Geochemical Stratigraphy of the Keweenawan Midcontinent Rift Volcanic Rocks with Regional Implications for the Genesis of Associated Ni, Cu, Co, and Platinum Group Element Sulfide Mineralization

2015 ◽  
Vol 110 (5) ◽  
pp. 1235-1267 ◽  
Author(s):  
R. R. Keays ◽  
P. C. Lightfoot
Keyword(s):  
Platinum Group Element ◽  
Volcanic Rocks ◽  
Group Element ◽  
Sulfide Mineralization ◽  
Midcontinent Rift ◽  
Platinum Group

Multiple S Isotopes and S Isotope Heterogeneity at the East Eagle Ni-Cu-Platinum Group Element Deposit, Northern Michigan

2020 ◽  
Vol 115 (3) ◽  
pp. 527-541
Author(s):  
E.K. Benson ◽  
E.M. Ripley ◽  
C. Li ◽  
B.W. Underwood ◽  
R. Mahin
Keyword(s):  
Platinum Group Element ◽  
Group Element ◽  
Spatial Proximity ◽  
Small Scale ◽  
Sulfide Mineralization ◽  
Exchange Reactions ◽  
Wide Range ◽  
Platinum Group ◽  
Northern Michigan ◽  
S Isotope

Abstract The East Eagle Ni-Cu-platinum group element deposit is a conduit-type deposit located in northern Michigan, in close spatial proximity to the currently producing Eagle deposit. Massive and semimassive (net-textured) sulfide mineralization at East Eagle occurs approximately 800 m lower in the stratigraphic sequence than that at Eagle and only ~200 m above the contact between Proterozoic and Archean rocks. Although sulfide mineralogy and textural types are similar at the two occurrences, there are important differences in their S isotope systematics. Massive sulfide mineralization at East Eagle is characterized by a relatively narrow range of δ34S values from 1.5 to 3.2‰. Semimassive sulfides show a similar range from 2.1 to 3.8‰. In strong contrast to these values, those from disseminated sulfides that border the massive and semimassive mineralization define a much larger range from –4.3 to 22.8‰. The much more restricted range in δ34S values recorded in the massive and semimassive sulfide mineralization compared to that of the disseminated mineralization is thought to reflect isotopic exchange reactions in the conduit involving accumulated sulfide and pulses of magma containing S of mantle origin. The ∆33S values of all three major types of sulfide mineralization at East Eagle are near 0‰, with most values between –0.03 and 0.03‰. Unlike ∆33S values from semimassive sulfide mineralization at Eagle, the ∆33S values at East Eagle show no, or very limited, evidence for the involvement of S derived from Archean sedimentary rocks. The wide range in δ34S values recorded in the disseminated mineralization provides strong evidence that S from Proterozoic sedimentary host rocks was involved in the mineralization; in some cases, as much as 85% of the S may have been of external origin. In addition to the wide range in δ34S values, the disseminated mineralization is characterized by spatially heterogeneous δ34S values. Meter-scale S isotope variations, as well as variations in Pt and Pd tenor, are consistent with multiple inputs of magma, each characterized by distinct S isotope ratios. Heterogeneity of several per mill at the centimeter scale indicates that the degree of supercooling exceeded the S diffusivity, preserving small-scale S isotope variability inherited from the sedimentary country-rock source. Elongate, branching plagioclase grains in many of the gabbroic rocks that host the disseminated sulfide mineralization are consistent with a rapid second stage of cooling.

Controls on the regional distribution of iron – nickel – copper – platinum-group element sulfide mineralization in the eastern Cape Smith Belt, Quebec

1994 ◽  
Vol 31 (1) ◽  
pp. 206-218 ◽  
Author(s):  
M. R. St-Onge ◽  
S. B. Lucas
Keyword(s):  
Platinum Group Element ◽  
Regional Distribution ◽  
Volcanic Rocks ◽  
Three Dimensional ◽  
Sedimentary Facies ◽  
Group Element ◽  
Eastern Cape ◽  
Platinum Group ◽  
High Level ◽  
Lava Lakes

