Sulphur source and genesis of polymetallic sulphide occurrences of the Ofoten district in the Central–North Norwegian Caledonides: evidence from sulphur isotopic studies

Fourteen stratiform, stratabound and vein-type sulphide occurrences in the Upper Allochthon of the Central–North Norwegian Caledonides have been studied for their sulphur, oxygen and hydrogen isotope composition. Depositional ages of host rocks to the stratabound and stratiform sulphide occurrences range from 590 to 640?Ma. The sulphides and their host rocks have been affected by polyphase deformation and metamorphism with a peak temperature of 650?°C dated to 432?Ma. A total of 104 sulphide and 2 barite samples were analysed for δ³⁴S, 16 whole-rock and quartz samples for δ¹⁸O and 12 samples of muscovite for δD. The overall δ³⁴S values range from ?14 to +31‰ with the majority of sampled sulphides lying within a range of +4 to +15‰. In most cases δ³⁴S within each hand specimen behaves in accordance with the equilibrium fractionation sequence, δ³⁴S_gn<δ³⁴S_cp<δ³⁴S_sph<δ³⁴S_py. A systematic increase in δ³⁴S from the vein sulphides (?8‰) through schist/amphibolite-hosted (+6‰) and schist-hosted (+7 to +12‰) to dolomite-hosted (+12 to +31‰) occurrences is documented. The δ³⁴S averages of the stratiform schist-hosted sulphides are 17 to 22‰ lower than in the penecontemporaneous seawater sulphate. The Bjørkåsen (+4 to +6‰) occurrence is a volcanogenic massive sulphide (VMS) transitional to sedimentary massive sulphide (SMS), exhalative, massive, pyritic deposit of Cu–Zn–Pb sulphides formed by fluids which obtained H₂S via high-temperature reduction of seawater sulphate by oxidation of Fe²⁺ during the convective circulation of seawater through underlying rock sequences. The Raudvatn, volcanic-hosted, disseminated Cu sulphides (+6 to +8‰) obtained sulphur via a similar process. The Balsnes, stratiform, ‘black schist’-hosted, pyrite–pyrrhotite occurrence (?6 to ?14‰) is represented by typical diagenetic sulphides precipitated via bacteriogenic reduction of coeval (ca. 600?Ma) seawater sulphate (+25 to +35‰) in a system open to sulphate supply. The δ³⁴S values of the Djupvik–Skårnesdalen (+7 to +12‰), Hammerfjell (+5 to 11‰), Kaldådalen (+10 to +12‰) and Njallavarre (+7 to +8‰) stratiform, schist-hosted, massive and disseminated Zn–Pb (±Cu) sulphide occurrences, as well as the stratabound, quartzite-hosted, Au-bearing arsenopyrite occurrence at Langvatnet (+7 to +11‰), suggest that thermochemically reduced connate seawater sulphate was a principal sulphur source. The Sinklien and Tårstad, stratabound, dolomite- and dolomite collapse breccia-hosted, Zn (±Cu–Pb) sulphides are marked by the highest enrichment in ³⁴S (+20 to +31‰). The occurrences ?are?assigned to the Mississippi-Valley-type deposits.?High δ³⁴S values require reduction/replacement of contemporaneous (ca. 590?Ma) evaporitic sulphate (+23 to +34‰) with C_org-rich fluids in a closed system. The Melkedalen (+12 to +15‰), stratabound, fault-controlled, Cu–Zn sulphide deposit is hosted by the ca. 595?Ma dolomitised Melkedalen marble. The deposit is composed of several generations of ore minerals which formed by replacement of host dolomite. Polyphase hydrothermal fluids were introduced during several reactivation episodes of the fault zone. The positive δ³⁴S values with a very limited fractionation (<3‰) are indicative of the sulphide-sulphur generated through abiological, thermochemical reduction of seawater sulphate by organic material. The vein-type Cu (±Au–W) occurrences at Baugefjell, Bugtedalen and Baugevatn (?8 to ?4‰) are of hydrothermal origin and obtained their sulphur from igneous sources with a possible incorporation of sedimentary/diagenetic sulphides. In a broad sense, all the stratiform/stratabound, sediment-hosted, sulphide occurrences studied formed by epigenetic fluids within two probable scenarios which may be applicable separately or interactively: (1) expulsion of hot metal-bearing connate waters from deeper parts of sedimentary basins prior to nappe translation (late diagenetic/catagenetic/epigenetic fluids) or (2) tectonically driven expulsion in the course of nappe translation (early metamorphic fluids). A combination of (1) and (2) is favoured for the stratabound, fault-controlled, Melkedalen and Langvatnet occurrences, whereas the rest are considered to have formed within option (1). The sulphides and their host rocks were transported from unknown distances and thrust on to the Fennoscandian Shield during the course of the Caledonian orogeny. The displaced/allochthonous nature of the Ofoten Cu–Pb–Zn ‘metallogenetic province’ would explain the enigmatically high concentration of small-scale Cu–Pb–Zn deposits that occur only in this particular area of the Norwegian Caledonides.     

