Breccia Matrix Research Articles

Alunite and the role of magmatic fluids in the Tambo high-sulfidation deposit, El Indio–Pascua belt, Chile

The Tambo high-sulfidation deposit, located within the El Indio–Pascua belt in Chile, produced almost 25 t (0.8 M oz) of gold from altered Tertiary rhyodacitic volcanic rocks. Episodic magmatic-hydrothermal activity in the district occurred over at least 4 my and is characterized by several stages of acid-sulfate alteration, including magmatic-hydrothermal, magmatic steam, steam-heated, and apparent supergene assemblages. Two stages of Au±Ag mineralization are recognized and are hosted in barite and alunite within hydrothermal breccias and veins. Isotopic compositions of fluid in alunite show a dominant magmatic signature, with only a variable 18O-enriched meteoric water component throughout the entire hydrothermal process. Alunite 40Ar/ 39Ar ages constrain the timing of alteration and the duration of the hydrothermal system. Pre-ore alteration occurred at about 10 to 11 Ma and was contemporaneous with the volcanism of the Tambo Formation. Alunite from this stage of alteration occurs in the matrix of barren breccias and as fine intergrowths of alunite–quartz±clays that selectively replaced feldspars and pumice fragments. The textural relationships combined with stable-isotope systematics suggest a magmatic-hydrothermal origin for the alunite, with a local magmatic steam overprint. Early ore-stage alunite (8.7±0.2 Ma) occurs with barite and gold±walthierite within open spaces of the breccia matrix, and has δ 34S values (24–27‰) typical of magmatic-hydrothermal alunite, reflecting equilibrium between aqueous H 2S and SO 4 2−. Fluid-inclusion ratios of H 2S/SO 2 (approximately 6) are consistent with ratios determined from stable-isotope data, and indicate reduced fluid conditions during ore deposition. Vapor-phase transport of Au, and deposition from condensed magmatic vapor rising from the brittle–ductile transition is inferred. Late gold coprecipitated with a third stage of alunite (8.2±0.2 Ma) that is characterized by nearly uniform chemical compositions and δ 34S values (1‰) similar to those for associated enargite, reflecting disequilibrium between H 2S and SO 4 2−. This third-stage of alunite is isotopically and chemically similar to that of post-ore, coarse, banded alunite±hematite-quartz veins that crosscut the breccias in the Tambo area. Analyses of fluid-inclusion gas from alunite in these veins indicate high SO 2 and disequilibrium CO 2–CO–CH 4–H 2 species, consistent with a magmatic-steam origin. The gases are also depleted in He, and the late gold-bearing alunite was probably precipitated from rapidly ascending SO 2-rich vapors that were flashed from condensed magmatic fluids located below the brittle–ductile transition. Alunite–huangite veins (8.6±0.4 Ma) were formed intermediate to the two mineralizing events. Steam-heated alunite (8.9±0.4 Ma), derived from the oxidation of H 2S within the vadose zone, is preserved at the highest elevations and overlaps with Stage 2 ore deposition. Post-mineral alunite±jarosite (7.3±0.1 Ma) occurs as apparent supergene crosscutting veins and overgrowths. Stable-isotope data, however, indicate a strong magmatic component to both near-surface and late-stage alunite-forming fluids. This dominant magmatic fluid signature at high levels during the waning stages of hydrothermal activity is attributed to climatic and physiographic conditions specific to the El Indio–Pascua belt during the Middle to Late Miocene.

Geology and SHRIMP U-Pb Geochronology of the Igarape Bahia Deposit, Carajas Copper-Gold Belt, Brazil: An Archean (2.57 Ga) Example of Iron-Oxide Cu-Au-(U-REE) Mineralization

A striking feature of the Carajas region, Brazil, is the clustering of a variety of different types of Cu-Au deposits. The most abundant in the belt are the >200 million metric tons (Mt) of Fe oxide Cu-Au-(U-REE) deposits, which, despite the variety of host rocks and different orebody morphologies, share a number of diagnostic features, including (1) intense Fe metasomatism leading to the formation of grunerite, fayalite, and/or Fe oxides (magnetite and/or hematite); (2) intense carbonate alteration (mainly siderite); (3) sulfur-poor ore mineralogy (chalcopyrite and bornite); (4) quartz-deficient gangue; (5) extreme low REE enrichment, and (6) enrichment in U and Co. The Igarape Bahia deposit is perhaps the best documented Fe oxide Cu-Au-(U-REE) deposit of the belt, containing about 219 Mt at 1.4 percent Cu and 0.86 g/t Au. The Cu-Au ore consists of steeply dipping breccia bodies that are hosted by hydrothermally altered metavolcano-sedimentary rocks. SHRIMP II zircon dating of the host metavolcanic rocks gives a 207Pb/206Pb age of 2748 ± 34 Ma. This suggests a correlation between the Igarape Bahia volcano-sedimentary sequence and the Grao Para volcanic rocks, which have published ages of ca. 2.75 Ga. SHRIMP dating of monazite from the matrix of ore-bearing magnetite breccias gives a 207Pb/206Pb age of 2575 ± 12 Ma, confirming the epigenetic nature of the mineralization and placing it ~175 m.y. after accumulation of the host volcano-sedimentary sequence. The 2575 ± 12 Ma SHRIMP age of hydrothermal monazite from the Igarape Bahia mineralization is indistinguishable from published conventional 207Pb/206Pb ages for zircons from the Archean A-type granites of the Carajas belt, indicating that mineralization processes at Igarape Bahia were temporally related to these A-type Archean granites. The wide range of highly radiogenic 87Sr/86Sr ratios (0.714–0.755) of carbonates from the Igarape Bahia deposit suggests multiple crustal sources, consistent with a magmatic-hydrothermal origin. SHRIMP dating of zircon xenocrysts recovered from crosscutting diabase dikes indicates a maximum 207Pb/206Pb age of ~2670 Ma, consistent with field evidence and the age of host rocks, but does not unequivocably constrain the age of the ores. The styles of hydrothermal alteration, mineralogy, and geochemistry of the Igarape Bahia ore, as well as published fluid inclusion and stable isotope data, support its classification as a member of the world-class Olympic Dam-type Fe oxide Cu-Au-(U-REE) group of deposits, as previously argued by several authors. The SHRIMP age of 2575 ± 12 Ma for hydrothermal monazite indicates that Igarape Bahia is an Archean example of this deposit group.

