Base Metal Occurrences Research Articles

Appraisal of subsurface structural model, a tool for understanding the influence of geodynamics in base metal occurrence within the Southern Benue Trough, southeastern Nigeria

Appraisal of subsurface structural model, a tool for understanding the influence of geodynamics in base metal occurrence within the Southern Benue Trough, southeastern Nigeria

Geochemical Mobility Associated to Gold and Base Metal Occurrences of Mangodara Sector, in Southern Burkina Faso, Banfora Greenstone Belts (West African Craton)

Geochemical Mobility Associated to Gold and Base Metal Occurrences of Mangodara Sector, in Southern Burkina Faso, Banfora Greenstone Belts (West African Craton)

Nd and Hf isoscapes of the Yilgarn Craton, Western Australia and implications for its mineral systems

The Yilgarn Craton, Western Australia is endowed with significant gold, nickel‑copper, Cu-Pb-Zn volcanic-hosted massive sulphide (VHMS) and iron ore deposits. Detailed knowledge of the spatial relationships between mineral systems and the crustal framework of the Yilgarn Craton may be powerful tools in the predictive exploration of undiscovered mineral systems. Both Nd and Hf isotopes have the potential to image crustal residence, mantle tapping structures, and juvenile input over both space and time. This study uses a recent compilation of Hf (1236 zircon analyses) and Nd (462 whole rock analyses) isotopes to image the architecture of the crust and compare this to the distribution of gold, nickel‑copper-platinum group elements (Ni-Cu-PGEs), Cu-Pb-Zn VHMS and iron ore deposits. Spatial correlation plots reveal a statistically significant relationship between Nd and Hf isotopic domains and the occurrence of gold, Ni-Cu-PGE, base metals, and iron confirmed by the coefficient of determination (R2) which measures how differences in metal occurrences can be explained by differences in the isotopic signature. Nickel occurrences show the strongest antithetic spatial correlation (R2 = 0.80) with Hf model ages, followed by gold (R2 = 0.58) and base metals (R2 = 0.35), which implies higher metal endowment in areas of more juvenile crust. In contrast, there is a significant positive spatial correlation (R2 = 0.49) between iron ore occurrence and older model ages. Isotopic data can also be plotted as Hf or Nd time-slices, where the strongest spatial correlation between gold, nickel, and base metal occurrence and juvenile input occurs at ca. 2700–2550 Ma. Generally, a model can be proposed where (1) base metals are more prevalent in juvenile crust because of its thinner nature, greater mantle input, high-heat flow, increased water-magma interaction; (2) iron ore is concentrated in regions of old, mature, and stable crust; (3) komatiite-hosted Ni-Cu-PGE systems are focused in juvenile rift zones at the edge of old crustal blocks; and (4) gold is also localised in juvenile crust at craton edges where several factors are localised, including orogenesis, occurrence of previously fertilised crust (i.e. Fe-rich lithologies, sanukitoids), favourable structures (potential inversion of rift structures), and ‘late’ alkaline magmatism. In gold, Ni-Cu-PGE, and base metal mineral systems, the ability of magmas to access higher crustal levels unimpeded appears vital in regional prospectivity and is itself a function of crustal configuration.

Gravity and Magnetic Survey in Southwestern Part of Cuddapah Basin, India and its Implication for Shallow Crustal Architecture and Mineralization

ABSTRACT Gravity and magnetic surveys were carried out in southwestern part of Cuddapah basin (CB) covering an area of ~3660 km2. Southwestern part of CB gained lot of attention after discovery of second most important uranium province in India. Some non-metallic industrial minerals and base metal occurrence are also reported in the study area. Present study with high spatial resolution gravity-magnetic survey aims to decipher detail basin geometry, nature of sediments, along with possible mineral deposits in SW part of CB. The gravity survey comprising short and long wavelength anomalies brought out sedimentary characteristics and basement architecture underneath the sediments. The long wavelength features of the gravity map shows gneissic basement, which is characterized by both basic and acid magmatic intrusive. Residual gravity anomaly map shows good correspondence with the exposed high density basaltic rock units and also brought out occurrence of concealed high density litho-units, which have significance for mineral prospecting. The magnetic map shows that both sediments and underneath basement are non-magnetic in nature except SW part of the study area, where study suggests occurrence of concealed mafic lensoid body. Euler solutions and combined gravity-magnetic modelling further facilitated for understanding of structural feature and basement geometry. Based on the integrated gravity and magnetic study mineral prospecting zones have been delineated for further detailed study.