Geochemistry and age of metamorphosed felsic igneous rocks with A-type affinities in the Willyama Supergroup, Olary Block, South Australia, and implications for mineral exploration

Leucocratic quartzofeldspathic gneisses form a significant proportion of the lower part of the Palaeoproterozoic Willyama Supergroup sequence in the Olary Block, South Australia and have correlatives in the adjacent Broken Hill Block. Field and geochemical data demonstrate that these rocks were originally rhyolitic volcanics and granite, with A-type affinities consistent with magma production during intracratonic rifting, supporting tectonic models proposed for the Willyama Supergroup in the Broken Hill Block. Although the rocks have characteristic high-field-strength element enrichment, many have undergone extensive pervasive pre- or syn-metamorphic sodic alteration and are typically rich in albite.

Sensitive high resolution ion microprobe (SHRIMP) U-Pb zircon data tightly constrain the depositional and early intrusive history. Zircons from an interpreted metavolcanic rock containing relict quartz phenocrysts yield an age of 1699 ± 10 Ma, whereas a metagranitoid sample has an age of 1703 ± 6 Ma. These results are compatible with recent geochronological data on felsic metavolcanic rocks from the Broken Hill Block (Page and Laing, 1992) and are indicative of widespread magmatism during deposition of the Willyama Supergroup. Nd signatures for the two Olary Block samples imply the presence of a significant component from a depleted mantle source.

The A-type metavolcanic rocks are locally associated with small iron formations, some of which grade into stratiform barite-rich horizons. Although potentially favourable for sediment-hosted exhalative PbZn mineralisation, the Fe- and Ba-rich units, along with transgressive vein and breccia occurrences of Fe oxides ± quartz ± pyrite cutting both the metavolcanic and metagranitoid rocks, may be more prospective for epigenetic Cu-Au mineralisation related to later metamorphic and/or magmatic events. Partial melting of the A-type suite during high grade regional metamorphism at ˜ 1600 ± 20 Ma led to the formation of local volumes of sodic granitoids and pegmatites containing U-Th-Ti-REE-F mineralisation.     

Stratiform and stratabound tungsten mineralisation in the Broken Hill Block,N.S.W.

The Early‐Middle Proterozoic Broken Hill Block contains three types of W occurrences, which show close stratigraphic control. All three types occur within a relatively narrow stratigraphic interval (the ‘Mine Sequence’ Suite of Stevens et al., 1980) comprising a highly variable group of metamorphosed silicic and mafic volcanics, clastic sediments, and exhalative and chemical sediments containing base metals. The first type includes occurrences of W and base metals in bedded calc‐silicate rocks. In the second type, W occurs in layered to non‐layered calcsilicate rocks associated with amphibolite; these are intimately associated in a narrow stratigraphic interval containing abundant, small, Broken Hill type deposits. The third type comprises stratabound, W‐bearing pegmatites, which have been remobilised from quartz‐feldspar‐biotite gneiss and bedded quartz‐tourmaline rocks. Tungsten has been mined only from the third type and only in small quantities. The three types of tungsten deposits show a close spatial relationship with stratiform and stratabound Pb‐Zn mineralisation, including the Broken Hill type. The Pb‐Zn and W deposits are inferred to be genetically related.     

