Exploration For Gold Deposits Research Articles

A study of Au-bearing-nanoparticle-enriched plants from the concealed gold deposits and their prospecting significance

This study analyzes Au-bearing nanoparticles in five plant species (Anemone vitifolia, Taraxacum mongolicum, Artemisia argyi, Larix gmelinii, and Equisetum arvense) from the Yangshan gold deposit. Numerous Au-bearing nanoparticles were discovered in the five predominant plants within the ore deposit. It was found that plants differ in terms of their absorption and tolerance of Au-bearing nanoparticles. Among the five dominant plant species, Anemone vitifolia exhibits the highest absorption and accumulation capacity for Au-bearing nanoparticles, making it an Au-bearing-nanoparticle-enriched plants. The Au-bearing-nanoparticle-enriched plants which can super enrich Au-bearing nanoparticles store more abundant and intuitive information about deeply concealed gold ore bodies. Nanoparticles with similar composition and morphology were found in both the root and stem specimens of the Anemone vitifolia. It is shown that the Au-bearing nanoparticles in the stem cells were probably transported from the root. Furthermore, we discuss the relationship between Au-bearing nanoparticles in plants and deep concealed gold deposits and the process whereby Au-bearing nanoparticles migrate from gold orebodies to plants. The Au-bearing nanoparticles can carry information from the concealed gold ore body to the surface medium, and then be absorbed by the roots of Au-bearing-nanoparticle-enriched plants, gaining access to the aboveground parts of the plants. Therefore, this paper advances a new technology for gold deposit exploration using Au-bearing nanoparticles for the enrichment of plants.

Genesis and prospecting significance of the Buziwannan gold-polymetallic ore deposit in the West Kunlun Orogen Belt, China: Constraints from mineral geochemistry and in situ S–Pb isotope analyses of sulfides

The Buziwannan gold-polymetallic deposit is located in Northwest China's West Kunlun Orogen Belt (WKOB). There are two metallogenic stages that have been identified: skarn and hydrothermal. In this research study, in situ mineral geochemistry and sulfur–lead isotopes are used to precisely constrain the genesis and ore-forming process.Pyrite is the most dominant sulfide, and four types have been identified. Py0 is a sedimentary euhedral crystal; Py1 is a subhedral grain occurring in skarn ores, and coexists with Po1, Apy1, Ccp1 and Gn1; Py2, which coexists with Apy2, Po2 and Ccp2, is a subhedral to irregular crystal; and Py3 is a colloform crystal. Gn2 is the most recent sulfide. Magnetite and Ccp1 LA–ICMPS analyses indicate a skarn origin. Py0 and Py1 have the highest Co/Ni ratios and Ni contents, respectively, and Au has positive correlations with Pb, Ag and As, according to the trace element compositions of pyrite (LA–ICPMS). With the exception of four spots, most pyrites have Au (0.01–30.23 ppm) and As (1.47–15,709 ppm) concentrations below the Au saturation line, indicating that the majority of gold is present as lattice bound Au+. Py3 has clearly higher Sb contents indicating that Sb-rich fluids were added.The δ34S values of sulfides show that the sedimentary Py0 and skarn sulfides have the highest and lowest δ34S values of +13.30 to +14.67 ‰ and +4.48 to +7.77 ‰, respectively, and other sulfides possess δ34S values that fall between the two members, indicating a mixed source of magmatic and sedimentary. The lead isotope values also indicate that the sulfides from the hydrothermal stage have a mixed Pb source of magmatic and sedimentary.The skarn ores are spatially and genetically related to the Triassic granodiorite, indicating a Triassic age for ore formation. The Triassic magmatic intrusions hold the potential for future gold deposit exploration in the WKOB.

