Textural character and chemistry of plagioclase and apatite in the Marathon Cu-PGE deposit, Ontario: Implications for mineralizing processes

Abstract

The Marathon deposit is hosted within the Two Duck Lake gabbro (TDLG) of the Mesoproterozoic Coldwell alkaline complex, and comprises three zones (Footwall Zone, Main Zone, and W Horizon), which have different textural, mineralogical, and geochemical characteristics. Plagioclase and apatite in the host gabbro units have complex textures and chemistry. Magmatic plagioclase is characterized by a strong positive Eu anomaly, and ?REE increases from core to rim. Plagioclase with replacement rims occurs adjacent to granophyric patches in the vicinity of the footwall, suggesting alteration by fluids exsolved from the granophyric melts. Apatite both predated and postdated plagioclase. Apatite crystals from the Main and Footwall zones generally have high Cl contents, whereas apatite from the W Horizon has low Cl content. Rocks with higher grades of mineralization in the Main and Footwall zones generally contain apatite with higher Cl contents, whereas, in the W Horizon, higher grades correlate with lower Cl in apatite. In addition, zoning, recrystallization textures, and the presence of primary fluid inclusions are more common in apatite from the Main and Footwall zones. ?REE in late apatite overlaps that of the core and rim of early apatite, suggesting that late apatite probably crystallized from a new influx of magma rather than from continued crystallization of a resident magma. The spatial variations in apatite Cl/F could be explained if compositionally distinct magma pulses were responsible for each mineralized zone, as most early apatite crystals are euhedral to subhedral, and magmatic. Intracontinental basaltic melts are, however, reported to be poor in Cl, suggesting local addition of Cl. Chalcopyrite in the Main Zone commonly replaces pyrrhotite and is intergrown with hydrous silicate minerals, suggesting Cu (re-)mobilization. Therefore, a zone-refining process in which volatiles, derived from footwall country-rock dehydration, migrated through the crystallizing gabbros and transported Cu to the Main Zone is an attractive model by which Cl could also be added to the system. The low Cl contents of apatite in the W Horizon can be explained if these fluids did not reach the W Horizon, or the W Horizon represents late-stage magma infiltration.