Evolution of fluorite-mica-feldspar veins: Evidences of a fossil geothermal system in the são Gabriel terrane and consequences for Pb-Zn-Cu metallogeny

Fluorite-mica-feldspar hydrothermal veins and stockworks host Pb-Zn-Cu sulphides in the São Gabriel Terrane, Southern Brazil. The veins fill second- and third-order structures in Neoproterozoic calcitic marbles on the Cambaizinho Complex. Veins are mostly symmetrically zoned showing, from borders to the centre, fluorite and sericite, muscovite, adularia and albite (minor quartz, calcite and iron oxides). Galena, sphalerite, chalcopyrite, pyrite and minor cassiterite are disseminated in the veins and concentrated in ore pockets. Vein petrography, mineral chemistry and microthermometric measurements in fluorite fluid inclusions indicate that the hydrothermal veins were formed in a shallow crust environment from epithermal fluids near 140 °C. Their origin is attributed to a fossil geothermal system related to the post-collisional alkaline volcanism and caldera system of the Acampamento Velho Formation (574-549 Ma). This hydrothermal system is interpreted as genetically associated to fluorite and Pb-Zn-Cu occurrences on rhyolites and miarolitic cavities of related alkaline granitoids, Pb-Zn-Cu anomalies on stream sediment and soil analyses and Au quartz veins hosted by alkaline granitoids accompanied by pyrite, sericite and fluorite alteration halos in the same region. Although there is no mineral deposit discovered so far, the studied veins linked to the other base metal occurrences would indicate that the northwestern São Gabriel Terrane may host a poorly preserved mineralized epithermal system.

Lithogeochemistry of Pegmatites at Broken Hill: An Exploration Vector to Mineralisation

A sampling program was undertaken to assess pegmatite geochemistry with the view to using the data to vector towards economic mineralisation. The work has shown that pegmatites hosted in high grade metamorphic rocks (granulite and upper amphibolite facies) are either leucogranites, quartz-feldspar-mica pegmatites or feldspar-rich pegmatites. In contrast those that occur within lower grade metamorphic rocks (lower amphibolite to greenschist facies) are feldspar poor and quartz-muscovite rich, locally highly sheared, with tourmaline on their margins and as replacements of surrounding host sediments.Element classification diagrams suggest a geochemical continuum from poorly evolved pegmatites hosted in the highest grade metamorphic rocks to highly evolved pegmatites in the lowest grade metamorphic rocks. Spatially, this trend is broadly from the south and southwest toward the north and northeast and from west to east.Economic element content is similarly divided into two broad groups; Pegmatites within higher grade metamorphic rocks host elevated lead-zinc-silver-manganese.Pegmatites within lower grade metamorphic rocks contain elevated tin, tungsten, lithium, niobium and tantalum. The presence of abundant tourmaline in association with these indicates highly elevated boron.Elevated base metals and silver in the granulite and amphibolite facies rock is attributed to the elevated nature of these elements in the metasedimentary protolith being largely the Broken Hill Group. Our interpretation suggests these are probably not allochthonous but are nearly in-situ bodies resulting from partial melting of the Broken Hill Group. Their chemistry reflects that of the surrounding rock. Analysis of the spatial distribution of pegmatites with elevated lead-zinc-silver-manganese indicates many of these are located close to known base metal and/or silver occurrences of the Broken Hill types (BHTs).In contrast the concentration of tin, tungsten, lithium and other incompatible elements in pegmatites which are allochthonous and intrude rocks of lower metamorphic grade, suggests these elements have accumulated as fractionation has taken place.This study has highlighted three areas of elevated base metal-silver within pegmatites where no known mineralisation is documented either as an occurrence or in historic exploration data. Systematic pegmatite sampling at Broken Hill has the potential to enable vectoring toward buried silver-lead-zinc ores of the Broken Hill type.

Geophysical Signature of the Southern Gurubang Base Metal Occurrence in South Eastern NSW

Ground-based, time-domain electromagnetic, magnetic and gravity datasets were obtained for the southern-section of the Gurubang VHMS deposit located approximately 15 km east of Cooma, NSW. The Gurubang deposit is hosted in a mid-late Silurian sequence of rock composed of shallow marine sediments and felsic volcanic rocks. The aim of this research was to ascertain the usefulness of high-resolution geophysical techniques in targeting and evaluating a small-scale polymetallic massive sulphide deposit, and to investigate how the detailed geophysics relates to the overall geological framework of the prospect area. The acquired data was analysed using a forward modelling approach. Due to the deposits high concentration of conductive minerals, a coincident loop time-domain electromagnetic 2D survey effectively delineated the sulphide mineralisation, and was useful in interpreting and adapting deposit parameters such as the azimuth, dip and strike length. Based on the physical nature of the target deposit, it was determined that high-resolution magnetic and gravity surveys would not be effective methods in directly delineating these smaller-scaled (10’s of m’s) mineral deposits. However, magnetics and gravity did prove effective in depicting the surrounding geology, including potential volcanic intrusions and basement lithologies and structures.

