Mid-Cretaceous Magmatic Evolution and Intrusion-Related Metallogeny of the Tintina Gold Province, Yukon and Alaska Craig J.R. Hart M.Sc. University of British Columbia 1995 B.Sc. McMaster University 1986 This thesis is presented for the degree of Doctor of Philosophy Centre for Global Metallogeny School of Earth and Geographical Sciences The University of Western Australia October 2004 1 Supervisors David I. Groves and Richard J. Goldfarb Collaborators John L. Mair, Lara L. Lewis, Moira Smith, Dan T. McCoy, Roger Hulstein, Paul Roberts, Tom Bundtzen Mike E. Villeneuve, Arne A. Bakke, David Selby, Chris Wijns 2 All work presented in this thesis is original and that of the candidate, unless otherwise specified, acknowledged or referenced. __________________________________ Craig J.R. Hart 18 October 2004 3 “There is nothing so sobering as an outcrop.” Francis Pettijohn (undated) The apical portion of the Tuna Stock as exposed in eastern Yukon. This slightly pera- luminous pluton is a member of the mid-Cretaceous Tungsten Plutonic Suite. 4 Abstract The Tintina Gold Province (TGP) comprises numerous (>15) gold belts and districts throughout interior Alaska and Yukon that are associated with Cretaceous plutonic rocks. With a gold endowment of ~70Moz, most districts are defined by their placer gold contributions, which comprise greater than 30 Moz, but four districts have experience significant increases in gold exploration with notable discoveries at Fort Knox (5.4 Moz), Donlin Creek (12.3 Moz), Pogo (5.8 Moz), True North (0.79 Moz), and Brewery Creek (0.85 Moz). All significant TGP gold deposits are spatially and temporally related to reduced (ilmenite-series) and radiogenic Cretaceous intrusive rocks that intrude (meta-) sedimentary strata. The similar characteristics that these deposits share are the foundation for the development of a reduced intrusion-related gold deposit model. Associated gold deposits have a wide variety of geological and geochemical features and are categorized as intrusion-centered (includes intrusion-hosted, skarns and replacements), shear- related, and epizonal. The TGP is characterized by vast, remote under-explored areas, unglaciated regions with variable oxidation depths and discontinuous permafrost, which, in combination with a still-evolving geological model, create significant exploration challenges. Twenty-five Early and mid-Cretaceous (145-90 Ma) plutonic suites and belts are defined across Alaska and Yukon on the basis of lithological, geochemical, isotopic, and geochronometric similarities. These features, when combined with aeromagnetic characteristics, magnetic susceptibility measurements, and whole-rock ferric:ferrous ratios define the distribution of magnetite- and ilmenite-series plutonic belts. Magnetite-series plutonic belts are dominantly associated with the older parts of the plutonic episode and comprise subduction-generated metaluminous plutons that are distributed preferentially in the more seaward localities dominated by primitive tectonic elements. Ilmenite-series plutonic belts comprise slightly-younger, slightly- peraluminous plutons in more landward localities in pericratonic to continental margin settings. They were likely initiated in response to crustal thickening following terrane collision. The youngest plutonic belt forms a small, but significant, magnetite-series belt in the farthest inboard position, associated with alkalic plutons that were emplaced during weak extension. Intrusion-related metallogenic provinces with distinctive metal associations are distributed, largely in accord with classical redox-sensitive granite-series. Copper, Au and Fe mineralisation are associated with magnetite-series plutons and tungsten mineralisation associated with ilmenite-series plutons. However, there are some notable deviations from expected associations, as intrusion-related Ag-Pb-Zn deposits are few, and significant tin mineralisation is rare. Most significantly, many gold deposits and occurrences are associated with ilmenite-series plutons which form the basis for the reduced intrusion-related gold deposit model. Among the mid-Cretaceous TGP plutonic suites, the Tombstone, Mayo and Tungsten suites are the most metallogenically prolific. The Tombstone suite is alkalic, variably fractionated, slightly late mafic phases, moderately reduced with titanite dominant, and has xenocrystic zircon. The Tungsten suite is peraluminous, entirely felsic, more highly fractionated, reduced with ilmenite i dominant, and has abundant xenocrystic zircon. Each suite has a distinctive petrogenesis. The Tombstone suite was derived from an enriched, previously-depleted lithospheric mantle, the Tungsten suite intrusions from continental crust including, but not dominated by, carbonaceous pelitic