Assemblage Of Pyrite Research Articles

The Role of a Transcrustal Shear Zone in Orogenic Gold Mineralization at the Ajjanahalli Mine, Dharwar Craton, South India

The Ajjanahalli gold mine is spatially associated with a Late Archean craton-scale shear zone in the eastern Chitradurga greenstone belt of the Dharwar craton, India. Gold mineralization is hosted by an ~100-m-wide antiform in a banded iron formation. Original magnetite and siderite are replaced by a peak metamorphic alteration assemblage of chlorite, stilpnomelane, minnesotaite, sericite, ankerite, arsenopyrite, pyrite, pyrrhotite, and gold at ca. 300° to 350°C. Elements enriched in the banded iron formation include Ca, Mg, C, S, Au, As, Bi, Cu, Sb, Zn, Pb, Se, Ag, and Te, whereas in the wall rocks As, Cu, Zn, Bi, Ag, and Au are only slightly enriched. Strontium correlates with CaO, MgO, CO2, and As, which indicates cogenetic formation of arsenopyrite and Mg-Ca carbonates. The greater extent of alteration in the Fe-rich banded iron formation layers than in the wall rock reflects the greater reactivity of the banded iron formation layers. The ore fluids, as interpreted from their isotopic composition ( δ 18O = 6.5–8.5‰ initial 87Sr/86Sr = 0.7068–0.7078), formed by metamorphic devolatilization of deeper levels of the Chitradurga greenstone belt. Arsenopyrite, chalcopyrite, and pyrrhotite have δ 34S values within a narrow range between 2.1 and 2.7 per mil, consistent with a sulfur source in Chitradurga greenstone belt lithologies. Based on spatial and temporal relationships between mineralization, local structure development, and sinistral strike-slip deformation in the shear zone at the eastern contact of the Chitradurga greenstone belt, we suggest that the Ajjanahalli gold mineralization formed by fluid infiltration into a low strain area within the first-order structure. The ore fluids were transported along this shear zone into relatively shallow crustal levels during lateral terrane accretion and a change from thrust to transcurrent tectonics. Based on this model of fluid flow, exploration should focus on similar low strain areas or potentially connected higher order splays of the first-order shear zone.

Volcanism, sedimentation and massive sulfide mineralization in a Late Cretaceous arc-related basin, Eastern Taurides, Turkey

Late Cretaceous arc volcanism and associated massive sulfide mineralizations occur in the Eastern Taurus Mountains. The mineralizations are related to felsic (dacitic–rhyolitic) volcanic rocks which, together with basement rocks, form the lowermost levels of a 5000 m thick volcano-sedimentary sequence, and are restricted to a few levels within the sequence. Basaltic rocks, which are intruded by tonalite and dacite, form the base of the volcano-sedimentary sequence. The dacitic–rhyolitic rocks are overlain by a volcano-sedimentary succession with andesitic–dacitic flows. The flows do not contain any mineralization. This succession starts with volcanic breccia and tuffs at the base, passing into crystal-rich sandstone, and pelagic mudstone with tuffite intercalations, and ends with marls intercalated with sandstones basalt and peridotite olistoliths. The felsic rocks bear stockwork and disseminated types of mineralization. The massive sulfide mineralization has a simple mineral assemblage of pyrite, chalcopyrite, bornite and sphalerite. Optical microscopic examination of pyrites from Derince Massive Sulfide Deposits reveals that the pyrites are commonly seen in framboidal and cubic forms. The geochemistry of the magmatic rocks and sedimentary facies associations indicate that the mineralizations in the Derince were developed in an intra-arc environment.

Geology, composition, and genesis of the Sukhoi Log noble metals deposit, Russia

With resources estimated at 1100 t Au at an average grade of 2.45 g/t, the black shale-hosted Sukhoi Log noble metals deposit is the largest concentration of gold in Russia. The Au mineralization and associated platinum group element (PGE) mineralization of the Sukhoi Log deposit is controlled by a tectonic zone along the axis of an overturned anticline located in metamorphosed carbon-bearing Upper Proterozoic sedimentary rocks. Organic compounds have been carbonatized by hydrothermal fluids. The centro-symmetric zoning of the mineralization is typical of infiltration metasomatism whereas disseminated and veinlet gold deposits accompanied by a quartz, carbonate, and pyrite assemblage were superimposed on sedimentary rocks. Nearly 90 minerals occur at Sukhoi Log including native metals, intermetallic alloys, sulfosalts, phosphates, tungstates, and oxides. Native gold is the main one mineral, but calaverite, hessite, petzite, and krennerite also occur. PGE mineralization is located on the periphery of the gold mineralization in the form of native platinum, Pt–Cu–Fe metal alloys, sperrylite, and cooperite. The mineral assemblage also includes native Fe, Cr, Cu, Ni, Sn, W, Ti, and Te, as well as alloys of these metals. The ore-forming process involved fluid temperatures of 130–395 °C, pressures of 0.2–2.4 kbar, and salt concentrations of 3.7–9.5 wt.% NaCl equiv. Mineralogical, geochemical, and geophysical studies were used to develop a three-stage genetic model of the deposit involving deposition of terrigenous and carbon-bearing terrigenous-carbonate rocks in an intraplate rift about 800 Ma followed by regional metamorphism at 520 Ma culminating in the development of palingenetic granites and hydrothermal fluids transporting contrasting assemblages of elements—(Cr, Ni, Ti, Pt+Pd) and (Sn, W, Mo, Zr, Au, Ag, Zn+Pb)—at 320 Ma.

