Electron paramagnetic resonance (EPR) spectroscopy in massive sulphide exploration, Rosebery mine area, western Tasmania, Australia

Electron paramagnetic resonance (EPR) spectroscopy of hot HNO₃ insoluble residues of rock powders is used as a new exploration technique for the volcanic-hosted massive sulphide (VHMS) deposit in the Rosebery mine area. The EPR signal intensities measured in 326.5±5 mT sweeps are strong in the altered rocks, and show a negative correlation with Ca, Na and Sr, and a positive correlation with K/Na, Rb/Sr and (K × Rb)/(Ca × Na × Sr). The EPR intensities measured in 326.5±100 mT sweeps show high values in the footwall pyroclastics, host rocks and hanging wall pyroclastics near and around the Rosebery deposit, and correlate positively with K, Fe, Mn, Ba, F, Rb, Zn, Pb and Zr. The Rosebery deposit and associated footwall alteration zone are located at the intersection of two elongated paramagnetic halos. The first is characterized by strong intensities of [AlO₄]° signals measured at magnetic flux density sweeps over 326.5±5 mT, trends NE–SW, and passes discordantly from the west to the east the White Spur Formation, altered footwall (footwall alteration zone), host rock of the Rosebery deposit, hanging wall and Mount Black Volcanics. The second, largely stratabound, halo is defined by strong intensities of Mn²⁺ sextets observed at magnetic flux density sweeps over 326.5±100 mT, runs N–S following the stratigraphic trend, and outlines the mineralized host rock and footwall alteration zone. It also extends toward the south into the unaltered footwall and hanging wall rocks. The first type of halo is considered to be related to wall rock alteration due to the VHMS mineralization processes as well to later Devonian metamorphism, and the second is thought to be related to massive sulphide mineralization alone.     

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 use of fluorine as a pathfinder for volcanic-hosted massive sulfide ore deposits

A multi-element geochemical study of the wall rocks of intermediate to felsic volcanic-hosted massive sulfide deposits was carried out to identify pathfinder elements which significantly enlarge the size of exploration targets. Drill core samples from the Crandon massive sulfide deposit in Wisconsin, and outcrop samples from the United Verde and Iron King deposits in Arizona, and from the Captains Flat, Mt. Costigan, and Wiseman Creek deposits in New South Wales, Australia were analyzed. Because anomalously high fluorine values have been described in several volcanic-hosted ore systems, fluorine was included in the study.All of the above deposits have patterns of fluorine enrichment around ore. Drill core samples from two noneconomic prospects within ten miles of the Crandon deposit contain background to only weakly anomalous fluorine values.At the large Crandon deposit (> 50 million tons of zinc, copper ore), fluorine enrichment extends approximately 320 m into the footwall rocks and at least 220 m into the hanging wall rocks. At the large United Verde deposit (> 50 million tons of copper, zinc ore), fluorine enrichment is recognizable in the footwall rocks at least 650 m from the ore. At the smaller Iron King deposit (five million tons production of zinc, lead, copper ore), fluorine enrichment extends for a distance of approximately 60 m into the footwall rocks. At the small deposits in New South Wales (< five million tons production of zinc, lead, copper ore), fluorine enrichment is easily recognizable, but with the samples collected, the limits of the anomalous patterns cannot be defined.Fluorine occurs in some hydrothermal systems unassociated with mineralization and is therefore not a specific signature of ore-forming processes. From the work completed, many massive sulfide deposits in volcanic rocks occur in hydrothermal systems which contain fluorine. On the basis of the data presented, if anomalously high fluorine values do exist in an exploration search area, the chances of finding a massive sulfide ore deposit are improved.Genetic models for volcanic-hosted massive sulfide ore deposits have concentrated on rock textures, alteration mineralogy, and geochemistry of the ore metals. From the data presented, fluorine should be considered as a component of massive sulfide systems in intermediate to felsic volcanic rocks, and should be considered as a possible complexing agent for the ore metals.     

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.     

