The basal dunite of the Precambrian mafic-ultramafic Näränkävaara intrusion: Petrogenetic considerations and implications to exploration

Abstract

Several mafic-ultramafic layered intrusions were emplaced in the Fennoscandian Shield during wide-spread mantle-sourced magmatism at 2.5–2.4 Ga. The Näränkävaara intrusion (surface area 5 × 30 km2), northeastern Finland, contains a 1.5–2 km thick basal dunite (not dated), and a 1.5 km thick layered series (2436 ± 5 Ma). A newly discovered marginal series between the basal dunite and the layered series indicates that the basal dunite is older, and highlights the need for further study on their relationship. Along its southern basement contact, the basal dunite contains a 200–300 m thick zone of olivine ortho- and mesocumulates, but the bulk of it is composed of olivine adcumulates and lesser olivine-orthopyroxene heteradcumulates. Based on whole-rock geochemistry, the basal dunite is divided into a low-Fe zone (average FeOt 10.2 wt% and Ni 2250 ppm) and a high-Fe zone (average FeOt 12.5 wt% and Ni 1700 ppm). Both zones have high MgO (32–47 wt%) and varying Cr (830–5160 ppm) and Al2O3/TiO2 (16–26). Textural and geochemical layering is similar along the 30 km strike of the basal dunite. A LREE-enriched high-MgO basaltic parental magma composition (13–18 wt% MgO) is inferred for the basal dunite from olivine–melt mixing trends in orthocumulates. The dunite exhibits at least two geochemical reversals as well as abundant low-porosity adcumulates, poikilitic chromite, and bimodal olivine, suggesting formation in a high-volume open magmatic system. Significant similarity in major and trace element compositions with the Näränkävaara layered series and the Burakovsky intrusion and Vetreny belt extrusives in Russian Karelia suggests that the basal dunite belongs to the Fennoscandian 2.5–2.4 Ga mafic layered intrusions. An Archean komatiitic origin for the dunite body cannot be completely ruled out, however. Distinct Ni-depletion in olivine is found in the basal dunite from the low-Fe zone to the high-Fe zone (3200 versus 2200 ppm). This depletion does not correlate with Fo contents, which suggests that it is not related to olivine fractionation. The basal dunite may thus have potential for Ni-(Cu-Co-PGE) sulfide mineralization.

Formation of poikilitic chromite in the basal dunite series of the 2.44 Ga Näränkävaara layered intrusion

“…Study materials are electron probe microanalysis (EPMA) results from chromites from a sampling profile across the SE block the BD (A in Fig. 1) (Järvinen et al 2020b). According to lithology, the profile is divided into three zones (BD-1 to BD-3 in Fig.…”

“…Chromite Fe 3+ # is sensitive to changes in the oxygen fugacity of its parental magma (Murck & Campbell 1986). Range of Fe 3+ # in the BD is 0.02-0.10 mol%, corresponding to approximately -6 to -9 log fO2 at the liquidus T (1350-1400 °C) of the BD parental magma (Murck & Campbell 1986;Järvinen et al 2020b). The lowest Fe 3+ # values indicate minimum oxygen buffer of -1 to -2 log unit below QFM.…”

“…Here, we describe coarse grained poikilitic chromites found in the 1.5-2 km thick basal dunite series (BD) of the mafic-ultramafic 2.44 Ga Näränkävaara layered intrusion, Northern Finland. These chromites are hosted in olivine adcumulates containing <Fo91 olivines, formed from parental magmas with, at most, 15-18 wt% MgO (Järvinen et al 2020b). High temperatures cannot easily be invoked to explain formation of these poikilitic chromites.…”

Geology of the Precambrian mafic-ultramafic Näränkävaara intrusion – Review of recent results

“…However, two major results contradict this hypothesis, and rather argue for a ~2.44 Ga related intrusive origin. First, the parental magmas of both series were determined by 1) direct analysis of the marginal series, 2) weighted average calculation of the layered series, and 3) modelled from the southern margin olivine orthocumulate of the basal dunite (inferred to represent olivine-melt mixes) (Järvinen et al 2020a(Järvinen et al & 2020b) -all three parental magmas show similar LREE-enriched high-MgO basaltic compositions with 13-18 wt% MgO (Fig. 2A), similar to other 2.5-2.4 Ga intrusions.…”

“…Whole-rock geochemistry can be used to separate the body into 10 subzones (layers) with Mg#'s decreasing from south to north towards the layered series (Fig. 1), however with several reversals to more primitive compositions (Järvinen et al 2020b).…”