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Sangster, D.F. (1996): Sandstone-Pb, in
Selected British Columbia Mineral Deposit Profiles, Volume 2 - Metallic
Deposits, Lefebure, D.V. and Hõy, T, Editors, British Columbia Ministry of
Employment and Investment, Open File 1996-13, pages 17-19.
IDENTIFICATION
COMMODITIES (BYPRODUCTS): Pb (Zn,
Ag).
EXAMPLES (British Columbia - Canada/nternational):
None in British Columbia; only two are known in Canada; Yava (Nova
Scotia) and George Lake (Saskatchewan), Laisvall (Sweden), Largentière (France),
Zeida (Morocco), Maubach and Mechernich (Germany).
GEOLOGICAL
CHARACTERISTICS
CAPSULE DESCRIPTION: Disseminated
galena with minor sphalerite, in transgressive basal quartzite or
quartzofeldspathic sandstones resting on sialic basement.
TECTONIC SETTING: Platformal
deposits commonly found in sandstones resting directly on basement (usually
cratonic) of sialic composition.
DEPOSITIONAL ENVIRONMENT / GEOLOGICAL
SETTING: Hostrocks were deposited in environments ranging from
continental fluvial to shallow marine or tidal beach. The most common
environment is one of mixed continental and marine character (i.e.,
paralic). Host rocks in most districts are succeeded by marine sediments,
suggestive of marine transgression onto the craton. Terrestrial organic
debris, ranging from trace to abundant, is present in most of the post-
Devonian deposits. Paleomagnetic data available in several districts
indicate a low paleolatitude position (0-30 ) for all deposits.
Paleoclimatic conditions ranged from warm arid to cool humid but in a
majority of cases, were semiarid and warm.
AGE OF MINERALIZATION:
Mineralization age has not been established with certainty; however,
deposits are found in rocks ranging from Middle Proterozoic to Cretaceous
age. Rocks of Late Proterozoic - Early Cambrian and Triassic ages contain
a majority of deposits of this type.
HOST/ASSOCIATED ROCK TYPES:
Hostrocks are grey or white (never red) quartzitic or quartzo-feldspathic
sandstones and conglomerates; they are rarely siltstone or finer grained
clastics. Sialic basement rocks, typically granites or granitic gneisses,
underly sandstone lead deposits. Shales and associated evaporites as beds,
nodules or disseminations are intercalated with the host sandstones.
DEPOSIT FORM: Orebodies are commonly
conformable with bedding in the sandstone, especially on a mine scale. In
detail, however, the ore zones may actually transgress bedding at a low
angle. Sedimentary channels in the sandstone are preferentially
mineralized; consequently, most deposits have a generally lensoid form. In
plan, ore zones tend to be sinuous and laterally discontinous. Ore zones
tend to be delimited by assay, rather than geological, boundaries.
Characteristically, a higher grade core is surrounded by material that
progressively decreases in grade outward. Rarely, higher grade zones occur
in, and adjacent to, steep faults; consequently, in these deposits, many
ore zones are narrow, lenticular bodies oriented at high angles to bedding.
TEXTURE/STRUCTURE: The preferred
site of ore minerals is as cement between sand grains resulting in
disseminated sulphide blebs or spots in massive sandstones or
concentrations of sulphides along the lower, more porous portions of
graded beds. The disseminated sulphides are not normally homogeneously
dispersed throughout the sandstone. Two very common textures are:
- i) spots, representing local
accumulations of galena, as much as 2 cm in diameter. Spots may be
randomly distributed in the sandstone or may show a slight preferential
alignment parallel to bedding;
ii) discontinous galena-rich streaks distributed parallel to bedding,
including crossbedding. Where carbonaceous material is present,
sulphides fill wood cells or replace cell walls. Concretionary-like
sulphide concentrations are abundant in some deposits. Epitaxial quartz
overgrowths on detrital quartz grains are very common and in some
deposits more abundant within or near ore zones than regionally.
Paragenetic studies indicate the epitaxial quartz predates galena.
ORE MINERALOGY (Principal and
subordinate): Galena, sphalerite, and pyrite, chalcopyrite and
various Ni-Co-Fe sulphides. Replacement of sulphides by secondary
analogues has been reported in one or more deposits.
GANGUE MINERALOGY (Principal and
subordinate): Silica, usually chalcedonic, and various carbonate
minerals constitute the most abundant non-sulphide cement.
ALTERATION MINERALOGY: If the
hostrocks were originally arkosic, pre-mineralization alteration
(sometimes referred to as "chemical erosion") of the host sandstones
commonly results in complete, or near-complete, destruction of any
feldspars and mafic minerals which may have been present. Otherwise,
alteration of quartz sandstone hosts is nil. Neomorphic formation of
quartz overgrowths and authigenic clay minerals, however, is a common
feature of these deposits; calcite and sulphates are less common cements.