An Exploration Model for A Gold Deposit in Kervian Area, Kurdistan Province, Iran, using a Combination of Geophysical Results with Geological Information and Other Exploratory Data

As many gold deposits are associated with sulfide zones, and the direct exploration of gold deposits using the geophysical methods is very difficult due to its low amount in the sub-surface, the direct exploration of sulfide zones by the geophysical electrical resistivity and induced polarization (IP) methods may lead to the indirect exploration of gold deposits. The gold deposit in the Kervian area is located in the Kurdistan shear zone, and is directly related to the sulfide, silica, and carbonate alteration units. After acquiring the resistivity and IP data, 2D modeling of the data is made in order to indirectly identify the gold-bearing zones in the surveyed area. As some of the identified geophysical anomalies indicating the sulfide zones may not be associated with the economic amounts of gold, in order to obtain an exploration pattern for the gold deposit in the studied area, a combination of the geophysical data modeling and interpretation results with the geological information and other exploratory data is used to reduce the uncertainty in identifying the gold-bearing zones in the studied area. Thus, modeling and interpretation of the geophysical data lead to identify the sub-surface anomalies as the locations of possible gold mineralization in the area, and then the drilling points are suggested in the area. Considering the geological studies and chemical analysis of the samples taken from the drilled boreholes crossing some of the geophysical anomalies, we conclude that the geophysical anomalies occurring inside the phyllite and carbonate units in the area can contain an economic amount of gold, and thus are recommended as the top priority for further exploration.

Geochemical and mineralogical diagnosis on gold ores: A case study from the Cadillac–Larder Lake Fault Zone, Abitibi, Canada

Numerous orogenic gold deposits have been discovered within the Archean Abitibi greenstone belt of Canada. These include the Francoeur, Wasamac, Astoria, Zulapa, Goldex, Lamaque, Lac Herbin, and Beaufor deposits, all of which are located within the Cadillac–Larder Lake Fault Zone (CLLFZ). These gold deposits are hosted by various lithologies and vary in terms of style of mineralization, sulfide abundance, and hydrothermal alteration. These variations mean that exploration for new deposits has been difficult in this area using common exploration criteria. This study presents the results of a principal component analysis (PCA)-based approach, combining geochemical and mineralogical data to quickly identify different types of gold mineralization within the CLLFZ, including assessing the use of this approach in mineral exploration.The first principal components in both classification methods used during this study (PCE1 and PCM1, representing element geochemical and mineralogical classification respectively) can efficiently discriminate between mineralization and barren/altered rocks. The second principal components (PCE2 and PCM2) can be used to constrain the host rocks (altered vs. barren) and style of mineralization (disseminated vs. vein-hosted), respectively. Finally, PCE3 and PCM3 give more detailed information on the nature of the mineralization in an area, possible reflecting variations in the composition of ore-forming fluids. All of the above indicates that the two PCE and PCM classification methods are complementary, which means that using approaches that combine these methods will provide a comprehensive understanding of the nature of studied samples and/or deposits.Information extracted from the results of the PCA undertaken during this study may be beneficial for mineral exploration in unknown districts, such as glaciated areas, where traditional geological fieldwork is challenging. This study also defines element and mineral mineralization indices based on PCE1 and PCM1, which enables evaluation of the extent and style of gold mineralization in an area. This study indicates that PCA-based automated mineralogical and geochemical classification methods could be highly useful in exploration for gold deposits.

Минералого-петрографическая характеристика месторождения Угахан

The article considers the mineralogical-petrographic and geochemical characteristics of the Ugahan gold ore deposit with the aim of creating a scheme of the mineralogical associations’ formation that is inscribed in the general geodynamic evolution of the Bodaibo region. The study has been carried out on the copyright material obtained in the research area during the fieldwork. The mineralogical-petrographic and analytical work has been carried out at the Research Sharing Center “Isotope-geochemical studies” of Vinogradov Institute of Geochemistry, the Siberian Branch of the Russian Academy of Sciences. The mineralogical and petrographic research has identified three generations of carbonate minerals, formed at different times and associated with different stages of the field formation. Within the deposit, the development of the regenerative quartz aggregates is widely observed. The phenomenon is especially widespread in the upper members of the Buzhuikhta formation that are composed of quartz sandstones. The development of the mineral associations in the deposit took place in the following order. At the early low-temperature stage, the framboid and small idiomorphic pyrite were formed (I and II). Later, the high-temperature minerals association developed, that included arsenopyrite, pyrrhotite, pyrite-III, native gold, galena, sphalerite, and gray copper ores. At the post-ore stage, the pyrite-IV crystallization in a quartz-carbonate rim occurred. A similar sequence of mineral association formation is observed at the Krasny and Sukhoy Log deposits, which suggests that the formation of ore mineralization of the Sukhoy Log type deposits is a result of similar ore-forming processes. The confinement of the Ugahan deposit to the Buzhuikhta formation allows us to consider this stratigraphic unit as promising in terms of further exploration for gold deposits. The content of arsenic (as the main gold-accompanying element in the Bodaibo region) in the deposits of the Buzhuikhta formation is an order of magnitude lower, which suggests that it should be excluded from the search characteristics within the boundaries of this stratigraphic level.

