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Cerro de Maimon |
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Dominican Republic |
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Cu Au Ag Zn
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Super Porphyry Cu and Au
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The Cerro de Maimón Cu-Au-Ag-Zn deposit is located 70 km NW of the Santo Domingo and 7 km east of the town of Maimón, in the
Monseñor Nouel Province of the Dominican Republic, on the island of Hispaniola (#Location: 18° 52' 37"N, 70° 14' 29"W).
The islands of Hispaniola (Haiti and Dominican Republic) and Cuba are largely the result of Cretaceous to Tertiary oblique convergence and northward subduction of oceanic crust on the leading edge of the thick oceanic plateau Caribbean Plate beneath these and the other islands of the oceanic Greater Antilles island-arc since ~135 Ma (Rojas-Agramonte et al., 2011; Pindell et al., 2012, and references therein). Subduction and related arc magmatism paused in Hispaniola and Cuba in the latest Cretaceous to earliest Tertiary as a result of volcanic arc-Caribbean Plate collision. Renewed arc magmatism accompanied the commencement of southward dipping subduction of oceanic crust from the north and continued until the Eocene when the final collision of the arc and the Bahamas Platform/Turks and Caicos Banks microcontinent to the north took place (Lewis and Draper, 1990; Mann et al., 1991; Rojas-Agramonte et al., 2006, 2008; Boschman et al., 2014). These two subduction events persisted for ~70 m.y. and included boninitic and island-arc tholeiitic series of dominantly Lower Cretaceous age (Primitive Island Arc) that evolved to calcalkaline and high-K calc-alkaline series during the Upper Cretaceous to Eocene (Donnelly and Rogers, 1980; Kerr et al., 1999; Jolly et al., 2001).
See the Northern Andes and Caribbean record for the regional setting and context.
The pre-Albian Maimón, Los Ranchos and Amina Formations on Hispaniola, are part of the oldest and chemically most primitive island-arc volcanism in the Caribbean region (Lewis and Draper, 1990; Lewis et al., 2002; Escuder-Viruete et al., 2007, 2009). The Maimón Formation occurs as a 9 x 73 km, NW-SE elongated belt that has been divided into two structural provinces aligned parallel to the trend of the belt: i). the Ozama Shear Zone to the SW, whose extreme deformation has obliterated most of the original igneous textures, and ii). the much less deformed Altar Zone to the NE (Draper et al., 1996; Lewis et al., 2000). Both zones have been metamorphosed to greenschist facies.
The Maimón Formation is in fault contact with the Loma Caribe peridotite and the Peralvillo Sur Formation (Lewis et al., 2002; Escuder-Viruete et al., 2007) to the southwest and with the Los Ranchos Formation to the northeast. The Loma Caribe peridotite, is interpreted to represent a serpentinised harzburgite with minor dunites, lherzolites and pyroxenites forming part of a dismembered ophiolite complex (Draper et al., 1996; Lewis et al., 2006; Proenza et al., 2007) that was tectonically emplaced over the Maimón Formation during the late Albian along a northward thrust resulting in obduction, deformation and metamorphism of the Maimón Formation, particularly in the Ozama shear zone. Maimón, Los Ranchos and Amina Formations are overlain by the shallow-water reefal Hatillo limestone composed of a massive micritic sequence (Kesler et al., 2005).
The Maimón Formation comprises a low-grade metamorphosed and variably deformed bimodal volcanic and volcaniclastic rocks containing scarce horizons of breccias and conglomerates. It includes a belt of well laminated sedimentary rocks, mainly fine-grained meta-tuffs, cherts, dark shales and limestones, which crops out in the North Central part of the formation and is conformable with the volcanic sequence (Torró et al., 2016; Kesler et al., 1991; Lewis et al., 2000).
The Cerro de Maimón deposit is located in the Ozama shear zone, in the southern branch of the Maimón Formation, in the immediate vicinity of the thrust contact with the Peralvillo Formation. Intense deformation, metamorphism and pervasive hydrothermal alteration, particularly in the footwall rocks, have largely obliterated primary textures of the igneous rocks. The least altered host protoliths were described as mafic to intermediate submarine volcaniclastic and volcanic rocks (Lewis et al., 2000). Quartz-sericite-pyrite schists are the dominant foot wall rocks, grading to chlorite-quartz-feldspar schists at depth. Thin graphitic and hematitic chert horizons are best developed in the western hanging wall (Watkins, 1990). A high concentration of quartz veins associated with strong hydrothermal alteration in the westernmost area have been interpreted to indicate that the western footwall zone might correspond to a sulphide stockwork, whereas the eastern foot wall rocks would be distal to the feeder zone (Lewis et al., 2000).
The ore-body is 1000 m long, ~300 m wide and averages 15 m in thickness, but locally reaches up to 40 m. The orebody dips 40°SW with a general steepening of the dip to the NW, but flattens to 20° down plunge (Watkins, 1990).
Mineralisation occurs in three types: i). a near-surface gold/silver rich oxide cap, ii). a supergene enriched sulphide zone, where the unaltered massive sulphide has been preferentially enriched in copper, and iii). the unaltered massive sulphide mineralisation deeper in the deposit, below the effect of weathering and where the copper to zinc ratio approaches 1:1. The oxide cap comprises goethite enriched in gold and silver and averages 30 m in thickness (Perilya website, 2020)
Ore textures are predominantly controlled by the degree of metamorphism and deformation, to the point where primary textures are obliterated. They are generally composed of aggregates of pyrite grains in a matrix of variable other sulphide and gangue minerals, with fabrics that range from matrix to grain-supported. A five-stage paragenetic sequence has been deduced as follows: Stages I and II involve hydrothermal alteration and massive sulphide mineralisation; Stage III - Metamorphism; Stage IV - Late Mineralisation; and Stage V - Supergene enrichment.
