SURFICIAL PLACERS
by Victor M. Levson
British Columbia Geological Survey
  

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 T.R. Giles. (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.