DESCRIPTIVE MODEL OF ALLUVIAL PLACER Sn

MODEL 39e

By Bruce L. Reed

DESCRIPTION Cassiterite and associated heavy minerals in silt- to cobble-size nuggets concentrated by the hydraulics of running water in modern and fossil streambeds.

GENERAL REFERENCES Hosking (1974), Taylor (1979), Sainsbury and Reed (1973).

GEOLOGICAL ENVIRONMENT

Rock Types Alluvial sand, gravel, and conglomerate indicative of rock types that host lode tin deposits.

Textures Fine to very coarse clastic.

Age Range Commonly late Tertiary to Holocene, but may be any age.

Depositional Environment Generally moderate to high-level alluvial, where stream gradients lie within the critical range for deposition of cassiterite (for instance, where stream velocity is sufficient to result in good gravity separation but not enough so the channel is swept clean). Stream placers may occur as offshore placers where they occupy submerged valleys or strandlines.

Tectonic Setting(s) Alluvial deposits derived from Paleozoic to Cenozoic accreted terranes or stable cratonic foldbelts that contain highly evolved granitoid plutons or their extrusive equivalents (see Model 14b, geochemical signature). Tectonic stability during deposition and preservation of alluvial deposits.

Associated Deposit Types Alluvial gravels may contain by-product ilmenite, zircon, monazite, and, where derived from cassiterite-bearing pegmatites, columbite-tantalite. Economic placers are generally within a few (<8) kilometers of the primary sources. Any type of cassiterite-bearing tin deposit may be a source. The size and grade of the exposed source frequently has little relation to that of the adjacent alluvial deposit.

DEPOSIT DESCRIPTION

Mineralogy Cassiterite; varying amounts of magnetite, ilmenite, zircon, monazite, allanite, xenotime, tourmaline, columbite, garnet, rutile, and topaz may be common heavy resistates.

Texture/Structure Cassiterite becomes progressively coarser as the source is approached; euhedral crystals indicate close proximity to primary source. Where a marine shoreline intersects or transgresses a stream valley containing alluvial cassiterite the shoreline placers normally have a large length-to-width ratio.

Ore Controls Cassiterite tends to concentrate at the base of stream gravels and in traps such as natural riffles, potholes, and bedrock structures transverse to the direction of water flow. The richest placers lie virtually over the primary source. Streams that flow parallel to the margin of a tin-bearing granite are particularly favorable for placer tin accumulation.

Geochemical Signature Anomalously high amounts of Sn, As, B, F, W, Be, W, Cu, Pb, Zn. Panned concentrate samples are the most reliable method for detection of alluvial cassiterite.

EXAMPLES

Southeast Asian tin fields (Hosking, 1974)
(Newell, 1971)
(Simatupang and others, 1974)
(Westerveld, 1937)