Geology and volcanic stratigraphy of the Canatuan and Malusok volcanogenic massive sulfide deposits, southwestern Mindanao, Philippines

The Canatuan and Malusok massive sulfide deposits are located near Siocon, Zamboanga del Norte, in southwestern Mindanao, Philippines. The Canatuan–Malusok area is underlain by the Jurassic–Cretaceous Tungauan schists, which form much of the Zamboanga Peninsula. The volcanic strata at Canatuan and Malusok can be traced for >7 km along strike and is host to at least three discrete massive sulfide bodies: Canatuan, Malusok and SE Malusok. Basal basaltic andesite volcanic rocks are generally chemically uniform and show only moderate alteration. The massive sulfide deposits occur in overlying rhyolitic to rhyodacitic volcanic rocks that are altered to a schistose assemblage of quartz, sericite, chlorite and pyrite. The alteration is texturally destructive but graded clastic beds are locally observed. Despite tropical saprolitic weathering, four lithogeochemical subunits of the felsic package are identified. Stratigraphic interleaving, however, has made correlation of these units over any significant distance difficult. The sulfide lenses are overlain by a few metres of felsic schists which locally contain manganese-bearing silicates and oxides that serve as a stratigraphic marker. Hangingwall andesitic volcaniclastic rocks are discontinuously preserved, although where present, they consist of regularly bedded mafic volcanic sandstones. The lateral continuity of a manganese-bearing marker and flanking felsic volcaniclastic intervals indicate that locally the volcanic strata form a homoclinal sequence. The Canatuan Au–Ag–Cu–Zn deposit consists of a gossan overlying a massive sulfide lens. The sulfides and gossan are flat lying and hosted within felsic volcanic rocks. The gossan is gold–silver-rich, and was formed by a combination of oxidation and volume collapse of the original sulfide lens. The sulfide minerals present below the current water table, are auriferous massive pyrite with base metal sulfides, with some supergene chalcocite. The transition from gossan to sulfides is very sharp, occurring at the water table. Massive sulfide deposits at Malusok are hosted in the same felsic sequence as Canatuan and they have similar base and precious metal contents. Only limited gossan has been found at Malusok. The bimodal nature of the volcanic rocks at Canatuan, together with their low HFSE contents, near-flat REE patterns and tholeiitic affinities, suggest that they formed in an intra-oceanic arc setting above a depleted mantle source. Mafic and felsic volcanic rocks of similar composition have been recovered from the Tonga-Kermadec and Izu-Bonin-Marianas island-arc systems in the western Pacific. Mafic rocks at Canatuan show no evidence for LILE enrichment that characterizes melts derived from metasomatized mantle under more mature arcs, suggesting that they are the product of a nascent, rather than a mature arc. There is no evidence from the REE, or other incompatible trace elements, that continental crust or evolved arc crust was involved in the generation of the Canatuan-Malusok volcanic rocks. Although it has been proposed that the Zamboanga metamorphic complex comprises microcontinental fragments of Eurasian affinity, our data do not support an evolved crustal setting for the Canatuan-Malusok volcanic rocks, which we suggest were derived from an intra-oceanic arc and subsequently accreted to the eastern Mindanao terrane.

Predicted equilibrium constants for solid and aqueous selenium species to 300 °C: applications to selenium-rich mineral deposits

In this study, an attempt has been made to assess aqueous speciation of selenium and solubility product constants of common selenides at elevated temperatures (up to 300 °C) by using various extrapolation methods. This study predicts that reduced selenium species are dominant species in many geological processes even under relatively oxidized conditions such as those dictated by the magnetite–hematite buffer. On the basis of extrapolated equilibrium constants and solubility product constants for common Se-bearing mineral phases, critical ∑Se/∑S ratios (molal ratios) in mineralizing fluids are proposed for independent selenium mineralization. The minimum ∑Se/∑S ratios in mineralizing fluids for independent selenium mineralization should be at least ∼10 −6, ∼10 −5 and ∼10 −4 at 100, 200 and 300 °C, respectively. For giant independent selenium deposits such as the La'erma and Qiongmo Au–Se deposits in the western Qingling mountains, and Yutangba Se deposits in Hubei Province, China, the mineralizing fluids have reached much higher ∑Se/∑S ratios ranging from ∼10 −1 to ∼10 −3 at 200 °C. This study also suggests that the equilibrium assemblage of pyrite–ferroselite among the common ore minerals requires the highest ∑Se/∑S ratios in mineralizing fluids, followed in decreasing order by the assemblages of stibnite–antimonselite, galena–clausthalite, cinnabar–tiemannite, and acanthite/argentite–naumannite. The assemblage of pyrite–ferroselite can also be formed under relatively oxidizing conditions where [∑H 2Se]/[∑H 2S] ratios can be high enough for the formation of independent ferroselite.