Discussion on the Minerogentic Series of Gold Deposit in Qinghai

Gold is one of the most important strategic minerals in Qinghai province. Based on the metallogenic characteristics, types, ore control factors and spatial distribution of gold deposits, the metallogenic regularities of gold deposits were put forward. It’s divided the pre-Cambrian, Early Paleozoic, Late Paleozoic, Mesozoic and Cenozoic the metallogenic series assemblages, and also their distribution features are discussed which focus on five metallogenic periods. According to the theory of metallogenic series of ore deposits, combining with characteristics of typical gold deposits, twenty metallogenic series are preliminarily divided in Qinghai province. It is pointed out some suggestions about prospecting and exploration of gold deposits in Qinghai.

Prospects for the exploration of gold deposits in the Golovanevsk suture zone of the Ukrainian shield

Prospects for the search for gold deposits within the limits of the Lypovenky-Sukhiy Tashlyk area of the Golovanivsk suture zone (GSZ) of the Ukrainian Shield (USh) have been considered, where in the 80―90s of the last century North Capitanian gold mineralization with indicative reserves of 27 tons of Au was discovered and drilling of more than 3000 wells with hydrotransportation of the core (CHC) in the crust of weathering of the Precambrian basement revealed signs of about 30 new massifs of rocks of hyperbasite composition. Drilling was directed at the search for deposits of silicate nickel, so was not paid attention enough to the presence of hypergene gold in more than half wells was not given enough. In this paper, the authors turned to this question, because the search for gold deposits within the USh is now in time. The work is based on the analysis and generalization of geological, archival laboratory data and results of detailed geophysical works at a scale of 1:10 000. The conclusion of previous studies confirmed that one of the important search criteria for gold mineralization is the presence of metasomatic, dynamometamorphic and skarn transformations in endocontact zones of massifs of basites and ultra-basites, as well as in the zones of intersection of such massifs with faults. Metasomatites and skarns have the largest weathering crust among other types of rocks, and the stem capacity of abnormal gold-bearing intervals may exceed 10 m. The largest number of ”gold-bearing” wells has just been recorded in the skarns, and the average content of Au in the weathering crusts of the main types of rocks in the research area is the largest one in the skarns and ultra-basites. The analysis of the maps of abnormal magnetic and gravitational fields allowed, in addition to the known here zones of faults in the northwest, to distinguish two meridional zones of shearing that coincide with the separate Lypovenky and Sukhiy Tashlyk areas of development of gold mineralization. Both areas of shearings consist of meridional and latitudinal echelon shears, many of which are bordered with bodies of basites and hyperbasites and accompanied by chains of gold anomalies. Shearing zones belong to the Subot-Moshorino phase of faulting (1.8―1.73 billion years ago), with which the active processes of ore formation of rare and noble metals are connected within the USh. Recommendations for further geological and geophysical investigations within the limits of Lypovenky―Sukhiy Tashlyk area of the GSZ have been given.

