Summary:
The Marimaca deposit is located within a belt of Mesozoic age copper deposits, known as the Coastal Copper Belt, which range in (pre-mining) size from Mantos Blancos, (~500 Mt) to Ivan (~50 Mt). These deposits, which are recognized as both “manto-type” and IOCG types, occur in a variety of host rocks and alteration associations and have different morphologies and structure.
The dominant rock types are intrusives from the “Naguayán Plutonic Complex” defined by Cortes et al. (2007). In Marimaca, the country rock is an equigranular to porphyritic monzodiorite intruded by a Dyke Swarm System consisting of various bimodal dyke episodes ranging in composition from gabbro to rhyodacite oriented NS to NE, NW and WNW and dipping 45-50° to 60-75° towards east and south-east.
The main Naguayán’ structural fabric, consists of a NS to NNE oriented sheeted-like fractures zone, including faults and dykes, controls the mineralization at Marimaca and can be observed for kilometers beyond the project area limits.
Deposit Types
Marimaca displays many characteristics of the IOCG mineralized system: primary mineralization consisting of chalcopyrite-magnetite and calco-sodic alteration. Recent perhaps low Au and Ag occurrence in the MAD 22 sulphide rich intercept confirms the deposit affiliation. Marimaca differs from typical coastal IOCG districts by the intense supergene alteration and mineralization.
The formation of the supergene blanket such as that discovered and evaluated at Marimaca has been not described in any other IOCG district. There is strong evidence that the actual oxide body was formed due to the successive oxidation of a previous secondary sulphide blanket. The lower parts of the oxide blanket: Mixed and Secondary Sulphide mineral zones existence, confirms this particular attribute of Marimaca as compared with other deposits from the northern Coastal Copper Belt.
Described remnants of previous enrichment or secondary sulphide were observed as chalcocite and covellite replacement textures of pyrite and chalcopyrite. Evidence of the oxidation process can be encountered in the Mixed zone, where zoned green and black copper oxides partially replace secondary sulphides. Mineralogic zoning and copper grade distribution in the blanket also suggest repeated events of lateral migration and accumulation. This process requires abundant pyrite to produce enough sulphuric acid, but as established the common IOCG system is low in pyrite, nevertheless as observed in the sulphide rich intercept of MAD-22, both Cu and Py rich sulphide phases characterize the primary mineralization at Marimaca, showing another singular variation in the common theme of the IOCG spectrum of mineral deposits.
Mineralization
The Marimaca deposit consists of a copper oxide (secondary or supergene) blanket, exposed at the surface that extends for approximately 1800 m along the NNW direction, 500 to 700 m wide and 200 to 350 m thick. Two thirds of the middle-upper part of the oxidized column correspond to copper oxides whereas the lower one-third corresponds to mixed and lesser secondary sulphide mineralization. Although general geometry is a blanket, the mineral zone interpretation was guided by the structural control, emerging the main structural orientations, especially the NS dipping east and the late NW to EW structural system.
The secondary sulfide zone consists of chalcocite and lesser covellite that occurs as fracture staining, sulfide coating, or massive replacement in breccias or veins (bands). Occurrence of massive chunks of chalcocite has been observed. Most of chalcocite occurs as pyrite replacement or coating, perhaps covellite is always replacing chalcopyrite. CuCN characteristic values are in the 45-75% range. A zonation of remnants zones of enriched sulfides is encountered beneath the central part of the blanket, and within mixed zones.
The chalcopyrite zones are not well defined due to a lack of enough drilling information. In some deep drill holes, massive occurrences of chalcopyrite are the most frequent. Some pyrite, these can be found in this sulfide zones. Most of the time, actinolite and magnetite are related to chalcopyrite occurrences.
Alteration
The most relevant alteration related to oxide mineralization is supergene, consisting of limonites, and minor clays mixed with copper oxides. Goethite and minor hematite are common limonites staining fractures or fill open fractures. Fault gouge is commonly composed by limonites mixed with, gypsum and rock flour. Jarosite is less common except as halos of some NW faults zones such as Manolo in the southern part of the area and some veins located towards the east.
The Marimaca hypogene background alteration consists of calco-sodic metasomatism. This is characterized by the replacement of mafics by actinolite and magnetite and the plagioclase and orthoclase by albite. The DIO and DDI units display biotite-magnetite replacement related to fine copper sulfide dissemination.
Mineralization related alteration consist of earlier actinolite-magnetite, which is characteristic of veins, feeders and rock banding, such type of alteration is common at district scale, and it is related to white albite-chlorite replacement and vein halo development. Strong albite is also related to the brecciation textures observed as related to veins and feeders, in some cases is associated with sericite and chlorite and in others, with hematite. Tourmaline has been observed related to the main feeder veins at the Atahualpa and La Atómica zones.
Because no direct relationship of wall rock alteration with copper mineralization or another metallurgical parameter at the oxide zones has been encountered, no detailed descriptions or studies concerning alteration have been completed to date. Nevertheless, the presence, abundance and mode of occurrence of certain alteration minerals such as albite, K Feldespar, actinolite, biotite, etc. has been detailed recorded in drill sample logging.