Characterization of ore-forming systems - advances and challenges

Economically viable concentrations of mineral resources are uncommon among the predominantly silicate-dominated rocks in Earth's crust. Most ore deposits that were mined in the past or are currently being extracted were found at or near Earth's surface, often serendipitously. To meet the future demand for mineral resources, exploration success hinges on identifying targets at depth, which, on the one hand, requires advances in detection and interpretation techniques for geophysical and geochemical data. On the other hand, however, our knowledge of the chain of events that lead to ore deposit formation is limited. As geoscience embraces an integrated Earth systems approach, considering the geodynamic context of ore deposits can provide a step change in understanding why, how, when and where geological systems become ore-forming systems. Contributions to this volume address the future resources challenge by: (i) applying advanced microscale geochemical detection and characterization methods; (ii) introducing more rigorous 3D Earth models; (iii) exploring critical behaviour and coupled processes; (iv) evaluating the role of geodynamic and tectonic setting; and (v) applying 3D structural models to characterize specific ore-forming systems

Precambrian geodynamics and ore formation: The Fennoscandian Shield

Compared with present-day global plate tectonics, Archaean and Palaeoproterozoic plate tectonics may have involved faster moving, hotter plates that accumulated less sediment and contained a thinner section of lithospheric mantle. This scenario also fits with the complex geodynamic evolution of the Fennoscandian Shield from 2.06 to 1.78 Ga when rapid accretion of island arcs and several microcontinent-continent collisions in a complex array of orogens was manifested in short-lived but intense orogenies involving voluminous magmatism. With a few exceptions, all major ore deposits formed in specific tectonic settings between 2.06 and 1.78 Ga and thus a strong geodynamic control oil ore deposit formation is suggested. All orogenic gold deposits formed syn- to post-peak metamorphism and their timing reflects the orogenic younging of the shield towards the SW and west. Most orogenic gold deposits formed during periods of crustal shortening with peaks at 2.72 to 2.67, 1.90 to 1.86 and 1.85 to 1.79 Ga. The ca. 2.5 to 2.4 Ga Ni-Cu PGE deposits formed both as part of layered igneous complexes and associated with mafic volcanism, in basins formed during rifling of the Archaean craton at ca. 2.5 to 2.4 Ga. Svecokarelian ca. 1.89 to 1.88 Ga Ni-Cu deposits are related to mafic-ultramafic rocks intruded along linear belts at the accretionary margins of microcratons. All major VMS deposits in the Fennoscandian Shield formed between 1.97 and 1.88 Ga, in extensional settings, prior to basin inversion and accretion. The oldest "Cyprus-type" deposits were obducted onto the Archaean continent during the onset of convergence. The Pyhasalmi VMS deposits formed at 1.93 to 1.91 Ga in primitive, bimodal arc complexes during extension of the arc. In contrast, the Skellefte VMS deposits are 20 to 30 million years younger and formed in a strongly extensional intra-arc region that developed on continental or mature arc crust. Deposits in the Bergslagen-Uusimaa belt are similar in age to the Skellefte deposits and formed in a microcraton that collided with the Karelian craton at ca. 1.88 to 1.87 Ga. The Bergslagen-Uusimaa belt is interpreted as an intra-continental, or continental margin back-arc, extensional region developed on older continental crust. Iron oxide-copper-gold (IOCG) deposits are diverse in style. At least the oldest mineralizing stages, at ca. 1.88 Ga, are coeval with calc-alkaline to monzonitic magmatism and coeval and possibly cogenetic subaerial volcanism more akin to continental arcs or to magmatic arcs inboard of the active subduction zone. Younger mineralization of similar style took place when S-type magmatism occurred at ca. 1.80 to 1.77 Ga during cratonization distal to the active N-S-trending subduction zone in the west. Possibly, interaction of magmatic fluids with evaporitic sequences in older rift sequences was important for ore formation. Finally, the large volumes of anorthositic magmas that characterize the Sveconorwegian Orogeny formed a major concentration of Ti in the SW part of the Sveconorwegian orogenic belt under granulite facies conditions, about 40 million years after the last regional deformation of the Sveconorwegian Orogeny, between ca. 930 and 920 Ma. (c) 2005 Elsevier B.V. All rights reserved