Genetic implications of pyrite chemistry from the Palaeoproterozoic Olary Domain and overlying Neoproterozoic Adelaidean sequences, northeastern South Australia

Sulphide mineralisation associated with rocks from the Palaeoproterozoic Olary Domain (OD) and overlying Neoproterozoic Adelaidean sequences has undergone a complex history of metamorphism and remobilisation. In this study, new trace element and sulphur isotopic analyses of pyrites from a large number of deposits and paragenetic generations are combined with an existing data set to build up a sequence of mineralising events linked to the tectonometamorphic evolution of the region. The typically high Co/Ni ratios (>10) indicate that early strata-bound pyrite precipitated from a volcanic-related fluid, which had fluctuating activities of the two metals during the early stages of the evolution of the Willyama basin. This period of mineralisation was followed by a diagenetic concentration of sulphide mineralisation at the horizon known as the Bimba Formation, which occurred as a result of the differing redox conditions between the upper and lower sequences in the Willyama Supergroup. During the Mesoproterozoic (1600 to 1500 Ma) Olarian Orogeny, metamorphic remobilisation of strata-bound pyrite resulted in an epigenetic signature; the trace element concentrations of this generation were controlled primarily by the proximity of mineralisation to the mafic intrusive bodies found throughout the terrane. Further reworking of existing sulphides during the Delamerian Orogeny and associated granitoid-intrusive rocks led to the formation of a new generation of epigenetic pyrite that occurs in quartz veins in the Adelaidean sequences and veins that crosscut Olarian fabrics in the Olary Domain. δ³⁴S results range from 16‰ to 11‰, but most data fall between 2‰ and 4‰. This association is suggestive of an initial uniform deep-seated crustal reservoir of sulphur, which has been repeatedly tapped throughout the metallogenic history of the region. The isotopic outliers can be explained by the input of biogenic sulphur or sulphur derived from oxidised, possibly evaporitic, sediments, respectively. Previous workers have invoked the Kupferschiefer and the Zambian Copperbelt as analogues to mineralisation processes in the Olary Domain. This study shows that δ³⁴S and trace element data are suggestive of some affinities with the aforementioned analogues, but a more likely link can be made between epigenetic remobilisation in the Olary region and the iron oxide copper gold (IOCG) style of mineralisation found at the nearby Olympic Dam deposit.     

Genesis of sediment-hosted stratiform copper–cobalt deposits, central African Copperbelt

The Neoproterozoic central African Copperbelt is one of the greatest sediment-hosted stratiform Cu–Co provinces in the world, totalling 140 Mt copper and 6 Mt cobalt and including several world-class deposits (10 Mt copper). The origin of Cu–Co mineralisation in this province remains speculative, with the debate centred around syngenetic–diagenetic and hydrothermal-diagenetic hypotheses.The regional distribution of metals indicates that most of the cobalt-rich copper deposits are hosted in dolomites and dolomitic shales forming allochthonous units exposed in Congo and known as Congolese facies of the Katangan sedimentary succession (average Co:Cu = 1:13). The highest Co:Cu ratio (up to 3:1) occurs in ore deposits located along the southern structural block of the Lufilian Arc. The predominantly siliciclastic Zambian facies, exposed in Zambia and in SE Congo, forms para-autochthonous sedimentary units hosting ore deposits characterized by lower a Co:Cu ratio (average 1:57). Transitional lithofacies in Zambia (e.g. Baluba, Mindola) and in Congo (e.g. Lubembe) indicate a gradual transition in the Katangan basin during the deposition of laterally correlative clastic and carbonate sedimentary rocks exposed in Zambia and in Congo, and are marked by Co:Cu ratios in the range 1:15.The main Cu–Co orebodies occur at the base of the Mines/Musoshi Subgroup, which is characterized by evaporitic intertidal–supratidal sedimentary rocks. All additional lenticular orebodies known in the upper part of the Mines/Musoshi Subgroup are hosted in similar sedimentary rocks, suggesting highly favourable conditions for the ore genesis in particular sedimentary environments. Pre-lithification sedimentary structures affecting disseminated sulphides indicate that metals were deposited before compaction and consolidation of the host sediment.The ore parageneses indicate several generations of sulphides marking syngenetic, early diagenetic and late diagenetic processes. Sulphur isotopic data on sulphides suggest the derivation of sulphur essentially from the bacterial reduction of seawater sulphates. The mineralizing brines were generated from sea water in sabkhas or hypersaline lagoons during the deposition of the host rocks. Changes of Eh–pH and salinity probably were critical for concentrating copper–cobalt and nickel mineralisation. Compressional tectonic and related metamorphic processes and supergene enrichment have played variable roles in the remobilisation and upgrading of the primary mineralisation.There is no evidence to support models assuming that metals originated from: (1) Katangan igneous rocks and related hydrothermal processes or; (2) leaching of red beds underlying the orebodies. The metal sources are pre-Katangan continental rocks, especially the Palaeoproterozoic low-grade porphyry copper deposits known in the Bangweulu block and subsidiary Cu–Co–Ni deposits/occurrences in the Archaean rocks of the Zimbabwe craton. These two sources contain low grade ore deposits portraying the peculiar metal association (Cu, Co, Ni, U, Cr, Au, Ag, PGE) recorded in the Katangan sediment-hosted ore deposits. Metals were transported into the basin dissolved in water.The stratiform deposits of Congo and Zambia display features indicating that syngenetic and early diagenetic processes controlled the formation of the Neoproterozoic Copperbelt of central Africa.     