Ore-Forming Processes in Irish-Type Carbonate-Hosted Zn-Pb Deposits: Evidencefrom Mineralogy, Chemistry, and Isotopic Composition of Sulfides at the LisheenMine

Lisheen is a strata-bound zinc-lead deposit formed during the Mississippian by replacement of hydrothermally dolomitized, grossly stratiform breccia bodies located near the base of the Carboniferous Waulsortian Limestone. It represents one of a number of carbonate-hosted massive sulfide ore deposits in the Irish ore field that, due to several unique features, have been classified as Irish type. Disseminated pyrite occurs in preore dolomite and around the margins of preore dolomite clasts within dolomite breccias. Early fine-grained sphalerite-pyrite mineralization occurs as infill of intergranular dolomite porosity. Locally, massive to semimassive iron sulfide is observed, mainly comprising pyrite with lesser marcasite. A complex polymetallic sulfide assemblage typifies the main ore stage, dominated by fine-grained disseminated, massive or colloform sphalerite and galena, with minor pyrite, chalcopyrite, arsenopyrite, tennantite, nickel- and cobalt-bearing minerals. Silver occurs in solid solution in tennantite, galena, and sphalerite. Dolomite and barite dominate the gangue, with lesser calcite. Main-stage mineralization involved the progressive replacement of preexisting iron sulfides and the dolomite breccias, initially by replacement of the breccia matrix and ultimately by replacement of clasts. Coarse crystalline sphalerite and euhedral galena crystals are generally restricted to fracture-fill mineralization or vugs within main ore-stage assemblages where they occur with euhedral dolomite and calcite. Barite intergrown with main ore-stage sulfides has δ 34S values of 14.3 to 18.1 per mil, consistent with the derivation of sulfate from coeval Carboniferous seawater. The δ 34S values for sulfides range from –44.1 to +11.8 per mil, with a mean value of –13.7 per mil, typical of the Irish ore deposits. The dominant low δ 34S signature is considered to be the result of bacterial reduction of coeval seawater sulfate. Extremely low δ 34S values, in the range of –38 to –44 per mil, are only observed in preore disseminated pyrite; such extreme fractionations are thought to be due to low bacterial sulfate reduction rates coupled with oxidative cycles in near-sea floor pore waters. Main ore-stage sulfides have δ 34S values in the range of –4 to –18 per mil, with a mode of –10 per mil, consistent with a typical bacterial fractionation from coeval seawater sulfate. Isotopic equilibrium between cogenetic sulfides is not observed. The bacteriogenic sulfur component was probably transported from bacterial colonies fringing the ore system by low-temperature brines. The δ 34S values of late ore-stage sulfides mainly range from –20.2 to +12.0 per mil, with the majority having relatively high values (mean = –3.0 ± 8.5‰, 1 σ ) interpreted as being due to the presence of a hydrothermal sulfur component, leached from the lower Paleozoic basement. For galena and sphalerite there is a general increase in δ 34S values with depth in the system, with time, and with proximity to east-west– and northwest-trending faults. These relationships suggest that input of hydrothermal sulfur from depth via fractures became increasingly important. Hydrothermal sulfur appears to be more important at Lisheen than the other major Irish deposits. Galena lead isotope analyses gave average 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb values of 18.183, 15.594, and 38.080, respectively. These data do not correlate with ore-stage, galena texture or δ 34S. The results are comparable to previous data from Lisheen and from Silvermines, 35 km to the west, implying a common lead source in the lower Paleozoic basement. The textural, mineral, chemical, and isotopic evidence suggests that main-stage ore was precipitated as a consequence of rapid supersaturation, caused by fluid mixing within the permeable dolomite breccias. This process involved relatively high temperature (ca. 240°C), metal-bearing solutions derived from a basement-equilibrated fluid reservoir (carrying Zn, Pb, Fe, Cd) and shallow, saline (ca. 25 wt % NaCl equiv) formation waters rich in bacteriogenic H2S. Minor metals (Cu, As, Ni, Co) are thought to have been stripped from the footwall Old Red Sandstone during hydrothermal alteration around fault conduits. The availability of abundant seawater sulfate, operation of open-system bacterial sulfate reduction, and episodic availability of free oxygen imply that ore formation cannot have occurred at significant depth below the paleosea floor. Cessation of mineralization was due to a cut-off of the sulfur-rich brine supply, possibly by deposition of impermeable hanging-wall sediments. This process of ore formation is consistent with evidence from the other economic Irish-type deposits in the ore field.

