Super Porphyry Cu and Au|
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The Cerro Quema high-sulphidation epithermal gold-copper deposit is located on the Azuero Peninsula, southwestern Panama, 9 km east of the settlement of Güerita, and ~200 km SW of Panama City (#Location: 7° 33' 10"N, 80° 32' 42"W).
Panama is located within a tectonic block that lies at the junction of the Caribbean, South American, Cocos, and Nazca plates (e.g., Duque-Caro, 1990; Kellogg et al., 1995; Harmon, 2005). A volcanic arc developed as a result of the eastward subduction of the ancient Farallon plate beneath the Caribbean plate, during the Late Cretaceous, with volcanic arc magmatism continuing until the Miocene (~23 Ma; Barckhausen et al., 2001; Werner et al., 2003; Lonsdale, 2005; Buchs et al., 2009, 2010; Pindell and Kennan, 2009; Wörner et al., 2009).
The accretion and obduction of seamounts and oceanic plateaux during the middle Eocene (Buchs et al., 2010) and collision of the Panama volcanic arc with the South American Plate during the middle to late Miocene (e.g., Keigwin, 1978; Trenkamp et al., 2002; Coates et al., 2004; Barat et al., 2012; 2014) produced a change in the subduction direction and the migration of the volcanic arc toward the north (Lissinna et al., 2002; Lissinna, 2005), with the present day Cordillera Central in north Panama being the expression of the active Panama volcanic arc. These changes also corresponded to the Farallon Plate splitting into the Cocos (west of North America) and Nazca (west of South America) plates, each with a different sense of movement, at ~23 Ma.
The Azuero Peninsula comprises an igneous basement overlain by fore-arc sediments (Buchs et al., 2011), and contains ~98 to ~40 Ma volcanic, plutonic, sedimentary and volcaniclastic rocks ranging (Del Giudice and Recchi, 1969; Bourgois et al., 1982; Kolarsky et al., 1995; Lissinna, 2005; Wörner et al., 2009; Buchs et al., 2010; Wegner et al., 2011; Corral et al., 2013).
Five distinct rock associations have been recognized in the Azuero Peninsula (Corral et al., 2016):
• Arc basement, composed of Aptian to Santonian (Late Cretaceous) basalts and pillow basalts with similar geochemical affinities to the Caribbean large igneous province, which is interpreted to be the basement to the arc sequence (Del Giudice and Recchi, 1969; Kolarsky et al., 1995; Hauff et al., 2000; Hoernle et al., 2002, 2004; Lissinna, 2005; Buchs et al., 2009; Corral et al., 2011).
• Azuero primitive volcanic arc, a regional scale heterogeneous suite, that comprises tholeiitic basalts and volcaniclastic rocks, locally interbedded with late Campanian-Maastrichtian hemipelagic limestones, which are equivalent to the proto-arc defined by Buchs et al. (2010), corresponding to an initial stages of arc volcanism.
• Azuero Arc Group, comprising calc-alkaline volcano-sedimentary, volcanic and arc-related intrusive rocks, representing Cretaceous and Paleogene volcanic arc rocks (Lissinna, 2005; Wörner et al., 2009; Buchs et al., 2010, 2011a; Wegner et al., 2011; Corral et al., 2011, 2013).
• Tonosí Formation, a middle Eocene to early Miocene, unconformably overlying sedimentary sequence (Recchi and Miranda, 1977; Kolarsky et al., 1995; Krawinkel and Seyfried, 1994; Krawinkel et al., 1999).
• Azuero accretionary complex, consisting of Paleocene to middle Eocene seamounts, oceanic plateaus, and mélanges accreted along the ancient subduction trench (Hoernle et al., 2002; Lissinna, 2005; Hoernle and Hauff, 2007; Buchs et al., 2011).
