Magmatic Cu-Ni-PGE-Au sulfide mineralisation in alkaline igneous systems: An example from the Sron Garbh intrusion,Tyndrum, Scotland

Magmatic sulfide deposits typically occur in ultramafic-mafic systems, however, mineralisation can occur in more intermediate and alkaline magmas. Sron Garbh is an appinite-diorite intrusion emplaced into Dalradian metasediments in the Tyndrum area of Scotland that hosts magmatic Cu-Ni-PGE-Au sulfide mineralisation in the appinitic portion. It is thus an example of magmatic sulfide mineralisation hosted by alkaline rocks, and is the most significantly mineralised appinitic intrusion known in the British Isles. The intrusion is irregularly shaped, with an appinite rim, comprising amphibole cumulates classed as vogesites. The central portion of the intrusion is comprised of unmineralised, but pyrite-bearing, diorites. Both appinites and diorites have similar trace element geochemistry that suggests the diorite is a more fractionated differentiate of the appinite from a common source that can be classed with the high Ba-Sr intrusions of the Scottish Caledonides. Mineralisation is present as a disseminated, primary chalcopyrite-pyrite-PGM assemblage and a blebby, pyrite-chalcopyrite assemblage with significant Co-As-rich pyrite. Both assemblages contain minor millerite and Ni-Co-As-sulfides. The mineralisation is Cu-, PPGE-, and Au-rich and IPGE-poor and the platinum group mineral assemblage is overwhelmingly dominated by Pd minerals; however, the bulk rock Pt/Pd ratio is around 0.8. Laser ablation analysis of the sulfides reveals that pyrite and the Ni-Co-sulfides are the primary host for Pt, which is present in solid solution in concentrations of up to 22 ppm in pyrite. Good correlations between all base and precious metals indicate very little hydrothermal remobilisation of metals despite some evidence of secondary pyrite and PGM. Sulfur isotope data indicate some crustal S in the magmatic sulfide assemblages. The source of this is unlikely to have been the local quartzites, but S-rich Dalradian sediments present at depth. The generation of magmatic Cu-Ni-PGE-Au mineralisation at Sron Garbh can be attributed to post-collisional slab drop off that allowed hydrous, low-degree partial melting to take place that produced a Cu-PPGE-Au-enriched melt, which ascended through the crust, assimilating crustal S from the Dalradian sediments. The presence of a number of PGE-enriched sulfide occurrences in appinitic intrusions across the Scottish Caledonides indicates that the region contains certain features that make it more prospective than other alkaline provinces worldwide, which may be linked the post-Caledonian slab drop off event. We propose that the incongruent melting of pre-existing magmatic sulfides or ‘refertilised’ mantle in low-degree partial melts can produce characteristically fractionated, Cu-PPGE-Au-semi metal bearing, hydrous, alkali melts, which, if they undergo sulfide saturation, have the potential to produce alkaline-hosted magmatic sulfide deposits.     

The opaque minerals and economic geology of the nemeiben ultramafic complex,saskatchewan, canada

The main rock types of the Nemeiben ultramafic complex form grossly concentric layers of clinopyroxenite, websterite, wehrlite and dunite. These rocks were partially amphibolitized late in the Hudsonian orogeny; consequently numerous relict primary textural and mineralogical features are preserved. Disseminated primary magmatic nickeliferous pyrrhotite, pentlandite, chalcopyrite, magnetite and chromite occur throughout this complex and local concentrations of sulfides occur. Textures, mineralogy and chemistry of these phases are indicative of a high temperature magmatic origin. Unmixing phenomena in pyrrhotite are attributable to post crystallization dissociation of a high temperature Fe-Ni-Cu-Co monosulfide solid solution. A secondary assemblage of fine-grained iron oxides, sulfides and native metals developed in altered ultramafic rocks. Magnetite and hematite, bravoite, violarite and millerite are among the minerals formed during serpentinization. Trace amounts of nickeliferous copper, native gold and silver occur in hematite veinlets and at the center of hematized former sulfide grains. Supergene alteration has affected most of the sulfides. Thus the most plausible explanation of the opaque minerals is that they represent a metamorphosed primary magmatic assemblage modified by supergene alteration.     

Metallogeny of the Nicholas-Denys Pb–Zn–Ag deposit,Bathurst Mining Camp,Canada

The Nicholas-Denys Pb–Zn–Ag deposit, located in the Bathurst Mining Camp (New Brunswick), consists of several pyrrhotite–sphalerite–galena sulfide lenses hosted by black mudstone of the Millstream Formation of the Fournier Group, deposited in an Ordovician backarc basin. The Nicholas-Denys sulfide lenses and hydrothermal alteration are conformable to the bedding-parallel S₁ regional foliation, and are sheared parallel to the Rocky Brook-Millstream shear zone, indicating a pre-Devonian deformation timing for mineralization. Reduced sulfur for Nicholas-Denys sulfides comes from bacterial reduction of Ordovician seawater sulfates in a system open to sulfates under partially oxygenated bottomwater conditions, with addition of magmatic sulfur from underlying mafic volcanic rocks. Lead was leached from the backarc basin sediments of the Millstream Formation mudstone and from underlying synvolcanic gabbros. The mineralizing fluid for Nicholas-Denys sulfides was reduced and acidic, favorable for precipitation of a pyrrhotite-rich mineralization. Characteristics of the Nicholas-Denys deposit are compatible with a SEDEX-type classification.     