Regional reconnaissance exploration rock geochemistry for massive sulphides, New Brunswick, Canada

In the Bathurst District of New Brunswick there are more than 50 known occurrences of base metal sulphide mineralization within an area of Palaeozoic volcanic-sedimentary rocks approximately bounded by the Rocky Turn deposit in the north, the Key Anacon deposit in the east, the Heath Steele deposit in the south, and the Devil's Elbow deposit in the west. Only four of these occurrences are, or have been, producing mines; 19 are classed as “major occurrences”. The area is highly prospective for massive sulphide deposits of the Brunswick Mining and Smelting and Heath Steele type; it would obviously be of considerable importance to define the zones within the sequence where major occurrences should be sought.To determine whether exploration rock geochemistry could be used on a regional reconnaissance scale, 419 samples of rhyolite from an area of 2000 km² (at an average density of one sample per 5 km²) were analyzed for total content of Cu, Pb, Zn, Ca, Mg, K, Na, Fe, and Mn. The data were processed by calculating the geometric mean of all samples in cells of approximately 10 km². Contrary to the relations documented on a mine scale (within one kilometre of major deposits), where the clearest halos are given by major elements, it is the ore elements that give the best regional patterns.The producing mines and the most important of the known occurrences all lie in zones where rhyolite contains less than 10 ppm Cu. Element ratios considerably enhance anomalous relations. The Zn:Pb ratio of the sulphides in the main deposits is 2.4–2.8, regardless of grade. It is demonstrated that all present and past producing mines and the most important known major occurrences lie within well-defined zones of Zn:Pb ratios of 2.4–2.8. Similarly, zones where the Pb:Cu ratio is > 3.0 and the Zn:Cu ratio is > 7.0 also define the most important deposits. These ore-element relations derived from a low sample-density survey define priority zones for detailed exploration for significant major massive sulphide deposits.     

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.     

Geology and wall rock alteration at the Hercynian Draa Sfar Zn–Pb–Cu massive sulphide deposit, Morocco

Draa Sfar is a siliciclastic–felsic, volcanogenic massive sulphide (VMS) Zn–Pb–Cu deposit located 15 km north of Marrakesh within the Jebilet massif of the western Moroccan Meseta. The Draa Sfar deposit occurs within the Sarhlef series, a volcano-sedimentary succession that hosts other massive sulphide deposits (e.g., Hajar, Kettara) within the dominantly siliciclastic sedimentary succession of the lower Central Jebilet. At Draa Sfar, the footwall lithofacies are dominated by grey to black argillite, carbonaceous argillite and intercalated siltstone with localized rhyodacitic flows and domes, associated in situ and transported autoclastic deposits, and lesser dykes of aphanitic basalt and gabbro. Thin- to thick-bedded, black carbonaceous argillite, minor intercalated siltstone, and a large gabbro sill dominate the hanging wall lithofacies. The main lithologies strike NNE–SSW, parallel to a pronounced S1 foliation, and have a low-grade, chlorite–muscovite–quartz–albite–oligoclase metamorphic assemblage. The Draa Sfar deposit consists of two stratabound sulphide orebodies, Tazakourt to the south and Sidi M'Barek to the north. Both orebodies are hosted by argillite in the upper part of the lower volcano-sedimentary unit. The Tazakourt and Sidi M'Barek orebodies are highly deformed, sheet-like bodies of massive pyrrhotite (up to 95% pyrrhotite) with lesser sphalerite, galena, chalcopyrite, and pyrite. The Draa Sfar deposit formed within a restricted, sediment-starved, fault-controlled, anoxic, volcano-sedimentary rift basin. The deposit formed at and below the seafloor within anoxic, pelagic muds.The argillaceous sedimentary rocks that surround the Draa Sfar orebodies are characterized by a pronounced zonation of alteration assemblages and geochemical patterns. In the more proximal volcanic area to the south, the abundance of medium to dark green chlorite progressively increases within the argillite toward the base of the Tazakourt orebody. Chlorite alteration is manifested by the replacement of feldspar and a decrease in muscovite abundance related to a net addition of Fe and Mg and a loss of K and Na. In the volcanically distal and northern Sidi M'Barek orebody alteration within the footwall argillite is characterized by a modal increase of sericite relative to chlorite. A calcite–quartz–muscovite assemblage and a pronounced decrease in chlorite characterize argillite within the immediate hanging wall to the entire Draa Sfar deposit. The sympathetic lateral change from predominantly sericite to chlorite alteration within the footwall argillite with increasing volcanic proximity suggests that the higher temperature part of the hydrothermal system is coincident with a volcanic vent defined by localized rhyodacitic flow/domes within the footwall succession.     

