Synmetamorphic lode-gold deposits in high-grade Archean settings

Lode-gold deposits that formed under conditions of the middle to upper amphibolite and granulite facies have been identified in the Yilgarn block of Western Australia. These deposits, which form a continuum with lower metamorphic grade (mesothermal) gold depos-its, show that at least some of the fluids responsible for mesothermal gold deposits originated in or below the middle crust. Studies of ¹⁸O suggest that fluid compositions were controlled by the wall rocks (at least for some elements). These more proximal gold deposits provide more information about deep fluid sources than do investigations of higher-level mineralization deposited from fluids strongly modified by wall-rock interaction during the early part of their ascent.