Mineralium Deposita (v.37, #8)

News from the Editors by Richard Goldfarb; Bernd Lehmann (683-683).

U–Pb Geochronology of VMS mineralization in the Iberian Pyrite Belt by Tucker C. Barrie; Yuri Amelin; Emilio Pascual (684-703).
A geochronology study using U–Pb isotope dilution TIMS analyses of zircon has been conducted to determine the ages of volcanic-associated massive sulfide (VMS) deposits in the Iberian Pyrite Belt (IPB), the world's most prolific VMS province. Ages have been determined for host rocks to four VMS systems that span the IPB: the giant Rio Tinto and Aljustrel districts in the central region, Lagoa Salgada to the west, and Las Cruces to the east. A sample of chloritized quartz porphyritic dacite/rhyolite in the footwall of the San Dionisio massive sulfide deposit of the Rio Tinto district is 349.76±0.90 Ma. This is taken as the best age estimate of the mineralization in the Rio Tinto district, probably the world's largest volcanogenic massive sulfide system. Two xenocrystic zircons from the same sample yielded 207Pb/206Pb ages of 414 and 416 Ma, which provide a minimum estimate for the age of the inherited component. A biotite tonalite from the Campofrio area, 3.5 km north of the center of the Rio Tinto district, is chemically similar to the felsic host rock protolith at Rio Tinto. The Campofrio sample has an age of 346.26±0.81 Ma, slightly younger and outside of the 2σ error for the Rio Tinto age; therefore, this phase of this intrusion was not a heat source for the hydrothermal system that formed the deposits of the Rio Tinto district. The Campofrio sample also has three zircon analyses with 207Pb/206Pb minimum ages of 534, 536, and 985 Ma, indicating inheritance from Ordovician and Neoproterozoic sources. In the Aljustrel VMS district, a U–Pb zircon age of 352.9±1.9 Ma characterizes the altered Green Tuff host rock of the Algares deposit, which is slightly older than the Rio Tinto age. Two zircons with 207Pb/206Pb ages of 531 and 571 Ma from this sample indicate inheritance from a Cambrian or older source. The age of mineralization at Lagoa Salgada is given by essentially identical ages of 356.21±0.73 and 356.4±0.8 Ma, for footwall and hanging wall samples, respectively. The hanging wall sample has two zircon analyses with 207Pb/206Pb ages of 464 and 466 Ma, indicating inheritance from an Ordovician or older source. The age for an altered dacite tuff sample from the hanging wall of the Las Cruces deposit is 353.97±0.69 Ma. One zircon analysis from the Las Cruces sample has a 207Pb/206Pb age of 1048 Ma, indicating inheritance from a Neoproterozoic source. These U–Pb ages refine the IPB geochronology provided by previous studies, and they suggest that either volcanism progressed toward the center of the IPB, or that volcanism was broadly static and the strata were progressively rifted to the margins during transtensional basin formation. The zircon inheritance provides direct evidence for Proterozoic to Ordovician sources, reflecting either basement rocks beneath the Phyllite–Quartzite Group during VMS formation in late Tournaisian times, or a Proterozoic to Ordovician detrital component in Phyllite–Quartzite Group source rocks. The presence of an older crustal component is consistent with VMS formation during rift development at a continental margin.
Keywords: Iberian Pyrite Belt Rio Tinto U–Pb geochronology Volcano-sedimentary formation Volcanogenic massive sulfide

The Cleo gold deposit, containing 4.25 Moz Au (37.1 Mt at 3.6 g/t), is the western part of a continuous orebody divided by a north-south tenement boundary. The eastern part is known as Sunrise, and together the Cleo and Sunrise deposits contain nearly 8 Moz Au in resources and past production. The focus of this study is the Cleo deposit, located 50 km south of Laverton in the Eastern Goldfields Province of the Yilgarn Craton of Western Australia, hosted in an Archean sequence dominated by volcaniclastic rocks. The Sunrise Shear Zone divides the sequence into hanging wall and footwall components. The gently north-west-dipping shear zone controls the orientation of shear-zone parallel ore zones, which characteristically involve pyrite replacement of magnetite in banded iron formation. Steeply-dipping multistage veins in the hanging wall and footwall define the Western Lodes, ore zones that are oriented parallel to adjacent rhyodacite–porphyry dikes. Western Lode veins are developed in all rock types, and commonly contain free gold, as well as pyrite, arsenopyrite, tennantite and chalcopyrite. In the footwall block, the margins of steeply east-dipping rhyodacite porphyry dikes form the main control on localization of the Western Lodes. In the hanging wall, the Western Lodes parallel a porphyry dike, both structures exploiting a favourable orientation in the stratigraphic sequence. Rhyodacite porphyry dikes exhibit strained margins in the Sunrise Shear Zone. Gold-bearing Western Lodes veins cut porphyry dikes and cut the Sunrise Shear Zone with minimal offset. The association of the ~2,675 Ma rhyodacite porphyry dikes with the Western Lodes ore zones is caused by the structurally favourable orientation of the dikes, and not to any direct genetic relationship between rhyodacite porphyry magma and ore fluids, as the latter post-dated the former.
Keywords: Australia Cleo deposit Eastern, Goldfield Province Orogenic gold Yilgarn Craton

