Exploration rock geochemistry — detection of trace element halos at heath steele mines (N.B., Canada) by discriminant analysis

The Heath Steele massive sulphide deposit in northern New Brunswick lies conformably within a sedimentary-volcanic sequence of probable Ordovician age which has been metamorphosed to the greenschist stage. The dominant sulphide mineral is pyrite, and the main economic minerals are sphalerite, galena, and chalcopyrite; the general grade of the ore is 5% Zn, 2% Pb, and 1% Cu.The distribution of Pb and Zn in acid volcanic rocks stratigraphically above the massive sulphides is compared with the distribution in similar rocks stratigraphically below the sulphides. Whereas there are discernable differences in the populations, there is also considerable overlap between them. To enable individual samples to be classified, linear discriminant functions were calculated for the two groups; Pb and Zn were found to be the most useful variables to separate the two populations. The functions were then tested on hanging wall and footwall samples not used in computing the functions. A halo region, extending about 1,200 ft above the sulphides and 4,000 ft along the same stratigraphic horizon as the sulphides was outlined by samples classified as “hanging wall”. Beyond the halo zone there is no significant difference in the distribution of Pb and Zn between the hanging wall and footwall acid volcanic rocks.The results demonstrate that rocks at Heath Steele, which show no evidence of mineralogical alteration attributable to mineralization, have a trace element halo of considerable extent spatially associated with the sulphides. If similar halos can be shown to be a general feature of massive sulphide deposits, the technique described should have wide application for exploration for deeply buried deposits of this type.     

Zn–Pb–Cu volcanic-hosted massive sulphide deposits: criteria for distinguishing brine pool-type from black smoker-type sulphide deposition

Eight Zn–Pb–Cu massive sulphide deposits that appear to have formed on the sea floor (seven in Spain, one in Tasmania) are believed to have been precipitated in brine pools, based on the salinities and temperatures of fluid inclusions in underlying stockworks. Comparing the geological features of these deposits with those of the Zn–Pb–Cu massive sulphide ores of the Hokuroku Basin, Japan, which have formed as mounds from buoyant fluids of low salinity, shows that brine pool deposits have: (1) potentially very large size and tonnage, and high aspect ratio, (2) higher Zn/Cu and Fe/Cu values, (3) no evidence of chimneys, (4) relatively abundant framboidal pyrite and primary mineral banding, (5) reduced mineral assemblages (pyrite-arsenopyrite/pyrrhotite), and minor or rare barite in the massive sulphide, (6) associated stratiform and/or vein carbonates, (7) relatively unimportant zone refining, (8) lack of vertical variation in sphalerite and sulphur isotopic compositions, and (9) evidence of local bacterial sulphate reduction. Application of these criteria to the Rosebery deposit in Tasmania, for which there are no fluid inclusion data, leads to the conclusion that the southern section was deposited as separate lenses in a brine-filled basin or basins. Other potential candidates include Brunswick no. 12 and Heath Steele (Canada), Woodlawn and Captains Flat (New South Wales), Hercules and Que River (Tasmania), and Tharsis and the orebodies at Aljustrel (Spain and Portugal). Recently published fluid inclusion data for Gacun (China) and Mount Chalmers (Queensland) suggest that not all ores deposited from highly saline fluids have reduced mineral assemblages.     

Lithogeochemistry of hypogene, supergene and leached cap samples, berg porphyry copper deposit, British Columbia