Study on Gold and base metal occurrence in Uluwai Prospect, Western Latimojong Mountain, South Sulawesi

Uluwai Prospect is located in the northern part of South Arm of Sulawesi along the eastern part of the Kalosi Fold Belt and Latimojong Mountain. The area is generally characterized by moderate to rugged topography area with elevation in the range of 700 to 1400 m above sea level in the mountainous complex called Latimojong Mountain Complex. The mineralization is characterized by a relatively simple sulphide ore mineral assemblage consists of pyrite, sphalerite and chalcopyrite. Samples were collected in areas showing abundant sulphide minerals where younger faults cut the bedding and foliation of country rocks. A number of silicified zones have been observed, as well as float material containing disseminated pyrite, chalcopyrite, and sphalerite with hematite, goethite and limonite. Some alteration types have been observed including sericitization, albitization, carbonatization and silisification. The samples collected indicated that the mineralisation is contained within metasedimentary (sandstone to mudstone) and greenschist. Geochemical analyses from 16 samples including 5 stream sediment samples indicated that the most promising mineralization occur in the prospect area are copper (Cu) and zinc (Zn). This is also supported by the abundance of chalcopyrite and sphalerite in some highly altered samples. Assaying of the collected samples revealed most of samples contain relatively low gold (Au) concentration. However, two samples contain 0.007 and 0.01 ppm of Au. In the mineralized area, Zn concentrations are up to 134 ppm, Cu up to 120 ppm and Pb up to 18 ppm and As up to 70 ppm. There is no clear relationship that exists between Au and the base metals except that one of the samples with highest Au values tend to have high Zn and As. This unclear pattern also shown by Cu, Pb and Zn. Base metal concentration in stream sediment samples show a relatively stable pattern than in rock samples. Arsenic tends to be elevated in base metal rich samples. Sb and Mo are relatively low in all sample type. However, Mo values will be high in the samples which contain highest Cu and Zn.

Geochemical and Geodynamic Constrain of Tholeiitic Volcanism and Related Intrusions in the Kampti Gold District, Southwest Burkina Faso: Implication for Mineral Exploration

The Kampti serie is a volcano-plutonic complex in the south-eastern corner of the Houndé belt, closed to the border with Ivory Coast. The stratigraphy comprises initially: a thick sequence of tholeiitic and pillowed basalt followed by pyroclastic projections derivated from bimodal volcanism; and flowing mostly to the south. The volcanic sequence is cross-cut by mafic cumulate body, stocks of gabbro, diorite, subvolcanic dykes and lately intrude by granitoid. It is bordered to the west by the pelitic schist of the Bambela basin with minor intercalation of Tarkwa type sediment. volcanoclastic facies is dominantly plagioclase-phyric (albite +/- oligoclase), zoned plagioclase has a core of anorthoclase. Secondary mineral infilled (quartz, kutnahorite, ripidolite, clinozoisite) of spherulites and oolite highlight a general low grade metamorphism of greenschist facies affecting the complex. Based on trace element chemistry, the tholeiitic rocks present flat REE pattern contrasting with the felsic rocks more enriched in LREE and depleted in HREE. The style of the magmatism in the Kampti serie is compatible with an island arc model, describe elsewhere in the birimian. Gold mineralisation and base metal occurrences associated to the nature of rocks and tectonics highlight a polymetallic district.

