Darryl D. Lindsay

Chuquicamata, one of the world's largest porphyry copper type deposits, is located in the Domeyko Cordillera in the Antofagasta region of northern Chile. The giant deposit is the largest of several situated within the strands of the West Fault System, a regional north-south strike-slip shear system paralleling the Late Cretaceous-Oligocene magmatic arc. The strike-slip system developed in response to plate tectonic interaction>between the subducting Nazca and South American plates and reflects variable rates and angles of convergence with the concomitant generation of porphyry magma bodies, some of which, like Chuquicamata, may have been emplaced into an active shear system.

Did the evolving structure play a significant role in ore control, making Chuquicamata a giant? Is there evidence of ductile deformation and of exhumation of the mineralizing system through to a brittle regime? Can the structural evolution of the deposit be linked with regional tectonics? These are some of the questions addressed during this project, taking advantage of the superb exposure afforded by a 4.5 x 3.0 x 0.7 km deep open pit. Detailed structural mapping and measuring of veinlet systems, fault systems and vein arrays within and around the mine during an 18 month period, were supplemented by study of drillcore, polished slabs, oriented polished thin sections, and limited geochronology

The examination and analysis of ductile deformational features, mineralized stockwork systems, and vein arrays indicate the emplacement and mineralization at Chuquicamata developed within an active dextral strike-slip fault zone that was exhumed through time.

1. Ductile deformation fabrics indicate top-to-the-SSW for low-angle reverse faults and dextral strike-slip for high-angle shear zones. Geometrical relationships between these fault sets are consistent with a strike-slip or wrench tectonic deformational system that parallels the NNE-trend of the host porphyry complex. Examination of the porphyry stockwork system at a deposit scale system suggests a randomness to mineralization, however, within sectors of the mine defined at structural domain scale, individual veinlets of the mineralized stockwork system show preferential orientations. The preferred orientations, representative of domains in the deposit, are N-S to NNE-SSW, NE-SW and WNW- ESE.
2. At the deposit scale, vein arrays associated with molybdenite, pyrite and enargite mineralization and syn- and post-mineralization fault system events crosscut the copper-bearing stockwork. These preferred orientations have a marked influence on the anisotropy of metal grade distribution within the deposit and are therefore of strategic importance to the operation. Mineralization of the vein arrays is related to fault slip and renewed magmato-hydrothermal activity. Fault systems and vein arrays define seven structural domains within the deposit that suggest at least two distinct deformational events. Domains of syn-mineralization fault-vein systems indicate continued mineralization within a dextral strike-slip deposit setting
3. Domains of brittle post- mineralization fault systems indicate an inversion of the strike-slip deformation zone from dextral to sinistral and a concentration of deformation and displacement along the regional West Fault.

Keywords:
Pages: 404
Supervisor: Marcos Zentilli

Awards

1998 C.I.M. Graduate Student Essay Competition (Metal Mining Division) - Second Prize