ized.
This has been due largely to the fact that the iron and manganese oxides
effectively stain and mask the zinc carbonate.

The Leadville ores occur as replacements and vein-fillings in a gently
faulted and folded Carboniferous limestone, in deposits of a general
tabular shape, parallel to the bedding but with very irregular lower
surfaces. The limestone is intruded by numerous sheets of porphyry,
mainly parallel to the bedding but sometimes cutting across it, against
the under sides of which most of the ore occurs. The primary sulphides
are believed to be genetically related in some fashion to these
porphyries. The older view was that the agents of deposition were
aqueous solutions from the surface above, which derived their mineral
content chiefly from the porphyries. Later views favor solutions coming
directly from the porphyries or deeper igneous sources. While in form
and association these ores are characteristic igneous contact deposits,
they lack the high-temperature silicates which are so distinctive of
many ores of this type.

The zinc ores of Franklin Furnace, New Jersey, belong in the group
associated with igneous agencies, but have certain unique features. They
consist of willemite and zincite, together with large amounts of
franklinite (an iron-manganese oxide) and silicates, in a pre-Cambrian
white crystalline limestone near its contact with a coarse-grained
granite-gneiss. The origin of the ores is obscured by later shearing and
metamorphism, but it seems best explained by replacement of the
limestone by heated solutions coming from the granitic mass. The view
has also been advanced that the ores originated in the limestone before
the advent of the igneous rocks. Secondary concentration is not
apparent.

FOOTNOTES:

[33] Ransome, Frederick Leslie, The copper deposits of Ray and Miami,
Arizona: _Prof. Paper 115, U.S. Geol. Survey_, 1919, pp. 12-13.

CHAPTER XI

GOLD, SILVER, AND PLATINUM MINERALS

GOLD ORES

ECONOMIC FEATURES

The principal and most essential use of gold is as a standard of value
and a medium of exchange. Gold has been prized since the earliest times
because of its luster, color, malleability, and indestructibility, and
has long been used as a trading medium. At present little of the metal
is actually circulated from hand to hand. Stocks of gold, however,
accumulated by governments and banking interests, form the essential
foundation of paper currency and of the v