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Ministry of Energy Mines and Responsible for Core Review

C - Placer

BC Profile # Deposit Type Approximate Synonyms USGS Model #
C01 Surficial placers Placer Au-PGE-Sn-diamond-mag-gar-gems 39a to e
C02 Buried-channel placers Paleochannel placers 39a to e
C03 Marine placers Off-shore heavy mineral sediments 39f*?
C04* Paleoplacer U-Au-PGE-Sn-Ti-diam-mag-gar-zir Quartz pebble conglomerate Au-U 29a

 

SURFICIAL PLACERS


C01
by Victor M. Levson
British Columbia Geological Survey
 

Levson, Victor M. (1995): Surficial Placers, in Selected British Columbia Mineral Deposit Profiles, Volume 1 - Metallics and Coal, Lefebure, D.V. and Ray, G.E., Editors, British Columbia Ministry of Employment and Investment, Open File 1995-20, pages 21-23.

 

IDENTIFICATION

 

SYNONYMS: Holocene placer deposits; terrace placers; fluvial, alluvial, colluvial, eolian (rare) and glacial (rare) placers.

 

COMMODITIES (BYPRODUCTS): Au, PGEs and Sn, {locally Cu, garnet, ilmenite, cassiterite, rutile, diamond and other gems - corundum (rubies, sapphires), tourmaline, topaz, beryl (emeralds), spinel - zircon, kyanite, staurolite, chromite, magnetite, wolframite, sphene, barite, cinnabar}. Most of the minerals listed in brackets are recovered in some deposits as the principal product.

 

EXAMPLES (British Columbia - Canada/International): Fraser River (Au), Quesnel River (Au), Tulameen district (PGEs); North Saskatchewan River (Au, Alberta, Canada), Vermillion River (Au, Ontario, Canada), Rivière Gilbert (Au, Québec, Canada), Klondike (Au, Yukon, Canada), Rio Tapajos (Au, Brazil), Westland and Nelson (Au, New Zealand), Yana-Kolyma belt (Au, Russia), Sierra Nevada (Au, California, USA), Goodnews Bay( PGE, Alaska, USA), Emerald Creek (garnet, Idaho, USA), Rio Huanuni and Ocuri (Sn, Bolivia), Sundaland belt (Sn, Thailand).

 

GEOLOGICAL CHARACTERISTICS

 

CAPSULE DESCRIPTION: Detrital gold, platinum group elements and other heavy minerals occurring at or near the surface, usually in Holocene fluvial or beach deposits. Other depositional environments, in general order of decreasing importance, include: alluvial fan, colluvial, glaciofluvial, glacial and deltaic placers.

 

TECTONIC SETTINGS: Fine-grained, allochthonous placers occur mainly in stable tectonic settings (shield or platformal environments and intermontane plateaus) where reworking of clastic material has proceeded for long periods of time. Coarse, autochthonous placer deposits occur mainly in Cenozoic and Mesozoic accretionary orogenic belts and volcanic arcs, commonly along major faults.

 

DEPOSITIONAL ENVIRONMENT / GEOLOGICAL SETTING: Surficial fluvial placer concentrations occur mainly in large, high-order, stream channels (allochthonous deposits) and along bedrock in high-energy, steep-gradient, low-sinuosity, single-channel streams (autochthonous deposits). Concentrations occur along erosional surfaces at the base of channel sequences. Alluvial fan, fan-delta and delta deposits are distinct from fluvial placers as they occur in relatively unconfined depositional settings and typically are dominated by massive or graded sands and gravels, locally with interbedded diamicton. Colluvial placers generally develop from residual deposits associated with primary lode sources by sorting associated with downslope migration of heavy minerals. Glaciofluvial and glacial placers are mainly restricted to areas where ice or meltwater has eroded pre-existing placer deposits. Cassiterite, ilmenite, zircon and rutile are lighter heavy minerals which are distributed in a broader variety of depositional settings.

 

AGE OF MINERALIZATION: Mainly Holocene (rarely Late Pleistocene) in glaciated areas; generally Tertiary or younger in unglaciated regions.

 

HOST/ASSOCIATED ROCK TYPES: Well sorted, fine to coarse-grained sands; well rounded, imbricated and clast-supported gravels.

