Massive Sulfide Orebodies Research Articles

Ophiolitic sulfide mineralization at Tilt Cove, Newfoundland; controls by upper mantle and crustal processes

Massive Cu-Zn sulfide (+ or - Au + or - Ag) orebodies are found at a number of localities in the Betts Cove ophiolite, the main ones being mined at Betts Cove and Tilt Cove. The mineralization is mainly pyrite and chalcopyrite, with lesser amounts of sphalerite, as well as traces of galena. Magnetite is locally abundant along with coarse specular hematite in the East mine. About 42,000 oz of gold was produced from both the massive ore and associated with sphalerite in banded and stringer sulfides within the ophiolitic pillow lavas. Mineralization is accompanied by intense black chloritization as well as more restricted stockwork pyrite and silica. The sulfide minerals exhibit the same tectonic fabrics as the host rocks, indicating their predeformation origin.Chemical data for 150 samples from the Tilt Cove area show primary chemical trends related to both olivine fractionation and advanced partial melting of the upper mantle. Silica, Fe, Mg, Ca, Na, and K were highly mobile during alteration with major variations related to gain or loss of silica--the lowest silica rocks approaching compositions of pure chlorite. Several trace elements were relatively immobile during alteration, including Ti, Zr, Y, P, Cr, and Ni. The lavas can be clearly separated into a high Zr group which overlaps samples from the ophiolite in the Betts Cove area and the compositional fields of mid-oceanic ridge basalts and island-arc tholeiites, and a low Zr group extremely depleted in both Ti and Zr. The low Zr samples, which are all from the Tilt Cove area where the largest massive sulfide orebodies are found, have a number of geochemical characteristics analogous to boninites. Rare earth element concentrations are lower in the lower Zr lavas, which have concave-upward patterns, than in the higher Zr lavas, which have convex-upward patterns.We endorse the suggestions of earlier workers that these lavas were formed by wet melting of depleted upper mantle caused by fluids produced through dehydration during subduction of previously metasomatized oceanic crust. We would go further, however, and suggest that these fluids were also enriched in base metals and sulfur, which were also incorporated into the lavas, either in solution or as immiscible sulfide droplets. The Tilt Cove orebodies were subsequently derived from these lavas by the generally accepted mechanism, i.e., leaching of metals by circulating seawater. It is estimated that the separate 3- to 5-million-ton sulfide orebodies at Tilt Cove were formed by leaching of approximately 2 km 3 of pillow lava containing approximately 300 ppm S, with no more than a approximately 200-m depth of circulation over a very broad area of approximately 12 km 2 . A smaller leaching volume would be adequate if higher concentrations of sulfur were available in the source rocks.

Genesis of, and exploration for, industrial and ore minerals

Report of the 11th AGM of the Mineral Deposits Studies Group held in the Department of Applied Geology, University of Strathclyde, Scotland, 19–21 December 1985. The meeting titled 'Genesis of, and exploration for, industrial and ore minerals' was attended by 92 participants (from academia and industry) and included a field trip, 38 papers and several poster sessions. On the first day M. J. Russell and A. J. Hall led a field trip to Tyndrum where dumps, veins and wall rocks were examined in drizzle. A keynote address on Black Smokers and a video taken by submersible were given by John Edmond. He described hydrothermal activity on intermediate and fast spreading ridges in the Eastern Pacific. The fluids are of sea-water origin, acid (pH about 3) and with temperatures around 350 °C. Chloride is the only anion and H 2 S concentrations reach 300 ppm whereas Cu and Zn are usually between 1 and 5 ppm. While the metalliferous sediments are analogues to ophiolite-type massive sulphide ore bodies no active example has been found that remotely compares to them in size. Most of the ore-forming metals are lost as 'black smoke' and the ore localization mechanism is a puzzle. Hydrothermal fluids from the Gulf of California are alkaline, contain major ammonium and are depleted in ore metals possibly suggesting that a sediment-hosted massive sulphide deposit is forming at depths. Precipitation may be caused by reaction of the ore fluids with biogenic carbonate tests and with ammonia produced by the thermocatalytic breakdown of

