OXYGEN AND IRON STABLE ISOTOPE SIGNATURES FROM THE MANTOVERDE IRON OXIDE-COPPER-GOLD DISTRICT, CHILE

Iron oxide-copper-gold (IOCG) deposits are found worldwide and commonly host economic amounts of their namesake metals. Despite more than a century of research, controversy persists regarding the source of their ore forming fluids. Suggested source reservoirs include magmatic-hydrothermal fluids, seawater, and/or meteoric waters that dissolved evaporites. Here, we focus on the Mantoverde IOCG district the Chilean Iron Belt in an effort to directly constrain the potential source reservoirs of the ore forming fluids. We report trace element concentrations and δ¹⁸O and δ⁵⁶Fe (⁵⁶Fe/⁵⁴Fe relative to IRMM-14) stable isotope data for magnetite and hematite from 14 samples along 230 meters of drill core from the northern section of Mantoverde. The average concentrations of [Ti+V] of magnetite average 0.12wt% (range 0.02-1.28wt%; n=276) and are comparable to other IOCG deposits. The [Ti+V] concentrations of hematite average 1.04wt% (range 0.03-3.11wt%; n=68) and are comparable to iron oxides in porphyry deposits and igneous rocks. The concentrations of [Al+Ca+Mn] of magnetite and hematite average 0.17 and 0.13wt%, respectively, and compare well with other IOCGs; however, [Al+Ca+Mn] contents are significantly variable (range 0.01-1.01 and 0.01- 0.5wt%, standard deviation 0.18 and 0.11wt% respectively). The δ¹⁸O values for magnetite range from 0.69 to 4.61‰ (average 2.99‰; n=9), and for hematite from -1.36 to 5.57‰ (average 0.60‰; n=4). The δ⁵⁶Fe values for magnetite range from 0.45 to 0.58‰ (average 0.54‰; n=7), and for hematite from 0.34 to 0.46‰ (average 0.40‰; n=2). These δ¹⁸O and δ⁵⁶Fe data overlap with values for magmatic-hydrothermal and igneous iron oxides, which globally range from δ¹⁸O = 0.09 to 4.0‰ and δ⁵⁶Fe = 0.06 to 0.80‰. The variability of [Ti+V], [Al+Ca+Mn] and values of δ¹⁸O below 0.09‰ likely reflect secondary hydrothermal alteration and/or dissolution and reprecipitation of iron oxides. These new data support a growing body of evidence that the Mantoverde IOCG district formed primarily by magmatic-hydrothermal fluids with little to no influence from meteoric waters.