Iron-oxide mineralisation magmatic processes and lineament development associated with deformation along the Atacama Fault System, Huasco Province, northern Chile

Leech, Desmond P. (2003) Iron-oxide mineralisation magmatic processes and lineament development associated with deformation along the Atacama Fault System, Huasco Province, northern Chile. (PhD thesis), Kingston University, .

Abstract

In this thesis spatial and temporal relationships are established between faulting and both pluton emplacement and iron-oxide mineralisation in the Huasco Province (28° to 29°S) of northern Chile. Landsat TM data are used to show a bimodal distribution of lineament orientations within the Chilean continental margin. NW-trending lineaments are most common in terms of both frequency and length. A subordinate set of elongate, interconnected NNE-trending lineaments belong to the NNE-trending Atacama Fault System (AFS) and the Chanarcillo fold¬and-thrust belt. Zones of brittle and ductile deformation are present along strands of the AFS. Early Cretaceous (ca. 120Ma) pluton emplacement occurred during transtensional deformation, and was facilitated by a combination of roof-uplift and floor-depression mechanisms along the AFS. REE concentrations in the plutonic complexes are moderately enriched, show negative Eu anomalies and moderate LREE: HREE ratios. These data imply fractionation from a plagioclase-rich, garnet-poor source. Iron-oxide mineral deposits are located at intersections between NW and NNE-trending structures. Mineralising fluids were trapped in and adjacent to NW and NNE-trending faults in the Los Colorados and Algarrobo iron-oxide deposits. In these deposits, magnetite [delta][sup]18(SMOW) values range from between +0.04 and +1.83 per mil. Coeval apatite has [delta][sup]18O(SMOW) values ranging from +5.28 to +8.32 per mil. [delta][sup]34(CDT) values from pyrite intergrown with magnetite range from -2.91 to +1.94 per mil. These data suggest that the fluids and metals responsible for mineralisation have close magmatic-hydrothermal affinities. Considerable volume increases <37%) are associated with mineralisation. Alteration that post¬dates mineralisation is associated with changing oxidation-state during cooling of the hydrothermal system. Fluid inclusion studies show that the late fluids belonged to a mixed H[sub]20-NaCI-CaCI[sub]2 system. The data presented here can be specifically applied to future exploration in the Chilean Iron Belt, and have a general relevance to the understanding' of the evolution of iron-oxide-(copper-gold) deposits.

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