Fe – Ni – Cu – platinum-group element (PGE) mineralization in the eastern Cape Smith Belt (Quebec) is hosted by ultra-mafic and differentiated mafic–ultramafic bodies (high-level sills and dykes, lava lakes, or channelized lava flows). These units of Early Proterozoic age are associated with both volcanic and sedimentary rocks (Povungnituk Group) that accumulated during continental rifting and a transitional continental to oceanic basalt sequence (Chukotat Group). Mineralization is found in two principal horizons. Important deposits occur in the Raglan horizon located near the tectonic boundary between the Povungnituk and Chukotat groups. A second mineralized zone, termed the Delta horizon, occurs within the Povungnituk Group and contains smaller sulfide showings. The distribution of proximal–distal sedimentary facies in the Povungnituk Group represents an important control on the localization, grade, and size of the sulfide deposits and showings. Deposits in the Raglan horizon are associated with relatively distal sedimentary units of the Povungnituk Group. In contrast, the Delta horizon occurs in a more proximal (quartz-rich) facies of the Povungnituk Group which is interlayered with volcanic rocks and marked by a profound decrease in the proportion of fine-grained, sulfuric beds relative to the Raglan horizon. Regional mapping has shown that the Cape Smith Belt is a thin-skinned thrust belt containing imbricates of Povungnituk Group and Chukotat Group rocks and thus controlling the regional extent of the mineralized horizons. Critically, the along-strike continuity of the Raglan horizon (85 km) is determined by the three-dimensional structural geometry of a late thrust fault.

Relationships among the Geordie Lake Cu-Pd deposit, alkaline basalt, and syenites in the Coldwell Complex, Midcontinent Rift, Canada

2021 ◽  
Vol 59 (6) ◽  
pp. 1571-1597
Author(s):  
David J. Good ◽  
Robert L. Linnen ◽  
Imran Meghji ◽  
Iain M. Samson ◽  
John McBride
Keyword(s):  
Trace Element ◽  
Platinum Group Element ◽  
Group Element ◽  
Primitive Mantle ◽  
Basalt Magma ◽  
Alkaline Basalt ◽  
Midcontinent Rift ◽  
Hydrothermal Processes ◽  
Platinum Group ◽  
Sulfide Assemblage

ABSTRACT The Geordie Lake Cu-Pd deposit is associated with troctolite at the base of the Geordie Lake intrusion, located near the center of the Coldwell Complex (1106.5 + 1.2 Ma). It is the only platinum group element deposit in the Midcontinent Rift associated with alkaline rocks. This study focuses on the long-standing questions regarding genetic relationships among the Geordie Lake gabbros, the Wolfcamp basalt, and the various syenites that make up the east-central portion of the Coldwell Complex. Primitive mantle-normalized trace-element patterns for the Geordie Lake intrusion are nearly flat from Th to Ce and show negative Sr, Eu, and Zr anomalies. Characteristic ratios for the Geordie Lake gabbro and troctolite include Th/Nb (0.12), La/Nb (1.1), La/Lu (150), La/Sm (6.9), Zr/Sm (18), and Gd/Yb (2.8). Trace-element patterns that are useful for determining petrogenesis for gabbros are similar to the Wolfcamp basalt and augite syenite with some key exceptions, notably the middle rare earth element and Zr abundances. Affects due to metasomatism or crustal contamination in Wolfcamp basalt and Geordie Lake gabbros and syenites are negligible. Results of Rayleigh fractionation modeling show (1) the Geordie Lake intrusion and Wolfcamp basalt are very similar but not directly related by crystallization, (2) the gabbros and basalt are not related to the syenites, (3) the lower augite syenite can be related to the upper augite syenite and amphibole quartz syenite by fractionation of a hypothetical crystal cumulate composed of orthoclase (78%), clinopyroxene (15%), olivine (1%), and titanomagnetite (6%). We conclude that the Geordie Lake intrusion, Wolfcamp basalt, and saturated syenites in the Coldwell were derived by separate partial melting events in a common mantle source. The origin of the sulfide mineralization is enigmatic because it exhibits characteristics of both magmatic and hydrothermal processes. The sulfide assemblage changes from disseminated bornite and chalcopyrite in the basal zone to pyrrhotite plus chalcopyrite in the upper zones. Sulfides occur as coarse blebs interstitial to fresh or partly altered silicates, or as very fine grains intergrown with clusters of biotite and actinolite. Primitive mantle-normalized platinum group element patterns exhibit a W-shape for Pd-Pt-Rh-Ir-Ni, indicating a relative depletion of Pt and Ir. The Cu/Pd ratios in the mineralized zones are within the range of mantle values (1000–10,000), Pd/Pt is 14–19, Pd/Rh is 91 + 37, and Pd/Ir >16,000. The Pd/Pt, Pd/Rh, and Pd/Ir are considerably higher than in the Wolfcamp basalt (<1, 17, and 75, respectively). If the sulfides are magmatic in origin, then either the Geordie Lake magma was, unlike the Wolfcamp basalt magma, depleted in Pt, Rh, and Ir, or these elements were selectively removed from the sulfide assemblage. Alternatively, Pd was enriched by late-stage hydrothermal processes. Additional work is recommended to constrain petrogenesis of the sulfides by detailed base-metal and TABS (Te, As, Bi, Sb, and Sn) element analysis.

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