22 - Plate Tectonics and Ore Deposits Study Notes

Plate Tectonics and Ore Deposits

Overview of Plate Tectonic Theory in Economic Geology

  • Plate tectonic theory has been extensively used in exploring economically viable mineral and hydrocarbon deposits.

  • It offers a framework to relate specific environments and spatial relationships to particular deposit types.

  • This has led to a classification system of economic deposits based on the geological processes or settings associated with plate tectonics.

    • Such classifications are becoming increasingly important as the search for smaller, concealed deposits is on the rise, as noted by Groves et al., 2005 in Economic Geology, volume 100, pages 203-224.

Classification of Economic Deposits Based on Plate Tectonics

  • A classification scheme is outlined based on the tectonic setting of ore formation:

    1. Autochthonous Deposits

    • Directly related to magmas at plate margins and interiors.

    1. Allochthonous Deposits

    • Related to plate-margin magmatism.

    1. Deposits Related to Sedimentary Basins

    • Formed by processes related to plate motions.

    1. Deposits Related to Climate and Paleolatitudes

    • Influenced by tectonic shifts affecting historical climate conditions.

  • Important to remember the evolution and variation of plate tectonic processes throughout geological time; for example, Archean deposits necessitate distinct frameworks for classification.

Development Stages of Ore Deposits

  • Fourfold division of deposit types based on formation stages:

    • RIFT Stage

    • Non-orogenic granites, Volcanogenic Massive Sulfides (VMS) including Cu-Zn, Sn-U-Nb-REE-F and Clastic-dominated Pb-Zn.

    • OCEAN Stage

    • Superior-type Banded Iron Formations (BIFs) from 2.6-1.8 Ga, comprised of layered igneous complexes producing Cr-Ni-Cu-PGE minerals.

    • CONVERGENCE Stage

    • Arc and back-arc related deposits, including carbonatites with U-Zr-Nb-REE deposits and other economic resources associated with accretionary prisms and porphyry systems.

    • COLLISION Stage

    • Associated with post-collision relaxation processes, forming Peraluminous granites and diverse ore deposits including Au, Li, and Sn in various geological settings.

Ore Mineral Formation

  • Ore minerals can form through both magmatism and the large-scale circulation of hydrothermal fluids that are energized by magmatic activities.

    • Specific minerals and their significance:

    • Apatite (P)

    • Pyrochlore (Nb)

    • Baddeleyite (U)

    • Monazite (REE + Th)

    • Zircon (Zr)

  • Initial continental rifting often leads to high geothermal gradients and results in the formation of alkaline, peralkaline, and carbonate-rich igneous rocks.

Continental Rift Contributions

  • Carbonatites from continental rifts contribute significantly to sodium carbonate and alkaline minerals in Lake systems, particularly in the East African Rift.

  • Economic deposits related to rift systems include:

    • Porphyry and vein-type Molybdenum deposits associated with subalkaline granites.

    • Copper-Nickel deposits related to mafic intrusions and hydrothermal processes.

    • Stratiform Cu deposits from rift-related sediments, likely formed during early marine transgression into continental interiors.

Proto-Ocean Deposits

  • Formation of narrow ocean basins between rifted continental fragments generates mineral deposits at mid-ocean ridge systems.

  • The Red Sea serves as a contemporary example of such activity.

  • There is circulation of hot brine at transform fault intersections at mid-ocean ridges, with potential economic Zn-Cu-Pb deposits being identified.

    • Example: "Atlantis II Deep," which features sulfide layers with Zn concentrations reaching up to 20 wt% and extending over significant areas.

Mid-Ocean Ridge Deposits

  • In mid-ocean ridge systems, hot water and gases flow through fractures in oceanic crust, leaching metal ions from host volcanic rocks at depth.

    • Metal ions are concentrated and redeposited at the ocean floor surface, creating deposits.

    • Types of deposits:

    • Black Smokers: Emitting water at 350-400 °C, precipitating metal sulfides (e.g., Au, Zn, Pb, Cu).

    • White Smokers: Emitting water at 30-330 °C, precipitating sulfates (e.g., Ba, Ca, Si).

  • Ores are dispersed from the MORs through sea-floor spreading and subsequently emplaced on land via terrane accretion.

Open-Ocean Deposits

  • Ore deposits may also occur in the open ocean, away from volcanic activities.

    • Example: Ferromanganese nodules form in oxidizing environments such as the abyssal pelagic zone through ion exchange processes, precipitating Mn2O3 from oceanic and interstitial waters.

Ophiolite-Hosted Ores

  • There's significant evidence of ore bodies forming within the oceanic lithosphere as it creates.

    • Indicators include ophiolites and massive sulfides like marcasite, chalcopyrite, and sphalerite found in or over layer-2 pillow lavas.

  • These deposits can form similar to red sea brines or from hydrothermal solutions enriched in metals circulating within basalts and dolerites.

    • Potential economic deposits of chromite are found within upper lithospheric mantle harzburgites and dunites.

Kimberlite and Diamond Formation

  • Kimberlite pipes represent critical magma-hosted mineral deposits for diamonds.

    • These are small intrusive bodies originating from the mantle, found in virtually all Archean cratons and throughout the Phanerozoic record.

    • The formation of kimberlite intrusions is often linked to times of heightened hotspot or mantle plume activity, notably during the supercontinent Gondwana's break-up.

  • Diamonds hold significant geological value as they are derived from some of the Earth’s oldest and deep-seated minerals and remarkable insights can be gleaned from their formation conditions.

Banded Iron Formations (BIFs)

  • Banded Iron Formations are notable Precambrian chemical sedimentary rocks characterized by alternating iron-rich and silica-rich layers, a composition generally retaining iron content of >15%.

    • Types:

    • Archean Type: Characterized by lower oxygen levels prior to the Great Oxygenation Event.

    • Proterozoic Type: Evolved during the Great Oxygenation Event, responding to changing oceanic conditions and atmospheric composition.

Deposits Related to Climate

  • Bauxite: This residual deposit is paramount for global aluminum supply, originating from in situ weathering of aluminosilicate minerals in tropical climates.

  • Evaporites: Build up in arid conditions through the evaporation of seawater, leading to mineral precipitation in a characteristic order.

    • They possess significant commercial use in the chemical industry, particularly as potash salts and lithium sources.

Economic Significance of Carbon Influx in Subduction Zones

  • Carbon is cycled through the mantle via subduction processes:

    • The primary reservoirs include subducting sediments, hydrothermally altered oceanic crust, and serpentinized mantle lithosphere.

    • As plates descend, the ascent of carbon occurs through a series of processes like fluid release, metamorphic reactions, and ultimately arc volcanism, contributing to atmospheric carbon levels.

The Impact of Plate Tectonics on Fossil Fuels

  • Sedimentary basins tied directly to plate movements facilitate fossil fuel formation:

    • Key elements include organic-rich layers, source heat, permeable pathways, and reservoir conditions, with plankton being the primary source of kerogens in sediments.

  • Conditions required for organic matter preservation and hydrocarbon maturation play crucial roles in global energy resource accessibility.

Summary of Mineralization Processes in Subduction Zones

  • Varied mineralization arises based on whether the lithosphere is oceanic or continental, with the most critical mineralizations originating from porphyry copper systems that are generally low-grade yet prevalent.

  • These systems are characterized by differing gold and molybdenum content when associated with different tectonic settings, emphasizing their diversity and significance in geological processes.