Petrogenesis of Rabor-Lalehzar magmatic rocks (SE Iran): Constraints from whole rock chemistry and Sr-Nd isotopes

The Urumieh-Dokhtar magmatic arc (UDMA) of Central Iran has been formed during Neotethyan Ocean subduction underneath Eurasia. The Rabor-Lalehzar magmatic complex (RLMC), covers an area ～1000?km² in the Kerman magmatic belt (KMB), SE of UDMA. RLMC magmatic rocks include both granitoids and volcanic rocks with calc-alkaline and adakitic signatures but with different ages.Miocene adakitic rocks are characterd by relatively enrichmented in incompatible elements, high (Sr/Y)_(N) (>40), and (La/Yb)_(N) (>10) ratios with slightly negative Eu anomalies (Eu_N/Eu*≈ 0.9), depletion in HFSEs, and relatively non-radiogenic Sr isotope signatures (⁸⁷Sr/⁸⁶Sr?=?0.7048–0.7049). In contrast, the Oligocene granitoids exhibit low Sr/Y (<20) and La/Yb (<9) ratios, negative Eu anomalies (Eu_N/Eu*?≈?0.5), and enrichment in HFSEs and radiogenic Sr isotope signatures (⁸⁷Sr/⁸⁶Sr?=?0.7050–0.7052), showing affinity to the island arc rocks. Eocene volcanic rocks which crusscut the younger granitoid rocks comprise andesites and dacites. Geochemically, lavas show calc-alkaline character without any Eu anomaly (Eu_N/Eu*?≈?1.0). Based on the geochemical and isotopic data we propose that melt source for both calc-alkaline and adakitic rocks from the RLMC can be related to the melting of a sub-continental lithospheric mantle (SCLM). Basaltic melts derived from a metasomatized mantle wedge might be emplaced at the mantle-crust boundary and formed the juvenile mafic lower crust. However, some melts fractionated in the shallow magma chambers and continued to rise forming the volcanic intermediate-mafic rocks at the surface. On the other hand, the assimilation and fractional crystallization in the shallow magma chambers of may have been responsible for the development of Oligocene granitoids with calc-alkaline affinity. In the mid-Late Miocene, following the collision between Afro-Arabia and Iranian block the juvenile mafic crust of UDMA underwent thickening and metamorphosed into garnet-amphibolites. Subsequent upwelling of a hot asthenosphere during Miocene was responsible for partial melting of thickened juvenile crust of the SE UDMA (RLM complex). The adakitic melts ascended to the shallow crust to form the adakitic rocks in the KMB.     

Experimental study of interaction between fluid-bearing basaltic melts and peridotite: A mantle-crustal source of trap magmas in the Norilsk area

The paper reports data on the geology and tectono-magmatic reactivation of the Norilsk area and on the stratigraphy and geochemistry of its volcanic sequence, with the discussion of the sources and genesis of the ore magmas and the scale of the ore-forming process. According to the geochemistry of the lavas and intrusive rocks (Ti concentration and the La/Sm and Gd/Yb ratios), two types of the parental magmas are recognized: high-Ti magmas of the OIB type (from bottom to top, suites iv, sv, and gd of phase 1) and low-Ti magmas (suites hk, tk, and nd of phase 2 and suites mr-mk of phase 3), which were derived from the lithospheric mantle. The magmatic differentiation of the parental low-Ti magma of the tk type into a magma of the nd type was associated with the derivation of an evolved magma of the nd type, which was depleted in ore elements, and an ore magma, which was a mixture of silicate and sulfide melts, protocrysts of silicate minerals, and chromite. Judging from their geochemical parameters, the intrusions of the lower Norilsk type were comagmatic with the lavas of the upper part of the nd suite, and the ore-bearing intrusions of the upper Norilsk type were comagmatic with the lavas of the mr-mk suites. When the ore-bearing intrusions were emplaced, their magmas entrained droplets of sulfide melt and protocrysts of olivine and chromite and brought them to the modern magmatic chamber. These protocrysts are xenogenic with respect to the magma that formed the intrusions. In certain instances (Talnakh and Kharaelakh intrusions), the moving magma entrained single portions of sulfide magma, which were emplaced as individual subphases. The experimental study of the peridotite-basalt-fluid system shows that mantle reservoirs with protoliths of subducted oceanic crustal material could serve as sources of relatively low-temperature (1250–1350°C) high-Ti magnesian magmas of the rifting stage from an olivine-free source.     

