Evolution of a volcanogenic hydrothermal system recorded by the behavior of LREE and Eu: Case study of the Key Tuffite at Bracemac–McLeod deposits, Matagami, Canada

Unraveling rare earth element signatures in hydrothermal carbonate minerals using a source–sink system

“…The alteration zones around this VMS deposit is characterized by REEY remobilization, with varying LREE enrichments and depletions on scales of tens to hundreds of meters (Genna et al, 2014). The mineralizing fluids were likely modified Archean seawater (Genna et al, 2014). The main solid source area during water-rock interaction for this subseafloor replacement mineralization is most likely the Watson Lake Rhyolite, in combination with gabbroitic dykes, the Bracemac Rhyolite and the unaltered tuffite.…”

“…A large sericitization zone (~250°C) surrounds the mineralization and is overprinted by a chloritization (~300°C) associated with a Zn-Cu-Ag mineralization. The alteration zones around this VMS deposit is characterized by REEY remobilization, with varying LREE enrichments and depletions on scales of tens to hundreds of meters (Genna et al, 2014). The mineralizing fluids were likely modified Archean seawater (Genna et al, 2014).…”

“…In this case, close association with secondary precipitates such as fluorite, thorianite and a REE-phosphate indicates limited mobilization (μm-scale), owing to the low solubility of the alteration products. Nonetheless, there are indications that REEY contained in allanite-Ce can be remobilized over much larger distances (tens of meters) by hydrothermal fluids around volcanogenic massive sulfide deposits (Genna et al, 2014). Because both monazite-Ce and allaniteCe contain REEY as major elements and are strongly LREE selective, coprecipitation with these minerals strongly influences the REEY availability for hydrothermal carbonate minerals.…”

“…The example below is based on an exemplary REEY study from Genna et al (2014), translated to a process-centered approach, while highlighting a few additional aspects. The 2.7 Ga old Bracemac and McLeod deposits in the Matagami mining camp (Canada) are hosted by an andesitic tuff.…”

“…While the glassy matrix of the source area is currently completely replaced by secondary minerals, it was possibly still present during mineralization, and all REEY contained in glass phases are expected to be released congruently (Möller and Giese, 1997). Genna et al (2014) combined whole rock REEY analyses of altered and unaltered andesitic host rock with LA-ICP-MS analyses of hydrothermal disseminated and vein calcite, rare ankerite associated with sulfide ores, finely disseminated fluor -(hydroxy-) apatite in the chloritized zone and primary or hydrothermal allanite. The unaltered host tuff outside the mineralized zone is used as a reference composition for REEY depletion and alteration around the deposits.…”