MINERAL    DEPOSITS    OF    NEW    MEXICO.  85 
LATER    SERIES. 
The  fissure  veins  in  volcanic-flow  rocks  are  the  most  recent  of  the  epigenetic  deposits  of 
New  Mexico.  These  surface  lavas  are  ordinarily  easily  recognizable  as  such  by  their  struc- 
ture, their  bedding,  and  the  topography  of  the  terranes.  They  consist  of  an  older  scries  of 
basalts,  rhyolites,  and  dacites,  less  commonly  andesites,  which  cover  the  eroded  surface  of 
the  intrusive  rocks  and  the  eruption  of  which  took  place  in  middle  or  late  Tertiary  time. 
Still  later — in  part  Quaternary — is  a  series  of  basalt  flows  which  is  not  known  to  harbor 
any  mineral  deposits.  The  surface  lavas  cover  but  a  small  part  of  New  Mexico.  The  earlier 
flows  formed  the  largest  volcanic  area  of  the  Territory,  and  connect  with  the  vast  lava  fields 
of  eastern  Arizona.  They  extend  through  northern  Grant,  southern  Socorro,  and  northern 
Sierra  counties,  though  not  without  interruption.  Smaller  areas  of  them  are  found  else- 
where. 
To  begin  from  the  north,  the  first  representatives  of  the  fissure  veins  in  surface  lavas  are 
found  at  Red  River  in  Taos  County,  where  they  cut  rhyolite  flows  and  contain  gold  with 
some  silver  in  quartz,  calcite,  and  fluorite  gangue.  Pyrite  and  other  sulphides  are  also 
present.  Farther  south,  at  Bland  or  Cochiti,  in  Sandoval  County,  are  gold-bearing  veins 
concerning  the  age  of  which  there  is  some  doubt,  but  which  probably  belong  to  this  class. 
The  gold,  accompanied  by  silver  sulphides,  is  present  in  minute  subdivisions  in  fine-grained 
almost  chalcedonic  quartz.  Further  representatives  are  the  Rosedale  veins  of  gold-bearing 
quartz  occurring  in  the  rhyolite  flows  of  southern  Socorro  County,  and  the  Chloride,  Phil- 
lipsburg,  Grafton,  and  Hillsboro  veins  in  Sierra  County,  which,  according  to  Dr.  C.  H.  Gordon, 
cut  andesite  flows  and  tuffs.  Most  important  of  all  are  the  deposits  of  the  Mogollon  Range, 
in  the  southwest  corner  of  Socorro  County,  which  are  believed  to  have  produced  several 
million  dollars  in  gold,  silver,  and  copper.  They  are  brecciated  fissure  deposits  of  large  size, 
carrying  finely  divided  gold  and  silver  bearing  sulphides  in  a  gangue  of  quartz  and  calcite, 
which  has  partially  replaced  the  breccia  fragments.  At  the  Cooney  mine  copper  minerals, 
mainly  bornite  and  chaicocite,  are  associated  with  quartz,  calcite,  and  fluorite  gangue. 
There  are  indications  that  the  vein-forming  action  during  this  last  epoch  has  persisted  up 
to  a  very  recent  time.  At  Socorro  a  system  of  silver-bearing  seams  of  barite  cuts  across 
rhyolite  at  the  base  of  the  escarpment  of  Socorro  Mountain,  along  which  hot  springs  of  con- 
siderable volume  now  issue.  The  Torrance  mine  on  this  belt  is  said  to  have  produced 
$200,000.  At  Ojo  Caliente,  Rio  Arriba  County,  a  small  silver  and  gold  bearing  vein  with 
fluorite  and  barite  gangue  appears  to  be  directly  connected  with  a  hot-spring  deposit  of  tufa 
laid  down  comparatively  recently  on  the  present  surface. 
Late  Tertiary  veins  are  confined  to  the  lava  flows  of  the  same  age,  and  there  is  strong 
evidence  that  they  were  deposited  by  hot  ascending  waters  shortly  after  the  consolidation 
of  these  lavas.  The  deposits  have  certain  broad,  common  characteristics;  gold  and  silver 
are  contained  in  finely  divided  form,  and  the  former  metal  has  rarely  been  concentrated  into 
important  placer  deposits. 
Fine-grained,  in  places  chalcedonic  quartz  is  characteristic  of  many  of  them,  as  is  often  a 
barite  and  fluorite  gangue.  Brecciation  of  the  walls  is  common,  while  regular  comb  or 
banded  structure  is  rarely  encountered.  Altogether  the  veins  bear  evidence  of  having  been 
formed  close  to  the  surface  and  of  having  suffered  little  erosion  since  the  time  of  their  depo- 
sition. A  notable  quantity  of  the  gold  and  much  of  the  silver  produced  by  the  Territory 
has  been  furnished  by  these  veins,  though  their  importance  has  not  equaled  that  of  the  older 
Tertiary  series  connected  with  the  deep-seated  intrusives. 
The  relations  of  the  New  Mexican  post-Cretaceous  metalliferous  deposits  to  igneous  rocks 
may  be  concisely  formulated  as  follows: 
1.  Intrusions  of  acidic  porphyries  and  granites  as  stocks  and  laccoliths  normally  produced 
metamorphism  and  accumulation  of  sulphides  of  copper,  lead,  and  zinc,  with  some  gold  and 
silver,  in  favorable  calcareous  strata  along  the  contact  planes. 
2.  Shortly  after  the  congealjng  of  the  intrusive  magmas  fissure  systems  were  developed 
in  the  stocks  and  laccoliths,  and  in  these  fissures  rich  gold-  and  silver-bearing  veins  were 
normally  deposited. 