In this paper we examine the influence of the development of supergene oxide and sulphide zones on the original hypogene geochemical patterns at the Berg deposit, British Columbia.The deposit, in the alpine zone of the Tahtsa Range, was logged (GEOLOG) and sampled in fourteen diamond drill holes along a N—S section and from outcrop where possible. Anomalous populations of major and trace elements were defined using log probability graphs and a sequential extraction (10% hydrochloric acid— ammonium oxalate — potassium chlorate/hydrochloric acid — nitric/perchloric acids) was used to study the distribution of elements between carbonate, oxide, sulphide and silicate phases.Core logs and assays show that primary ore minerals (chalcopyrite—molybdenite) extend from the outer part of the porphyry intrusion into the surrounding hornfels where the best grades of copper are found close to the intrusive contact. Maximum copper grades, however, result from development of a supergene enrichment blanket. Within the hypogene zone, principal lithogeochemical patterns reflect the differences in composition of the hornfels, originally intermediate to basic volcanics, and the intrusion, as well as the introduction of F and trace metals (Cu, Mo, Pb, Zn and Ag). Distribution of Ag is broadly comparable to that of Cu and Mo whereas anomalous Pb and Zn are present as peripheral haloes around the potential ore zones.Emergence of strongly acidic ground water and precipitation of iron oxides, indicate that leaching processes are active. Furthermore, although primary sulphides, associated with both their oxidation products and secondary sulphides, can still be found in surface samples, sequential extractions on drill core clearly indicate vertical redistribution of copper between oxide, carbonate and sulphide phases. Using ratios of metal concentrations to TiO₂, the surface concentrations of trace metals can be compared with those at depth and the relative amount of enrichment or depletion can be quantified. In highly leached sites the absolute concentrations of Cu, Mn and Zn are low which is reflected in TiO₂ ratios of <1. However, Mo, Pb and Ag are enriched at the same sites (TiO₂ ratios >1). In areas where physical erosion exceeds leaching (topographic lows) primary sulphides co-exist with secondary sulphide and oxide minerals. Here Cu, Mo, Pb, Zn, Mn and Ag are enriched. Fluorine is relatively unaffected by the leaching process. It would appear signature for a cale alkaline Cu and Mo porphyry deposit in an area where outcrop was intensely leached.     

Ground-water geochemistry in the abitibi volcanic belt of quebec

Large sections of the Abitibi volcanic belt of Archean age are covered by thick glacial and glacio-lacustrine overburden. To increase our knowledge of the geology and the economic potential of the basement, approximately 5000 ground-water samples were collected from wells, springs and drill holes over an area of about 20 000 km². Samples were collected at intervals varying from 150 to 500 m in settled areas. They were analysed for: Cu, Zn, Pb, Ni, Co, Cd, Cr, Mn, Fe, Mo, Hg, As, U, Na, K, Ca, Mg, Li, F, and Cl⁻ and pH.Several types of regional and local geochemical patterns were identified. However, since the apparent dispersion distance of trace elements from a point source is usually less than 1 km in ground waters, the local zones of geochemical activity having an area less than 15 km² are considered of greatest interest for exploration. The large regional patterns and geochemical complexes which consist of clusters of grouped, overlapping, and superimposed regional and local patterns are presumed to be related to particular geological or metallogenic contexts.The elements with the highest frequency of occurrence in the 240 identified local zones of geochemical activity are As, Ni and U which occur in more than 30% of these zones; they are closely followed by the groups Co, Cu, Mo and Zn, Hg, Pb, F⁻ which occur respectively in more than 25% and 20% of the zones. Element associations in these zones can be broken down into three major groups: the trace metals (Cu, Zn, Pb, Co, Cd, Mo), the major and associated elements (Na, K, Ca, Mg, Li, F⁻, Cl⁻) and the elements As, U and Ni which occur with elements of either of the first two groups.Ground-water samples were also collected in 21 mines. The results of this study suggest that the best indicator elements for base metal deposits are As, Mo and F, whereas As, Ni, F⁻ or Cl⁻ and Mn are the best indicators of gold deposits.The results of the ground-water survey have identified numerous new areas to explore. Anomalous concentrations of at least two of the best indicators occur in more than 33% of the 240 local zones of geochemical activity.     

Draa Sfar, Morocco: A Visean (331 Ma) pyrrhotite-rich, polymetallic volcanogenic massive sulphide deposit in a Hercynian sediment-dominant terrane