PGE distribution in massive sulfides from the P<Emphasis Type="SmallCaps">ACMANUS</Emphasis> hydrothermal field,eastern Manus basin,Papua New Guinea: implications for PGE enrichment in some ancient volcanogenic massive sulfide deposits

The distribution of platinum group elements (PGE) in Cu- and Zn-rich samples from the Roman Ruins and Satanic Mills vent sites in the PACMANUS hydrothermal field (Papua New Guinea) was studied and compared to that from selected ancient volcanogenic massive sulfide (VMS) deposits. Samples from the Satanic Mills site are enriched in Pd and Rh when compared to samples from Roman Ruins and reach highest values in active and inactive Cu-rich black smoker chimneys and chalcopyrite-cemented dacite breccias (up to 356 ppb Pd and up to 145 ppb Rh). A significant positive correlation was established between Cu and Pd and Rh in samples from both vent sites. Comparisons of chondrite normalized patterns and values of Pd/Pt and Pd/Ir ratios in Cu-rich sulfides and probable source rocks (felsic volcanic rocks/MORB) along with the evidence for a magmatic component in the PACMANUS hydrothermal system indicate that leaching of back-arc volcanic rocks together with addition of magmatic volatiles to the convecting hydrothermal system was the most important factor for PGE enrichment at PACMANUS and likely at some PGE-enriched ancient VMS deposits.An erratum to this article can be found at Editorial handling: B. Lehmann     

Significance of ancient sulfide PGE and Re–Os signatures in the mantle beneath Calatrava,Central Spain

Spinel lherzolite and wehrlite xenoliths from the Cenozoic Calatrava volcanic field carry the geochemical imprint of metasomatic agents that have affected the subcontinental lithospheric mantle beneath Central Iberia. Some xenoliths (mainly wehrlites) were enriched in REE, Sr, P, and CO₂ by silicic-carbonate-rich metasomatic melts/fluids, while others record the effects of subduction-related hydrous silicate fluids that have precipitated amphibole and induced high Ti/Eu in primary clinopyroxene. The petrographic observations and geochemical data suggest that interstitial glass in the xenoliths represent the quenched products of Si-rich melts that infiltrated the mantle peridotite shortly before the entrainment of the xenoliths in the host magmas that erupted ca 2 million years ago. During their infiltration, the metasomatic melts reacted with peridotite, resulting in silica enrichment, while remobilizing grains of iron-rich monosulfide solid solution (Fe-rich Mss) initially enclosed in, or intergranular to, primary olivine and pyroxenes. In situ laser ablation inductively coupled plasma-mass spectrometry analysis of single sulfide grains reveals that the Fe-rich Mss in glass shows platinum-group element (PGE) patterns and ¹⁸⁷Os/¹⁸⁸Os compositions identical to the Fe-rich Mss occurring as inclusions in, or at grain boundaries of primary silicates. Moreover, independent of its microstructural position, Fe-rich Mss exhibits PGE and ¹⁸⁷Os/¹⁸⁸Os signatures typical of Mss either residual after partial melting or crystallized directly from sulfide melts. Our findings reveal that young metasomatic melt(s)/fluid(s) may carry remobilized sulfides with PGE and Os-isotopic signatures identical to those of texturally older sulfides in the peridotite xenolith. These sulfides thus still provide useful information about the timing and nature of older magmatic events in the subcontinental mantle.     

A tale of elemental accumulation and recycling in the metamorphosed Keketale VMS-type Pb-Zn deposit,Altai Mountains

The elemental accumulation and recycling in the metamorphosed Keketale VMS-type Pb-Zn deposit of the Altai Mountains are presented in this study. Based on detailed fieldwork and microscopic observation, the formation of the deposit involved syngenetic massive sulfide mineralization and epigenetic superimposed mineralization. Different generations of iron sulfides (i.e., pyrite and pyrrhotite) with contrasting textural, elemental, and sulfur isotopic features were generated in primary mineralization (including hydrothermal iron sulfides, colloform pyrite) and secondary modification (including annealed iron sulfides, oriented iron sulfides, and vein-pyrite). It is revealed that the spatial variation in textures and elements of hydrothermal iron sulfides depends on the inhomogeneous fluid compositions and varied environment in VMS hydrothermal system. Both leached sulfur from the footwall volcanic rocks and reduced sulfur by the TSR process are regarded as important sulfur sources. Furthermore, large sulfur isotopic fractionation and negative δ³⁴S values were mainly caused by varied oxygen fugacity, and to a lesser extent, temperature fluctuation. The epigenetic polymetallic veins that contain sulfides and sulfosalts (e.g., jordanite-geocronite, bournonite-seligmannite, boulangerite) were considered as the products of metamorphic fluid scavenged the metal-rich strata. All things considered, it is indicated that two episodes of fluid with distinct origins were essential for the formation of the deposit. The predominant evolved seawater along with subordinate magmatic fluid mobilized metals from volcanic rocks and precipitated massive sulfides near the seafloor are vital for primary mineralization. The metamorphic fluid remobilized metals (i.e., FMEs: fluid mobile elements, e.g., Pb, As, Sb) from neighboring volcanic and pyroclastic rocks and destabilized them within the fractured zone are responsible for secondary mineralization, which enhances the economic value of the deposit. Accordingly, metal-rich Devonian strata had been successively swept by different origins of fluid, leading to progressively elemental enrichment and the formation of a large deposit. Furthermore, the current study enlightens that FME-bearing veins with economic benefits can be discovered near the metamorphosed VMS deposits.     

