Facies architecture, ore genesis (Zn-Pb-Cu) and structural evolution of the Paleoproterozoic Aijala–Metsämonttu area, SW Finland

Abstract

A field-based study was conducted in the metamorphosed and deformed Aijala–Metsämonttu area within the Orijärvi formation, part of the Uusimaa belt (1.9–1.88 Ga) in southern Finland. The study aimed to enhance understanding of the regional facies architecture and tectonic framework. Volcanic facies analysis, lithogeochemistry, and sulfide textural interpretations were utilized, allowing us to reassess the previously suggested exhalative model of the Metsämonttu sulfide deposit (1.5 Mt at 3.5 wt% Zn, 0.8 wt% Pb, 0.3 wt% Cu, 13.2 wt% S, 25 g/t Ag, and 1.4 g/t Au, production 1952–1974) to assist regional exploration.

Regionally, D₁ thrusting was the primary significant structural event, associated with the common occurrence of both L–S and L-tectonites. D₂ represents nearly orthogonal shortening, producing upright to steeply inclined F₂ folds. The Aijala–Metsämonttu area is dominated by a felsic volcaniclastic lithofacies association, originating from voluminous silicic magmas. The region features both primary and redeposited rhyolitic pyroclastic deposits intercalated with carbonates and iron formations, alongside subordinate dacite-rhyolite complexes and submarine mafic fissure vents. Geochemically, the rocks exhibit subalkaline to transitional magmatic affinities, indicating a volcanic arc system likely associated with early-stage rifting, extensional basin formation in an intra-arc or back-arc setting. The Metsämonttu succession primarily consists of resedimented syn-eruptive volcaniclastic deposits intercalated with sedimentary facies. The Metsämonttu sulfide deposit, hosted by metavolcanic rocks, marble, and skarn, contains semimassive to stringer-style polymetallic sulfides, controlled by stretching along D₁ recumbent fold axes (L-tectonite) and subsequent vertical stretching along D₂ limbs. The mineralization is considered a metamorphosed subseafloor carbonate replacement deposit, hosted within permeable polymict dacitic fiamme-lithic pumiceous breccia.