Draa Sfar is a Visean, stratabound, volcanogenic massive sulphide ore deposit hosted by a Hercynian carbonaceous, black shale-rich succession of the Jebilet terrane, Morocco. The ore deposit contains 10 Mt grading 5.3 wt.% Zn, 2 wt.% Pb, and 0.3 wt.% Cu within two main massive sulphides orebodies, Tazakourt (Zn-rich) and Sidi M'Barek (Zn–Cu rich). Pyrrhotite is by far the dominant sulphide (70 to 95% of total sulphides), sphalerite is fairly abundant, chalcopyrite and galena are accessory, pyrite, arsenopyrite and bismuth minerals are rare. Pyrrhotite is monoclinic and mineralogical criteria indicate that it is of primary origin and not formed during metamorphism. Its composition is very homogeneous, close to Fe₇S₈, and its absolute magnetic susceptibility is 2.10^− 3 SI/g. Ar–Ar dating of hydrothermal sericites from a coherent rhyolite flow or dome within the immediate deposit footwall indicates an age of 331.7 ± 7.9 Ma for the Draa Sfar deposit and rhyolite volcanism.The Draa Sfar deposit has undergone a low-grade regional metamorphic event that caused pervasive recrystallization, followed by a ductile–brittle deformation event that has locally imparted a mylonitic texture to the sulphides and, in part, is responsible for the elongated and sheet-like morphology of the sulphide orebodies. Lead isotope data fall into two compositional end-members. The least radiogenic end-member, (²⁰⁶Pb/²⁰⁴Pb = 18.28), is characteristic of the Tazakourt orebody, whereas the more radiogenic end-member (²⁰⁶Pb/²⁰⁴Pb  18.80) is associated with the Sidi M'Barek orebody, giving a mixing trend between the two end-members. Lead isotope compositions at Draa Sfar testify to a significant continental crust source for the base metals, but are different than those of the Hajar and South Iberian Pyrite Belt VMS deposits.The abundance of pyrrhotite versus pyrite in the orebodies is attributed to low fO₂ conditions and neither a high temperature nor a low aH₂S (below 10^− 3) is required. The highly anoxic conditions required to stabilize pyrrhotite over pyrite are consistent with formation of the deposit within a restricted, sediment-starved, anoxic basin characterized by the deposition of carbonaceous, pelagic sediments along the flank of a rhyolitic flow-dome complex that was buried by pelitic sediments. Deposition of sulphides likely occurred at and below the seafloor within anoxic and carbonaceous muds.Draa Sfar and other Moroccan volcanogenic massive sulphide deposits occur in an epicontinental volcanic domain within the outer zone of the Hercynian belt and formed within a sedimentary environment that has a high pelagic component. In spite of the diachronous emplacement between the IPB deposits (late Devonian to Visean) and Moroccan deposits (Dinantian), all were formed around 340 ± 10 Ma following a major phase of the Devonian compression.     

Lithogeochemistry of wainaleka Cu-Zn volcanogenic deposit, Viti Levu, Fiji, and possible applications for exploration in tropical terrains

A small Kuroko-type Cu-Zn deposit exhibiting metal zoning and alteration assemblages comparable with documented proximal volcanogenic deposits, occurs at the top of a felsic fragmental pile, mantling a large sodic rhyolite domal complex. The domal complex occurs within predominantly mafic to intermediate lavas and volcaniclastic rocks with low-potash island-arc tholeiitic affinities, representing the basal section of the early Tertiary (Eocene to middle Miocene) Wainimala Group near the southern coast of Viti Levu, Fiji.Lithogeochemical trends identified in analyses of rock chip samples from traverses across the domal complex reflect alteration zoning. Sodium, Ca and Sr are strongly depleted within the quartz-sericite foot-wall alteration zone (Zone I) 200 m below mineralization. Potassium, Rb and weaker Mn, Zn and Co depletion and Cu, Pb and Mg enrichment define clay-sericite (Zone II) and clay carbonate (Zone III) footwall alteration 600 to 1200 m below mineralization. Hanging-wall albite-chlorite-calcite-zeolite alteration (Zone IV) is accompanied by enhanced Zn, Pb, Co, Mn, Sr and Na values.Significant mine-scale lithogeochemical trends obtained from systematic sampling of a mineralized borehole section include K and Rb enrichment in the zone of strongest quartz-sericite alteration associated with mineralization and broad depletion of Mn, Na, Ca and Sr within altered footwall fragmental rocks. Minor Cu, Pb, Zn and Ag enrichment has accompanied low-grade propylitic alteration of hanging-wall rocks up to 50 m above mineralization. Analysis of weathered bedrock samples from traverses above the mineralized borehole section indicates that primary geochemical trends occur in the weathered zone. Outcropping gossan has strongly anomalous Cu (535 ppm-21.5%), Zn (3300 ppm-6.15%), Pb (420–8200 ppm), As (200–7000 ppm) and Hg (33–670 ppm) values.Application of lithogeochemistry as a follow-up exploration method in a tropical area such as Wainaleka was investigated as a possible replacement for ridge, spur and base-of-slope soil sampling techniques. Ridge-top auger samples and creek outcrop samples were collected at approximately 100 m intervals and a density of 70/km². Elements (including Cu, Pb, Zn, Mn, Rb, Sr, Na, K, Ca and Mg) were selected for analysis because of specific associations with mineralization and alteration, and low analytical costs. Single- and multi-element dispersions effectively outline mineralization and attendant alteration.     