Geometry, conditions and timing of off-axis hydrothermal metamorphism and ore-deposition in the Solea graben

The recent discovery of fossil spreading axes along the northern margin of the Troodos ophiolite1 provides a unique opportunity to compare a fossil seawater-hydrothermal system with systems of modern oceanic spreading centres2–6. Whereas previous accounts of ophiolite hydrothermal systems7–13 including Troodos14–16have elucidated many aspects of sea floor hydrothermal metamorphism, earlier workers have not placed ophiolite hydrothermal features within any specific spreading-centre structural framework. Most ophiolites exhibit complex histories of post-accretion deformation that mask or obliterate early structures related to seafloor spreading. The Troodos ophiolite is apparently still in the process of being 'emplaced'17 and most structural elements at Troodos appear to have originated beneath a Cretaceous sea floor1. We have discovered an extensive area of intense seawater-hydrothermal alteration within the Sheeted Dyke Complex of the Solea graben (Fig. 1). The metamorphic petrology and oxygen-isotope geochemistry of >100 dyke samples from this graben reveal the size and three-dimensional geometry of the hydrothermal system and yield estimates of temperature, salinity, oxygen-isotope composition and recharge-discharge geometry for fluids that transported the constituents of massive sulfide orebodies. Geometric discordance between the trends of the hydrothermally altered area and of earlier, seafloor spreading structures compels us to consider a post-spreading, off-axis origin for the orebodies of the Solea graben.

Geochemistry and palaeo-oceanography of metalliferous and pelagic sediments from the Late Cretaceous Oman ophiolite

Metalliferous and pelagic sediments are exposed within and above the extrusive successions of the Upper Cretaceous Oman ophiolite which, on the basis of mostly geochemical evidence, is believed to have formed in an incipient marginal basin setting located above a NE-dipping subduction zone. The ophiolitic extrusives document various volcano-tectonic settings which include the axial zones of a spreading ridge, fault-controlled seamounts and off-axis volcanic edifices. Most of the Fe, Mn and trace metal-enriched sediments studied are interpreted as precipitates formed by oxidation of solutions derived from high-temperature sulphide-precipitating vents. The trace element content (e.g. REE and Sr) was largely scavenged from seawater. The sediments are similar to the dispersed metalliferous sediments on the flanks of modern spreading ridges, and the ‘basal’ sediments of DSDP wells and of other ophiolite complexes (e.g. Troodos, Cyprus). Distinctive mound structures located low in the lavas are attributed to percolation of sulphide-rich solutions into already deposited metalliferous oxide sediments. The resulting iron-silica rock was probably originally precipitated as ferruginous silicates. Major massive sulphides formed off-axis at the base of intermediate-basic edifices of volcanic arc affinities. Fe, Mn and trace metal enrichment in the sediment cover of a flat-topped seamount of axial lavas is interpreted as a dispersion halo around the largest massive sulphide orebody which is situated 5 km away (Lasail). Small massive sulphide bodies are common in the axial lavas particularly along major seafloor fault zones. The metalliferous sediments, locally precipitated near these vents, are ferromanganiferous, but trace metal-depleted. The metalliferous and pelagic sediment cover of the extrusive successions, generally, documents waning hydrothermal input after volcanism ended in the area. A model is discussed in which the ophiolite was created at a spreading axis above a subduction zone dipping away from the Arabian continental margin. With progressive subduction this crust approached the margin. Initially, calcareous sediment accumulated above the calcite compensation depth (CCD), but then non-calcareous radiolarites were deposited as the ophiolitic crust approached the continental margin where the CCD was higher and marginal upwelling possibly enhanced productivity. As the edge of the Arabian continental margin entered the trench, the over-riding ophiolite was regionally uplifted allowing short-lived chalk accumulation above the CCD. This was followed by volcaniclastic deposition related to the tectonic emplacement.