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.     

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.     

Geochemical and mineralogical haloes about the Elura Zn-Pb-Ag orebody, western New South Wales

The Elura Zn-Pb-Ag deposit, situated 43 km NNW of Cobar, western New South Wales, is hosted by the C.S.A. Siltstone, a distal turbidite sequence. Deep weathering has given rise to a bleached quartz-muscovite-kaolinite rock to a depth of approximately 80 m. Weathered bedrock is mantled by a thin (0.3–2 m) layer of soil and transported overburden which contains thin layers of maghemite-bearing gravels. Outcrop in the area is extremely poor with the insignificant gossan subcrop covered by 0.2 m of soil. The water table is presently at a depth of about 80 m. Groundwater is saline with up to 2.5% total dissolved solids.Oxidation of the orebody has resulted in the formation of a gossan and ferruginization of wall rocks. Elements associated with ore and retained at high concentration in the gossan are Ag, As, Ba, Cu, Hg, Mo, Pb, Sb, (Se) and Sn; much of the Zn has been leached whilst Cd and Tl are below the detection limits. Silver, Cu and Hg have been partially leached and concentrated in the supergene zone. Ferruginous wall rock contains substantial amounts of Pb, As and Ba but other element contents are substantially lower than in the gossan.Secondary dispersion from the Elura orebody is largely restricted to an interpreted paleodrainage channel SW of the orebody and has occurred in two distinct periods. Mobile elements, particularly Zn, leached during gossan formation, occur in anomalous concentrations at or near the water table. More recently, the less mobile elements Pb, As, Bi, Hg and Sb have been leached from mechanically transported fragments of ferruginized wall rock and gossan by the saline groundwaters and occur as anomalies up to 150 m from the gossan in near-surface bedrock. Copper and Zn form broad low-contrast anomalies whilst Sn is retained within the gossan.Iron-rich bands, 50–1200 mm in thickness, which are common in the weathered zone about the Elura orebody, were formed by precipitation from groundwater passing along bedding planes, shears, fractures and cracks. They have higher As, Bi, Co, Cu, Mn, Ni, Zn, lower values of Ba and Sr, and similar Pb, Sb and Sn contents to the weathered siltstones. Iron may be derived from Fe-rich carbonates in the siltstones and be redeposited as goethite and minor hematite. These Fe-rich bands have trapped target and pathfinder elements which are believed to be from primary haloes rather than from the orebody or gossan.A two-stage mechanism for the formation of some secondary minerals within the weathered zone has been confirmed by stable light isotope studies. These studies have also shown that fractionation of S isotopes is minimal during gossan formation, and that a S isotopic halo in weathered bedrock may be used as an exploration tool.     

Ironstone-Gossan discrimination: Pitfalls of a simple geochemical approach — A case study from northeast Scotland