http://www.sciencedirect.com/science/article/pii/S1674987115000900

The Chertovo Koryto gold deposit(80 t Au at 1.84 g/t) in the Lena world-class province,Siberia,is hosted in a metamorphosed sequence of the Paleoproterozoic Mikhailovsk Formation that comprises the oldest black shale strata of the Baikal-Patom region.The mineralisation is confined to the thrust zone complicated with a conjugate anticline fold,zones of shearing and dislocation.The struaural position of the mineralisation is similar to that at the giant Sukhoi Log deposit in the neighbouring Mama-Bodaibo zone.In the latter,the isotope age data suggest that Khomolkho black shales,hosts to Sukhoi Log mineralisation,are of Ediacaran age and underwent prograde metamorphism during early Paleozoic.The geochemical composition of the terrigenous rocks that host Sukhoi Log,Chertovo Koryto,and a number of other deposits at the various stratigraphic levels throughout the Proterozoic sequence have much in common.They do not show elevated metal contents above the common black shale abundances,except for Au and As,which is at variance with the accepted view on diagenetic enrichment of black shales in the Lena province.The occurrence of sagenitic rutile in quartz and chlorite pseudomorphs after biotite and other petrographic observations provide evidence on a retrograde nature of the metamorphic mineral assemblages in the Mikhailovsk rocks.The sulphides are pyrrhotite and arsenopyrite with very minor pyrite at Chertovo Koryto,whereas pyrite is the predominant sulphide in the Sukhoi Log ore.Fluid inclusion data on both deposits emphasise a high-temperature nature of the mineralisation albeit revealing great contrast in the fluid composition.Sukhoi Log mineralisation was formed at mixing between low-salinity aqueous solutions and dense gaseous carbonic fluids,which facilitated effective gold scavenging and precipitation,as demonstrated by thermodynamic simulation.The precursory devolatilisation of the Mikhailovsk sediments at the prograde stage results in the paucity of gaseous carbonic fluid during retrograde metamorphism and mineralisation.The similarity in the styles and chemical parameters of mineralisation,and the predominant structural control of ore localisation within the same Precambrian regional tectonic unit support an idea that orogenic gold mineralisation in the Lena province was produced during a single early Paleozoic event.     

Remobilisation features and structural control on ore grade distribution at the Konkola stratiform Cu–Co ore deposit,Zambia

