Controls on copper and gold endowments of porphyry deposits

Porphyry deposits are the major natural source of copper and a significant natural source of gold, which are essential metals for our society. Porphyry deposits form at convergent margins both during subduction (syn-subduction, Andean-type deposits) and in post-subduction to post-collision and extensional geodynamic settings (post-subduction deposits). Syn-subduction porphyry deposits are typically associated with calc-alkaline magmas often characterized by high Sr/Y values (~50-150). In contrast, post-subduction deposits are mostly associated with variably alkaline magmas having lower Sr/Y values (~25-75). The reasons of the association of porphyry deposits with magmas having different geochemical affinities and of their widely variable Cu and Au endowments (from <1 to >100 Mt for Cu and from few tens to >2500 tons for gold) remain unconstrained.

Porphyry Cu-Au deposits define two distinct trends in plots of Au versus Cu endowments and Au endowment versus duration of the ore process (Chiaradia, 2020): one trend (Cu-rich) is characterized by steep Cu/Au endowment values (Cu/Au~250000) and an average low rate of Au deposition (~100 tons Au/Ma); the other trend (Au-rich) is characterized by low Cu/Au endowment values (Cu/Au~12500) and an average high rate of gold deposition (~4500 tons Au/Ma). The Au-rich trend is defined to the greatest extent by seven, alkaline magma-related, porphyry gold systems (>1100 tons Au) and subordinately by numerous calc-alkaline systems (<1300 tons Au). The Cu-rich trend is defined only by calc-alkaline magma-related porphyry systems.

Modelling of petrological and metal precipitation processes using a Monte Carlo approach suggests that, whereas Cu-rich porphyries are formed by large volumes of magma, Au-rich porphyries result from a better precipitation efficiency of Au. The specific association of the largest Au-rich porphyry deposits with variably alkaline magmas also points out that alkaline magma chemistry favours an upgrade of Au endowments.

 

References

Chiaradia, M. (2020) Gold endowments of porphyry deposits controlled by precipitation efficiency. Nature Communications 11, 248, https://doi.org/10.1038/s41467-019-14113-1.