134  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  1904.         [bull.  260 
Per  tod  of  mineralization. — At  Goldfield  the  ores  occur  in  bott 
rhyolite  and  andesite,  showing  that  tlie  mineralization  occurred  sub 
sequent  to  the  eruption  of  both  lavas.  At  Gold  Mountain  the  ore 
evidently  were  formed  after  the  eruption  of  the  rhyolite,  and  a 
Tonopah  the  eruption  of  the  earlier  (dacitic)  rhyolite  was  follows 
by  a  period  of  mineralization  which  produced  veins  showing  fre 
quently  a  larger  proportion,  of  gold  than  the  locally  more  importan 
veins  whose  formation  followed  the  eruption  of  the  earlier  andesih 
There  is,  therefore,  the  possibility  that  the  Goldfield  deposits  air 
identical  in  origin  with  the  later  series  of  veins  at  Tonopah.  Indeec  i 
there  are  at  Tonopah,  in  one  place  at  least,  mineralized  quartz  reej 
m  rhyolite  tuffs  which  have  the  same  peculiar  characteristics 
those  of  the  Goldfield  reefs,  and  these  Tonopah  deposits  have  afforde 
moderate  assays  showing  gold  and  no  silver. 
Nature  ami  origin  of  the  ore  deposit*. — The  veins  at  Goldfield  ai 
not  persistent  nor  well  defined.  The  outcrops  of  the  quartz  bodijii 
arc  irregular,  straggling,  branching,  and  apt  to  disappear  sudden" 
(fig.  9).  Neither  were  any  definite  systems  observed,  though  furth 
study  might  detect  them.  The  outcrops  may  even  be  nearly  circul 
or  crescent  ic,  and  frequently  they  are  roughly  lenticular  and  interim" 
tent.  The  quartz  itself  is  gray  and  jaspery,  and  is  due  almost  entire 
to  the  silicification  of  the  volcanic  rock  in  which  it  occurs.  Pra 
tically  no  ordinary  vein  quartz  was  observed. 
All  indications  show  that  this  silicification  (and  the  accompanyni 
mineralization)  is  the  work  of  hot  springs,  and  that  these  irregullj 
reefs  represent  the  horizontal  sections  of  columns  of  rocks  travers 
by  rising  columns  of  hot  water.  Had  the  rocks  been  strongly  fri  I 
tured  we  should  have  had  veins  like  those  of  the  early  andesite  I 
Tonopah,  which  are  also  due  to  hot-spring  action;  but  at  Goldfietl 
the  lack  of  such  a  fracture  system  resulted  in  this  curious  and  rati  I 
1 1  n  usual  type  of  deposit.  It  follows  that  the  quartz  bodies  will  prol  i 
bly,  as  a  rule,  extend  deeper  vertically  than  horizontally,  and  so  Inn 
roughly  the  nature  of  columns  or  pipes. 
Pay  shoots. — The  greater  part  of  one  of  these  jaspery  quartz  reef 
although  showing  disseminated  pyrite,  contains  little  or  no  vali 
in  gold.     Microscopic  investigation  has  shown  that  in  such  qua 
(lie  pyrite  is  often  probably  mainly  indigenous — that  is,  that  the  in 
sulphide  has  been  formed  by  the  action  of  the  sulphur  contained 
the  hot-spring  waters  upon  the  iron  silicates  contained  in  the  ho 
blende  and  biotite.     This  explains  the  absence  of  gold,  the  py]i 
having  the  same  origin   as  the  barren   pyrite  near  the   ore  boc  i| 
in  the  country  rock  at  Tonopah.     Within  some  of  these  barren  fl 
of  silicified  volcanic  rock  at  Goldfield,  however,  prospecting  has 
io  the  discovery  of  portions  containing  gold,  sometimes  in  large  qui; 
