Textural and trace element evolution of pyrite during greenschist facies metamorphic recrystallization in the southern Apuan Alps (Tuscany, Italy): Influence on the formation of Tl-rich sulfosalt melt

Several small pyrite ± baryte ± iron-oxide orebodies (Buca della Vena, Canale della Radice, Fornovolasco, Monte Arsiccio and Pollone) are hosted in the metamorphic rocks of the southern Apuan Alps, northern Tuscany, Italy. These deposits are exceptionally thallium-rich, expressed in part by assemblages comprising rare Tl-sulfosalts. Using a variety of techniques including laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) on fifty-four samples, the textural and trace element evolution of pyrite during greenschist facies metamorphism has been described. Five generations of pyrite have been recognised. Pyrite-1 (py1) and pyrite-2 (py2) (framboidal and colloform pyrite, respectively) likely represent pristine generations and are rich in trace elements. Pyrite-3 (py3) forms fine-grained disseminations often aligned with the metamorphic fabric and likely formed during the earliest stages of deformation associated with metamorphism. Pyrite-4 (py4) forms coarse, euhedral, inclusion-rich crystal clusters and likely formed as pristine pyrite generations began to recrystallize. Pyrite-5 (py5) is typically coarse, relatively ‘clean’ and often euhedral. It is interpreted to be the end-product of pyrite recrystallization, with grain size increasing and micro-inclusions being expelled late on the prograde path, then granoblastic annealed textures forming during retrograde cooling. Pyrite (especially primary pyrites; py1 and py2) is particularly rich in As (up to 17,400 ppm), Sb (up to 5100 ppm) and Tl (up to 4200 ppm), while Ni, Pb and Mn may be present above 1000 ppm, and Hg, Co, Cu, Zn, Mo, Bi and Ag may all be present above 100 ppm. Some trace elements, particularly As, Ni and Co, are commonly zoned in late metamorphic pyrite overgrowths. The incorporation of some trace elements into pyrite seems to have been facilitated by the presence of Sb in a similar way that As generally facilitates Au incorporation. Concentrations of most trace elements decrease from py3 to py4 at Fornovolasco and Monte Arsiccio, while in the other deposits the opposite trend is observed. Concentrations of most trace elements drop significantly from py4 to py5, where recrystallization of pyrite liberates a large proportion of As, Sb, Tl, Pb, Hg, Cu, Zn, Ag and Mn, likely during retrograde cooling and annealing. The release of these elements from pyrite during metamorphic recrystallization has directly facilitated the formation of late-stage sulfosalts, especially at Monte Arsiccio, where textural evidence suggests sulfosalt assemblages intimately associated with py5 were mobilised as melts during greenschist facies metamorphism. At Monte Arsiccio, around 75% of Tl hosted in early recrystallized pyrite is released upon complete recrystallization. As such, mass balance calculations show that all Tl contained in Tl-sulfosalts could be supplied from locally recrystallizing pyrite, and that in total, more than 250 tons of Tl could potentially have been liberated from the pyrite orebody at Monte Arsiccio during metamorphism. This study highlights the significant quantities of Tl and other metals that may be hosted in pyrite, and also the potential role that metamorphic recrystallization may play in mobilising and (re)-concentrating (or indeed dispersing) metals in a pyrite-dominant ore system.