Geochemical constraints on the genesis of the volcanic rocks in the southeast of Isfahan area, Iran

Late Miocene–Pliocene to Quaternary calc-alkaline lava flows and domes are exposed in southeast of Isfahan in the Urumieh Dokhtar magmatic belt in the Central Iran structural zone. These volcanic rocks have compositions ranging from basaltic andesites, andesites to dacites. Geochemical studies show these rocks are a medium to high K calc-alkaline suite and meta-aluminous. Major element variations are typical for calc-alkaline rocks. The volcanic rocks have SiO₂ contents ranging between 53.8% and 65.3%. Harker diagrams clearly show that the dacitic rocks did not form from the basaltic andesites by normal differentiation processes. They show large ion lithophile elements- and light rare earth elements (LREE)-enriched normalized multielement patterns and negative Nb, Ti, Ta, and P. Condrite-normalized REE patterns display a steep decrease from LREE to light rare earth elements without any Eu anomaly. These characteristics are consistent with ratios obtained from subduction-related volcanic rocks and in collision setting. The melting of a heterogeneous source is possible mechanism for their magma genesis, which was enriched in incompatible elements situated at the upper continental lithospheric mantle or lower crust. The geochemical characteristics of these volcanic rocks suggested that these volcanic rocks evolved by contamination of a parental magma derived from metasomatized upper lithospheric mantle and crustal melts.     

Lavas and their mantle xenoliths from intracratonic Eastern Paraguay (South America Platform) and Andean Domain, NW-Argentina: a comparative review

Protogranular spinel-peridotite mantle xenoliths and their host sodic alkaline lavas of Cretaceous to Paleogene age occur at the same latitude ≈26°S in central eastern Paraguay and Andes. Na- alkaline lavas from both regions display similar geochemical features, differing mainly by higher Rb content of the Paraguayan samples. Sr, Nd, and Pb isotope ratios are also similar with predominant trends from depleted to enriched mantle components. The mantle xenoliths are divided into two main suites, i.e. relatively low in potassium and incompatible elements, and high in potassium and incompatible elements. The suite high in potassium occurs only in Paraguay. Compositions of both suites range from lherzolite to dunite indicating variable “melt extraction”. Clinopyroxenes from the xenoliths display variable trace element enrichment/depletion patterns compared with the pattern of average primitive mantle. Enrichment in LREE and Sr coupled with depletion of Nb, Ti and Zr in xenoliths from both areas are attributed to asthenospheric metasomatic fluids affecting the lithospheric mantle. Metasomatism is apparent in the sieve textures and glassy drops in clinopyroxenes, by glassy patches with associated primary carbonates in Paraguayan xenoliths. Trace element geochemistry and thermobarometric data indicate lack of interaction between xenoliths and host lavas, due to their rapid ascent. Sr and Nd isotope signatures of the Andean and Paraguayan xenoliths and host volcanic rocks plot mainly into the field of depleted mantle and show some compositional overlap. The Andean samples indicate a generally slightly more depleted mantle lithosphere. Pb isotope signatures in xenoliths and host volcanic rocks indicate the existence of a radiogenic Pb source (high U/Pb component in the source) in both areas. In spite of the distinct tectonic settings, generally compressive in the Central Andes (but extensional in a back-arc environment), and extensional in Eastern Paraguay (rifting environment in an intercratonic area), lavas and host xenoliths from both regions are similar in terms of geochemical and isotopic characteristics.     

A geochemical aid to igneous rock type identification in deeply weathered terrain

A technique employing Ti, Zr and Cr to aid in the identification of primary igneous rock type in deeply weathered situations is described. The method is based on Ti/Zr ratio which is little affected either by primary alteration or weathering and adequately defines compositional fields for major igneous rock types. For volcanic rocks Ti/Zr ratios are rhyolite <4< dacite <12< andesite <60< basalt. Ultramafic rocks cannot be discriminated from mafic rocks by Ti/Zr ratio but are generally distinguished by high Cr.     