The Pierina epithermal Au–Ag deposit, Ancash, Peru: paragenetic relationships, alunite textures, and stable-isotope geochemistry

The Pierina high-sulfidation epithermal Au–Ag deposit (9°26.5′S; 77°35′W) was emplaced in the Middle Miocene into a hypabyssal-to-extrusive “pumice-tuff” and an underlying, older, dacitic flow-dome complex, both of which are cut by hydrothermal breccias and small dacitic domes. Stage I advanced argillic alteration generated a core of vuggy silica, focused in the tuff, and surrounded successively by zones of quartz–alunite, dickite±kaolinite±pyrophyllite, and illite–montmorillonite±kaolinite. Laser-ablation ICP-MS analysis of the sulfide minerals of the succeeding, Stage II, Cu (–Pb, Bi, Sb, Zn, As)–barite mineralization, largely confined to the vuggy-silica zone, reveals that both Au and Ag were introduced at that time. This assemblage was almost entirely obliterated during Stage III, when oxidation by low-temperature meteoric waters generated botryoidal hematite–goethite assemblages, which are now the main precious-metal hosts. Stage IV barite–acanthite mineralization shows limited temporal overlap with Stage III, but dominantly overprints the hematite–goethite assemblage. The deposit incorporates alunite exhibiting a wide range of modes of occurrence, grain size, and morphology. Disseminated alunite dominates Stage I alteration, which replaced phenocrysts and fragments, and locally hosts corroded, ≤40 μm alumino-phosphate-sulfate (APS) inclusions. The alunite shows limited Na substitution [molar Na/(Na+K)=<0.2], but alunite–natroalunite assemblages occur sporadically throughout the quartz–alunite alteration zone. Isotopic analysis yields δ 34S values of 16.6–31.0‰, consistent with a magmatic–hydrothermal origin. Rare occurrences of disseminated alunite and pyrite in textural equilibrium occur in unoxidized areas of the deposit, and yieldΔ 34S alu-py precipitation temperatures of 179 to 250 °C, with the majority below 200 °C. Disseminated alunite in shallower, oxidized portions of the deposit, where pyrite has been destroyed, yield δ 18O SO 4 values of 6.3‰ to 14.4‰. The lighter compositions occur along the axis of the quartz–alunite alteration zone, and imply precipitation at higher temperatures, albeit with some meteoric water involvement. Porcelaneous alunite (10–100 μm) forms the matrix of breccias that cut Stage I alteration. Sodium contents are low [Na/(Na+K molar≃0.1) and APS inclusions have not been identified. The δ 34S values of 21.8–27.1‰ are consistent with a magmatic–hydrothermal origin, but the δ 18O SO 4 values of 11.4‰ to 14.6‰ indicate deposition under cooler temperatures and the involvement of meteoric water that interacted substantially with igneous country-rocks. Coarse, open-space-filling alunite, which occurs as veins or coatings on breccia fragments, exhibits limited Na substitution, is not associated with natroalunite, and does not host APS inclusions. Its δ 34S and δ 18O SO 4 values of 12.9‰ to 26.2‰ and 6.5‰ to 8.6‰, respectively, imply a magmatic–hydrothermal origin and deposition at higher temperatures. Dike-like bodies and mantos of friable alunite have distinctive δ 34S (16.2–19.3‰) and δ 18O SO 4 (7.6–9.2‰) values, and may record a steam-heated environment. However, these compositions may also indicate loss of H 2S gas during oxidation, or overprinting of steam-heated alunite by magmatic–hydrothermal alunite (or vice versa). All forms of alunite exhibit higher δ 18O SO 4 values in the central part of the deposit, and the coherent isotopic chemistry casts doubt on the reliability of the widely accepted textural criteria for the origin of this mineral in hydrothermal systems.

Mineral-chemistry and stable-isotope constraints on the magmatism, hydrothermal alteration, and related PGE?(base-metal sulphide) mineralisation of the Mesoarchaean Baula-Nuasahi Complex, India

The Baula-Nuasahi Complex, on the southern flank of the Singhbhum Archaean nucleus in north-eastern India, exposes a series of Mesoarchaean igneous suites. These are (1) a gabbro–anorthosite unit, which is petrographically homogeneous, although mineral-chemistry data hint at a subtle eastward differentiation; (2) a peridotite unit (with three chromitite layers) together with (3) a pyroxenite unit which display cumulate textures, modal layering, and (for the peridotite unit) differentiation trends in both mineralogy and mineral chemistry; and (4) the Bangur gabbro (~3.1 Ga), which defines an oblong intrusion, crosscutting the older igneous suites in the southern part of the complex, with a curvilinear NW-trending apophysis, 2 km long and up to 40 m wide. Magmatic breccia comprising ultramafic and chromitite wall-rock clasts in a gabbro matrix is exposed at the contact of the main Bangur gabbro body and also forms the entire Bangur gabbro apophysis. Concentrations of platinum-group minerals (PGMs) are found where the breccia contains abundant chromitite clasts, and two types of platinum-group-element (PGE) mineralisation are recognised. Type 1 (Pt 1.1–14.2, Pd 0.1–2.1 ppm, with an average Pt/Pd=8–9) is a “contact-type” mineralisation which occurs in the breccia at the contact between the Bangur intrusion and its ultramafic host. The PGMs—Pt alloys (isoferroplatinum) and sulphides (braggite, malanite)—are enclosed by pyroxene and plagioclase, reflecting a magmatic origin. Significant wall-rock assimilation by the magma (giving rise to the Bangur gabbro) is indicated by changes in pyroxene composition and by the presence of relicts of chromite (from the host) now altered to secondary “ferritchromite” in the contact zone. Type 2 PGE mineralisation (Pt 0.3–1.6, Pd 1.8–6.0 ppm, with Pt/Pd~0.5–3.0) is restricted to the breccia apophysis of the Bangur gabbro where it occurs in the breccia matrix, associated with an intense hydrothermal alteration which does not exist in the contact zone. PGMs (PGE arsenides, tellurides, bismuthides and antimonides) and, where present, base-metal sulphides (BMSs) form intergrowths with hydrous silicates, reflecting a hydrothermal origin. Oxygen isotope geothermometry documents the main stages of hydrothermal alteration within a decreasing temperature range between 700–1,000 and 500–600 °C, and oxygen, hydrogen and sulphur isotopes show that the hydrothermal fluids were derived from the magma rather than an external source. Pervasive hydrothermal alteration in the breccia apophysis likely represents upward channelling of late-magmatic fluids along a narrow, near-vertical, subplanar conduit which led away from the main magma chamber. We suggest that Type 2 mineralisation was produced by late-magmatic hydrothermal remobilisation and reconcentration of Type 1 PGE mineralisation, and that the composition of the hydrothermal fluids controlled whether BMSs were enriched along with the PGMs.