The Mount Morgan gold-copper mine and environment, Queensland; a volcanogenic massive sulfide deposit associated with penecontemporaneous faulting

The Mount Morgan mine was closed in July 1981, having yielded 237,896 kg of gold (7,648,533 oz) and 360,616 metric tons of copper from 50 million metric tons of ore, an average recovered grade of 4.75 g/metric ton Au and 0.72 percent Cu. It has been Australia9s largest single lode gold orebody and is the fourth largest gold-producing area after the Kalgoorlie Golden Mile and the Bendigo and Ballarat areas (Woodall, 1979).The mine comprises the Main Pipe orebody of pyritic massive sulfides and the adjacent Sugarloaf orebody of siliceous stringer mineralization. When the host stratigraphy is reoriented with respect to regional dip and local faulting, the two orebodies are shown to form a pipelike volcanogenic massive sulfide configuration at a high angle to the stratigraphy.Minor stratiform pyritic mineralization adjacent to the stratigraphic top of the orebody grades downdip away from the orebody to a sequence of interbedded crystal tuffs and cherty rocks with some jasperoid beds.The associated alteration halo of the massive sulfide orebody has been intruded by the Late Devonian Mount Morgan Tonalite. The ore and alteration have been subjected to metamorphism and overprinted by later alteration and additional mineralizing phases, mostly related to the complex tonalite intrusion.The immediate host stratigraphy shows gross thickening and change of character on either side of the orebody, suggesting that the orebody formed along a penecontemporaneous fault. The fault appears to be part of a major resurgent cauldron structure.

Gahnite in the metamorphosed stratiform massive sulfide deposits of the Mineral District, Virginia, U.S.A.

Ferroan-gahnite, (Zn,Fe)Al2O4, is a common accessory mineral found in association with the metamorphosed volcanogenic massive sulfide ore bodies of the Mineral District, Virginia. Gahnite is present throughout the mineralized zones that contain pyrite, sphalerite, chalcopyrite, and sulfosalts. Although constituting only 1–2 modal percent of most samples and displaying not well-defined distribution pattern, gahnite is especially abundant at the margins of the mineralized zones and locally constitutes as much as 35 modal percent. The compositions vary slightly from one deposit to another but all lie within the range of 68 to 88 mol.% gahnite (ZnAl2O4), 8 to 25 mol.% hercynite (FeAl2O4), and 1–17 mol.% spinel (MgAl2O4). Gahnite occurs within a wide variety of mineral assemblages, with or without coexisting sphalerite.

Mineralogy, textures, and relative age relationships of massive sulfide ore in the West Shasta District, California

The Devonian massive sulfide orebodies of the West Shasta district in northern California are composed primarily of pyrite, with lesser amounts of other sulfide and gangue minerals. Examination of polished thin sections of more than 100 samples from the Mammoth, Shasta King, Early Bird, Balaklala, Keystone, and Iron Mountain mines suggests that mineralization may be divided into six paragenetic stages, the last five each separated by an episode of deformation: (1) precipitation of fine-grained, locally colloform and framboidal pyrite and sphalerite; (2) deposition of fine-grained arsenopyrite and coarse-grained pyrite, the latter enclosing tiny inclusions of pyrrhotite; (3) penetration and local replacement of sulfide minerals of stages 1 and 2 along growth zones and fractures by chalcopyrite, sphalerite, galena, ten-nantite, pyrrhotite, bornite, and idaite; (4) recrystallization and remobilization of existing minerals, locally increasing their size and euhedralism and promoting their aggregation; (5) deposition of quartz, white mica, chlorite, and calcite; and (6) formation of bornite, digenite, chalcocite, and covellite during supergene enrichment of several orebodies at the Iron Mountain mine. Despite regional greenschist facies metamorphism and local heating by intrusive bodies, enough of the original depositional features of the ore remain to suggest that the deposits in the district formed by processes similar to those that formed Kuroko- and Besshi-type massive sulfide deposits. Mineralogic and textural evidence do not support a second major episode of massive sulfide mineralization during the Permian.