The Azuero Peninsula is cut by several regional-scale subvertical faults, including the NW-trending Soná-Azuero fault zone, the east-trending Ocú-Parita fault and the Río Joaquín fault zone (Kolarsky et al., 1995; Buchs, 2008; Corral et al., 2011, 2013). The latter is 30 km long, and exhibits evidence of reverse dip-slip motion, and juxtaposes the Azuero igneous basement against the Azuero Arc Group (e.g., the Río Quema Formation). Secondary NW-trending regional structures, e.g., the Pedasí and the Punta Mala faults, both of which are sinistral strike-slip strructures, have disrupted the eastern Azuero Peninsula. In the central Azuero Peninsula mesoscale open folds with SW-plunging fold axes and moderate limb dips indicate dextral transpression with dominant reverse dip-slip motion (Corral et al., 2011, 2013).
The local stratigraphy is dominated by the Río Quema Formation, which hosts the Cerro Quema deposit (Corral et al., 2011; 2013). It comprises a late Campanian to Maastrichtian volcano-sedimentary sequence enclosed within the Azuero Arc Group, and is interpreted as the volcaniclastic apron to the Panama Cretaceous volcanic arc (Corral et al., 2013). The volcanic succession is exposed from the central to southeastern Azuero Peninsula, representing the fore-arc basin, between the subduction trench and the magmatic arc (e.g., Stern et al., 2012). The Río Quema Formation has been divided into three units, from the base (Corral et al., 2016).
• A lower unit containing andesitic lava flows and well-bedded crystal-rich sandstone to siltstone turbidites, interbedded with hemipelagic thin limestone beds.
• A limestone unit, a thick, light grey biomicritic hemipelagic limestone, intercalated with well-bedded cherts, thin-bedded turbidites and fine ash layers.
• An upper unit of volcaniclastic sedimentary rocks, interbedded with massive to laminar andesitic lava flows, dacite domes, dacite hyaloclastites and polymictic conglomerates. The dacites are characterised by quartz and up to 5 cm hornblende phenocrysts, as well as smaller plagioclase crystals, all set in a microcrystalline quartz-feldspar groundmass.
The total thickness of the Río Quema Formation is ~1700 m. It overlies both the Azuero igneous basement and the Azuero primitive volcanic arc, and is discordantly succeeded by the Tonosí Formation.
The Cerro Quema deposit occurs with in the centre of the Azuero Peninsula, and covers an area of ~20 km2. It is associated with an east-trending regional fault system, parallel to the Río Joaquín fault zone (Corral et al., 2011), and is hosted by the dacite dome complex of the Río Quema Formation. It comprises several separate orebodies, the principal of which are, from east to west, the Cerro Quema, Cerro Quemita and La Pava. Although mineralisation and hydrothermal alteration persist to the east (e.g., Cerro Idaida, Pelona and Peloncita), the economic potential of this zone is not well established (Corral et al., 2016).
40Ar/39Ar data of igneous rocks combined with biostratigraphic ages of the volcanic sequence indicate a maximum age of ~55 to 49 Ma (lower Eocene) for the Cerro Quema deposit. It was most likely caused by the emplacement of an underlying porphyry-like intrusion associated with the Valle Rico batholith in the Cretaceous-Paleogene fore arc (Corral et al., 2016).
Alteration and Mineralisation
Hydrothermal alteration at Cerro Quema appears to be predominantly restricted to dacite domes of the Río Quema Formation, influenced by the difference in permeability and porosity compared to other lithologies of the volcanosedimentary sequence (Corral, 2013). This alteration has an easterly trend, parallel to secondary structures of the Río Joaquín fault zone. Volcaniclastic sedimentary rocks and andesite lava flows fractured by these east-trending structures to both the east and west of Cerro Quema have also been weakly altered. The characteristic porphyritic texture of the dacites renders them easily distinguishable, even when hydrothermally altered. Despite the strong structural control, hydrothermal alteration also had a lithological influence, which is evident in the mushroom-shaped alteration domains at shallow levels, as at La Pava (Corral et al., 2016).