The massive sulphide lens and gangue are basically composed of quartz and white mica, which includes both sericite and well developed muscovite, and rare chlorite. Quartz that occurs as veins and aggregates of sutured crystals, often with feather shaped pressure shadows adjacent to larger pyrite grain edges are the product of stage III metamorphism. Other quartz veins are deduced to represent a late generation of stages IV and V as they contain angular sulphide fragments, including those of supergene origin. Aggregates of minute sericite needles are concentrated along layers and occur interspersed with quartz concentrations, whilst the schistosity is defined by bundles of tabular crystals of muscovite and minor chlorite of stage III that are locally crenulated. Muscovite bundles often wrap around pyrite grains. Barite accompanies other gangue minerals and sulphides among the pyrite grains or as infilling or inclusions within pyrite of stages II and III. Only local late hydrothermal sparry to blocky calcite is observed forming veins up to 0.5 mm in width cutting the bulk rock or partially flooding the groundmass among pyrite crystals (Torró et al., 2016).
Pyrite is the dominant sulphide, varying from cubic idiomorphic to completely rounded to very irregular. It has a wide variation in size, typically ranging from 250 µm to 1.5 mm, and occasionally up to 4.5 mm. During deformation, stage II pyrite crystals/grains were broken and individual crystals welded, with abundant cataclastic textures, microfracturing and 'blow-apart' structures, particularly where the interstitial material is quartz. Metablastic welded pyrite crystal aggregates frequent anneal fine to medium sized meta-crystal textures with triple junctions at 120°. In the outer sections of the sulphide lens, where the mineralisation is semi-massive, stronger deformation marked by muscovite wrapped around large pyrite crystals with rotation fabrics and the development of durchbewegn textures (Vokes, 1969). These observations are interpreted to imply that pyrite grains behaved as porphyroclasts during syn-metamorphic deformation (Torró et al., 2016).
Variable amounts of chalcopyrite, sphalerite, tennantite and galena are generally present throughout the width of the Cerro de Maimón massive sulphide mineralisation, with chalcopyrite and sphalerite being more abundant than tennantite, while galena is rare. Only local, subtle, chalcopyrite disease occurring as blebs, is seen in the sphalerite. Variable amounts of these sulphides occur along with gangue minerals interstitial to pyrite grains with a variety of textures indicative of plastic deformation and recrystallisation at lower temperature than pyrite (Pesquera and Velasco, 1993; Barrie et al.., 2010). The other four sulphides occur as both mono-minerallic and composite inclusions within pyrite. Annealing textures in complex bornite, chalcopyrite, sphalerite and galena mineralisation implies at least part of the bornite was formed prior to or during metamorphism (Torró et al., 2016 and references cited therein). Trace amounts of fine tellurides that are some tens of microns in size occur within voids in pyrite and are interpreted to have precipitated at late stages. They are predominantly Ag-tellurides (hessite, Ag2Te) and rare Bi- (hedleyite (Bi7Te3)/pilsenite (Bi4Te3)/ tellurobismuthite (Bi2Te23) and Pb-tellurides (altaite, PbTe). Hessite is largely concentrated along tetrahedrite margins, suggesting that they are alteration products of the latter (Torró et al., 2016). Discrete minute (~15 µm) electrum grains are crystallised in voids within pyrite or in the contacts between pyrite and the other sulphides. Electrum grains are systematically distributed in the outer parts of the sulphide lens, where deformation is more obvious. Supergene minerals include bornite, covellite-chalcocite and minor djurleite (Cu31S16), digenite (Cu9S5) and yarrowite (Cu9S8). Assemblages of covellite-chalcocite forming flame-like textures systematically replace bornite where supergene processes are well developed (Torró et al., 2016).
Production between 2008 and 2012 totalled (after Trout and Chavez, 2013):
2.367778 Mt of sulphide ore + 1.355665 Mt of oxide ore, for a total of 48 574 t Cu, 1.926 t of Au and 78.055 t of Ag.
NI 43-101 compliant Measured, indicated and inferred resources as of December 31st 2013 totalled (as quoted by Torró et al., 2016):
Sulphide ore -10.642 Mt @ 1.47% Cu, 0.78 g/t Au, 26.01 g/t Ag and 1.49% Zn, and
Oxide ore - 0.545 Mt @ 1.04 g/t Au and 11.80 g/t Ag.
The most recent source geological information used to prepare this decription was dated: 2016.
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
Cerro de Maimon
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Selected References:
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Torro, L., Proenza, J.A., Melgarejo, J.C., Alfonso, P., Farre de Pablo, J., Colomer, J.M., Garcia-Casco, A., Gubern, A., Gallardo, E., Cazanas, X., Chavez, C., Del Carpio, R., Leon, P., Nelson, C.E. and Lewis, J.F., 2016 - Mineralogy, geochemistry and sulfur isotope characterization of Cerro de Maimon (Dominican Republic), San Fernando and Antonio (Cuba) lower Cretaceous VMS deposits: Formation during subduction initiation of the proto-Caribbean lithosphere in fore-arc: in Ore Geology Reviews v.72, pp. 794-817.
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Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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