An Archaean submarine volcanic debris avalanche deposit, Yilgarn Craton, western Australia, with komatiite, basalt and dacite megablocks: The product of dome collapse

The Boorara Domain of the Kalgoorlie Terrane, Eastern Goldfields Superterrane, western Australia contains excellent exposure of Archaean felsic and ultramafic breccias characterised by facies associations interpreted to reflect a volcanic debris avalanche mode of deposition. Such Archaean volcanic deposits are typically difficult to identify due to poor preservation and exposure. However, primary volcanological and sedimentological features are preserved within the relatively low strain and low metamorphic grade (up to lower greenschist facies) Boorara Domain that allow accurate facies reconstruction. The breccia deposit is characterised by two clast populations. A ‘block facies’ comprised of metre- to decimetre-scale megablocks of dacite, basalt and komatiite is preserved within a ‘mixed’ matrix breccia facies of angular, coarse sand- to boulder-sized clasts. The megablocks preserve original stratigraphy and show fracturing and jigsaw-fit textures within the poorly sorted, unstratified, genetically related matrix. Overlying the volcanic debris avalanche deposit, are a series of stratified horizons. These deposits show evidence of hydraulic sorting within bedforms exhibiting normal grain-size grading and tractional scour and fill structures along their basal contacts. The stratified facies is interpreted to have been deposited by high concentration, high competency turbidity currents, triggered by slope stabilization slides in the source region. Primary contacts and volcanic textures preserved in decimetre-scale volcanic blocks allow reconstruction of the pre-collapse palaeovolcanological history of the source region. The volcanic debris avalanche deposit, together with the associated stratified sedimentary horizons, were produced by sector collapse of a submarine, dacitic volcanic dome. Contemporaneous komatiite intrusion into the dacite dome may have caused dome flank instability. However, the volcanic debris avalanche trigger is interpreted to be a post-lithification tectonic influence.

Photomosaics of the cathodoluminescence of 60 sections of meteorites and lunar samples

Cathodoluminescence (CL) petrography provides a means of observing petrographic and compositional properties of geological samples not readily observable by other techniques. We report the low‐magnification CL images of 60 sections of extraterrestrial materials. The images we report include ordinary chondrites (including type 3 ordinary chondrites and gas‐rich regolith breccias), enstatite chondrites, CO chondrites and a CM chondrite, eucrites and a howardite, lunar highland regolith breccias, and lunar soils. The CL images show how primitive materials respond to parent body metamorphism, how the metamorphic history of EL chondrites differs from that of EH chondrites, how dark matrix and light clasts of regolith breccias relate to each other, how metamorphism affects eucrites, the texture of lunar regolith breccias and the distribution of crystallized lunar spherules (“lunar chondrules”), and how regolith working affects the mineral properties of lunar soils. More particularly, we argue that such images are a rich source of new information on the nature and history of these materials and that our efforts to date are a small fraction of what can be done.

Impact lithologies and their emplacement in the Chicxulub impact crater: Initial results from the Chicxulub Scientific Drilling Project, Yaxcopoil, Mexico

Abstract— The Chicxulub Scientific Drilling Project (CSDP), Mexico, produced a continuous core of material from depths of 404 to 1511 m in the Yaxcopoil‐1 (Yax‐1) borehole, revealing (top to bottom) Tertiary marine sediments, polymict breccias, an impact melt unit, and one or more blocks of Cretaceous target sediments that are crosscut with impact‐generated dikes, in a region that lies between the peak ring and final crater rim. The impact melt and breccias in the Yax‐1 borehole are 100 m thick, which is approximately 1/5 the thickness of breccias and melts exposed in the Yucatán‐6 exploration hole, which is also thought to be located between the peak ring and final rim of the Chicxulub crater. The sequence and composition of impact melts and breccias are grossly similar to those in the Yucatán‐6 hole. Compared to breccias in other impact craters, the Chicxulub breccias are incredibly rich in silicate melt fragments (up to 84% versus 30 to 50%, for example, in the Ries). The melt in the Yax‐1 hole was produced largely from the silicate basement lithologies that lie beneath a 3 km‐ thick carbonate platform in the target area. Small amounts of immiscible molten carbonate were ejected with the silicate melt, and clastic carbonate often forms the matrix of the polymict breccias. The melt unit appears to have been deposited while molten but brecciated after solidification. The melt fragments in the polymict breccias appear to have solidified in flight, before deposition, and fractured during transport and deposition.

Impactites of the Yaxcopoil‐1 drilling site, Chicxulub impact structure: Petrography, geochemistry, and depositional environment

Abstract— The impact breccias encountered in drill hole Yaxcopoil‐1 (Yax‐1) in the Chicxulub impact structure have been subdivided into six units. The two uppermost units are redeposited suevite and suevite, and together are only 28 m thick. The two units below are interpreted as a ground surge deposit similar to a pyroclastic flow in a volcanic regime with a fine‐grained top (unit 3; 23 m thick; nuée ardente) and a coarse breccia (unit 4; ∼15 m thick) below. As such, they consist of a mélange of clastic matrix breccia and melt breccia. The pyroclastic ground surge deposit and the two units 5 and 6 below are related to the ejecta curtain. Unit 5 (∼24 m thick) is a silicate impact melt breccia, whereas unit 6 (10 m thick) is largely a carbonate melt breccia with some clastic‐matrix components. Unit 5 and 6 reflect an overturning of the target stratigraphy. The suevites of units 1 and 2 were deposited after emplacement of the ejecta curtain debris. Reaction of the super‐heated breccias with seawater led to explosive activity similar to phreomagmatic steam explosion in volcanic regimes. This activity caused further brecciation of melt and melt fragments. The fallback suevite deposit of units 1 and 2 is much thinner than suevite deposits at larger distances from the center of the impact structure than the 60 km of the Yax‐1 drill site. This is evidence that the fallback suevite deposit (units 1 and 2) originally was much thicker. Unit 1 exhibits sedimentological features suggestive of suevite redeposition. Erosion possibly has occurred right after the K/T impact due to seawater backsurge, but erosion processes spanning thousands of years may also have been active. Therefore, the top of the 100 m thick impactite sequence at Yaxcopoil, in our opinion, is not the K/T boundary.

