Zaruma - Portovelo
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The Zaruma-Portovelo low to intermediate sulphidation gold-silver vein complex is located ~160 km south of Guayaquil, and 55 km southeast of Machala in El Oro Province, SW Ecuador.
(#Location: 3° 41' 20"S, 79° 36' 31"W).
It is one of a number of Miocene porphyry and epithermal copper-gold mineral systems that occur in two main clusters, which define two mineral districts in western Ecuador, including Chaucha, Gaby-Papa Grande, Quimsacocha and Portovelo-Zaruma (Azuay-El Oro District), and Junín and Alpala (Imbaoeste District). These districts are ~440 km apart and located within 100 km of the Pacific coast, 200 km SSE of Guayaquil and 75 km north of Quito, in southwestern and northwestern Ecuador respectively. Other deposits are distributed along the Miocene porphyry belt between these two districts, and to the north and south into Colombia and Peru.
The Zaruma-Portovelo district has a long history of gold and silver mining. The Incas were already exploiting gold and silver in the area when Mercadillo, one of Pizzarro's force, followed the Rio Amarillo upstream to the Inca mine and founded the town of Zaruma in 1549 (Billingsley, 1926). Exploitation continued throughout the Spanish colonisation until 1870. Mining by a range of interests continued until 1984 when the mine was taken over by a state owned company. In 1984, thousands of poverty-stricken miners invaded the old pits and small-scale and artisan mining has been going on in the area until recently, when a modern mining operation was commenced. During its 450 year history, the district is estimated to have produced >155 tonnes of gold.
Tectonic and Regional Setting
For detail of the regional setting and geology, see the separate records for North Andes copper-gold province in Ecuador and the broader North Andes and Panama copper-gold province.
The 15 km long, 2 to 6 km wide, north-south oriented Zaruma-Portovelo vein system is bracketed to the north and south by the eastern portions of the ESE-trending Cangrejos-Zaruma intrusive belt and the WNW-ESE arc-transverse structural trend, the Piñas-Portovelo fault, respectively. The Piñas Fault is a major regional structure that cuts across the Andean structural grain and separates the Proterozoic-Palaeozoic and Mesozoic metamorphic terrain of the Amotape range to the south, from a Tertiary volcanic sequence to the north. The Tertiary volcanic rocks preserved on the northern side of the fault unconformably overlie Mesozoic metamorphic rocks of continental origin of the adjacent Eastern Cordillera (Cordillera Real). These preserved volcanic units show southward-thickening, reflecting growth sequences formed in a half graben during normal movement on the Piñas Fault (Coward, 2001; Spencer et al., 2002), as a consequence of a decrease in the rate of subduction of the Nasca plate beneath the South American plate at 37 Ma. North-south directed compression increased the rate of the plate convergence at ~20 Ma and caused the hanging wall of the Piñas Fault to bulge into the open, regional-scale WNW-ESE trending Cangrejos antiform, that paralels the Piñas Fault ~10 to 15 km to the north. A string of 26 to 16 Ma porphyry intrusions of dioritic to granodioritic composition, the Cangrejos-Zaruma Intrusive Belt, have intruded the hinge of this antiform.
Zaruma-Portovelo Vein System
Host rocks to the Zaruma-Portovelo vein system are 28.4 to 21.5 Ma, late Oligocene to early Miocene in age (Dunkley and Geybor, 1997; Spencer et al., 2002), and range in composition from calc-alkaline to alkaline, compatible with their emplacement in an extensional cordillera setting.
The Zaruma-Portovelo district is underlain by intrusions and mafic to intermediate volcanic rocks that comprise the Portovelo Unit of the Saraguro Group. The volcanic pile has been subdivided (Billingsley, 1926) into three units:
i). The Muluncay Series - the lowest part of the sequence, dominantly volcanic breccias, tuffs, lava flows and minor ignimbrites. The breccias include autobreccias, flow breccias, pyroclastic breccias and matrix-supported conglomerates which have been interpreted as mud-flow deposits;
ii). The middle part of the local stratigraphy consists of a sill-like medium-grained andesitic of the Portovelo Unit which constitutes the concordant limb of a lopolithic melanodioritic intrusion, similar in composition to the enclosing andesitic volcanic rocks;
iii). The uppermost unit, the Faique Series, consists of volcanic rocks which are similar to the Muluncay Series except that the breccias tend to be coarser grained.
Numerous intrusions are found within the deposit area, ranging from melanocratic diorite to quartz monzonite in composition. Many are fine grained and interpreted as high level, sub-volcanic in origin.
Four principal en echelon, generally north-south oriented vein sets, covering a total area of several tens of km2, are controlled and separated by SE trending, dextral thrust faults that dip 40 to 60°SW (Spencer et al., 2002). These four blocks of north-south oriented veins, each bounded to the NE and SW by NW-SE faults, are from north to south, the i). R. Nivel; ii). Mulancay; iii). Sesmo Colorado and v). Portovelo systems. The veins sets are progressively offset to the SE across the bounding fault on their southern margin. Structural fabrics suggest oblique, ductile, dextral thrust movement on the NW oriented faults. Veins are best developed in the immediate footwall of the northward vergent faults. Individual veins are usually bounded by dextral ductile chloritic shear zones. Most veins throughout the strike length of the Zaruma-Portovelo vein system have an average dip of 60°E. The mineralised portions of the main veins vary in width from 0.3 to 1.5 m (average 0.9 m).