rocks, and the Mayo suite from similar sedimentary crustal source, but is mixed with a distinct mafic component from an enriched mantle source. Each of these three suites has a distinctive metallogeny that is related to the source and redox characteristics of the magma. The Tombstone suite has a Au-Cu-Bi association that is characteristic of most oxidized and alkalic magmas, but also has associated, and enigmatic, U- Th-F mineralisation. The reduced Tungsten suite intrusions are characterized by world-class tungsten skarn deposits with less significant Cu, Zn, Sn, and Mo anomalies. The Mayo suite intrusions are characteristically gold-enriched, with associated As, Bi, Te, and W associations. All suites also have associated, but distal and lower temperature, Ag-Pb- and Sb-rich mineral occurrences. Although processes such as fractionation, volatile enrichment, and phase separation are ultimately required to produce economic concentrations of ore elements from crystallizing magmas, the nature of the source materials and their redox state play an important role in determining which elements are effectively concentrated by magmatic processes. Intrusion-related gold systems have an implied genetic relationship with a cooling pluton, but confident links are difficult to establish. U-Pb and Ar-Ar geochronology in the TGP has established main mineralizing events at circa 90 and 70 Ma, but precise temporal links between magmatism and mineralization have been difficult to confirm. An integrated geochronological approach to understand the magmatic-hydrothermal evolution of the Fort Knox, Clear Creek, Scheelite Dome, Mactung and Cantung intrusion-related deposits using SHRIMP zircon U-Pb, mica Ar-Ar, molybdenite Re-Os, and TIMS U-Pb data. TIMS U-Pb data, and SEM imagery of plutonic zircons indicate considerable inheritance and Pb-loss in the zircons. SHRIMP analyses avoid Pb-loss and inheritance and result in dates that are up to 4 m.y. older than the TIMS dates. Ar-Ar dates for magmatic biotite and hornblende indicate that most plutons cooled quickly, but Ar-Ar dates on hydrothermal micas from mineralization are 2-3 million years younger than the magmatic mica dates suggesting that Ar retention in hydrothermal micas may differ from that in magmatic micas. Re-Os molybdenite dates are in best agreement, and within the uncertainty of the SHRIMP dates for the host plutons. Direct comparisons of the absolute ages indicate magmatic-hydrothermal durations from 1.1 to 3.3 million years, which expand from 2.2 to 4.2 m.y. with the extremes of the uncertainties. These durations contrast thermal modeling which indicates rapid cooling to below ~250°C within 0.03 to 0.3 million years. Long-lived magmatic- hydrothermal systems may result from episodic replentishment of assembling magmatic plutons. Alternatively, the discrepancy may result from within-system uncertainties of geochronological decay constants and the compounding effects of decay constant errors across isotopic systems which conspire to increase the uncertainties of comparative dates. ii Contents Abstract ......................................................................................................................... i Contents ..................................................................................................................... iii List of Appendices ...................................................................................................... vii Acknowledgements ....................................................................................................viii Chapter One INTRODUCTION Preamble .....................................................................................................................1 Characteristics of IRGS .............................................................................................2 Concerns of Classif cation..........................................................................................2 Thesis Aims and Objectives .........................................................................................3 Thesis Organisation .....................................................................................................3 Supportive and Associated Contributions ....................................................................4 References ..................................................................................................................7 Chapter Two GEOLOGY, EXPLORATION and DISCOVERY in the TINTINA GOLD PROVINCE, ALASKA and YUKON Abstract ...................................................................................................................... 11 Introduction ...............................................................................................................