Geology of the Archean Intrusion-Hosted La-Grande-Sud Au-Cu Prospect, La Grande Subprovince, James Bay Region, Quebec(,

The Archean La Grande and Eastmain domains of the La Grande subprovince in the James Bay region of northwestern Quebec are the focus of renewed and extensive exploration as a result of recent major discoveries made in this region (e.g., ~8 Moz Au Roberto deposit). A number of significant Neoarchean Au ± base metal occurrences are present in the La Grande domain, including the La-Grande-Sud Au-Cu prospect (Zone 32, inferred resource of 4.2 million metric tons (Mt) at 2.1 g/t Au and 0.2 wt % Cu). The La-Grande-Sud Au-Cu prospect is hosted in the 2734 ± 2 Ma synvolcanic La-Grande-Sud Tonalite in the 2751 to 2732 Ma Yasinski Group volcanic rocks. The La-Grande-Sud Tonalite is a synvolcanic calc-alkaline intrusion emplaced in an arc-like tectonomagmatic setting. Timing relationships in mineralized zones as well as associated hydrothermal alteration zones support a pre- to early-D 1 origin for at least a part of the mineralization in the La-Grande-Sud Tonalite. Deformation events were responsible for the overprinting of the early mineralization by auriferous shear zone-related systems associated with D 2 . Evidence for a pre-D 2 and pre- to early-D 1 alteration and mineralization system includes: (1) a broadly concentric pattern defined by the hydrothermal alteration assemblages within the La-Grande-Sud Tonalite, which suggests that the hydrothermal alteration was, at least in part, controlled by the geometry of the intrusion rather than by the D 2 deformation; (2) the development of a weak biotite-bearing potassic assemblage (biotite-epidote-plagioclase-muscovite-calcite ± pyrite) that is gradually replaced outward from the center of the intrusion by a chlorite-bearing propylitic assemblage (chlorite-epidote-muscovite-plagioclase-calcite-pyrite), and locally overprinted by a sericitic alteration assemblage (muscovite-quartz-albite-chlorite-pyrite-carbonate), which represents the most intense alteration in the La-Grande-Sud Tonalite; (3) the presence of mineralized hydrothermal breccias along the margins of the intrusion; (4) deformed disseminated and stockwork-style Au-Cu sulfide mineralization consisting of pyrite, chalcopyrite, tennantite, arsenopyrite, pyrrhotite, Bi sulfosalts, and native Au ± Bi in the sericite, chlorite, and biotite facies alteration zones; (5) S 1 -parallel elongated tonalite clasts and sulfide veinlets in D 2 -folded and transposed hydrothermal biotite breccias; and (6) pyrite porphyroblasts that are elongated parallel to the S 1 fabric (syn-D 1 metamorphic recrystallization) and that were folded during D 2 . D 2 was associated with the development of conjugate auriferous extensional quartz-tourmaline veins (orogenic-style veins) that were superimposed on the pre- to early-D 1 intrusion-hosted mineralized system. Gold in these syn- to late-D 2 veins could have been remobilized from preexisting mineralization (disseminated sulfides) or was related to a late (syn-D 2 ) auriferous hydrothermal event. The intrusion-hosted Au mineralization of the La-Grande-Sud Au-Cu prospect illustrates that Archean synvolcanic intrusions, regardless of their size, can host a range of styles of alteration and mineralization, including both “early” and “late” mineralizing systems.

Mineral system analysis of the Mt Isa–McArthur River region, Northern Australia

The Mt Isa–McArthur region is renowned for a range of commodities and deposit types of world-class proportions. The region is described here in the context of a ‘mineral system,’ through consideration of processes that operate across a range of scales, from geodynamics and crustal architecture, to fluid sources, pathways, drivers and depositional processes. The objective is to improve targeting of Pb–Zn, Cu and Cu–Au deposits. Repeated extension and high heat flow characterise much of the history prior to 1640 Ma. The pre-Barramundi Orogeny (pre-1.87 Ga) metamorphic basement was the substrate on which a volcanic arc developed, focussed along the Kalkadoon-Leichhardt Belt. This is related to an inferred east-directed subduction between 1870 and 1850 Ma. From 1755 to 1640 Ma, three successive volcano-sedimentary basins developed, the Leichhardt, Calvert and Isa Superbasins, in an interpreted distal back-arc environment. The Isan Orogeny, from 1640 to 1490 Ma, overlapped with Isa Superbasin sedimentation, suggesting a transition from back-arc to a foreland basin setting. Most crustal thickening occurred in the Eastern Fold Belt, an area earlier characterised by thinned crust and deep marine environments. This region was deformed into nappe-like structures with high-temperature–low-pressure regional metamorphism and associated granites; the latter are absent from the Western Fold Belt. Metal deposition mainly occurred late in the history, with all known (and preserved) major base metal occurrences either hosted by Isa Superbasin rocks or formed during the Isan Orogeny. Earlier superbasins were potential fluid source regions. Sedimentary formation waters, metamorphic and magmatic fluids were present at prospect scale, while meteoric and possibly mantle sources are also implicated. The spatial distribution of metallogenic associations (i.e. iron oxide–copper–gold, Pb–Zn–Ag, U, Au) across the inlier may result from differences in the geodynamic make-up and evolution of the pre-1.87 Ga tectonic elements. Penetrative faults are interpreted as predominantly steeply dipping and to have acted as pathways for fluids, both in extension and compression. Fluid mixing was a potentially significant ore deposit control. Examples are drawn from the Ernest Henry iron oxide–copper–gold-related hydrothermal breccias in the east and from the Mt Isa Copper deposit in the west. Stress switching during late-stage deformation appears to have triggered a fluid mixing event that led to formation of the major copper deposits.