Reconnaissance geochemistry of the nechako plateau, British Columbia, using lake sediments

A lake sediment geochemical survey was undertaken over the Nechako plateau, British Columbia, to test application of the technique to regional exploration and compare its effectiveness with results obtained elsewhere. Organic-rich lake sediment samples were collected towards the centre of approximately 500 lakes on a helicopter-assisted survey covering some 16,000 km². Lakes overlying each of five major lithologies contain a distinctive suite of trace metals. Regional variations of Cu, Mo, Pb, Zn, Ni, Cr, Sr, Ba, Ag, Co, V and Ga can be related to differences in underlying geology. In addition, levels of Cu, Mo, Pb and Zn which exceed their regional thresholds are related to mineralization or lithologies favourable to mineralization. Although metal concentrations within a lake may vary up to 12x, anomalous conditions are not masked by this variability and the survey outlined all major sulphide occurrences known in the region.     

Comparison of interpretations of geochemical soil data by some multivariate statistical methods, Key Anacon, N.B., Canada

The nature of the information - and its usefulness - that can be obtained from some of the more common multivariate statistical techniques is illustrated by their application to H and B horizon soils analyzed for 16 variables from the vicinity of the Key Anacon massive sulphide deposit (New Brunswick, Canada) where the geochemical response is erratic and the contrasts low. The theoretical bases of the statistical techniques are given in an appendix.Stepwise linear discriminant functions are employed as a classifying technique on H horizon soils to identify a regional anomaly, and on B horizon soils to derive discriminant scores to define the location of the sulphide zone. R-mode nonlinear mapping (RNLM) indicates that the variables Hg, Cl⁻, and conductance are closely correlated with the organic carbon content; variations in the latter are clearly related to variations in the secondary environment.Q-mode NLM using Pb, Zn, Co, and Ni as variables (identified as good discriminators by stepwise linear discriminant function) identify “outlying” anomalous samples related to the mineralized zones. A principal component biplot (using the same variables employed in the Q-mode NLM) indicates essentially the same samples as being anomalous as the Q-mode NLM. The biplot technique has the added advantage of also indicating which variable(s) is responsible for any particular sample being identified as anomalous.The practical result of the investigation is that the zone of sulphide mineralization defined by drilling and from underground data is confidently identified and defined. A number of geophysical magnetic anomalies over the same stratigraphic horizon has a relatively weak or no geochemical response, although a few locations are defined as second priority targets for follow-up work.     

The mineralogy and chemical composition of the Woodlawn massive sulphide orebody

The main Woodlawn ore lens is a polymetallic, massive sulphide deposit’ with pyrite the major constituent, variable sphalerite, galena and chalcopyrite, and minor arsenopyrite, tetrahedrite‐tennantite, pyrrhotite and electrum. The silicate gangue minerals are chlorite, quartz, talc and sericitic mica. Other mineralization in the vicinity consists of footwall copper ore in chlorite schist and several smaller massive sulphide lenses. The predominant country rocks are felsic volcanics and shales, with abundant quartz, chlorite and mica, and talc in mineralized zones.

An important textural feature of the massive ore is the fine compositional banding. Bands, which vary in thickness from a few tens of micrometres to several millimetres, are produced by variations in the sulphide content. Post‐depositional metomorphism and minor fracturing have only slightly modified this banding.

Apart from the major element constituents—Pb, Zn, Fe, Cu and S—the ore is characterized by significant (100–1000 ppm) values for Ag, As, Cd, Mn, Sb and Sn, and lower (1–100 ppm) values of Au, Bi, Co, Ga, Hg, Mo, Ni, Tl. In and Ge. Variations in the base‐metal sulphide content, the gangue mineralogy, and trace elements, are used to separate the orebody into hanging‐wall and footwall zones. The hanging‐wall zone shows a more variable trace element content, with higher Tl, Sn, Ni, Mn, Ge and Sb, but lower Ag, Cd, and Mo, than the footwall zone.

In general style of mineralization, mineralogy, and chemistry, the Woodlawn deposit resembles other volcanogenic massive sulphide deposits in eastern Australia, in New Brunswick in Canada, and the Kuroko deposits of Japan.     

