Structural Control of Ore Deposits: The Role of Pre-Existing Structures on the Formation of Mineralised Vein Systems

Chauvet A

doi:10.3390/min9010056

Abstract

The major role played by pre-existing structures in the formation of vein-style mineral deposits is demonstrated with several examples. The control of a pre-existing decollement level on the formation of a crustal extension-related (collapse) gold deposit is first illustrated in the Quadrilátero Ferrífero from Brazil. Shear zone and decollement structures were also examined and shown to control veins formation by three distinct processes: (i) re-aperture and re-using of wrench shear zones in the case of Shila gold mines (south Peru); (ii) remobilisation of metal in volcanic-hosted massive sulphide (VHMS) deposit by subsequent tectonic events and formation of a secondary stockwork controlled by structures created during this event (Iberian Pyrite Belt, Spain); (iii) formation of economic stockwork by contrasting deformation behaviours between ductile black schist versus brittle more competent dolomite (Cu-Ifri deposit, Morocco). Two examples involve changing of rheological competence within zones affected by deformation and/or alteration in order to receive the mineralisation (case studies of Achmmach, Morocco, and Mina Soriana, Spain). The last case underscores the significance of the magmatic–hydrothermal transition in the formation of mesothermal gold deposits (Bruès mine, Spain). All these examples clearly demonstrate the crucial role played by previously formed structures and/or texture in the development and formation of ore deposits.

Highlights

We can distinguish (i) vein opening during flexural folding of multilayers with contrasted lithology with a void being created within competent levels byopening of the coeval axial plane cleavage (Figure 1a); (ii) gaps formed by differential shearing due to fracturing and boudinage of more competent layers leading to stockwork development (Figure 1b); (iii) aperture controlled by extensional relay associated with either fibrous/comb veins or breccia veins (Figure 1c)
Detailed study of geometry and composition of vein associated with ore deposits, combined with mineralogical and textural constraints, is indispensable in order to understand the mode of formation of mineralised systems
Vein geometry indicates left-lateral shearing, but the opposite conclusion is deduced when taking into account the fact that the veins and, the secondary ones, are characterised by aperture and stress-free textures that are not consistent with the classical status of what we call a cleavage

Summary

Introduction

Numerous studies have been devoted to the process of vein formation mainly because of their significance in term of tectonics and deformation (stress and strain determination), e.g., [1,2,3,4,5], and because of their significant economic interest in the case of metal-bearing veins, e.g., [6,7,8].Several works have concentrated on the external geometry of veins and their relationships with the mode of opening and, the local or regional stress field during vein formation [9,10,11].Complementary studies have integrated information that can be deduced from vein infilling textures, such as the classical tripartite division in syntaxial (inward growth), antitaxial (outward growth) and stretching veins (complex pattern with no consistent growth direction) [12,13,14]. Numerous studies have been devoted to the process of vein formation mainly because of their significance in term of tectonics and deformation (stress and strain determination), e.g., [1,2,3,4,5], and because of their significant economic interest in the case of metal-bearing veins, e.g., [6,7,8]. Several works have concentrated on the external geometry of veins and their relationships with the mode of opening and, the local or regional stress field during vein formation [9,10,11]. In vein-type deposits, the study of the nature and texture of vein infilling is important because it lies at the base of the ore-forming process itself. The latest contributions [15,16,17] only deal with the internal texture of ore deposits without considering the (external) geometry of the veins themselves

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