Zambales ophiolite,Philippines

The Acoje massif is part of a mafic-ultramafic complex, the Zambales ophiolite, and is a fragment of Mesozoic oceanic crust. This paper documents the occurrence and phase relations of sulfides and associated phases in the critical zone of the Acoje massif. The Acoje critical zone (ACZ) forms the basal cumulate sequence of the massif and consists of a variably serpentinized lower ultramafic zone and a relatively less altered upper mafic zone. Two distinct sulfide associations have been identified: (1) a troilite (±pyrrhotite)-dominated group hosted by the mafic zone and (2) a pentlandite-dominated group hosted by the ultramafic zone. Troilite-dominated assemblages represent the original mineralogy of magmatically precipitated sulfides in the entire cumulate sequence. The pentlandite-dominated group appears to have evolved from the primary magmatic sulfides during low-temperature re-equilibration. The paragenetic evolution from the magmatic assemblage to the low-temperature assemblage appears to have proceeded as follows: (1) S-rich hexagonal pyrrhotite+pentlandite+chalcopyrite (or cubanite)+magnetite, (2) S-poor hexagonal pyrrhotite+pentlandite+intermediate solid solution (iss) phase (and/or cubanite)+magnetite, (3) troilite (or mackinawite)+pentlandite+iss+magnetite, (4) troilite (or mackinawite)+pentlandite+iss+native Cu+magnetite, (5) pentlandite+native Cu+magnetite, and (6) pentlandite+native Cu+Fe-Ni alloy+magnetite. This evolutionary trend, in conjunction with the observed textural, chemical, and sulfur-isotopic relations, indicates that the native metal and alloy phases in the ACZ were produced by low-temperature reduction of the primary magmatic sulfides. Correlations between sulfide assemblages and coexisting silicate-hydrosilicate-oxide assemblages further indicate that this alteration occurred during retrograde serpentinization of the Acoje massif. Two end-member models that could explain the inferred low-temperature mineralogic evolution of the ACZ sulfides are described: (1) an isothermal reduction model and (2) a non-isothermal equilibration model. Both isothermal and non-isothermal effects apparently were involved in the development of variably reduced sulfide-oxide-metal assemblages from the initial magmatic sulfides.     

Accessory iron and nickel minerals from the Mt. Poputnaya ultramafic massif,eastern Kamchatka,Russia

Oxides, sulfides, arsenides, native metals, and intermetallic compounds are accessory ore minerals from the rocks of the Mt. Poputnaya ultramafic massif. The Fe–Ni phases containing 55.3–82.3 wt % Ni are the most abundant among them. Magnetite, pyrrhotite, Co–Fe and Fe–Ni phases, and native iron are the comparatively high-temperature minerals, whereas heazlewoodite, orcelite, dienerite, and native copper are formed at low temperatures. The found minerals result from serpentinization at 500°C and below.     

High-temperature carbonate minerals in the Stillwater Complex, Montana, USA

High-temperature carbonate minerals have been observed in association with sulfide minerals below the platiniferous Johns-Manville (J-M) reef of the Stillwater Complex in a stratigraphic section that has been previously shown to be characterized by unusually Cl-rich apatite. The carbonate assemblage consists of dolomite with exsolved calcite in contact with sulfide minerals: chalcopyrite and pyrrhotite in the Peridotite Zone; and pyrrhotite with pentlandite, pyrite and chalcopyrite in Gabbronorite Zone I of the Lower Banded Series. A reaction rim surrounds the carbonate–sulfide assemblages, showing an alteration of the host orthopyroxene to a more calcium-enriched, Fe-depleted pyroxene. The calcite–dolomite geothermometer yields a minimum formation temperature as high as 950 °C for the unmixed assemblages. Iron and manganese concentrations exceed the range seen in carbonatite and mantle xenolith carbonates and are distinctly different from the nearly pure end-member carbonates associated with greenschist-grade (and lower) assemblages (e.g., carbonate veins in serpentinite) that occur locally throughout the complex. The association of high-temperature carbonates with sulfides beneath the J-M reef supports the hydromagmatic theory which involves a late-stage chloride–carbonate fluid percolating upwards, dissolving PGE and sulfides and redepositing them at a higher stratigraphic level. Characterization of the processes which form strategically important metal deposits, such as the J-M reef of the Stillwater Complex and the analogous Merensky reef of the Bushveld Complex in South Africa, could potentially lead to better exploration models and, more broadly, a deeper understanding of the cooling and compositional evolution of large bodies of ultramafic and mafic magma and of carbonatites, on both a local and a regional scale.     

