e minerals probably owe their origin to hot magmatic
solutions, as suggested by the close association of the ores with the
igneous rock, the presence of minerals containing chlorine, fluorine,
and boron, and the development in the limestone of dense silicates and
mineral associations characteristic of hot-water alteration. The
manganese ores are mined principally in the oxidized zone. Rich silver
ores are found below the water table, but mainly in veins independent of
the manganese deposits.

At Butte, Montana, a little high-grade manganese material has been
obtained from the unoxidized pink manganese carbonate, which is a common
mineral in some of the veins. It is associated with quartz and metallic
sulphides and is similar in origin to the copper ores of the same
district (pp. 201-202).

The lower-grade and the more ferruginous manganese ores are of a
somewhat similar origin to the principal high-grade ores, in that they
represent surface concentrations of the oxides from smaller percentages
of the carbonates and silicates in the rocks below. Deposits of this
nature have been derived from a wide variety of parent rocks--from
contact zones around igneous intrusions, from fissure veins of various
origins, from calcareous and clayey sediments, and from slates and
schists. The manganese and manganiferous iron ores of the Cuyuna
district of Minnesota, the largest source of low-grade ores in this
country, were formed by the action of weathering processes on
sedimentary beds of manganese and iron carbonates constituting "iron
formations." The process is the same as the concentration of Lake
Superior iron ores described elsewhere.

Manganese, like iron, is less soluble than most of the rock
constituents, and tends to remain in the outcrop under weathering
conditions. To some extent also it is dissolved and reprecipitated, and
is thus gathered into concretions and irregular nodular deposits in the
residual clays. In some cases it is closely associated with iron
minerals; in others, due to its slightly greater solubility, it has been
separated from the iron and segregated into relatively pure masses. With
manganese, as with iron, katamorphic processes are responsible for the
concentration of most of the ores. The ores are in general surface
products, and rarely extend to depths of over a hundred feet.

CHROME (OR CHROMITE) ORES

ECONOMIC FEATURES

The principal use of chrome ores is in the making of the alloy
ferroc