Browsing by Subject "Central Lapland Greenstone Belt"

The mafic-ultramafic Kevitsa intrusion, located within the Central Lapland Greenstone Belt in Northern Finland, hosts a disseminated Ni-Cu-PGE deposit. Drillhole KVX018 penetrates through the intrusion, intersecting its bottom contact at 1772 meters and is associated with relatively low resistivity at the bottom of the intrusion. The KVX018 drillhole is the deepest drilled into the intrusion so far and the observed low resistivity zone is unique for the study area. Previous studies have shown the bottom contact of the Kevitsa intrusion to be associated with seismic reflections and possible mineralization. This paper studies the characteristics of the bottom contact of the Kevitsa intrusion from the drill core KVX018 and interprets the origin of the low resistivity and its relationship with mineralogy. From geochemical and petrophysical characteristics, four layers with different characteristics were observed within the studied section: footwall, contact zone, lower cumulates and upper cumulates. The lower cumulates were found to be strongly contaminated by elements associated to hydrothermal fluids from country rocks. The contamination was observed for 125 meters upwards from the basal contact as elevated concentrations of e.g. lithium, lanthanum, rubidium and potassium, and footwall rocks close to the contact were found to be depleted in these elements. The contact zone was found to be strongly altered by silicification and albitization. Hydrothermal fluid activity at the bottom contact was also observed by epidote alteration of plagioclase feldspar. Contact zone mineralization was observed and it was found to be false ore type with Ni tenor of 2.28 %. Upwards from contact mineralization, the mineralization was found first to change into local low-grade Ni-PGE ore and then into normal ore on top part of the studied drill core section. Ultramafic intrusive rocks were observed to be pervasively altered by amphibole alteration locally into a degree where in many rocks, alteration had overprinted the primary mineralogy and textures to be undistinguishable. Alteration intensity was found to increase downwards within the lowermost part of the intrusion. Salt minerals were observed by eye on the surface of some samples and by X-Ray Diffraction in one sample. XRD studies indicated nitratine and sylvite minerals present in the studied sample. These salt minerals are presented commonly in evaporites and their presence indicates an evaporitic source. Resistivity of rocks is generally affected by e.g. sulfide content, salinity, porosity and alteration. Resistivity and chargeability were found to be correlative, indicating resistivity to correlate also with presence of sulfide minerals. However, after depth of 680 meters, resistivity decreases without a correlating trend in other petrophysical properties. This paper concludes that the observed low resistivity is resulted from a presence of salt and sulfide minerals as well as alteration intensity.

Talc is a problematic alteration mineral at the Kevitsa Ni-Cu-(PGE) mine in Sodankylä, Finland, and its distribution and control were assessed in this thesis. Kevitsa is a polymetallic mine hosted in an ultramafic intrusion, extracting Ni, Cu, Co, Au, Pt and Pd, which are of increasing importance in green energy technologies. Talc – a common alteration product in ultramafic rocks – detrimentally interferes with the recovery of copper in the flotation stage of ore processing when concentrations exceed 5 wt. %, thus affecting the economics of mine operations. It was found different talc concentrations had different spatial associations and controls, with three dominant styles identified, and a multi-stage genesis of talc alteration is proposed. The talc styles identified in the study are as follows: (style 1) pervasive talc-chlorite alteration, (style 2) talc-dolomite alteration haloes proximal to dolomite veins and (style 3) talc on brittle structures, associated with magnetite. Low values of talc between 0.2-0.5 wt.% (style 1) were found to have no preferential spatial distribution, occurring as background alteration throughout the intrusion. Intermediate values (between 1-5 wt. %) were associated with late brittle fractures and structures (style 3), with a notable association with the NE-flt-rv1 fault zone. Style (2) was found to have a dominant structural control, specifically being associated with north-south trending structures. Dominant structures with this association identified are NS-flt1_flt-002 and NS-flt-2_flt-009. Highest values (commonly exceeding >10 wt. %) manifest themselves as alteration haloes proximal to veins, where talc-carbonate replaces the intercumulus mineral phases. Here it is proposed that ‘low talc’ alteration, style (1), was the first talc association to occur, generated by late magmatic fluids or regional metamorphism accompanying amphibole and serpentine alteration. The association observed as style (2) was likely generated by the infilling of north-south trending structures by carbonate-talc veins through metasomatism by a CO2 rich metamorphic fluid, perhaps delivered by a deep-seated structure, often generating talc values in excess of 10 wt.%. The third stage is proposed to be talc enrichment via meteoric fluid percolation, after exhumation. This generated talc along brittle structures associated with magnetite style (3), and talc-carbonate concentrations may also be upgraded at this stage. Further enrichment of talc is observed at the surface, attributed to freeze thaw-cycles of permafrost upgrading talc values. The identification of these processes and controls on talc will not only have implications for the economics of Kevitsa as high talc zones can be avoided, but findings may have useful applications for mining of similar deposits in the Central Lapland Greenstone belt such as the nearby Sakatti Cu-Ni-(PGE) project, when it enters production.