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Re-Os MOLYBDENITE AGES FROM THE ARCHEAN YELLOWKNIFE GREENSTONE BELT: COMPARISON TO U-Pb AGES AND EVIDENCE FOR METAL INTRODUCTION AT ~2675 Ma

Luke Ootes^,* and David R. Lentz

Department of Geology, University of New Brunswick, Box 4400, Fredericton, New Brunswick, Canada E3B 5A3

Robert A. Creaser

Department of Earth and Atmospheric Sciences, University of Alberta, 1-26 Earth Sciences Building, Edmonton, Alberta, Canada T6G 2E3

John W. F. Ketchum and Hendrik Falck

Northwest Territories Geoscience Office, Box 1500, 4601b-52nd Avenue, Yellowknife, Northwest Territories, Canada X1A 2R3

Corresponding author: e-mail, luke_ootes{at}gov.nt.ca

The late Archean Yellowknife greenstone belt in the southwestern Slave province hosts world-class orogenic gold deposits with total production of 15 million ounces (Moz) of gold. The timing of gold mineralization is estimated to be ca. 2600 Ma, based on the relative timing of D₂ structures and associated formation of deposit-hosting shear zones. The majority of studies on the deposits indicate that gold was emplaced by metamorphically driven hydrothermal fluids depositing gold in chemical and structural traps. Alternatively, based on ore mineral compositions, proximity to plutons, and increased gold grades in the vicinity of quartz-feldspar porphyry dikes, it has been suggested that the gold deposits may also be related to the intrusions.

In this study, we present two new Re-Os molybdenite ages from two intermediate to felsic plutonic rocks and compare them with U-Pb zircon crystallization ages. The Duckfish aplite contains intergrown molybdenite and yields a Re-Os model age of 2611 ± 7 Ma, the same as a U-Pb zircon crystallization age of 2611.2 ± 1.5 Ma for the aplite and within assigned error of a previously reported 2608 ± 3 Ma U-Pb titanite age for the Duckfish pluton. The subvolcanic Ryan Lake pluton has synplutonic quartz veining with intergrown molybdenite that yields an Re-Os model age of 2679 ± 8 Ma (repeat analysis of 2675 ± 7), within assigned error of the 2671 +6/–5 Ma U-Pb zircon crystallization age of the pluton. In both cases, the Re-Os and U-Pb chronometers survived later low- to moderate-temperature, low-pressure contact and regional metamorphism.

The ca. 2675 Ma Ryan Lake pluton and associated quartz-feldspar porphyry dikes have gold contents significantly higher than the least-altered ore-hosting mafic volcanic rocks, but the pluton is significantly older than the ca. 2600 Ma relative age of the gold deposits. The age data and the fact that an early pluton carries elevated gold suggest that similar plutons and associated quartz-feldspar porphyry dikes may have fertilized the greenstone belt prior to formation of the orogenic gold deposits. We suggest that when ca. 2600 Ma metamorphically derived fluids interacted with these older rocks, the gold budget in the hydrothermal fluid could have been increased, as evidenced by increased gold grades near porphyries. An intrusion-related, early introduction of gold into the Yellowknife greenstone belt may have implications for other Archean gold deposits where there is a spatial relationship between gold and early plutonic rocks.