Abstract

In humid tropical fan deposits of the late Pleistocene age in southwestern Colombia, the mean gold grain size is anomalously coarser than the associated mean magnetite grain size. This anomaly is even greater when equivalent diameter mean sizes are compared, which indicates that the gold is deposited under different flow conditions than the magnetite. Entrainment sorting concentrated the gold in the deeper parts of the channel during prolonged intervals when the flow velocity was above the entrainment velocity of magnetite and near that of gold (30-60 cm/sec). Magnetite was preferentially concentrated in sand bars in more shallow reaches of the channel where flow velocities were slightly lower (15-35 cm/sec). After the gold was concentrated, most of the magnetite was added to the deposit of cobbles when magnetite-rich sand bars migrated over the cobble deposits. In most cases, later flood events stripped away these sand bars and deposited more cobbles, resulting in a thick sequence of cobble gravel with gold and magnetite concentrated in the cobble matrix at certain intervals. The frequent high flow events in humid tropical fans are conducive to this type of placer development.

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