A CHARACTERIZATION OF THE CERRO NEGRO NORTE FE-OXIDE (CU-AU) DISTRICT, COASTAL CORDILLERA, NORTHERN CHILE

The intrusion-related Cerro Negro Norte (CNN) Fe-oxide (± Cu-Au) deposit is hosted in andesites and diorites of the early to middle Cretaceous Coastal Cordilleran arc of northern Chile. Tabular and irregularly shaped magnetite orebodies are localized on splays and fractures of the regional NNE striking Atacama Fault Zone (AFZ). Production from this district was ~100 MT @ ~ 65 wt. % Fe.

Early Na-Ca alteration assemblages associated with Mag ± apatite ± Py ± Cp ore include actinolite, marialitic scapolite, oligoclase, titanite, and epidote. Na-Ca alteration is extensive (> 4 km²), locally pervasive in the district, and is locally associated with granodiorite dike emplacement. The alkali-rich alteration and sulfide poor mineralization at CNN is characterized by metasomatic exchange of major, minor, and trace elements (added Fe, Na, Ca, Cl, P, REE) between andesitic and diorite host rocks and saline hydrothermal fluids. Intrusion-heated fluids converge along the AFZ, and dikes, and may have been derived either from sea water or evaporitic water trapped in sedimentary rocks of the protoarc. Later carbonate (dolomite) alteration is also localized along NE faults.

Overprinting hydrothermal assemblages such as Tm-Qtz-sericite (± breccias), associated with granodiorite dikes, and chlorite-calcite-Tm-Qtz assemblages are related to Py ± Cp,± hematite and Cu-Au mineralization. Supergene minerals include goethite + Cu-carb and Cu-oxide. Inferred Cu-Au estimates are ~1 MT @ ~ 1g/T Au and 0.25 wt. % Cu. Late alteration assemblages may contain a component of magmatic saline fluids generated by observed monzodiorite-granodiorite dikes and pluton emplacement.

Massive Mag ore and associated Na-Ca alteration assemblages were deposited at high temperatures (~ 500 to 600^oC), with igneous intrusions providing heat but not necessarily fluids and metals. Later moderate to low temperature Cu-Au mineralization (sulfide + oxide) replaces Mag, and records the transition to more brittle faulting, with NW ± re-activated NNE structural control, and a greater proportion of magmatic fluids, sulfur (³⁴dS_py=-1 ‰), and metals.

Abbreviations: Carbonate (Carb); Chalcopyrite (Cp); Magnetite (Mag); Tourmaline (Tm); Pyrite (Py); Quartz (Qtz).