Gold and Trace Element Release From Pyrite During Prograde Metamorphism of Carbonaceous Metapelites: Implications for Orogenic Gold Formation

Abstract

The Neoproterozoic carbonaceous metapelites of the Mandhali Formation in the inner Lesser Himalaya contain diagenetic pyrite with above-background levels of Au (>200 ppb) when analyzed by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). These metapelites exhibit progressively metamorphosed zones (from south to north), ranging from the chlorite-muscovite zone (~372°C), to the muscovite ± biotite (~520°C) zone, to the greenschist-amphibolite transition (~570°C, >5 kbar). Within this regional framework, we document a hitherto unknown paragenesis of pyrite growth from diagenesis to peak metamorphism along with a localized phase of contact metamorphism, which predates regional metamorphism. A comparative assessment of pyrite trace element concentrations from the progressive metamorphic zones reveals that >80% of Au and ~65% of As is released from diagenetic pyrite during early chlorite dehydration at ~372°C, that is, in the chlorite-muscovite zone. Further into the muscovite ± biotite zone, at the terminal chlorite breakdown stage (480°–520°C), pyrite still occurs as the major sulfide with subordinate pyrrhotite. However, by this stage, ~93% of Au; ~75% of As; and ≥90% of Mo, Cd, Pb, Sb, W, Cu, Bi, Te, and Tl are released from pyrite. With the transition of pyrite to pyrrhotite (500°–550°C), almost all trace elements are released from pyrite into metamorphic fluids, except Co that is incorporated into the metamorphic pyrrhotite. This documented grain-scale elemental mobility is also reflected in bulk-rock Au assays; that is, mean Au of 9.8 ppb in the chlorite-muscovite zone and 1.1 ppb in the muscovite ± biotite zone. Our results suggest that pyrite recrystallization during chlorite dehydration is relatively more significant than pyrite-pyrrhotite transition in terms of Au and As release, and by extension, orogenic gold formation.