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Diamond Exploration

Alluvial Deposits

3.2 - PALEOPLACERS
SYNONYMS: Buried-channel placer deposits; paleochannel deposits; fluvial and alluvial placers.
COMMODITIES (BYPRODUCTS): Mainly Au and PGE {also Cu, Ag, garnet, cassiterite, rutile, diamond and other gems: corundum (rubies, sapphires), tourmaline, topaz, beryl (emeralds), spinel; zircon, kyanite, staurolite, chromite, magnetite, ilmenite, barite, cinnabar}. Most of the minerals listed in brackets are recovered as byproducts.

GEOLOGICAL CHARACTERISTICS

Detrital gold, platinum group elements, diamonds and other heavy minerals occurring in buried valleys (typically with at least several metres of overlying barren material, usually till, clay or volcanic rocks), mainly as channel-lag and gravel-bar deposits. See description of surficial placers (C01) for general information about alluvial placer deposits.
TECTONIC SETTINGS: Coarse-grained, paleochannel placer Au deposits occur mainly in Cenozoic and Mesozoic accretionary orogenic belts and volcanic arcs, commonly along major faults that may also control paleodrainage patterns. PGE-bearing deposits commonly associated with accreted and obducted oceanic terrains. Fine-grained paleoplacers also may occur in stable tectonic settings (shield or platformal environments) where reworking of clastic material has proceeded for long periods of time.
DEPOSITIONAL ENVIRONMENT / GEOLOGICAL SETTING: Mainly incised paleochannels in mountainous areas including: high-gradient (generally >0.05, less commonly >0.1), narrow bedrock-floored valleys (paleogulches); high-level, abandoned tributary valleys with intermediate gradients (typically 0.01 to 0.1); large, buried trunk valleys (on the order of 100 m deep, a few hundred metres wide and >1 km long) with low channel gradients (generally <0.02 in mountainous reaches and <0.001 in plateau areas); channels buried in modern alluvial valleys with gradients similar to the modern streams. The first two settings are dominated by high-energy, low-sinuosity, single-channel, coarse-grained autochthonous placer deposits, whereas the latter two are characterized by autochthonous and allochthonous placers deposited in wandering gravel-bed river, braided stream and alluvial fan environments. In most paleochannels, coarse-grained placer concentrations occur mainly along channel floors or along other erosional surfaces such as at the base of cut-and-fill sequences; in meandering stream environments finer grained placers also occur along point bar margins and in other areas of slack water.
AGE OF MINERALIZATION: Tertiary and Pleistocene. Older paleoplacers (excepting the Proterozoic Witwatersrand placers) are rare, due to poor long-term preservation of deposits in high-relief, subaerial environments. Pleistocene paleoplacer deposits in British Columbia generally predate at least the last glaciation.
HOST/ASSOCIATED ROCK TYPES: Coarse (pebble to boulder), rounded gravels (or conglomerate), commonly with sandy interbeds or lenses. Gravels usually imbricated, clast supported, open work or with a sandy matrix, and typically with abundant resistant rock types (quartzite, vein quartz, chert, basalt, granite) and minor, less resistant, lithologies (shale, siltstone, schist, etc.). Au placers are commonly associated with rock types hosting epithermal or mesothermal vein deposits. PGE placers occur with ultramafic hostrocks. Paleoplacers can be buried under a variety of materials, including glacial till, glaciolacustrine silts and clays, glaciofluvial sands and gravels, marine sediments and basalt flows.
DEPOSIT FORM: Highly variable and laterally discontinuous; paystreaks typically thin (< 2 m), lens shaped and tapering in the direction of paleoflow; usually interbedded with barren sequences.
TEXTURE/STRUCTURE: Typically well rounded, flattened flakes or plates of low sphericity; coarse, more spherical nuggets common in high-gradient channels; fine (flour) gold common in distal stream reaches; evidence of primary crystal structure very rare.
ORE MINERALOGY (principal and subordinate): Au nuggets, flakes and grains and PGE minerals, (Cu, Ag, and various industrial minerals and gemstones).
GANGUE MINERALOGY: Quartz, pyrite and other sulphides and in many deposits subeconomic concentrations of various heavy minerals, especially magnetite and ilmenite.
ALTERATION MINERALOGY: Fe and Mn oxide precipitates common. Clay alteration of unstable clasts and matrix in some deposits.
ORE CONTROLS: Dominant controls on the geographic distribution of ore include the location of paleodrainage channels, proximity to bedrock sources, and paleorelief. Paleochannels are locally controlled by faults and less resistant rock units. Stratigraphically, placers accumulate mainly at the base of erosional successions along unconformities overlying bedrock or resistant sediments such as basal tills or glaciolacustrine clays. Overlying bedded gravel sequences generally contain less placer minerals and reflect bar sedimentation during aggradational phases. Sedimentologic factors controlling placer accumulations are discussed in Profile C01 (Surficial Placers).
GENETIC MODEL: For an explanation of formation of alluvial placers see surficial placers (C01). Placer deposits are buried when base level rises or channel abandonment occurs. Factors inducing these changes include glaciation, volcanism, stream capture and cutoff, or rising sea level.
ASSOCIATED DEPOSIT TYPES: Paleochannel placer deposits are associated with alluvial fan and fan-delta paleoplacer deposits in some areas (see comments below). Autochthonous fluvial and alluvial placers commonly derive from hydrothermal vein deposits. PGE placers are associated with Alaskan-type ultramafics.
COMMENTS: Alluvial fan and fan delta paleoplacer sequences comprise a distinct subtype of buried placer deposits. They occur in relatively unconfined depositional settings compared to paleochannel placer deposits and typically are dominated by massive or graded, poorly sorted gravels and sands, locally with interbedded diamicton. They are generally lower grade and larger volume than fluvial deposits but they contain relatively uniform placer concentrations. Paleofan deposits are mainly local in origin as indicated by high clast angularity and local derivation. Placer minerals occur in both poorly sorted debris-flow sediments and interstratified fluvial gravels and sands. Concentrations are commonly highest at sites of subsequent fluvial degradation.

