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Chumbo (Zinco) em arenitos

by D.F. Sangster
Geological Survey of Canda, Ottawa


Ref: Pb, galena, areanito, paleoalto

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.



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).


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).


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).


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).


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.

DEPÓSITOS - 26/04/2004 18:34:00

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