The alteration pattern at Cerro Quema comprises an inner 30 to 230 m wide core of vuggy quartz, sandwiched within a zone of local advanced argillic quartz-alunite and pyrophyllite alteration over a width of 30 to 200 m, enclosed within a widespread 100 to 400 m wide kaolinite, illite and illite/smectite-bearing argillic alteration zone. An outer propylitic alteration halo has only been observed in rare drill core samples, surrounding the argillic alteration zone. These alteration types may be summarised as follows, after Corral et al. (2016);
• Vuggy quartz, the innermost alteration zone which forms irregular, generally vertical, funnel- or tabular-shaped bodies, commonly found on top of mineralised zones. Intervals of massive quartz and silicified breccias are also present in this zone. The vuggy quartz is composed of a groundmass of microcrystalline anhedral quartz grains, enclosing disseminated pyrite, barite and minor rutile, with traces of sphalerite. At depth, it also contains disseminated pyrite, chalcopyrite, enargite and tennantite. Its texture is characterised by voids which preserve the crystal morphology of hornblende and plagioclase, some of which are partially filled by drusy quartz, pyrite, and rutile.
• Advanced argillic alteration zone occurs as an irregular halo surrounding the vuggy quartz alteration, and has different mineralogical expressions depending on its occurrence (i.e., surface/subsurface). Exposed quartz-alunite alteration at La Pava is associated with a massive quartz-cemented breccia zone, with only minor alunite which is very fine grained within the breccia cement. At other locations at the La Pava, Chontal Edge and Cerro Quema occurrences, a more representative advanced argillic alteration assemblage at surface is characterised by quartz, dickite, pyrophyllite, barite, illite and minor diaspore altering massive and brecciated dacite. Clay minerals, dickite, pyrophyllite and illite, replaced hornblende and plagioclase, and are also found in the breccia as cement. Barite occurs along fractures and within the breccia cement. Disseminated pyrite is characteristic of the advanced argillic alteration zone. At depth, the assemblage comprises quartz, alunite-natroalunite, aluminum phosphate-sulphate minerals, dickite, pyrophyllite, barite and rutile, associated with hydraulic breccias.
• Argillic alteration is defines a halo surrounding both the vuggy quartz and advanced argillic alteration domains. This envelope generally encloses the vuggy quartz zone with a sharp contact, whilst the contact with the advanced argillic zone is gradational. It is recognised as a whitish-grey, hydrothermally altered rock which typically preserves the original volcanic textures and comprises an assemblage of quartz, kaolinite, illite and illite-smectite with minor chlorite, which replaced hornblende and plagioclase crystals. Locally, disseminated pyrite is found in this zone.
The assemblage within the argillic alteration domain are zoned outward from the mineralised centres, with kaolinite is dominating proximal to ore, grading outward to kaolinite±illite, then to ±illite-smectite, whilst kaolinite±smectite±chlorite-smectite and chlorite have been recognised in distal locations. At La Pava, there are subvertical pipe-like structures where dacites with hornblende and plagioclase phenocrysts have been replaced by advanced argillic quartz, dickite, barite and pyrite alteration and crosscut the argillic altered rocks.
• Propylitic assemblages constitute the most distal alteration halo, affecting dacites, andesites and volcaniclastic sedimentary rocks. These assemblages are characterised by chlorite, epidote, carbonate, rutile, pyrite and chalcopyrite, with minor hematite and magnetite. Hornblende is partially to completely altered to chlorite and epidote, and plagioclase to carbonate. The latter also occurs as patches and veinlets. Minor pyrite, chalcopyrite, rutile, magnetite and hematite have replaced hornblende, as well as occurring as disseminations. This zone has a transitional contact with the argillic alteration zone, where clay minerals have partially overprinted propylitic alteration assemblages.
