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1.
A comparative study of the gossans from ten base-metal deposits in southern Africa included the establishment of geochemical criteria for regional evaluation of gossans and ironstones. Factors such as ore composition, nature of gangue, element mobility, depth of weathering, degree of erosion and groundwater chemistry are discussed with respect to gossan geochemistry.Areal geochemical zonation, relative concentration ranges, element correlations for gossan versus sulphide and scattergrams comparing different gossans were all utilized to examine geochemical criteria for gossan classification. Of the elements studied, Ba (as barite) and Pb are the least mobile, Cu and Ag are variously retained in gossan, whereas Zn and Cd are generally dispersed.Gossans derived from such a wide variety of ore types contribute several multivariate populations to the total data set. Principal components analysis was consequently of little value in separating gossan suites from barren ironstones. Stepwise discriminant analysis successfully distinguished base-metal- from pyrite-derived gossans and ferricretes, discriminated among gossans from different ore provinces and classified individual gossans within base-metal provinces. Discriminant functions commonly comprised only 2 to 6 elements. Characteristic multi-element signatures for the various gossans were subsequently applied to the regional evaluation of ironstone in southern African exploration.  相似文献   

2.
The Wadi Bidah Mineral District of Saudi Arabia contains more than 16 small outcropping stratabound volcanogenic Cu–Zn–(Pb) ± Au-bearing massive sulphide deposits and associated zones of hydrothermal alteration. Here, we use major and trace element analyses of massive sulphides, gossans, and hydrothermally altered and least altered metamorphosed host rock (schist) from two of the deposits (Shaib al Tair and Rabathan) to interpret the geochemical and petrological evolution of the host rocks and gossanization of the mineralization. Tectonic interpretations utilize high-field-strength elements, including the rare earth elements (REE), because they are relatively immobile during hydrothermal alteration, low-grade metamorphism, and supergene weathering and therefore are useful in constraining the source, composition, and physicochemical parameters of the primary igneous rocks, the mineralizing hydrothermal fluid and subsequent supergene weathering processes. Positive Eu anomalies in some of the massive sulphide samples are consistent with a high temperature (>250°C) hydrothermal origin, consistent with the Cu contents (up to 2 wt.%) of the massive sulphides. The REE profiles of the gossans are topologically similar to nearby hydrothermally altered felsic schists (light REE (LREE)-enriched to concave-up REE profiles, with or without positive Eu anomalies) suggesting that the REE experienced little fractionation during metamorphism or supergene weathering. Hydrothermally altered rocks (now schists) close to the massive sulphide deposits have high base metals and Ba contents and have concave-up REE patterns, in contrast to the least altered host rocks, consistent with greater mobility of the middle REE compared to the light and heavy REE during hydrothermal alteration. The gossans are interpreted to represent relict massive sulphides that have undergone supergene weathering; ‘chert’ beds within these massive sulphide deposits may be leached wall-rock gossans that experienced silicification and Pb–Ba–Fe enrichment from acidic groundwaters generated during gossan formation.  相似文献   

3.
Lead isotope analyses were performed on 26 polymetallic massive sulphide deposits of the Iberian Pyrite Belt, as well as on overlying gossans and associated volcanic rocks. All the massive sulphide deposits (except for Neves-Corvo), and nearly all the volcanic rocks show very similar isotopic compositions grouped around 18.183 (206Pb/204Pb), 15.622 (207Pb/204Pb) and 38.191 (208Pb/204Pb), indicating that most of the ore deposit lead was derived from the same continental crust environment as the associated volcanic rocks. The isotopic compositions are representative of the average south Iberian crust during the Devonian to Early Carboniferous (Dinantian), and their constancy implies a homogenization of the mineralizing fluids before the deposition of the massive sulphides from hydrothermal fluids circulating through interconnected regional fracture systems. This isotopic constancy is incompatible with multiple, small, independent hydrothermal cells of the East Pacific Rise type, and fits much better with a model of hydrothermal convections driven by “magmatic floor heating”. Neves-Corvo is the only south Iberian massive sulphide deposit to have a heterogeneous isotopic composition with, in particular, a highly radiogenic stanniferous ore (206Pb/204Pb of the cassiterite is >18.40). A model of lead mixing with three components is proposed to explain these variations: (1) one derived from the Devonian to Early Carboniferous (Dinantian) continental crust that generated all the other massive ores; (2) an Eohercynian stanniferous mineralization partly remobilized during the formation of the massive sulphides, but independent of them; and (3) a Precambrian continental crust component. The juxtaposition of three different sources places Neves-Corvo in a specific paleogeographic situation that could also explain its mineralogical specificity. The geodynamic context that best explains all the obtained isotopic results is one of an accretionary prism. The fact that lead isotope signatures of the gossans are almost identical to those of the underlying massive sulphides means that this technique could be a useful exploration tool for the Iberian Pyrite Belt.  相似文献   