Spatial distribution and trace element geochemistry of laterites in Kunche area: Implication for gold exploration targets in NW, Ghana

Kunche area is located in NW Ghana within the Wa-Lawra greenstone Birimian belt and is composed mainly of volcaniclastics, metasediments and some intrusive granitoids. The area is covered with laterites making it difficult to identify exploration targets. In this study, we describe a new methodology based on statistical tools and trace element geochemistry to map the pathfinder elements of gold deposits in lateritic context. However, the results of this study are specific to a particular context and cannot be extrapolated without supplementary studies to all the lateritic areas. In this respect, a total of 67 lateritic samples were collected from residually weathered materials and their spatial distribution was determined by means of the GIS-based kriging interpolation method. The samples collected vary from detrital to residual laterites/duricrusts and are hosted in volcaniclastic rocks. ICP-MS and XRF techniques were used to determine the element concentrations of the samples. The trace element geochemical data were analyzed using bivariate and multivariate geostatistical analysis to establish relationships among elements. Fe-oxides such as goethite and hematite and clay minerals like kaolinite are the main secondary minerals of the concretionary reddish lateritic samples. All the analyzed elements showed asymmetrical distribution rather than normal distribution. Spearman correlation shows that Cu, Pb, S, As, and Ag have moderate to strong positive correlation with Au. From the multivariate geostatistical analysis, three element associations; a) Fe, Pb, S, Co, Cr; b) Ni, Y, Rb, Sr, Zn, and c) Ca, Cu, Mn, Ti, Zr, As, Au, Ag were observed. Threshold values of selected elements were determined using the median absolute deviation (MAD) method, which indicates possible anomalous concentrations in the laterites for Pb (≥48 ppm), Cu (≥46 ppm), As (≥134.2 ppm), and Ag (≥0.42 ppm). Multi-element mapping indicates that Pb + Cu + As + Ag is the most ideal association in the exploration of gold deposits. It reveals ellipsoidal anomalies comparable to the Au distribution map that suggest dispersion and accumulation of the pathfinder elements in the area. The geochemical anomalies are mainly restricted to the environment of the residual laterites in the Kunche area and we recommend that exploration programs should be focused in such areas.

Mineralogical characteristics of silver minerals from the Dongyang Gold deposit, China: Implications for the evolution of epithermal metallogenesis

The Dongyang gold deposit is located in Fujian Province in southeastern China, mainly hosted by a NW–SE trending Late Jurassic porphyritic rhyolite, and classified as a low-sulfidation epithermal gold deposit. The units associated with the gold mineralization in this area are highly altered and have undergone sericitic and argillic alteration as well as silicification, with the latter two types of alteration especially associated with gold enrichment. The deposit contains pyrite, marcasite, and arsenopyrite, with lesser amounts of chalcopyrite, galena, sphalerite, and silver minerals. The sulfides and silver minerals within the deposit provide insight into the evolution of the ore-forming fluid and the processes that caused the gold enrichments to form in the study area. In addition, the geochemical similarity of gold and silver means these elements tend to migrate and precipitate simultaneously, in turn indicating that sulfides and silver minerals can provide insights into the processes that concentrate gold. Here, we present chemical compositional, and crystal structural data for sulfides and silver minerals within the Dongyang gold deposit by scanning electron microscopy (SEM), electron microprobe analyser (EMPA), X-ray diffraction (XRD) and laser Raman microprobe, and use these data to gain insights into the characteristics and evolution of the fluids that formed the deposit. We also outline key concepts for future gold deposit exploration in this region. Four types of pyrite in the deposit are recognized based on arsenic content and crystal morphology, evolving from early As-poor pyrite (Py1) to As-rich pyrite (Py2) and/or arsenopyrite, to non-cubic As-poor pyrite (Py3), and finally to cubic As-poor pyrite (Py4), with an inverse correlation between sulfur and arsenic concentrations. Pyrite crystal morphologies vary from earlier pentagonal dodecahedron forms to later cube forms, indicating a decrease in degree of supersaturation (as controlled by the activity of dissolved Fe and sulfide) and/or temperature conditions for pyrite growth. In addition, the presence of As-poor marcasite associated with early pyrite (Py1) suggests that the early ore-forming fluid was relatively acidic. Arsenopyrite is generally concentrated in the shallower parts of the deposit, indicating that later hydrothermal fluids were enriched in arsenic. This study also identified a number of key silver mineral phases that formed in a sequence from chalcopyrite, to a galena–fahlore–allargentum–dyscrasite–polybasite–electrum assemblage, to stephanite, and finally to chlorargyrite. Chalcopyrite replaced by fahlore suggests increased oxygen fugacity and decreased temperature. Fahlore Sb / (As + Sb) (>90%) and Ag / (Ag + Cu) ratios (6%–27%) are indicative of relatively high temperatures (>200 °C), and allargentum is thought to form in elevated pH conditions (pH > 7). Stephanite and chlorargyrite precipitated under relatively low temperature (<200 °C). Finally, the presence of chlorargyrite suggested that silver was transported as chloride complexes, indicating relatively high oxygen fugacity. The sequence of silver mineral precipitation suggests that the ore-forming fluids showed a temporal increase in pH and oxygen activity, and a decrease in temperature and sulfur fugacity. This indicates that the hydrothermal fluids that formed the deposit can be divided into early and late stage fluids, with the former being a relatively higher-temperature and higher‑sulfur fugacity fluid compared with the lower-temperature and higher‑oxygen-fugacity late-stage fluid. Finally, the presence of silver minerals with high gold contents (up to 2 wt%), as well as arsenian pyrite and arsenopyrite, may be useful indicators of prospective areas for gold exploration in this region.

