Mobilisation of deep crustal sulfide melts as a first order control on upper lithospheric metallogeny

Abstract

Magmatic arcs are terrestrial environments where lithospheric cycling and recycling of metals and volatiles is enhanced. However, the first-order mechanism permitting the episodic fluxing of these elements from the mantle through to the outer Earth's spheres has been elusive. To address this knowledge gap, we focus on the textural and minero-chemical characteristics of metal-rich magmatic sulfides hosted in amphibole-olivine-pyroxene cumulates in the lowermost crust. We show that in cumulates that were subject to increasing temperature due to prolonged mafic magmatism, which only occurs episodically during the complex evolution of any magmatic arc, Cu-Au-rich sulfide can exist as liquid while Ni-Fe rich sulfide occurs as a solid phase. This scenario occurs within a 'Goldilocks' temperature zone at ~1100-1200 °C, typical of the base of the crust in arcs, which permits episodic fractionation and mobilisation of Cu-Au-rich sulfide liquid into permeable melt networks that may ascend through the lithosphere providing metals for porphyry and epithermal ore deposits.

Fig. 1. Geological setting of lower crustal cumulate bodies in arcs.

A Generalized cross-section of a subduction zone, with associated metasomatism and magmatism (after Richards, Wilkinson) showing the location of the lower crustal MASH zone. B Schematic representation of sulfide supersaturated mafic-ultramafic cumulates in the MASH zone. Note the general isotherms for the base of the crust in this zone indicate a probable temperature range of above 1000 °C within the cumulates after emplacement. C Geological map of the Ivrea Zone, northern Italy, showing the generalized stratigraphy and the location of the Isola and Sella Bassa sampling sites. CMB Cossato–Mergozzo–Brissago. Modified after^,.

Fig. 2. ZEISS Mineralogic maps and quantitative sulfide assemblages of polished thin sections from the Isola Sill.

A an orthopyroxene-olivine cumulate with interstitial sulfides of pyrrhotite (po), pentlandite (pn) and chalcopyrite (cpy); B an orthopyroxene cumulate with interstitial sulfides. Quantitative data and sulfide ratios are also presented in Supplementary Data 3.

Fig. 3. Sulfide inclusions in primary silicates from the Isola Sill and Sella Bassa.

A Sulfide inclusion with pyrrhotite (po) and pentlandite (pn) exposed on the cut surface in cumulus orthopyroxene (opx) from Isola; B Sulfide inclusions with pyrrhotite and pentlandite exposed on the cut surface in cumulus orthopyroxene from Sella Bassa; C Sulfide inclusion with pyrrhotite and pentlandite exposed on the cut surface in intercumulus amphibole (amp) from Sella Bassa; D, E Examples of time-resolved analysis spectra from LA-ICP-MS analyses of inclusions from the Isola Sill and Sella Bassa, respectively, showing separate peaks in Cu, and Ni+Co, representing spatially separated chalcopyrite (cpy) and pentlandite grains within pyrrhotite.

Fig. 4. Precious metal-bearing minerals associated with sulfides in the samples from the Isola Sill and Sella Bassa.

A Palladian-melonite (Pd-mel) inclusion in interstitial pyrrhotite (po) bleb in an orthopyroxene (opx) cumulate from the Isola Sill; B Pt-Pd-bearing melonite in interstitial pyrrhotite-pentlandite (pn) bleb from Sella Bassa; C Palladian-melonite in chalcopyrite (cpy) vein with minor sphalerite (sph) from amphibole (amp)-bearing cumulate from Sella Bassa; D Cluster of moncheite (mon) grains and sperrylite (sp) with chalcopyrite altered to tremolite (trm) from an amphibole-rich cumulate from Sella Bassa; E Au-Ag alloy associated with cobaltian pentlandite (Co-pn) in a chalcopyrite-pyrrhotite vein from Sella Bassa.

Fig. 5. ZEISS Mineralogic maps of clinopyroxene-amphibole-orthopyroxene cumulates and quantitative (wt%) definition of sulfide assemblages in polished thin sections from Sella Bassa.

A Interstitial sulfides dominated by pyrrhotite (po) and pentlandite (po) (sample IV18-SB1); B Interstitial pyrrhotite-pentlandite blebs and a vein of chalcopyrite (cpy)-pyrrhotite-pentlandite (sample IV18-SB4); and C Interstitial sulfides dominated by pyrrhotite and pentlandite with minor chalcopyrite and a vein of dominantly chalcopyrite (Sample IV18-SB3). Quantitative data and sulfide ratios are also presented in Supplementary Data 3.

Fig. 6. Proposed scenarios for lower crustal sulfide crystallization and mobilization.

A Modeling results of progressive sill emplacement at the base of the crust in the Ivrea Zone show the rise in overall temperature on the emplacement of successive sills. Approximate relative timings of the Isola Sill and Sella Bassa are shown. Note that each magma pulse is emplaced at 1100 °C, though cools quickly; and a significant melt fraction only becomes present when the overall temperature of the complex reaches the silicate solidus. At 1 GPa, Cu-sulfide melt is present and mss crystalline between 1090 and 1160 °C. B Schematic representation of the evolution, fractionation, and mobility of sulfide liquid in lower crustal cumulates. See text for description of Steps 1–8. px pyroxene, amp amphibole, mss monosulfide solid solution, iss intermediate solid solution, cpy chalcopyrite, pn pentlandite, po pyrrhotite. Approximate positions of steps 1–8 from B are indicated in bold on A.