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Lagoa Salgada |
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Portugal |
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Zn Pb Cu Au Ag
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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All papers now Open Access.
Available as Full Text for direct download or on request. |
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The Lagoa Salgada zinc-lead-copper-silver-gold volcanic hosted massive sulphide (VHMS) deposit is the northwestern most of the known significant Cu-Zn orebodies of the Iberian Pyrite Belt (IPB), located in Portugal, some 90 km NW of Neves Corvo and 80 km SE of Lisbon.
The IPB is one of three domains that now make up the South Portuguese Zone (SPZ), the southwestern-most terrane of the Variscan orogen on the Iberian peninsula. The proto SPZ collided obliquely with the Ossa Morena terrane to the northeast during the Variscan orogeny, producing a strike-slip tectonic regime that resulted in the opening of a series of pull-apart basins within the SPZ terrane, and in turn triggered the submarine volcanism of the IPB.
The Iberian Pyrite Belt contains a sequence of 1000 to 5000 m of late Palaeozoic rocks, comprising, from the base:
(i) the Phyllite-Quartzite Group (base unknown) composed of a monotonous detrital sequence of alternating dark gray shales and quartz sandstone;
(ii) the overlying Volcano Sedimentary Complex that hosts the VHMS deposits (including Lago Salgada) within a stratigraphic sequence that is grouped into three felsic volcanic cycles separated by two mafic pulses, collectively reaching a thickness of up to 1300 m (true thickness) near volcanic centres, characterised by a large diversity of volcanic and sedimentary facies. This felsic-mafic volcanic sequence is interbedded with shale (~75% shale and ~25% felsic and mafic volcanic rocks) and some chemical sediments, dated as Late Famennian to Early-Late Visean (Late Devonian to Early Carboniferous). The massive sulphide deposits are hosted by the felsic volcanic units and/or black shales. The felsic volcanic centres were built up as a variable number of effusive and explosive volcanic episodes and consist mainly of felsic lavas and domes (dominantly of quartz and feldspar-phyric rhyolitic and dacitic compositions) that may have intercalated thick pyroclastic units sourced from the lavas and/or domes. The volcanic centres have marginal aprons of abundant bedded volcaniclastic units that gradually pass outwards into shales with non-volcanic provenance that are the dominant rock type of the complex.
(iii) the Baixo Alentejo Flysch Group an up to 3,000 m thick turbiditic sequence that comprises shales, litharenites and rare conglomerates of Lower to Upper Carboniferous age.
Variscan deformation within the IPB stratigraphic sequence is characterised by south- to southwest-verging folds, corresponding to a thin-skinned foreland fold and thrust belt. Low-grade regional metamorphism displays a northward increase from zeolite to greenschist facies.
The Lagoa Salgada orebody is hosted within a northwest-southeast aligned 3.5 x 1 km wedge of the Volcano Sedimentary Complex of the IPB, concealed beneath ~135 m of palaeo-fluvial fan sediments of the Tertiary Sado basin. It comprises two distinct zones, a central stockwork and a massive sulphide lens several hundred metres to the northwest. The orebody was eroded prior to the deposition of the Tertiary cover, with a hematite gossan zone of oxidised massive sulphides below the unconformity overlying a zone of supergene enrichment. The deposit is folded, faulted and interpreted to occur mostly on the subvertical-overturned and intensely faulted limb of a southwest-verging anticline, and is further offset by an east-west-trending Alpine-age fault in the north, with a 50 m downthrow of the northern block, and unknown horizontal displacement.
The Central Stockwork Zone comprises veins and semi-massive lenses of sulphide that are dominantly hosted by the Lower Volcanic Unit (LVU) a thick (up to 250 m), strongly chloritised quartz-phyric rhyodacite unit within a thick (>700 m) sequence of feldspar- and quartz-phyric, and quartz-phyric rhyodacites, with intercalations of siltstone, the base of which has not been intersected. In contrast to the other deposits of the IPB, the massive sulphide ore is not found in close proximity to sedimentary rocks, and is hosted by a seperate feldspar- and quartz-phyric rhyodacite, the Upper Volcanic Unit (UVU) that dominates the sequence. These two rhyodacites are clearly distinguished by their phenocryst content, with the former geochemically corresponding to a more evolved series than the latter. U-Pb isotope determinations yield ages of 356.4±0.8 and 356.21±0.73 Ma for the feldspar- and quartz-phyric, and quartz-phyric rhyodacite volcanic units, respectively. Lithogeochemical studies have clasified the LVU volcanic rocks as dacitic and the UVU as andesitic.
The quartz-phyric rhyodacite of the LVU hosting the Central Stockwork Zone is dominated by a coherent facies that is intercalated with and grades into an overlying monomictic rhyodacitic breccia facies. Intervals of the coherent rhyodacite facies are up to 150 m thick, with ~7 modal % of embayed euhedral to subhedral, 5 mm-long quartz phenocrysts in a flow banded groundmass, characterised by 1 to 10 mm thick alternating dark and pale bands that may contain abundant chlorite wisps and abundant relics of recrystallised spherulites. Pale bands are mainly microcrystalline sericite, with accessory quartz and feldspar, while the dark bands are composed of microcrystalline quartz, feldspar, and chlorite with accessory sericite, although overprinting irregular domains of dark (chlorite-rich) and pale (sericite-rich) assemblages are probably the result of hydrothermal alteration. The intercalated and overlying, up to 50 m thick, monomictic rhyodacitic breccia facies, which encloses the best-developed sulphide stockwork of the Central Stockwork Zone, consists of massive, clast-supported intervals of irregular and polyhedral rhyodacite clasts. These clasts have similar textures to the coherent rhyodacite facies and their shapes and groundmass textures suggest that fragmentation of the rhyodacite is probably a consequence of autobrecciation. This upper breccia interval is in fault contact with the overlying, moderately sericite altered shale.
