Two-step gold mobilization in metamorphic terranes: A refined metamorphic model for Orogenic Gold

At the source of an orogenic gold deposit, Au and As are inferred to be mobilized during prograde metamorphic breakdown of diagenetic pyrite into pyrrhotite. However, the exact timing and mechanisms of Au and trace element release from diagenetic pyrite is still elusive. Here we investigate trace element chemistry (LA-ICP-MS) of sulfides (diagenetic pyrite, metamorphic pyrite and pyrrhotite) and bulk Au contents from middle greenschist to lower amphibolite facies metapelites (Neoproterozoic Mandhali Formation from NW Indian Lesser Himalayas). Our results show that more than 80% of Au and 65% of As are released from diagenetic pyrite during early metamorphic recrystallization in the middle greenschist facies. In the lower amphibolite facies, pyrite has been completely metamorphosed and has already lost 93% Au, 75% As and the bulk of Pb, Sb, Cu, Bi and most other trace elements prior to breakdown into pyrrhotite. Despite late and incomplete pyrite to pyrrhotite transition in the amphibolite facies, Au was also mobilized from the bulk rock (from 9.0 ppb to 1.1 ppb mean bulk Au) indicating: (1) even a partial pyrite to pyrrhotite transition can result in regional Au mobilization, and (2) Au mobilization occurs in a two-step process involving release from diagenetic pyrite into the rock matrix during metamorphic recrystallization followed by later mobilization in metamorphic fluids.