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Diamond Exploration

Alluvial Deposits

SYNONYMS: Beach, coastal or shoreline placers; offshore placer deposits; coastal dune placers (rare).
COMMODITIES (BYPRODUCTS): Ti (ilmenite, rutile), Zr (zircon), Sn, Au, PGEs (locally Ag, Th, REE, monazite, yttrium, magnetite, garnet, diamonds and other gems).


Detrital gold, ilmenite, cassiterite, platinum group elements, diamonds and other heavy minerals occurring at the present or paleoseafloor surface. They usually occur in Holocene raised or submarine beach or strandline deposits along wave-dominated shorelines, but can also be found in coastal dunes, drowned fluvial channels, or as offshore relict lag concentrations.
TECTONIC SETTINGS: Placers occur mainly along cratonic margins where reworking of clastic material has proceeded for long periods of time. The margins of Cenozoic and Mesozoic accretionary orogenic belts and volcanic arcs are also important settings.
DEPOSITIONAL ENVIRONMENT / GEOLOGICAL SETTING: Marine placers form in exposed, shoreline or near shore environments in areas of active winnowing by waves and long-shore or tidal currents. They occur along present beaches and are also preserved as relict submerged deposits or raised strandlines that formed during glacially induced, low or high sea level stands. Beach placers accumulate mainly in the upper foreshore and backshore depositional environments. Geological settings include sand spits, barrier islands, coastal dunes, buried marine scarps, drowned fluvial deposits and submerged residual or lag deposits overlying bedrock or till.
AGE OF MINERALIZATION: Mainly Holocene (rarely Late Pleistocene) in glaciated areas; generally Tertiary or younger in unglaciated regions.
HOST ROCK TYPES: Well sorted, medium to coarse-grained sands overlying fine-grained shallow marine deposits; some lag gravel concentrations over till or bedrock.
DEPOSIT FORM: Paystreaks follow strandlines in shoreline environments and are thin (often <1 m), long (>100 m, often > 1 km) and narrow (<50 m); usually interbedded with barren sequences; titaniferous sands are up to 20 m thick in Queen Charlotte Sound.
TEXTURE/STRUCTURE: Au is typically very fine grained (< 0.5 mm diameter), well rounded, flattened and of high fineness; coarser Au (~1 mm diameter) occurs in relict lag gravels.
ORE MINERALOGY (principal and subordinate): Native Au, ilmenite, rutile, cassiterite, PGEs, zircon, magnetite (Ag, gemstones, garnet, monazite, various industrial minerals).
GANGUE MINERALOGY: Quartz, pyrite and other sulphides and in many deposits subeconomic concentrations of various heavy minerals.
WEATHERING: Leaching (e.g. Fe from ilmenite) and destruction of unstable minerals may result in residual enrichment of the deposit.
ORE CONTROLS: Heavy mineral concentrations occur along stable shorelines where long- term sorting and winnowing by wave or current action occurs; richest pay streaks usually follow strandlines marked by beach gravels or coquina accumulations; common over clay beds, till or bedrock; occurrence often controlled by the extent of onshore placer or bedrock sources.
ASSOCIATED DEPOSIT TYPES: Coastal placer concentrations commonly associated with present or former fluvial or deltaic surficial placers (CO1).


GEOCHEMICAL SIGNATURE: Anomalous concentrations of Au, As, Fe, Sn, Ti, Zr, REE, Th, Y and U in shoreline or near shore sediments. GEOPHYSICAL SIGNATURE: Ground penetrating radar useful for delineating the geometry, structure and thickness of sandy shoreline deposits. Shallow seismic, electromagnetic, induced polarization, resistivity and magnetometer surveys are locally useful (e.g. IP anomalies from ilmenite). OTHER EXPLORATION GUIDES: Panning and other methods of using gravity sorting to identify concentrations of gold, ilmenite, zircon, rutile, magnetite or other heavy minerals.


TYPICAL GRADE AND TONNAGE: Deposits are typically high tonnage (0.1 to 100 Mt) but low grade (e.g. 0.05-0.25 g/t Au, 50-200 g/t Sn); higher grade deposits are small (e.g. Graham Island beach deposits 120 m long, 15 m wide and 15 cm thick [1000 t ] contain up to 20 g/t Au and 70 g/t Pt). Surface relict gravels offshore of Nome contain 920 ppb Au. Placer concentrations are highly variable both within and between individual deposits.
ECONOMIC LIMITATIONS: The main economic limitations to mining surficial placer deposits are the typically low grades and their location near or below the water table. Offshore placers may occur as much as 100 m below present sea level. Environmental concerns have placed severe restrictions on development in many areas.
IMPORTANCE: Beach placers account for a significant part of the world's Ti production (mainly from Australia, India, Brazil and Florida) and are an important source of Au, zircon, magnetite, garnet, monazite and diamonds.
EXAMPLES: Graham Island (PGE, Au), Queen Charlotte Sound (Au); Country Harbour (Au, Nova Scotia, Canada), Nome (Au, Alaska, USA), Bermagui (Au, Australia), Westland and Nelson provinces (Au, New Zealand), Starke (ilmenite, Florida, USA), Atlantic beaches ( ilmenite, zircon, Brazil), Sherbo deposit; rutile (Sierra Leone), Rosetta sand spit (ilmenite, magnetite, zircon, Egypt), Kerala and Quilon (ilmenite, zircon, monazite, India).

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Rafal Swiecki, geological engineer email contact

This document is in the public domain.

March, 2011