 

DEPOSIT FORM: In fluvial environments 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: Grain size decreases with distance from the source area. Gold typically fine grained (< 0.5 mm diameter) and well rounded; coarser grains and nuggets rare, except in steep fluvial channel settings where gold occurs as flattened flakes. Placer minerals associated with colluvial placer deposits are generally coarser grained and more angular.

 

ORE MINERALOGY (Principal and subordinate): Au, PGE and cassiterite (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 such as magnetite and ilmenite.

 

ALTERATION MINERALOGY: Fe and Mn oxide precipitates common; Ag-depleted rims of Au grains increase in thickness with age.

 

ORE CONTROLS: In fluvial settings, placer concentrations occur at channel irregularities, in bedrock depressions and below natural riffles created by fractures, joints, cleavage, faults, foliation or bedding planes that dip steeply and are oriented perpendicular or oblique to stream flow. Coarse- grained placer concentrations occur as lag concentrations where there is a high likelihood of sediment reworking or flow separation such as at the base of channel scours, around gravel bars, boulders or other bedrock irregularities, at channel confluences, in the lee of islands and downstream of sharp meanders. Basal gravels over bedrock typically contain the highest placer concentrations. Fine-grained placer concentrations occur where channel gradients abruptly decrease or stream velocities lessen, such as at sites of channel divergence and along point bar margins. Gold in alluvial fan placers is found in debris- flow sediments and in interstratified gravel, sand and silt. Colluvial placers are best developed on steeper slopes, generally over a weathered surface and near primary lode sources. Economic gold concentrations in glaciofluvial deposits occur mainly along erosional unconformities within otherwise aggradational sequences and typically derive their gold from older placer deposits.

 

GENETIC MODEL: Fluvial placers accumulate mainly along erosional unconformities overlying bedrock or resistant sediments such as basal tills or glaciolacustrine clays. Basal gravels over bedrock typically contain the highest placer concentrations. Overlying bedded gravel sequences generally contain less placer minerals and reflect bar sedimentation during aggradational phases. Frequently the generation of more economically attractive placer deposits involves multiple cycles of erosion and deposition.

 

ASSOCIATED DEPOSIT TYPES: Fluvial placers commonly derive from hydrothermal vein deposits and less commonly from porphyry and skarn deposits. PGE placers are associated with Alaskan-type ultramafics. Allochthonous fluvial placers are far traveled and typically remote from source deposits.

 

EXPLORATION GUIDES

 

GEOCHEMICAL SIGNATURE: Anomalous concentrations of Au, Ag, Hg, As, Cu, Fe, Mn, Ti or Cr in stream sediments. Au fineness (relative Ag content) and trace element geochemistry (Hg, Cu) of Au particles can be used to relate placer and lode sources.

 

GEOPHYSICAL SIGNATURE: Ground penetrating radar especially useful for delineating the geometry, structure and thickness of deposits with low clay contents, especially fluvial terrace placers. Shallow seismic, 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: Panning and other methods of gravity sorting are used to identify concentrations of gold, magnetite, hematite, pyrite, ilmenite, chromite, garnet, zircon, rutile and other heavy minerals. Many placer gold paystreaks overlie clay beds or dense tills and in some camps these ‘false bottom’ paystreaks are important.

 

ECONOMIC FACTORS

 

TYPICAL GRADE AND TONNAGE: Deposits are typically high tonnage (0.1 to 100 Mt) but low grade (0.05-0.25 g/t Au, 50-200 g/t Sn). Placer concentrations are highly variable both within and between individual deposits.

 

ECONOMIC LIMITATIONS: The main economic limitations to mining surficial placer deposits are typically low grades and most deposits occur below the water table. Environmental considerations are also an important limiting factor as these deposits often occur near, or within modern stream courses.

 

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. Recorded placer production has represented 3.5% of B.C.’s total gold production in the last twenty years. Prior to 1950, it was approximately 160 000 kg. Actual production was significantly larger. Placer mining continues to be an important industry in the province with annual average expenditures of more than $30 million over a survey period from 1981 to 1986. Shallow alluvial placers also account for a large part of world tin (mainly from SE Asia and Brazil) and diamond (Africa) production.

