146  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  1904.        [bull.  260. 
EFFECT     OF     WATERS     PRODUCING     THE     TONOPAH     RHYOLITE. 
DACITE  VEINS   ON   EARLIER  FORMED  VEINS. 
Although  as  a  rule  a  decomposed  andesite  seems  to  have  presented 
a   formidable   barrier   to   the   circulating  waters   accompanying  the< 
Tonopah  rhyolite-dacite,  in  some  places  the  waters  must  have  trav- 
ersed  the  andesite.     Indeed,  it  is  along  the  brittle  andesite  veins 
and  silicified  adjacent  andesite  that  fractures  and  fissures  must  hav£ 
been  most  easily  formed  at  this  period.     In  the  case  of  the  Tonopah 
Extension,  the  earlier  andesite  vein  has  been  reopened,  and  along,1;, 
the   hanging   wall   a    new    vein   of   barren   jaspery   quartz    formed  | 
This   is   probably   due   to   waters   of  the   rhyolite-dacite   period   oi 
mineralization.     In  this  case  the  new  quartz  is  relatively  barren  asij 
compared  with  the  old.     It  is  evident,  however,  that  the  effect  of  such 
solutions  must  have  been  to  dissolve  a  great  deal  of  the  gold  andi 
silver  contained  in  the  earlier  veins,  and  naturally  to  reprecipitatrti 
it  elsewhere.     Thus  the  ores  might  be  reprecipitated  in   a  concenn 
trated   form.     This  very  likely  has  been  the  case  in  the  MontamiJ 
Tonopah,   where  the  original   vein  has  been   reopened,   and   in   thi 
fissure  thus  formed  minerals  similar  to  those  in  the  older  vein,  bu 
richer  in  gold  and  silver,  have  been  precipitated  in  crustified  form  j 
It  is  reasonable  to  assume  that  this  may  have  been  the  work  of  tb 
waters  of  the  rhyolite-dacite  period,  of  the  same  kind  and  characte: 
as  those  to  which  the  barren  quartz  hanging-wall  portion  of  the  veil 
in  the  Tonopah  Extension  is  due, 
Again,  it  is  natural  that  such  waters  may  have  dissolved  some  o  J 
the  metallic  contents  of  the  older  veins,  and  instead  of  precipitating  j 
them  within  these  veins  may  have  carried  them  out  and  deposited' 
them  elsewhere,  as,  for  example,  in  the  veins  of  the  rhyolite-dacitfi y 
forming  bunches  of  high-grade  ore  in  these  usually  barren  veimnjl 
This  may  be  the  explanation  of  the  comparatively   small   amoun  j, 
of   rich   ore    found    in    some    of   the   rhyolite-dacite    veins,    as,   fo  k 
instance,   in   the    Desert    Queen    and    the    MacNamara.     These    ar  I 
practically  the  only  veins  of  this  period  in  the  district  that  con  j 
tain  high-grade  ore,  and  both  these  veins  lie  near  rich  earlier  andesit 
veins.     Veins  in  the  rlryolite-dacite  that  lie  farther  away  from  th  L 
earlier  andesite  veins  are  frequently  large,  but  are  typically  of  lo\    ■ 
orade  or  barren. 
These  veins  are  plainly  the  results  of  ascending  hot  waters.  an< 
represent  the  effects  of  the  Tonopah  rhyolite-dacite  eruptions,  bavin;  L . 
the  same  relation  to  these  eruptions  that  the  earlier  andesite  vein 
did  to  the  eruptions  of  the  earlier  andesite,  The  characteristic  lac)  ^ 
of  definition  and  persistence  in  these  veins  as  compared  with  th 
veins  in  the  earlier  andesite  shows  that  at  the  time  they  were  forme* 
no  definite  fracture  zones  were  available  as  channels,  so  that  th 
'!li 