Gold mineralization of the Hutti mine, southern India, is situated in closely spaced laminated quartz veins and associated alteration haloes along steeply dipping shear zones within a sequence of rather uniform amphibolites. Intense shearing has resulted in large-scale mylonitization of the wall rocks. Anastomosing shear zones, with intervening lensoid bodies of unsheared amphibolites, are characteristic features of the deposit. The general pattern of symmetrical alteration comprises a distal zone of chlorite-rich rock, with a proximal biotite-rich zone adjacent to laminated quartz veins. Arsenopyrite thermometry yielded a temperature range of 350–477 °C for the biotite alteration zone, which preceded the formation of the laminated quartz veins. Mass balance calculations on the alteration zones indicate a gradual mass and volume loss during alteration. The alteration is accompanied by intense potash metasomatism and addition of sulfur, which resulted in the formation of arsenopyrite, pyrrhotite, and pyrite. Results of fluid inclusion studies suggest that low salinity (3.9–13.5 wt% NaCl equivalent) H2O–CO2 rich fluids were responsible for gold-rich laminated quartz vein formation in the Hutti deposit. These fluids constituted a later counterpart of the protracted fluid activity that first formed the biotite alteration zone. The estimated P–T values range from 1.0 to 1.7 kbar at 280–320 °C. These data, along with the alteration assemblages and the characteristic gold–sulfide association, both in the altered wall rock and laminated quartz veins, suggest that gold, transported as reduced bisulfide complexes, was deposited in response to sulfidation reactions in the wall rocks. Comparison of P–T conditions of formation of gold–quartz veins at Hutti with two other large gold deposits in the eastern Dharwar Craton, namely Kolar (1.8 kbar/280 °C) and western Ramagiri (1.45–1.7 kbar/240–270 °C), indicates broadly similar lode-gold forming conditions in the Dharwar Craton.
Keywords: India Orogenic gold Hutti Fluid inclusions

