Geochemical constraints on the origin of sulfide mineralization in the Duke Island Complex, southeastern Alaska

Abstract

[1] Magmatic Cu-Ni-PGE sulfide mineralization associated with mafic to ultramafic igneous rocks is rare in convergent plate settings. However, recent exploration in the Duke Island Complex of southeastern Alaska has detected horizons of sulfide mineralization primarily in the olivine clinopyroxenite unit. The composition of the sulfides, recalculated to 100% sulfide, averages 0.48% Ni and 1.4% Cu. The Duke Island Complex was generated by the emplacement of multiple pulses of variably fractionated magma ranging in composition from picrite to ankaramite that originated in deeper staging chambers. The presence of sulfides in the complex indicates that fO 2 conditions were more reduced than those commonly ascribed to arc-related magmas. The mass of sulfide mineralization together with the sulfur isotopic values indicates that external sulfur was added to the parent magmas at staging chambers. Os isotopic data from sulfides and C isotopic compositions of graphite in the olivine clinopyroxenites also indicate that the parental magmas were contaminated by crustal material. However, oxygen isotopic values of clinopyroxene together with the REE data suggest that bulk assimilation of country rocks has not occurred and the contamination was selective, involving C-and S-bearing fluids or graphitic sulfides. The observed enrichment in Cu and S isotopic values of the sulfides is best explained by assimilation of sulfides from metal-bearing rock types such as black shales or volcanogenic massive sulfide deposits that occur in terranes above the mantle wedge.

Peridotite enclaves hosted by Mesoarchaean TTG-suite orthogneisses in the Fiskefjord region of southern West Greenland

“…Hanghøj et al [48]. However, elevated Fe is rather common for ultramafic cumulates [83,61,73,115,58]. A cumulate origin may be supported by the field observations of ductile deformation of laterally extensive, parallel layers of chromitites within the dunites, although some mantle sections also display such layered chromitites.…”

Oxygen isotopic variability associated with multiple stages of serpentinization, Duke Island Complex, southeastern Alaska

“…Because of its ultramafic lithology, concentric arrangement of the lithological zones (although not as pronounced as in most other cases), relatively small size and a proposed funnel-shaped cross-section, the Duke Island Complex is categorized as an Alaskan-type complex (Taylor and Noble, 1969;Irvine, 1974;Johan, 2002). The three important rock-types (Irvine, 1974;Saleeby, 1992;Butler et al, 2001;Thakurta et al, 2008a) at the Duke Island Complex are: (1) Dunite: This occurs in a large outcrop in the northern part of the Judd Harbor area. Small outcrops of dunite are also found locally in the Hall Cove area.…”

“…The presence of both fresh rocks and those that are highly altered permits an evaluation of the serpentinization process. Cu-Ni-PGE sulfide mineralization has recently been discovered in the Complex (Thakurta et al, 2008a), and relatively low-temperature hydrothermal alteration may have been important in the mobilization and transport of sulfur and metals away from sites of primary localization. In addition, perturbation to radiogenic isotope systems such as Sm-Nd (Saleeby, 1992) and Re-Os (Thakurta et al, 2008a) has been proposed to in part be a function of hydrothermal processes.…”

“…Cu-Ni-PGE sulfide mineralization has recently been discovered in the Complex (Thakurta et al, 2008a), and relatively low-temperature hydrothermal alteration may have been important in the mobilization and transport of sulfur and metals away from sites of primary localization. In addition, perturbation to radiogenic isotope systems such as Sm-Nd (Saleeby, 1992) and Re-Os (Thakurta et al, 2008a) has been proposed to in part be a function of hydrothermal processes. Wenner and Taylor (1974) analyzed the O and H isotope compositions of four serpentine samples from the Duke Island Complex.…”

SIMS U-Pb zircon dating and Re-Os isotopic analysis of the Hulu Cu-Ni deposit, eastern Tianshan, Central Asian Orogenic Belt, and its geological significance

“…Orogenic convergent plate settings have so far attracted relatively little exploration effort based on theoretical considerations (Maier et al 2008). However, evidence for an enhanced sulfide potential of magmatic-arc settings has begun to accumulate in the last two decades with the discovery of Cu-Ni ores in the Early Proterozoic Sally Malay intrusion of Australia (Hoatson and Blake 2000), the Early Carboniferous Aguablanca intrusion of Spain (Pina et al 2006), the Late Carboniferous-Early Permian Kalatongke intrusion of Xinjiang in NW China (Han et al 2007), the Duke Island Complex in southeastern Alaska (Thakurta et al 2008), the Tati and Selebi-Phikwe belts of eastern Botswana (Maier et al 2008) and the Xiangshan intrusion in NW China (Han et al 2010). The distribution and geodynamic setting of the Hulu Complex are similar to those of the Alaskan-type intrusions.…”