  Wyo.,  deposits  given  by 
iSpencer,c  and  in  their  general  appearance  they  also  resemble  those  upon  which  the  gold  and 
■copper  mines  of  the  Southern  Appalachian  States  are  working. 
a  In  the  Silver  Crown  and  Sweetwater  districts.    See  Lindgren,  W.,  The  gold  production  of  North 
America:  Trans.  Am.  Inst.  Min.  Eng.,  vol.  33,  p.  839. 
6  Spencer,  A.  C,  Prof.  Paper  U.  S.  Geol.  Survey,  No.  25,  1904,  p.  56. 
cOp.  cit.,  p.  53. 
Bull.  285—06 6 