The Marymia gold deposit, comprising two orebodies, Keillor 1 and Keillor 2, is at the northern end of the Plutonic Well greenstone belt in the Marymia Inlier, in the southern Capricorn Orogen, just north of the Yilgarn craton. The Marymia Inlier is a discrete fault-bounded Archean gneiss–granitoid–greenstone domain surrounded by sedimentary basins that were formed and variably metamorphosed and deformed during several Palaeoproterozoic orogenic cycles. The greenstone sequence at Marymia is stratigraphically and geochemically similar to greenstone sequences in the Yilgarn craton, but was subjected to further deformation and metamorphism in the Palaeoproterozoic. Late Archean deformation (D1–D2) was ductile to brittle–ductile in style, whereas Palaeoproterozoic deformation was predominantly brittle. Equilibrium mineral assemblages indicate that peak amphibolite-facies metamorphism (540–575 °C, <3 kb) was overprinted by greenschist-facies metamorphism (300–360 °C). Petrographic textures indicate that prograde metamorphism was coeval with D1–D2, with peak metamorphism early to syn D2. Gold mineralisation at Marymia is hosted in metamorphosed tholeiitic basalts and banded iron formation. On a gross scale, the distribution of gold is controlled by D2 folds and shear zones. Lithological contacts with strong rheological or chemical contrasts provide local controls. Gold-related alteration comprises subtle millimetre- to centimetre-wide zones of silicification with variable amounts of quartz, hornblende, biotite, K-feldspar, plagioclase, calcite/siderite, scheelite, titanite, epidote, sulfide and telluride minerals. Quartz veins are generally narrow and discontinuous with low total volume of quartz. Gold is sited in the wall rock, at vein salvedges or within stringers of wall rock within veins. There are two distinct opaque-mineral assemblages: pyrite–pyrrhotite–chalcopyrite–galena and hessite–petzite–altaite–Bi-telluride–galena. Ore samples are variably enriched in Ag, Te, Pb, W, Cu, S and Fe reflecting heterogeneity of the ore mineralogy. Structural timing and temperature of formation of alteration and ore minerals support deposition of gold during late peak amphibolite-facies metamorphism from neutral to alkaline (pH=5–6), moderately oxidising (log ƒ O2≈–21–22) and CO2-bearing (X CO2 ≈0.2) fluids. The total sulfur content of the fluid is estimated at 1mΣS. Lead isotope compositions support derivation of lead from within the local greenstone sequence. Gold lodes were deformed by faults and shear zones in the Palaeoproterozoic, with only limited remobilisation. Subeconomic, carbonate vein- and breccia-hosted base metal mineralisation is locally hosted within Palaeoproterozoic fault zones, which clearly cut gold lodes. Base-metal-related alteration is characterised by intense carbonatisation, chloritisation, and albitisation of the mafic host rocks. Mineral assemblages are consistent with formation at greenschist facies conditions. Lead isotope compositions support crystallisation at ca. 1.7 Ga from lead that is similar in composition to earlier gold-related galena.
Keywords: Amphibolite-facies Base-metals Capricorn Orogen, Marymia Inlier Orogenic gold

Microthermometric studies of sphalerite-hosted fluid inclusions from fracture fillings in felsic igneous rocks of modern polymetallic massive sulfide mineralization from the JADE field (Central Okinawa Trough, Japan) and from mounds on the seafloor in the North Fiji basin reveal evidence for sub-critical two-phase separation of seawater. Chemical analyses of the fluid inclusions indicate intense water–rock interaction between the hydrothermal fluids and their host rocks. The analyses indicate that sub-critical phase separation has a much larger effect on the partitioning of chloride and bromide into liquid and vapor phase than previously reported for vent fluids. This study also shows that, for the first time, chlorine isotope fractionation correlates with Cl and Br fractionation. Using combined Cl/Br ratios and δ37Cl values allows us to discount Br-enriched organic matter or halite precipitation as responsible for the formation of Br-rich fluids. Moreover, the same parameters exclude any significant magmatic component in the mineral-forming fluids as previously proposed for similar modern sulfide deposits and other massive sulfide deposits in the geological record.
Keywords: Chlorine isotopes Fluid inclusions Massive sulfides Phase-separation Seafloor mineralization

The giant Imiter silver deposit: Neoproterozoic epithermal mineralization in the Anti-Atlas, Morocco by Alain Cheilletz; Gilles Levresse; Dominique Gasquet; Moulay Azizi-Samir; Rachid Zyadi; Douglas A. Archibald; Edward Farrar (772-781).
The world-class Imiter silver deposit, in the Anti-Atlas Mountains of Morocco, is a Neoproterozoic epithermal vein deposit genetically associated with a felsic volcanic event, and formed within a regional extensional tectonic regime. Rhyolitic volcanism related to ore formation has been dated at 550±3 Ma by ion-probe U/Pb on zircons. The economic silver mineralization is superimposed on an older, discrete base-metal assemblage associated with calc-alkaline granodioritic magmatism. The magmatism is dated at 572±5 Ma by ion-probe U/Pb dating on zircons, and by 40Ar/39Ar dating on hydrothermal muscovites. In the Anti-Atlas Mountains, the Precambrian–Cambrian transition appears as an important period for the formation of major, productive precious-metal deposits associated with volcanic events and extensional tectonics. The Imiter silver deposit constitutes a Precambrian analogue to modern epithermal deposits.
Keywords: Epithermal Silver Extensional tectonics Geochronology Neoproterozoic Morocco

Keywords: Epithermal Gold Tapajós Province Amazon Craton Brazil

Keywords: Andes Adakite Flat slab Lower crust Porphyry copper

Keywords: Adakitic magmatism Normal calc-alkaline magmatism Porphyry copper deposits Northern Chile

Keywords: Age-dating research Lead–zinc deposits Mississippi Valley Reply