Located in northeast Scotland, the Lecht manganiferous ironstone occurs as several minor and one principal outcrop within deeply weathered Dalradian meta-sediments. The distribution of these shows is controlled primarily by an underlying porous breccia pipe and not by Dalradian stratigraphy or faulting, as previously suggested. The deposit is composed principally of goethite and cryptomelane, with minor hematite, ramsdellite, pyrolusite, lithiophorite, chalcophanite and woodruffite. The ironstone is enriched in several target and pathfinder elements, particularly Zn and Ba which are primarily concentrated in the manganese oxides. Detailed examination of the geochemistry demonstrates that the enrichments are actually more typical of non-economic ironstones (particularly bog-ore) than gossans (a conclusion supported by field, textural and mineralogical evidence), illustrating the danger of relying upon simple geochemical surveys alone for ironstone-gossan discrimination. No relict sulphides, secondary ore minerals, native metals, gangue minerals or “boxwork” textures were observed in either hand specimen or polished section. The morphology and textures of the Lecht ironstone are typical of those observed in bog-iron ores and in weathered profiles.The Lecht ironstone is considered to have been derived from prolonged weathering of the local Dalradian meta-sediments. These are enriched in target and pathfinder elements and are regarded as a prospective sequence. Cementation of the subsequent regolith by solutions rich in iron, manganese and other elements, combined with bog-ore formation and penetration of the breccia pipe by these solutions, produced the complex and varied morphology and geochemistry seen in the deposit today. The Lecht deposit may represent the distal manganiferous expression of a goldrich zinc-lead exhalative deposit hosted by the Dalradian meta-sediments of the region.     

Facies architecture of the felsic lava‐dominated host sequence to the Thalanga massive sulfide deposit,Lower Ordovician,northern Queensland

The Thalanga volcanic‐hosted massive sulfide deposit occurs in the Cambro‐Ordovician Mt Windsor Subprovince in northern Queensland. The orebody comprises steeply dipping, stratiform, sheet‐like, polymetallic massive sulfide lenses. Overall, the volcanic facies architecture at Thalanga is dominated by quartz‐ and/or feldspar‐phyric lavas and synvolcanic intrusions that comprise coherent facies and in situ and resedimented autoclastic facies. Systematic phenocryst logging (mineralogy, abundance, size) has been used to discriminate separate emplacement units of rhyolite in the footwall and dacite in the hangingwall. Some of the petrographically different rhyolite and dacite types can also be distinguished using immobile‐element geochemistry. Rhyolitic lavas and intrusions in the footwall are weakly to strongly altered. Apparent clastic textures resulting from hydrothermal alteration and metamorphism are widely developed in the coherent facies. Genuine clastic textures are characterised by clasts with randomly oriented internal laminar or banded fabric (e.g. rotated, flow‐laminated clasts), marked and consistent differences in quartz phenocryst abundance and/or size range between clasts and matrix, and normal grading. Mass‐flow‐emplaced, rhyolitic breccia units delineate palaeo‐sea‐floor positions in the footwall that are potentially prospective for exhalative massive sulfide mineralisation. A comparison of the distribution of clastic and coherent facies with the geometry of strongly altered zones in the footwall indicates that intense hydrothermal fluid flow was independent of the facies arrangement. The massive sulfide lenses conformably overly altered footwall rhyolite and occur in a distinctive facies association which includes coarse quartz‐phenocryst‐rich rhyolitic sills with peperitic contacts and crystal‐rich polymictic breccia. The hangingwall to the orebody consists of largely unaltered dacitic lavas and synvolcanic intrusions and minor dacitic pumice breccia, dacitic breccia and polymictic volcanic breccia. The facies architecture shows that the Thalanga massive sulfide deposit formed in a below‐storm‐wave‐base depositional environment on top of an elevated, lava‐dominated, rhyolitic volcanic centre. A modern analogue for the setting of the Thalanga massive sulfide is the PACMANUS hydrothermal field on the crest of the dacite lava‐dominated Pual Ridge in the eastern Manus backarc basin (Papua New Guinea).     

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.     

