Gold Mineralization in Archean Banded Iron Formation, Third Portage Lake Area, Northwest Territories, Canad

Abstract Oxide-facies banded iron formation in the Third Portage Lake area is hosted by ultrama?c and calc-alkaline volcanic rocks and related sedimentary rocks of the Archean (2.8 Ga) Woodburn Lake Group, 80 km north of Baker Lake. These rocks record at least two phases of metamorphism/metasomatism. Greenschist facies metamorphism, in the temperature range of 370°C to 520°C and at pressures above 2.3 kbar, has affected all rocks on a regional scale. Later mineralization and alteration of banded iron formation (BIF) and associated volcanic rocks resulted from metasomatic ?uids which carried Mg, K, Ca, S, As, Cu and Au. These ?uids were con?ned to ductile to brittle, cross-cutting and layer-parallel micro-shears developed in the thickened hinge region of a shallow plunging recumbent fold. Metasomatic reactions yielded cummingtonite + biotite-bearing assemblages which are associated with strongly mineralized iron formation. Pyrrhotite is the dominant sul?de phase and occurs within ductily deformed iron formation where it pervasively replaces magnetite. Minor ?ne-grained chalcopyrite and arsenopyrite generally occur as inclusions within pyrrhotite and recrystallized magnetite, and rarely as individual grains. Pyrite is restricted to brittlely deformed iron formation, and to the contact between iron formation and altered ultrama?c rocks, where it is associated with quartz veins. Pyrite typically replaces pyrrhotite. Native gold is associated with both pyrrhotite and pyrite, but higher gold concentrations are associated with pyrite. Grunerite/Ca amphibole + stilpnomelane and sparse pyrrhotite and pyrite characterize weakly mineralized iron formation. Within adjacent ultrama?c rocks, metasomatic reactions yielded assemblages of interlayered Ca-amphibole + biotite, and talc + Ca-amphibole + carbonate + biotite. K-Ar geochronology on metasomatic biotite from mineralized BIF yielded a 1791 ± 32 Ma age. An epigenetic model is favored for the emplacement of gold. The age of the gold mineralization in the Third Portage Lake area is uncertain, but may be Proterozoic based on the preliminary isotopic age and by analogy with Proterozoic gold deposits elsewhere in this part of the Churchill Structural Province.
Keywords: Mineralization, Gold mineralization, Northwest Territories, Portage Lake, Metamorphism.
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Summary: The Geological Society has its own quarterly journal called Exploration
and Mining Geology, for the publication of Canadian and international papers on
applied aspects of mineral exploration and exploitation, including mineral
deposit geology, geochemistry, and geophysics, mining geology, mineral resource
appraisal and estimation methods, environmental geology, and case histories. The
editor of the journal is Jeremy P. Richards
Publication: Exploration & Mining Geology
Issue: 1
Volume: 5
Year: 1996
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Summary: The Na-feldspar orebodies in Sardinia occur along structures belonging to a regional ENE-WSW system that developed during the late stages of the Hercynian orogeny, and that was reactived during the Alpine orogeny. The main petrographic facies include granitoids, albitized granitoids, albitites (more or less rich in quartz), and albitites containing Mg minerals. Field observations and petrological and geochemical data point toward a metasomatic origin for the albitite orebodies.
Albitization...
Publication: Exploration & Mining Geology
Author(s): R. BORNIOLI, S. FADDA, M. FIORI, S.M. GRILLO
Keywords: Albite deposits, Mineralogical evidence, Geochemical evidence, Na-feldspar orebodies, albitizing fluids
Issue: 1
Volume: 5
Year: 1996
Text
Summary: The East Sullivan Stock (ESS) is a small, composite felsic alkaline pluton of presumed late Archean age in the Val d’Or mining district. Three main facies have been distinguished: monzonite
sensu stricto, porphyritic monzonite, and green trachytic porphyry. This monzonitic group of
rocks is of relatively homogeneous composition with high abundances of Na2O, K2O, Ba, Sr, Rb, Zr,
Mo and LREE. The alkaline ESS is distinguished from other igneous units in the Val d’Or mining
district, such as the...
Publication: Exploration & Mining Geology
Author(s): MEHMET F. TANER
Keywords: East Sullivan Stock, Gold-Copper, exploration, Hydrothermal solution
Issue: 1
Volume: 5
Year: 1996
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Summary: The Ni-rich Epidote Zone of the Fraser Mine at Sudbury is characterized by alteration assemblages rich in epidote, actinolite and magnetite which are roughly concentrically zoned and up to hundreds of meters in length. These zones form discrete areas within the igneous Footwall Breccia below the Sudbury Igneous Complex. The assemblages grade from epidotized Footwall Breccia matrix, through actinolite-bearing rocks with few vestiges of original Footwall Breccia textures, into the most...
Publication: Exploration & Mining Geology
Author(s): CATHARINE E.G. FARROW
Keywords: Geochemistry, Geochemical evolution, Frasier Mine.
Issue: 1
Volume: 5
Year: 1996
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