Unraveling the Western Churchill Province Paleoproterozoic Gold Metallotect: Constraints from Re-Os Arsenopyrite and U-Pb Xenotime Geochronology and LA-ICP-MS Arsenopyrite Trace Element Chemistry at the BIF-Hosted Meliadine Gold District, Nunavut, Canada

Abstract

The gold endowment of the western Churchill Province is conventionally attributed to reworking and attendant metamorphism during the Trans-Hudson Orogeny (1.9-1.8 Ga). The Meliadine gold district is regarded as a type example of this inferred Paleoproterozoic gold metallotect and also represents one of Canada's largest emerging orogenic greenstone-and banded iron formation (BIF)-hosted gold districts (2.8 Moz contained Au in reserves and total resource of 5.8 Moz Au). The largest gold deposits are cospatial with the Pyke fault and associated quartz (± ankerite) veining, which cuts Neoarchean (ca. 2.66 Ga) plutonic and supracrustal rocks comprising the Rankin Inlet greenstone belt.Meliadine gold occurs as inclusions within idioblastic arsenopyrite crystals, at sulfide crystal boundaries, and/ or as sulfide fracture fills in hydrothermally altered, sulfidized, and veined BIF. Clusters of gold, Bi-Mo telluride, chalcopyrite, and galena inclusions characterize high-grade ore zones and are spatially associated with sieve-textured arsenopyrite domains. These microtextural relationships suggest that incipient sulfide recrystallization liberated gold, which, along with other precious and base metals, was redistributed during late, fluidassisted and deformation/metamorphic-driven remobilization. Late precious and base metal enrichment is also demonstrated by in situ laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) arsenopyrite mapping. Hydrothermal xenotime crystals occur with gold in low-strain microtextural sites and yield U-Pb ages at ca. 1.86 Ga, suggesting that remobilization was concomitant with Trans Hudson orogenesis.New Re-Os arsenopyrite model ages range from 2.3 to 1.8 Ga and document a hitherto unrecognized, pre-1.86 Ga hydrothermal activity. The range of Re-Os model ages tends to support partial open-system behavior and/or mixing of disparate arsenopyrite generations that are evident from microtextural observations and in situ LA-ICP-MS element mapping. Replicate analyses of the two most rhenium rich arsenopyrite samples yield reproducible Re-Os model ages at ca. 2.27 and 1.90 Ga. These rhenium-rich samples are also gold poor and likely yield ages that predate gold remobilization and subsequent enrichment along arsenopyrite crystal boundaries and fractures. We suggest that pre-1.86 Ga hydrothermal activity was likely critical to the gold endowment of the district, but is obscured by reworking during the later stages of the Trans-Hudson Orogeny at 1.86 Ga. If correct, these new ages imply that the Paleoproterozoic gold metallotect, which is recorded across the western Churchill Province, in fact comprises multiple, temporally distinct gold events. Differentiating between disparate Paleoproterozoic gold events represents a critical step for effective mineral exploration in the western Churchill Province and at other reworked Archean terranes.

Results of LA-ICP-MS sulfide mapping from Algoma-type BIF gold systems with implications for the nature of mineralizing fluids, metal sources, and deposit models

“…A minor sulfide-rich event may have occurred before precipitation of apy1, as rare pyrrhotite and pyrite inclusions are present in apy1 (Carpenter 2004). Elemental mapping of arsenopyrite (Carpenter 2004;Lawley et al 2015c) confirms the above paragenetic interpretation in the part of the area sampled.…”

“…14b, c). These variable elemental associations suggest a model that involved a prolonged hydrothermal history with mobilization of earlier gold, as proposed by Lawley et al (2015c).…”

“…Due to the highly variable level of previous knowledge and types of analyses available for samples from these deposits (Janvier et al 2015;Janvier 2016;Lawley et al 2015c;Oswald et al 2015), LA-ICP-MS mapping was done on pyrite for the Meadowbank deposit, whereas only LA-ICP-MS traverse mode analyses were done for arsenopyrite from the Meliadine gold district and on pyrrhotite and arsenopyrite from the Musselwhite deposit.…”

“…The earlier results of LA-ICP-MS elemental mapping of arsenopyrite (Carpenter 2004;Lawley et al 2015c) suggested gold mineralization at the Meliadine deposit in the Churchill Province was related to its early introduction during the Archean growth of coarse-grained apy1 and was subsequently remobilized during subsequent deformation/metamorphism (Wagner et al 2007) and pressure-solution processes accompanying the Trans-Hudson Orogeny (i.e., 1.86-1.85 Ga). Thus, concentration of Au in later stage recrystallized apy2 and along fractures may relate to the effect of the Trans-Hudson Orogeny.…”

Laser-Induced Breakdown Spectroscopy—An Emerging Analytical Tool for Mineral Exploration

“…To address these analytical challenges, there is a growing interest in two-dimensional (2D) element mapping, which provides geochemical information in a complete geologic context with all of the adjoining mineral phases. With careful calibration using standards of known composition, these geochemical images can provide fully quantitative results for the targeted area of interest [109,110]. Interpreting geochemical data in a geologic context provides critical constraints on the relative timing relationships between microscale features, which can, in turn, be used to unravel overprinting hydrothermal histories.…”