Compositions of magmatic hydrothermal fluids determined by LA-ICP-MS of fluid inclusions from the porphyry copper–molybdenum deposit at Butte, MT

We analyzed 85 fluid inclusions from seven samples from the porphyry Cu–Mo deposit in Butte, MT, using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). The Butte deposit formed at unusually great depth relative to most porphyry deposits, and fluid inclusions in deep veins trapped a low-salinity, CO₂-bearing, magmatically derived, supercritical fluid as a single aqueous phase. This fluid is interpreted to be the parent fluid that cooled, decompressed, unmixed, and reacted with wall rock to form the gigantic porphyry Cu deposit at Butte. Few previous analyses of such fluids exist.Low-salinity, aqueous fluids from the earliest veins at Butte are trapped in deep veins with biotite-rich alteration envelopes (EDM veins). These veins, and the Butte quartz monzonite surrounding them, host much of the Butte porphyry Cu mineralization. Twenty fluid inclusions in one EDM quartz vein are dominated by Na, K, Fe (from 0.1 to 1 wt.%) and contain up to 1.3 wt.% Cu. These inclusions contain only small amounts (tens of ppm) of Pb, Zn, and Mn, and typically contain Li, B, Ca, As, Mo, Ag, Sn, Sb, Ba, and W in less than detectable quantities. The abundance of Cu in early fluids indicates that a low-salinity, Cu-rich, aqueous ore fluid can be directly produced by aqueous fluid separation from a granitic magma. Similar inclusions (eight) in an early deep quartz–molybdenite vein with a K-feldspar selvage have similar compositions but contain significantly less Cu than most inclusions in the biotite-altered vein. Analyzed inclusions in both veins contain less than detectable concentrations of Mo even though one is molybdenite-bearing.Low-salinity, CO₂-bearing aqueous fluids are also trapped in pyrite–quartz veins with sericitic selvages. These veins cut both of the above vein types and contain inclusions that were trapped at lower pressure and temperature. Thirty-nine inclusions in two such veins have compositions similar to early fluids, but are enriched by up to a factor of 10 in Mn, Pb, and Zn relative to early fluids, and are slightly depleted in Fe. Many of these inclusions contain as much or more Cu than early fluids, although little chalcopyrite is found in or around pyrite–quartz veins.Eighteen halite-bearing inclusions from three veins from both chalcopyrite-bearing and barren veins with both K-silicate and sericitic selvages were analyzed as well. Halite-saturated inclusions are dominated by Na, K, Fe, and in some inclusions Ca. Whereas these inclusions are significantly enriched in Ca, Mn, Fe, Zn, and Pb, fluids in all three veins contain significantly less Cu than early, high temperature, low-salinity inclusions.Analyses of all inclusion types show that whereas bulk-salinity of the hydrothermal fluid must be largely controlled by the magma, fluid–rock interactions have a significant role in controlling fluid compositions and metal ratios. Cu concentrations range over an order of magnitude, more than any other element, in all four samples containing low-salinity inclusions. We infer that variations are the result of fluid trapping after different amounts of fluid–rock reaction and chalcopyrite precipitation. Enrichment, relative to early fluids, of Mn, Pb, and Zn in fluids related to sericitic alteration is also likely the result of fluid–rock reaction, whereby these elements are released from biotite and feldspars as they alter to sericite. In halite-bearing inclusions, concentrations of Sr, Ca, Pb, and Ba are elevated in inclusions from the pyrite–quartz vein with sericitic alteration relative to halite-bearing inclusions from unaltered and potassically altered samples. Such enrichment is likely caused by the breakdown of plagioclase and K-feldspar in the alteration envelope, releasing Sr, Ca, Pb, and Ba.     

The distribution of Hg, Ba, Cu and Zn in the vicinity of cupriferous sulfide deposits, Troodos complex, Cyprus

A geochemical rock- and soil-sampling program was carried out in the vicinity of eight concealed “Cyprus type” deposits, occurring in marginal mafic to intermediate metapillow lavas of the Troodos Ophiolite Complex. The mineralization of massive and stockwork sulfide ore is characterized by the predominance of pyrite, intergrown with less chalcopyrite and minor amounts of sphalerite.Background values of Hg are in the range of 8–12 ppb for soils and 3–6 ppb for surface rocks. Anomaly/background ratios of 10:1 (soils) and 5:1 (rocks) have been found only, where Hg migrated along channels formed by faults cutting shallow-seated mineralization. Here, Hg sometimes shows significant correlations with Cu, Zn, Ba and exceptionally with Co. However in one case an Hg anomaly in soils and surface rocks was detected directly over a deposit. The use of Hg as indicator element for these types of deposits is therefore limited. Buried mineralization may be delineated more distinctly by Cu, Zn and Ba.     