Distinctive features of Late Palaeozoic massive sulphide deposits in South China

More than 20 sediment-hosted massive sulphide deposits occur in Late Palaeozoic basins in South China. These deposits are accompanied by a certain amount of volcanic rocks in the host sequence and are economically important for their Cu, Pb, Zn, Au and Ag reserves. The deposits and their host strata were commonly intruded by Mesozoic granitoids. Remobilisation of sedimentary ores and magmatic hydrothermal overprinting processes resulted in the coexistence of massive sulphides with vein-, skarn- and porphyry-type orebodies in the same region or within a single deposit. The ore-containing basins occur in different tectonic settings. The Lower Yangtze basin occurs on a passive continental margin, where the deposits are high in Cu and Au with minor Pb and Zn and recoverable Ag, Co and Mo. The ores have a lower concentration of radiogenic lead, and δ³⁴S values close to zero. Fluid inclusions are highly saline and Na-rich. Fluids and metals of the Lower Yangtze Region are interpreted to have been derived essentially from deep sources including the Precambrian basement. By contrast, basins of the Nanling Region formed in an intracontinental setting developed on a folded Caledonian basement. These deposits are higher in Pb, Zn, Sn and W, as well as Cu, with recoverable Ag, Sb, Hg, U, Bi, Tl and Mo. The ores are characterised by a higher concentration of radiogenic lead and a wide variation of δ³⁴S composition. Fluid inclusions have lower salinities and higher K⁺/Na⁺ ratios. Fluids are considered to have been sourced substantially from seawater by convection. Metals for the Nanling deposits were essentially derived from the Caledonian basement by leaching. The contrast in ore composition between these two regions appears to have been controlled by differences in basement composition of the ore-forming basins.     

Heavy-mineral concentrates from rocks in exploration for massive sulphide deposits

A number of programs have investigated the use of rock geochemistry in the search for volcanogenic massive sulphide deposits in the Canadian Shield. Regional-scale studies have been successful in differentiating productive from nonproductive volcanic cycles. Wall-rock studies have successfully delineated alteration halos related to the mineralizing event. While an alteration halo has been identified around the South Bay massive sulphide deposit, this halo does not extend far enough from the deposit to be useful for reconnaissance purposes. The authors therefore tested the possibility of enhancing detection of a primary trace-element halo by using the heavy mineral fraction of the rocks.The geochemical dispersion of trace elements in the heavy-mineral fraction of rocks was investigated around the South Bay massive sulphide deposit, in the Superior Province of the Canadian Shield. Approximately 270 samples were ground to 74–500 μm (−35 +200 mesh) and separated using the heavy liquid bromoform. Following removal of the magnetic fraction, the samples were further pulverized, and analyzed by atomic absorption spectrophotometry for Cu, Pb, Zn, Ag, Fe, Mn, Co and Ni. Corresponding whole-rock samples were analyzed to provide for a comparative study with the whole-rock geochemistry.Analysis of the heavy-mineral fraction of rocks revealed strong and extensive halos of Cu, Pb, Zn and Ag persisting in some cases up to 10 km along strike away from the South Bay Deposit. By comparison, in the whole-rock data, halos of Pb, Ag and Zn were detected no farther than 1–2 km away from the deposit. Furthermore, trace-element content in the whole rocks appeared to be dominated by rock type; either multivariate statistical techniques, or separation of the data by rock type, was necessary to distinguish the anomaly related to mineralization. Trace-element content in the heavy-mineral concentrates was dominated by the presence of the sulphide minerals pyrite, chalcopyrite, and sphalerite, thus directly reflecting mineralization.Use of the heavy-mineral fraction of the rock eliminates the dilution effects of quartz and feldspar, allowing enhancement of trace-metal concentrations in sulphide minerals, and the delineation of strong and extensive halos of Cu, Pb, Zn, Ag and Mn around the South Bay massive sulphide deposit. While the cost of preparation of heavy-mineral separates is higher than that for whole-rock samples, the anomaly clearly defined by the trace-element content of the heavy fraction avoids the need for costly major-element and subsequent statistical analysis, and increases target size by an order of magnitude. The heavy-mineral fraction obtained from rocks shows great potential as an exploration guide to volcanogenic massive sulphide deposits.     

Geochemical dispersion in wallrocks associated with the Norbec deposit, Noranda, Quebec

