Semi-weekly herald., 1913-03-06
FORMATION OF THE
LA PLATA DISTRICT

mountains, is one Of the greatest Of
any districts in the United States
Taking as a starting point the heac
waters of the La Plata river where
the mountains are composed princi
pally of the Dolores formation with
actions of monzonite, diorite and o
secondary andesite porphyry. At the
present time there has been nothing
developed worth mentioning but the
Cumberland mine and from all indi-
cations it is in the zone of one OF
the monzonite upheavals with bands
and assures of the altered andesite
Along the strike of the altered ande-
site and monzonite well defined fs-
sures of white quartz, varying IN
thickness from few inches to several
feet, carries the principal part of
the values. From all indications this
Fart of the district is practically on
the north end of the main voncan,c ac-
ton of the La Plata district. Going
south from the Cumberland the for
nation has very little change, on the
west slope from the La Plata river
until Borren gulch is reached, while OF
the east slope there has been greater
action both glacier and volcanic, from
Lewis gulch south the mines are To-
Gated in an area of sedementary rocks
intruded by sheets and dykes of green
l, x.rhtrh in, I'm ~ hy
dark basic dykes. The sheets ars
simply overflows from dykes whilc the
dykes are filed gashes from some old
crater. The sedementary rocks con-
sist If red calcarious clay shaie, rcd
sand stone and conglomerates. The
later grade into each other hori-
zontally and vertically, no single men,-
Der persisting for any considerable
distance. These rocks belong to he
cries generally known as the red beds
md to the lower diversion of tina,
series the Cutler formation. The in.
truding bodies are diorite, monzonits
and andersite, porphyries, light gray
| sh green rock with white field 'pai
and dark hornblend phonocrusts. The
porphyry sheets or overflows vary ir
thickness, while they generally follow
the bedding plans of the rocks which
they intrude, frequently they are
prosecuting and often irregular.
The porphyry dykes vary in size
but have the same litologic appear
ance as the sheets or overflows. The
basic dykes which cut the green por'
phyries are very dark and of dense
texture.
1t is very evident that the por
phyries are of later period than tht
Red Beds" which they intrude ir
this particular locality. Both ths
sedementary and igenous rocks are
jointed and fissured in every direction
There is we'll defined set of fis
sures running a few degrees south OF
east in several of the mines which
appear to be of an earlier age ant
also older than some of the intrusiv,
rock.
The sandstones, which are usual,
red and pliable, are considerably met
amorphosed in the vicinity Of the ore
deposits where they have been in
durated by the deposition OF silica ant
their color has been changed to ligh
gray. 1t appears that the iron oxide
to which their usual red color is due
was either removed or altered.
The shales, too, are more or less 81
licifed and their color is similarly
changed so that it is sometimes diff
cult to distinguish shals from sand
stone.
The fracture of the sediments wher,
they are bracciated are sharp ane
clear cut, suggesting that their in
duration partly preceded the move
ments which crushed them. Sine,
their induration appears to be du,
to the action Of circulating water
rather than to contact metamorphisn
induced by the heat of the porphyry
it is evident that there was some frac
turing before and some after the met
| amorphism of the sediments.
The metaliferous minerals of the Ii
Plata district are the tellurides o.
gold and silver-silvanite, petite ane
probably calanerites, native colds, ma
tive quicksilver, amalgam, freibergite
tennantite, stephanite and other sui
phantimouides and sulphur cyanides OF
silver, pyrite marasite, chalcophyrit.
galena, zinc bled hunatjte and magma
tite.
l have just touched on the genera
geology of the entire district and moi
one any particular locality or mine
taking into consideration that the ma
jority Of the workings are simply OF
untitled gashes, stringers or glacier de
posits and in very few instances tina
there has not been any work done OF
the main action or fissure.
There have been five distinct actioni
Of volcanic matter which causes tn.
country to be scanned and gashed II
every direction, the two main actioni
causing the deposition Of mineral be