Geochemical alteration halos around the Mount Morgan gold-copper deposit, Queensland, Australia

The Mount Morgan Au-Cu pyritic massive sulphide deposit occurs in a north-trending belt of Middle Paleozoic volcanic rocks located in south-central Queensland. The host rocks for the deposit are a normal sequence of rhyolitic tuff that have a north-northwest regional strike and easterly dips of 20° to 30°. The tuff contains thin units of chert, jasperoid and carbonate.The Mount Morgan deposit was represented by a zone of sulphide mineralization 600 m long, 100–200 m wide and 300 m deep that transects stratigraphy and can be divided into: (1) an oxidized zone, characterized by a hematitic, Au-enriched gossan with minor stratiform sphalerite-argillite; and (2) a primary zone which can be subdivided into an upper zone of greater than 50% sulphide minerals (Main Pipe orebody), and a lower siliceous stockwork zone with approximately 20% sulphide minerals (Sugarloaf orebody). Pyrite is the most abundant sulphide mineral in both the upper and lower primary zones with lesser pyrrhotite and accessory chalcopyrite, sphalerite and gold. A zone of silicification forms an envelope around the orebody and extends stratigraphically downwards in a pipe-like zone for greater than 750 m. The orebody contained 67 Mt of 4.87 g/t Au and 0.70% Cu.The distribution and variation of between 7 and 29 elements and specific conductance were examined in 1252 samples of the host rocks taken from diamond drill core and surface outcrop. The host rocks in the immediate vicinity of the deposit are marked by the development of three distinct but overlapping chemical and mineralogical zones representing an outward progression from the most intensive to a less intensive alteration. A 50-m-thick siliceous inner zone of intensely altered rocks, depleted in all investigated elements except Si, surrounds the orebody. This zone passes outward into a 100-m-thick middle zone of dominantly sericite-pyrite characterized by high concentrations of K, Fe, Cu and Co. The sericite-pyrite zone, in turn, passes into an outer 100-m-thick chlorite zone with high Fe, Mg, Mn and Zn concentrations. High concentrations of H₂O⁺ are associated with the sericite-pyrite zone and the chlorite zone. The alteration pipe underlying the Mount Morgan orebody is characterized by depletions in Na, Ca and K and enrichments in Fe and Mg. A non-economic pyrite body contained within the alteration pipe has spatially restricted enrichment halos of Fe, Mg, Zn, Cu and Co.     