Geology and Mineralization Styles of the George Fisher Zn-Pb-Ag Deposit, Mount Isa, Australia

The George Fisher deposit contains one of the world’s largest in situ Zn resources (108 Mt at 93 g/t Ag, 5.4% Pb, and 11.1% Zn). It is situated 25 km north of Mount Isa and is hosted by the ~1655 Ma Urquhart Shale in the Western fold belt of the Mount Isa inlier. George Fisher is Zn rich compared to Mount Isa and Hilton and is distinguished by a paucity of syn- to late-tectonic Cu mineralization and associated silica-dolomite alteration. The deposit contains 11 mineralized stratigraphic intervals that include intercalated pyritic siltstones and banded mudstones with abundant layer-parallel nodular and planar carbonate bands, separated by thick, barren, medium-bedded mudstones. The Zn-Pb-Ag mineralization is hosted predominantly by pyritic siltstones within each stratigraphic interval and occurs as strata-bound lenses that subparallel bedding, bifurcate, and pinch out along the extent of the main economic zone. Four styles of mineralization have been differentiated at the deposit. Layer-parallel, disseminated sphalerite comprises low-grade occurrences of Zn, whereas the bulk of ore is contained within sphalerite and galena veins and breccias that record a prolonged and texturally complex history of ore emplacement. The earliest mineralization is represented by bedding-parallel, vein-hosted sphalerite which was emplaced prior to earliest folding of the host-rock sequence (GFD2). Layer-parallel disseminated sphalerite formed as a halo to veins as alteration and infill after secondary carbonates in host rocks and nodular carbonate layers that transgress stratigraphy at a district scale. Breccia-hosted sphalerite ore was emplaced during GFD2 to GFD4 fold development at the deposit and represents in situ deformed sphalerite veins and disseminated mineralization. Galena occurs as infill in veins and matrix of breccias that postdate the bulk of sphalerite and were emplaced during GFD4, late in the tectonic history and in part synchronous with late tectonic Cu mineralization event in the district. The continuity of ore lenses, covariation of Zn and Pb grades at the deposit scale, and similarities to other sediment-hosted ore systems discount the possibility that this paragenetic complexity represents separate Zn and Pb mineralization episodes prior to and at the culmination of regional folding at George Fisher, respectively. The late textural setting of galena is interpreted as the result of preferential concentration of Pb into new structural sites during deformation. The range of sphalerite and galena mineralization styles at George Fisher is interpreted to record subsurface emplacement of Zn and Pb, possibly during diagenesis or associated with early weak brittle deformation in at least semiconsolidated sediments, followed by extensive textural modification of the orebody during the folding history of the deposit.

Zn-Pb mineralisation in the Silvermines district, Ireland: a product of burial diagenesis

Carbonate-sulphide cement stratigraphic relationships in the host rock and ore have been used to constrain the age of mineralisation at the Silvermines zinc-lead-barium deposit. The base-metal sulphides post-date planar dolomite and replace stylolites. Furthermore, the pre-mineralisation planar dolomites also replace stylolites. These and other diagenetic observations indicate that the base-metal sulphides formed at burial depths greater than 800 m, but probably predate the Variscan deformation (since pressure shadows overgrow base metal sulphides). This indicates that the sulphides are of epigenetic origin, constraining the age of mineralisation to between the late Chadian (≈347 Ma) and the late Westphalian (≈307 Ma). However, the most likely age for mineralisation, (based on widespread macro-stylolite development) is Asbian (≈339 Ma) or younger. No evidence of synsedimentary sulphides (in the form of hydrothermal chimneys, vent faunas, or sulphides intergrown with marine cements) was observed at Silvermines. Mineralised breccias (black matrix breccias), late-stage internal sediments, and dissolution zones within the carbonate cements all appear to be produced by hydrothermal karsting that occurred during the mineralisation process. Fluid inclusion homogenisation temperatures for ore-stage calcites (up to 300 °C) approach the peak temperature estimates derived from regional maturation parameters (270 to 310 °C from conodont alteration indices and vitrinite reflectance). This suggests that homogenisation temperatures represent maximum heating temperatures (probably during Variscan time) rather than mineralisation temperatures.

