New study proposes ‘Goldilocks’ temperature zones as factories for generation of mineral deposits

Current understanding of the formation of porphyry deposits

Porphyry deposits are a major source of copper (Cu), gold (Au), and molybdenum (Mo) (Fig. 1). These deposits contribute 75%, 50%, and 20% of the Cu-, Mo-, Au produced in the world, respectively¹.

Figure 1. Molybdenite (dark grey molybdenum-sulphide mineral) hosted in quartz (grey) veins in a porphyry deposit (Source²)

Typically, porphyry deposits form in magmatic arcs (oceanic and continental) above subduction zones in a series of processes beginning with (i) the expulsion of metal-enriched fluids from the subducting slab into the mantle wedge resulting in partial melting of the mantle wedge, followed by, (ii) the migration and stalling of such magmas in the magma chambers in the mid to lower crust, and subsequently, (iii) the migration of metal-rich fluids produced during the crystallisation of magmas in magma chambers into the upper crust where they form pipe- and dyke-like porphyry deposits ^3,4 (Fig. 2).

Figure 2. Geological setting for the formation of porphyry deposits (Source⁵)

Knowledge gaps and new contributions

The formation of porphyry deposits is fairly understood. However, the mechanisms controlling the transfer of metals from the mantle/lower crust to the upper crust in magmatic arcs where there are likely to form porphyry deposits remain elusive ⁶.

A new study published in Nature Communications proposes a mechanism in an attempt to constrain conditions conducive for such metal transfer events⁷. The study investigates magmatic sulphides hosted in magmas located in the melting-assimilation-storage-homogenisation (MASH) zones in the lower crust (Fig. 2). The samples used in the study were collected from the Ivrea Zone, a rare geological archive host to lower crustal cumulate rocks accumulated at the base of continental crust now rotated by almost 90^o. The study found that the process central to fluxing of metals (Cu and Au in the case of porphyry systems) is the “Goldilocks” temperature window (1090-1160 ^oC) in the lower crust whereby the two metal-hosting sulphide phases exist in different physical forms, a solid Ni-Fe-rich monosulphide solution (mss) and Cu-Au-rich sulphide melt. Such a temperature window allows the fractionation and mobilisation of the Cu-Au-rich sulphide melts which ascend into the upper crust and possibly form economic deposits (e.g., porphyry deposits), and the trapping of Ni-Fe-rich material in the cumulate residue.

 

References and further reading

1.           Sillitoe, R. H. Porphyry Copper Systems. Econ. Geol. 105, 3–41 (2010).

2.           Rüegg, P. How ore deposits are formed. ETH Life ethlife.ethz.ch/archive_articles/121120_erzlagerstaetten_per/index_EN.html (2012).

3.           Hedenquist, J. W. Lowenstern, J. B. The role of magmas in the formation of hydrothermal ore deposits. Nature 370, 519–527 (1994).

4.           Richards, J. P. Magmatic to hydrothermal metal fluxes in convergent and collided margins. Ore Geol. Rev. 40, 1–26 (2011).

5.           Wilkinson, J. J. Triggers for the formation of porphyry ore deposits in magmatic arcs. Nat. Geosci. 6, 917–925 (2013).

6.           Sillitoe, R. H. Porphyry and Hydrothermal Copper and Gold Deposits: A Global Perspective. in (ed. Porter, T. M.) 21–34 (PCG Publishing, 1998).

7.           Holwell, D. A. et al. Mobilisation of deep crustal sulfide melts as a first order control on upper lithospheric metallogeny. Nat. Commun. 573, 1–12 (2022).