晚二叠世峨眉山地幔柱岩浆成矿作用

晚二叠世峨眉山地幔柱岩浆作用同时形成了Cu-Ni-PGE硫化物矿床和V-Ti-Fe氧化物矿床等不同类型的岩浆矿床。从硫化物矿床的PGE富集型、Cu-Ni-PGE富集型到Cu-Ni富集型,再到钒钛磁铁矿矿床,成矿基性-超基性岩体中基性岩石比例逐渐增加,PGE含量降低。铜镍铂族硫化物矿床具Nb和Ta负异常,岩浆流体组分含量较高,含有较高的H₂;而钒钛磁铁矿矿床具Nb、Ta和Ti正异常,Zr和Hf负异常,岩浆流体组分含量较低,含有较高的H₂O、CO₂和H₂。两类矿床强不相容元素和轻稀土元素(LREE)富集,Sr-Nd同位素组成与峨眉山玄武岩的演化趋势一致。Sr-Nd-Os-C-He同位素组成揭示岩浆上升过程中经历了不同程度的地壳混染,高钛玄武岩和钒钛磁铁矿矿床成矿岩体的地壳混染程度较低,部分低钛玄武岩和铜镍硫化物矿床存在明显的地壳混染。这两类岩浆矿床的形成与峨眉山地幔柱玄武岩浆有关,岩浆介质环境中H₂含量较高,V-Ti-Fe 氧化物矿床的形成与分离结晶、高含量的水和氧逸度的升高有关,Cu-Ni-(PGE)硫化物矿床的形成与还原性流体介质、结晶分异和地壳混染作用有关。     

The use of the “immobile” elements Zr and Ti in lithogeochemical exploration for massive sulphide deposits in the precambrian pecos greenstone belt of northern New Mexico

Mineralogical and geochemical data of surface rocks of the Mid-Proterozoic Pecos greenstone belt in Northern New Mexico and results of drill core data of the Jones Hill deposit are presented and discussed within the geological framework derived from detailed geological mapping. Over 800 rock samples were analyzed for 25 elements (all major and several trace elements) and the lithologies were mineralogically and petrographically determined. The results show that the stratigraphic sequence of the Jones Hill deposit is composed of felsic pyroclastics, exhalites and pyroclastics of mafic and mixed felsic-mafic origin, upon which are superimposed certain alteration effects.In volcanic rocks of felsic origin Mg, Fe, Mn, Cu, Pb, Zn, As are enriched and Na, Ca, Sr are clearly depleted when compared with their unaltered source rocks (Table I). Zirconium and Ti are found to be relatively immobile during alteration and subsequent intrusive, deformational and metamorphic events.Alteration trends, best defined by the Na₂O/MgO ratios and systematic metal zoning, confirm that the stratigraphic sequence is overturned. Lithogeochemical anomalies outline a target which is bigger (over 300 m) and more precisely defined than do other methods, mapping of alteration assemblages or soil geochemistry.New important aspects have evolved from this study, which are applicable to rock geochemical exploration and the recognition of alteration/mineralization anomalies in general. Ratios of Zr/TiO₂ can be shown to reflect the primary pre-alteration-premetamorphic composition of metavolcanic rocks (including shallow intrusives, flows, tuffs, and volcaniclastics). This ratio also enables the determination of the original composition of variably altered rocks of identical parentage (source rock). This determination is a fundamental prerequisite for the recognition and quantification of alteration trends. Direct applications are, for example, that apparently similar chlorite schists derived through hydrothermal alteration, metamorphism, and deformation Zr/TiO₂ ratio of 0.04 (and greater); however chlorite schists derived from mafic volcanic sources and metamorphosed without much hydrothermal alteration have a much lower Zr/TiO₂ ratio of 0.0075 (and less). The altered rock related to massive sulphide alteration can thus be recognized and distinguished from similar rocks of different origin. In addition, the relative concentrations of the immobile elements Zr and Ti are useful in making stratigraphic correlations in structurally complex terrain such as the Pecos greenstone belt.The assumption of a relative immobility of these elements in samples from Jones Hill is supported by the following observation:

1. (1) The TiO₂ and Zr values of Jones Hill rocks fall in narrow clusters (Fig. 1) as opposed to a random scatter plot.

2. (2) The element abundances (ratios) are very similar to: (a) granite (Type A), fresh felsic volcanics (Type A) of the same area and same age (Table I) and basalt (Type B) of the same area; and (b) fresh basalts and rhyolites reported from young volcanic island arcs.

3. (3) Highly altered chlorite schists directly underlying the main ore horizon and within the ore horizon, display Zr/TiO₂ ratios identical to less altered felsic rocks in the foot wall.

These rock geochemical methods can be demonstrated to be applicable at all scales ranging from regional reconnaissance through local prospect evaluation down to detailed drill core interpretations during deposit delineation stages.     