Melonite group minerals and other tellurides from three Cu–Ni–PGE prospects, Eastern Desert, Egypt

Melonite group minerals and other tellurides are described from three Cu–Ni–PGE prospects in the Eastern Desert of Egypt: Gabbro Akarem, Genina Gharbia and Abu Swayel. The prospects are hosted in late Precambrian mafic–ultramafic rocks and have different geologic histories. The Gabbro Akarem prospect is hosted in dunite pipes where net-textured and massive sulfides are associated with spinel and Cr-magnetite. Michenerite, merenskyite, Pd–Bi melonite and hessite occur mainly as inclusions in sulfides. Typical magmatic textures indicate a limited role of late- and post-magmatic hydrothermal processes. At Genina Gharbia, ore forms either disseminations in peridotite or massive patches in hornblende-gabbro in the vicinity of metasedimentary rocks. Actinolitic hornblende, epidote, chlorite and quartz are common secondary silicates. Sulfide textures and host rock petrography suggest a prolonged late-magmatic hydrothermal event. Michenerite, merenskyite, Pd–Bi melonite, altaite, hessite, tsumoite, sylvanite and native Te are mainly present in secondary silicates. The Abu Swayel prospect occurs in conformable, lens-like mafic–ultramafic rocks in metasedimentary rocks and along syn-metamorphic shear zone. The sulfide ore and host rocks are metamorphosed (amphibolite facies; 550 to 650 °C, 4 to 5 kbar) and syn-metamorphically sheared. Melonite group minerals are represented by merenskyite and Pd–Bi melonite. Other tellurides comprise hessite, altaite and joséite-B. Melonite group minerals and tellurides occur as inclusions in mobilized sulfides and along cracks in metamorphic garnet and plagioclase.The different geological history of the three prospects permits an examination of the role played by magmatic, late-magmatic and metamorphic processes on the mineralogy of melonite group minerals and diversity of tellurides. The contents of PGE and Te in the ore and temperature of crystallization control the mineralogy and compositional trends of the melonite group minerals. Crystallization of the melonite group minerals over a wide range of temperatures in a Te-rich environment enhances the elemental substitutions. Merenskyite dominates the mineralogy of the group at low Te activity, while Pd–Bi melonite is the common phase at high Te activity.     

Lead isotopes in sulfides from the Stillwater Complex, Montana: evidence for subsolidus remobilization

Isotopic ratios of Pb in sulfide minerals (primarily pyrrhotite, chalcopyrite, and pentlandite) from a suite of samples from the platiniferous J-M Reef of the Stillwater Complex were measured to elucidate the temporal and genetic relationship between sulfides and host silicate minerals. Results indicate that sulfides and coexisting plagioclases are generally not in isotopic equilibrium, that both sulfides and feldspars record highly radiogenic initial ratios at 2.7 Ga, and that a component of “post-emplacement” radiogenic Pb has mixed with common Pb in the sulfides. A model involving introduction of radiogenic Pb carried by fluids derived from sources external to the complex is favored. Analyses of the lead isotopic composition of sulfides in veins which cut the complex indicate that a significant fraction of the radiogenic lead which was added to the sulfides was externally derived during an extensive hydrothermal episode, associated with Proterozoic regional metamorphism around 1.7 Ga. The possibility that some fractions of the radiogenic Pb may have been derived from primary minerals altered during the low-grade metamorphism cannot be discounted. The amount of radiogenic lead added is variable and in some cases negligible. There is a good correlation between the lead isotope composition and the nature of the secondary mineral assemblage. Sulfides and plagioclases in samples that show little or no alteration of the primary minerals are generally in isotopic equilibrium and preserve isotope ratios consistent with magmatic crystallization at 2.7 Ga. Samples with the most radiogenic sulfides contain abundant secondary minerals (serpentine, talc, actinolite, chlorite and zoisite) associated with greenschist facies metamorphism. Some of the radiogenic Pb in the sulfides can be removed by progressive stepwise leaching. However, in most samples recrystallization of sulfides during metamorphism has mixed common Pb and radiogenic Pb throughout the crystal structure such that, in these samples, stepwise leaching does not recover initial Pb isotopic ratios. Plagioclases are much more resistant to low temperature recrystallization and in almost all cases, stepwise leaching reveals the initial lead isotopic composition. The reactivity of sulfides over a wide temperature range enhances their utility in understanding not only the processes involved in their formation at the time of magmatic emplacement but also postmagmatic processes which were important in the redistribution and enrichment of platinum group elements (PGE) within the ore zone. Received: 30 December 1998 / Accepted: 16 June 1999     

First Finds of Platinum and Palladium Minerals in Sulfide Ores of the Khudolaz Intrusive Complex (Southern Urals)

Forms of occurrence of platinum (sperrilite, moncheite) and palladium (Sb-michenerite, Pd–Bi phase) minerals in intrusive rocks of the Khudolaz differentiated complex have been studied. Platinum minerals were identified in disseminated Cu–Ni sulfide ores from ultramafic olivine–hornblende rocks of the Khudolaz complex, whereas palladium minerals were found in ores from olivine–hornblende gabbroids. The structural arrangement of grains as inclusions in sulfides of the primary magmatic association testifies that they were formed as a result of segregation of platinum group elements, which partitioned into the composition of sulfides during low-temperature mineral formation process at the late-magmatic stage.     

