Igneous Rocks and Ore Deposits

• Geology for Engineers - Winter 2025 - Day 6
• Ore deposits become economical based on factors like access, concentration processes, and technology.

Ore Deposits Linked to Igneous Rocks

• Some ore deposits are directly linked to igneous rocks, suggesting a genetic association between magmatic processes and ore formation.
• Hydrothermal deposits systems are developed around magma chambers, often in silicic volcanic terranes.
• Layered Mafic Intrusions (LMIs) can host significant ore deposits due to the separation and concentration of elements during magmatic crystallization.
• Impact structures, such as the Sudbury impact, can create unique geological environments conducive to ore formation.
• Granitic pegmatites are known for rare element mineralization due to extreme fractionation of magma during their formation.
• Kimberlites are the primary source of diamonds, transported from the mantle to the Earth's surface through volcanic pipes.
• Carbonatites, rare igneous rocks, can be enriched in rare earth elements and other valuable minerals.
• Porphyry copper deposits are large-scale deposits associated with intrusive igneous rocks and hydrothermal activity.
• Volcanogenic Massive Sulfide (VMS) deposits are linked to seafloor volcanism and the activity of black smokers, which precipitate sulfide minerals.

Magmatic-Hydrothermal Deposits

• These deposits are associated with both magmatic and hydrothermal processes
• Intrusion-related deposits are associated with felsic, plutonic rocks.
• Porphyry copper-molybdenum-gold (Cu-Mo-Au) deposits can form in these intrusion-related settings.
• Seafloor volcanism (mafic, volcanic) can lead to the formation of Volcanic Hosted (VMS) deposits containing copper, lead, zinc, gold, and silver.
• Hot springs can deposit gold and silver.
• Epithermal deposits contain gold and silver.
• Mesothermal veins contain gold and silver.

Mineral Deposit Accessibility

• Mineral deposits of igneous origin are typically rooted deeply but are brought closer to the surface through weathering and erosion.
• The ease of discovering concentrated resources renders them as economical reserves, such as metal-bearing VMS, diamonds, and gold.

Seeing Deep into the Earth

• Gravity imaging measures changes in rock density by looking at variations in gravity.
• Seismic imaging provides a way to see deep into the Earth.
• Magnetotelluric imaging is a geophysical method used to image the Earth's subsurface.
• 3D imaging overlays geophysical methods in the Rouyn-Noranda transect.

Origin versus Occurrence

• The origin of an ore deposit relates to the primary geologic environment of formation, such as diamonds forming at 250 km depth in the mantle.
• The occurrence of an ore deposit relates to where the mineral is found today, such as diamonds found in kimberlite pipes after transportation to the surface.

Placer Deposits

• Placer deposits form due to varying flow speeds in a river channel.
• The water separates minerals by specific gravity, concentrating heavier minerals in slower-moving sections.
• Winnowing concentrates heavy minerals such as galena, native gold, and diamonds.
• Examples of placer deposits: Klondike Gold Rush, Witwatersrand South Africa

Coté Gold, Gogama, ON

• Coté Gold is a low-grade bulk-tonnage Archean deposit (lode Au deposit) with minute amounts of gold contained in the rock.
• It includes porphyry Cu-Au, syenite-associated disseminated gold, and reduced Au-Bi-Te-W intrusion-related deposits.
• Detailed mapping, sampling, and geophysical surveys have revealed hydrothermal alteration, targeting gold-rich areas.
• The Harquail School of Earth Sciences played an instrumental role in its discovery in 2010.

Serpentine Alteration

• Mantle peridotites overlying subduction planes are continuously hydrated during slab dehydration, becoming enriched in specific elements.
• Serpentinites are transported downward and break down at ~100 km depth, releasing H₂O.
• Ultramafic minerals (olivine and orthopyroxene) are unstable under low-pressure and temperature conditions.
• As a result, magnesium (Mg), iron (Fe), and trace amounts of Ni are detached from silicate minerals.
• The resulting products are serpentine, iron oxides, and Ni minerals.
• Olivine and orthopyroxene react with hot water to liberate cations (Fe2+, Mg2+, Ni2+).

Low-Grade Ni Deposit

• Lode deposits are metal-bearing deposits containing minute amounts of the element of interest, often due to lack of concentrating processes.
• The Crawford Deposit has at least 5 times lower concentrations of Ni compared to Sudbury deposits (>2 wt% Ni).
• Serpentine type is added due to hydrothermal alteration of ultramafic rocks, resulting in a few secondary minerals dominated by serpentine.
• The hydration of Ni-bearing olivine paired with desulfurization results in remobilization of nickel.
• Silicate nickel trapped inside olivine is concentrated in secondary sulfides and alloys.
• The ore would be mined in an open-pit concept by bulk tonnage

Day 6 Summary

• It is crucial to focus on near-surface ore deposits.
• Geological processes concentrate resources, making them economical.
• Finding deposits requires knowing rock types, associations, age, and structures.
• Low concentrations of resources can be economical with adapted mining methods (open pits).
• Geophysical methods (gravity, seismic, magnetotelluric) are invaluable for understanding mineral resource extent.