Textural,mineralogical and stable isotope studies of hydrothermal alteration in the main sulfide zone of the Great Dyke,Zimbabwe and the precious metals zone of the Sonju Lake Intrusion,Minnesota, USA

Stratigraphic offsets in the peak concentrations of platinum-group elements (PGE) and base-metal sulfides in the main sulfide zone of the Great Dyke and the precious metals zone of the Sonju Lake Intrusion have, in part, been attributed to the interaction between magmatic PGE-bearing base-metal sulfide assemblages and hydrothermal fluids. In this paper, we provide mineralogical and textural evidence that indicates alteration of base-metal sulfides and mobilization of metals and S during hydrothermal alteration in both mineralized intrusions. Stable isotopic data suggest that the fluids involved in the alteration were of magmatic origin in the Great Dyke but that a meteoric water component was involved in the alteration of the Sonju Lake Intrusion. The strong spatial association of platinum-group minerals, principally Pt and Pd sulfides, arsenides, and tellurides, with base-metal sulfide assemblages in the main sulfide zone of the Great Dyke is consistent with residual enrichment of Pt and Pd during hydrothermal alteration. However, such an interpretation is more tenuous for the precious metals zone of the Sonju Lake Intrusion where important Pt and Pd arsenides and antimonides occur as inclusions within individual plagioclase crystals and within alteration assemblages that are free of base-metal sulfides. Our observations suggest that Pt and Pd tellurides, antimonides, and arsenides may form during both magmatic crystallization and subsolidus hydrothermal alteration. Experimental studies of magmatic crystallization and hydrothermal transport/deposition in systems involving arsenides, tellurides, antimonides, and base metal sulfides are needed to better understand the relative importance of magmatic and hydrothermal processes in controlling the distribution of PGE in mineralized layered intrusions of this type.     

Lead Isotopic Composition and Lead Source of the Huogeqi Cu-Pb-Zn Deposit, Inner Mongolia, China

The Huogeqi orefield located on the northern side of Mt. Langshan, Inner Mongolia occurs in the Middle Proterozoic Langshan Group metamorphic rocks, and the orebodies arc stratiform. In the past twenty years, many Chinese geologists have conducted researches on the Huogeqi Cu-Pb-Zn deposit, but there has been still a controversy on its origin. Some advocate that the deposit is of sedimentary-metamorphic rcworking origin, some hold that it is of sea-floor SEDEX origin, and others have a preference for magmatic superimposition origin. The crux of the controversy is that there is no common understanding about the source of ore-forming materials. In this paper, the Pb isotopic compositions of regional Achaean-Early Proterozoic basement rocks, various types of sedimentary- metamorphic rocks and volcanic rocks in the mining district, Late Proterozoic and Hercynian magmatic rocks arc introduced and compared with the orc-lead composition, so as to constrain the source of the ore lead. The result indicates that （1） sulfides in the ores have homogeneous Pb isotopic compositions, showing a narrow variation range. Their ^206pb/^204pb ratios arc within a range of 17.027- 17.317; ^207Pb/^204pb ratios, 15.451-15.786 and ^208Pb/^204pb ratios, 36.747-37.669; （2） the Pb isotopic compositions of the regional Achaean-Early Proterozoic basement rocks arc characteristic of the old Pb isotopic composition at the early-stage evolution of the Earth, which varies over a wider range, reflecting significant differences in Pb isotopic compositions of the ores. All this indicates that the source of ore lead has no bearing on the basement rocks; （3） the sedimentary-metamorphic rocks in the mining district arc characterized by highly variable and more radiogenic Pb isotopic compositions and their Pb isotopic ratios arc obviously higher than those of ores, demonstrating that ore lead did not result from metamorphic rcworking of these rocks; （4） Pb isotopic compositions of Late Proterozoic diorite-gabbro and Hercynian granite are higher than those of ores. Meanwhile, the Pb isotopic compositions of sulfides in the small-sized strata-penetrating mineralized veinlets formed at later stages arc completely consistent with that of sulfides in stratiform-banded ores, suggesting that these veiniets arc the product of autochthonous rcworking of the stratiform-banded ores during the period of metamorphism and the late magmatic superimposition-mineralization can be excluded; （5） amphibolite, whose protolith is basic volcanic rocks, has the same Pb isotopic compositions as ores, implying that ore lead was derived probably from basic volcanism. So, the source of ore-forming materials for the Huogeqi deposit is like that of the volcanic massive sulfide （VMS） deposits. However, the orebodies do not occur directly within the volcanic rocks, and instead they overlie the volcanic rocks, showing some differences from those typical VMS-type deposits.     

Regional-scale hydrothermal alteration in the Central Blake River Group,western Abitibi subprovince,Canada: implications for VMS prospectivity

