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 Energy 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 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. |