Hydrothermal alteration in metasedimentary rock-hosted orogenic gold deposits, Reefton goldfield, South Island, New Zealand

Orogenic or mesothermal quartz lodes in lower Palaeozoic Greenland Group metasedimentary rocks of the Reefton area have produced 67 tonnes (t) of gold prior to 1951, and recent exploration has identified new gold resources in several deposits, including the largest past producers, Blackwater and Globe-Progress. The metasedimentary rocks consist of alternating sandstone and mudstone beds that were metamorphosed to lower greenschist facies prior to being hydrothermally altered adjacent to the quartz lodes. The sandstones are feldspathic litharenites averaging Q65–F10–R25, with detrital grains of quartz, rock fragments, muscovite, and plagioclase and biotite that were altered to albite and chlorite, respectively, during metamorphism. Accessory minerals are graphite, apatite, zircon, tourmaline and titanite. Hydrothermal alteration of the sandstones has developed a mineral assemblage of K-mica, carbonate (dolomite, ankerite, ferroan magnesite and magnesian siderite), chlorite, pyrite and arsenopyrite. The abundance of hydrothermal chlorite is greater at Blackwater than at the other prospects studied. Hydrothermal alteration associated with the quartz lodes is marked by bleaching, magnesian siderite spots, disseminated arsenopyrite and pyrite and thin carbonate, quartz and sulphide veins. These trends are accompanied by increasing concentrations of S, As and Sb and decreasing Na, and a decrease of Fe and Mg in K-mica. The alkali alteration indices 3K/Al (representing K-mica) and Na/Al (representing albite) generally show antipathetic trends, with 3K/Al increasing near the lodes and Na/Al decreasing. These trends reflect the replacement of albite by K-mica. Carbonate alteration indices CO2/(Ca + Mg +Fe) and CO2/[Ca + Mg + Fe –0.5(S + As)] quantify the abundance of hydrothermal carbonates, but they show variable correlation with the lodes. They increase the width of the alteration halo in the hanging wall of the lodes at the Globe-Progress and General Gordon prospects, but the peak values are as far as 150 m from the lodes. By contrast, peak values of the carbonate alteration indices are within 10 and 2 m of the lodes, respectively, at the Merrijigs and Blackwater deposits. Data show that for deposits with wide hydrothermal alteration halos, such as at the Globe-Progress and General Gordon prospects, the use of a suite of geochemical indicators can assist exploration by indicating trends in hydrothermal alteration that provide vectors to mineralisation. They also increase the size of the exploration target. By contrast, the alteration halo of the Blackwater deposit is restricted to within less than 5 m of the quartz lode and, therefore, the geochemical indicators are of more limited assistance to exploration.

Hypogene violarite of exsolution origin from Mount Keith, Western Australia: field evidence for a stable pentlandite–violarite tie line

AbstractIn most documented occurrences, violarite (FeNi2S4) occurs as a product of the supergene alteration of primary pentlandite or millerite. Earlier experimental phase relations studies predicted the possible existence of a stable violarite–pentlandite tie line, though there has been little field evidence supporting this hypothesis, and the preferred topology in the Ni-Fe-S system involves a pyrite–millerite tie line. This paper documents the occurrence of violarite-pentlandite±pyrite assemblages which, on the basis of mineral chemistry and textural evidence, appear to be hypogene. Primary cobaltian violarite (with 2.1–13.2 wt.% Co) occurs as lamellae in pentlandite in the MKD5 nickel sulphide orebody at Mount Keith, central Western Australia. These lamellae are interpreted to be of exsolution origin. Cobalt is preferentially partitioned into violarite, resulting in high Ni:Co ratios in the associated pentlandite relative to pentlandite in violarite-free assemblages. Hypogene violarite-millerite±pentlandite assemblages were also noted. In all hypogene assemblages, violarite differs in both textural and mineral chemical characteristics from supergene violarite from the upper portions of the MKD5 orebody. The implications of the assemblages for the known low-temperature phase relations in the Ni-Fe-S-(Co) system are discussed.

Geology and gold–molybdenum porphyry mineralisation of the Archean Taschereau–Launay plutons, Abitibi, Quebec

Porphyry gold deposits constitute a new challenge for mineral exploration in Archean greenstone belts, yet the relationships between intrusive rocks and mineralisation remain poorly understood. The Messegay gold occurrence is situated in the Taschereau–Launay plutonic complex of the Northern Volcanic Zone of the Abitibi greenstone belt. The Taschereau intrusion (2718±2 Ma) represents a complex network of sills composed of tonalite with minor amounts of diorite, monzonite and gabbro. The Launay granite, which intrudes the Taschereau pluton, is late-orogenic and its emplacement was controlled by an early reverse ductile N–S shear zone. Intrusive rocks have been subjected to four episodes of alteration: epidote–chlorite–carbonate, magnetite–hematite–microcline, albite–pyrite, and sericite–quartz. Shear zone hosted and metasomatic Au–Mo mineralisation is mainly associated with albite–pyrite assemblages, but also occurs within zones of intense magnetite–hematite–microcline and sericite–quartz alteration. The Messegay gold occurrence is interpreted as a new type of disseminated gold–molybdenum porphyry deposit associated with a zone of magmatic differentiation in a magnetite-series pluton.