Chemistry of the 350°C hot springs on the crest of the East Pacific rise at 21°N

Hydrothermal fields on submarine spreading centres were first studied systematically during dives of the deep submersible ALVIN on the crest of the Galapagos Ridge in 86°W in the spring of 1977. While the exiting waters had temperatures only about 20°C above that of the ambient water column detailed analysis of their chemistry showed them to be formed by mixing of cold sea water (as “ground-water”) with a hydrothermal endmember of approximate temperature 350°C. Subsequently fields of hot springs with this temperature were found on the crest of the East Pacific Rise at 21°N by ALVIN in 2 600 metres water depth. Reconnaissance water sampling of these systems was made in November 1979 and a detailed study has just been completed (November 1981).The 350°C solutions are completely depleted of their original sea-water concentrations of Mg and SO₄. They are acid with a pH (25°C, 1 atmos) of 3.6 and an acidity of 400 μeq/kg. They contain about 7 mmol/kg of H₂S. The isotopic composition of this sulphur and the arsenic to sulphur ratio in the solutions indicate that about 85% of it is of igneous origin. The “soluble elements” Li, K and Rb are strongly enriched over the sea-water values, as are Ca and Ba. Sr is present at close to the sea-water concentrations however the isotopic compositon is identical to that of the basalts. The exiting solutions are clear and homogeneous super-critical fluids of in situ density approximately 0.65 g/cm³. Velocities in the throat of the orifices are around 1.5 m/sec. The iron concentrations are 1.8 mmol/kg and the Fe/Mn ratio is about 3. The reconnaissance samples gave Zn of 120 μol/kg and Cu and Ni of about 15 μol/kg.Upon mixing with sea-water the hot springs precipitate a voluminous black “smoke” predominantly composed of fine-grained FeS. Anhydrite is precipitated around the throat of the orifice producing chimney-like constructional features up to 10-m high. As these grow vertically the anydrite is replaced by sulphide minerals. The outer surface of the chimneys is colonized by several species of worms that secrete mats of tubes, up to several centimetres in diameter, composed of a tough organic material. Lateral growth of the chimneys via leaks in their walls leads to precipitation of sulphide minerals in a morphology controlled by the organic mats. All the numerous extinct sulphide deposits in the area have this characteristic surface texture.The active deposits on the EPR are unlike ophiolite type massive sulphides chemically, mineralogically and texturally. However, they do represent the primary precipitate. It appears that during lateral growth and coalescence of the chimneys in a given field the original deposit is reworked chemically as the 350°C solutions stream through the disequilibrium rapidly precipitated material. A “zone refined” substrate results consisting of coarsely crystalline, permeable relatively pure pyrite. This secondary deposit is, of course, capped with juvenile chimneys. It is these that probably constitute the ochres, the oxidized surficial zones of massive sulphides historically worked for silver and other elements present at only trace levels in the bulk deposit.     

Volcanic lithogeochemistry and alteration at the Delbridge massive sulfide deposit, Noranda, Quebec

The Delbridge orebody occurs within a thick sequence (> 1 km) of porphyritic to aphyric massive rhyolite and rhyolite breccia of the Archean Blake River Group. The orebody produced ≈ 370,000 tonnes grading 0.61% Cu, 9.6% Zn, 110 g/t Ag and 2.1 g/t Au (1969–1971). The footwall consists of massive quartz porphyritic rhyolite mantled by proximal rhyolite breccias. An irregular chloritic alteration pipe with mineralization is subvertical to the ore lens. The orebody occurs at a thick cherty horizon within rhyolite breccia, and is overlain by a succession of mafic debris flows, porphyritic to aphyric massive rhyolite flows, and finally andesite. The main alteration assemblage in the rhyolite units is quartz-albite-sericite-chlorite-carbonate. Immobile element plots and rare-earth element data indicate that the footwall rhyolite flows and proximal breccias are tholeiitic to transitional (Zr/Y = 3.5–5.5; La_N/Yb_N = 1.7–2.6), whereas hangingwall rhyolite flows are mildly calc-alkaline (Zr/Y = 6.5–7.5; La_N/Yb_N = 2.8–3.8). These two rhyolite types also have separate alteration lines in Ti-Zr space and in various immobile element plots. The identification of chemically different rhyolites above and below the orebody provides markers that can be identified and traced even where strongly altered. An intrusive rhyolite mass in the footwall is chemically identical to the hangingwall aphyric rhyolite flows, and is interpreted as the feeder to these flows. Calculated mass changes in the footwall rhyolite commonly are large, and result from major silica change (±30%), significant loss of Na₂O + CaO, and important additions of K₂O and FeO + MgO. The margins of the pipe show net mass gain, whereas the interior of the pipe shows net mass loss. Hangingwall rhyolite shows mass changes that generally are much smaller than in the footwall. Felsic rocks in the silica-sericite alteration zone up to ≈ 200 m from the orebody have high δ¹⁸O values of 10–12‰, reflecting low-temperature alteration. The orebody occurs near the contact between a mainly tholeiitic rhyolite footwall and an overlying sequence of mildly calc-alkaline rhyolite then andesite.     