 

REFERENCES

 

Boyle, R.W. (1979): The Geochemistry of Gold and its Deposits; Geological Survey of Canada, Bulletin 280, 584 pages.
Giusti, L. (1986): The Morphology, Mineralogy and Behavior of "Fine-grained" Gold from Placer Deposits of Alberta, Sampling and Implications for Mineral Exploration; Canadian Journal of Earth Sciences, Volume 23, Number 11, pages 1662-1672.
Herail, G. (Editor) (1991): International Symposium on Alluvial Gold Placers, Abstract Volume; La Paz, Bolivia.

Levson, V. M. and Giles, T.R. (1993): Geology of Tertiary and Quaternary Gold-bearing Placers in the Cariboo Region, British Columbia. B. C. Ministry of Energy, Mines and Petroleum Resources, Bulletin 89, 202 pages.

Levson, V.M. and Morison, S.R. (in press): Geology of Placer Deposits in Glaciated Environments; in Glacial Environments - Processes, Sediments and Landforms, Menzies, J., Editor, Pergamon Press, Oxford, U.K., 44 pages.

Minter, W.E.L. (1991): Ancient Placer Gold Deposits; in Gold Metallogeny and Exploration, Foster, R.P., Editor, Blackie, pages 283-308.

Morison, S.R. (1989): Placer Deposits in Canada; in Quaternary Geology of Canada and Greenland, Fulton, R.J., Editor, Geological Survey of Canada, Geology of Canada, Number 1, pages 687-694.

Sutherland, D.G. (editor) (1991): Alluvial Mining; Institution of Mining and Metallurgy, Elsevier Applied Science, London, 601 pages.

 

BURIED-CHANNEL PLACERS


C02

by Victor M. Levson and Timothy R. Giles
British Columbia Geological Survey

 

Levson, Victor M. and Giles, Timothy R. (1995): Buried-Channel Placers, in Selected British Columbia Mineral Deposit Profiles, Volume 1 - Metallics and Coal, Lefebure, D.V. and Ray, G.E., Editors, British Columbia Ministry of Energy and Mines, Open File 1995-20, pages 25-28.

 

IDENTIFICATION

 

SYNONYMS: Paleoplacer 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.

 

EXAMPLES (British Columbia and Canada/International): 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).

 

GEOLOGICAL CHARACTERISTICS

 

CAPSULE DESCRIPTION: Detrital gold, platinum group elements 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 terranes. 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 in British Columbia and other countries because of difficulties in locating deposits and high overburden to ore stripping ratios.

 

REFERENCES

 

Boyle, R.W. (1979): The Geochemistry of Gold and its Deposits; Geological Survey of Canada, Bulletin 280, 584 pages.

Johnston, W. A. and Uglow, W.L. (1926): Placer and Vein Gold Deposits of Barkerville, Cariboo District, British Columbia; Geological Survey of Canada, Memoir 149, 246 pages.

Kartashov, I.P. (1971): Geological Features of Alluvial Placers; Economic Geology, Volume 66, pages 879 - 885.

Levson, V.M. (1991): Geologic Controls on Alluvial Gold Mining in the Cariboo District, British Columbia, Canada; in Alluvial Mining, Institution of Mining and Metallurgy, London, pages 245 - 267.

Levson, V.M. (1992): The Sedimentology of Pleistocene Deposits Associated with Placer Gold Bearing Gravels in the Livingstone Creek Area, Yukon Territory; in Yukon Geology - Volume 3, Bremner, T., Editor, Indian Affairs and Northern Development, Yukon Exploration and Geological Services Division, Whitehorse, pages 99-132.

Levson, V.M. and Giles, T.R. (1993): Geology of Tertiary and Quaternary Gold-Bearing Placers in the Cariboo Region, British Columbia; B. C. Ministry of Energy, Mines and Petroleum Resources, Bulletin 89, 202 pages.

Levson, V.M. and Blyth, H. (1994): Applications of Quaternary Geology to the Study of Glacial Placer Deposits; A Case Study of the Otter Creek Paleoplacer, British Columbia; Quaternary International, Volume 20, pages 93-105.

Levson, V.M. and Morison, S.R. (in press): Geology of Placer Deposits in Glaciated Environments; in Glacial Environments - Processes, Sediments and Landforms, Menzies, J., Editor, Pergamon Press, Oxford, U.K., 44 pages.

Minter, W.E.L. (1991): Ancient Placer Gold Deposits; in Gold Metallogeny and Exploration, Foster, R.P., Editor, Blackie, pages 283-308.