Application of Three-Dimensional Computer Modeling for Reservoir and Ore-Body Analysis: ABSTRACT

Three-dimensional computer modeling of reservoirs and ore bodies aids in understanding and exploiting these resources. This modeling tool enables the geologist and engineer to correlate in 3 dimensions, experiment with various geologic interpretations, combine variables to enhance certain geologic attributes, test for reservoir heterogeneities and continuity, select drill sites or perforation zones, determine volumes, plan production, generate geologic parameters for input to flow simulators, calculate tonnages and ore-waste ratios, and test sensitivity of reserves to various ore-grade cutoffs and economic parameters. All applications benefit from the ability to update rapidly the 3-dimensional computer models when new data are collected. Two 3-dimensional computer modeling projects demonstrate these capabilities. The first project involves modeling porosity, permeability, and water saturation in a Malaysian reservoir. The models were used to analyze the relationship between water saturation and porosity and to generate geologic parameters for input to a flow simulator. The second project involves modeling copper, zinc, silver, gold, and specific gravity in a massive sulfide ore body in British Columbia. The 4 metal models were combined into one copper-equivalence model and evaluated for tonnage, stripping ratio, and sensitivity to variations of ore-grade cutoff. End_of_Article - Last_Page 262------------

Selective extraction techniques in exploration for volcanogenic sulphide deposits, Eastern Black Sea region, Turkey

Selective extraction techniques employing 0.05 M EDTA, 1 M hydroxylamine hydrochloride (Hxl) in acetic acid and 0.157 M ammonium oxalate in oxalic acid were used to assess the mode of occurrence of the trace elements in the various geochemical sample media and to increase the anomaly contrast over the concealed massive ore bodies in the the Eastern Black Sea region of Turkey. In hematitized pumice tuffs, an exhalate horizon overlying the Madenköy massive sulphide ore body, the trace elements such as As, Cu, Zn, Pb and Mo are partly associated with sulphides and partly occur in clays, and Fe- and Mn-oxides. The extraction data on stream sediments suggest that a hot nitric acid attack is the best digestion procedure for detecting mechanical dispersion trains in regional geochemical programmes. Comparison of the extractabilities of metals in soils from mineralized areas with those from the background Sirtköy soils by the different extraction procedures demonstrates that EDTA, and to lesser extent Hxl are the best extraction procedures for increasing the anomaly/background ratio. These two extraction procedures were found to also enhance Zn and Pb anomalies over the cover rocks of the Madenköy massive ore body relative to the largely exposed stockwork-impregnation zones. Among the EDTA, Hxl and ammonium oxalate extractions, only the last procedure extracted considerable amounts of As from the soil samples. Vapor-phase analysis suggests that mercury occurs mainly in sulphide form which is leachable only by ammonium oxalate solution that also leaches adsorbed mercury. The EDTA and the Hxl solutions only extracted adsorbed mercury. These three extractants have no effect on either mercuric oxide or mercuric sulphate. Arsenic, a very useful pathfinder in this region, can only be partly extracted from the pumice tuff and soil samples by ammonium oxalate solution.

Extreme 18O-enriched volcanics and 18O-evolved marine water, Aljustrel, Iberian Pyrite Belt: transition from high to low Rayleigh number convective regimes