A fluid inclusion study of uranium and copper mineral systems in the Murphy Inlier, Northern Australia

The Palaeoproterozoic Murphy Inlier is situated at the southern end of the McArthur Basin in Northern Australia. The inlier contains over 50 uranium, copper, tin and base metal occurrences. Fluid inclusion studies were carried out on samples of quartz veining from the uranium and copper deposits as well as from the basement rocks to determine the composition of the fluids and to investigate how uranium and copper were transported in these fluids. Four types of fluid inclusions were observed in this study; Type A vapour-rich inclusions with ≥30 vol.% vapour, Type B two phase aqueous inclusion with ≤20 vol.% vapour, Type C multiphase inclusions with one or more solid phases and, Type D liquid-only inclusions. At least three different fluids were identified in the Murphy Inlier. There is a CaCl 2 ± LiCl-rich brine, a NaCl-rich brine and a low salinity fluid. The fluids can also be grouped into a high temperature (typically homogenising above 210 °C) population and a low temperature (typically homogenising below 240 °C) population. Depending on location, the high temperature fluid may be enriched in CO 2, N 2, or CH 4. In the uranium deposits, gas-rich inclusions are dominated by CO 2 indicating that these fluids are relatively oxidized, while in the copper deposits both CO 2 and CH 4 are present indicating that these fluids are more reduced. The low temperature population of Type B inclusions have a mode of homogenisation at 190 °C in the uranium deposits and a mode at 120 °C in the copper deposits. Similarly, Type C inclusions have a mode of homogenisation at 235 °C in the uranium deposits and 170 °C in the copper deposits. Variations in the composition of the inclusions suggest that at least two stages of fluid mixing occurred. Firstly, there was mixing between a CaCl 2 ± LiCl-rich brine and a NaCl-rich brine to produce a fluid of intermediate composition. This fluid then mixed with a low salinity fluid. Geochemical modeling has shown that both uranium and copper can be transported in the same fluid at high f O 2 and moderate to high chloride concentrations. In the proposed model for mineralisation, uranium and copper were leached from the volcanics or sediments in the McArthur Basin and were simultaneously transported in the oxidised, Na-Ca-Li-bearing saline fluids which had undergone mixing within the basin. Uranium precipitated when this fluid was reduced, either by reaction with Fe-rich mafic volcanics, carbonaceous rocks, or by mixing with a CH 4-bearing, low salinity fluid derived from the basement. Copper remained in the fluid until further changes in salinity, f O 2 or pH occurred, most probably as a result of mixing with lower salinity meteoric fluids. The fluid may have continued to cool to near surface temperatures as evidenced by the trapping of liquid-only fluid inclusions in some veins.

Molybdenite in Pomona Island Granite at Lake Manapouri, Fiordland

A small occurrence of molybdenite (MoS2) mineralisation has been discovered in the weakly A-type Pomona Island Granite on the shorelines of Lake Manapouri in eastern Fiordland. The disseminated appearance of molybdenite and the absence of quartz veins indicates that mineralisation is probably the product of magmatic and/or hydrothermal activity related to pluton crystallisation at c. 157 Ma, and not younger (c.128–116 Ma) shear zone-related mesothermal mineralisation as has been recently described from the Murchison Mountains to the north and Stewart Island to the south. Although apparently not of economic grade, the Pomona Island Granite locality is regionally important because it is the first direct pluton-related Mo-mineralisation event to be recognised in eastern Fiordland. This occurrence adds to the growing number and known styles of base metal occurrences within the Jurassic–Cretaceous magmatic arc (Outboard Median Batholith) that formed on or near the New Zealand Gondwana margin. Furthermore, the wide distribution of essentially uninvestigated A-type granites in the Outboard Median Batholith means that there may be further Mo-mineralised localities awaiting discovery.

