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Evolution of the Late Cretaceous Whistler Au-(Cu) porphyry corridor and magmatic-hydrothermal system, Kahiltna terrane, southwestern Alaska, USA

University of British Columbia

The Whistler Corridor is located in the Alaskan Range 150 km northwest of Anchorage. Hosted by the regionally extensive Kahiltna flysch terrane, the Whistler Igneous Suite (WIS) volcano-magmatic sequence is calc-alkalic, metaluminous, and exhibits an arc related trace element signature. Extrusive rocks comprise andesite flows, volcaniclastic rocks, and hypabyssal dykes and sills. Intrusive rocks are dioritic with two major phases. An initial phase associated with porphyry mineralisation was dated by zircon U-Pb (CA-TIMS) at 76.4 ± 0.3 Ma. A later unmineralised phase had previously been determined by hornblende Ar-Ar at 75.5 ± 0.3 Ma. Mineralised diorite exhibits Nb/Y ratios >1.1 distinct from unmineralised diorite (Nb/Y<1.1). Of several porphyry occurrences the largest is the Whistler deposit hosting an indicated and inferred resource of 3.13 Moz Au and 769 Mlbs Cu. The main Au-Cu zone is characterised by feldspar-stable albite-magnetite (sodic-ferric) and K-feldspar-magnetite (potassic) alteration associated with magnetite (M-) and quartz (A-, B-) veins. High-temperature albitic alteration was characterised using energy dispersive spectroscopic (EDS) analyses of previously unidentified alteration. A peripheral zone of quartz-sericite-pyrite (phyllic) alteration is associated with quartz (D1-3) and pyrite (D4-5) veins. Sphalerite and galena in D3-veins define an overprinting Zn-Pb zone. An intermineral intrusive phase in the core of the deposit is associated with a magmatic-hydrothermal breccia hosting the highest-grade Cu-Au zone. Locally, shallow-level equivalent of D3-veins comprise colloform and crustiform textured intermediate-sulphidation Pb-Zn-Ag-Au veins. Sulphide δ³⁴S isotopes range from 0.4-7.7‰ (xˉ=3.8‰; σ=1.3‰). δ³⁴S in M/A-veins is 1.7‰; B-veins 3.4‰; D1-3 veins 3.7‰; D4-5 veins 4.9‰; and E-veins 6.5‰. δ³⁴S values increase temporally due to the preferential fractionation of ³⁴S into increasingly acidic, reduced fluids. Chlorite and sericite overprint feldspar-stable alteration. Short-wave infrared spectroscopy and X-Ray diffraction demonstrate higher temperature, more crystalline sericite in phyllic than chlorite-sericite alteration. A lack of a negative Nb-Ta anomaly, consistently positive sulphide δ³⁴S isotopes, and oxidised magnetite-series igneous rocks suggest a lack of crustal contamination. Thus, at ca. 76 Ma Whistler represents the earliest, least crustally contaminated, porphyry occurrences of the Late Cretaceous magmatic epoch in SW Alaska.

Text

2014-08-26

Attribution-NonCommercial-NoDerivs 2.5 Canada

http://hdl.handle.net/2429/50184

Geological Sciences

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/2.5/ca/