The Late Archean Blake River Group is a thick succession of predominantly mafic volcanic rocks within the southern zone of the Abitibi greenstone belt. It contains a number of silicic volcanic centers of different size, including the large Noranda volcanic complex, which is host to 17 past-producing volcanogenic massive sulfide deposits. The Noranda complex consists of a 7- to 9-km-thick succession of bimodal mafic and felsic volcanic rocks erupted during five major cycles of volcanism. Massive sulfide formation coincided with a period of intense magmatic activity (cycle III) and the formation of the Noranda cauldron. Hydrothermal alteration in these rocks is interpreted to reflect large-scale hydrothermal fluid flow associated with rapid crustal extension and rifting of the volcanic complex. The alteration includes abundant albite, chlorite, epidote and quartz (silicification), which exhibit broad stratigraphic and structural control and correlate with previously mapped whole-rock oxygen isotope zonation. The Mine Sequence volcanic rocks are characterized by abundant iron-rich chlorite (Fe/Fe+Mg >0.5), hydrothermal amphibole (ferroactinolite) and coarse-grained epidote of clinozoisite composition (<10 wt% Fe ₂O ₃). Volcanic rocks of the pre-cauldron sequences, which contain only subeconomic stringer mineralization, are characterized by less abundant chlorite and mainly fine-grained epidote (>10 wt% Fe ₂O ₃) lacking the clinozoisite solid solution. Alteration in the Mine Sequence volcanic rocks persists along strike well beyond the limits of the main ore deposits (as far as several tens of kilometers) and can be readily distinguished from greenschist facies metamorphic assemblages at a regional scale. The lack of similar alteration in the pre-cauldron sequences is consistent with limited ¹⁸O-depletion and suggests that the early history of the volcanic complex did not support large-scale, high-temperature fluid flow in these rocks. Comparisons with a much smaller, barren volcanic complex in nearby Ben Nevis township reveal important differences in the alteration mineralogy between volcanoes of different size, with implications for area selection during regional-scale mineral exploration. The Ben Nevis Complex consists of a 3- to 4-km-thick succession of mafic, intermediate and felsic volcanic rocks centered on a small subvolcanic intrusion. Alteration of the volcanic rocks comprises mainly low-temperature assemblages of prehnite, pumpellyite, magnesium-rich chlorite (Fe/Fe+Mg <0.5), iron-rich epidote (>10 wt% Fe ₂O ₃) and calcite. Actinolite ± magnetite alteration occurs proximal to the intrusive core of the complex, but the limited extent of this alteration indicates only local high-temperature fluid circulation adjacent to the intrusion. A distal zone of carbonate alteration is located 4–6 km from the center of the volcano. Although iron-bearing carbonates are present locally within this zone, the absence of siderite argues against a high-temperature origin for this alteration. These observations do not offer positive encouragement for the existence of a fossil geothermal system of sufficient size or intensity to have produced a large massive sulfide deposit.     

金川超大型铜镍硫化物矿床的铂族元素地球化学特征

对金川超大型铜镍岩浆硫化物矿床岩石、矿石的铂族元素地球化学特征研究表明 ,金川岩体的平均Cu/Pd值远大于原生地幔岩浆的Cu/Pd值 ,说明其岩石为因硫化物析离而失去Pd的岩浆所结晶 ;且岩石的PGE具有部分熔融趋势 ,与地幔橄榄岩接近 ,这些均指示存在岩浆熔离作用。该矿床岩石、矿石的PGE球粒陨石标准化分布模式比较对应 ,均可分为两种类型 ,反映了岩浆多次侵入、熔离分异同时成岩成矿的特征。另外 ,PGE S关系分析表明其成岩成矿过程中有少量地壳物质混染。PGE地球化学特征参数还指示了其高镁拉斑玄武质母岩浆的性质。     

Source component mixing controls the variability in Cu and Au endowment along the strike of the Eastern Andean Cordillera in Peru

Mississippian arc magmatic suites of the Au-rich Pataz and Cu-dominated Montañitas regions in Peru reveal distinct modes of magmatic-hydrothermal petro- and metallogenesis. The distinction is remarkable due to their broad contemporaneity (336–322 Ma), arc-parallel position, and close distance (<?50 km) to each other. In both arc regions, petrography, geochemistry, and the tectonic setting of magmatic suites suggest a rapid switch from syn-collisional/compressional to post-collisional/extensional (with ‘A₂-type’ signature) emplacement regime. Rocks of the Au-rich Pataz region originate from mixed sources with a contribution from the mantle (εHf?>?0 and δ¹⁸O of ~?5.3‰) and assimilated old crust (variously low εHf and δ¹⁸O >?5.3‰). The ultimate source of Au in the mineralised Pataz batholith was oxidised (fO₂ at FMQ buffer; based on zircon trace chemistry) and alkali-, LILE- and HFSE-enriched, most likely represented by the metasomatised mantle. The syn-extensional emplacement of the relatively reduced (ΔFMQ?<?0), but unmineralised, A₂-type suite involved assimilation of reduced crust. Associated, reduced, magmatic-hydrothermal fluids infiltrated the Au-bearing batholith suite and effectively mobilised and transported and concentrated Au. In the Montañitas region, rocks are oxidised (ΔFMQ?>?0) and dominantly mantle derived without significant incorporation of crustal material. Samples from the Cu-mineralised suites indicate the additional contribution of a δ¹⁸O <?5.3‰ source, potentially melted layer-2 gabbro. In addition, the elevated whole-rock La/Yb and Sr/Y ratios are compatible with minor addition of slab-derived material, which may have enhanced Cu endowment in this region. Late-magmatic, oxidised fluids derived from the younger A₂-type suite controlled Cu mobilisation and concentration, while Au behaved largely refractory. In general terms, it is postulated that source mixing in continental arcs is a first-order control of contrasting Cu and Au endowment and that sequential intrusion processes facilitate late-magmatic-hydrothermal mobilisation and concentration of specific metal assemblages.     