Pre-1.8 Ga tectono-magmatic evolution of the Kalkadoon–Leichhardt Belt: implications for the crustal architecture and metallogeny of the Mount Isa Inlier, northwest Queensland, Australia

New geochemical, U–Pb zircon and Nd–Hf isotope data for felsic and mafic components of the Kalkadoon–Leichardt Belt (KLB) in the Mount Isa Inlier of northwest Queensland confirm that the evolution and tectonic make-up of this belt prior to 1.8 Ga was closely aligned with that of the Western Fold Belt. Like pre-1.8 Ga magmatic rocks of the Western Fold Belt, those in the KLB are characterised by late Archean to Paleoproterozoic crustal residence ages (TDM ca 2.6–2.3 Ga) but the geochemical data suggest stronger within-plate affinities, implying that the KLB intrusions underwent a greater degree of crustal assimilation and/or were emplaced further inboard of the active subduction margin. A new SHRIMP U–Pb zircon age of ca 1.86 Ga from a felsic intrusion in the northern KLB confirms earlier age constraints for the main phase of the Kalkadoon Granite. Nd isotope patterns for intrusions in the Western Fold Belt and the KLB are distinct from those of ca 1.7–1.5 Ga intrusions in the Eastern Fold Belt. This isotopic discrepancy suggests the Eastern Fold Belt underwent a separate evolutionary history and was accreted to the WFB/KLB at some stage prior to, or during, the ca 1.86 Ga Barramundi Orogeny. The occurrence of widespread, isotopically homogeneous ca 1.72 Ga magmatism (Argylla Event) across the entire Mount Isa Inlier implies that amalgamation of the allochthonous Eastern Fold Belt with the North Australian Craton was completed by that time. The significantly elevated metallogenic potential of the Eastern Fold Belt for ca 1540–1500 Ma Fe-oxide–copper–gold, relatively low endowment of the Kalkadoon–Leichhardt Belt, and predominance of ca 1650–1550 Ma base metal occurrences in the Western Fold Belt, are all consequences of the tectonic interplay and lithospheric processes that controlled the evolution of the Mt Isa Inlier prior to ca 1.8 Ga.

The Drenchwater deposit, Alaska: An example of a natural low pH environment resulting from weathering of an undisturbed shale-hosted Zn–Pb–Ag deposit

Abstract The Drenchwater shale-hosted Zn–Pb–Ag deposit and the immediate vicinity, on the northern flank of the Brooks Range in north-central Alaska, is an ideal example of a naturally low pH system. The two drainages, Drenchwater and False Wager Creeks, which bound the deposit, differ in their acidity and metal contents. Moderately acidic waters with elevated concentrations of metals (pH ⩾ 4.3, Zn ⩽ 1400 μg/L) in the Drenchwater Creek drainage basin are attributed to weathering of an exposed base-metal-rich massive sulfide occurrence. Stream sediment and water chemistry data collected from False Wager Creek suggest that an unexposed base-metal sulfide occurrence may account for the lower pH (2.7–3.1) and very metal-rich waters (up to 2600 μg/L Zn, ⩽ 260 μg/L Cu and ⩽89 μg/L Tl) collected at least 2 km upstream of known mineralized exposures. These more acidic conditions produce jarosite, schwertmannite and Fe-hydroxides commonly associated with acid-mine drainage. The high metal concentrations in some water samples from both streams naturally exceed Alaska state regulatory limits for freshwater aquatic life, affirming the importance of establishing base-line conditions in the event of human land development. The studies at the Drenchwater deposit demonstrate that poor water quality can be generated through entirely natural weathering of base-metal occurrences, and, possibly unmineralized black shale.

Gold potential of the Mpanda Mineral Field, SW Tanzania: evaluation based on geological, lead isotopic and aeromagnetic data

Gold and base metals of the Mpanda Mineral Field (MMF) is the focus of this paper. Gold veins and gold-bearing base metal occurrences are structurally controlled by conjugate sets of NW–SE and E–W trending faults and/or shear zones that crosscut high-grade metamorphic rocks and post-kinematic intrusions. It was anticipated that Palaeoproterozic country rocks could have been potential host rocks for the gold mineralisation in this area. This argumentation was based on Pb model ages of various deposits from the MMF. Recent fieldwork and Pb isotopic results presented herein indicate that epigenetic gold and base metal vein-type mineralisation in the MMF is post-Palaeoproterozoic. Our Pb isotope study concentrates on constraining the sources of metals in gold-bearing quartz reefs and base metal occurrences. Pb isotopes of whole rocks and minerals indicate that mineralisation was emplaced during the Neoproterozoic, contemporaneous with the intrusion of alkaline granites and carbonatite complexes (e.g., Sangu–Ikola carbonatite complex) at ∼720 Ma. The source of Pb in the mineral occurrences is compatible with that characteristic of the Palaeoproterozoic host rocks. Aeromagnetic data suggest that the gold-bearing, NNW–SSE trending area continues to the north beyond Mpanda town. Pb isotope results and aeromagnetic data have significant implications for future exploration programs within the region, in that the search should potentially focus on the defined geophysical borders and trendlines, and on Neoproterozoic, rather than Palaeoproterozoic vein systems.