4.
Twelve massive sulphide deposits from the Iberian Pyrite Belt (IPB) show well-preserved iron caps, some of which were mined during the last century to recover precious metals (e.g., Tharsis, Rio Tinto, San Miguel). Field observations and correlation assays between the distinct mineral sequences at different deposits suggest that all the gossans were developed under similar conditions and have undergone the same geological events. All the gossans have a mushroom-like morphology in sharp contact with the underlying massive sulphide orebodies. In most cases these are located over an apparent supergene enrichment zone rich in secondary sulphides. Some gossans extend into tongues of alluvial heterolithic breccias consisting of eroded transported gossans displaced as far as several hundred meters away from their sources. The distribution of major minerals throughout the gossan profiles (goethite, hematite, quartz and jarosite) and the statistical analysis of the geochemical data distinguish three separate zones, with gradual contacts roughly parallel to the current topography: (1) the lower zone dominated by goethite and subordinate jarosite, with significant enrichment in S, As, P, Pb, Sn, Sb, Ag and Au; (2) the middle or principal zone dominated by goethite and lacking jarosite, which is depleted in S, and As, as well as heavy and precious metals; and (3) the upper zone near the surface, mainly composed of hematite and quartz with only weak anomalies in P, Pb and Sn. The origin and variations occurred in the profiles are explained by a three-stage process. This involves an initial acidic stage of gossan development centred on the oxidation of sulphides that lead to the formation of the first Fe-rich oxyhydroxides and sulphates (mainly goethite and jarosite, respectively). Over time, a progressive stage of maturity is reached progressively downwards through the gossan profile due to the intensification of the oxidation and leaching processes. The ongoing gossan formation produced alteration and reprecipitation of pre-existing oxyhydroxides, the loss of the majority of the previously sorbed heavy metals, and a major dilution of trace elements especially in the zones near the surface. The main results of this stage of formation are the production of heavy metal-depleted oxyhydroxides, most commonly goethite and hematite, and the disappearance of jarosite. Subsequently, local uplift of the gossanous rocks by neotectonic movements facilitated the rejuvenation of the oxidation of the ores. This final stage complicated the previously developed zonation with the formation of jarosite in mature areas. Possible major breaks in this gossan development ocurred in Messinian times (7–8 Ma) and at the beginning of the Early Quaternary (1–2 Ma?).  相似文献   

5.
The South Australian portion of the Willyama Inliers hosts a diversity of small sulphide and uranium deposits and numerous outcropping gossans. This fact, together with geological similarities to the adjacent Broken Hill Block has led to extensive exploration. A broad classification distinguishes two main types of sulphide mineralisation: 1) stratiform iron sulphide-dominated (±Cu, Zn, Co) deposits which occur widespread within specific stratigraphic intervals, and stratabound occurrences of syn-depositional to diagenetic origin which show some structural control; 2) syn-tectonic to post-peak metamorphic replacement and vein-type deposits (Fe-Cu-Au and Cu-Zn-Pb), which are hosted by fractures and within faults and shear zones. These occurrences show no stratigraphic control and are not spatially related to type 1 mineralisation. Late-stage deposits also differ from stratiform/stratabound mineralisation in their texture, mineral assemblage and geochemical composition. Much of the sulphide mineralisation in the Olary Block has been interpreted as resulting from rift-associated syn- to diagenetic processes, such as hot spring exhalations and base metal precipitation along reduction-oxidation interfaces. Subsequent granitic intrusive, high grade metamorphic and multiphase deformation events would have induced remobilisation and redeposition of sulphides in a variety of epigenetic modes. However, a detailed petrographic and geochemical study of sulphide mineralisation in the Olary Block demonstrates that due to the lack of abundant pervasive fluids, translocation and modification of preexisting sulphides were restricted to less than a few centimetres. Instead, widespread syn-tectonic to epigenetic (i.e., post-peak metamorphic) mobilisation of ore constituents occurred to form retrograde sulphide mineralisation as well as multiple generations of late-stage vein deposits. These epigenetic deposits are genetically unrelated to synsedimentary and diagenetic occurrences, an aspect of significance for exploration in the Olary Block. Temporal separation of peak metamorphism in deeper crustal levels from its occurrence in shallow levels, periodic tectonic disturbances and repeated seismic pumping are processes believed to have resulted in intermittent mobilisation of ore constituents from a deep-seated metasedimentary reservoir.  相似文献   

