Geochemical exploration for base metal sulphide deposits in an arid environment (eastern Namaqua Metamorphic Province), South Africa

The massive sulphide deposits at Areachap and Kantienpan Cu-Zn Mine are hosted by a ~1600 Ma old volcano sedimentary succession known as the Areachap Group, in the eastern part of Namaqua Metamorphic Province, South Africa. The deposits were affected by a complex deformation and metamorphic history and represent examples of upper amphibolite to granulite grade metamorphosed volcanic-hosted massive sulphide (VHMS) deposits. The host rocks of both ore deposits are peraluminous-gneisses and the dominant sulphide minerals in the ore zone are pyrite, pyrrhotite sphalerite, and chalcopyrite and trace amounts of galena. Other ore related minerals include barite and anhydrite. The metamorphic minerals in the alteration zones at the Areachap and Kantienpan VHMS deposits are characterized by the presence of plagioclase, almandine and pyrope, enstatite and clinoenstatite, cummingtonite and gedrite, cordierite, sillimanite, and retrograde chlorite and chamosite. Lithogeochemical methods are widely used in exploration geochemistry to identify the primary alteration zones related to VHMS mineralization, as these zones are often exposed, while the massive sulphide ore body itself may be concealed. Especially in areas that were not affected by high grade metamorphism and intensive deformation. Some of these methods include the variation in the relative abundance of major element concentrations throughout the rock successions, mineral chemistry of silicates and spinel minerals near the ore zone, and normative compositions of the rock successions. However, the application of these methods is limited by complex geology in regional metamorphic terranes, such as the Namaqua Metamorphic Province. Therefore, in addition, three of the more advanced lithogeochemical approaches, known as the Isocon method, the Box Plot and Pearce Elemental Ratios, are combined here and adapted for application in such regions. Based on the mineral chemistry, it is evident that plagioclase is more Ca-rich adjacent to the ore zone, pyroxene has the highest relative Mg* ratio (Mg* ratio =100 x cationic ratio of Mg / (Mg + Fe + Ca)), the almandine and pyrope components of garnet are high and the spessartine and grossular components are low. In addition, the Mg-rich variety of mica (phlogopite) is more common near the ore zone and the peraluminous nature of the footwall zone is revealed by the presence of gahnite. Cordierite and retrograde chlorites show the highest Mg#’s (Mg# = Mg/ (Mg + Fe)) in the ore zone. In addition to the above, Pearce Element Ratio analyses of cordierite, pyroxene and garnet may be used to define proximity to sulphide mineralization. Geochemically, the ore zone and alteration zones at Areachap and Kantienpan VHMS ore deposits display a high peraluminous ratio (Al2O3 / (Na2O+K2O+CaO)) confirming the peraluminous nature of these zones as indicated by the mineral chemistry discussed above. The intervals identified in sampled borehole core with low CaO and Na2O and with high MgO and K2O contents represent the alteration zone in the original footwall rocks of the deposit. Isocon studies have shown that the alteration zones at the Areachap and Kantienpan deposits are enriched in Mg, Fe (total), S, Zn, Si, Co and F and depleted in Na, Ca, Sr, Ni, V and La. Elements that behaved relatively immobile include Zr, Ti, P, Mn, Al, Y, and U. The box plot, alteration index versus the chlorite-carbonate-pyrite index, was originally proposed to illustrate the combined effects of hydrothermal and diagenetic alteration and is based on characteristic primary mineral reactions in regions not affected by regional metamorphism. It is demonstrated here that these primary mineral reactions are preserved in a unique set of metamorphic minerals, and that the box plot can be modified for high-grade metamorphic rock types. When samples with very high Mg contents (MgO>>K2O, AI>90% and CCPI>98%) are plotted in the box plot they may be classified as representative of anomalous areas that are highly prospective. Samples with high Mg contents (MgO>K2O, AI>64% and CCPI>93%) may be considered representative of areas that may be classified as of moderate priority in an exploration programme. The findings of the mineral chemical and geochemical investigations of the footwall alteration at the Kantienpan, Areachap and Prieska Cu-Zn ore bodies are used to define various statistical factors. The applicability of these factors in lithogeochemical exploration is demonstrated by calculating the respective factor scores for a regional lithogeochemical data set. It is demonstrated how these factors could be used to identify samples collected from localities that are highly prospective for the discovery of concealed VHMS style mineralization. Based on the statistical analyses of the regional data set, the altered rocks may be distinguished from the metapelitic rocks by their high scores for the alteration factor and low scores for the peraluminous factor. The peraluminous rocks may be separated from the hornblende-gneisses by their high scores for the peraluminous factor, and from the amphibolites by their very low ortho-amphibolite factor scores and high peraluminous factor scores. The variation, of trace elements in the surface calcrete layer that conceals the mineralization in the studied areas, displays the geochemical signature of mineralization, but the concentrations of Cu, Zn and Pb are much lower at and near surface and increase down depth within the profiles. The absolute concentrations and peak to background ratios of the elements of interest at the surface therefore depend on the thickness of the underlying calcrete layer in the area. Two methods, a total analysis (x-ray fluorescence, XRF) and partial extraction (NH4EDTA solution), were applied in to evaluate results, which would be successful and commercially viable in a general exploration programme, using regolith samples. The results of the two methods above were then compared to another patented partial extraction method (mobile metal ion, MMI) on a data set previously reported on by Rossouw (2003). At Kantienpan, where the sand cover is very shallow to absent, dispersion appears to be more related to the secondary redistributions of gossaniferous clasts, than to dispersion of mobile metal ions on the surface of sand particles. The XRF method shows a wider dispersion halo here than methods based on partial extraction. Whereas, at Areachap, where a relatively thick sand (approximately one metre) covers the calcrete layer, partial extraction (based on a NH4EDTA solution extraction) results in a larger, recognizable, dispersion halo than that detected by XRF. The MMI results show a larger span for Zn, followed by NH4EDTA and finally XRF. For Cu, the NH4EDTA method exhibits the largest span followed by XRF and then MMI. The anomalous Cu, Zn and Pb contents extracted by partial extraction methods from the wind blown sand deposits indicate that these elements were derived from the ore minerals related to the massive sulphide deposits. However, Mn and Fe contents analysed by XRF also show high values that could not be only related to derivation from massive sulphide ores. Some of the high concentrations of these elements in the sand cover is ascribed to the weathering of other iron-rich rock forming minerals.Thesis (PhD (Geology))--University of Pretoria, 2006.Geologyunrestricte