The metamorphosed mafic-ultramafic complex of Mokuro, Ilesha Schist Belt, southwestern Nigeria

In the Proterozoic Schist Belt of Nigeria, lenticular bodies of metabasites and meta-ultramafics are frequently intercalated within staurolite bearing metapelitic schists. Such a metamorphosed mafic-ultramafic complex is particularly well exposed in the Mokuro riverbed between the towns of Ife and Ilesha. These outcrops display contact relationships with the surrounding metasediments, as well as between the individual mafic and ultramafic rock types. The most common mafic rocks are indistinctly layered amphibolites, accompanied by apatite rich amphibolites and massive amphibolites, in part rich in ilmenite and pyrrhotite. Among the generally massive ultramafic rocks, nearly monomineralic amphibole rocks predominate, while chlorite-amphibole, talc-chlorite-amphibole and talc bearing olivine-chlorite-amphibole rocks occur in subordinate amounts.Field, textural and geochemical evidence suggest that the mafic-ultramafic complex derived from a thick, structurally differentiated basaltic sill that contained doleritic portions in its interior. Slow cooling rates in these inner parts enabled crystal settling with the formation of ultramafic cumulates. Due to the enrichment of volatiles during the crystallisation process, higher amounts of apatite and sulphides, as well as late magmatic amphibole, were formed in parts of the mafic-ultramafic body.Mineral assemblages in the mafic-ultramafic complex testify to a metamorphic overprint under amphibolite-facies conditions. Thermodynamic modelling in the system CMFASH leads to an estimated P–T range of 1.5–3 kbar and 550–620°C for the metamorphic peak assemblage talc-olivine-chlorite-Ca amphibole-orthoamphibole.     

A laser ablation inductively coupled plasma mass spectrometry study of the distribution of chalcophile elements among sulfide phases in sedimentary and magmatic rocks of the Duluth Complex,Minnesota, USA

Nickel-copper sulfide deposits occur in the basal unit of the Partridge River Intrusion, Duluth Complex (Minnesota, USA). Many lines of evidence suggest that these sulfides are formed after assimilation of the proterozoic S-rich black shales, known as the Bedded Pyrrhotite Unit. In addition to S, black shales are enriched in Te, As, Bi, Sb and Sn (TABS) and the basaltic magma of the intrusion is contaminated by the partial melt of the black shales. The TABS are chalcophile and together with the platinum-group elements, Ni and Cu partitioned into the magmatic sulfide liquid that segregated from the Duluth magma. The TABS are important for the formation of platinum-group minerals (PGM) thus their role during crystallization of the base metal sulfide minerals could affect the distribution of the PGE. However, the concentrations of TABS in magmatic Ni-Cu-PGE deposits and their distribution among base metal sulfide minerals are poorly documented. In order to investigate whether the base metal sulfide minerals host TABS in magmatic Ni-Cu-PGE deposits, a petrographic and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) study has been carried out on base metal sulfide and silicate phases of the Partridge River Intrusion, Duluth Complex.Petrographic observations showed that the proportions of the base metal sulfide minerals vary with rock type. The sulfide assemblage of the least metamorphosed Bedded Pyrrhotite Unit from outside the contact metamorphic aureole consists of pyrite with minor pyrrhotite plus chalcopyrite (<5%), whereas within the contact aureole the sulfide assemblage of the Bedded Pyrrhotite Unit rocks consists dominantly of pyrrhotite (>95%) with small amount of chalcopyrite (<2%). The sulfide mineral assemblage in the xenoliths of the Bedded Pyrrhotite Unit and in the mafic rocks of the basal unit contains two additional sulfides, pentlandite and cubanite.Our LA-ICP-MS study shows that sulfides of the Bedded Pyrrhotite Unit are rich in TABS; consistent with these S-rich black shales being the source of TABS that contaminated the mafic magma. Most of the TABS are associated with sulfides and platinum-group minerals in the rocks of the Bedded Pyrrhotite Unit from the contact aureole, the Bedded Pyrrhotite Unit xenoliths and the mafic rocks of the Duluth Complex. In addition to these phases the laser maps show that silicate phases, i.e., orthopyroxene and plagioclase contain Sn and Pb respectively. In contrast, in the least metamorphosed samples of the Bedded Pyrrhotite Unit from outside the contact aureole although the pyrite contains some TABS mass balance calculations indicates that most the TABS are contained in other phases. In these rocks, galena hosts significant amounts of Te, Bi, Sb, Sn and Ag and few very small grains of Sb-rich phases were also observed. The host phases for As were not established but possibly organic compounds may have contributed.     