EXPLORATION GUIDES


GEOCHEMICAL SIGNATURE: Anomalous concentrations of Au, Ag, Hg, As, Cu, Fe and Mn in stream sediments. Gold fineness (relative Ag content) and trace element geochemistry (Hg, Cu) can be used as a signature to identify lode sources.
GEOPHYSICAL SIGNATURE: Shallow seismic refraction and reflection techniques are useful for delineating paleochannel geometry and depth to bedrock. Electromagnetic, induced polarization, resistivity and magnetometer surveys are locally useful. Geophysical logging of drill holes with apparent conductivity, naturally occurring gamma radiation and magnetic susceptibility tools can supplement stratigraphic data.
OTHER EXPLORATION GUIDES: Exploration should focus on sites of natural overburden removal, such as along glacial meltwater channels, and areas underlain by Tertiary fluvial deposits. Buried placers are commonly preserved below glacial lake sediments, on the lee-side of bedrock highs where glacial erosion was minimal and along narrow valleys oriented transversely to the regional ice-flow direction. Airphoto interpretation and satellite imagery data can aid exploration for buried valley placers. Concentrations of magnetite, hematite, pyrite, ilmenite, chromite, garnet, zircon, rutile and other heavy minerals can be used to indicate placer potential.

ECONOMIC FACTORS


TYPICAL GRADE AND TONNAGE: Placer concentrations in fluvial deposits are highly variable both within and between individual deposits. In paleochannel gold placers, grades of 0.5 to 5 g/m3 Au are typical, although grades as high as 75 g/m3 Au are reported. The values, however, do not include overburden dilution factors which can reduce grades tenfold or more. Deposit sizes are also highly variable, ranging from 1000 t to 10 Mt.
ECONOMIC LIMITATIONS: The main economic limitation to locating, evaluating and mining paleochannel placer deposits is the thick overburden which results in high stripping ratios. Over-consolidation of tills and other sediments due to glaciation makes overburden stripping difficult and is a major limitation inhibiting exploitation of these buried deposits.
IMPORTANCE: Placer gold deposits account for more than two-thirds of the world's gold reserves and about 25% of known total production in British Columbia. Buried-channel placers have been under developed because of difficulties in locating deposits and high overburden to ore stripping ratios.
EXAMPLES: Williams Creek (Au, 093H 119), Bullion (Au, 093A 025), Lightning Creek (Au, 093H 012), Otter Creek (Au, 104N 032), Spruce Creek (Au, 104N 034); Chaudière Valley (Au, Québec, Canada), Livingstone Creek (Au, Yukon, Canada), Valdez Creek (Au, Alaska, USA), Ballarat (Au, Victoria, Australia), Bodaibo River (Au, Lena Basin, Russia), Gibsonville (Sn, New South Wales, Australia), Ringarooma (Sn , Tasmania, Australia).

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March, 2011