PROCJENA UTJECAJA GEOMETRIJE PUKOTINA NA PROPUSNOST, PRIMJER LABORATORIJSKOGA ISPITIVANJA I NUMERIČKOGA MODELIRANJA

The geometry of fractures includes orientation, spacing, aperture are among the parameters affecting permeability in rocks. Studying the effect of fractures geometry on the permeability in a laboratory scale requires the selection of a suitable sample in terms of physical and mechanical properties. Therefore, in this study, fibrous fiber was selected due to low water absorption and permeability as well as its non-brittle behavior and flexibility. In order to investigate the effect of fracture geometry on the permeability, 1, 2, 3, and 4 fractures with spacing greater than 50 mm, 50 mm, 25 mm, and 15 mm and with orientations of 0, 15, 30, 45, and 60 degrees to the horizon in the sample were created. The fractures did not come into contact with the surface of the sample .The results showed that the permeability raises exponentially with increasing orientation and decreasing the spacing. This situation is mostly seen in fractures with orientations larger than 30 degrees. Also, the permeability measured in the laboratory was compared with the results obtained from the numerical method of distinct elements and UDEC software. The results showed an error of about 10-15%, which is well-matched between the permeability obtained from the laboratory and the numerical method.Geometrija pukotina uglavnom obuhvaća orijentaciju, razmak i promjer, kao varijable koje određuju propusnost stijena. Studije koje se bave izučavanjem geometrije pukotina na laboratorijskim uzorcima imaju preduvjet odabira prikladnih uzoraka za ocjenu fizičkih i mehaničkih svojstava. Stoga je ovdje odabrano vlakno s malom apsorpcijom vode i propusnošću te sa svojstvima nekrtosti, odnosno savitljivosti. Ispitane su 1, 2, 3 i 4 pukotine s razmacima većim od 50 mm, od 50 mm, od 25 i od 15 mm te orijentacijama od 0, 15, 30, 45 i 60 stupnjeva od vodoravne ravnine. Pukotine nisu bile u dodiru s površinom uzorka. Propusnost je rasla eksponencijalno, prateći porast kuta orijentacije i smanjivanje razmaka među pukotinama. To je najbolje opaženo s pukotinama pod kutom većim od 30 stupnjeva. Propusnost dobivena laboratorijski uspoređena je s rezultatima izmjerenim metodom konačnih elemenata i programom UDEC. Dobivena je pogrješka od 10 do 15 % čime je dokazano dobro podudaranje laboratorijskih i numeričkih rezultata

The effect of particle size distribution on the geochemistry of stream sediments and heavy minerals in the Kuh-Zar copper-gold mineralization, South of Damghan

