Heterogeneous Evolution and Distribution of Mineral Deposits Through Time

Uneven distribution of mineral deposits through space and time reflect evolution of the earth in terms of hydrosphere-atmosphere, changes in global heat flow and trends in plate tectonic settings and (Barley & Groves, 1992; Cawood & Hawkesworth, 2015; Groves, Condie, Goldfarb, Hronsky, & Vielreicher, 2005). Their formation and preservation are specific to a particular time in the Earth’s history associated with the prevailing conditions within that time. The two factors evolution of hydrosphere-atmosphere and the changes in global heat flow are related to specific. The former which include deposit such as iron formations and Pb-Zn deposits for which transportation of metals is strongly dependent on the oxidation-reduction conditions in the atmosphere and hydrosphere (Barley & Groves, 1992; Cawood & Hawkesworth, 2015). Similarly, the changes in global heat flow from a from a very hot earth to progressively cooler earth has influenced the formation and distribution of komatiites associated with nickel deposits and the Kambalda-type (Barley & Groves, 1992; Cawood & Hawkesworth, 2015).

The last factor which is the long-term tectonic trends is much more complex as it involved the supercontinent cycle. Spatial and temporal distribution of mineral deposits are important in understanding the processes that were in action within their tectonic environments when they formed (Cawood & Hawkesworth, 2015). The tectonic settings and the type of mineral deposit has a strong influence on the formation, preservation and distribution of the mineral deposits (Cawood & Hawkesworth, 2015; D. I. Groves & Bierlein, 2007). Super-continental cycle which involves the cyclic amalgamation and break-up of supercontinents plays an important role in the formation of the types of deposits driven by the process of tectonism which involves convergence, collision, and extension (Cawood & Hawkesworth, 2015; Skinner, 2005; Spencer et al., 2015). It should be noted that there are deposits which fall in more than one tectonic setting such as Sedimentary-Exhalative (SEDEX) and Mississippi-Valley Type (MVT) deposit (D. I. Groves & Bierlein, 2007). A. (Orogenic-convergent settings) B. (Anorogenic settings)Ore type 3Ga 2Ga 1Ga P Ore type 3Ga 2Ga 1Ga P

Before we get into details about the tectonic settings and their age-related preservation potential let’s first look at the theory behind subcontinental lithospheric mantle (SCLM). SCLM is the layer within mantle lithosphere below continental crust which influence thickening of the lithosphere (David I. Groves et al., 2005). The buoyancy of the SCLM in the Archean relative to the denser one of Proterozoic favoured the preservation of early-formed deposits (D. I. Groves & Bierlein, 2007; David I. Groves et al., 2005). The high global heat flow and stabilised Archean craton together with buoyancy of SCLM allowed for peak preservation potential of deposits types as Platinum Group Elements-PGE in layered intrusions, diamonds in alkaline pipes and Iron-Oxide Cu-Au (IOCG) (Cawood & Hawkesworth, 2015; D. I. Groves & Bierlein, 2007; Hawkesworth, Cawood, & Dhuime, 2013). The mantle basic magmatic deposits such as PGEs lie towards the central Archean craton, the basic magma forms due to upwelling of hot mantle plume beneath the Archean SCLM (Barley & Groves, 1992; D. I. Groves & Bierlein, 2007). Another set of deposits of deep alkaline magmatism associated with diamonds which form during Neoarchean and become common in younger igneous host rocks as they are more susceptible to weathering (D. I. Groves & Bierlein, 2007).

Stages of calc–alkaline magmatism resulted in porphyry Cu–Au deposit formation (Skinner, 2005). Convergent plate margins are sites of major continental growth and are fertile settings for the formation of mineral deposits (Cawood & Hawkesworth, 2015; Hawkesworth et al., 2013). In convergent zones, preservation potential does not only reflect supercontinental cycle bias but also the function of level of emplacement which influences the tendency of erosion of the deposits and so the subsequent preservation (Cawood & Hawkesworth, 2015). Major deposit types include epithermal Au–Ag and porphyry Cu–Mo-Au which forms in magmatic arc settings. Orogenic gold, that forms in convergent margins is associated with orogenic events while Epithermal Ag–Au and porphyry Cu deposits older than Mesozoic are not so common (Barley & Groves, 1992; Cawood & Hawkesworth, 2015).

Gold-bearing deposits that form in convergent margins (e.g., porphyry-skarn-epithermal Cu-Mo Au-Ag systems) are prone to exhumation and erosion, they form through the collision of their host arcs with continental blocks and are preserved in ages older than Mesozoic (Barley & Groves, 1992; D. I. Groves & Bierlein, 2007). The transition from plume-influenced buoyant plate tectonics to modern-style plate tectonics, with the change from buoyant to negatively buoyant subcontinental lithospheric mantle, strongly influenced the patterns of preservation of other deposits, for example, orogenic gold and Volcanic-Massive Sulphides (VMS) deposits (David I. Groves et al., 2005). VMS deposits occur in oceanic lithosphere in either midocean ridge or supra-subduction zone environments (Cawood & Hawkesworth, 2015). They are incorporated into the continental record through accretion events associated with periods of ocean closure and continental assembly/terrane accretion and hence correspond with cycles of supercontinent assembly (Cawood & Hawkesworth, 2015). These deposits are preferentially associated with break-up and dispersal stages of the cycle, such as Gondwana in the early Palaeozoic and Pangea in the Mesozoic (Skinner, 2005).