Morison, S.R. (1989): Placer Deposits in Canada. in Quaternary Geology of Canada and Greenland, R.J. Fulton, Editor, Geological Survey of Canada, Geology of Canada, Number 1, pages 687-694.

Reger, R.D. and Bundtzen, T.K. (1990): Multiple Glaciation and Gold-placer Formation, Valdez Creek Valley, Western Clearwater Mountains, Alaska; Alaska Department of Natural Resources, Division of Geological and Geophysical Surveys, Professional Report 107, 29 pages.

Smith, N.D. and Minter, W.E.L. (1980): Sedimentologic Controls of Gold and Uranium in Two Witwatersrand Paleoplacers; Economic Geology, Volume 75, pages 1-14.

Shilts, W.W. and Smith, S.L. (1986): Stratigraphy of Placer Gold Deposits; Overburden Drilling in Chaudière Valley, Quebec; Geological Survey of Canada, Paper 86-1A, pages 703-712.

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MARINE PLACERS


C03
by Victor M. Levson
British Columbia Geological Survey

 

Levson, Victor M. (1995): Marine Placers, in Selected British Columbia Mineral Deposit Profiles, Volume 1 - Metallics and Coal, Lefebure, D.V. and Ray, G.E., Editors, British Columbia Ministry of Employment and Investment, Open File 1995-20, pages 29-31.

 

IDENTIFICATION

 

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).

 

EXAMPLES (British Columbia - Canada/International): 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).

 

GEOLOGICAL CHARACTERISTICS

 

CAPSULE DESCRIPTION: Detrital gold, ilmenite, cassiterite, platinum group elements 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 nearshore 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 (C01).

 

EXPLORATION GUIDES

 

GEOCHEMICAL SIGNATURE: Anomalous concentrations of Au, As, Fe, Sn, Ti, Zr, REE, Th, Y and U in shoreline or nearshore 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.

 

ECONOMIC FACTORS

 

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.

 

REFERENCES

 

Barker, J.C., Robinson, M.S. and Bundtzen, T.K. (1989): Marine Placer Development and Opportunities in Alaska; Mining Engineering, volume 42, pages 551-558.

Barrie, J.V. (1990): Contemporary and Relict Titaniferous Sand Facies on the Western Canadian Continental Shelf; Continental Shelf Research, Volume 11, pages 67-80.

Barrie, J.V. (1994): Western Canadian Marine Placer Potential. Canadian Institute of Mining, Metallurgy and Petroleum, Bulletin, Volume 87 (977), pages 27-30.

Barrie, J.V. Emory-Moore, M., Luternauer, J.L. and Bornhold, B.D. (1988): Origin of Modern Heavy Mineral Deposits, Northern British Columbia Continental Shelf. Marine Geology, Volume 84, pages 43-51.

Emory-Moore, M. (1991): Placer Gold Potential of the Northern Newfoundland Shelf; Geological Survey of Canada, Open File 2417, 113 pages.

Hein, F.J., Syvitski, J.P.M., Dredge, L.A. and Long, B.F. (1993): Quaternary Sedimentation and Marine Placers along the North Shore, Gulf of St. Lawrence; Canadian Journal of Earth Sciences, Volume 30, pages 553-574.

Laznicka, P. (1985): Empirical Metallogeny - Depositional Environments, Lithologic Associations and Metallic Ores - Volume 1: Phanerozoic Environments, Associations and Deposits, Part A; Elsevier Science Publishers, Developments in Economic Geology, 19, 1002 pages.

Nelson, C.H. and Hopkins, D.M. (1972): Sedimentary Processes and Distribution of Particulate Gold in the North Bering Sea; U.S. Geological Survey, Professional Paper 689, 27 pages.


*  Note:  All BC deposit profile #s with an asterisk have no completed deposit profile.  USGS deposit model #s with an asterisk had no published model in the late 1990s
 

Examples of Placer Deposits

BC Profile # Global Examples B.C. Examples
 C01 North Saskatchewan River (Saskatchewan), Nome (Alaska) Fraser River, Quesnel River, Graham Island
 C02 Livingstone Creek (Yukon), Valdez Creek (Alaska) Williams Creek, Otter Creek, Bullion mine
C03 New South Wales & Queensland (Australia) Middlebank (off north end of Vancouver Island)
C04* Elliot Lake & Blind River (Ontario), Witwatersrand (South Africa) Mulvehill