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

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.Minerals 2019, 9, 56 2 of 22 improve mining exploration strategies. I will present and discuss seven case studies of metal-bearing veins with different modes of formation that highlight the role of pre-existing features on their development. This aspect seems to be frequently underestimated, at least in the case of vein deposits, and this work aims to demonstrate its significance in the development of exploration and exploitation programs. The re-using of some previously formed structures has, in that case, a significant but passive role with respect to the formation of the economic feature. This concept is already exemplified in another contribution of this volume [18]. Each of the seven cases presented herein include a brief overview of the regional geology and deformation history, followed by a detailed geometrical and textural analysis of ore-bearing veins and a regional-scale genetic model that integrates these data. The relationships between neo-formed structures versus pre-existing ones will be highlighted in each case as well as its implications for regional vein distribution and, consequently, exploration programs.

Sidi M'Barek: a representative example of the Moroccan massive sulfide deposits

“…10ac). Hence the cleavage serves as a receptacle for chalcopyrite concentration in a late deformation event that causes dilation parallel to a pre-existing cleavage (Chauvet et al 2019). The evidence clearly supports the pyrrhotite-rich mineralization as being overprinted and further remobilized by syn-to late-tectonic pyrite-and chalcopyrite-rich mineralization.…”

Alteration and Mineralization Products of the Zannone Giant Pockmark (Zannone Hydrothermal Field, Central Tyrrhenian Sea)

“…In hydrothermal explosions, the rounded shape of clasts is determined by reworking (abrasion and milling) during their genesis (e.g., References [49,50] and references therein). Actually, it might have been originated also by other reworking processes, such as those related to sediment transport (e.g., fluvial transport in the subaerial environment); however, this process seems to be unlikely as the clasts show a well-preserved altered outer rim and occur at depth > 120 m.…”

Editorial for Special Issue “Structural Control of Mineral Deposits: Theory and Reality”

“…These publications mostly concerned vein internal infilling textures [10,11], the vein formation model, with the contribution and controversy of the crack seal, dissolution-precipitation, diffusion, and seismic-valves mechanisms (e.g., [12][13][14][15][16]). In his review, Chauvet [17] discussed of some of these concepts, in order to highlight the role and the significance of pre-existing structures in the formation of vein-style deposits.…”

“…Funedda et al [19] and Fridovsky et al [20] also used a detailed description of the relationships between mineralization and deformation in deformed domains, such as the Variscan domain of Sardinia and the Verkhovansk-Kolyma folded region of NE Russia. Funedda et al [19] pay close attention to the opening process of structures that will serve as traps for mineralised fluid catching, a fact that is fundamental in any tectonic understanding of a mineralised vein system [17]. Fridovsky et al [20] also proposed a pluri-deformational model associated with multiple stages of mineralisation formation.…”

“…In addition, it has been demonstrated that stockwork within VHMS environments can result in subsequent syntectonic veining instead of earlier veins related to feeder zones [31]. Indeed, the observation of stockwork within a VHMS context needs to be considered with particular attention because of the possible coexistence of the two types of stockwork: the one related to the feeder zone and the other the result of subsequent deformation [17]. It has been suggested that the second event and associated metal may contribute significantly to a relative enrichment in VHMS environment.…”