Quartz-eye keratophyric tuffs at Aljustrel, S. Portugal, Iberian Pyrite Belt possess unusually high, uniform whole rock δ-values up to 18.1%. with a mean of 16.7 ± 0.7%.. Because the quartz eye cores have δ 18O 12.0 to 13.5%. the felsic tuffs may have had an original whole rock oxygen isotope composition of 10 to 11%. The deduced enrichments of up to +8%. in keratophyric derivatives is attributed to isotope exchange with abundant marine water under temperatures which diminished to ≤ 100°C, during thermally driven convective cooling. The isotopic uniformity requires elevated permeability of ∼ 10 −8 cm 2, such that the second critical Rayleigh number was exceeded, and drifting, non steady state convection cells dominated, with relatively smooth thermal structures. In mineralised counterparts of the tuffs beneath the Feitais-Estacao Zn-Pb-Cu massive sulphide orebodies, whole rock δ-values are ~12%. and quartz (13.3–15.4%.)-chlorite (3.2–6.3%.) fractionations correspond to temperatures of 220–270°C, and a calculated fluid δ 18O of 1.4 to 5.7%. This local isotopic overprinting in vent domains of the regionally high 18O was induced by a stable, second stage convective regime imposed by drastic reduction of permeability accompanying capping of the geothermal discharge by hydrothermal cherts, and/or progressive spilitisation of the tuffs. At lower water/rock coupled with higher temperatures, the recirculating marine water underwent variable 18O enrichment up to 5.7%. by exchange with the high 18O tuffs. An upwards increase in δ 18O quartz and δ-quartz reflects progressive cooling of the geothermal discharge from 15.4%. 270°C in the stockwork, through 18.3%. 220–240°C in the orebody, to 20.1%. 110–130°C in overlying hydrothermal cherts, probably induced by entrainment of ambient marine bottom water in sub-seafloor aquifers.

The Balmat-Edwards zinc-lead deposits; synsedimentary ore from mississippi valley-type fluids

Reconstruction of some aspects of the initial environment of ore deposition through detailed S, C, and O isotope studies of the Fowler orebody, and through reconnaissance trace element and S isotope studies of sphalerite concentrates and composite samples of ore from 22 orebodies; and (2) evaluation of the effects of high-grade metamorphism on the stable isotope geochemistry of both the ore and gangue minerals of a massive sulfide orebody. The delta 34 S values of pyrite, sphalerite, and galena in the Fowler orebody range from 11.5 to 17.5 per mil; anhydrite associated with ore ranges from 25.6 to 27.0 per mil. Average equilibration temperatures calculated from the delta 34 S differences between coexisting sulfur-bearing phases are 388 degrees C for pyrite-sphalerite, 338 degrees C for sphalerite-galena, 329 degrees C for pyrite-galena, and 602 degrees C for anhydrite-pyrite; all pairs display narrow temperature distributions. The delta 13 C and delta 18 O values of carbonates from the Fowler orebody range from -1.6 to 1.4 per mil and from 24.8 to 27.0 per mil, respectively; quartz delta 18 O values range from 23.0 to 27.0 per mil. The delta 34 S values of sphalerite concentrated from composite samples of the orebodies range from 12.7 to 14.7 per mil. This narrow range contrasts sharply with the wide variability of associated evaporitic anhydrite lenses, which range from 8 to 28 per mil. A stratigraphic increase in the delta 34 S value of the evaporitic anhydrite at Balmat, and large sulfide iron-formation occurrences near Balmat, imply that bacterial sulfate reduction occurred on an unusually large scale during Balmat sedimentation. The presence of such an increase in a sedimentary section may constitute a valuable exploration guide.--Modified journal abstract.

Simulation as a tool for planning of exploration for deep seated base metal deposits

Exploration for massive sulphide ore bodies at great depths in the Skellefte district in northern Sweden is expensive and relies heavily on extensive drilling operations. Different drilling strategies have been studied by mean simulation. The bedrock structure, the targets and the drilling procedure have been described by mathematical models. These models are based on known facts from the field. For some significant geological parameters assumptions about the statistical properties are made. A number of different drilling strategies are designed. Drilling campaigns are simulated in the model and the expected outcomes are calculated and compared. A case study is presented. Quantitative estimates of the optimal drilling pattern are obtained, the influence on the expected outcome when the geological parameters are varied as well as the cost-benefit relation for investments in the project are estimated.