Magnetic Exploration of Structurally Controlled Mineralization at Low Latitudes: A Case from the Masara Gold District, Mindanao, Philippines

Abstract Magnetic data transformations are applied to the high-resolution magnetic data from the Masara gold district in the southern Philippines to enhance features related to a porphyry copper prospect and epithermal gold deposit. Using several filtering methods and structural mapping techniques, we were able to highlight the signal coming from a shallow chlorite-sericite alteration zone. Although the epithermal veins are mainly composed of nonmagnetic minerals, calculation of the tilt derivative of the magnetic grid revealed linear features that are coincident with the surface projection of the gold-bearing veins. Furthermore, dip directions of the magnetic sources were determined using Euler deconvolution, and they were found to coincide with the dip directions of the veins as determined by structural measurements at different underground levels. Aside from showing the utility of the magnetic methods in epithermal gold deposit exploration, this study also demonstrates that challenges of processing and interpreting magnetic anomalies in areas near the equator can be overcome. This research outlines a processing workflow that can be adopted for further investigation of other mineralized areas.

Application of EH4 Electromagnetic Imaging System in Gold Ore Exploration

EH4 electromagnetic imaging system is relatively advanced electromagnetic exploration instrument at home and abroad with the advantage of great exploration depth, intuitive and portable etc. In this paper, on the basis of the systematical collection of predecessors' research data and combined with the present situation of domestic and foreign researches, the author introduces the working principle of the EH4 electromagnetic imaging system, field-working method, scope, and possibly, and some geological problems that may be solved in gold prospecting. Apart from this, through exploration cases of in Zhangsangou Gold mining in Heilongjiang province, Yuerya gold deposit in Hebei province and in 460 gold mine in region of Beishan of Gansu, this paper illustrates that the effect of EH4 system in gold deposit exploration is remarkable.

Role of surface phenomena in concentrating incompatible elements: Au in pyrite from hydrothermal clays at thermal fields in southern Kamchatka

The paper reports data on pyrite obtained using microscopic, physicochemical, and analytical techniques. The major mineral concentrating Au in clays at the Upper Koshelevskoe and East Pauzhetka hydrothermal fields is pyrite, which contains Au evenly distributed in its crystals (0.07–0.25 ppm) and Au bound to the surface of its crystals (0.8–6.8 ppm). The clear correlations between the concentration of equally distributed Au and the topological surface area of single crystals and the absence of correlations with the bulk specific BET surface area rule out purely adsorption mechanisms of Au accommodation at the surface of pyrite crystals, because otherwise the concentration of the minor element would have been proportional to the actual surface area but not the geometric one. In contrast to what is typical of high-temperature hydrothermal systems, at hydrothermal fields Au is an element highly compatible with pyrite. This may be explained by changes in the growth mechanisms of pyrite crystals and the transition to their growth via incorporation of colloid and subcolloid particles. Low-temperature pyrite in hydrothermal clays exhibits certain geochemical features important for exploration for gold deposits related to modern and ancient hydrothermal systems. This pyrite differs from higher temperature pyrite at ancient gold deposits in bearing sulfoxide sulfur species on the surface of its crystals instead of monosulfide species and also in having a less ordered and dense structure, higher porosity, and a globular topography of the surface of crystals.