Preferential development of VMS deposits appears to be associated with periods in the Wilson cycle of elevated sea levels associated with continental dispersal after Gondwana break-up in the early Palaeozoic and in post-Pangean Mesozoic times (Skinner, 2005). Extensional settings where thinning and extension may be related to hotspot activity. Anorogenic granites such as those of the Bushveld Complex (Sn, W, Mo, Cu), pyroxenite–carbonatite intrusions such as Phalaborwa (Cu–Fe–P–U–REE), and kimberlites (diamonds), and VMS represent ore deposit types formed in this setting (Skinner, 2005). As continental rifting extends to the point that incipient oceans begin to open basaltic volcanism marks the site of a midocean ridge and this site is also accompanied by exhalative hydrothermal activity and plentiful VMS deposit formation. Such settings also provide the environments for chemical sedimentation and precipitation of banded iron-formations (Skinner, 2005). Precambrian manganese deposits show a similar temporal pattern to BIF as most of the deposits are related to enrichments of manganiferous BIF with manganese carbonate layers (David I. Groves et al., 2005).

Extension environments include VMS, Ni–Cu sulphide, Fe-oxide–Cu–Au and CD Pb–Zn deposits (D. I. Groves & Bierlein, 2007). Their general distribution is similar to orogenic gold deposits with peaks in the Neoarchean and late Paleoproterozoic and a more continuous distribution in the Phanerozoic but with significant peaks in the early and mid-Palaeozoic corresponding with assembly of Gondwana and Pangea (Barley & Groves, 1992; David I. Groves et al., 2005). This temporal association with orogenic gold reflects their common formation in back-arc basin settings and the higher preservation potential of this association in the long-term rock record than mid-ocean ridge environments (Cawood & Hawkesworth, 2015). Mid-ocean ridges are the culmination of extensional processes. Exhalative activity at these sites gives rise to “black-smoker” vents that provide the environments for the formation of Cyprus type VMS deposits (Cawood & Hawkesworth, 2015). The basalts which form at mid-ocean ridges also undergo fractional crystallization at sub-volcanic depths to form podiform chromite deposits as well as Cu–Ni–PGE sulfide segregations (Skinner, 2005)

Geodynamic settings such as CD Pb–Zn deposits occur in extensional settings, including rift and passive margins, back-arc basins and intracratonic rifts. The major pulse of mineralization of this type is recorded at the end of the Paleoproterozoic to early Mesoproterozoic (Cawood & Hawkesworth, 2015). This time frame corresponds with breakup of Nuna and the start of the Rodinian cycle and thus does not readily fit with the preservation bias model outlined above (D. I. Groves & Bierlein, 2007). Intracontinental sags as part of the environmental deposits may account for their preservation. Fe-oxide–Copper–Gold (IOCG) occupy a variety of extensional settings within pre-existing cratons and are tied to pulses of anorogenic alkaline or A-type magmatism close to the margins of the cratons or to lithospheric boundaries within the cratons (Cawood & Hawkesworth, 2015). The development of IOCG deposits in intracontinental settings and the relationship with mantle derived magmatism means that their temporal distribution is not directly related to the supercontinent cycle (Barley & Groves, 1992).

The IOCG deposit type has been expanded to include many different styles of iron-oxide–rich mineralization that formed in a variety of tectonic settings, but only those deposits with significant copper- and iron-bearing sulphides and gold resources are considered here (David I. Groves et al., 2005). Precambrian deposits, protected from uplift and erosion in the centres of buoyant cratons. The first appearance of iron-oxide copper-gold (IOCG) deposits at ~2.55 Ga closely follows development of early Precambrian subcontinental lithosphere mantle (Sawkins, 1989). Gold-bearing deposit types also display different temporal distributions related to the change from a more buoyant plate tectonic style in the early hotter Earth to a modern plate tectonic style typical of the Phanerozoic (David I. Groves et al., 2005). The temporal distribution of economically significant Precambrian IOCG deposits shows major peaks in the latest Archean (ca. 2.57 Ga), Paleoproterozoic (ca. 2.05 Ga; e.g., Palabora), and Mesoproterozoic (ca. 1.59 Ga; e.g., Olympic Dam) that are significantly offset from the main periods of crustal growth at ca. 2.7 and 1.9 Ga. Giant Precambrian IOCG deposits also appear to have required the pre-existence of buoyant Archean (and/or Paleoproterozoic) subcontinental lithospheric mantle for their formation and subsequent preservation (David I. Groves et al., 2005).