On diagenesis, dolomitisation and mineralisation in the Irish Zn-Pb orefield

Marine calcite cementation and lithification of Carboniferous carbonate sediments hosting Zn-Pb mineralisation in the Irish orefield occurred at or near the seafloor. A relatively early, fine-grained, grey replacive dolomite, preferentially developed in micrite, is widely developed in the Waulsortian Limestone Formation, the main host to mineralisation, and is pervasive in the southeastern Midlands in proximity to the Leinster Massif. This dolomite formed after the first four main stages of calcite cementation but probably also developed within tens of metres of the seafloor as evidenced by incorporation of clasts of dolomite in intraformational sedimentary breccias. Later, coarse-grained white dolomite preferentially replaced coarser components of the Waulsortian Limestone and infilled residual vuggy porosity. Whilst some of this coarse dolomite may be related to the fine replacive dolomite event, a common spatial association with fault zones, coupled with primary fluid inclusion data, suggest that a significant proportion of this phase precipitated during the onset of fault-controlled subsidence and widespread hydrothermal circulation within the Irish Midlands area. Fluids up to ~250 °C and 10–15 wt% NaCl equivalent, sourced from a Lower Palaeozoic basement-equilibrated fluid reservoir, infiltrated the carbonate sequence via faults and fractures. The more localised development of dolomite-cemented breccias (white matrix breccias) that are frequently associated spatially with mineralisation was a consequence of the increased focusing of these hydrothermal fluids. Ore formation was broadly synchronous with development of the white dolomite breccias but only happened where mixing occurred between the hydrothermal ore-fluids and localised, near-surface reservoirs of low-temperature, H2S-rich brine. In the Waulsortian, this process led to the precipitation of a distinctive black dolomite that forms a broad halo to massive sulphides. Although ore-stage sulphides postdate significant diagenesis of the host rocks, and often display "epigenetic" textures, the fact that much of the cementation occurred soon after carbonate deposition means that mineralisation does not have to have formed after significant burial. In fact, the occurrence of clasts of hydrothermal dolomite and sulphides in intraformational debris-flow breccias is only consistent with mineralising processes occurring in the near-seafloor environment, relatively soon after host-rock deposition. The regional development of a distinctive pink dolomite associated with faults and fractures was a post-ore event, and is considered to mark a regional brine migration linked to the onset of the Variscan orogeny. The development of this new tectonic and flow regime may have been responsible for the cessation of economic mineralisation in Ireland.

CHLORINE ENRICHMENT AND HYDROUS ALTERATION OF THE SUDBURY BRECCIA HOSTING FOOTWALL Cu Ni PGE MINERALIZATION AT THE FRASER MINE, SUDBURY, ONTARIO, CANADA

An assemblage of hydrous, Cl-bearing alteration-induced minerals is present within 150 m of footwall-style Cu–Ni–PGE sulfide mineralization hosted by the Sudbury Breccia at the Fraser Copper zone, Fraser mine, Onaping–Levack area, Ontario. The pyroxene hornfels to hornblende hornfels contact-metamorphic aureole of the Sudbury Igneous Complex (SIC) contains less than 15 modal % hydroxysilicates. Near the mineralized areas, a diagnostic assemblage of secondary minerals of the albite–epidote hornfels facies is present in the matrix of the Sudbury Breccia; it contains 25 to 80% actinolite + chamosite as well as epidot e + sodic plagioclase (An2.8–42) + quartz ± titaniferous magnetite ± biotite ± K-feldspar ± titanite. The Cl content and Cl/(Cl + F) values of amphibole and biotite in this matrix are elevated within 150 m of mineralization, and increase toward known occurrences of sulfides. Parameters of K, Na, and A-site occupancy correlate with Cl content in actinolite in such matrix material. The value of Mg# is lower in biotite in the matrix near mineralized areas and is inversely correlated with Cl/(Cl + F) in the bioti te, reflecting Mg–Cl or Fe–F avoidance. Calculated temperatures of amphibole–plagioclase equilibration for the diagnostic hydrous assemblage are between 442 to 540 ± 75°C. The assemblage formed during a retrograde metamorphic event during or after footwall Cu–Ni–PGE mineralization, and after passage of peak conditions of contact metamorphism at the base of the SIC. The enrichment of Cl in this assemblage was the result of the increased amounts of free Cl relative to F in the equilibrating fluid , and favorable cation proportions for incorporation of Cl into the structure of hydroxysilicate minerals. The Cl-bearing assemblage accounts for a significant portion of the bulk halogen content of Sudbury Breccia matrix. It is unique to zones of Sudbury Brec cia that host mineralization, and hence may be used to identify prospective areas.

P-T-X conditions of hydrothermal fluids and precipitation mechanism of stibnite-gold mineralization at the Wiluna lode-gold deposits, Western Australia: conventional and infrared microthermometric constraints

The Wiluna lode-gold deposits are located in the Archean Wiluna greenstone belt, in the northern sector of the Norseman-Wiluna belt in the Yilgarn Craton of Western Australia. They are hosted in subgreenschist facies meta-basalts, and controlled by the Wiluna strike-slip fault system and associated shear veins and breccias. The 13 individual lode-gold deposits have produced around 115 t Au from 1901 to 1946 and 1986 to today. Historically, they also produced 38.3 t As and 3.5 t Sb. Gold formed in two stages: stage 1 gold-pyrite-arsenopyrite is finely disseminated in the wallrock and breccia fragments, whereas stage 2 gold-stibnite is located in massive shear veins and breccia matrix, as fracture-fill and in banded-colloform textured veins. Stibnite-gold orebodies only occur in some of the deposits (e.g., Moonlight and northern part of the West Lode) and also display a restricted vertical extent, being preserved only in the uppermost 200 m of stibnite-bearing lodes. Petrographic, conventional, and infrared microthermometric and laser-Raman analysis on stibnite-bearing quartz veins and breccias reveal that the antimony- and gold-rich hydrothermal fluid was of mixed H2O-NaCl-CO2±CH4 type. Microthermometric measurements reveal maximum homogenization temperatures of 340 °C (average 290±25 °C), and a wide range of salinities between 0.2 and 23 eq. wt% NaCl. Aqueous-carbonic fluid inclusions contain variable XCO2+CH4 (0.03 to 0.82), with the carbonic phase containing a maximum XCH4 of 0.21. Combined petrographic and microthermometric evidence suggests that the fluid inclusion properties reflect fluid immiscibility of a low-salinity, medium XCO2+CH4, homogeneous parent fluid at about 290 °C and pressures between 700 and 1,700 bar. Fluid immiscibility was triggered by cyclic pressure release during fault-zone movement. The decompression (adiabatic cooling) of the hydrothermal fluids shifted the ore fluid to lower temperatures, significantly reduced the degree of stibnite undersaturation, and caused stibnite to precipitate. The deposition of stibnite reduced the ore-fluid H2S concentration, thereby destabilized gold bisulfide complexes in solution, and caused gold precipitation locally. This mechanism explains the intimate spatial association of stibnite and gold in quartz veins and breccias in the stibnite-gold orebodies at Wiluna.

