4D modelling the Fennoscandian Shield Skellefte and Vihanti-Pyhäsalmi districts

ProMine final conference,

23-25 April 2013, Levi, Finland

Fennoscandian Shield

Pyhäsalmi-Vihanti ore district

The main volcanic units of the Pyhäsalmi-Vihanti ore district Vihanti group (~1925 -1920 Ma) • felsic to mafic early arc volcanism • abundant volcaniclastics • calc silicate rocks • subvolcanic felsic intrusion Pyhäsalmi group (~1930-1925 Ma) • bimodal felsic and mafic early arc volcanism • lavas, breccias and porphyries • subvolcanic felsic intrusion

Vihanti model

Pyhäsalmi model

3D/4D Modeling methods for regional and semi-regional 3D models

Bedrock observations Geological analogies from the surrounding geology, theories and assumptions of the geological events and absolute age determinations

Magnetic maps

Geophysical surveys, drillholes and structural geological inference!

Bouguer maps

Drill hole data Geological maps

Geophysical maps Geological analogies from the surrounding geology, seismic sections and other geophysical surveys

Drill hole data Surpac strings

Surpac solids

Semi-regional 3D model of Vihanti

Regional model of the RLZ

Semi-regional model of Pyhäsalmi

Methods used for Pyhäsalmi 3D/4D- models

• Lithogeochemistry • Deep penetrating geophysics

– Seismics – Magnetotellurics

• Sructural analysis • Modeling with goCad and Surpac-

programs

? ? W E

Z=1000m

Inversion model of regional gravity data

Vihanni and Kuuhkamo models

The contact between Lampinsaari volcanites and granodiorite based on seismic data and geological maps

Vertical faults, NW dipping undefined discontinuity, the brown surface marks the possible lower boundary for the granodiorite underlied by volcanites and calc-silicate rocks, modeled using GeoModeller software Vihanti Zn and pyrite ore

1 km

N

Pyhäsalmi 3D- model

Regional structural evolution

4D-Pyhäsalmi

Deposition of voluminous felsic volcanics, intrusion of felsic&mafic synvolcanic intrusions, hydrothermal alteration, FORMATION OF MASSIVE VMS ORE

Deposition of overlying tholeiitic volcanic rocks and minor pyrrhotite rich mineralizations

Movement and deformation on thrust carries deposit over unaltered material, formation of sheath fold

Further folding & regional metamorphism at amphibolite facies

Refolding of the earlier structures into upright position, intense subvertical shearing +late pegmatites

Skellefte district Regional scale Semi-regional scale Deposit scale

Structural and facies analysis in the Vargfors syncline

• Combination of structures

and sedimentary stratigraphy

• Syn-extensional sedimentation along growth faults

• Inversion and formation of asymmetric synclines

Regional scale structural analysis

Regional scale structural analysis

• N: dominating coaxial deformation at shallower crustal level

• S: high metamorphic conditions and non-coaxial deformation at deeper crustal level

• Strain partitioning during one SSE-NNW transpressional event OR two separate events with contrasting bulk shortening directions

Deposit scale 3D-model

Simplified 3D-models of VMS ore bodies illustrating varyiations in shape and orientation

Semi-regional and regional scale - Structural data from field mapping

in 3D (sparse plugin for GoCAD) - Interpolation along map traces - Cutting of surfaces

Uncertainty modelling

Uncertainty modelling

Observed in field, mine or drillcore Interpreted from geophysical data (reflection seismic-, IP-, resitivity-, MAG-, or MT-study) Interpreted from structural data or extrapolated from geophysical data Inferred / unknown

3D-model of the Skellefte district

Prospectivity modelling

Correlation between distribution and shape of VMS deposits, and regional deformation patterns

Prospectivity modelling

4D modelling

- Schematic illustration of listric and related transfer faults, the formation of volcanic and sedimentary rocks and VMS deposits during crustal extension

- Subsequent deformation of rocks and transposition of ore bodies and the effect of different block-rotations, subsequent erosion and deformation between the neighboring fault-bound compartments and the tectonic transposition of the ore bodies