spencer.]  THE    JUNEAU    GOLD    BELT,   ALASKA.  35 
where  the  Gold  Creek  placers,  which  first  called  attention  to  the 
region,  were  derived  from  it,  and  where  several  lode  mines  are  now 
being  developed.  Here  mineralization  is  rather  irregularly  distributed 
through  the  black  slates  in  a  zone  averaging  somewhat  more  than  800 
feet  in  width,  running  parallel  with  the  easily  recognized  outcrop  of 
the  greenstone,  which  forms  the  effective  foot  wall  of  the  vein  system. 
This  vein  complex  is  well  defined  from  the  middle  slope  of  Juneau 
Mountain,  where  recent  prospecting  has  been  in  progress,  through 
the  Ebner,  Humboldt,  Alaska- Juneau,  and  Alaska-Perseverance  prop- 
erties to  Sheep  Creek  divide  and  thence  southeastward  across  the  basin 
of  Sheep  Creek,  a  total  distance  of  about  5  miles. 
In  the  northwest  half  of  the  portion  of  this  zone  which  lies  in  Gold 
Creek  an  important  feature  is  the  occurrence  of  several  dikes  of 
highly  altered  and  sometimes  mineralized  rock,  commonly  known  as 
diorite  or  brown  rock.  The  original  character  seems  to  have  been 
gabbro,  but  the  rock  has  suffered  so  complete  metasomatic  alteration 
that  its  original  nature  is  seldom  observable. 
When  these  dikes  are  present  the  quartz  occurs  mostly  in  the  form 
of  oblique  gash  stringers,  which  often  recur  along  the  contacts 
between  the  dikes  and  the  slates.  Locally  auriferous  sulphides, 
mostly  pyrrhotite,  impregnate  the  bod>^  of  the  igneous  rock  between 
the  veinlets  of  quartz,  but  the  values  are  principally  in  the  veins. 
The  free-milling  ores,  which  are  confined  to  the  northwestern  mines 
of  the  lode  system  in  Gold  Creek,  are  said  to  contain  only  small 
amounts  of  silver,  the  bullion  sometimes  running  as  low  as  1  dollars 
of  silver  in  1,000. 
In  the  southeastern  part  of  Gold  Creek  the  brown  diorite  dikes  are 
smaller  and  more  irregular,  and  while  many  cross-cutting  stringers 
are  present  in  the  slate,  a  large  part  of  the  veining  takes  the  form  of 
interleaved  stringers  and  irregular  bunches  following  the  slaty  cleav- 
age. Here  only  a  small  amount  of  pyrrhotite  is  found,  the  sulphides 
being  principally  galena  and  sphalerite,  with  a  small  amount  of  chal- 
copyrite.  The  proportion  of  silver  varies  from  three  to  six  times  the 
gold  by  weight. 
Formerly  in  working  the  deposits  of  Gold  Creek  the  quartz  was 
picked  out  and  milled  b^  itself,  but  this  was  not  found  profitable,  and 
Jit  is  now  realized  that  the  only  possibility  of  economic  operation  lies 
in  the  direction  of  mining  and  milling  a  large  tonnage  of  unsorted 
[material. 
The  average  values  of  the  ores  mined  in  a  large  way  and  unsorted 
is  very  low,  but  the  strength  and  persistence  of  the  lode  system  leads 
to  the  belief  that  both  mineralization  and  values  will  continue  to  any 
depth  likely  to  be  reached  in  mining. 
i'j||  The  estimated  output  of  Gold  Creek  lode  mines  and  placers  to  date 
lis  $1,500,000. 