Surficial clay assemblage associated with the hydrothermal activity of Bouillante (Guadeloupe, French West Indies)

Surficial clay minerals as indicators of present-day geothermal activity were studied around Bouillante, Guadeloupe, French West Indies. The study is based on widespread sampling (>100 surface samples) in a zone extending 5 km north and 9 km south from Bouillante. Three types of mineral associations were distinguished: (1) dioctahedral smectites (in which the ‘beidellite–nontronite’ behavior predominates), with zeolites, calcite±quartz±kaolinite; (2) ordered illite–smectite mixed-layered clay with adularia, silica±calcite; and (3) kaolinite–smectite mixed-layered clays±halloysite±kaolinite±smectite±silica. The first association characterizes steaming grounds and areas subjected to hydrothermal fluid infiltrations along fractures or breccia zones in marine and subaerial environments. The second association was only identified in the matrix of epithermal breccias. This assemblage appears as a tracer of high-temperature fluids and is a good indicator of the proximity to the geothermal reservoir. The third assemblage was found both within and beyond the bounds of the known geothermal area, suggesting a pedogenic origin. We interpret the occurrence of the three assemblages in the Bouillante geothermal area as a background of clayey tropical weathering (kaolinite–smectite) with a hydrothermal overprint (dioctahedral smectites and illite–smectite).

Mud volcanoes in the Alboran Sea: evidence from micropaleontological and geophysical data

During the BASACALB-TTR9 cruise of the R/V Professor Logachev (1999), two mud volcanoes (called Marrakech and Granada) were discovered in the southern sector of the mud diapir province in the West Alboran Basin (WAB). This paper presents micropaleontological and geophysical data on these mud volcanoes from gravity core samples, sidescan sonar (OKEAN) images and high-resolution seismic lines. Mud breccia recovered from the Granada mud volcano is matrix-supported with well-consolidated clasts of limestone, marlstone, claystone, siltstone, sandstone and mudstone, whereas mud breccia from the Marrakech mud volcano contains unconsolidated clasts. The mud breccia matrix contains abundant Miocene calcareous nannofossils (CN), together with Pliocene–Pleistocene species and reworked late Cretaceous and Paleocene–Eocene species. CN dating indicates that clasts in the mud breccia derive from late Cretaceous, Paleocene, Eocene, and probable Miocene sediments. These data suggest that the mud diapirs and mud volcanoes in the WAB can be derived from the olistostromes of Unit VI, the basal stratigraphic sequence in the Alboran Sea basin. Unit VI consists of lower Miocene sediments that incorporated late Cretaceous and Paleocene–Eocene materials and basement-derived rock fragments. The mud volcanic deposits are covered by a thin drape of pelagic marls, suggesting that these two volcanoes are currently inactive. Structures determined on high-resolution seismic profiles across mud volcanoes and surrounding diapirs correspond to the late-stage, Pliocene-to-Quaternary diapir development. This stage is thought to have developed during a compressional tectonic setting that produced folding and wrench tectonics throughout the basin. Mud ascent at that time resulted in active diapirism and mud volcanoes on the seafloor.

Age of the Baula PGE mineralization (Orissa, India) and its implications concerning the Singhbhum Archaean nucleus

Abstract Two types of platinum group element (PGE) mineralization are identified in the mafic facies of the Baula Complex intruding the Gabbro-Anorthosite Unit of the Singhbhum Archaean nucleus. The first, Pt-dominated, is magmatic—the PGM are included in the magmatic silicates of a gabbro-norite intrusion (the Bangur Gabbro) cutting the Complex’s ultramafic facies and are found preferentially where the gabbro incorporates clasts of dunite and chromitite near the gabbro/ultramafics contact. The second, Pd-dominated, is hydrothermal—the PGM are associated with hydrous silicates located in the gabbro matrix of a magmatic breccia in which the blocks are from the Baula ultramafic facies. The originating fluids for the second type of mineralization were exsolved from the magma and concentrated PGE and volatile elements; we see no evidence of remobilization or external fluid contamination. A structural study has demonstrated that (a) the breccia was provoked by the intrusion of the upper part of the Bangur Gabbro, and (b) the breccia matrix corresponds to the Bangur Gabbro. Consequently, both types of PGE mineralization are associated with the same magmatic event. A zircon age of 3122±5 Ma has been obtained for the main Bangur Gabbro intrusion, and similar ages (3123±7 and 3119±6 Ma) have been obtained on the PGE-mineralized gabbro matrix of the breccia. This supports the hypothesis of a genetic link between the Bangur Gabbro with its Pt-dominated PGE and the breccia with its Pd-dominated PGE, giving a mean weighted age of 3121±3 Ma for the mineralization. Sm–Nd data on eight gabbro samples of the Baula Complex yielded a similar age of 3205±280 Ma. The dating of these events places them close to the broad-scale stabilization of the Singhbhum Archaean nucleus, estimated at 3.1 Ga, and defines one of the oldest PGE mineralizing events ever recorded. The Bangur Gabbro shows many affinities with the Gabbro-Anorthosite Unit of the Singhbhum Archaean nucleus in terms of age of emplacement and isotopic composition. It may be, therefore, that the Bangur Gabbro actually forms part of the Gabbro-Anorthosite Unit and that a fundamental control of its two types of PGE mineralization is the presence of ultramafic clasts.

