THE  GOLDEN  CURRY  MINE,  ELKHORN,  MONTANA"
GEOLOGY

An  interesting  deposit  of  magmatic  gold-  and  copper-bearing  pyrrho-
tite (non-nickeliferous,  however)  occurs  in  a  marginal  "basic"  segre-
gation of  the  Boulder  quartz  monzonite  batholith.  The  deposit  thus
differs  from  the  normal  type  of  magmatic  pyrrhotite  deposits  in  respect
to  the  geological  relations  and  the  character  of  the  mother  rock.  The
mineral  composition,  the  inclusion  of  the  ore  in  a  subsilicic  rock,  and  the
relatively  unaltered  condition  of  the  ore  and  country  rock,  give  this
deposit  the  characteristics  of  a  magmatic  ore.  It  is  possibly  the  unmeta-
morphosed  equivalent  of  the  lenses  of  pyrrhotite  in  "basic"  layers  of
gneiss,  and  is  an  indication  that  certain  of  these  puzzling  occurrences  are
altered  magmatic  segregations.
The  Boulder  batholith  is  well  known  to  students  of  ore  deposits
because  it  was  accompanied  and  followed  by  extensive  mineralization,
and  especially  because  it  encloses  the  enormous  copper  deposits  at  Butte.
The  Golden  Curry  mine  at  Elkhorn  is  in  a  region  of  intense  min-
eralization,54 which  occurred  in  and  near  the  roof  of  the  batholith.  In
this  property  there  are  two  types  of  ore  deposits:  (i)  a  contact
deposit  between  quartz  monzonite  and  limestone,  consisting  of  magnetite
and  some  chalcopyrite  accompanied  by  garnet;  (2)  the  magmatic  pyrrho-
tite deposit.  The  latter  occurs  250  feet  from  the  contact  mentioned
above,  as  the  sulfid  rich  portion  of  a  lens  of  fresh  monoclinic  pyroxene,
pyrrhotite,  and  chalcopyrite.  This  is  surrounded  by  a  border  zone,  which
grades  into  the  normal  quartz  monzonite  by  the  addition  of,  first,  pla-
gioclase,  then  hornblende,  quartz,  and  the  accessory  minerals  of  the
quartz  monzonite,  and  finally  biotite  and  orthoclase.  According  to


53  Knopf,  A. — A  magmatic  sulphide  ore-body  at  Elkhorn,  Mont.     Econ.  Geol.