The Konkola deposit is a high grade stratiform Cu–Co ore deposit in the Central African Copperbelt in Zambia. Economic mineralisation is confined to the Ore Shale formation, part of the Neoproterozoic metasedimentary rocks of the Katanga Supergroup. Petrographic study reveals that the copper–cobalt ore minerals are disseminated within the host rock, sometimes concentrated along bedding planes, often associated with dolomitic bands or clustered in cemented lenses and in layer-parallel and irregular veins. The hypogene sulphide mineralogy consists predominantly of chalcopyrite, bornite and chalcocite. Based upon relationships with metamorphic biotite, vein sulphides and most of the sulphides in cemented lenses were precipitated during or after biotite zone greenschist facies metamorphism. New δ³⁴S values of sulphides from the Konkola deposit are presented. The sulphur isotope values range from −8.7‰ to +1.4‰ V-CDT for chalcopyrite from all mineralising phases and from −4.4‰ to +2.0‰ V-CDT for secondary chalcocite. Similarities in δ³⁴S for sulphides from different vein generations, earlier sulphides and secondary chalcocite can be explained by (re)mobilisation of S from earlier formed sulphide phases, an interpretation strongly supported by the petrographic evidence. Deep supergene enrichment and leaching occurs up to a km in depth, predominantly in the form of secondary chalcocite, goethite and malachite and is often associated with zones of high permeability. Detailed distribution maps of total copper and total cobalt contents of the Ore Shale formation show a close relationship between structural features and higher copper and lower cobalt contents, relative to other areas of the mine. Structural features include the Kirilabombwe anticline and fault zones along the axial plane and two fault zones in the southern limb of the anticline. Cobalt and copper behave differently in relation to these structural features. These structures are interpreted to have played a significant role in (re)mobilisation and concentration of the metals, in agreement with observations made elsewhere in the Zambian Copperbelt.     

Wallrock alteration at the western system of the CSA mine,cobar, New South Wales,Australia

Petrological and geochemical characteristics of the sequence from chloritic siltstone through “siliceous siltstone” to “elvan”, which occurs approaching ore on the hanging wall of the Western System in the CSA Mine, Cobar, indicate its formation by extensive dilational silicification together with minor addition of S and K very close to the ore. The manner whereby more chloritic members of this sequence developed from ‘background’ muscovite-rich siltstones remote from mineralization is less clear. “Elvan” is not an exhalative chert. The distribution of wallrock alteration products and characteristics of the ores do not support suggestions that Cobar orebodies are remobilized stratiform deposits. Alteration preceded epigenetic ore emplacement in the early stages of a regional metamorphic episode which took place soon after Early Devonian turbidite sedimentation without associated igneous activity. Mineralization is attributed to diagenetic dewatering or metamorphic dehydration of the sediment pile.     

Origin of giant stratabound deposits of magnesite and siderite in Riphean carbonate rocks of the Bashkir mega-anticline, western Urals

In the Bashkir mega-anticline (western Urals) stratabound magnesite, siderite, fluorite and base-metal deposits are hosted by a sequence of Riphean sediments with a thickness of more than 12 km. The giant deposits of siderite (Bakal) and sparry magnesite (Satka) belong to the largest known mineral deposits of this type on Earth but are still disputed with respect to their origin. Both the Fe- and Mg-carbonate ores are clearly characterized by mimetic preservation of sedimentary and diagenetic textures of the host carbonate sediments, giving evidence of epigenetic metasomatic replacement. In the stratiform magnesite deposits of Satka, O- and C-isotopes, REE pattern and the lithostratigraphic position of the ore, point to the reflux of early diagenetic Mg-rich brines being responsible for the selective replacement of brecciated dolomite. The Bakal siderite deposits are hosted by Lower Riphean carbonate rocks and are controlled by a Lower to Middle Riphean unconformity marked by deep erosion and subsequent transgression-related sedimentation of coarse clastics. Their independence of carbonate lithofacies and their trace element distribution are indicative of metasomatic processes. Fe-bearing fluids have probably been generated by low-grade metamorphic (catagenetic) devolatization from underlying argillites, causing the metasomatic formation of large siderite ore bodies in the Bakal carbonates due to the focusing of the fluid flow by the overlying Mid-Riphean coarse clastics.     

安徽铜陵冬瓜山铜、金矿床两阶段成矿模式

冬瓜山铜金矿床包括层状硫化物矿体、矽卡岩型和斑岩型矿体。层状硫化物矿体具层状形态和层控特征，矿石具块状、层纹状和揉皱状构造。燕山期岩浆岩及其岩浆流体对层状矿体进行了叠加和改造，改变了其结构构造、矿物组合和矿石成分，并在其上叠加蚀变和矿化。层状矿体中的铜是由含铜流体交代块状硫化物矿石形成的。冬瓜山铜金矿床经历了两次成矿作用：第一成矿阶段．在石炭纪中期，海底喷流作用形成了块状硫化物矿床，矿石成分以硫、铁矿为主；第二成矿阶段。燕山期岩浆侵人，一方面岩浆热液与围岩相互作用发生矽卡岩化、硅化、钾长石化、石英绢云母化和青磐岩化，形成矽卡岩型和斑岩型矿体，另一方面岩浆流体对块状硫化物矿体进行叠加改造，致使块状硫化物矿体富集铜等成矿物质。     