Geochemistry of hydrothermal alteration at the Qolqoleh gold deposit, northern Sanandaj–Sirjan metamorphic belt, northwestern Iran: Vectors to high-grade ore bodies

The Qolqoleh orogenic gold deposit in the northern part of the Sanandaj–Sirjan metamorphic belt in northwestern Iran is hosted by a steeply dipping sequence of greenschist facies Cretaceous volcano–sedimentary rocks, including mafic to intermediate metavolcanic rocks, sericite and chlorite schist, and marble. Geochemical and petrochemical data including the ∑REE, (La/Yb)N and Eu/Eu* ratios were obtained from country rocks, ore-enveloping alteration zones, and mineralized zones to assess the nature of the trace element and rare earth element (REE) interaction between the wall rock and the mineralizing fluid.Quartz–sulfide veins at the deposit are characterized by a pyrite–pyrrhotite–chalcopyrite–sphalerite–arsenopyrite–native gold assemblage. Alteration halos border the mineralized zones and broadly comprise: (1) an outer carbonate–chlorite alteration zone in all rock types, particularly in chlorite schist; (2) a middle sericite–carbonate alteration zone in the sericite schist; and (3) an inner quartz–sulfide alteration zone in sericite schist and mafic to intermediate metavolcanic rocks.The geochemical data indicate that the concentrations of Al2O3, P2O5, TiO2, Y, and Zr are relatively constant, suggesting that these elements were the least mobile during hydrothermal activity. Using Al2O3 as the immobile component, there is evidence for mobility of trace elements, particularly light REE, TiO2, and Zr in the altered wall rocks. The altered rocks show a relatively light REE depletion ((La/Yb)N≅9.41), which clearly correlates with the grades of gold mineralization and intensity of the alteration (3ppm Au). The depletion of light REE is best indicated by a decrease in (La/Yb)N as shown by ratios of 10.5 to 11.8. Wall rock decarbonation reactions during infiltration of the mineralizing fluid resulted in differential mobilization of REE, from a fluid with initially low REE content.The overall trace element geochemistry of the altered wall rock is controlled by the initial composition of the wall rocks and the ore-fluid composition. Hydrothermal ore-forming fluids are recognized as CO2-rich near-neutral reduced fluids with high values of H2S, K, and S content. Observed variability in alteration halos at the Qolqoleh deposit points to major differences in REE and trace element content in original host rocks that have interacted with a relatively similar ore fluid. Therefore, depending on the composition of each host rock lithology, the geochemistry of hydrothermal alteration (e.g., ∑REE content and (La/Yb)N ratios) and alteration mineralogy including the carbonate–sericite–quartz–sulfide assemblages may be used as a primary tool for lithogeochemical exploration for gold deposits in northwestern Iran.

Soil geochemical prospection for gold deposit in the Arzular area (NE Turkey)

The Arzular gold mineralization is located at the southern part of the Black Sea Tectonic Unit in northern Turkey. This belt hosts several high-potential epithermal gold deposits and it is highly probable that there are a number of undiscovered occurrences in the region. The presence of intensely altered areas, suitable structural elements and acidic intrusions indicates that the region has an important potential for epithermal gold. Soil geochemistry surveys are widely used for the exploration of buried mineral deposits. In this work the applicability of soil geochemistry surveys for exploration of buried mineral deposits was tested using various statistical methods. In this respect, concentrations of 16 elements have been assayed in about 50 soil samples collected from the Arzular area. Soil samples were taken from the B horizon. The data revealed elevated concentrations for gold (0.20–259.2 μg/kg), silver (23–26,972 μg/kg), arsenic (1.7–301.8 mg/kg), copper (54.09–170.7 mg/kg), zinc (5.1–261.3 mg/kg) and lead (2.6–385.45 mg/kg). Gold is significantly correlated with elements such as Sb, Cd, Pb, Ag, As, Cu and Mo. Relatively high anomaly contrasts between Au, As, Ag and Sb and their coinciding anomalies along an E–W trending fault at the eastern part of the field might indicate that As, Ag and Sb could be used as a pathfinder element for exploration of gold deposits. Factor analysis was applied to elements in soil samples and then distribution diagrams were prepared using the factor scores obtained. In a distribution diagram for mineralization-associated factor score, two anomaly fields along the E–W trending fracture line and one anomaly field at the eastern part were distinguished. The largest anomaly in an ellipsoidal shape with long axis of 350 m was obtained from the southeast part of the area. Anomaly patterns show that factor scores might yield suitable and net results for the exploration. NE–SW trending fracture system is not related to mineralization and is younger than E–W trending fracture line. However, the E–W trending fault system controls the mineralization; therefore, the eastern extent of this fault zone needs to be investigated.

