Metal Observations

Metals on an Expansion Tectonic Earth are considered in the context of metallogenic epochs and metallogenic provinciality. A metallogenic epoch is a regularly recurring sequence of events that gives rise to a cyclicity in mineral deposition during geological history. A metallogenic province is a specific region possessing a notable concentration of a certain metal or metals linked with common characteristic features.

Metallogenic Epochs

On an Expansion Tectonic Earth, tectonic settings are global scale and characteristic metallogenic epochs correlate globally. In this context tectonic development suggests a common heritage for both lithosphere and metals. It has been shown elsewhere that crustal development on an Expansion Tectonic Earth is dominated by horizontal motion, including basement extension and ensialic orogenesis associated with high heat flow within a global network of crustal weakness. Characteristic peaks in the abundance of specific styles of metallic mineralization is then reflected in the changing crustal development with time, the evolution of the hydrosphere and atmosphere, and the secular decrease in global heat flow.

Metallogenic epochs on an Expansion Tectonic Earth are considered in relation to the continental and oceanic crustal development and subsequent tectonic and stratigraphic history. Because the metal data are not age dated, these epochs cannot be modelled in any detail so the MRDS metal data are simply modelled for the present-day (Pliocene), Permian, and Cambrian small Earth models. This modelling reflects metal distributions and associations persisting at the end of the Precambrian, just prior to the end-Permian supercontinental breakup, and where these same metals are located today.

The correlation of metallogenic epochs and tectonic development on an Expansion Tectonic Earth is typically categorised by the following tectonic settings:

• Late Archaean stabilisation of continental crust characterised by high global heat flow and magmatism.

• Intracratonic rifting, extension and tensile fragmentation of the proto-Archaean crust consisting of stages of early rifting with Au-conglomerate basins, BIF basins, layered intrusions hosting PGE, Cr, V-Ti-Fe and Ni-Cu; early-Proterozoic greenstone belts with vein Au, sulphide Cu-Au deposits; and late rifting with layered intrusions hosting Ni-Cu, PGE and Cr.

• Basement crustal extension and rifting generating ophiolite associated Cu-Co-Zn massive sulphide deposits, Ni-Cu deposits and continental arcs with Zn-Cu-Pb-Ag-Au volcanogenic massive sulphide (VMS), porphyry Cu-Au-Mo, BIF, and Kiruna-type Fe deposits.

• Cratonisation, basement crustal extension and intracratonic rifting characterised by rifted basins with sediment-hosted Zn-Pb-Cu and granitoids with Sn, Mo, W, Cu, Fe and REE.

• Formation of vast, elongate, slowly sinking basins during the mid- to late-Proterozoic characterised by metalliferous carbonaceous argillites and marls, often associated with bedded phosphorites, Cu-Pb and Zn-Pb base metals in a predominantly oxygenated hydrosphere.

Archaean Metallogenesis

Mineralization during the Archaean was extremely varied, with a direct relationship between metal type and host rock composition or environment of deposition. Metals include those that favour development or association with ultramafic-mafic magmatic rocks (mainly gold, nickel, chrome, asbestos), those that occur predominantly in mafic to felsic magmatic successions (mainly massive sulphide deposits containing copper, zinc, silver, and gold, as well as iron-formations), those that favour development in sedimentary sequences (mainly iron-formations, limestone, barite, and placer deposits of gold and tin) and mineralization restricted to granitic rocks.

Proterozoic Metallogenesis

The Archaean-Proterozoic boundary is a significant interface between two substantially different crust-forming regimes: a primitive Archaean and a more evolved Proterozoic. The late-Archaean to early-Proterozoic transition was an episodic event characterised by a change in the composition of granite rocks to potassium-rich granites displaying a negative europium-anomaly. During that time Archaean cratonic regions yielded enormous amounts of clastic and chemical sedimentary material to both platforms and continental margins, estimated at 43 kilometres thickness in places.

The Proterozoic Era on an Expansion Tectonic Earth was a period of steady to rapidly accelerating expansion resulting in crustal extension and sediment accumulation within a network of intracratonic sedimentary basins. Vast, elongate, slowly sinking basins were the dominant tectonic feature of the late-Proterozoic. Archaean cratonic regions yielded clastic material to both platforms and proto-continental margins, and sedimentation in these epi-continental basins and seas provided environments for concentration of base metals and chemical sediments, including BIF, carbonate, and silica-rich rocks.

Phanerozoic Metallogenesis

In most places of the world there was no break between the Proterozoic and Phanerozoic rock associations, with sedimentation continuing across the time boundary. The late-Proterozoic, characterised by heat dissipation through rifts and mobile zones, evolved into the Palaeozoic orogenic belts producing a variegated range of metal deposits. Most Palaeozoic metal deposits lie within orogenic belts, and Mesozoic and Cenozoic orogenic deposits in particular are aligned with present plate boundaries, supporting the ascendancy of new mechanisms of crustal behaviour during the Palaeozoic and later eras.

On an Expansion Tectonic Earth Palaeozoic continental crustal extension and epi-continental sedimentation culminated with the progressive break-up and dispersal of continents during the late-Permian. Palaeozoic sedimentation then shifted from epi-continental settings to marginal basins located around the perimeters of the newly formed continents. Long chains of volcanic island arcs around the margins of the continents were associated with initiation of mid-ocean-rifting, as well as rift-bordered and back-arc basins and large shallow epi-continental seas. Most Phanerozoic mineral deposits lie within orogenic belts and Mesozoic and Cenozoic deposits in particular are aligned with present plate boundaries.

•	Late Archaean stabilisation of continental crust characterised by high global heat flow and magmatism.
•	Intracratonic rifting, extension and tensile fragmentation of the proto-Archaean crust consisting of stages of early rifting with Au-conglomerate basins, BIF basins, layered intrusions hosting PGE, Cr, V-Ti-Fe and Ni-Cu; early-Proterozoic greenstone belts with vein Au, sulphide Cu-Au deposits; and late rifting with layered intrusions hosting Ni-Cu, PGE and Cr.
•	Basement crustal extension and rifting generating ophiolite associated Cu-Co-Zn massive sulphide deposits, Ni-Cu deposits and continental arcs with Zn-Cu-Pb-Ag-Au volcanogenic massive sulphide (VMS), porphyry Cu-Au-Mo, BIF, and Kiruna-type Fe deposits.
•	Cratonisation, basement crustal extension and intracratonic rifting characterised by rifted basins with sediment-hosted Zn-Pb-Cu and granitoids with Sn, Mo, W, Cu, Fe and REE.
•	Formation of vast, elongate, slowly sinking basins during the mid- to late-Proterozoic characterised by metalliferous carbonaceous argillites and marls, often associated with bedded phosphorites, Cu-Pb and Zn-Pb base metals in a predominantly oxygenated hydrosphere.