Genesis of the Mamut Porphyry Copper Deposit, Sabah, East Malaysia

Abstract: The Mamut deposit of Sabah, East Malaysia, is a porphyry type Cu‐Au deposit genetically related to a quartz monzonite (“adamellite”) porphyry stock associated with upper Miocene Mount Kinabalu plutonism. The genesis of the Mamut deposit is discussed based on petrology of the intrusives in the Mount Kinabalu area combined with ore– and alteration–petrography, fluid inclusion and sulfur isotope studies. Groundmass of the adamellite porphyry at Mamut is rich in K which suggests vapor transport of alkaline elements during the mineralizing magmatic process, while the groundmass of the post‐ore “granodiorite” porphyry at Mamut contains small amounts of normative corundum suggesting depletion in alkaline elements at the root zone of the magma column. Sub‐dendritic tremolitic amphibole rims on hornblende phenocrysts in the Mamut adamellite porphyry suggest interaction between the mineralizing magma and the exsolved fluids. Occurrences of clinopyroxene microphenocrysts and pseudomor‐phic aggregates of shredded biotite and clinopyroxene after hornblende phenocrysts in the barren intrusives imply lower water fugacity and decreasing in water fugacity, respectively. Compositional gap between the core of hornblende phenocrysts and the tremolitic amphibole rims and those in the groundmass of the Mamut adamellite porphyry suggests a decrease in pressure. Higher X_Mg (=Mg/(Mg+Fe) atomic ratio) in the tremolitic amphibole rims in the Mamut adamellite porphyry compared to those of the barren intrusions suggests high oxygen fugacity. High halogen contents of igneous hydrous minerals such as amphiboles, biotite and apatite in the Mamut adamellite porphyry suggest the existence of highly saline fluids during the intrusion and solidification of the mineralizing magma. Fluid inclusions found in quartz veinlet stockworks are characterized by abundant hypersaline polyphase inclusions associated with subordinate amounts of immiscible gaseous vapor. Both Cu and Au are dispersed in disseminated and quartz stockwork ores. Chalcopyrite and pyrrhotite as well as magnetite are the principal ore minerals in the biotitized disseminated ores. Primary assemblage of intermediate solid solution (iss) and pyrrhotite converted to the present assemblage of chalcopyrite and pyrrhotite during cooling. Subsequent to biotitization, quartz veinlet stockworks formed associated with retrograde chlorite alteration. The Cu‐Fe sul–fides associated with stockwork quartz veinlet are chalcopyrite and pyrite. Overlapping Pb and Zn and subsequent Sb mineralizations were spatially controlled by NNE‐trending fractures accompanying the phyllic and advanced argillic alteration envelope. Sulfur isotopic composition of ore sulfides are homogeneous (about +2%) throughout the mineralization stages. These are identical to those of the magmatic sulfides of Mount Kinabalu adamellitic rocks.     

Sulfide Oxidation and Production of Gossans,Ashanti Mine,Ghana

At the Justice mine, in the Ashanti goldfields of southwestern Ghana, chemical weathering of gold- bearing sulfide-rich lodes has produced a series of characteristic mineralogical and geochemical features that are diagnostic. In this type of gold mineralization, the most abundant sulfides are arsenopyrite, pyrite, pyrrhotite, and chalcopyrite with minor bornite and sphalerite. Gold occurs predominantly as native gold, spatially associated and chemically bound with arsenopyrite. Elsewhere gold-silver tellurides are present in quartz veins. During sulfide oxidation, arsenopyrite is replaced by amorphous and crystalline Fe-Mn arsenates, goethite, hematite, and arsenolite in box- and ladderwork textures. In the extremely weathered gossans exposed at surface or in exploration pits, goethite, hematite, and scorodite are present as pseudomorphs of oxidized arsenopyrite, which can be used as a visual pathfinder for gold-arsenic mineralization. As with arsenopyrite, pyrite and pyrrhotite alteration produces boxwork and ladderwork textures with the sulfide replaced by goethite, hematite, and a complex limonite. Copper sulfides and goethite replace bornite and chalcopyrite in ladder-type textures. With more intensive weathering, this assemblage is replaced by cuprite, goethite, and hematite. Gold mineralogy in the gossan is complex, with evidence of in situ precipitation of supergene gold as well as alteration of hypogene native gold. The concentration of pathfinder elements decreases in the gossan as a result of supergene leaching. Mass- balance calculations confirm that gossan production largely is in situ and, consequently, the hypogene geochemical dispersion patterns are preserved even though the proportion of many elements decreases as intensity of weathering increases.