Exploration of gold deposit in the Seta area, Kamishihoro town, Eastern Hokkaido

Exploration of gold deposit in the Seta area, Kamishihoro town, Eastern Hokkaido

Deciphering the Complex Fluid History of a Greenstone-Hosted Gold Deposit: Fluid Inclusion and Stable Isotope Studies of the Giant Mine, Yellowknife, Northwest Territories, Canada

Mesothermal, greenstone-hosted gold deposits are typically products of complex hydrothermal systems that involved multiple fluids at various times throughout their histories. The Giant mine is an example of such an extremely complicated system, including (1) multiple, local and regional gold-depositing events; (2) at least two styles of gold ore introduction in the mine area, including both refractory, sulfide-hosted and free-milling, vein-hosted ores, whose relative timing is enigmatic; (3) fluid overprinting associated with deposition of multiple generations of postore vein- and vug-filling minerals; and (4) postore deformation and recrystallization of ore veins, especially along faults. Three main stages of quartz-carbonate mineralization are recognized in the Giant mine. Stage I encompasses deposition of dominant refractory, sulfide-hosted ores and subordinate free-milling, vein-hosted gold ores. Refractory, metavolcanic rock-hosted orebodies are connected to metasedimentary rocks east of the mine by an east-dipping alteration zone characterized by a depletion in Na and enrichments in K, Ag, As, S, and Sb. Quartz veins within the wall-rock alteration zone have δ 18 O values that decrease systematically from 14.7 per mil in deeper metasedimentary rocks toward 11.6 per mil in shallower metabasalts in the mine. This decrease is interpreted to indicate that 18 O-enriched ore fluids originated in deeper, metasedimentary rocks and reacted extensively with wall rocks along the entire extent of their flow paths before depositing dominantly refractory gold ores within more 16 O-enriched, shallower, Ti-rich tholeiitic metabasalts. Quartz ± carbonate veins related to these gold ores were deposited from H 2 O-CO 2 -NaCl fluids with T h values of 180° to 360°C and salinities of 4 to 9 wt percent NaCl equiv. Evidence of sporadic fluid unmixing indicates that gold was deposited at temperatures near 350°C and pressures of 1 to 2 kbars. The δ 18 O values (SMOW) of vein quartz (11.6–14.7‰) and calcite (8.6–14.1‰) within the ore-related wall-rock alteration zone indicate deposition from fluids ( δ 18 O water = 4.4–9.8‰) that equilibrated with metasedimentary and metavolcanic rocks during greenschist metamorphism. The δ 18 O values (8.6–11.4‰) of vein quartz that locally contains free gold, hosted in metavolcanic rocks outside of the ore-related alteration zone, indicate deposition from fluids with lower δ 8 O water values of 2.8 to 5.6 per mil. The difference in ranges of δ 18 O water values may indicate that multiple events were responsible for gold-bearing quartz veining, or alternatively, that fluids depositing veins within and outside of the alteration zone represented distinct fluid reservoirs that evolved through reaction with isotopically distinct rocks in their source regions (i.e., 18 O-enriched metasedimentary vs. 16 O-enriched metavolcanic rocks). Postore (stage II) carbonate veins associated with minor Pb-Zn-Sb-Ag mineralization were deposited from highly saline NaCl-CaCl 2 brines with T m values of –32° to –36°C and T h values of 72° to 273°C. The δ 18 O values of these carbonates (20.6–26.1‰) indicate that their parent brines ( δ 18 O water = 9–14‰) also equilibrated with metasedimentary and metavolcanic rocks. Subsequent dissolution of these carbonate veins created abundant vuggy porosity. Late (stage III), primarily vug-filling dolomite ± stibnite, overgrown by scalenohedral calcite, was deposited from dilute fluids ( h values of 95° to 115°C. The δ 18 O values of dolomite (17.4 toward 13.4‰) and latest calcite (10.9‰) indicate deposition from progressively less evolved meteoric waters with decreasing δ 18 O values from 1.1 toward –6.9 per mil. Refractory ores in the main alteration zone and gold-bearing quartz veins contained therein formed from chemically similar CO 2 -H 2 O-NaCl ore fluids whose oxygen isotope compositions are consistent with a metasedimentary source. These two styles of gold mineralization appear to be part of the same mineralizing event in which the style of mineralization was dictated by the mechanism of ore deposition. Refractory, sulfide-hosted gold mineralization resulted from reaction of mineralizing fluids with metavolcanic wall-rock Fe 2+ . Free-milling, gold-bearing quartz vein ores were deposited as a result of fluid unmixing, with loss of H 2 S to the vapor phase. Gold-bearing quartz veins outside of the main wall-rock alteration zone in the Giant mine were also deposited by unmixing of similar H 2 O-CO 2 -NaCl fluids. However, these fluids were isotopically distinct from those within the alteration zone and may not be part of the refractory, sulfide-hosted gold-depositing event. They may instead represent a separate mineralizing event whose fluids and ore-forming constituents were derived solely from within metavolcanic rocks. This overprinting of one event on the other may have been a necessary condition for the development of world-class gold deposits in the Yellowknife district. The important roles of both metavolcanic and metasedimentary source rocks may explain why some smaller greenstone belts, with limited volumes of metavolcanic rocks, can host substantial economic gold mineralization and has important implications for regional resource evaluation and exploration for gold deposits in greenstone belts.