CEMENTATION, HYDROTHERMAL ALTERATION, AND Zn-Pb MINERALIZATION OF CARBONATE BRECCIAS IN THE IRISH MIDLANDS: TEXTURAL EVIDENCE FROM THE COOLEEN ZONE, NEAR SILVERMINES, COUNTY TIPPERARY--A DISCUSSION

Sir: The origin of mineralization-associated breccias in the Irish Carboniferous is crucial to an understanding of Irish-type Zn-Pb deposits. One of the key lines of evidence that has been used to support a synsedimentary component to the Irish-type deposits has been the presence of mineralized clasts within the breccias, which were interpreted as being of sedimentary origin (Andrew, 1986; Ashton et al., 1986, 1992). However, if these breccias are, instead, of hydrothermal origin, then arguments for a synsedimentary mineralization component are invalidated (e.g., Peace and Wallace, 2000; Reed and Wallace, 2001). In this context Lee and Wilkinson (2002) have made a timely and important contribution to this problem of breccia origin. Lee and Wilkinson (2002) have advanced a case for mineralized sedimentary breccias at the Cooleen zone, Silvermines, suggesting that the black matrix breccias may have originally been debris flow breccias. We have no disagreement with Lee and Wilkinson’s basic observations, but suggest that these observations better support a hydrothermal origin for the brec-cias, as has previously been suggested for black matrix brec-cias elsewhere in Ireland (e.g., Hitzman et al., 1992). Specifically, we suggest that the breccias are the result of hydrothermal karsting that was directly associated with sulfide mineralization. Hydrothermal breccias are associated with mineralization at many localities throughout the Irish Midlands, and it is important that these breccias are not confused with genuine sedimentary breccias. The morphology and paragenesis of the breccias described by Lee and Wilkinson (2002) have a number of features in common with epigenetic hydrothermal breccias described from elsewhere in the Irish ore field, and from localities outside Ireland. Mineralized breccias from Ireland have been described from Galmoy, Lisheen, Silvermines, and …

Exploration at the Kuscayiri Au (Cu) prospect and its implications for porphyry-related mineralization in western Turkey

The Kuscayiri Au (Cu) deposit is localized along EW-trending linear silica-quartz structures or pods within altered Lower Miocene volcanics that are underlain by medium to high level Mesozoic granodiorite intrusions. The deposit is characterized by disseminated or fracture-controlled pyrite (up to 12%), variable amounts of silicification, hematite–pyrite matrix breccias produced by intense fracturing of siliceous rocks and moderate to weak saccharoidal to crystalline quartz stringer stockwork veining in small breccia pipes. An early quartz–sericite alteration event was focused along the EW-trending structures and resulted in the precipitation of pyrite. A later, more spatially extensive, advanced argillic alteration event overprinted the quartz–sericite event. The highest gold content is restricted to the high-temperature portion of the alteration zone characterized by stockwork or replacement quartz associated with sericite–pyrite–alunite–pyrophyllite. Trace gold and associated magnetite, arsenopyrite, realgar, specular hematite and chalcopyrite also occur within the smectite–illite/smectite–kaolinite–chlorite alteration facies. During the Tertiary, widespread subaerial to subaqueous arc volcanism developed in western Turkey, and numerous occurrences of high/low sulfidation epithermal systems are associated with major NW–SE extensional deformation. The resultant NE–SW-trending grabens appear to be highly favorable host environments for the formation of porphyry-related gold–copper systems. The Kuscayiri mineralization may represent a lithocap with Au (Cu), the top of one such system yet to be discovered.

The Carbonate-Hosted Lisheen Zn-Pb-Ag Deposit, County Tipperary, Ireland

The Lisheen Zn-Pb-Ag deposit, located in County Tipperary in southeast-central Ireland, consists of a group of strata-bound pyritic sulfide lenses containing 22 Mt of ore with an overall grade of 11.5 percent Zn, 1.9 percent Pb, 26 g/t Ag, and 16 percent Fe hosted in Lower Carboniferous (Early Mississippian) carbonate rocks. Sulfide bodies at Lisheen are restricted to the Waulsortian mudbank complex and the Lisduff oolite unit of the Ballysteen limestone, which compose the stratigraphically lowest, nonargillaceous carbonate horizons in the local Lower Carboniferous succession. The deposit is located within a major right-stepping relay zone of a segmented normal fault system. Sulfide bodies occur adjacent to the normal faults; thickest development of sulfides generally occurs at points of maximum throw. Metal zoning is well developed relative to these structures. The Waulsortian mudbank complex and portions of the overlying succession were subjected to an early, premineralization, regional-scale dolomitizing event that increased permeability. The regional dolostone produced by this event is cut and replaced by hydrothermal dolomite, referred to as black matrix breccia and white matrix breccia, associated with mineralization. Hydrothermal dolomitization was most intense in the Waulsortian mudbank complex, though it locally affected rocks from the Silurian basement to more than 200 m above the level of major sulfide precipitation. The majority of the Lisheen deposit consists of Fe-Zn-Pb-Ag sulfides hosted in the black matrix breccia, regional dolomite and, to a much lesser extent, undolomitized limestone of the Waulsortian complex. Sulfide bodies at Lisheen consist of pyrite-marcasite, sphalerite, and galena with subsidiary sulfosalt minerals, arsenopyrite, and copper sulfides. Sulfides replace carbonate minerals and infill dissolution cavities. Gangue minerals intergrown with sulfides include dolomite, calcite, barite, and quartz. Mineral textures indicate a regular paragenetic sequence of sulfide deposition from early precipitation of colloform iron sulfide containing minor sphalerite and galena through deposition of a complex assemblage of sphalerite, galena, iron sulfides, sulfosalt minerals, and copper sulfides. Distribution of alteration and sulfide zones indicates that the Lisheen deposit formed as a prograding hydrothermal system. Textural and mineralogic evidence suggests that the hydrothermal fluids were relatively acidic and that reaction with the carbonate host rocks was a primary reason for sulfide precipitation. On the basis of the probable age of faulting and crosscutting relationships of the regional and hydrothermal dolomite, the maximum age of mineralization at Lisheen was at least 10 m.y. after host-rock deposition.