Deposits Formed by Sedimentary and Surficial Processes - Metals and Society: An Introduction to Economic Geology - page 115

Table 5.2 Physical properties of minerals in placer deposits (From Garnett and Bassett ( 2005 ))
Mineral

Tenacity Grindability b

Commodity Density
(g/cm 3 )

Moh's
hardness

Remarks

Diamond a Diamond 3.5 10 Brittle 1 Very hard but only

moderate density;
brittle

Gold Au 15-19 2.5-3.0 Malleable 11 Very dense, soft,

malleable

Platinum PGE 14-19 4.0-4.5 Malleable 10 Dense, moderately soft,

malleable

Cassiterite Sn 6.5-7.1 6-7 Brittle 6
Rutile Ti 4.2-4.3 6-6.5 Brittle 3 Heavy mineral in beach
sands
Zircon Zr 4.7 7.5 Brittle 2 Heavy mineral in beach
sands
Monazite Th, REE 5-5.3 5-5.5 Brittle 9 Heavy mineral in beach
sands
Ilmenite Ti 4.7 5.5-6.0 Brittle 8 Heavy mineral in beach
sands
Garnet Abrasive 3.5-4.3 6.5-7.5 Brittle 4 Industrial mineral
a Minerals arranged in order of their survivability in the fluvial environment
b Resistance to grinding in laboratory ball-mill tests

layer that may become enriched in valuable metals. This process is called
supergene alteration or supergene enrichment.

5.2 Placer Deposits

A placer ore body is deposit of sand, gravel or soil containing eroded particles of
valuable minerals. These minerals are able to survive and become concentrated in
the surface environment because of their chemical and physical properties, as
illustrated in Table 5.2 . They are not necessarily thermodynamically stable but
their rates of reaction are slow compared with the duration of erosion, sedimentary
transport and deposits. An example is diamond, which, although thermodynami-
cally unstable at low pressure (James Bond had it wrong, diamonds are not
“forever”), is well able to survive long enough to be transported from their
kimberlitic source to a site of deposition in offshore gravels. Other minerals in
placer deposits are gold, which occurs in the native or metallic form, and oxides or
silicates such as rutile, ilmenite, zircon, and monazite, which are the sources of Ti,
Zr, Nb and other high-technology metals. Uraninite is stable only under reducing
conditions such as those that existed in the atmosphere, oceans and rivers of the first
part of Earth history, and placer deposits of these minerals formed only in the
Archean and early Proterozoic.
Diamond, uraninite and the Zr-Ti oxides are moderately to extremely hard,
which allows them to resist abrasion as the grains are released from their host