Sr isotope stratigraphy across the Paasivaara PGE reef of the Penikat intrusion, Northern Finland:insights into the genesis of reef-type PGE mineralization
Abstract. Layered mafic-ultramafic intrusions host the largest reserves of feasible PGE commodities. Worldwide rare in the geological record, these deposits are well-constrained in a few large igneous provinces. Despite extensive research on layered intrusions and related PGE ores, the models regarding the PGE ore-formation are diverse, involving both magmatic and hydrothermal processes, and the subject remains largely debatable.
In Europe, research efforts around the Baltic Large Igneous Province (BLIP) has doubled due to the new geopolitical strategy of the European Union in their search for new sources of critical metals. Global politics have placed Finnish mafic-ultramafic layered intrusions in the spotlight.
Penikat is a Paleoproterozoic 2.44 Ga layered intrusion part of the east-west trending Tornio-Näränkävaara belt in northern Finland. The exposed intrusion is 23 km long and 1.5 km to 3.5 km on the current erosional level. Based on seismic and gravimetric data, it extends to 2.5 km deep. The intrussion is split into five westward-dipping blocks: Sompujärvi, Kilkka, Yli-Penikka, Keski-Penikka, and Ala-Penikka.
Traditionally, Penikat is divided into five megacyclic units (MCU I-MCU V), hosting three main PGE reef horizons: the Sompujärvi Reef (SJ), located to the basal part of MCU IV; the Ala-Penikka Reef (AP), occurring within the lower proportion of unit IV; and the Paasivaara Reef (PV), which defines the hanging wall contact from the MCU IV to the MCU V.
The present study comprises 700 m of stratigraphy in the Ala-Penikka block and focuses on the Paasivaara reef (PV). This platinum-enriched reef appears at the Transition Zone (TZ), along the MCU IV-V boundary. To gain new insight into Penikat’s petrogenesis, we utilize fourteen thick section samples from historic drill cores, both intersecting the PV reef, and two additional samples made after hand specimens. The sections were reviewed using petrologic microscopy before subjecting them to geochemical analyses.
From these sections, we obtained new in-situ Sr isotope data in plagioclase grains by using the LA-MC-ICP-MS laser ablation technique at the GTK facilities in Espoo, Finland. The procedure can track sub-grain scale heterogeneities from core to rim. As radiogenic ratios are rarely affected by melting or crystallization, subgrain analyses are a powerful tool for tracing the contributors in a magmatic system.
In addition, EPMA was used to analyze the plagioclase composition, with dedicated attention to the anorthite content in the assayed grains, at the University of Oulu, Finland. The anorthite content analysis was then compared to the LA-MC-ICP-MS results to assess the magmatic differentiation undergone by the plagioclase crystals.
The results support crystallization stability in most of MCU IV (Sr(i) ~ 0.7027 ± 0.0005) and MCU V (Sr(i) 0.7032 ± 0.0005 ). Nonetheless, data record a systematic, gradual increase in radiogenic Sr signature in the sparsely 38 m between MCU IV and V. In addition, rim to core distributions register more than a single Sr(i) signature at a subgrain scale in TZ samples. The variability in the Sr signature at the Transition Zone converges with a sharp An86 content spike at the PV reef boundary, arguing for a primitive magma contributor.
The results confirm affinity between the SHMB type magma, related to plume magmatism, and the parental magma of Penikat. The Transition Zone would be a product of binary and asymmetric magma mixing, each with contrasting isotopic lineages and varying degrees of crustal contamination. The discussion supports magma ascension from a staging chamber as a likely origin of the PV reef, thus supporting the new magma pulse hypothesis. Finally, the present study is compared with previous Sr(i) stratigraphy in Bushveld, showing similarities between the UPZ-MZ transition and the Transition Zone in Penikat. The detection of similar Sr(i) may lead to new PGE discoveries in the future