Re and Os concentrations in arc basalts: The roles of volatility and source region fO2 variations

Olivine and spinel compositions, major elements (including ferric and ferrous iron), S, Re and Os contents have been measured for a suite of primitive (most >6 wt% MgO) basalts from the Trans Mexican Volcanic Belt (TMVB), including the western Mexican volcanic belt, the Michoacan-Guanajuato Volcanic Field, Sierra Chichinautzin, Pico de Orizaba region, Palma Sola, San Martin Tuxtlas, and the eastern alkaline province (EAP). Sulfur contents at sulfide saturation were calculated to determine whether the measured S contents are representative of sulfide saturated liquids. Most of the samples have S contents much lower than expected for sulfide saturation. A few have higher contents than calculated perhaps due to the presence of sulfate in the measured total sulfur (i.e., more oxidized samples). Comparison of the TMVB samples along with previously analyzed MORB, OIB, BABB and arc samples reveals a continuum of Re and Os contents that is best explained by variation in oxygen fugacity—and thus sulfide stability—in the source region. High Re and Os magmatic suites are best explained by derivation by melting of oxidized mantle, where sulfide is no longer stable and Re and Os behave incompatibly, whereas low Re and Os magmatic suites are derived from melting of relatively reduced mantle where sulfide is stable, and Re and Os behave compatibly. Intermediate examples abound, and arc magmas span a wide range of Re and Os concentrations due to variation of fO₂ in the source during genesis of arc magmas. Low Re magmatic suites are furthermore potentially affected by volatility which can lower Re by a factor of 3-5.     

莱芜张家洼铁矿磁铁矿LA-ICP-MS微量元素特征及其对成矿过程的制约

莱芜张家洼铁矿位于华北克拉通东缘的鲁西地区,矿石成因类型为夕卡岩型铁矿。矿体赋存在早白垩世高镁闪长岩与奥陶系马家沟组灰岩及白云岩接触带附近。本文通过对莱芜岩浆和热液磁铁矿电子探针(EPMA)以及激光剥蚀电感耦合等离子体质谱(LA ICP MS)分析,探讨磁铁矿微量元素组成及变化规律对成岩和成矿作用的指示,为揭示张家洼铁矿的矿床成因及其成矿流体演化过程提供重要制约。分析结果表明,莱芜岩浆磁铁矿与热液磁铁矿相比明显富集Ti、V、Cr等亲铁元素,相对富集Nb、Ta、Zr、Hf等高场强元素以及Sn、Ga、Ge、Sc等中等相容元素,Mg、Al、Mn、Zn、Co显著富集于热液磁铁矿中。Ti、V、Cr以及Mg、Al、Mn、Zn在岩浆和热液中具有不同的地球化学行为,Ti、V、Cr从熔体中进入磁铁矿主要受温度、分配系数以及fO2控制。Mg、Al、Mn、Zn主要受控于水岩反应和后期绿泥石+碳酸盐脉的交代,这些元素通过类质同象替换富集于热液磁铁矿中。Co在热液磁铁矿中除了受水岩相互作用和后期流体交代的影响外,硫化物的出现会导致Co含量急剧降低。Si、Ca、Na及Sr、Ba在岩浆和热液磁铁矿中的地球化学行为非常一致。Ti Ni/Cr图能够用于区分岩浆和热液磁铁矿,莱芜岩浆磁铁矿中Ti含量较高且Ni/Cr比值≤1,热液磁铁矿Ti含量较低且绝大多数Ni/Cr比值≥1。张家洼热液磁铁矿可分为早、晚两个阶段：早期阶段包括(1)早期原生粒状磁铁矿和(2)早期次生磁铁矿;晚期阶段包括(3)晚期原生磁铁矿和(4)晚期次生磁铁矿。原生磁铁矿具有典型的三联点结构特征;次生磁铁矿受后期热液交代影响表现为多空隙,通常呈不规则状、树枝状、骸晶以及交代残余结构。磁铁矿微量元素生动记录了成矿流体演化过程,从早期到晚期、从原生到次生都显示Mg、Al、Mn、Zn包括Co含量持续升高,表明成矿流体可能朝着富集这些微量元素的方向演化。后期流体的交代导致绿泥石蚀变为磁铁矿,连续水岩相互作用和后期流体的交代以及绿泥石直接蚀变是导致热液磁铁矿富集Mg、Al、Mn、Zn等元素的主要原因。热液磁铁矿晚期孔隙较为发育,孔隙度的增加促使更多的流体和磁铁矿发生反应。热液磁铁矿的微量元素不仅能够反映矿床形成的物理化学条件,而且可以反映围岩性质以及水岩相互作用过程。