X-ray computed tomography: A modern Aladdin’s lamp to discover orogenic gold deposits
The Suurikuusikko gold deposit in the Central Lapland Greenstone Belt of northern Finland is the largest primary gold-producing mine in Europe, located on the strike-slip Kiistala shear zone (KiSZ). Gold is refractory, occurs either as lattice-bound or submicron-scale gold inclusions, and is mainly locked within arsenopyrite. A small proportion of visible gold also occurs along fractures. In this study, we used lab-based and synchrotron radiation–based X—ray computed microtomography and nanotomography, respectively, to visualize and quantify rock volumes in a 3D space. This rapidly evolving non-invasive technology provides a holistic approach of textural analysis that eliminates interpretative procedures associated with 2D methods. The 3D volume can be used to create virtual cross-sections at any desired angle or a 3D rendering, and quantitatively analyzed with image processing software, which act as “Genie of the Aladdin’s lamp”. The technique revealed a kinematic history and a number of in situ quantitative aspects including size, shape, spatial distribution, and geometrical orientation of arsenopyrite and pyrite in a highly altered host-rock matrix. Quantitative data highlight the spatial orientation of the long axes (X) of arsenopyrite crystals. Most crystals show a preferred orientation reflecting deformation-induced crystallization and/or remobilization and reprecipitation, where three dominant classes can be recognized based on the spatial distribution of arsenopyrite crystals. Each class can be related to the regional-scale deformation events. The method significantly helped in classifying the mineralized (gold-bearing) vs. unmineralized clusters of arsenopyrite and in depicting the geometry of the ore zones. The workflow of analyzing orogenic gold from micro- to nanoscale offers an indispensable new method in characterizing 3D textural settings of ores, which could be further integrated with trace element analysis using LA-ICP-MS and in situ XRF imaging. Different ages were obtained from low- and high-strained samples using LA-ICP-MS U-Pb monazite and xenotime geochronology ranging from ca. 1930 to 1794 Ma, corresponding to the different tectonic events of the Svecofennian orogeny. The ca. 1930 to 1900 Ma age group related to the basin inversion and D1 thrusting phase along the KiSZ. We infer that the primary gold-bearing fluids were infiltrated along the thrust splays during this event. The ca. 1834-1794 Ma age group complements the strike-slip kinematics of the gold-bearing (refractory) arsenopyrite crystals. The youngest age of ca. 1774 Ma represents the late semi-brittle event, where the lattice bound or submicron gold inclusions exsolved from the arsenopyrite crystal structure and precipitated along secondary fractures.
Date: 6 December 2024, 12:00 (Friday, 8th week, Michaelmas 2024)
Venue: Department of Earth Sciences, South Parks Road OX1 3AN
Venue Details: Seminar Rooms
Speaker: Dr Muhammad Sayab (The Geological Survey of Finland GTK)
Organising department: Department of Earth Sciences
Part of: Earth Sciences departmental seminars
Booking required?: Not required
Booking url: https://www.earth.ox.ac.uk/event/x-ray-computed-tomography-a-modern-aladdins-lamp-to-discover-orogenic-gold-deposits
Audience: Public
Editor: Maria Petrunova