The problem remains of discriminating between auriferous and non-auriferous gossans, or those produced by pedological concentration of iron. Although mineral textures such as box-and ladderwork replacement and mineral pseudomorphs are useful field criteria, the most reliable guide for evaluation still is trace-element geochemistry. By use of multi-element discriminant analysis, gossans of different origins can be distinguished (along with their surface expression) from ironstones and barren lateritic soils. In regional reconnaissance studies, the evaluation of trace-element geochemistry as a discriminant along with field mapping may indicate gold potential of even extremely altered products of mineralization and, in so doing, provide a basis for the classification of weathered samples.     

Sulfides associated with podiform bodies of chromite at Tsangli,Eretria, Greece

Small bodies of pyrrhotite, chalcopyrite, minor pentlandite, and magnetite occur at the peripheries of podiform bodies of chromite in ultramafic ophiolitic rocks at Tsangli, Eretria, central Greece. Banding of magnetite and sulfide within the bodies is reminiscent of magmatic banding. A magmatic origin has been proposed for similar sulfide masses in the Troodos ophiolite (Panayiotou, 1980). The compositions of the host rocks, chromite, and of the sulfides have been investigated. On average, the sulfide mineralization, recalculated to metal content in 100% sulfide, contains 0.55% Ni, 5.15% Cu, 0.29% Co, 9 ppb Pd, 179 ppb Pt, 16 ppb Rh, 112 ppb Ru, 31 ppb Ir, 58 ppb Os, and 212 ppb Au. These metal contents, particularly the high Cu/(Cu+Ni) ratio of 0.78 and the Pt/(Pd+Pt) ratio of 0.95, are inconsistent with the sulfides having reached equilibrium with their Ni rich host rocks at magmatic temperatures and accordingly it is concluded that they are not of magmatic origin. The average ³⁴S value of the sulfide bodies is +2 while that of a sample of pyrite from country-rock schist is –15.6. These values are inconclusive as to the origin of the sulfur. It is suggested that the sulfides have been precipitated by hydrothermal fluids, possibly those responsible for the serpentinization of the host rocks. The source of the metals may have been the host rocks themselves.     

Thoughts on the Jiaodong Gold Province of China:Towards a Tectonic Model

The Jiaodong gold province is situated in the eastern Sino-Korean Platform within the so-calledJiaoliao Uplift. The basement rocks are Archaean and Proterozoic metamorphic rocks. Mesozoic sedimentary andvolcanic cover occur within extensional basins. Intrusive rocks are dominated by Mesozoic granitoid, with interme-diate-acid and basic dyke swarms. The structures form an E-W-trending anticlinorium in the basement complex, andlarge-scale NE-SW-and NNE-SSW-trending fault zones of Mesozoic age. The gold mineralization is associated withthe Mesozoic faults and related secondary fractures in the granites or granite-basement contacts. The mineralizationtypes are quartz-vein type and wall-rock alteration type. Wall-rock alteration is very well developed around the orezones. Alteration minerals include quartz, sericite (and fuchsite), pyrite, calcite, chlorite, hematite, rutile and graph-ite. The ore assemblage is uniform in all deposits, including pyrite, chalcopyrite, galena, sphalerite, arsenopyrite,pyrrhotite, gold, electrum, hessite, petzite, magnetite, molybdenite, tetrahedrite and wolframite. Mesozoic collisionand subduction between the South China and North China continental blocks contributed to formation of the Meso-zoic granitoid intrusions. The granitic magma is considered to be derived from partial melting of the crust throughunderplating processes. Gold was remobilised from basement rocks and deposited in fracture zones by the high-temperature fluids associated with these processes.     

Ore-bearing hydrothermal metasomatic processes in the Elbrus volcanic center, the northern Caucasus, Russia