Regional fluid flow and the genesis of Irish Carboniferous base metal deposits

The Irish carbonate-hosted base metal deposits have long been an enigmatic subclass of deposit. Some of the Irish deposits (Harberton Bridge, Allenwood) are clearly epigenetic, hosted in breccia pipes and have close affinities to Mississippi Valley-type deposits. Others, are characterised by stratabound and sometimes stratiform mineralisation (Abbeytown, Navan), while a third group is associated closely with concordant dolomitic breccias and cavity fill mineralisation at the base of carbonate mud mounds (Lisheen, Ballinalack). When the stratigraphic and textural evidence is reviewed, it is apparent that all of the base metal mineralisation occurred in already compacted sediments. Hence, mineralisation probably occurred at depths of at least several 100 m depth below the sea bed. The mineralisation occurred either during transtensional Lower Carboniferous basin development, or subsequently, during the onset of Variscan shortening. Fluids may have been derived from dewatering of Variscan-driven deformation to the south of the carbonate platform, with fluid flow through major fracture zones and basal clastic aquifers. Alternatively, high heat flow produced by Lower Carboniferous extension may have driven the mineralising system. Syn-genetic models are extremely difficult to sustain.     

Contrasting Pb isotopes of Archaean and Palaeoproterozoic sulphide mineralisation, Disko Bugt, central West Greenland

Sulphide separates from mineralisation in Archaean and Palaeoproterozoic host rocks of the Disko Bugt area, central West Greenland, have been analysed for trace elements and Pb isotopes. Isotopic compositions of lead from sulphide separates of Archaean supracrustal rocks show wide variations. Archaean semi-massive sulphides and sulphides separated from felsic metavolcanites yield an errorchron age of 2821 +77/−82, with a model μ₁ value of 7.36; this is comparable to the estimated age of the supracrustal rocks. The two most prominent mineralised sites, the Andersen and Eqip prospects, have their own unique Pb isotope pattern; the Andersen prospect is considered to represent the result of an upper crust of Palaeoproterozoic process. The sulphide separates of Palaeoproterozoic epigenetic mineralisation hosted in shear and fault zones in the supracrustal rocks has a common origin, e.g. linked to a metamorphic peak and/or hydrothermal alteration. Gold-bearing samples indicate a local origin for associated sulphides; no regional processes seem to be involved in the formation of the gold occurrences. Received: 17 March 1997 / Accepted: 8 July 1997     

Some evidence for the base-metal potential of the Pretoria Group: stratigraphic targets,tectonic setting and REE patterns

The Timeball Hill and Silverton Formations of the 2.1–2.3 Ga Pretoria Group have regional lithological associations which are thought to have been favourable for the genesis of stratiform sulphide deposits. The observed association of carboniferous and pyritic black shales, tuffaceous material, stromatolitic carbonates and inferred turbidity current deposits is common in stratiform sulphide deposits of the sedimentary exhalative group. Massive sulphides in the Silverton Formation are compatible with a syngenetic brine discharge, probably related to deep fracture systems. The basal shales of the Timeball Hill Formation are significantly enriched in base-metals and Ba. Interlayered tuff beds at this stratigraphic level have PGE-contents of up to 1 g/t. The REE-geochemistry of Pretoria Group sedimentary rocks supports hydrothermal activity as an important factor in both stratigraphic units.     

Uraniferous diagenetic xenotime in northern Australia and its relationship to unconformity-associated uranium mineralisation

