Dr. Stephen Piercey: Evaluating the interplay of magmatism... (07-04-2017)

 

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Dr. Stephen Piercey, Memorial University of Newfoundland

Hutchison Lecture

Evaluating the interplay of magmatism, tectonics, and basin redox in the genesis of the Wolverine volcanogenic massive sulfide (VMS) deposit, Yukon, Canada

Vendredi 7 avril 2017 à 15h30 / Friday, April 7, 2017, 3:30 pm

Redpath Museum Auditorium, 859 Rue Sherbrooke O, Université McGill

Résumé / Abstract :

The Wolverine volcanogenic massive sulfide (VMS) deposit, Yukon, is a unique natural laboratory to study the interrelationships of magma evolution, tectonics, and basin redox in the genesis of VMS deposits in volcanic- and sediment-rich extensional basins. Pre-VMS (~352 Ma) quartz-feldspar porphyries (QFP) have continental crust-like Nb/Ta ~12, εNdt = -7.1 to -11.5 (average = -10.6), and εHft = -12.2 to -20.8 (average = -16.5). The syn-VMS (~347 Ma) feldspar porphyries have higher high field strength element (HFSE) and rare earth element (REE) concnetration, and higher Nb/Ta (~17), εNdt = -7.8 to -8.1 (average = -8.0), and εHft = -13.6 to -18.0 (average = -14.8). Both suites have and Proterozoic (to Archean) depleted mantle model ages (1.59-2.58 Ga). In situ U-Pb on zircons illustrate that while some ages are close to previously reported concordant TIMS ages, most samples have evidence of inheritance with ages ranging from 348-381.7 Ma for the QFP suite and 368.9-370.5 Ma for the FP suite. In situ εHft values for the zircons range from -11.5 to -21.0 (average -15.3) and -11.6 to -26.0 (average = -18.7) for the QFP and FP, respectively. The chemical and isotopic shifts from the QFP to younger FP suite reflects the varying contributions from evolved continental crust versus juvenile basaltic melts, and can be accommodated within an evolving continental back-arc basin in which there was a progressive increase in mantle input as a result of upwelling of juvenile basaltic material beneath the back-arc basin as it opened. Notably, the upwelling of mafic magmatism and greater mantle components in the syn-VMS FP suite also coincided with higher temperature felsic magmatism and VMS deposit genesis.

Shales intimately associated with the deposit, and with the porphyritic rhyolites, have redox sensitive trace element signatures (e.g., Mo-U, Corg-Ni systematics; Ni-V-Cr-Mn systematics) that indicate deposition under anoxic to sub-oxic conditions that were periodically euxinic with free H2S in the water column and the unconsolidated sediment pile. In contrast, shales proximal to the mineralized horizon have rare earth element and Y (REY) systematics (Ce/Ce*<<1 and Y/Ho>27) similar to oxygenated seawater. The Ce/Ce* values have a negative correlation with P2O5 and suggest a particulate shuttle control by detrital apatite that precipitated in the upper, oxygenated water column where it inherited an oxygenated seawater REE signature, which was then transported to the deeper water and deposited as detrital grains. The Ce/Ce* and Y/Ho values also correlate with CO2 and carbonate content of the shales. Moreover, the shales with the strongest oxygenated REY signatures and CO2-enrichment coincide with the strongest euxinic signatures. This paradox can be reconciled by enhanced deposition of apatite coincident with deposit formation, coupled with a late hydrothermal overprint on the shales from low temperature, CO2-rich (oxygenated?) hydrothermal fluids (i.e., high Y/Ho and Ce/Ce*<<1) in a vent-proximal environment.

The presence of high temperature magmatism, extensional geodynamics, and a H2S-rich near-vent environment were critical for generating the Wolverine hydrothermal system and the resultant deposition of mineralization. Identification of similar tectonic environments with similar geological and geochemical features is critical for finding new resources along evolving continental margins.