6.
In the Rajpura-Dariba belt zinc and lead mineralization was established on the basis of gossan and opinions were given that the deposits are stratiform and syngenetic. During last two decades metamorphism and volcanic processes have been attributed for the generation of exhalative and remobilized lead-zinc ore bodies. Subsequent exploration was based on the concept of successive borehole planning around positive intersections. Areas having surface indications of base metal mineralization and their extensions have been already explored. Recent deep drilling by Hindustan Zinc Limited (HZL) in this area has intersected rich ore lodes, thereby confirming depth ward continuity of ore zones at some places. It indicates possibility of searching hidden deposits by innovative techniques. Therefore, today, there is a challenge before geoscientists to discover new deposits in this area by an exploration technique which should have low risk, consume less time and it should be cost effective.  相似文献   

7.
Lead isotope ratios of galena from the carbonate-hosted massive sulphide deposits of Kabwe (Pb-Zn) and Tsumeb (Pb-Zn-Cu) in Zambia and Namibia, respectively, have been measured and found to be homogeneous and characteristic of upper crustal source rocks. Kabwe galena has average isotope ratios of 206/204Pb = 17.997 ± 0.007, 207/204Pb = 15.713 ± 0.010 and 208/204Pb = 38.410 ± 0.033. Tsumeb galena has slightly higher 206/204Pb (18.112 ± 0.035) and slightly lower 207/204Pb (15.674 ± 0.016) and 208/204Pb (38.276 ± 0.073) ratios than Kabwe galena. The isotopic differences are attributed to local differences in the age and composition of the respective source rocks for Kabwe and Tsumeb. The homogeneity of the ore lead in the two epigenetic deposits suggests lead sources of uniform isotopic composition or, alternatively, thorough mixing of lead derived from sources with relatively similar isotopic compositions. Both deposits have relatively high 238U/204Pb ratios of 10.31 and 10.09 for Kabwe and Tsumeb galenas, respectively. These isotope ratios are considered to be typical of the upper continental crust in the Damaran-Lufilian orogenic belt, as also indicated by basement rocks and Cu-Co sulphides in stratiform Katangan metasediments which have a mean μ-value of 10.25 ± 0.12 in the Copperbelt region of Zambia and the Democratic Republic of Congo (formerly Zaire). The 232Th/204Pb isotope ratios of 43.08 and 40.42 for Kabwe and Tsumeb suggest Th-enriched source regions with 232Th/235U (κ-values) of 4.18 and 4.01, respectively. Model isotopic ages determined for the Kabwe (680 Ma) and Tsumeb (530 Ma) deposits indicate that the timing of the mineralisation was probably related to phases of orogenic activity associated with the Pan-African Lufilian and Damaran orogenies, respectively. Galena from the carbonate-hosted Kipushi Cu-Pb-Zn massive sulphide deposit in the Congo also has homogeneous lead isotope ratios, but its isotopic composition is comparable to that of the average global lead evolution curve for conformable massive sulphide deposits. The μ (9.84) and κ (3.69) values indicate a significant mantle component, and the isotopic age of the Kipushi deposit (456 Ma) suggests that the emplacement of the mineralisation was related to a post-tectonic phase of igneous activity in the Lufilian belt. The isotope ratios (206/204Pb, 207/204Pb, 208/204Pb) of the three deposits are markedly different from the heterogeneous lead ratios of the Katangan Cu-Co stratiform mineralisation of the Copperbelt as well as those of the volcanogenic Nampundwe massive pyrite deposit in the Zambezi belt which typically define radiogenic linear trends on lead-lead plots. The host-rock dolomite of the Kabwe deposit also has homogeneous lead isotope ratios identical to the ore galena. This observation indicates contamination of the Kabwe Dolomite Formation with ore lead during mineralisation. Received: 8 September 1997 / Accepted: 21 August 1998  相似文献   

