Geological framework of the mineral deposits of the Collahuasi district, Regi‚àö‚â•n de Tarapac‚àö¬8, Chile

The Collahuasi district, Chile (21ºS, 68º45'W) is the northernmost of four major Eocene-Oligocene porphyry-epithermal mineral camps distributed along a 650 km-long segment of the central Andean margin. The district hosts the Rosario, Ujina, Quebrada Blanca and Copaquire porphyry copper-molybdenum deposits, which together account for a global resource of ~77 Mt of Cu and 6,500 Mlb of Mo. The copper-rich La Grande high sulfidation epithermal vein system, adjacent to the Rosario porphyry, contains a further 8 Mt of Cu. The district also contains small Au- and Ag-bearing epithermal deposits and exotic copper deposits. Collahuasi is unique among the northern Chilean porphyry copper districts in that the deposits are primarily hosted within the Carboniferous to Triassic volcano-sedimentary units (The Peine Group) that formed the basement to the Paleogene arc. New mapping of 120 km2 (60%) of the district, petrology, whole rock chemistry, 26 new laser ablation zircon U-Pb ages and six new 40Ar/39Ar step heating ages constrain a detailed new Permian to Eocene stratigraphy and intrusive history that is coherent across the district. The Peine Group stratigraphy at Collahuasi is here divided into 43 lithostratigraphic members, which comprise four broad facies associations. The stratigraphy and related intrusive rocks reflect distinct periods of accommodation of deposited materials and petrogenesis during Peine Group time. The lowermost Peine Group (~310-290 Ma) records explosive, mostly subaerial eruptions that resulted in the formation of voluminous dacitic to rhyolitic ignimbrites. A significant period of creation of sedimentary accommodation space commenced at ~290 Ma, and was accompanied by effusive subaqueous eruption of basaltic andesites in the Rosario-La Grande area. Polylithic epiclastic conglomerates and granular sandstones interbedded with the basaltic andesites were deposited as local mass flows entered the adjacent subbasin. By about 285 Ma, the early basin had filled, and creation of accommodation space shifted westward across the Monctezuma Fault. Dacitic, and lesser andesitic lavas were erupted, followed by voluminous dacitic and rhyodacitic ignimbrites. During periods of volcanic quiescence, laminated microbial limestones and minor evaporites were deposited in a lacustrine environment. The same time coincides with a change in the bulk magma chemistry after which younger magmas are commonly reddish rather than grey and contain little magmatic ilmenite. A volcanic hiatus between 270 and 260 Ma coincided with intrusion of voluminous equigranular medium grained intermediate to felsic plutons. Disharmonic open, upright 'crumple' folding of the Peine group was associated with the emplacement of these intrusions. The uppermost Peine Group was erupted in the earliest Triassic (~248 Ma) and includes various felsic domes, flows and proximal explosive volcaniclastic facies. These formed at two centres, interpreted to be located on the Domeyko Fault and the northern Monctezuma Fault, respectively. At around the same time, rare, thick dykes of porphyritic monzogranite, and a swarm of relatively primitive basaltic andesite dykes were emplaced in the eastern half of the district. These record the onset of slab rollback and the waning of magmatism in the Choyoi Arc. Thickness variations and movement of deposcentres relative to major faults imply that all of the faults that define the modern fracture architecture of the Collahuasi district originated as growth faults during deposition of the Peine Group. Faults and vein arrays were mapped at 1:300-500 in selected domains across and/or around the perimeter of each of the six principal mineral deposits, and in trenches and excavations adjacent to the deposits. The relative timing of successive events is constrained by the hydrothermal vein, fault and breccia paragenesis in each system. There is a broad distinction between the structural architecture of two pairs of deposits formed approximately two million years apart. The 37-36 Ma Copaquire and Quebrada Blanca porphyries are located on NNE trending second-order faults that splay from the 1000 km-long Domeyko Fault. Accommodation of intrusions, breccias, and early stage alteration and mineralisation occurred along this generation of faults, as well as on 3rd order ENE-trending splays at Quebrada Blanca. Early stage veining in both deposits includes important sheeted or subhorizontal sets. These mineralised complexes are therefore interpreted to have intruded during dextral transpression along the Domeyko Fault. In contrast, the ~35 Ma Ujina, and ~34 Ma Rosario deposits are located near the intersection of major NNW-trending Triassic dykes with multiple second order Peine-age faults. At Ujina, vein arrays are strongly influenced by older fracture sets, but define grossly concentric arrays indicative of formation during conditions of very low differential horizontal stress.