Introduction The Kuh-Zar copper-gold mineralization is located in 110 km south of Damghan at Torud-Chah Shirin volcanic-plotonic belt. Stream sediments are used as useful technique in the regional geochemical exploration. Mineralogy, geochemistry and particle size of stream sediments reflecting the composition of source rocks, mechanical and chemical weathering, morphological and hydrological features of the basin, sorting, and climate, as well as several other factors. It is important to consider the influence of geochemical and mineralogical controls on particle size distribution of stream sediments. Studies of distribution of trace elements in relation to the size fraction of stream sediments generally show that several elements, including Mo, Cu, Zn, Mn, and Fe are concentrated in the finest fractions of the sediments. Therefore the majority of stream sediment surveys have been based on the collection of < 200 µm materials. The Forum of European Geological Survey standard sieve mesh is < 150 µm. However, in present study geochemical distribution of elements investigate in various size fractions of stream sediments to obtain optimum mesh size.  Material and methods In order to achieve the scope of this study were collected samples from stream sediments, igneous rocks, and silicic veins. The number of 11 thin, thin-polished, and polished samples was studied by optical microscope. To study the effect of particle size distribution on stream sediment geochemistry, 6 stations was selected on the base of lithological, alteration, mineralization, tectonic, and watershed criteria. Each silt sample in every site consisted of 25 sub-samples that were collected along some 30–50 m from active part of stream channel. Silt samples at the field have been screened by a sieve of 2 mm to remove coarse sand. Each sample has been screened with a series sieve from 0.063 to 2 mm (ASTM codes). The ratio of size fractions was determined by weighing of each fraction. All of size fractions were digested in HNO3+HCl (aqua regia) and then analyzed for multi-elements by Varian 735-ES ICP-OES at Zarazma laboratory in Tehran. For measuring the concentrations of Au, fire assay preparation method was employed and the final aliquote was analyzed by Perkin-Elmer 5300 AAS at Zarazma laboratory. Along with silt geochemical samples, 6 heavy mineral and 3 lithogeochemical samples are also studied. Finally, based on the results interpretations have been made.  Discussion of Results & Conclusions  The Kuh- Zar is one of the most important prospecting areas for copper-gold in the northeast of Iran. Geology of the area consists of intermediate to mafic lava with middle-upper Eocene age. The Oligocene granodiorite and diorite were injected into Eocene volcanic series. Intrusive rocks lead to alteration and mineralization of copper and gold. The study of particle size distribution shows that 2-0.425 mm and 0.180-0.125 mm size fractions are forming the maximum and minimum weight percent of stream sediments, respectively. Geochemical data surveying demonstrate that the Au, Ag, As, Cu, Pb, Sb, Mo, and S are considerably enriched at the 4, 5, and 6 sampling stations. Concentration of these elements in 0.425-0.180 mm, 0.180-0.125 mm, and 0.125-0.063 mm mesh size is more than any other fraction. The < 0.150 mm is not representative size fraction of sediments in this area. Gold concentrate in a distance about 700 meters from mineralized source rocks in the Kuh-Zar stream sediments, whilst optimum distance for concentration of Cu, Pb, Ag, As, Sb, and Mo is about 1200 meters. The heavy mineral technique same geochemical surveys suggest that the 0.425 to 0.063 mm size fractions are useful for prospecting of gold and associated elements in the Kuh-Zar area

Geochemical evaluation of Pabdeh Formation in Nosrat field, southeast Persian Gulf using Rock- Eval VI pyrolysis

The present study was performed on 59 drillhole cuttings from Pabdeh Formation in Nosrat oil field using Rock- Eval VI pryrolysis. Geochemical analysis indicated that Pabdeh Formation possesses poor to good hydrocarbon potential. Plotting S1 against TOC suggests that samples were not affected by polluting substances such as crude oil and lubricants while drilling operation. Jones organic fancies diagram shows B-BC area indicating that Pabdeh Formation was deposited in marine anoxic to oxic environments. HI vesus Tmax shows that most samples initially have had type II kerogen and now reflecting a mixture of type II to III kerogen (capable of generating oil) that have already entered oil generation window. In addition, S1+S2 versus TOC plot also suggests that Pabdeh Formation can be considered as a capable hydrocarbon generating source rock in the study area

Applying the box plot to the recognition of footwall alteration zones related to VMS deposits in a high-grade metamorphic terrain, South Africa, a lithogeochemical exploration application

