Summary:
Due to its unique structural and supergene mineralization characteristics, Marimaca, situated in the historic Naguayán District, stands out as an anomaly among occurrences of this type of IOCG mineralization.
The dominant rock type is the “Naguayán Plutonic Complex,” an equigranular to porphyritic monzodiorite intruded by a de swarm system consisting of various bimodal dyke episodes ranging in composition from gabbro to rhyodacite. The system is oriented NS to NE, NW, and WNW and dips 45-50° to 60-75° towards east and south-east.
The main fabric consists of a sheeted-like fracture zone, including faults and dykes, oriented NS to NNE, that controls the mineralization at Marimaca and can be observed for kilometers beyond the project area limits.
Background rock alteration consists of Na-Ca metasomatism. The mineralization is associated with the alteration of albite, actinolite, chlorite, and iron oxides. The NS to NE parallel fracture and related veins control chalcopyritemagnetite-rich primary mineralization.
Intense, extensive and pervasive events of supergene oxidation have produced the actual copper oxide blanket that forms the mineral deposit at Marimaca. The surface data indicate that copper oxides are controlled by a highly fractured host rock, resulting in a high-permeability background generated by the superposition of multiple fracturing events and dyke intrusions. The oxidation resulted from the alteration of a previous secondary sulphide-enriched blanket that produces zonation from Brochantite-Atacamite at cores immediately surrounding the secondary sulphides' remnant patches and successive external halos of predominant chrysocolla and further external wad.
Compared with other belt deposits, Marimaca is fully hosted by intrusive rock units that become extremely permeable thanks to intense fracturing. Thus, the mineralization style differs significantly from the neighbouring typical volcanichosted Manto-type copper deposits. At the same time, although the nature of primary mineralization is IOCG, developing a consistent secondary blanket makes a difference compared to the typical IOCG from the Coastal Cordillera (Espinoza et al., 1996; Kojima et al., 2009).
Deposit Descriptions
The Marimaca deposit is located within the Coastal Copper Belt, a belt of Mesozoic-age copper deposits.
The host rock in Marimaca is the “Naguayán Stock,” an equigranular monzodiorite that grades to diorite in part, cut by monzodiorite porphyries and various systems of dacitic and dioritic dykes (NE, NS, NW, and WNW orientation).
The most important structural feature of Marimaca is a system of sub-parallel, planar, pervasive, and persistent fractures occurring along an NS elongated structural belt. These fractures give the rock an appearance of “pseudostratification” composed of centimetric to decametric sub-parallel “sheeted-like” fractures. A WNW to NW system of late faults is important and creates additional permeability favourable for forming an oxide blanket.
The Marimaca deposits consist of a copper oxide blanket, exposed at the surface extending for approximately 1,600 m along the NNW direction, 400 m to 500 m wide and 200 m to 300 m thick. Two-thirds of the middle-upper part of the oxidized column corresponds to copper oxides, whereas the lower one-third corresponds to mixed and lesser chalcocite mineralization. Although general geometry is a blanket, the interpretation of the mineral zone was guided by structural control, especially the NS dipping east and the late NW to EW structural system.
The mineralogy of the oxide zone consists of Brochantite, Atacamite, Chrysocolla and wad occurring as disseminations and impregnation of fractures in the parallel band system with an NS orientation, but also in diagonal fault systems with NE and NW orientations. The subjacent mixed zone consists of copper oxides and remnants of chalcocite and covellite, minor pyrite and chalcopyrite. The secondary sulphides carry mostly sooty chalcocite, replacing pyrite and covellite after chalcopyrite.
The Marimaca alteration consists of metasomatism with little evidence of destructive hydrothermal alteration. The calc-sodic (Na-Ca) metasomatism is background alteration, while albitization and chlorite are alteration minerals related to mineralization. Some K-spar and biotite are also observed. In the oxide zone, the limonite, mostly goethite, is associated with copper mineralization.
Marimaca displays many characteristics of the IOCG mineralized system: primary mineralization consisting of low pyrite and chalcopyrite-magnetite, calco-sodic alteration; however, no Au occurrences are recorded or observed. Marimaca differs from typical coastal IOCG districts in terms of intense supergene alteration and mineralization.
Deposit Model
The metallogenic setting the area consists of “manto-type” copper deposits hosted by La Negra Formation volcanics, as well as some IOCG-affiliated vein districts, hosted by Jurassic intrusive (Espinoza et al., 1997; Maksaev and Zentilli, 2002, Sillitoe and Perello, 2005). Towards the eastern border there are some porphyry-type copper systems of Late Jurassic to lower Cretaceous age. The recent discovery of the Cachorro copper deposit (Arriagada, 2021), described as like Mantos Blancos style mineralization is re-opening the ground for exploration of larger manto-type copper deposits in the region.
Deposit Type
Marimaca exhibits numerous characteristics of the IOCG mineralized system, including primary mineralization featuring chalcopyrite-magnetite, and calc-sodic alteration. Recent findings of potentially low Au and Ag occurrences in the MAD-22 sulphide-rich intercept confirm the deposit's affiliation. Marimaca is distinguished from typical coastal IOCG districts by its intense supergene alteration and mineralization.
Marimaca may suggest a new type of IOCG deposit, which is still under characterization. This potentially new type of deposit could have significant exploration potential in the future. Additional data and studies are required to develop reliable models and to better understand the system's evolution.