Alluvial Placer Deposits in South America
The Choco as well as other districts of Colombia were worked since early times with what was known as batea and calabash pans. The placers are river and bench gravels that yielded a great amount of Au/Pt. The country rocks are Tertiary conglomerates, sandstones, shales and a variety of kinds of volcanics and serpentinized basic intrusives. Quartz veins with gold, silver, lead and zinc can be found in abundance at the meeting areas amid tertiary granite intrusives and andesites, greenstones and metasediments.
The alluvial placers in this district are distinguished by especially fine gold particles that are disseminated in sands and gravels mostly taken from Roraima Plateau sandstones and conglomerates. The beginning of gold remains to be open to discussion as a relative deficiency of main deposits barely gives explanation for the existence of very great and extensive amounts of gold placers. The pay streaks are very rich in higher areas of Roraima plateaux and reduce when going downstream. As in Colombia, platinoids can be seen in the gravels.
In resent placers gold can often times be found along with small to moderate amounts of native platinoids, mainly in terrains set apart by ultra basic or basic intrusives with associated Ni-Cu-Co sulphide deposits. Besides gold, there are a lot of placers with platinoids that have alluvial diamonds in economical grades.
There are three sources of gold that can be found in this area and are placed in order of value:
- 1st is a wide-ranging paleoplacer or conglomerate inside the Roraima plateaux, and this yields coarse gold that often times shows crystallized forms of 960 in fineness.
- 2nd are basic intrusives or gabbroic rich quartz that have a lot of pyritization and sericitization, intrusives. The gold that is taken from this source can be recognized by very fine round particles, which are of 870 to 920 in fineness and are dispersed in the gravels. The occurrence of secondary pyrite in pay streaks and nonexistence of any mechanical abrasions or flattening implies that chemical transport is implicated. The source is disseminated pyrite inside the intrusive bodies.
- 3rd are Precambrian greenstones related gold quartz, gold quartz pyrite and quartz gold tourmaline veins that have a fineness of 860.