INSIGHTS ON IRON OXIDE-APATITE AND IRON OXIDE-COPPER-GOLD DEPOSITS FROM O AND H ISOTOPES IN ACTINOLITE

Disagreement persists over genetic models for iron oxide-apatite (IOA) and iron oxide-copper-gold (IOCG) deposits. To shed new light, we analyzed O and H isotopes in gangue actinolite from five IOA deposits and one IOCG deposit in southeast Missouri, USA, and compiled these with all other published data for the two deposit types. The global compilation represents 22 deposits in seven districts spanning Archean to Cretaceous time.

Actinolite compositions for IOA deposits all overlap on a δ18O vs δD plot. Several deposits show concave upward arrays that suggest equilibration with a fluid of fixed isotopic composition at temperatures mostly in the range 350 to 600°C. The implied fluid δD values are consistently near -30‰ which suggests that fluids were exsolved from magmas but remained in contact with the melts rather than escaping continuously (cf, accumulated magmatic water model of B Taylor, 1992). This contrasts with the isotopic records observed in porphyry-, skarn-, and pegmatite forming systems, all of which have been invoked as analogs for IOA systems. The actinolite data may, however, be consistent with models involving magnetite-vapor bubble aggregates (Knipping et al, 2015).

In contrast to actinolites in IOA deposits, actinolites in IOCG deposits fall into distinct regions on the plot. Higher-than-IOA δD values are seen in Peruvian examples and in IOCG prospects in the Olympic Dam district, Australia. This is consistent with fluid derivation partly from evaporated seawater, as suggested by others based on independent evidence. IOA-like compositions at Candelaria, Tjårrojåkka Cu, and Boss could reflect an IOA-like magmatic-hydrothermal formation mechanism but may also reflect fluid mixing or superposed signatures of magmatic and evaporative fluid events. Lower-than-IOA δ18O values in the Sossego and Sequeirinho deposits, Brazil, could reflect meteoric, connate, and/or exchanged waters of other origins. Two other deposits (both in Missouri) have compositions resembling the Brazil compositions: the hematite-replaced IOA at Iron Mountain and the magnetite-dominant IOCG at Boss. Overall, actinolite O and H isotopes support the idea that IOCG deposits, collectively and individually, reflect diverse fluid sources whereas IOA deposits reflect a common fluid source and common genetic mechanism.