The Goldfield news. [volume] (Goldfield, Nev.) 1904-1911, 1908-12-12

been an abstraction of iron which may
have been sufficient to form the pyrite
in the actual ore, upon the superstition
that the dissolved iron, like the silica,
migrated toward the assures
Aspects of the Altered and Mineralized
Rocks
One of the most important points for
the intelligent prospector of the future
is that he must be able to distinguish
original unaltered igneous rocks from
the altered aspects and alteration prod
ucts. 1t is difficult for the ~prospector,
engineer, or wen the geologist, at first
glance, to realize that vast outcrops of
white, rhyolitic looking rocks were
originally black andesitic porphyries.
which have been altered by volcanic
vapors; and still more difficult to be-
lieve, as the writer has seen, that areas
of pulverulent gypsiferous adobe soil
are still another residual extreme of the
same process.
The following descriptions of the
character and alteration of the volcanic
rocks by fumarolic vapors are given by
Ransome:
"Although comparatively unaltered,
representatives of most of the rocks
named may be found within the dis
trict, yet the metamorphism which they
have undergone over much of the area
is of a most conspicuous kind. Dark
pyroxene-andesites, dacite and rhyolite
have alike been changed to nearly
white aggregate of secondary minerals.
Such products, as a rule, retain only
traces of the original textures of the
igneous rocks from which they have
been derived. To detect and interpret
these traces requires patient field OBJ
servation of the progressive stages of
alteration supplemented by mieroseop-
zeal study.
"The altered rock at Goldfeld is
light gray, fecked with numerous duh
white spots which represent the original
feldspar phenocrysts. The few quartz
phenocrysts are unchanged, but the fer
romagnesian or, more briefly, 'femic
minerals have wholly disappeared.
Pyrite in small, disseminated crystals
is abundant
The alteration (alunitization) is char
acteristically associated with unoxidized
ores, and gold, pyrite, tetrahedrite, bis
muthnite and the other minerals found
in such ores were deposited at the same
time that the ~neighboring rocks were
changed to quartz, %alunite and kaolinite.
From comparative chemical analyses
of the altered and unaltered rocks, Ran
some concludes that:
cit appears that the altered rock has
lost a little of it's silica, more of its
alumina, nearly all of it's magnesia and
lime, three-fourths of its soda and two
thirds of it's potash. On the other hand,
the water has greatly increased, a large
amount of sulphnric anhydrite has been
added and the iron has been converted
into pyrite."
Three types or stages of alteration
are recognizable at Goldfeld, as stated
by Ransome:
"First, where the chemical activity
has been most intense, the rocks nave
been changed to porous, fine-grained ag-
gregates consisting essentially of
quartz. This is the material of which is
composed the hundreds of craggy
points and lodges which are one of the
most characteristic features of the to-
Biography of the district. Although
most OF these lodges, so far as known,
contain no ore bodies, yet until recently
no ore has been found that was not in
or alongside such quartz rock.
"Some of the discoveries at Goldfeld,
made in 1906, however, show that very
rich bodies of ore occur at places where
there is no silicious outcrop.
"The second type of alteration is one
in which the rock is changed to a com
p~aratively soft, light-colored mass of
quartz, kaolinite, %alunite and pyrite. At
any given locality the boundaries be-
tween the rocks representing two kinds
of alteration may be fairly sharp; but
1.-THE proportions of q~uartz( kaolinite
and alunite vary widely and conse-
quently, over the field at large, grada-
lions in composition and hardness may
be found between these two typical
metamorphic products.
IN thirp type of alteration has eff
fected less conspicuous results thar the
other two, and it's products are not
closely associated with the ores. It con
sists IN the development of calcite.
quartz, chlorite, epidote and pyrite, at
the expense of the original minerals and
groundmass, and is thus propylitic in