Increasing emphasis is being given to the search for blind mineral deposits and the presence of geochemical dispersion halos in wallrocks may significantly enhance the chances of exploration success. Accordingly, the nature of wallrock dispersion associated with the Lake Dufault Cu-Zn-Au-Ag massive sulphide deposit, Noranda, Quebec, was examined to see if any such patterns existed. On the basis of relatively well defined geology, the area offered an opportunity to establish the relation between geochemical alteration and metallogeny, texture, structure and mineralogy.Anomalous element contents spatially related to mineralization are confined to the stratigraphic footwall (Waite Rhyolite) where Na₂O, SiO₂ and possibly CaO depletions and total Fe₁ Mn and MgO enrichments can be related to the hydrothermal vent. Dispersion halos 150–500 m in diameter are at least twice as wide as the obvious mineralogical alteration halo. Cu, Zn, Ag and S distributions show roughly similar patterns which are not obviously related to the main alteration pipe but do reveal broad enrichment zones in the general area of mineralization.The dispersion appears to be partly controlled by texture. Because of greater primary permeability, the halos produced in rhyolite breccia are wider and better defined than those in more massive rhyolites. On a wider scale (10 km²) dispersion patterns are related to the fault and fracture systems of the area.As many as five stages of metamorphism have affected the area, altering all rocks to some extent. Samples can be classified petrographically into alteration groups, both related and unrelated to mineralization. Comparison of these groups reveals the inadequacy of certain elements for outlining alteration related to mineralization. For example, the range of K₂O in relatively unaltered rhyolite overlaps the composition of all other groups, including those related to mineralization. Similarly apparent is that the distribution of certain elements, e.g., Na₂O, may adequately define one part of an alteration zone such as the sericite-rich zone, but not another.It is concluded that careful use of lithogeochemistry in conjunction with detailed geological mapping and core logging can provide information otherwise obtainable only by time-consuming petrographic analysis. Thus it can be a potent weapon in the search for blind ore deposits.     

Chemical characteristics of groundwater around two massive sulphide deposits in an area of previous mining contamination, Iberian Pyrite Belt, Spain

A detailed chemical study of groundwater was carried out to elucidate the processes controlling the oxidation and dissolution of sulphide minerals at two massive sulphide deposits in the Iberian Pyrite Belt (IPB), i.e. the mined La Zarza deposit and the unmined Masa Valverde deposit. It was found that major-element compositions varied according to the hydrological regime, La Zarza being in a relatively high area with groundwater recharge (and disturbance due to the human factor) and Masa Valverde being in a relatively low area with groundwater discharge. The variations mainly concern pH, Eh, SO₄ and Na concentrations. Metal concentrations were determined (a) by ICP-MS after filtration, and (b) in some cases by voltammetric measurement of Cu, Pb, Zn, Cd and Mn using the Voltammetric In situ Profiling (VIP) System, which allows detection of only the mobile fractions of trace elements (i.e., free metal ions and small labile complexes a few nanometers in size). If one compares the results obtained by each of the two methods, it would appear that the groundwater shows significant enhancement of metal solubility through complexing with organic matter and/or adsorption onto colloids and/or small particles. In areas of sulphide oxidation, however, this solubility enhancement decreases according to Cu>Zn>Cd>Pb. Under very low redox conditions, the attained metal concentrations can be several orders of magnitude (up to 10⁸–10⁹ for Cu and 10²–10³ for Pb) larger than those expected from equilibrium with respect to sulphide minerals as calculated with the EQ3NR geochemical code; Zn concentrations, however, are close to equilibrium with respect to sphalerite. The implication of these results is discussed with respect both to mineral exploration and to environmental issues.     

The Palaeoproterozoic Kristineberg VMS deposit, Skellefte district, northern Sweden, part I: geology