Petrological significance of manganese carbonate inclusions in spessartine garnet and relation to the stability of spessartine in metamorphosed manganese-rich rocks

Idioblastic spessartine garnet pervasively developed in Mn-rich rocks and impure manganese carbonate ore at the Lower Proterozoic Nsuta manganese deposit, Western Ghana, contains abundant inclusions of micritic and microconcretionary carbonates and, to a lesser extent, quartz. Detailed mineralogical and microprobe studies indicate all the carbonate phases (i.e. carbonate inclusions in garnet, carbonates coexisting with garnet and carbonates not directly in contact with garnet, the latter hereafter referred to as matrix carbonates) lie within the rhodochrosite–kutnahorite solid solution series, i.e. ~Mn55–80(Ca + Mg)20–45CO3 to Ca42(Mn + Mg)58(CO3)2. Minor compositional differences occur in the various carbonate phases, but partition of major elements among coexisting phases indicate most carbonate minerals strongly fractionate Ca and Mg over coexisting spessartine. The nature, composition and textural relationship of coexisting minerals and inclusions in porphyroblastic spessartine indicate that the latter formed from metamorphic reactions in which rhodochrosite and/or kutnahorite and quartz were consumed, in part corroborating earlier observations on a rhodochrosite precursor for spessartine. Spessartine formation is thus envisaged to have taken place when the predominantly Mn carbonate–quartz assemblage became unstable in the presence of minor amounts of an unknown aluminous phase. Because all the carbonates appear to be low-temperature phases with no indications of significant recrystallisation or homogenisation, it could be argued that the spessartine + rhodochrosite ± kutnahorite ± quartz ± pyrite assemblage stabilised during very low-grade greenschist facies metamorphism under relatively low but uniform f O2 conditions. These observations also suggest the stability field of spessartine could extend to relatively lower temperatures than currently envisaged.

Regional Fluid Flow and Gold Mineralization in the Dalradianof the Sperrin Mountains, Northern Ireland

Gold vein mineralization occurs in the metamorphosed and deformed Dalradian (Neoproterozoic) rocks of the Sperrin Mountains, Northern Ireland. Two structures exerted a control on the location of the mineralization; the north-south Omagh lineament and the west-northwest-east-southeast Curraghinalt lateral ramp in the footwall of the northeast-southwest Omagh thrust. These are Caledonian structures resulting from the thrusting of Dalradian rocks over a possibly still active Ordovician arc. Cathodoluminescence microscopy distinguishes four phases of vein quartz in the Curraghinalt gold prospect. Fluid inclusion studies and stable isotope geochemistry have defined the probable fluids responsible for the precipitation of each quartz phase and associated sulfide and precious metal mineralization. The initial phase (Q1) appears to have been associated with the main Caledonian metamorphic event (ca. 470 Ma) and is nonauriferous. The second phase (Q2) forms an extensive cement to brecciated early quartz and is believed to have involved a fluid (~15 wt % CO 2 , 10 wt % NaCl + KCl equiv) with a significant magmatic component of 470 to 400 Ma, which underwent phase separation and dilution with a cooler formation water. This process resulted in precipitation of the main phase of gold mineralization characterized by an assemblage of electrum, pyrite, arsenopyrite, chalcopyrite, tennantite-tetrahedrite, and various tellurides. Similar fluids are observed on a regional scale, concentrated within the hanging wall of the Omagh thrust, indicating an extensive fluid-flow event. The relative abundance of gold at the Curraghinalt and Cavanacaw prospects is thought to be due to higher fluid fluxes in favorable zones of dilation and closer proximity to the fluid source. The deposit was subsequently reactivated with the precipitation of later quartz (Q3-Q4) from a formation water believed to be resident in the Dalradian metasediments, which mixed with a low-temperature, high-salinity basinal brine, probably during Carboniferous basin inversion. Brine flow resulted in the remobilization of earlier electrum, reducing its fineness, and also introduced base metal sulfides, carbonates, and barite. Again, brine flow is localized by the Omagh thrust, indicating the long-lived role of this structure in controlling regional fluid migration.

Petrography, mineralogy and geochemistry of the Zarshuran Carlin-like gold deposit, northwest Iran

Gold mineralisation at Zarshuran, northwestern Iran, is hosted by Precambrian carbonate and black shale formations which have been intruded by a weakly mineralised granitoid. Granitoid intrusion fractured the sedimentary rocks, thereby improving conditions for hydrothermal alteration and mineralisation. Silicification is the principal hydrothermal alteration along with decalcification and argillisation. Three hydrothermal sulphide mineral assemblages have been identified: an early assemblage of pyrrhotite, pyrite and chalcopyrite; then widespread base metal sulphides, lead-sulphosalts and zoned euhedral arsenical pyrite; and finally late network arsenical pyrite, massive and colloform arsenical pyrite, colloform sphalerite, coloradoite, and arsenic–antimony–mercury–thallium-bearing sulphides including orpiment, realgar, stibnite, getchellite, cinnabar, lorandite and a Tl-mineral, probably christite. Most of the gold at Zarshuran is detectable only by quantitative electron microprobe and bulk chemical analyses. Gold occurs mainly in arsenical pyrite and colloform sphalerite as solid solution or as nanometre-sized native gold. Metallic gold is found rarely in hydrothermal quartz and orpiment. Pure microcrystalline orpiment, carbon-rich shale, silicified shale with visible pyrite grains and arsenic minerals contain the highest concentrations of gold. In many ways Zarshuran appears to be similar to the classic Carlin-type sediment-hosted disseminated gold deposits. However, relatively high concentrations of tellurium at Zarshuran, evidenced by the occurrence of coloradoite (HgTe), imply a greater magmatic contribution in the mineralising hydrothermal solutions than is typical of Carlin-type gold deposits.

