Description:
Gold placers, or deposits such as gravel and sand which contain
notable concentrations of gold, all result from the slow milling
and concentration processes incident to natural erosion of
pre-existing gold-bearing rocks. The origin of many gold placers
is traceable directly to auriferous veins, lodes, or replacement
deposits which, in most instances, were not of high grade.
According to Emmons, placers are not apt to form from goldbearing
outcrops that contain abundant manganese, iron sulphides,
and chlorides, unless precipitating agents such as calcite,
siderite, rhodochrosite, pyrrhotite, chalcocite, nepheline, olivine,
or leucite are abundant, or unless erosion is very rapid. In other
words, the gold may be dissolved and carried below by means
of natural chlorination processes that are established when solutions
containing chlorides, together with sulphuric acid from the
oxidation of iron sulphides, act upon manganese dioxide; but this
process is neutralized if precipitating agents are present, and
may be ineffective if erosion is very rapid