The geology of the New Insco copper deposit, Noranda district, Quebec

The New Insco copper deposit is an Archean volcanogenic massive sulfide orebody situated on the northwestern edge of the Noranda mining district, in the Superior Province of the Canadian Shield. The ore occurs near the base of a thin unit of pillowed and brecciated basalt that overlies rhyolitic volcaniclastic rocks. Both the basalt and rhyolite are heavily chloritized and sericitized and form a zone of alteration that is depleted in SiO2, Na2O, and CaO and enriched in FeOT, MgO, K2O, Cu, Zn, and S. The units are separated from an underlying massive rhyolite by a diorite intrusion and from an overlying series of pillowed basalts by a fault.The orebody is a pyrrhotite-rich lens of massive sulfide, containing conformable bands of disseminated chalcopyrite and pyrite, that is interlayered with lenses of pyrite–chalcopyrite-bearing siliceous ore. The massive ore is interpreted to have been deposited in a distal environment with pyrrhotite as a primary mineral. It was recrystallized during regional metamorphism, which imparted a polygonal–granular texture to the pyrrhotite and formed overgrowths on pyrite crystals. Following Pleistocene glaciation, pyrrhotite was extensively replaced by pyrite, marcasite, iron oxides, carbonates, and hydroxides.

The lead and strontium isotope geochemistry of metalliferous sediments associated with Upper Cretaceous ophiolitic rocks in Cyprus, Syria, and the Sultanate of Oman: Discussion

Gale et al. presented lead isotope analyses for 18 samples of metalliferous sediments associated with Upper Cretaceous ophiolitic rocks in Cyprus, Syria, and the Sultanate of Oman. They concluded that essentially 100% of the lead in some samples from Oman was leached from the underlying basaltic oceanic crust and that some samples from Syria contained comparatively little basaltic lead. They attributed the linear distribution of lead isotopic values to mixing of less radiogenic lead from mid-ocean ridge basalts with more radiogenic lead from sea water as represented by manganese nodules. The purpose of this discussion is to show that the least radiogenic lead component was not from typical midocean ridge basalts, but most likely from ophiolitic volcanic and intrusive rocks. Conclusions as to the relative importance of basaltic and "sea-water" sources to the lead in these sediments, and (by inference) in associated massive sulfide orebodies, are premature in view of the lack of sufficient isotopic data on the associated ophiolitic volcanic rocks. Gale et al. stated on page 1296 that the lead isotope data ".. . form linear arrays extending from values typical of mid-ocean ridge basalts (MORB) . . . ." This is not true. On standard 207~b/2"~b-206~b/204~b and 208~b/2"~b-206~b/2@'Pb plots showing the actual measured values for MORB (Figs. 1,2) there is minimal overlap between the values they report for metalliferous sediments and the MORB data (Sun 1974; Chych and Tatsumoto 1975; Sun et al. 1975; Tatsumoto 1978; Mattinson 1979). Therefore typical MORB values cannot represent the required end member. Only the isotopic values for a few samples from Syria and a few from Oman overlap with the main field of MORB data. However, lack of overlap on a plot of 208~b/206~b207~b/206~b (Fig. 3), with the minor exception of that between the Oman values and those for Indian Ocean ridge basalts, suggests that the overlap on the other plots may be due to 2"Pb error. Furthermore, the three plots

Massive Sulfide Orebodies: The Buchans Orebodies . Fifty Years of Geology and Mining. E. A. Swanson, D. F. Strong, and J. G. Thurlow, Eds. Geological Association of Canada, Toronto, 1981. x, 350 pp., illus., + loose maps in slipcase. $36. The Geological Association of Canada Special Paper 22.

Massive Sulfide Orebodies: <i>The Buchans Orebodies</i> . Fifty Years of Geology and Mining. E. A. Swanson, D. F. Strong, and J. G. Thurlow, Eds. Geological Association of Canada, Toronto, 1981. x, 350 pp., illus., + loose maps in slipcase. $36. The Geological Association of Canada Special Paper 22.

Massive Sulfide Orebodies: The Buchans Orebodies . Fifty Years of Geology and Mining. E. A. Swanson, D. F. Strong, and J. G. Thurlow, Eds. Geological Association of Canada, Toronto, 1981. x, 350 pp., illus., + loose maps in slipcase. $36. The Geological Association of Canada Special Paper 22.