An integrated mineral exploration programme in the Takengon tenement, Aceh magmatic arc, north Sumatra

A multidisciplinary approach led to the discovery of several gold and base metal occurrences during the 1996–1998 exploration campaign in the Takengon tenement of the Aceh magmatic arc, north Sumatra. Mineralization types included: (i) Au + base metal-bearing quartz veins; (ii) porphyry Cu-style mineralization; (iii) skarn-like Au and Cu-sulphide mineralization; and (iv) granite-hosted molybdenite veins. Regional structural interpretation of synthetic aperture radar and Landsat Thematic Mapper satellite images revealed prominent NNW–SSE and NNE–SSW trending fault zones that cross-cut the region and that were later shown to be spatially related to mineralization. Results from stream sediment analyses delineated several areas that were anomalous in Au±Cu, Pb, Zn, Mo and As, and mineralized bedrock was encountered in some of these areas during a subsequent lithogeochemical survey. An airborne geophysical survey defined a broad region characterized by a low magnetic response with locally high magnetic relief, and moderate to high potassium (K) but low thorium (Th) counts. In addition, the geophysical signals outlined smaller isolated areas that exhibit moderate magnetic anomalies and variable low to high K values. An integrated map with superimposed geological, geochemical and geophysical data demonstrates that results from the various prospecting techniques were generally complementary. Where mineralized and altered rocks occur, the adjacent sediments are metal-bearing (Au±Cu±Pb±As) and the geophysical responses are magnetic, and moderately to highly radioactive (K). In one case in which stream sediments were not auriferous, the discovery of Au- and base-metal-bearing quartz veins was possible through a combined exploration approach consisting of pathfinder elements (As and Pb), elliptical magnetic lineament, moderate to high K anomalies, and regional structures.

Geochemistry of oil inclusions in sulfide-related calcites—fingerprinting the source of the sulfate-reducing hydrocarbons of the Pb–Zn carbonate-hosted Jubilee deposit of Nova Scotia, Canada

Sulfide precipitation in the context of carbonate-hosted base metal deposits has been previously explained by numerous processes including SO 4 reduction in the presence of hydrocarbons. This model has been suggested for numerous deposits although clear criteria to support the model have not been systematically provided. Numerous oil-inclusions are encompassed by fibrous calcite crystals in finely laminated Carboniferous limestone at the base of the Windsor Group, a unit that hosts numerous base metal occurrences in Nova Scotia, particularly the hydrocarbon-rich Jubilee Pb–Zn deposit in Cape Breton Island. Oil from two inclusion-rich samples from this deposit have been characterised by gas chromatography, gas chromatography–mass spectrometry, gas chromatography–isotope ratio mass spectrometry and bulk stable C isotopes. As established in the authors' former publications, the Jubilee deposit is a clear metallogenic case for which the ground preparation and mineralisation stages involved reduction of SO 4 by hydrocarbons. Here, the question of potential sources of these hydrocarbons is addressed. It is postulated that the hydrocarbons that were trapped in the sulfide-related calcites at the deposit, correlate with previously characterised oil seeps, and it is demonstrated that their source is not the marine host-carbonates but stratigraphically deeper lacustrine formations of the Horton Group.

Base Metal Skarns and Au Occurrences in the Southern GaspeAppalachians: Distal Products of a Faulted and DisplacedMagmatic-Hydrothermal System along the Grand Pabos-Restigouche Fault System