Precaldera, caldera, and postcaldera cycles are recognized in the geological evolution of the Pleistocene-Holocene Elbrus volcanic center (EVC). During the caldera cycle, the magmatic activity was not intense, whereas hydrothermal metasomatic alteration of rocks was vigorous and extensive. The Kyukyurtli and Irik ore-magmatic systems have been revealed in the EVC, with the former being regarded as the more promising one. The ore mineralization in rocks of the caldera cycle comprises occurrences of magnetite, ilmenite, pyrite and pyrrhotite (including Ni-Co varieties), arsenopyrite, chalcopyrite, millerite, galena, and finely dispersed particles of native copper. Pyrite and pyrrhotite from volcanics of the caldera cycle and dacite of the Kyukyurtli extrusion are similar in composition and differ from these minerals of the postcaldera cycle, where pyrite and pyrrhotite are often enriched in Cu, Co, and Ni and millerite is noted as well. The composition of ore minerals indicates that the hydrothermal metasomatic alteration related to the evolution of the Kyukyurtli hydrothermal system was superimposed on rocks of the caldera cycle, whereas the late mineralization in rocks of the postcaldera cycle developed autonomously. The homogenization temperature of fluid inclusions in quartz and carbonate from crosscutting veinlets in the apical portion of the Kyukyurtli extrusion is 140–170°C and in quartz from geyserite, 120–150°C. The temperature of formation of the chalcopyrite-pyrite-pyrrhotite assemblage calculated using mineral geothermometers is 156 and 275°C in dacite from the middle and lower portions of the Malka lava flow and 190°C in dacite of the Kyukyurtli extrusion. The hydrothermal solutions that participated in metasomatic alteration of rocks pertaining to the Kyukyurtli ore-magmatic system (KOMS) and formed both secondary quartzite and geyserite were enriched in fluorine, as evidenced from the occurrence of F-bearing minerals-zharchikhite, ralstonite, α-ralstonite, and fluorite-identified in these metasomatic rocks for the first time. By analogy with porphyry Cu-Mo deposits in Chile and the United States, the ore mineralization of the KOMS may be classified by composition and textural and structural attributes as a supraore level of porphyry copper genetic type. The volcanic rocks of the KOMS and the EVC as a whole are enriched in Ag, Mo, Zn, As, Sb, Se, and Ba. Judging from the scale of argillic alteration and taking into account the data on porphyry Cu-Mo ore-magmatic systems of the Greater Caucasus, veined Pb-Zn ore mineralization may be expected in the propylitic zone at a depth down to 1000 m from the present-day erosion level of the KOMS. Stringer-disseminated Au-Ag, Cu, and Cu-Mo ore mineralization of the upper part of the porphyry ore-magmatic system related to subvolcanic dacitic intrusions may be localized somewhat deeper.     

Sulfide petrology of basal chilled margins in layered sills of the Archean Deer Lake Complex,Minnesota

Sulfide minerals in amounts up to 3 vol% are found in basal, chilled marginal zones of two layered peridotite-pyroxenite-gabbro sills in the Early Precambrian Deer Lake Complex, northcentral Minnesota. The sulfides occur interstitially to silicate minerals, and consist of pyrrhotite with minor exsolved cobaltian pentlandite, chalcopyrite, gersdorffite, and marcasite±pyrite as an alteration product of pyrrhotite.The basal chilled units (3–6 m) of the sills are divisable into three zones based primarily on textures. The lowermost zone is an equigranular basalt, whereas the overlying zone is characterized by skeletal, spinifex-like actinolite after clinopyroxene. The upper zone of the basal margins contains elongate and swallow tail plagioclase, and is barren of sulfide minerals.Electron microprobe analyses of sulfide minerals and modal data suggest that sulfide bulk compositions at 1,100–1,000 ° C represent a pyrrhotite solid solution and a coexisting Cu-rich sulfide liquid. Cooling of the Cu-rich liquid and low temperature transformations are thought to have produced chalcopyrite or chalcopyrite plus pyrrhotite. The sulfide minerals have reequlibrated to temperatures near 300 ° C or less.Analyses of sulfur content and ³⁴S values suggest that assimilation of sulfur from adjacent country rocks was the principal mechanism responsible for anomalous concentrations of sulfides in the basal chilled margins. The distribution of sulfides in the peridotite-pyroxenite-gabbro portions of the sills, and calculations of settling rate preclude an origin involving gravitational settling of immiscible droplets through the magma body.     

Ore Mineralization in Ultramafic and Metasomatic Rocks of the Ospin–Kitoi Massif,Eastern Sayan

In the Ospin–Kitoi ultramafic massif of the Eastern Sayan, accessory and ore Cr-spinel are mainly represented by alumochromite and chromite. Copper–nickel mineralization hosted in serpentinized ultramafic rocks occurs as separate grains of pentlandite and pyrrhotite, as well as assemblages of (i) hexagonal pyrrhotite + pentlandite + chalcopyrite and (ii) monoclinal pyrrhotite + pentlandite + chalcopyrite. Copper mineralization in rodingite is presented by bornite, chalcopyrite, and covellite. Talc–breunnerite–quartz and muscovite–breunnerite–quartz listvenite contains abundant sulfide and sulfoarsenide mineralization: pyrite, gersdorffite, sphalerite, Ag–Bi and Bi-galena, millerite, and kuestelite. Noble metal mineralization is represented by Ru–Ir–Os alloy, sulfides, and sulfoarsenides of these metals, Au–Cu–Ag alloys in chromitite, laurite intergrowth, an unnamed mineral with a composition of Cu₃Pt, orcelite in carbonized serpentinite, and sperrylite and electrum in serpentinite. Sulfide mineralization formed at the late magmatic stage of the origination of intrusion and due to fluid–metamorphic and retrograde metasomatism of primary rocks.     