8.
Gravimetry was the main exploration method used in this high risk area to the north of the Grândola fault, in the Iberian Pyrite Belt (IPB). It showed several targets, among which the Valverde-Lagoa Salgada area stood out. Here, in August 1992, a polymetallic massive sulphide orebody was found, named Lagoa Salgada. According to the geological characteristics of the area and to the nature of the surveyed targets, other exploration techniques were used, namely: geology, magnetometry, resistivity, electrical sounding, magneto-telluric, seismic and mechanical drilling. The deposit was found under a Tertiary cover, 128?m in depth, complete with a 15?m thick gossan, caused by paleoalteration, with a supergene enrichment zone. This gossan gave way to massive sulphides, with intersections, at times, exceeding 60?m. They are limited towards the footwall by an important fault with strong associated kaolinization, which is interpreted as a reverse fault, placed on the inverse limb of an anticlinal structure. Throughout the deposit, the chemical composition of the mineralization showed great variation reaching significant values of Zn, Pb, Sn, Cu, As, Hg, Sb, Cd, Au and Ag. In the initial phase the deposit has an estimated tonnage of 5?Mt. It is related to a large gravity anomaly with the general orientation NW-SE, which was investigated by 15 boreholes made by the Instituto Geológico e Mineiro. The drilling cut an important volcanic centre of an acid to intermediate nature, around which intense hydrothermal activity was associated with contemporaneous sulphide mineralization. The gravity anomaly has two nuclei, a NW and a central one, about 450?m apart, where a pyrite orebody with polymetallic sulphides and stockwork sulphide mineralization are developed. This discovery led to an extensive exploration project that confirmed the mining potentialities of the NW sector of the IPB, to the north of Grândola fault, where previously no polymetallic sulphide occurrences were known.  相似文献   

9.
The Camaquã Cu (Au, Ag) and Santa Maria Pb-Zn (Cu, Ag) deposits are the largest base-metal deposits discovered so far in the sedimentary clastic sequences of the Neoproterozoic-age Camaquã Basin. The origin of the Camaquã-Santa Maria deposits has been the centre of dispute, with three alternative genetic hypotheses proposed: a syngenetic model, a diagenetic model, and a magmatic hydrothermal model. In detail, this mineralization has been suggested to be related to sedimentary clastic-diagenetic processes, volcanic-related processes, or deep granitic magmatism.Reevaluation of previous data and new studies in the area yield the following conclusions: (1) mineralization is fracture-controlled and magmatic-hydrothermal in origin rather than stratiform syngenetic or diagenetic; (2) the temperature of deposition of the main ore minerals was 210 to 300°C; (3) the ∂34S of sulphides of around 0% indicates an external magmatic-hydrothermal source of sulphur; (4) Pb isotope ratios of sulphides indicate that metals were derived at the end of the Brasiliano Cycle from a large crustal source with very primitive Pb and that (5) the age of mineralization is 594 Ma, as constrained by U/Pb SHRIMP (Sensitive High Resolution Ion Microprobe) determinations on zircons of the Lavras Granite.Thus, the Camaquã and Santa Maria deposits are interpreted to be of magmatic-hydrothermal origin, with the metals derived from an old crustal-basement source during the end of Dom Feliciano Collisional Orogeny, at 594 Ma, late in the Brasiliano Cycle.The interpretation above is critical for base-metal exploration in the Sul Riograndense Shield. Previous exploration methodologies mainly followed models based on a sedimentary hypothesis for the origin of the deposits. However, the occurrence of mineralization along fractures within specific wall-rocks requires consideration of alternative exploration parameters. These include: (1) ancient EW- and NW-trending regional fractures and their intersections, which are potential structural sites for base-metal mineralization, and (2) preferential wall-rock sites, either rocks with high initial porosity or secondary fracture-induced porosity or soluble rocks which are susceptible to replacement processes. Post-collisional plutonism of the Dom Feliciano Orogeny was the most likely heat source, and also the source of sulphur and base metals. Gravity surveys and alteration studies would be useful to determine the presence of intrusive bodies at depth which may have the potential to host porphyry-style Cu-Au deposits.  相似文献   