In the southern Gaspe Appalachians, base metal and Au occurrences are spatially associated with the Acadian dextral strike-slip Grand Pabos and Restigouche faults. Mineralization occurs as veins, replacement zones, and disseminations along the main structures and subsidiary faults with orientations compatible with the dextral strike-slip kinematics along the main faults. Limestone of the Upper Ordovician-Lower Silurian White Head Formation hosts numerous base metal skarns and Au occurrences that are surrounded by halos of argillaceous alteration. The Reboul prospect is a type example of base metal skarns and consists of three types of mineralization: (1) early strata-bound disseminated Cu-Zn-Ag associated with marble-hornfels and skarn, (2) northeast- and northwest-trending (Cu-Zn-Pb-Ag-(Au) and Ag-Cu-Zn-Pb-(Au-Sb)) veins, parallel and perpendicular to the regional folding, and (3) late Ag-Au-Zn-Pb mineralized zones in quartz breccia veins parallel to the Grand Pabos fault. Crosscutting relationships, fluid inclusion microthermometry, mineralogy of the three types of mineralization, and alteration patterns were used to define the timing of mineralizing processes which are (1) prograde metamorphism, (2) metasomatism and hydrothermal alteration with high-temperature skarn formation and Cu-Zn-Ag mineralization, (3) retrograde alteration with Cu-Zn-Pb-Ag-(Au) and Ag-Cu-Zn-Pb-(Au-Sb) veins, and (4) late Au precipitation at low temperature. The Saint-Andre-de-Ristigouche prospect is the best example of Au mineralization spatially associated with a base metal skarn in the area. It consists of Au-Sb-As extensional veins and stockwork hosted by limestones. Felsic dikes associated with base metal skarns and a magmatic-hydrothermal breccia at the Reboul prospect are evidence for magmatic activity coincident with the mineralizing events. Argillaceous alteration, high-temperature and high-salinity fluid inclusions, and the Pb isotope compositions of galena are compatible with a model linking the skarns and Au occurrence to Early to Middle Devonian magmatic activity at depth along the Grand Pabos and Restigouche faults. A palinspastic pre-Acadian strike-slip reconstruction of the southern Gaspe Appalachians shows that all mineral occurrences hosted by the White Head Formation belong to the same mineralized area that might represent distal components of a magmatic-hydrothermal system at depth.

Lead isotope signatures of sedimentary rocks as a tool for tracing ore lead sources: a study of base-metal and barite occurrences in Jameson Land Basin, central East Greenland

Numerous sediment-hosted base-metal and barite occurrences are found within the post-Caledonian Jameson Land basin in East Greenland. A study was undertaken to evaluate the sources of lead in several base-metal occurrences by comparing the lead isotope signatures of sedimentary and intrusive rocks from the basin with ore lead signatures established in earlier studies. To differentiate mineral phases and simulate various degrees of natural hydrothermal processes the samples were leached sequentially with HCl, H2O2 (only in the case of organic-rich samples), HNO3 and HF + HNO3.The lead isotopic compositions of the sedimentary rocks were found to vary according to mineralogical paragenesis, and with origin and geographical position of the adjacent basement. All HF-soluble lead (after leaching with HCl and HNO3) in coarse-grained arkoses and conglomerates plotted on well-defined mixing lines in both uranogenic and thorogenic diagrams, whereas all other analyses were more radiogenic. Since feldspar is the dominant lead-carrying phase in the HF fraction of the coarse-grained clastic rocks, the mixing lines are referred to as the feldsparlead trends and are interpreted to reflect the presence of detrital feldspars from two well-defined basement sources. The radiogenic end of the feldspar-lead trend (high 207Pb/204Pb and 208Pb/204Pb ratios) is believed to represent feldspars derived by the breakdown of Caledonian granites on account of isotopic overlap with these. The lower end of the feldspar-lead trend more probably represents pre-Caledonian metasediments.Lead isotopes from the base-metal occurrences in the western part of the Jameson Land basin plot on the feldspar-lead trends, strongly suggesting that feldspars from the thick sequence of Devonian to Lower Permian continental clastic deposits are the source of ore lead. The position on the feldspar-lead trend of the individual west Jameson Land mineral occurrences varies according to the local geological environment. The Mesters Vig Pb-Zn veins plot nearest the low 207Pb/204Pb and 208Pb/204Pb end of the feldspar-lead trends in accordance with the scarcity of Caledonian granites in the nearby basement, whereas mineralization further to the south has more 'Caledonian granite-lead' incorporated.Mineral occurrences in east Jameson Land are slightly more radiogenic than those of west Jameson Land, showing a strong affinity towards the Caledonian lead component. The east Jameson Land ore lead defines a trend extending from the upper end of the feldspar-lead trends towards more radiogenic values. This trend away from the feldspar-lead fields probably reflects the fact that the Carboniferous-Permian sequence in the east is much thinner than in the west owing to the overall architecture of the basin. The east Jameson Land ore lead is interpreted as being derived from various local source rocks.