Magma chamber dynamics, mineralisation, and growth of the 2.44 Ga Koillismaa Intrusion, the Fennoscandian Shield: Evidence from the Pintamo Block

A ~1150 m-long diamond drill hole through the full stratigraphy of the Pintamo Block of the 2.44 Ga Koillismaa Layered Mafic Intrusion provides new insights into magma chamber dynamics. The drill core records at least two major compositional reversals which are attributed to injections of progressively more primitive magma from a deeper staging reservoir. Following these reversals, the stratigraphy records a transition from open- to closed-system behaviour. Three Cu–Ni–PGE sulphide‑mineralised units and one Fe–Ti–V oxide‑mineralised unit are present in the drill core. The sulphide‑mineralised units formed during magma replenishment events, whereas Fe–Ti–V oxide mineralisation developed during Fenner-type differentiation in the final stages of chamber evolution, progressing roof‑down and floor‑up towards the Sandwich Horizon. Stratigraphic correlations across the intrusion show that these features are laterally continuous, developed early in the chamber’s evolution, indicating that the intrusion originally comprised two partially connected sub-chambers, with the Pintamo Block representing the thinner, distal part of the main chamber. The middle stratigraphy of the Pintamo Block is laterally continuous with the main chamber, whereas the lowermost and uppermost units are absent, indicating that the chamber reached its lateral extent early and subsequently thickened through progressive floor subsidence along reactivated basement faults. Floor tilting generated angular discordances in the lower stratigraphy, while enhanced in situ crystallisation along the roof in thicker parts of the chamber accounts for systematic thinning and local absence of the uppermost units in distal areas. Progressive thickening of the chamber produced a gently inclined roof surface, promoting migration of buoyant residual melt towards the thinner distal portion of the chamber and thereby influencing the growth and thickness distribution of the roof sequence. ​