A succession of near-orthogonal horizontal tectonic shortenings in the Paleoproterozoic Central Lapland Greenstone Belt of Fennoscandia: constraints from the world-class Suurikuusikko gold deposit

M Sayab,J Välimaa,J Kuva,T Niiranen,D Aerden,F Molnár

doi:10.1007/s00126-019-00910-7

M Sayab, J Välimaa + Show 4 more

Open Access

https://doi.org/10.1007/s00126-019-00910-7

Copy DOI

Abstract

The relative and absolute timing of orogenic gold deposits in complex structural settings are active and challenging topics of research, especially in Precambrian greenstone belts. The Suurikuusikko gold deposit in Central Lapland Greenstone Belt is currently the largest primary gold producing deposit in Europe, located on a slight bend of the strike-slip Kiistala shear zone (KiSZ). Gold is refractory and locked inside arsenopyrite and pyrite. In this study, different structural features were investigated along the KiSZ from the recently stripped Etelä pit, which is the southern extension of the Suurikuusikko ore body. Our data source ranges from aeromagnetic to high-resolution aerial images, X-ray computed tomography scans of selected rock samples and regional geological and geophysical datasets. The KiSZ has recorded five discrete deformation phases, spanning between ca. 1.92 and 1.76 Ga. The refractory gold at the Suurikuusikko deposit formed during E-W contraction related to the D1 thrusting phase. This was followed by a N-S shortening event (D2), where most of the strain was taken up by the northern and southern thrusts. Tectonic vectors then switched from N-S to NE-SW and, as a result, dextral strike-slip regime (D3) commenced along the KiSZ. This event exsolved invisible gold from the sulfides and remobilized it along with fractures. A near-orthogonal switch of the regional stress regime from NE-SW to NW-SE flipped the kinematics of the KiSZ from dextral to sinistral (D4). The last deformation phase (D5) produced widespread veining under E-W contraction and secured gold mineralization at the Iso-Kuotko gold deposit within the KiSZ.

Highlights

IntroductionThe sample Kittilä-02 was collected from the mylonite zone (location 06; Fig. 3c) to determine the plunge direction of the mylonitic shear and further study its kinematics
Research during the past two decades has shown that the localization of world-class orogenic gold deposits in metamorphic belts are intimately related to the repetitive reworking of crustal-scale compressional shears (e.g., Groves et al 1998, 2018; Goldfarb et al 2005; Tomkins 2013; Groves and Santosh 2016; Sayab et al 2016)
The dextral strike-slip shearing observed in volcanic rocks at the margin of the Kiistala shear zone (KiSZ), within the Etelä pit, is a result of this tectonic activity and we place this event within D3 (Fig. 12c), dated by Lahtinen et al (2018) at ca. 1.88–1.87 Ga

Summary

Introduction

The sample Kittilä-02 was collected from the mylonite zone (location 06; Fig. 3c) to determine the plunge direction of the mylonitic shear and further study its kinematics This can be deduced from the long axes of porphyroclasts with respect to the mylonitic foliation (Bell and Johnson 1992). At locations 20 and 21, a dextral shear sense is indicated by crenulation asymmetry or porphyroclasts These relationships suggest that the sinistral strike-slip movement recorded in a 2m wide dark mylonitic band in the middle of the shear zone by mylonitic foliation Sm-2 is younger than the dextral Sm-1, and recorded in volcanic rocks closer to its margins. The kinematics and relative timing between the strike-slip regime and thrusting phase of the KiSZ are further discussed below

Findings

Discussion

Conclusions