Perhaps this technique is one and the same as "dry winnowing."  The
California Division of Mines and Geology publication The Elephant as They
Saw It (1949, still in print) provides an engraving of this process in
Figure 5, page 46.  It involved using a blanket to throw the crushed ore in
the air in a heavy wind.  The lighter fraction blew away, while the gold
was retained.  Not a very efficient technique, to be sure.