Metal deposits that formed and got preserved in orogenic belts resulting from convergence of tectonic crusts peaked during the late Archean (Barley & Groves, 1992) . This was due to the presence of higher heat flow, thicker oceanic crust, and an anoxic atmosphere together with numerous arc-related orogenic deposits in the late Archean, reflects the preservation of greenstone belts in stable shield areas of the world (Skinner, 2005). Archaean cratons are often enriched with a range of mineralization types including, orogenic gold, komatiite-hosted nickel and banded iron formation (BIF) (Jenkin et al., 2015). Late Archean formation of buoyant subcontinental lithospheric mantle was particularly important in the preservation of the earlier formed deposit types within cratonic margins and in providing crucial conditions for the formation of others (David I. Groves et al., 2005). In terms of Atmosphere- hydrosphere, during the Archean the atmosphere contained very little free molecular oxygen (although the actual amounts are still strongly debated) and what little did exist was the result of inorganic dissociation of water vapor. A reduced atmosphere in the Archean helps to explain many of the features of ore formation at that time, including the widespread mobility of Fe2+, the development of banded iron-formations (BIF), and the preservation of detrital grains of uraninite and pyrite in sedimentary sequences such as those of the Witwatersrand and Huronian basins (Skinner, 2005).

The concentration of deposits in the Late Archean and late Paleoproterozoic to early Mesoproterozoic appears to have formed near craton margins during alkaline magmatism derived from previously metasomatized mantle lithosphere (David I. Groves et al., 2005). Ni-Cu sulphide deposits are intracontinental rifting associated with break-up of supercontinents which shows temporal distribution related to rifting of Archean (D. I. Groves & Bierlein, 2007). These deposits have high magnesium content due to the hot conditions of the Earth in which it formed. The assembly of the first large continents during middle Proterozoic was associated with metal deposits which form in anorogenic continental basins (barley & Groves, 1992). Mineralisation includes, volcanic-massive sulphides (VMS), komatiites-nickel and porphyry copper deposits. The rise in sea levels between 1.8 and 1.6 Ga resulted to the preservation of large marine platforms and intracontinental basins. Within the continent, anorogenic magmatism occurred followed by fragmentation (Barley & Groves, 1992; D. I. Groves & Bierlein, 2007). Between 1.3 and 1.0 Ga also called the Grenville orogeny, orogenic activity together with continental amalgamation resulted to copper deposits in intracontinental rifts (Barley & Groves, 1992). The Bushveld Complex and Phalaborwa carbonatites were emplaced within the continental crust during the 2.0 Ga and was stabilised by 3.0 Ga (Sawkins, 1989). Future metal exploration can be enhanced by the understanding of the above-mentioned factors that contribute to different styles of deposits and their distribution. Behaviour, type and concentration of mineral deposits reflect their formational environment (Kesler & Ohmoto, 2006). These provide key understanding of magma evolution, tectonic processes and the state of atmosphere-hydrosphere (Barley & Groves, 1992).

Reference list:

1. Barley, M. E., & Groves, D. I. (1992). Supercontinent cycles and the distribution of metal deposits through time. Geology, 20(4), 291–294. https://doi.org/10.1130/0091-7613(1992)0202.3.CO;
2. Cawood, P. A., & Hawkesworth, C. J. (2015). Temporal relations between mineral deposits and global tectonic cycles. Geological Society, London, Special Publications, 393(1), 9–21. https://doi.org/10.1144/SP393.1
3. Groves, D. I., & Bierlein, F. P. (2007). Geodynamic settings of mineral deposit systems. Journal of the Geological Society, 164(1), 19–30. https://doi.org/10.1144/0016-76492006-065
4. Groves, D. I., Condie, K. C., Goldfarb, R. J., Hronsky, J. M. A., & Vielreicher, R. M. (2005). 100th Anniversary special paper: Secular changes in global tectonic processes and their influence on the temporal distribution of gold-bearing mineral deposits. Economic Geology, 100(2), 203–224. https://doi.org/10.2113/gsecongeo.100.2.203
5. Hawkesworth, C., Cawood, P., & Dhuime, B. (2013). Continental growth and the crustal record. Tectonophysics, 609, 651–660.
6. Jenkin, G. R. T., Lusty, P. A. J., McDonald, I., Smith, M. P., Boyce, A. J., & Wilkinson, J. J. (2015). Ore deposits in an evolving Earth: an introduction. Geological Society, London, Special Publications , 393(1), 1–8. https://doi.org/10.1144/SP393.14
7. Kesler, S. E., & Ohmoto, H. (2006). Evolution of Early Earth’s Atmosphere, Hydrosphere, and Biosphere: Constraints from Ore Deposits (Vol. 198). Geological Society of America.
8. Sawkins, F. J. (1989). Anorogenic felsic magmatism, rift sedimentation, and giant Proterozoic Pb-Zn deposits. Geology, 17(7), 657–660.
9. Skinner, B. J. (2005). Introduction To Ore-Forming Processes,. American Mineralogist (Vol. 90). https://doi.org/10.2138/am.2005.426
10. Spencer, C. J., Thomas, R. J., Roberts, N. M. W., Cawood, P. A., Millar, I., & Tapster, S. (2015). Crustal growth during island arc accretion and transcurrent deformation, Natal Metamorphic Province, South Africa: New isotopic constraints. Precambrian Research, 265, 203–217. https://doi.org/10.1016/j.precamres.2015.05.0114