Mineralization and regional alteration at the Mons Cupri stratiform Cu–Zn–Pb deposit, Pilbara Craton, Western Australia

The Mons Cupri Cu–Zn–Pb deposit is hosted by conglomerate of the ca. 2,965 Ma Cistern Formation, which forms part of the Bookingarra Group in the Central Pilbara tectonic zone of the Pilbara Craton. The deposit has two distinct mineralized zones: a 5- to 10-m-thick, stratiform, Zn–Pb-rich semi-massive sulphide lens, which overlies a funnel-shaped Cu-rich stringer zone. The deposit is located 5–20 m stratigraphically below the contact of the Cistern Formation with the overlying Rushall Slate. A high-Mg basalt (the Comstock Member) is located near the base of the Rushall Slate. Regional alteration mapping indicates that feldspar-destructive alteration facies are restricted to the vicinity of the deposit, with intense chloritic alteration restricted to the stringer zone and a narrow cross-cutting zone that can be traced for 700 m to the east, in the footwall of the deposit. Feldspar-destructive zones are well developed in the Comstock Member, indicating that mineralization occurred after deposition of the Rushall Slate. The Mons Cupri stratiform semi-massive sulphide lens is hosted by a conglomerate, and textural evidence indicates that this lens formed by replacement. These observations, combined with Pb isotope data, suggest that the Mons Cupri deposit formed epigenetically, at least 30 Ma after deposition of its host rock.Editorial Handling: B. Gemmell     

Towards a volcanic–structural balance: relative importance of volcanism, folding, and remobilisation of nickel sulphides at the Perseverance Ni–Cu–(PGE) deposit, Western Australia

Perseverance is a world-class, komatiite-hosted nickel sulphide deposit situated in the well-endowed Leinster nickel camp of the Agnew–Wiluna greenstone belt, Western Australia. The mine stratigraphy at Perseverance trends north-northwest (NNW), dips steeply to the west, and is overturned. Stratigraphic footwall units lie along the western margin of the Perseverance Ultramafic Complex (PUC). The PUC comprises a basal nickel sulphide-bearing orthocumulate- to mesocumulate-textured komatiite that is overlain by a thicker, nickel sulphide-poor, dunite lens. Hanging wall rocks include rhyodacite that is texturally and compositionally similar to footwall volcanic rocks. These rocks separate the PUC from a second sequence of nickeliferous, E-facing, spinifex-textured komatiite units (i.e. the East Perseverance komatiite). Past workers argue for a conformable stratigraphic contact between the PUC and the East Perseverance komatiite and conclude that the PUC is extrusive. This study, however, clearly demonstrates that these komatiite sequences are discordant, implying that the PUC may have intruded rhyodacite country rock as a sill with subsequent structural juxtaposition against the East Perseverance komatiite. Early N–S shortening associated with the regional D_I deformation event (corresponding to the local D_P1 to D_P3 events at Perseverance) resulted in the heterogeneous partitioning of strain along the margins of the competent dunite. A mylonite developed in the more ductile footwall rocks along the footwall margin of the PUC, while isoclinal F₃ folds, such as the Hanging wall limb and Felsic Nose folds, formed in low-mean stress domains along the fringes of the elongated dunite lens. Strata-bound massive and disseminated nickel sulphides were passively fold thickened in hinge areas of isoclinal folds, whereas basal massive sulphides lubricated fold limbs and promoted thrust movement along shallowly dipping lithological contacts. Massive sulphides were physically remobilised up to 20 m from their primary footwall position into deposit-scale fold hinges to form the 1A and Felsic Nose orebodies. First-order controls on the geometry of the Perseverance deposit include the thermomechanical erosion of footwall rocks and the channelling of the mineralised komatiitic magma. Second- or third-order controls are several postvolcanic deformation events, which resulted in the progressive folding and shearing of the footwall contact, as well as the passive fold thickening of massive and disseminated sulphide orebodies. Massive sulphides were physically remobilised into multiple generations of fold hinges and shear zones. Important implications for near-mine exploration in the Leinster camp include identifying nickeliferous komatiite units, defining their three-dimensional geometry, and targeting fold hinge areas. Fold plunge directions and stretching lineations are indicators of potential plunge directions of massive sulphide orebodies.