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Freitag, Katja, 2000

Geology and structure of the Lower southwest Orebody, Greens Creek Mine, Alaska

Bibliographic Reference

Freitag, Katja, 2000, Geology and structure of the Lower southwest Orebody, Greens Creek Mine, Alaska: Golden, Colorado, Colorado School of Mines, Ph.D. dissertation, 248 p., illust., maps (some color).


The Greens Creek Zn-Pb-Ag-(Cu-Au) volcanogenic massive sulfide deposit, in the Alexander terrane in southeastern Alaska, formed by structurally controlled hydrothermal activity in a back-arc basin environment. Numerous sulfide orebodies are located discontinuously along the contact between highly altered mafic volcanic/volcaniclastic stratigraphic footwall rocks and stratigraphically overlying graphitic metasiltstones and metashales. Subsequent to formation, the orebodies were multiply deformed during accretionary and, later, wrench tectonic activity. The complex structure of the deposit includes open to isoclinal folds that are refolded by open to closed folds, low-angle faults, and high-angle strike-slip faults. Detailed rib maps and constructed plan maps, cross sections and long sections were used to define the structural geometry and style, and enable a nonrigorous palinspastic reconstruction of the Lower Southwest orebody. Petrographic, electron backscatter diffraction, and stable isotope studies were used to characterized sulfide and carbonate assemblages. The earliest deformation features, inferred from ore thicknesses and high Au and Cu grades, are late Triassic, syndepositional normal (?) faults (Dmineral deformation). During diagenesis (D0 deformation), early 'ladder' veins formed perpendicular to metasiltstone beds and, during partial subduction of the Alexander terrane in the Jurassic, these beds were offset along small thrust faults (D1 deformation). Late Jurassic to early Tertiary accretion of the Alexander terrane resulted in south-southwest- to southeast-plunging, open to isoclinal folds (D2 deformation) and southeast plunging, open folds (D3 deformation), with orebody thickness and footwall and ore lithologies controlling fold geometry. The D4 deformation event signified a change from brittle to ductile deformation, and formed thrust faults and gentle folds. Starting in the Eocene, wrench tectonics resulted in high-angle faults (D5), which locally reactivated the thrust faults. Despite intense folding, most pyrite, galena, and sphalerite textures are predominantly syndepositional, with local remobilization of galena, sulfosalts, chalcopyrite, and gold into fragmented pyrite, and only subtle deformation textures observed along open to isoclinal fold limbs. A comparison to sulfide textures of undeformed and deformed volcanogenic massive sulfide deposits indicates that sulfide textures at Greens Creek are similar to deformed massive sulfides that have been metamorphosed to lower greenschist facies conditions. Palinspastic reconstruction of the Lower Southwest orebody reveals spatially distinct, coeval units of massive sulfide and hydrothermal, mineralized dolomite. Although stockwork zones are not found beneath the orebody, locally developed, thickened sections of the sulfide and dolomite units, which coincide with highest grade:thickness ratios and highest Cu and Au grades, are interpreted to indicate proximity to hydrothermal fluid vent sites.

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