Organic petrology, chemical composition, and reflectance of pyrobitumen from the El Soldado Cu deposit, Chile

Solid pyrobitumen (residual petroleum) is found intimately associated with sulfides in several manto-type (strata-bound) Cu deposits of central Chile. El Soldado (lat 32°38.8′S, long 71°06.7′W) is one of the largest deposits of this type (>130 Mt @ 1.5% Cu) and is hosted by volcanic and subvolcanic rocks of the Cretaceous Lo Prado Fm. Detailed organic petrology of authigenic sulfide inclusions, degassing vesicles, and anisotropic pyrobitumen and graphitic carbon allowed the elucidation of a complex diagenetic history of a biodegraded reservoir in faulted volcanic and subvolcanic rocks. Due to the overmaturity of the host sequence, biomarker techniques (aliphatic and aromatic) were unsuitable to investigate the composition of the pyrobitumen. However, microprobe and laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) analyses, Rock-Eval pyrolysis, and 13 C NMR techniques were used to ascertain the composition of the pyrobitumen. Spatial variations in the maturity of the pyrobitumen were assessed using both random reflectance ( R o) and rotational reflectance ( R omax and R omin). The data are consistent with the following evolution for El Soldado: As a result of burial in the Cretaceous back-arc basin, petroleum was generated from organic-rich shales in the underlying lower Lo Prado Fm. and migrated into primary and structural porosity in the host rocks, the predominantly volcanic upper Lo Prado Fm. Low-temperature (<90°C) biodegradation of petroleum developed an early assemblage (Stage I assemblage) of pyrite (±sphalerite±chalcopyrite). Geopetal structures related to pressure degassing of semisolid petroleum suggest that migration occurred while the strata were horizontal, at ca. 130 Ma. Continued basinal burial led to thermal degassing, increased maturation and contraction of the petroleum into pyrobitumen. Around 20 Ma later an influx of high-temperature (ca. 250–350°C) Cu-rich fluids induced by regional granitoid intrusion (Stage II assemblage) replaced preexisting pyrite and pyrobitumen. The mineralizing fluids altered the pyrobitumen physically and chemically: (a) increasing R o to greater than ca. 5% in the core of the orebodies against a background level of ca. 3%; (b) developing anisotropy (nongranular, fine to coarse mosaic, and coarse flow mosaic) within the pyrobitumen and locally graphitized the pyrobitumen, developing graphitic carbon; and (c) incorporating Cl (±Cu±Fe) into the pyrobitumen during the development of anisotropy.

The role of organic matter in the genesis of the El Soldado volcanic-hosted manto-type Cu deposit, Chile

Solid bitumen (residual petroleum) occurs intimately associated with sulfides in several manto-type Cu deposits of central Chile. El Soldado (32 degrees 38'S; 71 degrees 04'W) is one of the largest manto-type deposits (>130 million metric tons1.5% Cu) and is hosted by rhyolites and andesites of the Lower Cretaceous Lo Prado Formation.As a result of basinal burial in the Early Cretaceous back-arc basin, petroleum was generated from organic-rich shales in the underlying lower Lo Prado Formation and migrated into primary and structural porosity in the upper Lo Prado Formation. Migration was controlled by extensional basinal faults. Low-temperature (<90 degrees C) biodegradation of petroleum developed an early stage I assemblage of (mostly framboidal) pyrite (+ or - sphalerite + or - chalcopyrite). Geopetal structures related to pressure degassing of semi-solid petroleum suggest that migration occurred while the strata were horizontal, at ca. 130 Ma. Continued basinal burial led to thermal degassing, increased maturation, and solidification of the petroleum.Low-grade regional (pumpellyite-prehnite) metamorphism of the sequence was followed, at ca. 110 to 103 Ma, by the influx of high-temperature (ca. 300 degrees C), Cu-rich fluids concurrently with emplacement of the Cretaceous batholith into the basin. Basinal fluids were focused into a now tilted sequence by a brittle shear (north-south to north-northwest) fault system. Copper sulfides (stage II assemblage) replaced preexisting pyrite and bitumen, forming zoned orebodies with a central chalcocite-hematite zone, followed outward by bornite-chalcocite, chalcopyrite-bornite, and pyrite-chalcopyrite zones. The mineralizing fluids altered the bitumen thermally and chemically as follows: (1) reflectance (R o ) increased up to ca. 5 percent in the core of the orebodies against a background of ca. 3 percent; (2) anisotropy (nongranular, microscale, and domain) developed and the bitumen was locally graphitized; (3) chlorine (+ or - Cu + or - Fe) was incorporated into the bitumen affecting its reflectance. The source of Cu was most likely the overlying oxidized volcanic pile; the Cu was transported as chloride (CuCl) complexes, precipitating where the CuCl-rich solutions reacted with bitumen (adsorption/reduction) and pyrite from the stage I assemblage.The study demonstrates that in the Lower Cretaceous basin of Chile, degraded petroleum reservoirs can be important controls for metallic mineralization derived from hydrothermal solutions of different sources, particularly if biodegradation generated pyrite.