The Kristineberg volcanic-hosted massive sulphide (VMS) deposit, located in the westernmost part of the Palaeoproterozoic Skellefte district, northern Sweden, has yielded 22.4 Mt of ore, grading 1.0% Cu, 3.64% Zn, 0.24% Pb, 1.24 g/t Au, 36 g/t Ag and 25.9% S, since the mine opened in 1941, and is the largest past and present VMS mine in the district. The deposit is hosted in a thick pile of felsic to intermediate and minor mafic metavolcanic rocks of the Skellefte Group, which forms the lowest stratigraphic unit in the district and hosts more than 85 known massive sulphide deposits. The Kristineberg deposit is situated lower in the Skellefte Group than most other deposits. It comprises three main ore zones: (1) massive sulphide lenses of the A-ore (historically the main ore), having a strike length of about 1,400 m, and extending from surface to about 1,200 m depth, (2) massive sulphide lenses of the B-ore, situated 100–150 m structurally above the A-ore, and extending from surface to about 1,000 m depth, (3) the recently discovered Einarsson zone, which occurs in the vicinity of the B-ore at about 1,000 m depth, and consists mainly of Au–Cu-rich veins and heavily disseminated sulphides, together with massive sulphide lenses. On a regional scale the Kristineberg deposit is flanked by two major felsic rock units: massive rhyolite A to the south and the mine porphyry to the north. The three main ore zones lie within a schistose, deformed and metamorphosed package of hydrothermally altered, dominantly felsic volcanic rocks, which contain varying proportions of quartz, muscovite, chlorite, phlogopite, pyrite, cordierite and andalusite. The strongest alteration occurs within 5–10 m of the ore lenses. Stratigraphic younging within the mine area is uncertain as primary bedding and volcanic textures are absent due to strong alteration, and tectonic folding and shearing. In the vicinity of the ore lenses, hydrothermal alteration has produced both Mg-rich assemblages (Mg-chlorite, cordierite, phlogopite and locally talc) and quartz–muscovite–andalusite assemblages. Both types of assemblages commonly contain disseminated pyrite. The sequence of volcanic and ore-forming events at Kristineberg is poorly constrained, as the ages of the massive rhyolite and mine porphyry are unknown, and younging indicators are absent apart from local metal zoning in the A-ores. Regional structural trends, however, suggest that the sequence youngs to the south. The A- and B-ores are interpreted to have formed as synvolcanic sulphide sheets that were originally separated by some 100–150 m of volcanic rocks. The Einarsson zone, which is developed close to the 1,000 m level, is interpreted to have resulted in part from folding and dislocation of the B-ore sulphide sheet, and in part from remobilisation of sulphides into small Zn-rich massive sulphide lenses and late Au–Cu-rich veins. However, the abundance of strongly altered, andalusite-bearing rocks in the Einarsson zone, coupled with the occurrence of Au–Cu-rich disseminated sulphides in these rocks, suggests that some of the mineralisation was synvolcanic and formed from strongly acidic hydrothermal fluids. Editorial handling: P. Weihed     

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.     

The application of stepwise discriminant analysis to geochemical data from the host rocks of the Sullivan Pb-Zn-Ag deposit, Kimberley, B.C., Canada

The Sullivan Pb-Zn-Ag massive sulphide deposit in southeastern British Columbia occurs within middle Proterozoic argillite, siltstone and quartz wacke of the Purcell Group. Rock samples were collected from the hangingwall and footwall of the eastern section of the mine and from outcrop up to 50 km from the Sullivan deposit. The samples were analyzed for Cu, Pb, Zn, S, Mn, Ba, Fe, K, Ca, Na and specific conductance. A stepwise discriminant analysis applied to the analytical data determined the group of variables that differentiate between hangingwall, footwall and outcrop or “Background” samples. Hangingwall and footwall rock samples were most effectively discriminated from “background” rock samples on the basis of specific conductance with Cu, Pb, S, Na, and Ba selected as less efficient discriminators. The variables that discriminate hangingwall from footwall rock samples are Cu, Zn and S. The selection of the discriminating variables in each case can be explained in terms of the chemical changes that occur as a result of host rock alteration and sulphide deposition during the mineralizing event at the Sullivan deposit.Stepwise discriminant analysis was used to reduce a number of potential pathfinder variables to an optimum group of pathfinder variables. These optimum pathfinders represent the variables that most effectively differentiate the host rocks of the Sullivan deposit from rocks outside of the mineralized zone that apparently do not contain massive sulphide mineralization.     

Tourmaline in the central Swedish ore district

More than 40 recently discovered tourmaline occurrences have been investigated in the Mid-Proterozoic Bergslagen ore district of central Sweden. Some are spatially associated with ores, others with zones of leaching, remobilization and migmatization. Among the tourmaline-bearing ore deposits are the Dammberg ZnPb-Fe sulphide deposit, the Sala Pb-Zn-Ag deposit, the Dalkarlsberg, Pershyttan and Håksberg Fe oxide deposits, the Leja Cu deposit, and the Zinkgruvan Zn-Pb-Ag deposit. Tourmaline has been recorded a) as tourmalinites and tourmaline-bearing chemical sediments; b) in tourmaline-bearing skarns; c) in tourmaline-quartz veins; d) as disseminations along the foliation in schists; e) in tourmaline pegmatites; f) in tourmalinized haloes in metavolcanites along tourmaline pegmatites; and g) in late joints. Tourmalinites, tourmaline-bearing chemical sediments and tourmaline-bearing skarns are spatially associated with sulphide and oxide mineralizations. The dravite components in these tourmalines are proportional to the size of Zn-Pb sulphide mineralizations. Tourmalines from quartz veins close to and within ore deposits contain high Zr and Cr contents. With increasing distance away from these deposits, the Zr and Cr contents fall significantly. Tourmalines from pegmatites have inherited a number of trace element enrichments through partial melting and assimilation of volcaniclastic sediments into granitic melts. Despite magmatic homogenization, Zn contents in these tourmalines reflect the proximity of Zn-Pb-sulphide deposits, decreasing away from them. Tourmalines from late joints with Zn contents above the 100 ppm level are also indicative for the proximity of Zn-Pb sulphide mineralizations. Thus, some trace elements in these tourmalines may represent suitable exploration tools.     

