Geodynamic Reconstruction of the Winston Lake Greenstone Belt and VMS Deposits: New Trace Element Geochemistry and U-Pb Geochronology

Abstract

The ca. 2720 Ma Winston Lake greenstone belt of the Wawa-Abitibi terrane, Superior province, is located on the northern margin of the Wawa subprovince and has important regional tectonostratigraphic implications for the development of the Neoarchean Superior province. The metallogenic importance of the belt is evident from the presence of the Winston Lake, Pick Lake, and Zenith volcanogenic massive sulfide (VMS) deposits. However, the chemostratigraphic evolution has not been fully examined at the greenstone belt scale, nor have the VMS deposits been examined from the perspective of the belt-scale petrogenesis and volcanic reconstruction. This study (1) reviews the geology and VMS mineralization in the Winston Lake greenstone belt, and (2) reconstructs its geodynamic setting, and its magmatic and metallogenetic history using new geological, trace element, and Nd isotope data constrained by new U-Pb geochronology. The belt is subdivided into two main lithotectonic assemblages: the Winston Lake Assemblage composed of tholeiitic to calc-alkalic bimodal volcanic lithofacies, and the Big Duck Lake Assemblage composed of alkalic to tholeiitic mafic flows and sills. The contact between the Winston Lake and Big Duck Lake Assemblages is intruded by the differentiated Zenith gabbro, which hosts the 0.16 Mt Zenith VMS orebody as a series of xenoliths. The variably altered felsic lithofacies of the Winston Lake Assemblage host the 3.1 Mt Winston Lake Main and 1.3 Mt Pick Lake Zn-rich VMS deposits. The felsic volcanic lithofacies of the Winston Lake Assemblage are FIII-type rhyolites and have a bimodal distribution of Zr/Ti indicating two distinct magmas formed by different temperatures of partial melting. Mafic lithofacies of the Winston Lake Assemblage are typically calc-alkalic to transitional between calc-alkalic and tholeiitic in composition. They have arc to back-arc trace element contents with distinctive negative Nb and Ti anomalies, enrichment in the light rare earth elements (LREE), and higher Th/Nb ratios than the mafic rocks from the Big Duck Lake Assemblage. Mafic strata from the Big Duck Lake Assemblage have trace element characteristics similar to mature back-arc tholeiitic basalts, and OIB-like or oceanic plateau alkalic basalts with high Nb/Th and Ti/V ratios. The chemostratigraphy of the Winston Lake greenstone belt is consistent with a geodynamic model where arc rifting is followed by back-arc development. Nd isotope data indicate that strata from the Winston Lake and Big Duck Lake Assemblages are juvenile and show no evidence of contamination by significantly older crust. Therefore, crustal contamination signatures in the mafic flows from the Winston Lake Assemblage must have formed by contamination from crust that is of similar age and supports a comagmatic relationship between the mafic and felsic strata. The mafic flows of the Big Duck Lake Assemblage represent a mature back-arc setting with evidence for enriched plume-generated basalts in the upper mafic unit. The Winston VMS deposit formed during the early rifting of the arc and the timing is tightly constrained by the hydrothermally altered 2721.2 ± 0.9 Ma VMS-hosting felsic strata and younger, postmineralization, 2719 ± 4 Ma Zenith gabbro. The Zn-dominated VMS mineralization suggests that temperature of the hydrothermal fluid during discharge was relatively cool (<300°C) and did not maintain sufficiently high temperatures to precipitate significant amounts of copper relative to zinc. This is likely due to adiabatic cooling of the fluid because of a low-pressure (<0.5 kb) sea-floor environment and/or permeable volcaniclastic strata. A U-Pb age of emplacement of 2692.1 ± 0.7 Ma from the syndeformation quartz-feldspar porphyry in the Big Duck Lake Assemblage indicate it is comagmatic with syndeformation magmatic events typical of other greenstone belts along the northern margin of the Wawa-Abitibi terrane. This event is interpreted to represent the timing of terrane accretion of the Wawa-Abitibi terrane to the Superior province.