Massive Sulfide Orebodies: <i>The Buchans Orebodies</i> . Fifty Years of Geology and Mining. E. A. Swanson, D. F. Strong, and J. G. Thurlow, Eds. Geological Association of Canada, Toronto, 1981. x, 350 pp., illus., + loose maps in slipcase. $36. The Geological Association of Canada Special Paper 22.

Imaging from Space: X-Ray Astronomy with the Einstein Satellite . Proceedings of a meeting, Cambridge, Mass., Jan. 1980. Riccardo Giacconi, Ed. Reidel, Boston, 1981 (distributor, Kluwer Boston, Hingham, Mass.). viii, 330 pp., illus. $44.50. Astrophysics and Space Science Library, vol. 87.

Imaging from Space: <i>X-Ray Astronomy with the Einstein Satellite</i> . Proceedings of a meeting, Cambridge, Mass., Jan. 1980. Riccardo Giacconi, Ed. Reidel, Boston, 1981 (distributor, Kluwer Boston, Hingham, Mass.). viii, 330 pp., illus. $44.50. Astrophysics and Space Science Library, vol. 87.

Comparison of sulphide deposits from the East Pacific Rise and Cyprus

Submarine sulphide deposits of the East Pacific Rise, discovered in 1978, at the mouth of the Gulf of California are compared here with fossil massive sulphide ore bodies. Two main types of massive sulphide ore bodies are defined in relation to the associated volcanism. The Cyprus ore shows striking analogies with East Pacific Rise deposits. The comparison is based on mineralogical observations and textural facies evidence. These similarities suggest that the genetic processes were identical.

Conductor identification in tropical terrains—Case histories from the Itapicuru greenstone belt, Bahia, Brazil

An airborne electromagnetic (EM) and magnetic survey was conducted to prospect for massive sulfide ore bodies in the Itapicuru greenstone belt in the state of Bahia, Brazil. Because tropical weathering is intense in this region, most of the recorded airborne EM anomalies were caused by a conductive overburden. Standard procedures to select targets for ground investigation (follow‐up) were found only partially satisfactory because such procedures resulted in anomalies in conductive environment being largely ignored. Because of this difficulty, a new scheme of target selection was developed which is based on a statistical analysis of conductance within a given lithologic unit. Four case histories of ground geophysical surveys, which were carried out during the follow‐up of airborne anomalies are described. The case histories focus on problems likely to be encountered in environments of intense tropical weathering. In areas covered by a conductive overburden, the EM response is sensitive to a proper choice of field equipment, frequency, and coil separation. As a result of the geophysical survey, bodies of volcanogenic massive sulfides were found in the Itapicuru greenstone belt, but so far none is of economic importance.

Sulphur‐isotope study of the massive sulphide orebody at Woodlawn, New South Wales

Abstract 34S/32S ratios have been measured in a suite of samples from the stratabound, volcanogenic massive sulphide deposit at Woodlawn, N.S.W. 34S values for the sulphides vary as follows: in the ore horizon, pyrite +6.7 to +9.2%. (mean +8.1‰), sphalerite +5.2 to +8.6‰. (mean +6.9‰), chalcopyrite +6.4 to +7.0‰ (mean +6.7‰) and galena +2.8 to +5.5‰ (mean +4.4‰); in the vein mineralization, the host volcanics—pyrite +8.7 to +11.4%. (mean +9.8‰), sphalerite +7.8 to + 10.3‰ (mean +9.2‰), chalcopyrite; +8.8 to +10.1‰ (mean +9.2‰) and galena +6.9 to +7.2‰ (mean +7.1‰). Barite from the upper ore horizon levels has an isotopic composition of +30.0‰, consistent with its having originated from Silurian ocean sulphate. The general order of 34S enrichment in the sulphides is pyrite > chalcopyrite ∼ sphalerite > galena. Isotopic fractionations in the systems galena/sphalerite/pyrite and chalcopyrite/pyrite indicate an equilibration temperature of 275–300°C. This temperature is considered to represent that of sulphide deposition.