Natural Science in Archaeology

3.6 Unconsolidated Deposits 63
Table 3.5 Size categories of sedimentary particles
Size (mm) Name
Gravel Boulder
256
64 Cobble
32 Pebble Very large
16 Large
5 Medium
4 Small
2 Granule
1 Sand Very coarse sand
1/2 Coarse sand
1/4 Medium sand
1/8 Fine sand
1/16 Very fine sand
1/32 Mud Silt Coarse
1/64 Medium
1/128 Fine
1/256 Very fine
1/512 Clay Coarse
1/1024 Medium
1/2048 Fine
1/4096 Very fine
3.6.2 Placer Deposits
Placer deposits are surficial mineral deposits formed by gravity concentration of
material that has weathered out of bedrock. Placers concentrate gold, cassiterite
(tin oxide), other dense metallic minerals, and many precious stones (including diamond).
Flowing water is the most effective separator of light from heavy materials,
and the most common type of placer deposit is formed in alluvial systems where
there is a break in the slope of the river course. Placer deposits form when the
stream velocity is abruptly diminished and the heavier materials are dropped and
concentrated in the bottom gravels. Dipping foliated rock or rocks that have irregular
surfaces or “riffles” are most effective in trapping the heavy materials
One can judge what minerals will be concentrated in placer deposits by looking
at their respective specific gravities as follows: quartz 2.65, feldspar 2.55–2.75,
mafic silicates 2.9–3.4, corundum 4.0, magnetite 5.1, cassiterite 6.4–7.1, gold 19.3.
In addition to having high density, the minerals recovered from placer deposits are
those that are resistant to chemical and mechanical weathering.
Interestingly, the purity of placer gold is generally greater than the gold in the
primary deposit from which it was derived. This results in part from the fact that
surface waters dissolve the silver alloyed in the gold. The gold grains that collect in
placer deposits vary considerably in size. Generally, large nuggets remain near the
primary gold deposit and show little effect of transportation. Gold in larger stream
channels is usually fine to medium-fine in size.