Geochemical Evolution of the Epidote Zone, Fraser Mine, Sudbury, Ontario: Ni-Cu-PGE Remobilization by Saline Fluid

Abstract 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 intensely altered rock, a magnetite-rich facies. Pentlandite, pyrrhotite and pyrite are the dominant sul?de minerals; chalcopyrite is minor. The strongest geochemical differences compared to original Footwall Breccia occur where epidote and actinolite dominate the alteration assemblage (EPDZact facies). The most intense alteration facies are typi?ed by low Cu/(Cu+Ni) values of <0.1. Strong positive correlations exist between Ni and Co, Cu and Au, and among Ir, Os, Ru and Rh. Chondrite-normalized Ni - PGE - Au - Cu plots display an overall increase in all elements from the least to the most intensely altered facies except for Au and Cu. As the proportion of actinolite increases in the EPDZact facies, the chondrite-normalized pro?les ?atten considerably due to low Au and Cu contents. Abundances of Ni, Ir, Ru and Rh almost ubiquitously increase with degree of alteration. In contrast, Cu, Zn, Ag and Au contents are low in almost all facies. It is suggested that saline hydrothermal ?uids moved through the Epidote Zone formation and became more oxidized as they reacted with the Footwall Breccia and with footwall units, including ultrama?c rocks. As this trend continued, magnetite-rich zones eventually developed. The occurrence of calcite locally in these magnetite-rich zones suggests that the pH of the ?uid increased, causing Cu solubility to drop as activity in the hydrothermal system waned. Sudden changes in ?uid composition such as this may have been due to an in?ux of ?uid from bounding ultrama?c rocks. Hydrothermal activity was focused along the margins of footwall ultrama?c units. Individual hydrothermal cells may have formed in the Footwall Breccia along pre-existing structures or sul?de concentrations. Metals may have been scavenged from sul?de-bearing Footwall Breccia and possibly Sublayer rocks. Nickel and some PGE were deposited in the Epidote Zone, whereas Cu, Au and Ag continued outward into the footwall where they were deposited as Cu-rich footwall veins.
Keywords: Geochemistry, Geochemical evolution, Frasier Mine.
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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: 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...
Publication: Exploration & Mining Geology
Author(s): A.E. ARMITAGE, R.S. JAMES
Keywords: Mineralization, Gold mineralization, Northwest Territories, Portage Lake, Metamorphism.
Issue: 1
Volume: 5
Year: 1996
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