Stratabound, uraniferous diagenetic xenotime cements provide a minimum depositional age of 1,632±3 Ma for the sedimentary Birrindudu Group in the Killi Killi Hills, Tanami Region in northern Australia. The age of xenotime formation is broadly coeval with that recently proposed (1,650–1,600 Ma) for uranium mineralisation in the unconformity-associated deposits of the Pine Creek Inlier, northern Australia, and Athabasca Basin, Canada. The geological setting and formation model for the uraniferous xenotime crystals are similar to those widely proposed for unconformity-associated uranium deposits, suggesting a genetic link between the two. However, xenotime formation in the Birrindudu Group occurred during an apparently earlier stage of diagenesis, compared to late diagenetic formation of unconformity-associated uranium deposits. This could be explained by variations in the thickness of sediment cover and diachronous diagenesis across the basin, at the time of the basin-wide uranium mobilisation event, herein dated at ca. 1,630 Ma. In such a scenario, stratabound uraniferous xenotime cements could represent the remote distal zones of a more deeply buried, uranium mineralising system. Alternatively, the xenotime layer represents a precursor to, or a source for, later unconformity-associated ore deposition. In this case, the presence of diagenetic uraniferous xenotime in an area prospective for unconformity-associated uranium mineralisation would be an indication of, and still provide an approximate age for, uranium mobilisation within the cover sequence. Xenotime is a far more robust mineral than uraninite for U–Pb geochronology and can potentially provide a more reliable and precise timeframe for uranium mineralisation and subsequent recrystallisation events if present in the immediate uranium-ore environment.     

块状硫化物矿石中硫化物的压溶和增生及成矿意义——以加拿大西部矿床为例

加拿大西部块状硫化物矿石普遍地发生过硫化物的压溶和增生。增生作用根据增生体的成分可以分为同质增生和异质增生，根据动力环境可以分为静态增生和动态增生。三晶嵌接结构可以是静态增生的产物。压溶和增生是块状硫化物矿床成岩和变质过程中的重要作用。脉石矿物的压溶可使原生矿石就地加富，硫化物的压溶可使成矿物质发生再活化。增生可促进矿质沉淀。富含硫化物的地层之所以能成为地球化学障而有利于后期热液叠加和层控矿床的形成，硫化物晶芽的增生是一种重要机制。     

四川省拉拉铜矿田赋矿变质岩原岩恢复及其对找矿的指示

四川拉拉铜矿是中国著名的大型富铜矿床,其矿体均赋存于河口群变质岩系中,具有典型的层控特征,因此对这套变质岩开展原岩恢复工作能更好地限定其矿床成因,也可对区域下一步的深边部找矿提供指示。通过对赋矿变质岩开展了系统的岩相学和岩石地球化学研究,结果显示其原岩为一套泥质粉砂岩-细碧岩的岩石组合。其中,黑云石英片岩、二云石英片岩及白云石英片岩的原岩可能为泥质粉砂岩,而石英钠长岩、变火山角砾岩及变火山凝灰岩的原岩可能为细碧岩。河口群形成的构造环境可能为裂谷环境。结合矿体似层状、透镜状的层控特征及已有的成岩成矿年龄数据,认为该矿床存在早期VMS（火山成因块状硫化物矿床）型铜矿化,根据其围岩类型,可能属于"别子型"成矿亚类。本区此类矿床下一步找矿工作的方向为铁镁质火山岩与硅质碎屑岩的界面。     

扬子地台西缘大渡河谷超大型层状铅锌矿床地质地球化学特征及成因

20世纪90年代以来，四川地矿局207地质队及地方采矿业在扬子地台西缘大渡河谷沿岸，相继发现了黑区—雪区、红花、白熊沟、核桃坪、养善坪—中溪坪—宝水溪—双凤沟等层状铅锌矿床及矿点。其中，黑区—雪区矿床的矿体地表露头长达6000m，估算的铅锌资源量达370万吨，达到超大型矿床的规模。不同的矿床、矿点的分布构成了长度大于50km的铅锌矿化带，预测铅锌资源量有望达1000万吨以上。各矿床、矿点产于上震旦统—下寒武统灯影组顶部麦地坪段含磷硅质白云岩的相同层位，矿化与硅质岩及角砾状白云岩层关系密切。矿床中发育层状、纹层状、条带状、同沉积角砾状、层间揉皱等构造，沉积特征明显。矿床的地质、地球化学特征显著区别于区域上以往发现的产于地台盖层碳酸盐岩中具有明显后生成因特点的密西西比河谷型（MVT型）铅锌矿床，也有别于产于中元古界褶皱基底中的火山沉积块状硫化物型（VMS型）铅锌矿床。笔者的研究表明，与铅锌矿密切共生的硅质岩为典型的海底热水沉积产物，铅锌矿床属于海底喷流—沉积成因（SEDEX型）；成矿作用受晚震旦世—早寒武世北东东向延伸的汉源—峨眉凹陷及同生断裂带的控制。该类层状铅锌矿床可能是扬子地台西缘铅锌矿聚区中最具找矿潜力的矿床类型。     

