Abstract

The Au-rich massive to semimassive sulfide lenses of the LaRonde Penna deposit (58.8 Mt at 4.31 g/t Au) are stacked in a steeply dipping, southward-facing homoclinal volcanic sequence forming a continuous, differentiated volcanic succession composed of two main formations: the ca. 2700 Ma Hébécourt Formation and the 2701 to 2698 Ma Bousquet Formation, which corresponds to the uppermost segment of the Blake River Group. The Hébécourt Formation is composed of regionally extensive LREE-depleted ([La/Sm]N ∼0.9) tholeiitic, basaltic to andesitic, massive to pillowed flows that formed a submarine stratum on which the Bousquet Formation was emplaced. The Bousquet Formation is further divided into a lower member and an upper member. The lower member of the Bousquet Formation is composed of feldspar and quartz-phyric tholeiitic felsic (Zr/Y ∼3.4, Zr/TiO2 ∼860) sills and extensive effusive and volcaniclastic mafic to intermediate and tholeiitic to transitional rocks. The upper member is mainly characterized by submarine, coalesced dacitic to rhyodacitic autoclastic flows that are cut and/or covered by rhyodacitic and rhyolitic domes and/or partly extrusive cryptodomes and by intermediate to mafic sills and dikes.

Mafic to intermediate and tholeiitic to transitional (Zr/Y ∼2.3–5) rocks of the Bousquet Formation are characterized by a low Zr/TiO2 ratio (<60), moderately enriched chondrite-normalized LREE and MREE ([La/Sm]N ∼2.2–2.7) patterns, flat HREE ([Gd/Lu]N ∼1.2–2) patterns, and negative Nb, Ta, Zr, and Hf anomalies. Felsic transitional to calc-alkaline (Zr/Y ∼5–8) rocks of the upper member of the Bousquet Formation are characterized by a moderate Zr/TiO2 ratio (∼250–615), high incompatible element contents, LREE-enriched patterns ([La/Sm]N ∼3.2–6.6), flat HREE patterns ([Gd/Lu]N ∼1–1.4), pronounced negative Nb, Ta, and Ti anomalies, and positive Zr and Hf anomalies. The Nd isotope signature of six separate LaRonde Penna deposit host units (εNd ∼3–3.4) suggests that they were generated by partial melting of depleted upper mantle and/or juvenile material (mafic crust) or by a combination of those two processes. The sequence is interpreted to reflect the progression from diapirism of depleted upper mantle associated with underplating by mafic-ultramafic magma and assimilation and magmatic differentiation (assimilation-fractional crystallization) at midcrustal levels in subsidiary magmatic chambers within a ca. 2721 Ma, relatively thick, juvenile or immature mafic ± felsic arc–back-arc crust in an intermediate setting between back-arc basin and volcanic-arc environments. This setting, compatible with the inferred geodynamic setting for the southern Abitibi belt, could be responsible, at least in part, for the Au enrichment of the volcanic massive sulfide (VMS) deposits of the Doyon-Bousquet-LaRonde mining camp.

This study shows that Archean HREE-depleted and high Th, transitional to calc-alkaline dacite, rhyodacite, and rhyolite, referred to as FI and FII type, such as those associated with the LaRonde Penna deposit, can be important hosts for VMS and Au-rich VMS and may be as prospective as the tholeiitic or FIII-type rhyolite-bearing sequences.

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