The Ishkinino Co-Cu massive sulfide deposit hosted in ultramafic rocks of the Main Ural Fault Zone, the southern Urals

The results of study of the Ishkinino Co-Cu massive sulfide deposit hosted in ultramafic rocks of the Main Ural Fault Zone are discussed. The ore field is localized in a fragment of Early Devonian accretionary prism composed of oceanic and island-arc tectonic sheets. The antiform structure of the ore field was formed at the collision stage in the Late Devonian. The primary ore was deposited near the bottom in the environment of the accretionary prism at the island-arc stage of evolution, whereas the superimposed ore mineralization was related to the collision stage. The primary ore is composed of massive, stringer-disseminated, and clastic varieties with two mineral assemblages of sulfides and oxides. The superimposed stringer-disseminated ore mineralization is represented by Co-Ni-Fe arsenides and sulfoarsenides, native gold, Bi and Te minerals, and late sulfides and oxides. Loellingite, safflorite, rammelsbergite, and krutovite were identified in the massive sulfide ore for the first time in the Urals. The geochemical attributes of Co-Ni minerals serve as indicators of superimposed processes. Chromites contained in rocks and ore correspond to Cr-spinel of suprasubduction ultramafic rocks in chemical composition. It is suggested that sulfide ore may be found in the accretionary prisms of the presently active island arcs composed of ultramafic sheets.     

Mineral Compositions and Equilibria in the Metamorphosed Iron Formation of the Gagnon Region, Quebec, Canada

Forty-seven specimens of the Wabush Iron Formation were collectedfrom ten outcrop areas. Twenty-five specimens contain the assemblage(1), quartz+clinopyroxene+calcite with or without orthopyroxene,grunerite, magnetite, ankerite, and siderite. Five specimenscontain assemblage (2), quartz+clinopyroxene+actinolite+calcite+magnetite+hematite,and two contain assemblage (3), quartz+orthopyroxene+actinolite+magnetite+hematite.In three specimens of assemblage (1), graphite occurs in theabsence of magnetite; pyrrhotite and pyrite occur separatelyor together in specimens with assemblage (1). Thirty-nine clinopyroxenes, 38 orthopyroxenes, 18 grunerites,7 actinolites, 16 calcites, 1 ankerite, and 1 siderite wereanalyzed for iron, manganese, and calcium by X-ray emissionspectrography. Magnesium contents were estimated by assumingstoichiometric proportions. Minerals occurring with hematite show low Fe/(Fe+Mg) ratios,and those in the other assemblages show higher values with awide range of variation. In orthopyroxene, Fe/(Fe+ Mg) rangesfrom 0·17 (with hematite) to 0·77. Regularity in the distributions of Fe, Mn, and Ca between pairsof coexisting minerals shows that equilibrium was attained inmost of the rocks studied. This regularity is also accomplishedin the distribution of Mn between calcite and coexisting silicatesas well as between the silicates themselves. Small differencesin the distributions of Ca and Fe depend on both outcrop areaand mineral assemblage. Phase rule considerations suggest that the specimens with dolomite-ankeriteor magnesitesiderite do not represent equilibrium assemblages.Variations in orthopyroxene compositions in assemblages withpyrite or pyrrhotite, or both, and magnetite indicate non-equilibrationof sulfides with silicates. The presence of the oxygen buffer,magnetite+hematite, attests to the immobility of oxygen duringmetamorphism. Within each outcrop area, over which the temperature and pressureare assumed to have been uniform, variations in the compositionsof the silicates in the sub-assemblages quartz+ orthopyroxene+gruneriteand quartz+orthopyroxene+clinopyroxene+calcite indicate gradientsof µ_H2O µ_CO2 and respectively. As characterizedby the composition of orthopyroxene, both gradients are relativelylow along strike, and high across strike. The direction of gradientsacross strike is almost without reversals, which is consistentwith intergranular diffusion of H₂O and CO₂. Phase rule restrictionsfor a majority of assemblages are not in accord with the simultaneousimposition of µ_H2O and µ_CO2 gradients on the rocks,nor the formation of an H₂O-CO₂ fluid phase during metamorphism.     

