Abstract
Cretaceous porphyry copper deposits of northern Chile (28º-29º30’ S) are genetically related with dacitic to dioritic porphyries and they represent a still poorly-explored target for Cu resources. The porphyries correspond to stocks distributed into two separated discontinuous NS trending belts of different age. The location of these porphyries is generally adjacent to orogen-parallel major fault systems that extend along the studied segment and also have a marked temporal relationship with deformation events registered along these structures. A first episode of Cu-bearing porphyry emplacement took place between 116 and 104 Ma (Mina Unión or Frontera, Cachiyuyo, Punta Colorada, Dos Amigos, Tricolor porphyries). These Early Cretaceous dacite to diorite porphyries are spatially associated with the eastern segments of the Atacama Fault System, which records sinistral transpression that started at 121 Ma producing ground uplift, consequent denudation and exhumation of the Early Cretaceous magmatic arc. This resulted in a change from marine to continental deposition with an angular unconformity in the site of the back-arc basin after of eastward migration of the deformation around 112-110 Ma. At the scale of the continental margin, this deformation is correlated with early stage of the Mochica Orogenic event described in Perú. A second episode of Cu-bearing porphyry emplacement occurred between 92 and 87 Ma (Elisa, Johana, Las Campanas and La Verde deposits), which are spatially and temporally associated with the regional-scale Las Cañas-El Torito reverse fault, active between 89 and 84 Ma, during the Peruvian Orogenic Phase. This fault up thrust to the west part of the Chañarcillo Group rocks (Lower Cretaceous) over the younger upper levels of the Cerrillos Formation (Upper Cretaceous). The integrated geological mapping and geochemical data of the Early to Late Cretaceous volcanic rocks indicates that both Early Cretaceous sinistral transpression and Late Cretaceous east-west compression were not significant in promote changes in magma genesis, except for slight changes in trace element ratios (increase in Th/Ta, Nb/Ta and La/Yb) suggesting that the Late Cretaceous deformation event produced only slightly increase in crustal thickness (>40 km), but far from being comparable to major Cenozoic orogenic phases, at least along the magmatic arc to back-arc domains in the study area. Finally, our study give insights about regional geological parameters that can be used as a first order guide for exploration of Cu resources along Cretaceous magmatic belts of northern Chile, where both Early and Late Cretaceous Cu-bearing porphyry intrusions are restricted to a large structural block bounded to the west and east by Cretaceous fault systems.
Highlights
The distribution of metallic ore deposits along the southern central Andes has been typically characterized as various margin-parallel NS trending metallogenic belts of specific ages and broad geological setting (e.g., Sillitoe, 1981, 1991, 2003; Camus, 2003; Sillitoe and Perelló, 2005)
We present new constrains on the age of some of these Early and Late Cretaceous Cu-Au porphyries located between 28o and 30o S, throughout seven new zircon U-Pb ages, which were obtained in the Geochronology Laboratory of University of Tasmania and the Isotope Geology Laboratory of SERNAGEOMIN, with the aim of refining the crystallization age of these ore-bearing dacite porphyries
After the transpression ceased in the arc at 117 Ma and moved to the back-arc, both areas were ubiquitously invaded by magmas and hydrothermal fluids between 116 and 110 Ma, as is registered by the dacite and diorite intrusives related to porphyry copper-like mineralization, and dike swarms that crosscut older alteration zones related to Iron oxide-copper-gold (IOCG) and iron oxide-apatite (IOA) deposits (Travisany et al, 1995; Creixell et al, 2012)
Summary
The distribution of metallic ore deposits along the southern central Andes has been typically characterized as various margin-parallel NS trending metallogenic belts of specific ages and broad geological setting (e.g., Sillitoe, 1981, 1991, 2003; Camus, 2003; Sillitoe and Perelló, 2005). Along the Domeyko Cordillera, in northern Chile, porphyry copper deposits of EoceneOligocene age are distributed in a longitudinal belt, spatially associated with the Eocene-Oligocene arc and the transpressive/compressive Domeyko Fault System (Boric et al, 1990), which points to a major structural control on the magmatism and mineralization for this time span (e.g., Mpodozis and Cornejo, 2012). A younger belt of world-class porphyry-Cu-Mo deposits of Late Miocene age is located east of the Eocene-Oligocene arc, along the high Andes of central Chile, including the Los Pelambres, Río Blanco-Los Bronces, Los Sulfatos and El Teniente porphyry copper deposits. These ore deposits are spatially associated to the Neogene magmatic arc, developed in a thickened crust. The case is different for a still not well known belt of Cretaceous Cu-porphyries recognized at north Chile (e.g., Reyes, 1991; Maksaev et al, 2007, 2010; Sillitoe and Perelló, 2005; Richards et al, 2017) which are largely smaller and lower in grade relative to Cenozoic deposits, and are spatially and temporally associated to Creixell et al / Andean Geology 47 (3): 469-507, 2020
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