Summary:
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.
Project Geology
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.
Geological Setting and Mineralization
The Marimaca deposit is a structurally controlled copper oxide mantle, exposed at surface, extending ~1800m NNW, 500–700m wide, and 200–350m thick. The upper two-thirds of the oxidized column contain copper oxides; the lower third hosts mixed mineralization with minor secondary sulphides. The host is a coarse-grained monzodiorite to quartz diorite intrusive, dominated by plagioclase. Key structures include an NS foliation dipping east and a late NW–EW fault system.
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.
The formation of the supergene blanket, such as that discovered and evaluated at Marimaca, has not been described in any other IOCG district. There is strong evidence that the actual oxide body was formed due to the oxidation of a previous sulphide blanket. Remnants of this blanket were encountered and consisted of chalcocite and covellite replacement of pyrite and chalcopyrite. The oxidation process can be evidenced in the Mixed zone, where green coppers and black copper oxides partially replace secondary sulphides. The blanket's mineralogic zoning and copper grade distribution suggest repeated lateral migration and accumulation events. This process requires abundant pyrite to produce enough sulphuric acid, but as established, the IOCG system is low in pyrite. It is possible that a very rich and pervasive chalcopyrite, stronger than pyrite primary mineralization and a long-lived process of oxidation, can explain the formation of the Marimaca’s uncommon secondary blanket.
Deposit Types
IOCG veins districts are hosted by Jurassic intrusive. Marimaca, located at the old Naguayán District is an anomaly in the context of this type of IOCG mineralization occurrences. Marimaca is reasonably close enough to be assigned to this type of geology and mineralization, given their special structural and supergene mineralization features. Despite their economic importance, little is known about the IOCG mineralization.
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.
The formation of the supergene blanket, such as that discovered and evaluated at Marimaca, has not been documented in any other IOCG district. There is compelling evidence that the oxide body was created through the successive oxidation of a previous secondary sulphide mineralization. The lower sections of the oxide blanket, comprising Mixed and Secondary Sulphide mineral zones, highlight this characteristic of Marimaca compared to other northern Coastal Copper Belt deposits (Marimaca, 2023).
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.