Ames et al dating Sudbury

INTRODUCTION

• Sudbury Structure: An enigmatic geological feature with controversies regarding its formation:

  • Possible formation from meteorite impact (Dietz, 1964; Grieve et al., 1991).

  • Alternative theories include explosive volcanism (Muir, 1984) or a combination of both, including impact-induced magmatism (Naldrett, 1984).

• Models for Impact Cratering: Comprised of seven stages:

  1. Initial impact

  2. Compression

  3. Rarefaction and attenuation

  4. Excavation

  5. Ejection and fallback

  6. Mechanical modification

  7. Hydrothermal and chemical alteration (Kieffer and Simonds, 1980; Melosh, 1989).

• Impact-Induced Hydrothermal Events:

  • Documented (McCarville and Crossey, 1996; Allen et al., 1982).

  • Timing of ground or seawater influx into hot craters and hydrothermal alteration is still unclear.

  • Most terrestrial impact craters show minor alteration; Sudbury is a notable exception (Ames and Gibson, 1995).

• Research Focus: Integrating mapping of the Sudbury structure with geochronology to:

  • Constrain the age of impact and hydrothermal episodes.

  • Identify sources for xenocrystic zircons and establish connections between the hydrothermal system and impact.

GEOLOGIC SETTING OF THE SUDBURY STRUCTURE

• Location: Straddles the Archean Superior Province and Proterozoic Southern Province boundary.

• Components: Comprised of:

  • 60 x 30 km Sudbury igneous complex

  • Interior Whitewater Group

  • Brecciated footwall rocks

• Granulite Facies Rocks:

  • Levack Gneiss Complex forms a northern arcuate belt in the footwall of the Sudbury igneous complex.

  • Exhumation occurred in two stages:

    • Late Archean uplift (2648 to 2625 Ma).

    • Final exhumation during the 1850 Ma impact event (Wodicka, 1997).

• Emplacement Ages: Sudbury igneous complex and xenoliths dated at 1850 Ma (Krogh et al., 1984), confirming the age of meteorite impact.

SAMPLING AND GECHRONOLOGY

• U-Pb Geochronologic Data:

  • Links hydrothermal activity with the 1850 Ma impact event.

  • Constrains complex impact processes to less than 4 million years.

• Alteration Types in the Onaping Formation:

  • Semiconformable alterations including:

    • Silicification

    • Albitization

    • Chloritization

    • Calcitization

    • Complex feldspathization

  • Directly related to Zn-Cu-Pb ore deposits.

• Hydrothermal System Age: 1848.4 +3.8/–1.8 Ma.

REGIONAL HYDROTHERMAL ALTERATION

• Alteration Patterns: A comprehensive circulation system of hydrothermal fluid altered rocks in the Onaping Formation, leading to:

  • Stacked alteration zones characterized by:

    • Silicification

    • Albitization

    • Chloritization

    • Calcitization

    • Feldspathization (Ames and Gibson, 1995; Ames et al., 1997).

• Base-Metal Deposits: Associated with alteration zones in the Onaping Formation and the Vermilion Formation, extending into overlying shales.

• Importance of Study: Understanding the mechanics of large-scale impact craters like Sudbury can provide insights into other craters, such as Chicxulub and Vredefort.

DISCUSSION

• Onaping Formation Timing:

  • Emplacement constrained to 1851–1847 Ma due to Shocked zircons.

• Heat Source for Hydrothermal Activity:

  • Derived from the 1850 Ma impact event.

  • Hydrothermal systems were short-lived, lasting from tens to hundreds of thousands of years.

• Conclusion: The presence of distinct stratigraphic sequences indicates more complex processes than mere fallback breccia. The impact-induced hydrothermal system yielded alteration zones and mineralizations comparable to volcanic regions.