Pre-sandstone weathering of granitic basement, as evidenced by the
presence of paleoregolith and the destruction of feldspar and mafic
minerals, has been observed beneath several deposits.
ORE CONTROLS: 1. Sialic basement;
those with average lead content greater than ~30 ppm are particularly
significant. 2. Basal portion of grey or white (not red) quartzitic
sandstone of a transgressive sequence on sialic basement. The "cleaner"
portions, with minimum intergranular material, are the preferred host
lithologies because they are more porous. 3. Channels in sandstone,
especially on the periphery of the sedimentary basin. These channels may
also be evident in the basement.
GENETIC MODEL: Groundwater transport
of metals leached from lead-rich basement, through porosity channels in
sandstone; precipitation of metals by biogenically- produced sulphide. A
genetic model involving compaction of brine-bearing basins by over-riding
nappes has been proposed for deposits in Sweden.
ASSOCIATED DEPOSIT TYPES: Sandstone
Cu and sandstone U (D05).
EXPLORATION GUIDES
GEOCHEMICAL SIGNATURE: Stream
sediment and soil geochemical surveys; analyze for Pb and Zn.
GEOPHYSICAL SIGNATURE: Induced
polarization anomalies (?)
OTHER EXPLORATION GUIDES: Epitaxial
quartz overgrowths are abundant, especially within and near the ore zones.
Host sandstones deposited at low paleolatitudes. Sialic basement with high
lead content (>30 ppm). Basal quartz sandstone of a transgressive
sequence, overlying basement. Channels in sandstone as evidenced by
thickening, lateral conglomerate-to-sandstone facies changes, etc.
Permeable zones in sandstone (i.e., “cleanest” sandstone, minimum of
intergranular clayey material).
ECONOMIC FACTORS
TYPICAL GRADE AND TONNAGE: Deposits
range in grade from 2 to 5% Pb, 0.2 to 0.8% Zn, 1 to 20 g/t Ag; most are
less than 10 Mt in size. Because of the disseminated nature of the ore,
tonnages and grades can be markedly affected by changes in cut-off grades.
At Yava, for example, at cut-off grades of 1, 2, and 3%, tonnages and
grades are as follows: 71.2 Mt at 2.09% Pb, 30.3 Mt at 3.01%, and 12.6 Mt
at 3.95%, respectively.
ECONOMIC LIMITATIONS: Because of the
typically low Pb grades and the general paucity of byproduct commodities,
this deposit type has always been a minor player in the world's base metal
markets.
IMPORTANCE: In some countries where
other sources of Pb are limited, sandstone-Pb deposits have constituted
major national resources of this metal (e.g. Sweden).
REFERENCES
Bjírlykke, A. and Sangster, D.F. (1981):
An Overview of Sandstone-Lead Deposits and their Relation to Red-bed
Copper and Carbonate-Hosted Lead-Zinc Deposits; in Economic Geology 75th
Anniversary Volume, 1905- 1980, Skinner, B.J., Editor, Economic Geology
Publishing Co., pages 179-213.
Bjírlykke, A., Sangster, D.F. and Fehn,
U. (1991): Relationships Between High Heat-producing (HHP) Granites
and Stratabound Lead-Zinc Deposits; in Source, Transport and Deposition of
Metals, Proceedings of the 25th Anniversary Meeting, Pagel, M. and Leroy,
J.L., Editors, Society of Geology Applied to Mineral Deposits,
pages 257-260.
Bjírlykke, A. and Thorpe, R.I. (1983):
The Source of Lead in the Olsen Sandstone-Lead Deposit on the Baltic
Shield, Norway; Economic Geology, Volume 76, pages 1205-1210.
Rickard, D.T., Wild‚n, M.Y., Marinder,
N.E. and Donnelly, T.H. (1979): Studies on the Genesis of the Laisval
Sandstone Lead-Zinc Deposits; Economic Geology, Volume 74, pages
1255-1285.
Sangster, D.F. and Vaillancourt, P.D.
(1990): Paleo-geomorphology in the Exploration for Undiscovered
Sandstone-lead Deposits, Salmon River Basin, Nova Scotia; Canadian
Institute of Mining and Metallurgy, Bulletin, Volume 83, pages 62-68.
Sangster, D.F. and Vaillancourt, P.D.
(1990): Geology of the Yava Sandstone- Lead Deposit, Cape Breton
Island, Nova Scotia, Canada; in Mineral Deposit Studies in Nova Scotia,
Volume 1, Sangster, A.L., Editor, Geological Survey of Canada,
Paper 90-8, pages 203-244. |