Interpretation of geochemical soil surveys by block averaging

In many cases the spatial distribution patterns of elements in soils - especially where geochemical response is weak - is erratic and incoherent and poorly defines underlying mineralized zones; frequency distributions of elements are characteristically positively skewed. Geochemical soil survey data for Cu, Pb, and Zn from around the Key Anacon massive sulphide deposit in New Brunswick, Canada, are used to illustrate an interpretative technique useful in these situations. Calculation of mean element values of all samples within non-overlapping blocks eliminates random background noise and enhances significant spatial trends. Frequency curves of the block means tend to normality if the spatial distribution is random; the frequency curves emphasize the skewness or become polymodal if the spatial distribution is systematic. The only critical step in this simple interpretative procedure is the determination of the appropriate block size for the particular sample density relative to the areal size of the geochemical response to the target.     

Stratigraphy and base metal mineralization in the Otavi Mountain Land, Northern Namibia—a review and regional interpretation

Sediment-hosted base metal sulfide deposits in the Otavi Mountain Land occur in most stratigraphic units of the Neoproterozoic Damara Supergroup, including the basal Nosib Group, the middle Otavi Group and the uppermost Mulden Group. Deposits like Tsumeb (Pb–Cu–Zn–Ge), Kombat (Cu–Pb–Zn), Berg Aukas (Zn–Pb–V), Abenab West (Pb–Zn–V) all occur in Otavi Group dolostones, whereas siliciclastic and metavolcanic rocks host Cu–(Ag) or Cu–(Au) mineralization, respectively. The Tsumeb deposit appears to have been concentrated after the peak of the Damara orogeny at around 530 Ma as indicated by radiometric age data.Volcanic hosted Cu–(Au) deposits (Neuwerk and Askevold) in the Askevold Formation may be related to ore forming processes during continental rifting around 746 Ma. The timing of carbonate-hosted Pb–Zn deposits in the Abenab Subgroup at Berg Aukas and Abenab is not well constrained, but the stable (S, O, C) and Pb isotope as well as the ore fluid characteristics are similar to the Tsumeb-type ores. Regional scale ore fluid migration typical of MVT deposits is indicated by the presence of Pb–Zn occurrences over 2500 km² within stratabound breccias of the Elandshoek Formation. Mulden Group siliciclastic rocks host the relatively young stratiform Cu–(Ag) Tschudi resource, which is comparable to Copperbelt-type sulfide ores.     

Brine pool deposition for the Zn–Pb–Cu massive sulphide deposits of the Bathurst mining camp, New Brunswick, Canada. I. Comparisons with the Iberian pyrite belt

The Ordovician Zn–Pb–Cu massive sulphide ore deposits of the Bathurst mining camp share many features with those of the Devonian/Carboniferous Iberian pyrite belt, particularly the tendency to large size (tonnage and metal content); shape, as far as can be determined after allowing for deformation; metal content, particularly Fe/Cu, Pb/Zn and Sn; mineral assemblages (pyrite + arsenopyrite ± pyrrhotite and lack or rarity of sulphates); sulphide textures (particularly framboidal pyrite); lack of chimney structures and rubble mounds; irregular metal or mineral zoning; and the low degree of zone refining compared to Hokuroku ores. The major differences between the provinces are the lack of vent complexes and the presence of Sn–Cu ores in the Iberian pyrite belt. There are also similarities in the geological setting of the two camps: both lie within continental terranes undergoing arc-continent and continent–continent collision, and in each case massive sulphide mineralisation followed ophiolite obduction; the ore deposits are associated with bimodal volcanic rocks derived from MORB and continental crust and marine shales; and mineralisation was locally accompanied or followed by deposition of iron formations.Fluid inclusion data from veins in stockworks from at least six of the Iberian massive sulphide deposits point to sulphide deposition having taken place in basins containing mostly spent saline, ore-forming fluids (brine pools), and it is suggested that most of the major features of the Bathurst deposits can be explained by similar processes. The proposed model is largely independent of ocean sulphate and O₂ content, whereas low values of each are requisites for the current, spreading-plume model of sulphide deposition in the Bathurst camp.