10.
An assessment is made of the use of lead isotopes in soils as an exploration tool for concealed lead-rich base metal massive sulphide deposits, using four case histories in southeast Australia as examples. Case history 1 covers soil profiles (< 1 m depth) unrelated to mineralization in which the lead isotope ratios exhibit variation with depth and are different from those for massive sulphide deposits. Case history 2 covers six shallow (< 0.4 m depth) profiles from two traverses over the concealed Elura Zn-Pb-Ag deposit in central western New South Wales. For one traverse over completely concealed (blind) sulphides, no isotopic or geochemical signature of the orebody could be detected. In two profiles from the other traverse, for depths of 0.15 m and 0.3–0.45 m, the lead isotopic ratios in the soils are identical with those in the ore (the target isotope ratios). Case history 3 covers two occurrences of Silurian volcanogenic Cu-Zn-Pb mineralization at the Benambra prospect in northeast Victoria. The isotopic ratios for the soils are identical with those in the sulphides and the isotopic signature of the sulphides is also detectable in traverses where no obvious surface anomaly is developed. Case history 4 is an undrilled anomaly in Silurian shales and felsic volcanics where the isotope ratios in B horizon soils are consistent with those of the target isotopic signature.The information from these case histories suggests that the lead isotopic analysis of soils will be a useful exploration tool for concealed mineralization, although at the present stage of research it appears that its application may be limited to shallowly buried (< 100 m) targets or deeply buried targets with an extensive oxidation profile. Its main advantage over conventional geochemical techniques is its ability to discriminate between similar geochemical anomalies arising from different styles of mineralization, as illustrated by an example from Benambra.  相似文献   

11.
The Wilga and Currawong copper-zinc massive sulphide deposits in northeastern Victoria occur within a sequence of Silurian volcanics and sediments. The Wilga deposit which was discovered in mid 1978 consists of a single lens while the Currawong deposit, discovered in early 1979, consists of at least two lenses.The first indication of the presence of base metal mineralization in the area was provided by an assessment of stream sediment geochemical data contained in open-file Exploration Licence reports at the Victorian Department of Minerals and Energy.The massive sulphide mineralization does not outcrop, but the ore horizons are weakly mineralized and give rise to stringer gossans as far as 150 m up dip from ore grade mineralization. These can be identified by their trace element chemistry (anomalous values of Bi, Fe, As, Au, Pb, Hg, Se, Co, Ag and Mn) corresponding to the trace element signature of both stringer and massive sulphides.Soils in the area are essentially skeletal and residual with some colluvial movement on the steeper slopes. The soils are highly anomalous in Cu, Pb, and Zn over the projected horizon of the Wilga mineralization and the No. 2 lens at Currawong.The stream sediment responses at both Wilga and Currawong result from a combination of chemical and elastic dispersion. Downslope from the surface expression of the Wilga mineralization a spring discharges directly into the Tambo River. The spring has a very low pH and is rich in base metals resulting in enhanced metal values in both stream water and stream sediments.Analyses of selected samples of the more prominent vegetation species have failed to show a clear relationship to the mineralization.  相似文献   

12.
Most magmatic-hydrothermal Cu deposits are genetically linked to arc magmas. However, most continental or oceanic arc magmas are barren, and hence new methods have to be developed to distinguish between barren and mineralised arc systems. Source composition, melting conditions, the timing of S saturation and an initial chalcophile element-enrichment represent important parameters that control the potential of a subduction setting to host an economically valuable deposit. Brothers volcano in the Kermadec island arc is one of the best-studied examples of arc-related submarine magmatic-hydrothermal activity. This study, for the first time, compares the chemical and mineralogical composition of the Brothers seafloor massive sulphides and the associated dacitic to rhyolitic lavas that host the hydrothermal system. Incompatible trace element ratios, such as La/Sm and Ce/Pb, indicate that the basaltic melts from L’Esperance volcano may represent a parental analogue to the more evolved Brothers lavas. Copper-rich magmatic sulphides (Cu?>?2 wt%) identified in fresh volcanic glass and phenocryst phases, such as clinopyroxene, plagioclase and Fe–Ti oxide suggest that the surrounding lavas that host the Brothers hydrothermal system represent a potential Cu source for the sulphide ores at the seafloor. Thermodynamic calculations reveal that the Brothers melts reached volatile saturation during their evolution. Melt inclusion data and the occurrence of sulphides along vesicle margins indicate that an exsolving volatile phase extracted Cu from the silicate melt and probably contributed it to the overlying hydrothermal system. Hence, the formation of the Cu-rich seafloor massive sulphides (up to 35.6 wt%) is probably due to the contribution of Cu from a bimodal source including wall rock leaching and magmatic degassing, in a mineralisation style that is hybrid between Cyprus-type volcanic-hosted massive sulphide and subaerial epithermal–porphyry deposits.  相似文献   

