The development of a LA-ICP-MS method to identify and quantify PGE tracers in unconventional mineral phases in ultramafic metasomatic Ni-Cu-PGE sulfide deposits

The Ivrea-Verbano Zone (IVZ) in northwestern Italy is a cross-section of lower crust and upper mantle that was uplifted from the base of the crust to the surface during the Alpine orogeny, where it is exposed today. The IVZ hosts ultramafic harzburgitic pipe intrusions that are mantle-sourced, and host complex metasomatic assemblages of hydrous silicates (e.g., phlogopite and amphibole) and Ni-Cu-PGE mineralization consisting of pyrrhotite (FeS_1-x), pentlandite (Fe,Ni)₉S₈, and chalcopyrite (CuFeS₂). The oldest pipe at Fei di Doccio, emplaced at ~277 Ma, and the youngest pipe at Valmaggia, emplaced at ~249 Ma, represent long-lived fluid and metal transfer from the mantle to the lower crust. Both pipes are characterized by two physical and chemical domains: the outer rim of the pipe that exhibits a higher degree of metasomatism and higher modal abundance of sulfide, and the inner core of the pipe that exhibits a lesser degree of metasomatism and lower modal abundance of sulfide. While the pipes host varying degrees of sulfide mineralization, they also host platinum-group elements (PGE; e.g., Ru, Rh, Pd, Os, Ir, Pt) in the form of platinum-group mineral-alloys (PGM; e.g., merenskyite) and as trace elements hosted within sulfide minerals.

This project aims to develop a method using the PGE in pentlandite and associated phlogopite to determine the capacity and efficiency of mantle-sourced metasomatic fluids in transporting and mineralizing PGE.

Using LA-ICP-MS, the two domains (rim and core) of the two pipes will be analyzed separately to determine 1) if pentlandite and phlogopite PGE analysis can be used to determine the efficiency of a fluid in transporting PGE, and 2) if analyzing the physical chemical domains (rim vs. core) can be a useful tool in determining fluid flow and PGE transport and entrapment in an intrusive body. This work aims to develop a LA-ICP-MS method to obtain high quality data to answer the preceding questions.

Header image: Ni-Cu sulfide ore featuring pentlandite/violarite (white, cracked), chalcopyrite (yellow), and various accessory magnetite and spinel (light to medium grey) in a harzburgite pipe from the Ivrea-Verbano Zone in NW Italy. FOV = 1 mm