Graduate Thesis Or Dissertation

Plate 1RaabAlexander2002.pdf

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  • The intrusion-related Cerro Negro Norte 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 NINE striking Atacama Fault Zone. Production from this district was [approximately]100 MT @ [approximately] 65 wt. % Fe. Early Na-Ca alteration assemblages associated with magnetite ± apatite ± pyrite ± chalcopyrite ore include actinolite, marialitic scapolite, oligoclase, titanite, and epidote. Na-Ca alteration is extensive (>4 km² in area), 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, Rare Earth Elements) between andesitic and diorite host rocks and halite-saturated saline hydrothermal fluids preserved as inclusions. Intrusion-heated fluids converge along the Atacama Fault Zone, and dikes, and may have been derived either from seawater or evaporitic water trapped in sedimentary rocks of the protoarc. Younger, cross-cutting hydrothermal assemblages such as tourmaline-quartzsericite (± breccias), associated with granodiorite dikes, and chiorite-calcite-tourmalinequartz assemblages are related to pyrite ± chalcopyrite ± hematite and Cu-Au mineralization. Supergene minerals include goethite, Cu-carbonates and Cu-oxide. Later carbonate (dolomite) alteration is also localized along northeast-striking faults. Inferred Cu-Au estimates are [approximately] 1 MT @ [approximately] 1 g/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 magnetite ore and associated Na-Ca alteration assemblages were deposited at high temperatures ( 500 to 6000 C), with igneous intrusions providing heat but not necessarily fluids and metals. Later moderate to low temperature Cu-Au mineralization (sulfide + oxide) replaces magnetite, and records the transition to more brittle faulting, with NW ± re-activated NNE structural control, and a greater proportion of magmatic fluids, sulfur ([delta]³⁴S[subscriptpy] = -1 0/00), and metals.
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