Economic potential of the ultramafic rocks of Jamaica and Tobago: two contrasting geological settings in the Caribbean

Ultramafic rocks in Jamaica are dunites with minor lherzolite, often serpentinised, and are part of a dismembered ophiolite complex. In Tobago, dunites, wehrlites, pyroxenites and hornblendites form the lower part of a plutonic complex of island arc affinity. The mineral assemblages and chemistry reflect these differences. Chromite in Jamaica is high in Al and Mg, whereas in Tobago it is rich in Fe, as in Alaskan-type intrusives. Ni-Cu-PGE assemblages in Jamaica are pentlandite, with later low temperature heazlewoodite, awaruite and native copper, the latter with Pt and Pd. In Tobago an assemblage of pentlandite, pyrrhotite, pyrite and chalcopyrite is much less affected by later alteration. PGE phases also occur. The dunites in Jamaica have sufficient MgO to be a potential source of olivine. The higher Fe in olivine from Tobago indicates that olivine cumulates in plutonics from island arc settings are a less suitable source of the mineral. Ni-laterites in Jamaica are unlikely because of high topographic relief. The prospect for Ni-laterites in Tobago is low as there is little Ni in the olivines. Chrysotile asbestos, talc and magnesite are absent in both islands. This is probably a consequence of the lack of secondary serpentine recrystallisation to form fibrous chrysotile veins, the deep tectonic level and lack of hydrothermal circulation for magnesite to form, and the absence of metamorphic/metasomatic events and/or late stage extension tectonics which might have yielded talc.

Alteration-mineralization zoning and fluid inclusions of the high sulfidation epithermal Cu-Au mineralization at Zijinshan, Fujian Province, China

The Zijinshan mine in Fujian Province in the southeast of China is the first recognized example of high sulfidation epithermal Cu-Au mineralization of Cretaceous age in mainland China. The Cu-Au mineralization occurs around a dacitic volcanic pipe in the central part of a calc-alkaline, trachydacitic to dacitic volcanic dome of Cretaceous age that has intruded an S-type Jurassic granite. The Cu-Au orebodies are subparallel, northwest-trending breccias, veins, and massively silicified rocks which have developed along extensional fractures in wide hydrothermal alteration zones. At depth the dacite porphyry pipe changes downward into a potassic-altered granodiorite porphyry associated with porphyry copper-style mineralization, indicating a close spatial and temporal association between high sulfidation Cu-Au and porphyry copper mineralization.Zoning of hydrothermal alteration and ore mineralization assemblages at Zijinshan is typically recognized from the deep and central zones toward shallower and outer portions within the epithermal system. A deep sericite + quartz + pyrite assemblage in the phyllic alteration zone gives way upward to a dickite + quartz + pyrite + or - zunyite + or - sericite assemblage in the dickite alteration zone and successively to the alunite + quartz + pyrite assemblage in the funnel-shaped alunite alteration zone at depths between 350 and 1,200 m from the land surface. High sulfidation copper mineralization is typically developed within the alunite alteration zone and shows a gradual change of ore mineralogy from digenite and enargite preferentially at deeper levels to covellite and gold in shallow levels. The silicic alteration and associated gold mineralization are restricted at shallow depths (less than 350 m). Vertical metal zoning in the sequence Au-Ag-As --> Cu --> Cu-Pb-Zn is also recognized within the silicic and alunite alteration zones.Paleogeographic reconstruction indicates depths of 2 was probably formed by phase separation of a deep magmatic fluid and subsequently ascended northwestward. The upwelling magmatic vapors condensed into meteoric water at shallow levels (higher than 1,700 m below the paleosurface), forming the high sulfidation epithermal fluids (

Age of Cu(-Fe)-Au mineralization and thermal evolution of the Punta del Cobre district, Chile

The Punta del Cobre belt is located 15 km south of Copiapo, northern Chile. It comprises several Cu(-Fe)-Au deposits in the Punta del Cobre and Ladrillos districts, east of the Copiapo river, and the Ojancos Nuevo district, with the new Candelaria mine, and Las Pintadas district, west of the river. The mineralization in the Punta del Cobre belt is characterized by a simple hypogene mineral assemblage of chalcopyrite, pyrite, magnetite, and hematite. Average ore grades are 1.1 to 2% Cu, 0.2 to 0.6 g/t Au, and 2 to 8 g/t Ag. Massive magnetite occurs as veins and irregularly shaped bodies. The ore is spatially associated with alkali metasomatism and in particular with potassic alteration. The Cu(-Fe)-Au deposits are hosted mainly in volcanic rocks of the Punta del Cobre Formation (pre-upper Valanginian) that underlie Neocomian limestones of the Chanarcillo Group. This region experienced backarc basin formation in the Neocomian, uplift and granitoid intrusions in the middle Cretaceous, and eastward migration of the magmatic front of about 30 km between middle Cretaceous and Paleocene. To determine the timing of ore deposition and to reconstruct parts of the thermal history of the Punta del Cobre district, in the eastern part of the belt, we have obtained 40Ar/39Ar incremental-heating and Rb-Sr analyses of mineral and whole-rock samples. An 40Ar/39Ar incremental-heating experiment on hydrothermal biotite, formed synchronous with the Cu(-Fe)-Au mineralization, yielded an inverse isochron age of 114.9 ± 1.0 Ma (all errors reported at ±2σ), consistent with a Rb-Sr isochron of 116.8 ± 2.7 Ma calculated from 7 whole-rock samples. These data are interpreted to represent the age of potassic alteration that accompanies mineralization. Ore formation temperatures of 400 °C to 500 °C were previously estimated based on paragenetic relationships. Shearing at the Candelaria deposit occurred after ore deposition and before the main stage of batholith emplacement. Published K-Ar ages for the middle Cretaceous batholith near the Punta del Cobre belt range from 119 to 97 Ma. Our data suggest that the mineralization is related to the earlier stages of batholith emplacement. The biotite age spectrum indicates that the Punta del Cobre district was not affected by temperatures above ∼300 °C–350 °C, the closure temperature for argon in biotite, during the contact metamorphic overprint produced by later emplaced batholithic intrusions. Whole-rock 40Ar/39Ar ages are considerably younger; incremental-heating experiments yielded an inverse isochron age of 90.7 ± 1.2 Ma and weighted mean plateau ages of 89.8 ± 0.6 Ma and 89.5 ± 0.6 Ma. These samples are dominantly K-feldspar, for which we assume an argon closure temperature of ∼150 °C, thus they give the age of cooling below ∼150 °C–200 °C.

