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I 
84  CONTRIBUTIONS   TO    ECONOMIC    GEOLOGY,  1905. 
by  narrow  fissures,  and  replaced  along  the  fissure  walls  by  copper  and  zinc  sulphides. 
Veins  in  the  porphyry  carry  copper  and  gold,  with  smaller  amounts  of  silver,  lead,  and  zinc. 
These  phenomena  of  contact  metamorphism  with  sulphide  formation,  but  without  much 
gold  and  silver,  are  repeated  from  one  end  of  the  Territory  to  the  other  along  the  bound 
aries  of  intrusive  stocks  and  laccoliths.  The  contact  deposits  are  almost  without  exception 
accompanied  by  the  formation  of  systems  of  fissure  veins,  rich  in  gold  and  silver,  whic 
chiefly  cut  the  intrusive  rock,  but  also  penetrate  across  the  contact.  The  veins  have  cer- 
t  ain  broadly  marked  characteristics.  They  are  regular,  with  absence  of  general  brecciation 
in  the  filling  quartz  predominates  and  is  apt  to  be  coarsely  crystalline,  sometimes  showing 
comb  structure;  a  banded  structure  by  deposition  is  rather  common.  In  some  cases 
there  may  be  doubt  as  to  the  age  of  these  veins,  but  enough  is  known  about  them  to  say 
that  as  a  group  they  antedate  the  late  Tertiary  volcanic  flows. 
Some  of  the  copper  and  lead  and  all  of  the  zinc  produced  from  the  ores  of  the  Territory 
have  been  derived  f rom  contact-metamorphic  deposits.  A  very  large  part  of  the  gold  taken 
from  the  Territory  has  directly  or  indirectly — by  placers — been  obtained  from  the  older 
fissure  veins  just  described.     Much  silver  also  comes  from  this  source. 
It  is  certain  that  the  development  of  these  deposits  can  not  be  correlated  with  the  ordi- 
nary circulation  of  surface  waters,  and  it  appears  to  the  writers  that  the  facts  very  strongly 
support  the  view  that  the  sulphides,  as  well  as  the  gold  and  silver,  were  contained  in  the 
intrusive  magma ;  the  baser  metals  were  most  readily  given  off  at  the  contacts  during  the 
intrusion;  the  gold  and  silver  deposition  was  in  the  main  a  later  process;  these  meta 
were  concentrated  in  fissures  which  were  opened  in  the  intrusive  rocks  shortly  after  thei 
consolidation,  and  the  veins,  like'  the  whole  intrusive  mass,  have  been  subjected  to  erosion 
At  the  time  of  vein  formation  the  presently  accessible  parts  were  deeply  buried. 
A  number  of  irregular  lead  deposits  occur  as  replacements  in  the  Ordovician  or  Devo- 
nian limestones  in  Sierra  County,  the  most  prominent  localities  being  Cooks  Peak,  Ilermosa, 
and  Kingston;  also  in  Grant  County,  where  the  principal  producer  is  at  Granite  Gap,  in 
the  Peloncillo  Range  In  most  cases  these  ore  bodies  are  close  to  masses  of  intrusive  J 
porphyries,  although  they  are  not  contact  deposits.  In  some  cases  they  have  assuredly 
been  caused  by  the  same  waters  which  filled  the  fissure  veins  shortly  after  the  congealing 
of  the  intrusive  magma. 
Several  of  the  large  silver  deposits  of  Grant  County  form  irregular  replacements  of  pre- 
Carboniferous  limestone  and  in  most  cases  are  closely  associated  with  dikes  or  larger  bodies 
of  porphyry.  At  Silver  City  the  ores  are  found  in  Silurian  limestone  immediately  below 
the  Devonian  shale.  At  Georgetown  and  Lone  Mountain  the  limestone  just  below  these] 
shales  is  likewise  the  ore  horizon.  At  Lake  Valley  Doctor  Gordon  found  that  a  thin  bed 
at  the  base  of  the  lower  Carboniferous  (Mississippian)  limestones  carries  the  ore.  In  all 
these  cases  the  principal  ore  is  the  chloride  of  silver,  although  partially  decomposed  galena, 
rich  in  sliver,  is  present  in  most  places. 
The  important  lead,  zinc,  and  copper  deposits  of  the  Magdalena  Range  near  Socorro 
have  been  studied  in  considerable  detail  by  Dr.  C.  II.  Gordon,  and  were  also  visited  by  the 
authors  of  this  paper.  The  sulphides  form  replacements  in  the  westward-dipping  lime- 
stone beds  on  the  western  side  of  the  range;  associated  minerals  are  magnetite  and  pyrox- 
ene. The  deposits  are  of  contact-metamorphic  origin  and  have  probably  been  induced 
by  the  intrusion  of  a  granite-porphyry  or  monzonitic  porphyry  which  outcrops  above  andll 
below  the  mines.  The  deposits  are  found  at  the  boundary  of  porphyry  and  limestone 
and  along  particularly  susceptible  horizons,  as  below  the  Silver  Pipe  limestones.  Replace! 
ment  by  sulphides  extends  up  for  several  hundred  feet  from  the  contact. 
In  some  instances  veins  cutting  porphyry  are  not  accompanied  by  contact  metamorphic 
deposits.  In  the  Jicarilla  district,  in  Lincoln  County,  narrow  seams  with  gold-bearing 
pyrite  have  furnished  considerable  placer  gold.  No  sediments  are  known  here.  At  White 
Oaks  well-defined  veins  and  irregular  sheeted  zones  carry  rich  auriferous  pyrite  with  some 
quartz.  Some  of  these  veins  extend  into  the  Cretaceous  sediments.  At  Steeple  Rock,  in 
western  Grant  County,  veins  in  porphyry,  carrying  gold  and  some  silver,  are  covered  by  later 
flows  of  rhyolite.     No  sedimentary  rocks  were  seen  here. 