Gold is found as submicroscopic disseminated grains and as invisible gold within pyrite (Corral et al., 2011), whilst copper occurs in association with hypogene chalcopyrite, enargite, bornite and tennantite, and supergene covellite and chalcocite. Gold and copper mineralisation is predominantly associated with the vuggy quartz and advanced argillic alteration zones. However, minor gold and copper occurrences have also been found in the argillic and propylitic alteration zones.
• Hypogene mineralisation is generally developed below an oxidised zone, although metre scale outcrops are exposed at surface. The most abundant sulphide at Cerro Quema is pyrite, although there is a group of other accompanying sulphides also associated with the Au-Cu mineralisation. Hypogene mineralisation has been split into five stages, with stages 3 and 4 being the main ore-forming events.
- Stage 1 comprises disseminated, fine-grained, idiomorphic and subidiomorphic pyrite, accompanied by rutile and barite in voids and groundmass, with minor enargite, tennantite and chalcopyrite at depth. Trace sphalerite occurs disseminated in the groundmass.
- Stage 2 is represented by disseminated pyrite in the cement of a hydraulic breccia, in association with alunite-natroalunite, dickite and traces of chalcopyrite.
- Stage 3 comprises pyrite, chalcopyrite, enargite and tennantite veinlets which crosscut stages 1 and 2. Textures indicating replacement of pyrite by enargite, enargite by tennantite, and tennantite by chalcopyrite are seen in the veinlets. Bornite is found as a trace mineral.
- Stage 4 occurs as ~5 cm thick breccia bands which are composed of pyrite, chalcopyrite and minor enargite. These breccia bands crosscut all the previous stages.
- Stage 5 is largely an intermediate-sulphidation phase, occurring as 5 to 10 cm thick base metal sulphide-rich veins, composed of pyrite, quartz and barite, together with minor chalcopyrite, sphalerite and galena.
• Supergene mineralisation and alteration - tropical weathering has affected fresh and hydrothermally altered rocks to depths of ~150 m, largely controlled by rock permeability, which at Cerro Quema is afforded by the vuggy quartz, hydrothermal breccias, fracture zones and hyaloclastites.
Weathering of the high-sulphidation ores has produced a thick quartz- and iron oxide-rich zone overprinting the primary sulphide zone in the upper part of mineral bodies. This oxidation zone is characterised by vuggy quartz containing abundant hematite and goethite within the groundmass, replacing the cement of hydrothermal breccias, and filling voids in the vuggy quartz zone. Supergene jarosite, kaolinite, halloysite and gypsum also occur in fractures, vugs and breccia matrix, with remnant hypogene pyrite, barite and rutile.
Supergene enrichment has deposited secondary Cu-bearing minerals such as chalcocite and minor covellite below the oxidation zone, replacing chalcopyrite, tennantite and enargite as well as filling small fractures. The enrichment factor of the oxide zone compared to the sulphide zone is 2.41 and 0.61 respectively, for gold and copper (Corral, 2013). At Cerro Quema, the oxidation zone has higher gold grades (up to 2400 ppb Au), and the enrichment zone has higher copper grades (up to 1% Cu).
Indicated + inferred mineral resource are estimated to be 30.86 Mt @ 0.73 g/t Au, containing 22.64 tonnes of Au, including 2.392 t Au equiv. of Cu ore (Corral et al., 2016).
This record is paraphrased from Corral et al. (2016).
The most recent source geological information used to prepare this summary was dated: 2016.
This description is a summary from published sources, the chief of which are listed below.
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Corral, I., Cardellach, E., Corbella, M., Canals, A., Gomez-Gras, D., Griera, A. and Cosca, M.A., 2016 - Cerro Quema (Azuero Peninsula, Panama): Geology, Alteration, Mineralization, and Geochronology of a Volcanic Dome-Hosted High-Sulfidation Au-Cu Deposit : in Econ. Geol. v.111 pp. 287-310|
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