Archaean precious-metal hydrothermal systems, Dome Mine, Abitibi Greenstone Belt. II. REE and oxygen isotope relations

1979 ◽  
Vol 16 (3) ◽  
pp. 440-458 ◽  
Author(s):  
R. Kerrich ◽  
B. J. Fryer
Keyword(s):  
Oxygen Isotope ◽  
Greenstone Belt ◽  
Precious Metal ◽  
Igneous Rocks ◽  
Lode Gold ◽  
Quartz Veins ◽  
Hydrothermal Quartz ◽  
Abitibi Greenstone Belt ◽  
Chemical Sediments ◽  
Gold Bearing

The Porcupine District, Abitibi Greenstone Belt, is one of the most extensive areas of Archaean auriferous mineralisation. At least two stages of lode-gold emplacement are recognised: the first stage involves gold-bearing carbonate–chert chemical sediments within the lower mafic volcanic sequence; the second stage is represented by auriferous hydrothermal quartz veins which postdate deformation of the greenstone assemblage and transect diverse host rocks.Rare-earth element (REE) concentrations in the stratiform carbonates are typical of the distinctive patterns recorded for Archaean chemical sediments. Chert in these rocks has a δ18O value averaging 17.1‰, implying exchange from heavier 18/16 ratios during diagenesis and metamorphism. Metabasic volcanic rocks and quartz–feldspar porphyry stocks with background gold abundances have mean whole-rock δ18O values of 9.1‰ and 10.7‰ respectively. This enrichment in 18O relative to primary igneous rocks is attributed to oxygen isotope exchange with seawater at low temperatures during fluid transport through the oceanic crust.Quartz in all of the five hydrothermal vein systems present has a δ18O of 14‰ to 15‰, and quartz-muscovite fractionations are 3.4‰ to 3.8‰. Ambient temperatures of mineralisation are estimated to have been 400 °C to 450 °C, from oxygen isotope thermometers, fluid inclusion filling temperatures, and metamorphic mineral assemblages. The calculated δ18O of the mineralising solutions is~10‰, implying fluids of metamorphic origin. REE patterns in hydrothermal quartz veins suggest that they have been derived from high-temperature solutions in equilibrium with source rocks having relatively flat (chondrite normalised) REE distributions, such as tholeiitic and komatiitic volcanics. Adjacent to hydrothermal veins, quartz in igneous rocks approaches isotopic equilibrium with vein quartz, at 15‰, and whole-rock δ18O values for metabasalts shift to ~11‰, implying extensive water-rock interaction. Strong depletions in heavy REE of metabasic schists adjacent to veins provides further evidence for pervasive hydrothermal alteration. The Eu enrichment of all lode gold deposits analysed at Dome Mine is consistent with the reduced state of the solutions involved in their deposition, as recorded by the predominance of Fe2+. The gold-bearing veins are believed to have formed by focussed flow of fluids outgassed at the greenschist–amphibolite transition. Source volumes for Au in the Porcupine District exceed 600 km3, the carrier fluid volume for mineralisation was 60–90 km3, the Au solute concentration in the low nanogram mL−1 range, and transport distances were of the order of 10 km. Such veins may be the precursors of precious-metal-bearing chemical sediments if fluids debouche into the hydrosphere.

Archaean precious-metal hydrothermal systems, Dome Mine, Abitibi Greenstone Belt. I. Patterns of alteration and metal distribution

1979 ◽  
Vol 16 (3) ◽  
pp. 421-439 ◽  
Author(s):  
B. J. Fryer ◽  
R. Kerrich ◽  
R. W. Hutchinson ◽  
M. G. Peirce ◽  
D. S. Rogers
Keyword(s):  
Greenstone Belt ◽  
Precious Metals ◽  
Hydrothermal Systems ◽  
Tectonic Deformation ◽  
Hydrothermal Quartz ◽  
Abitibi Greenstone Belt ◽  
Two Stages ◽  
Host Rocks ◽  
Chemical Sediments ◽  
Gold Bearing