13.
A study of the trace-element content of ironstones of the Yilgarn Shield of Western Australia has shown that statistical groupings of Cu, Ni, Zn, Pb, Mn and Cr may be used to distinguish between nickel sulphide gossans from other ironstones. These groupings are supported by petrological and geological observations as well as drilling information. The paper also explains the use of Cu, Ni, Zn, Pb, Mn and Cr in establishing geochemical signatures for various lithologic types.  相似文献   

14.
Exploration of Zn-rich sulphide deposits at Leadville, northern Lachlan Fold Belt, New South Wales, for over two decades has been largely on the premise that the mineralisation represents felsic volcanic-hosted massive sulphides (VHMS). Deposits are hosted by ?Silurian felsic metavolcanic, psammopelitic and calcareous metasedimentary rocks which have been intruded by the late Carboniferous I-type Gulgong Granite. Evidence for an epigenetic replacement (skarn) origin of the deposits, rather than representing metamorphosed volcanogenic massive sulphides, includes the proximity of evolved granitic intrusives and reactive carbonate rocks, a skarn mineral assemblage (with characteristic prograde and retrograde stages), lack of textural or lithological indications of an exhalative origin, and gossan and sulphide compositions consistent with Zn-Pb skarns and atypical of Lachlan Fold Belt VHMS deposits. Furthermore, sulphide lead isotope ratios are significantly more radiogenic than signatures for VHMS deposits in the Lachlan Fold Belt. Carbonate δ13C and δ18O and sulphide δ34S values are consistent with the interaction of magmatic hydrothermal fluids with Palaeozoic carbonate rocks and a largely magmatic source of sulphur. It is concluded that the Leadville deposits are of skarn type, genetically related to the Gulgong Granite.  相似文献   

15.
The Eastern Highlands of Australia have probably been in existence since the Late Cretaceous or earlier and so there has been ample time for mature gossan profiles to form over outcropping volcanogenic Zn–Pb–Cu mineralisation in the eastern Lachlan Fold Belt. The mature gossan profiles are characterised by the upward progression from supergene sulfides to secondary sulfates, carbonates and phosphates into a Fe-oxide dominated surficial capping which may contain boxwork textures after the original sulfides (as at the Woodlawn massive sulfide deposit). However, the region has locally been subjected to severe erosion and the weathering profile over many deposits is incomplete (immature) with carbonate and phosphate minerals, especially malachite, being found in surficial material. These immature gossans contain more Cu, Pb and Zn but lower As, Sn (and probably Au) than the mature gossans. Although Pb is probably the best single pathfinder for Zn–Pb–Cu VHMS deposits of the eastern Lachlan Fold Belt, Ag, As, Au, Bi, Mo, Sb and Sn are also useful, with most of these elements able to be concentrated in substantial amounts in Fe oxides and alunite–jarosite minerals.  相似文献   

16.
The Spanish-Portuguese Pyrite Belt covers a large area in the SW part of the Iberian Peninsula from Seville to the westcoast of Portugal. Total reserves of aprox. 1.000 million tons of massive sulphide ores have an average content of 46% S, 42% Fe, and 2–4% Cu+Pb+Zn. The stratiform sulphide deposits and accompanying manganese mineralizations are of synsedimentary-exhalative origin. They occur in a Lower Carboniferous, geosynclinal, volcanic-sedimentary rock sequence, strongly folded during the Hercynian Orogeny. A brief outline of the regional geology of this ore province is given, and the geology of three mining districts is described: Lousal (Portugal), La Zarza and Tharsis (Huelva Province, Spain). A close relationship between sulphide and manganese ores with the submarine, acid alkaline volcanism is emphasized. Solfataric activity is responsible for the formation of sulphides in the final stages of volcanic extrusions. The ore concentration in big deposits (ore-lenses with up to 100 million tons of massive sulphides) has been due to inflows of sulphide muds and/or detrital sulphides into newly formed depressions of a contineously changing seafloor topography due to volcano tectonic movements.  相似文献   