Veins to the north of Zaruma contain dog-tooth quartz, intergrown with bands of dark, Fe-chlorite, epidote, tourmaline and magnetite, precipitated from high-temperature, possibly porphyry-related fluids, at a depth of 900 m, passing upwards through an intermediate argillic zone to an assemblage of pyrophyllite-diaspore-topaz-tourmaline/dumortierite (a high temperature advanced argillic alteration zone or lithocap; Sillitoe, 2000). Veins textures in the northern 10 km of the trend, do not change appreciably along strike, although in the southern part of the trend, compositions change gradually from quartz, to quartz-calcite to calcite dominated over a strike distance of ~2 km. This results in crudely banded gold-bearing quartz-carbonate-base metal veins related to low-temperature fluids dominating the southern part of the vein system between Zaruma and Portovelo. Similalrly, a vertical zonation in composition from quartz dominated deeper portions of veins to calcite dominated assemblages near surface was observed, in particular, within the Portovelo fault block of veins. At the surface, on ridges and hills in the SW of the district, advanced argillic assemblages of vuggy quartz, kaolinite, dickite, pyrophyllite and minor alunite are evident defining a roughly tabular area of lower temperature advanced argillic alteration. Overall, alteration styles associated with the veins show a transition from high temperature assemblages in the NE portion of the area, centred on the El Poglio porphyry stock within the Cangrejos antiform (part of the Cangrejos-Zaruma intrusive belt), to low temperature zones towards the southwest and south, culminating in an illite-smectite assemblage in contact with the veins adjacent to the Piñas-Portovelo fault (Spencer et al., 2002).
The northern veins are dominated by relatively higher temperature assemblages with disseminated chalcopyrite-pyrite-magnetite in K-silicate altered El Poglio porphyry andbanded quartz-chalcopyrite-pyrite-magnetite-dark-green chlorite within veins in the R-Nivel vein block. The next vein block to the south, the Muluncay en echelon set, is characterised by quartz-chalcopyrite-pyrite, progressing to quartz-chalcopyrite-sphalerite-galena in the Sesmo-Colorada vein block, and a sphalerite-dominant, quartz-carbonates-sphalerite-pyrite-galena±chalcopyrite assemblage in the Portovelo en echelon set. A similar zonation is evident in an east-west traverse, with higher temperature assemblages in eastern and central part of the district dominated by chalcopyrite changing into sphalerite-dominated to galena-dominated assemblages in the easternmost part of the district (Kalinaj, 2004). A zonation is also evident within the individual vein blocks, e.g., in the Sesmo Colorado block, barren quartz-pyrite veins are found to the north and east, with chalcopyrite (overprinted by supergene chalcocite and covellite) increasing in the quartz veins to the south and west, with increased galena and sphalerite in the southern- and westernmost veins. At the highest levels in this same vein system, the veins contain sphalerite, galena, pyrite and chalcopyrite. The total sulphide content of the northern quartz-pyrite veins is 5 to 10%, but increases to as much as 30 and 20 to 50% in the southern and western sphalerite dominated veins respectively. However, calcite dominated veins are typically sulphide-poor, containing ~5% sulphide, mainly pyrite-galena-sphalerite (Spencer et al., 2002).
The gold and silver distribution differs from that of the base metals, with the best concentrations closely related to the footwall of southerly dipping, NW-SE trending thrust faults that separate the four main vein blocks detailed above, especially in the northern en echelon sets of R-nivel, Muluncay and Sesmo-Colorada. In these blocks, the Au-Ag mineralisation within the veins is concentrated in NW-SE elongated, 2 to 3 by ~1 km envelopes immediately to the north of, and parallel to, the southern bounding fault of each vein block. In contrast, the gold in the Portovelo mine, which accounted for around 90% of the gold production from the district, came from the central part of the Portovelo en echelon vein set, forming a zone that parallels the veins and extends almost to the both the NE and SW bounding faults. The gold-barren zone adjacent to the south bounding Piñas Fault is possibly due to this area lying within the low-temperature illite-smectite alteration zone found in the SW of the vein system, as described previously (Kalinaj, 2004).
With silver minerals (polybasite and acanthite) native gold occurs as inclusions in chalcopyrite, sphalerite and galena. Gold also shows corrosion textures on sphalerite and galena grains, infilling exfoliation fractures in galena crystals and infilling micro-fractures within pyrite crystals. It is also frequently observed in association with dark-green chlorite in the northern part of the Zaruma District (Kalinaj, 2004).
The volcanic stratigraphy that hosts the veins dip at ~30° SW, on the southwestern limb of the Cangrejos anticline, with lines of evidence that suggest his tilting postdated the emplacement of the mineralised veins which dip 40 to 60°SW (Spencer et al., 2002).
Historic production in Zaruma District is estimated to be ~155 tonnes (5 Moz) of Gold and 375 t (12 Moz) of Silver (Kalinaj, 2004; Spencer et al., 2002).
Canadian NI 43-101 compliant mineral resource estimates of the Zaruma Gold Field (Zaruma and Portovelo district vein systems), in 2004 and 2005 are reported by Dynasty Metals and Mining Inc. (2012) as follows:
Measured - 1.568 Mt @ 13.93 g/t Au;
Indicated - 0.915 Mt @ 13.87 g/t Au;
Inferred - 3.382 Mt @ 12.72 g/t Au
The most recent source geological information used to prepare this summary was dated: 2005.
This description is a summary from published sources, the chief of which are listed below.
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References to this deposit in the PGC Literature Collection:
Spencer, R.M., Montenegro, J.L., Gaibor, A., Perez, E.P., Mantilla, G., Viera, F. and Spencer, C.E., 2002 - The Portovelo-Zaruma mining camp, southwest Ecuador; porphyry and epithermal environments: in SEG Newsletter April, 2002, No. 49, p. 1, pp. 9-14.|
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