The Porcupine District, Abitibi Greenstone Belt is one of the most extensive areas of Archaean auriferous mineralization. At least two stages of lode gold emplacement may be recognized. The first involves gold-bearing ferroan dolomite layers with subordinate chert, sulphides, and graphite deposited as laterally extensive chemical sediments at interflow horizons within the mafic volcanic sequence. The second stage is represented by major gold-bearing hydrothermal quartz–albite–dravite veins which transect diverse host rocks including the carbonate chemical sediments. Differences between gold-bearing chemical sediment and auriferous hydrothermal veins, in terms of texture, mineralogy, and nature of inclusions, together with considerations of chemistry are not compatible with local derivation of veins from enveloping chemical sediments or adjacent host rocks. The chemical sediments display slump structures and predate all tectonic deformation. In general, auriferous hydrothermal quartz veins transect bedding and/or schistosity, and are at a low state of internal strain. They appear to have been emplaced late during the second regional fold episode.Au, Ag, and Pd average 10, 2, and 0.1 ppm respectively in ore types at the Dome mine; representing concentration factors of 10 000,40, and 10 times background values in unmineralized metabasalt, and primary igneous rocks worldwide. Au and Ag are inhomogeneously distributed.Mineralized metabasic rocks adjacent to vein stockworks have Ti/Zr and Ti/Al2O3 ratios comparable to tholeiitic basalts, but display variable enrichment or depletion of silica, systematic depletion in Na2O, and where intensely altered significant extraction of calcium. The low Ni and Cr contents of the carbonate layers, together with low Ti/Zr ratios (43–78) of the carbonates and their enveloping mafic schists, are not consistent with the hypothesis that these auriferous rocks are carbonated ultramafics. Massive banded quartz–fuchsite–dravite veins have Cr and Ni abundances averaging 350 ppm, implying hydrothermal transport of these elements. Ti/Zr ratios of 120, together with high Mg, Cr, and Ni abundances in magnesite–dolomite–quartz– chlorite schists which host the banded veins are compatible with a primary komatiitic composition. Mineralized metabasic rocks are reduced (Fe2+/ΣFe = 0.9) relative to rocks with primary background abundances of precious metals (Fe2+/ΣFe = 0.7). This change of oxidation state implies that large volumes of reducing hydrothermal solutions were involved in vein mineralization.

IV. On a peculiar pasty form of Silica, from a cavity in Gold-bearing Quartz

1880 ◽  
Vol 4 (17) ◽  
pp. 42-43
Author(s):  
A. French
Keyword(s):  
Suspended Matter ◽  
Precious Metal ◽  
Quartz Veins ◽  
First Impression ◽  
Long Time ◽  
Gold Bearing

The alluvium over an area of about 50 square miles around Lead-hills in Lanarkshire, is auriferous. In many places the precious metal may be rendered visible after fifteen or twenty minutes washing with the primitive wooden trough employed by the local gold-seekers. Frequently nuggets have been found weighing from one to four or five pennyweights, and these are often either contained in pieces of loose quartz, or have quartz fragments attached to them; there are therefore good reasons to believe that the gold found in the red stratum of clay lying immediately above the rock has been derived from the numerous quartz veins which traverse the district. The author was one day searching, along with some friends, for gold quartz in situ, and while examining a vein which crosses a gulch in the hills, called the “Gold Scars,” found cavities in the quartz filled with a peculiar pasty substance, which appeared at first sight like minute scales of silver, which had been precipitated from a solution. Our first impression was that it was silver, but its behaviour in water soon cleared away the delusion; it rendered the water exceedingly turbid and white, and the suspended matter was a long time in settling.

Metallogenesis of the Abitibi greenstone belt of Canada: a contribution from the analysis of trace lead in sulfide minerals

1989 ◽  
Vol 26 (12) ◽  
pp. 2529-2540 ◽  
Author(s):  
Etienne Deloule ◽  
Clément Gariépy ◽  
Bernard Dupré
Keyword(s):  
Greenstone Belt ◽  
Igneous Rocks ◽  
Lava Flows ◽  
Ore Formation ◽  
Abitibi Greenstone Belt ◽  
Isotopic Compositions ◽  
Leaching Experiments ◽  
Iron Formations ◽  
Different Sources

Pb-isotopic compositions are reported for 12 ore localities within the late Archean Abitibi greenstone belt. The studied samples carry massive or disseminated sulfides hosted in a variety of materials, including mafic–ultramafic igneous rocks, felsic lavas, porphyries, and sedimentary iron formations. Repeated leaching experiments on these sulfides frequently revealed the presence of a radiogenic Pb component, which is attributed to in situ decay of U and Th. The leaching experiments make it possible, in some cases, to separate the radiogenic Pb from the initial Pb included in the minerals. Six Pb–Pb isochrons formed by the analyses on leachates and residues show little evidence of secondary perturbations and yield ages that are, within error, similar to those determined for the supracrustal assemblage. This implies that the ores were concentrated synchronously with the main phases of magmatic activity, close to 2.7 Ga.The initial isotopic compositions of the sulfide specimens point to the existence of two different sources of metals: (i) juvenile, mantle-derived igneous rocks and (ii) older recycled supracrustal series. Ore formation frequently involves mixing of metals from these two sources in variable proportions. The initial isotopic composition of these two reservoirs is best evaluated by examining the composition of sulfides associated with komatiitic lava flows and with sedimentary iron formations, respectively.

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