Understanding Porphyry Cu-Mo Deposits

Porphyry copper-molybdenum deposits are significant sources of copper and molybdenum. These deposits are characterized by low-grade, large-volume ore bodies associated with intrusions of igneous rocks.

Magma Association: Calc-alkaline and I-type

The question highlights that these specific deposits are linked to particular types of magma:

• Calc-alkaline magmas: These are common in convergent plate boundary settings and are typically generated during processes involving the melting of the Earth's mantle wedge above a subducting oceanic plate.
• I-type magmas: This classification refers to magmas derived from the melting of igneous source rocks in the crust (meta-igneous). They are often found in subduction-related settings as well.

The formation of Porphyry Cu-Mo deposits is strongly tied to the magmatic processes occurring within these specific magma types.

Tectonic Setting: The Crucial Link

The formation environment, or tectonic setting, is critical for the generation and emplacement of the magmas that host these deposits. The question asks where these calc-alkaline or I-type magmas are typically generated.

Analyzing the Options for Porphyry Deposits

Let's examine the different tectonic settings provided:

• Mid-oceanic ridges: These are divergent plate boundaries where new oceanic crust is formed. The magmatism here is typically tholeiitic and basaltic, not usually associated with the calc-alkaline magmas required for porphyry Cu-Mo deposits.
• Subduction zones: This is where one tectonic plate slides beneath another. As the oceanic plate subducts, it releases water, causing melting in the overlying mantle wedge. This generates calc-alkaline magmas that rise to form volcanic arcs and associated intrusions. These are the prime locations for the generation of magmas hosting Porphyry Cu-Mo deposits.
• Continental rifts: These are areas where continents are being pulled apart (divergent boundaries). The magmatism here can be varied but is not typically the calc-alkaline type associated with porphyry deposits.
• Oceanic islands: These can be formed by hotspots (like Hawaii) or along mid-ocean ridges. Hotspot volcanism is usually alkaline or tholeiitic, and ridge volcanism is tholeiitic, neither being the primary source for porphyry Cu-Mo deposits.

Conclusion on Porphyry Deposit Genesis

Based on the known geological associations, the calc-alkaline or I-type magmas required for the formation of Porphyry Cu-Mo deposits are generated in environments adjacent to subduction zones, specifically within the magmatic arcs that form above them.