The Central Stockwork Zone is distributed over an area of 2 km2 where the host volcanic rocks have undergone hydrothermal chlorite-quartz-sulphide alteration, especially near the associated massive sulphides, with a gradual outward transition to less-altered zones of chlorite-sericite-carbonates-quartz-sulphides and quartz-carbonates-sulphide assemblages. The more chloritic alteration is more pronounced in the in the feldspar- and quartz-phyric volcanic rocks than the quartz-phyric rhyodacite where sericite-quartz-carbonate-sulphide assemblages dominate. Pyrite is the principal sulphide within the Central Stockwork Zone, characterised by strong deformation and subsequent recrystallisation, with associated sulphosalts being more common than in the massive sulphide zone to the NW. Sphalerite (with chalcopyrite disease and very fine inclusions of cassiterite) and tetrahedrite(-tennantite) are finely intergrown with pyrite and are found interstitial to accompanying coarse euhedral arsenopyrite. Stannite and fine pyrrhotite locally occurs as very small inclusions within pyrite and tetrahedrite(-tennantite). The Central Stockwork Zone mostly contains non-economic mineralisation, with only local, metre-width shear zones containing brecciated semimassive pyrite, arsenopyrite and quartz. Cu grades in these zones reach a maximum of 1.4 wt.% in weakly to unaltered quartz-phyric rhyodacite, although most are below 1 wt.% Cu. This zone has only minor Pb, Zn and Sn.
The UVU sequence hosting and intercalated with the Northwestern Massive Sulphide lens zone is a thick (up to 100 m) feldspar- and quartz-phyric rhyodacitic unit that both overlies and is partially a lateral equivalent to the LVU quartz-phyric rhyodacitic unit hosting the central stockwork to the southeast. The feldspar- and quartz-phyric rhyodacite is typically sericite and chlorite altered and comprises thin bands of coherent rhyodacite that grade to much thicker intervals (up to 50 m) of monomictic feldspar-quartz-phyric rhyodacitic breccia. The coherent facies are evenly feldspar-phyric, with ~20 and ~5 modal % of feldspar and quartz phenocrysts respectively. The monomictic feldspar- and quartz-phyric rhyodacitic breccia is dominated by thick clast-supported intervals, characterised by jigsaw-fit and clast-rotated clasts, some of which are dominantly altered to sericite, and others to chlorite. It has been intepreted to represent a hyaloclastite developed from a massive lava flow. This breccia typically hosts a well-developed sulphide stockwork, with the veins preferentially in the breccia matrix, overlain by a massive sulphide lens. The volcanic units and massive sulphide lenses are commonly overlain by either a 50 cm-thick discontinuous and irregular hydrothermal chert layer (grading outward into jasper) or a thick band of strongly chlorite-sericite altered shale.
The Northwestern Massive Sulphide lens zone, which comprises massive polymetallic sulphides and related chloritised stockwork mineralisation, averages between 25 to 30 m in true thickness and has a wide variation in metal content, averaging 0.35% Cu, 3.22% Pb, 4.43% Zn, 0.40% Sn, 72.52 g/t Ag and 0.95 g/t Au, with lesser amounts of Hg, As and Sb. Cu is >1% in the associated stockwork zone, with reduced concentrations of Sn. Pb and Zn values reach maximums of ~23 and 20 wt.% respectively in some drill intersections through the massive sulphide lenses. The mineralogy includes a wide variety of ore minerals, with pyrite being dominant, accompanied by insterstitial sphalerite, arsenopyrite, tetrahedrite(-tennantite), galena, löllingite, chalcopyrite, cassiterite, stannite, chalcocite, neodigenite, covellite, enargite, bornite, bournonite, meneghinite and minor bismuth sulphosalts, plus pyrrhotite. Pyrite occurs as clusters of small (10 to 15 µm) recrystallised individual crystals, with remnants of primary colloform, atoll and framboidal textures observed in less-deformed rocks. Annealing textures are common.
The palaeogossan is characteristically a highly siliceous, lead-rich leached cap of reddish- to ocher-coloured material, mostly consisting of goethite and hematite after pyrite, with local fine fractures filled with native Ag-Hg amalgams. It is underlain by a zone of supergene enrichment, represented by an assemblage of chalcocite, neodigenite, covellite and bornite. The supergene enrichment zone has an upper limonite-rich and a lower siliceous-, Au-, Ag-rich zone. The latter has calculated resources of up to 0.28 Mt @ 0.3% Cu, 5.6% Pb, 0.3% Zn, 60.6 g/t Ag, 1.0 g/t Au and up to 1.4% Sn.
Published NI 43-101 compliant resources at December 2011, using a 3.5 Zn equiv.% cut-off (Portex Minerals website, 2012) were:
Indicated resource - 2.9 Mt @ 2.94% Pb, 3.40% Zn, 0.34% Cu, 54.72 g/t Ag, 0.82 g/t Au = 7.18 Zn equiv.%;
Inferred resource - 1.6 Mt @ 2.50% Pb, 1.80% Zn, 0.35% Cu, 51.00 g/t Ag, 0.78 g/t Au = 5.54 Zn equiv.%;
This summary is mostly drawn from, and in places closely paraphrases, de Oliveira et al., 2011.
The most recent source geological information used to prepare this decription was dated: 2011.
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.
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de Oliveira D P S, Matos J X, Rosa C J P, Rosa D R N, Figueiredo M O, Silva T P, Guimaraes F, Carvalho J R S, Pinto A M, Relvas J R M S and Reiser F K M, 2011 - The Lagoa Salgada Orebody, Iberian Pyrite Belt, Portugal: in Econ. Geol. v.106 pp. 1111-1128
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