Alteration zones (more commonly foot wall alteration zones) are related to volcanic-hosted massive sulfide (VMS) deposits and represent unique features that may be targeted during exploration. Of these, the chloritic foot wall alteration pipe is the most extensive and characteristic of VMS deposits. This feature is geochemically identified by a strong relative enrichment in aluminium and magnesium and a coupled depletion in calcium and sodium, giving rise to chloritic rocks in the primary environment of formation. During high grade regional metamorphism such chloritic precursor rock types are replaced by an unusual mineral paragenesis, typically containing magnesium rich cordierite, phlogopite, orthoamphiboles or orthopyroxenes and aluminium rich minerals such as sillimanite and corundum. This suggests that the unusual geochemical features of the alteration zone, retained during the deformation and metamorphism, should be recognisable in lithogeochemical exploration. The massive sulfide deposit in the eastern part of the metamorphic Namaqua Province, South Africa, at Areachap, Kantienpan and the defunct Prieska Cu–Zn Mine are hosted by a Mid-Proterozoic volcano sedimentary succession known as the Areachap Group. These deposits were affected by a complex deformation and metamorphic history and represent examples of upper amphibolite to granulite grade metamorphosed VMS deposits. The application of the known lithogeochemical methods is especially complicated where the geology is not well understood, due to the poor rock exposure of complexly deformed and metamorphosed areas, such as in the eastern part of the Namaqua Province. The box plot presents a more readily applicable lithogeochemical method to characterize and identify the alteration process, but it was designed for relatively un-metamorphosed environments. It is demonstrated here that the box plot may also be applied to high-grade metamorphic terrains and that the mineral phases used in defining the boxplot in low grade metamorphic environments may be replaced by their equivalents in high grade metamorphic terrains. The compositional trends of the metamorphic minerals themselves may be used in defining the boxplot for high grade metamorphic terrains. These include the transition of: annite to phlogopite; grossular to almandine or pyrope, augite to enstatite or clinoenstatite, and hornblende to gedrite or cummingtonite. Close to the ore zone, the relative Mg content of pyroxene, cordierite and biotite are higher than further away from this zone. It could be demonstrated that the changes in the mineral compositions are gradational when comparing unaffected rocks with progressively more altered wall rocks. Conclusions based on an application of the isocon method demonstrate that primary footwall alteration zones in the Areachap Group's VMS deposits are characterized by elemental depletion of Na2O, CaO, Sr, Ni, V and La and enrichment of MgO, Fe2O3(total), S, Zn, SiO2, Co and F. It is shown that the whole rock compositions of rocks that were independently identified as the metamorphic equivalents of altered rocks, using the isocon method, plot in the correct place in the box plot for high grade regionally metamorphosed terrains. This establishes the box plot as an effective and practical tool for lithogeochemical exploration for VMS deposits in complexly deformed high grade metamorphosed terrains.Please see Appendix for supplementary dataPartly funded by CERMOD at the University of Pretoria.http://www.elsevier.de/chemernf201

Prediction of shear wave velocity using empirical correlations and artificial intelligence methods

Good understanding of mechanical properties of rock formations is essential during the development and production phases of a hydrocarbon reservoir. Conventionally, these properties are estimated from the petrophysical logs with compression and shear sonic data being the main input to the correlations. This is while in many cases the shear sonic data are not acquired during well logging, which may be for cost saving purposes. In this case, shear wave velocity is estimated using available empirical correlations or artificial intelligent methods proposed during the last few decades. In this paper, petrophysical logs corresponding to a well drilled in southern part of Iran were used to estimate the shear wave velocity using empirical correlations as well as two robust artificial intelligence methods knows as Support Vector Regression (SVR) and Back-Propagation Neural Network (BPNN). Although the results obtained by SVR seem to be reliable, the estimated values are not very precise and considering the importance of shear sonic data as the input into different models, this study suggests acquiring shear sonic data during well logging. It is important to note that the benefits of having reliable shear sonic data for estimation of rock formation mechanical properties will compensate the possible additional costs for acquiring a shear log

Identification of reservoir fractures on FMI image logs using Canny and Sobel edge detection algorithms

Because of the significant impact of fractures on production in hydrocarbon reservoirs, identification of these phenomena is a very important issue. Image logs are one of the best tools for revealing and studying fractures in reservoir and researcher can get lots of information about geological features in wells, by studying and analyzing these logs. In this research, two approaches have been used to determine the fractures in two wells A and B located in one of the oil fields in southwest of Iran. In the first approach, using Geolog software (version-7), after processing and correction of raw image log data, the number, position, dip, extension, layering, density and expansion of fractures have been identified. In the second approach, considering that the fractures in FMI images have edges, the Canny and Sobel filters as edge detection operators in image processing have been used to detect fractures in these images