New mineralogical and isotopic constraints on Main Zone-hosted PGE mineralisation at Moorddrift, northern Bushveld Complex

The northern limb of the Bushveld Complex, South Africa contains a number of occurrences of platinum-group element (PGE) mineralisation within Main Zone rocks, whereas the rest of the complex has PGE-depleted Main Zone units. On the farm Moorddrift, Cu–Ni–PGE sulphide mineralisation is hosted within the Upper Main Zone in a layered package of gabbronorites, mottled anorthosites and thin pyroxenites. Our observations indicate that a 10-m-thick, ‘reef-style’ package of mineralisation has been extensively ‘disturbed’, forming a mega breccia which in some localities may distribute mineralised rocks over intersections of over 300 m. The sulphides are made up of pyrrhotite, pentlandite and chalcopyrite, heavily altered around their margins and overprinted by secondary pyrite. Platinum-group mineral assemblages typical of primary magmatic deposits, with Pt and Pd tellurides and sperrylite, are present in the ‘reef-style’ package, whereas there is a decrease in tellurides and an increase in antimonides in the ‘disturbed’ package, interpreted to be related to hydrothermal recrystallization during veining and brecciation. Sulphur isotopes show that all sulphides within the mineralised package on Moorddrift have a crustal signature consistent with local country rock sediments of the Transvaal Supergroup. We interpret the mineralisation at Moorddrift as a primary sulphide reef, likely produced as a result of the mixing of crustally contaminated magmas in the Upper Main Zone, which has been locally disrupted post-crystallisation. At present, there are no firm links between Moorddrift and the other known PGE occurrences in the Main Zone at the Aurora and Waterberg projects, although the stratigraphic position of all may be similar and thus intriguing. Nonetheless, they do demonstrate that the Main Zone of the northern limb of the Bushveld Complex, unlike the eastern and western limbs, can be considered a fertile unit for potential PGE mineralisation.     

Lead isotopic signatures at the Klein Aub Mine,Namibia — Implications for mineralisation models

Pb and U isotopic data have been obtained for stratabound, sediment-hosted Cu-mineralisation and igneous, volcanic and sedimentary rocks from other stratigraphic levels in the vicinity of the Klein Aub Mine, Namibia, with the aim of investigating possible sources and metal emplacement mechanisms. The results indicate the existence of a pervasive Cu-sulphide mineralisation event, and support metal introduction during a syn-metamorphic (epigenetic) phase. Some indications are also found of another, earlier and possibly syn- or diagenetic, mineralisation event but this aspect requires confirmation by further study. Differences in the degree of radiogenicity of the Pb characterize the various mineralisation styles, the epigenetically mineralised laminite containing Pb of a more radiogenic nature than the other laminites, but also having lower absolute concentrations of Pb and U. A mineralisation age of ca. 600 Ma is indicated but the Pb data can neither confirm nor disprove the conclusions of Borg (1988) and Borg and Maiden (1987) that the basalt of the Doornpoort Formation is a source of the Cu in the Klein Aub deposits.     

Copper deposits in the basal breccias and volcano-sedimentary sequences of the eastern ligurian ophiolites (Italy)

A large number of the copper deposits, associated with the eastern ligurian ophiolites, are linked to the volcano-sedimentary sequences and, in minor amounts, to the ophiolitic basal breccias partially covering the intrusive rocks (ultramafites and gabbros). Some of these Fe-Cu-Zn mineralizations were selected because of their well defined stratigraphic and structural features, which, together with the textural and mineralogical characters and the trace elements in sulphides support the following considerations: - the stratiform mineralizations in the basal breccias show evidence of a sedimentary deposition preceding any consistent emplacement of a volcanic cover; - the stratabound deposits between the volcanic series and the sedimentary cover originate from hydrothermal or volcanic (exhalative) processes; - the stratabound deposits inside the volcanic pile show textural and mineralogical evidence of a volcanic origin with a relatively high temperature of formation; - the stockwork mineralizations, with their epigenetic characters, are of hydrothermal origin, probably related to subsea-floor convection cells.