Geology, lithogeochemistry, and alteration of the Battle volcanogenic massive sulfide zone, Buttle Lake mining camp, Vancouver Island, British Columbia

Volcanogenic, polymetallic massive sulfide deposits within the Buttle Lake mining camp occur within the felsic volcanic and volcaniclastic rocks of the Myra formation that immediately overlies Price formation andesite. These units comprise the lower exposed part of the Paleozoic Sicker Group in the Buttle Lake uplift in central Vancouver Island. The Sicker Group represents a volcanic island arc that forms the base of the allochthonous Wrangell terrane.The Battle Main zone contains 2.3 million tons (Mt) of proven and probable reserves grading 1.3 g/t Au, 25.3 g/t Ag, 2.5 percent Cu, 0.5 percent Pb, and 13.2 percent Zn. Overall, the Buttle Lake camp hosts geologic reserves of about 12.5 Mt. The Battle Main massive sulfide zone occurs within the H-W horizon, the lowest member of the Myra formation, immediately above the Price formation andesite.Price formation andesite is over 300 m thick in the vicinity of the camp and consists of feldspar + or - pyroxene porphyritic basaltic andesite flows. Synvolcanic block faulting of this unit formed the regional-scale Buttle Lake camp basin with a strike length of at least 10 km. Local sub-basins in this major structure contain the sulfides and felsic volcanic rocks of the H-W horizon.The H-W horizon is a 15- to 200-m-thick felsic package that extends throughout the basin. It consists of eight members, three of which represent periods of massive sulfide deposition. From the stratigraphic base to the top of these members are (1) the Battle Main massive sulfide lens, (2) fine rhyolitic tuffaceous deposits, (3) H-W mafic sills, (4) Gap massive sulfide lens, (5) coarse rhyolite pyroclastic deposits, (6) rhyolite tuffaceous and cherry sediments, (7) Battle Upper massive sulfide lenses, and (8) a tripartite rhyolite flow-dome complex consisting of quartz porphyritic rhyolite, quartz-feldspar porphyritic rhyolite, and green quartz-feldspar porphyritic rhyolite. Volcaniclastic units in the felsic package evolved with interfingering of subaerial and submarine pyroclastic processes. Flow-dome units evolved by fractionation of quartz and feldspar.Alteration, evaluated petrographically and with Pearce element ratios, is most intense close to synvolcanic and synmineral feeder faults that channeled solutions through the Price formation andesite proximal to the Battle Main massive sulfide lens. Envelopes to the feeder faults have distinctive Mg addition anomalies and contain the mineral assemblage pyrite > sericite > quartz > chlorite. Discharge feeder stockworks immediately below the Battle Main massive sulfide lens contain varying proportions of sericite, quartz, and pyrite but lack significant chlorite. Deposition of the H-W horizon rhyolitic units on top of the Battle Main massive sulfide lens did not halt mineralization. Rather, fluids continued to percolate upward through the newly deposited rhyolitic units and deposited the Battle Upper lenses close to the new sediment-water interface. Alteration below the Battle Upper lenses consists of quartz, sericite, and pyrite with minor galena, sphalerite, and tennantite. Overall, hydrolysis of feldspar in both the Price formation and the H-W horizon resulted in a halo surrounding the ore lenses marked by addition of K and concomitant Na and Ca depletion.

