Placer Deposits

Under construction

Placer deposits are accumulations of valuable minerals that form as a result of natural weathering and erosion processes. The causative factors in formation of placers are gravity separation and sedimentary processes. In respect of placer types, a distinction must be drawn between residual placers, eluvial placers, stream or alluvial placers, beach placers, and eolian placers. Placers minerals must be mechanically stable and chemically stable or chemically unreactive to avoid mechanical and chemical breakdown. They are generally hard and lack cleavage. To accumulate in placers the minerals must be crucially denser than quartz, the main constituent of sand and gravel (quartz specific density 2.65 g/cm³). Minerals mined from placer deposits range in density from about 3.5 g/cm³ (for example diamond) to gold with about 14 - 19.3 g/cm³ to platinum-group metals with densities up to 20 g/cm³.

 

Minerals that mined from placer deposits include gold and platinum-group metals (PGM), diamonds and gems (garnet, sapphire, ruby), tin (cassiterite), niobium-tantalum (coltan, i.e. columbite-tantalite), tungsten (wolframite), rare-earth elements and thorium (monazite), titanium (ilmenite and rutile), uranium, zirconium (zircon), and iron sands with high concentrations of magnetite.

Quartz vein with pyrite and gold being weathered  and formation of residual and eluvial placers.

Chart on the formation of alluvial fans. Alluvial fans are fan- or cone-shaped deposits of detritus that form at a break in slope at the edge of an alluvial plain. They are formed by deposition from fluvial detritus and the sediment comes from an erosional realm  adjacent to the basin. Sediments on alluvial fans are typically poorly sorted and include abundant gravel-size detritus and sometimes they can contain boulders. 

Conceptual model of a alluvial fan developed at the mouth of a major river flowing from a source area in the NW and debouching into a shallow-water intermontane lake. The general geometry of the fan, the various fan facies of deposition, the location of coarse clastics and algal mats, the arrangement of varying zones of enrichment in gold, the portion of the fan subjected to transgression and winnowing, and the approximate dimensions of a typical fan are shown. The plus signs designate relative quantities of gold (modified from GUILBERT & PARK 1986).

Concentration of gold and heavy minerals often occur at the following places: (1) sudden drop-off into deeper and larger volume of water; (2) in cracks/crevices in stream bed; (3) behind rock bars or outcrop of harder rocks across the stream; (4) in pot holes/rock holes; (5) downstream from tributary; (6) when stream channels widens; (7) behind large boulders and riffles (obstructs); (8) behind islands; (9) where the slope of the stream becomes less; (10) behind sharp curves where the water bounces several times against the banks; and (11) at gravel bars (point bars) that form at the inside banks of meander loops.

Concentration principle 1:  Sudden drop-off into deeper and larger volume of water where the flow velocity is less than upstream.

Concentration principle 2: Gold is deposited in crevices, cracks, joints or fractures in stream bed. 

The base of an alluvial fan above the Paleozoic basement (slate) where the gold content are highest, Spain. Note the big cobbles and boulders. 

Concentration principle 3a: Gold is deposited were the stream bed  exhibits natural riffles and rock bars, i.e. resistant beds, dykes or veins.

Concentration principle 3b: Were obstructions are in stream bed and prevent the stream flow, the concentrating effects of eddies are of importance. 

Concentration principle 5: 

downstream from tributary

Concentration principle 7: behind large boulders

Concentration principle 9: When stream channel widens

This test diagram shows how gold accumulate in a river. In this test, they used galena (black) instead of gold, but the results are comparable. In rapid streams, streaks and tails of coarser "heavies" are dropped in slack water velocity "shadows" in several characteristic locations (after BOYLE 1979 and GUILBERT & PARK 1986). Note how the sharpness and length of the curve makes a radical difference in the formation of paystreak deposits.