SUBDUCTION AND COLLISIONAL TECTONISM: IMPLICATIONS FOR MAGMA GENESIS AND PORPHYRY COPPER-GOLD-MOLYBDENUM MINERALIZATION

Porphyry copper ore deposits are found near sites of subduction around the Pacific basin. Nearly all deposits in Arizona and along the Andes are Cu-Mo systems whereas many in the western Pacific (e.g., Grasberg) and British Columbia are Cu-Au systems. While both types form along convergent margins, the origin of the chemical dichotomy is problematic. It is suggested that porphyry copper orebodies are products of two fundamentally different types of convergent margin magmatism. Cu+Mo systems form where subduction is steady, but plate dip is shallowing and the locus of magmatism is migrating. Cu+Au systems form where the jamming of subduction zones causes collisional delamination – a process that causes a short-lived magma generation event due to the rupture and breakoff of the descending plate. A major portion of the magma, including most Cu, in both settings is derived from melting in the asthenosphere. Cu+Mo deposits can form during subduction because magma generation also involves the melting of Mo-rich oceanic shale. Cu+Au deposits can form during collisional tectonism also involves the partial melting of metasomatized lithospheric mantle generates magma that is potassic and oxidized. The metal abundances in both types of convergent margin magmas are not extraordinary. The determinative factor is the formation of bubbles of saline fluid deep (3+ km) along the sides of cooling stocks. The fluid concentrates the metals proportionate to their abundance in the magma, which have higher Mo/Au ratios in subduction-generated magmas and higher Au/Mo ratios in collisional delamination magmas. The ascent of buoyant magma along the walls of a stock causes bubbles to grow to the point they can rise on their own to charge cupolas at the top with metal-rich fluid. Porphyry copper-type orebodies can form where the focused discharge from a fluid-charged cupola is a combination of steady pervasive infiltration and episodic injection along fractures opened because pull-apart extension occurs on the connection between strike-slip faults that were active during devolatization. The faulting and extension also probably generated pathways for emplacement of stock-forming pulses of intrusion.