Hydrothermal processes in partially serpentinized peridotites from Costa Rica: evidence from native copper and complex sulfide assemblages

Native metals and metal alloys are common in serpentinized ultramafic rocks, generally representing the redox and sulfur conditions during serpentinization. Variably serpentinized peridotites from the Santa Elena Ophiolite in Costa Rica contain an unusual assemblage of Cu-bearing sulfides and native copper. The opaque mineral assemblage consists of pentlandite, magnetite, awaruite, pyrrhotite, heazlewoodite, violarite, smythite and copper-bearing sulfides (Cu-pentlandite, sugakiite [Cu(Fe,Ni)₈S₈], samaniite [Cu₂(Fe,Ni)₇S₈], chalcopyrite, chalcocite, bornite and cubanite), native copper and copper–iron–nickel alloys. Using detailed mineralogical examination, electron microprobe analyses, bulk rock major and trace element geochemistry, and thermodynamic calculations, we discuss two models to explain the formation of the Cu-bearing mineral assemblages: (1) they formed through desulfurization of primary sulfides due to highly reducing and sulfur-depleted conditions during serpentinization or (2) they formed through interaction with a Cu-bearing, higher temperature fluid (350–400 °C) postdating serpentinization, similar to processes in active high-temperature peridotite-hosted hydrothermal systems such as Rainbow and Logatchev. As mass balance calculations cannot entirely explain the extent of the native copper by desulfurization of primary sulfides, we propose that the native copper and Cu sulfides formed by local addition of a hydrothermal fluid that likely interacted with adjacent mafic sequences. We suggest that the peridotites today exposed on Santa Elena preserve the lower section of an ancient hydrothermal system, where conditions were highly reducing and water–rock ratios very low. Thus, the preserved mineral textures and assemblages give a unique insight into hydrothermal processes occurring at depth in peridotite-hosted hydrothermal systems.     

Ore minerals and geochemical characterization of the Dungash gold deposit,South Eastern Desert,Egypt

The Dungash historic gold mine is located in the South Eastern Desert of Egypt. The gold-bearing quartz veins are hosted by the metavolcanic and metavolcaniclastic rocks along an ENE–WSW trending shear zone. Alteration types recorded in the wall rocks are sericitization, silicification, carbonatization, chloritization, sulfidization, ferruginization, and listwanitization. The ore mineral assemblage comprises arsenopyrite, pyrite, native gold, pyrrhotite, sphalerite, chalcopyrite, and galena. The primary sulfide mineral assemblage formed during a hypogene hydrothermal stage whereas anglesite and goethite occur as secondary supergene phases. Microthermometric fluid inclusion analysis revealed that the auriferous quartz precipitated from a moderately saline (5 to 11.22 wt% NaCl_equiv) solution at temperatures above the recorded homogenization temperatures (T _h), which range from 380 to 177 °C. The minimum pressures of trapping are between 350 and 400 bars. The fluid evolution during mineralization is explained by mixing of a magmatic fluid with meteoric waters. Initially, the high temperature and moderately saline magmatic fluid dominated and progressively became diluted with meteoric waters. Highest gold content is recorded in the carbonatized zone and the quartz veins. However, gold content in the carbonatized zone of the footwall exceeds several times its content in the quartz veins and the carbonatized zone of the hanging wall.     

铂族元素矿物共生组合(英文)

由于铂族元素能有效地降低汽车尾气的污染 ,其需求量日益增加 ,对铂族元素矿床的寻找已是当务之急。着重从矿物矿床学角度对铂族元素的矿物共生特点进行了探讨。铂族元素可呈独立矿床产出 ,主要产于基性超基性层状侵入体、蛇绿岩套及阿拉斯加式侵入体中。铂族元素也伴生于铜镍矿床中 ,该类铜镍矿床主要与苏长岩侵入体、溢流玄武岩及科马提岩有关。产于基性超基性层状侵入体中的铂族矿物有铂钯硫化物、铂铁合金、钌硫化物、铑硫化物、铂钯碲化物、钯砷化物及钯的合金。这些铂族矿物可与硫化物矿物共生 ,也可与硅酸盐矿物共生 ,还可与铬铁矿及其他氧化物矿物共生。产于蛇绿岩套中的铂族矿物主要是钌铱锇的矿物 ,而铂钯铑的矿物则较少出现 ,这些铂族矿物可呈合金、硫化物、硫砷化物以及砷化物 4种形式出现。产于阿拉斯加式侵入体中的铂族矿物主要有铂铁合金、锑铂矿、硫铂矿、砷铂矿、硫锇矿及马兰矿等少数几种 ,其中铂铁合金与铬铁矿及与其同时结晶的高温硅酸盐矿物共生 ,而其他的铂族矿物则与后来的变质作用及蛇纹岩化作用中形成的多金属硫化物及砷化物共生。产于铜镍矿床中的铂族矿物主要是铂和钯的矿物。产于基性超基性层状侵入体、蛇绿岩套及阿拉斯加式侵入体中的铂族矿物的共同特点是它们均与铬铁矿?