17.
The Sarcheshmeh copper deposit is one of the world's largest Oligo-Miocene porphyry copper deposits in a continental arc setting with a well developed supergene sulfide zone, covered mainly by a hematitic gossan. Supergene oxidation and leaching, have developed a chalcocite enrichment blanket averaging 1.99% Cu, more than twice that of hypogene zone (0.89% Cu). The mature gossans overlying the Sarcheshmeh porphyry copper ores contain abundant hematite with variable amounts of goethite and jarosite, whereas immature gossans consist of iron-oxides, malachite, azurite and chrysocolla. In mature gossans, Au, Mo and Ag give significant anomalies much higher than the background concentrations. However, Cu has been leached in mature gossans and gives values close or even less than the normal or crustal content (< 36.7 ppm). Immature gossans are enriched in Cu (160.3 ppm), Zn (826.7 ppm), and Pb (88.6 ppm). Jarosite- and goethite-bearing gossans may have developed over the pyritic shell of most Iranian porphyry copper deposits with pyrite–chalcopyrite ratios greater than 10 and therefore, do not necessarily indicate a promising sulfide-enriched ore (Kader and Ijo). Hematite-bearing gossans overlying nonreactive alteration halos with pyrite–chalcopyrite ratios about 1.5 and quartz stringers have significant supergene sulfide ores (Sarcheshmeh and Miduk). The copper grade in supergene sulfide zone of Sarcheshmeh copper deposit ranges from 0.78% in propylitized rocks to 3.4% in sericitized volcanic rocks, corresponding to the increasing chalcopyrite–pyrite or chalcocite–pyrite ratios from 0.3 to 3, respectively. Immature gossans with dominant malachite and chrysocolla associated with jarosite and goethite give the most weakly developed enrichment zone, as at God-e-Kolvari. The average anomalous values of Au (59.6 ppb), Mo (42.5 ppm) and Ag (2.6 ppm) in mature gossans associated with the Sarcheshmeh copper mine may be a criterion that provides a significant exploration target for regional metallogenic blind porphyry ore districts in central Iranian volcano–plutonic continental arc settings. Drilling for new porphyry ores should be targeted where hematitic gossans are well developed. The ongoing gossan formation may result in natural acidic rock drainage (ARD).  相似文献   

18.
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.  相似文献   

19.
The Cu-Zn-Pb-Ag sulphide deposit at Teutonic Bore is a stratiform deposit of apparent volcanogenic origin in Archaean metabasaltic rocks in the Yilgarn Block of Western Australia. The area has moderate topographic relief, and has been weathered to a depth of about 75 m, which is about 40 m below the present water table. Part of the deposit, before mining commenced, was exposed at the surface as a gossan, samples of which contained between 500 and 1000 ppm Zn, Cu and Sb, and in excess of 1000 ppm Pb, Sn and As.The primary ore has weathered to supergene sulphides, then to an oxide assemblage containing abundant secondary ore minerals, then to leached oxides, and finally to surface gossan. The strong geochemical signature of the gossan is attributed partly to elements chemically combined with Fe oxides, partly to the presence of several stable secondary ore minerals, and partly to the presence of unweathered primary minerals. Diagnostic boxwork textures, except for those of pyrite, are not well developed.  相似文献   

20.
Thousands of silica gossans are exposed at the top of the Lower Cretaceous basalt hills in the Paraná volcanic province, strongly indicating the presence of calcite, amethyst and agate geode deposits along with native copper mineralization. The Embrapa silica gossan in the northwestern portion of the province (Campo Grande region) is an excellent example of such novel geological structure in the continental flood basalts. This silica gossan has a size of 450 × 350 m standing out as a treeless area in the densely wooded savanna and makes part of the stratigraphy of six basalt flows of Paranapanema intermediate-Ti chemical type. The base of the volcanic column is constituted by two Pitanga types and the overlying column is Paranapanema type. Every basalt flow has a silicified sand layer or breccia at the top and these are fed by abundant sand dikes. The Anel Viário Norte (AVN) flow is the most intensely altered by hydrothermal fluids producing voluminous secondary calcite infillings in the amygdales and fractures. In this region the basalts contain higher copper content than the average of the volcanic province. The studied silica gossans display negative anomalies in gamma spectrometry as a response to K, U and Th depletion during alteration. We propose a new exploration methodology by observing GoogleEarth images complemented with field studies and geochemistry to readily locate favorable areas for amethyst and agate geode deposits and native copper mineralization.  相似文献   

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