Modelling of equilibrium and non equilibrium segregation to grain boundaries : R.G. Faulkner (Loughborough University of Technology, Loughborough, UK)

The volcanic geothermal system of Olkaria in Kenya has been extensively drilled for development purposes. The well data show that the reservoir is liquid-dominated. Often downhole temperatures in thermally stabilized wells follow the boiling point curve with depth to at least 2 km. In one part of the field (Olkaria East), a vapor-dominated cap existed above the liquid reservoir when the first deep wells were sunk in the area. Many of the producing wells have excess discharge enthalpy, i.e. the enthalpy is higher than that of steam-saturated water at the temperature of producing aquifers. In some cases, inflow from feed zones in the vapor cap may contribute to the excess enthalpy. Geothermometer results indicate, however, that excess enthalpy is dominantly produced by phase segregation in the depressurization zone around producing wells, the liquid being partially retained in the aquifer by its capillary adsorption onto mineral grain surfaces while the steam flows into the wells. The flashed water discharged from wells is of the Na–Cl or Na–HCO3 types containing 50–1200 ppm of Cl. The most abundant dissolved constituent (average on a molal basis) in the aquifer fluid is CO2, followed by Na, Cl, Si, F, K and H2S. Modelling of aquifer fluid compositions using the phase segregation model reveals that the concentrations of only one of the major reactive components (CO2) may be externally controlled, i.e. by its flux to the hydrothermal fluid. All the other reactive components (Al, Ca, F, Fe, K, Mg, Na, Si, S) are controlled by close approach to local equilibrium with hydrothermal minerals. In the Olkaria West and Domes sectors, the CO2 aquifer fluid concentration is controlled by its flux from the magma heat source, but in other parts of the Olkaria field, it closely approaches equilibrium with the epidote–prehnite–calcite–quartz hydrothermal mineral assemblage. The mineral assemblage pyrite–pyrrhotite–magnetite controls aquifer water H2S and H2 concentrations. Departure from equilibrium is larger for individual hydrothermal minerals than for mineral pairs and especially mineral assemblages. In the case of mineral pairs, the only aqueous species entering the reactions are either a pair of ions or gases, but for minerals assemblages it is a single gas only. If a gas is in equilibrium with a mineral assemblage, all its minerals should also be in equilibrium with the gas. There are several reasons for the observed larger departure from equilibrium between solution and individual minerals than between mineral pairs and assemblages:(1) the stoichiometry of the reactions, (2) in the case of iron, inadequate quality of thermodynamic data on its hydrolysis constants and(3) changes in fluid component concentrations between undisturbed aquifer and wellhead due to precipitation/dissolution reactions. This last mentioned process is most important for components present in low concentration in the fluid. Specifically for Ca-bearing minerals, the apparent under-saturation in the reservoir water is considered to be an artefact caused by removal of Ca from solution by calcite precipitation in the depressurization zone around wells where the water is strongly degassed by its extensive boiling. The initial vapor fraction in the reservoir fluid has been estimated from the modelled H2S and H2 concentrations in that fluid assuming their concentrations to be fixed by specific mineral-solution equilibria in the initial aquifer liquid water. The results indicate that very small amount of vapor is present in the initial aquifer fluid, or 0.03% by weight on average (a few % by volume). This number is thought to carry considerable uncertainty when all the errors and approximations made in the calculations are taken into account. Yet, some vapor must be present, generated by depressurization boiling of rising hot water.

A thermodynamic prediction on the stability of the nukundamite + chalcopyrite and bornite + pyrite assemblages

AbstractA thermodynamic prediction of the Gibbs free energy of formation (ΔGfo) of nukundamite (empirical composition Cu5.5FeS6.5) was made in order to specify whether the nukundamite + chalcopyrite or the bornite + pyrite assemblage is stable in the Cu-Fe-S system. The results of calculations using previously reported data of ΔGfo values of some Cu-Fe-sulphide minerals in equilibrium with nukundamite indicate that the total free energy of the nukundamite + chalcopyrite assemblage is appreciably higher than that of the bornite + pyrite assemblage in the temperature range 250–400°C. This means that nukundamite + chalcopyrite is a metastable assemblage under common ore-forming conditions.The occurrence of nukundamite is not uncommon in the Fijian kuroko deposits in contrast to the Japanese kuroko deposits. A thermochemical treatment for this phenomenon leads to the interpretation that the black ore containing nukundamite in the Fijian deposit was formed under relatively highsulphidation and low-pH conditions. This suggestion is in good agreement with the present experimental result that the bornite + pyrite assemblage was produced in the temperature range 350–250°C by using near-neutral hydrothermal solutions.

Características das mineralizações auríferas no grupo Serra do Itaberaba, Guarulhos-SP

A seqüência metavulcano-sedimentar do Grupo Serra do ltaberaba encerra várias ocorrências de ouro, das quais a de Tapera Grande é a melhor conhecida. Os principais tipos de mineralizações auríferas podem ser agrupados como: a) singenético, stratabound, com o ouro disseminado nas rochas metavulcanoclásticas básicas e intermediárias e nos metaexalitos (metacherts com sulfetos e turmalinitos) e b) epigenético, associados a veios de quartzo sulfetados, superimposto ao anterior ou geneticamente vinculados às zonas de cizalhamento. O primeiro tipo é caracterizado pela assembléia pirrotita e pirita com calcopirita subordinada e pela granulação muito fina do ouro livre, enquanto que no segundo tipo o ouro ocorre associado à calcopirita, covelina e calcosina, mas de modo subordinado, pois é mais comum sob a forma livre. Análises químicas revelaram que os teores de cobre, chumbo, zinco e prata são mais elevados nos minérios do tipo epigenético, enquanto no tipo singenético tem-se a presença de paládio e altos teores de tungstênio. Diferentemente do que comumente ocorre em depósitos similares, o arsênio não foi detectado em teores significativos.