TECTONIC CONTROLS ON PORPHYRY CU AND EPITHERMAL DEPOSITS AT CONVERGENT CONTINENTAL MARGINS

Convergent continental margins host a range of deposit types of which porphyry Cu (PCD) and epithermal deposits of high-sulfidation (HS) and intermediate-sulfidation (IS) types are common in the magmatic arc. All deposits form from intermediate-composition arc magmas emplaced in the shallow crust. PCDs however form during distinct and short-lived episodes of the arc. The same also must apply to metal-bearing HS deposits, because of their genetic link to a PCD, despite the widespread formation of intense but barren acid leached zones in volcanic edifices. Furthermore, there are times during an arc’s history when HS systems or when IS systems dominate. Where contemporaneous, there is little geographic overlap between HS and IS systems although HS systems may evolve to IS mineral assemblages. Where HS and IS systems are spatially related, there is a temporal difference between them. Vein geometry indicates fundamentally different stress conditions during deposit formation. PCD veins have consistent orthogonal or conjugate angular relations, indicative of low differential horizontal stress at the time of formation. Similarly, HS deposits are controlled by faults older than host volcanic rocks and ore reflects flow along orthogonal fluid conduits. In contrast, veins with a common strike and length of up to several kms define IS deposits, indicating a differential horizontal stress during formation. Low-differential horizontal stress environment seemingly favorable for PCD/HS deposits corresponds to parts of an arc or periods during arc activity where there was limited deformation synchronous with magmatism. These conditions are conducive to the uninterrupted formation of an upper crustal magma chamber, and vertical hydrothermal fluid escape into the PCD or HS environment. Such tectonomagmatic conditions are also rare and should be short lived. Conversely, LS deposits are favored during times of differential horizontal stress and active deformation coincident with magmatism. Deformation disrupts the fluid seal around the magma chamber, allowing fluid to escape to the hydrosphere.