Page 180 - The Geology and Ore Deposits of Sierra County, New Mexico - Bulletin 10

MINING DISTRICTS 177
Twenty-five and Thirty stopes, fracturing and folding of the
limestone have been more important than elsewhere, and conse-
quently these stopes reach a maximum height for the district.
This difference in the manner of replacement has led some ob- servers to believe that mineralization was due to two distinct
types of solutions, an acid or siliceous type in the southern and southwestern part, which formed the Bridal Chamber, Twenty-
five Cut and Thirty Stope ore bodies, and a more basic type in
the area to the northeast; which formed the ores of the Strieby- Apache-Miles Standish-Virginia stopes and the Black Bird
workings. In the area surrounding the Emporia incline the two
solutions are supposed to have mingled, with the result that an intermediate type of ore was deposited.
It is thought that the primary ore-forming solutions in the
district had their origin in the same deep seated source from
which the "Porphyrite" or monzonite porphyry emanated, and that these solutions migrated upward along fractures in the
limestone and along the monzonite-limestone contacts, followed the contact between the Blue and Crinoidal limestones over large
areas, and from these moved upward again along minor fractures
in the
-
Crinoidal limestone. At first the solutions were high in
silica and contained small amounts of manganese, iron and silver. The cherty replacement deposits that they formed contained
about 75 per cent silica, 3 per cent iron, 6 per cent manganese
and 2 to 5 ounces of silver a ton. Later the solutions were higher in calcium carbonate and silver, and during this period they de-
posited their load of silver and iron- and manganese-bearing cal- cite in the cracks and fractures formed within the cherty
replacement bodies by the gradual elevating and tipping of the
fault block within which the ore is found. The primary silver minerals were stephanite, proustite, pyrargyrite, and argentifer-
ous galena locally, while the primary source of the manganese and iron was principally the iron- and manganese-bearing calcite,
or ankerite, and the iron and manganese content of the chert
beds. Some pyrite was also deposited.
The ore deposits owe their commercial importance entirely
to the oxidation, transportation, and secondary precipitation and enrichment of the silver, which has taken place since the sedi-
mentary fault block was elevated and laid bare by the erosion of the overlying extrusive rocks. The primary sulphantimonides
of silver have been oxidized and converted to cerargyrite, embo- lite and native silver. Galena has been converted to cerusite,
vanadinite and wulfenite. The ankerite has been converted to
manganese oxides, limonite and manganiferous calcite. Calcite, coating cerargyrite and manganese minerals and as veinlets, is
a secondary product. The oxidizing solutions have migrated down the dip slope just over or through the chert bed and have
formed rich ore along the east-west fractures of the block. Some