Application of lead isotopes to geochemical exploration of gold deposits in Baoban Hainan Province, China

Lead isotopes have been widely applied in geochemical exploration and evaluation of ore deposits, as well as in ascertaining the age of mineralization and the source of ore fluids. Long-term practice showed that the method of lead isotope targeting is somewhat efficient for macroscopic evaluation of forecasting areas, but not powerful enough for forecasting concealed orebodies. As the contents of U and the variation of U/Pb ratio sharply decrease with depth in the lithosphere, U-Th-Pb isotopic differentiation must have occurred during the crust-mantle evolution. Lead isotopic ratios show a wide variation range, varying in the front of mineralization and shallow-derived ores, but maintaining very stable in the major orebody and being usually close to the average isotopic composition of the crust and mantle of the continent block from which the ores were derived. Therefore, the lead isotopic composition can serve as a measure for identifying the position of mineralization. The lead isotope geochemistry was applied to the exploration and evaluation of the Baoban gold deposits of Hainan Province, China. The analytical results of ore veins and adjacent rocks showed that there is a correlation between the lead isotope data and the position of orebody. Based on the experience from the Baoban gold deposits and other ore deposits in Yunnan Province, an exploration principle has been established, that is, positive anomalies of lead isotope eigenvectors for prospecting deep-seated orebodies and negative anomalies of eigenvectors for enlarging lateral exploration surrounding the known deposit. The ore beds in the Erjia and Beiniu mining districts should be assigned to the deep part of the orebody and those in the Tuwaishan mining district should be the shallow part, so ore beds corresponding to those in the Erjia and Beiniu mining districts may be found in the Tuwaishan mining district.

Characterization of Geochemical Distributions Using Multifractal Models

The use of multifractals in the applied sciences has proven useful in the characterization and modeling of complex phenomena. Multifractal theory has also been recently applied to the study and characterization of geochemical distributions, and its relation to spatial statistics clearly stated. The present paper proposes a two-dimensional multifractal model based on a trinomial multiplicative cascade as a proxy to some geochemical distribution. The equations for the generalized dimensions, mass exponent, coarse Lipschitz–Holder exponent, and multifractal spectrum are derived. This model was tested with an example data set used for geochemical exploration of gold deposits in Northwest Portugal. The element used was arsenic because a large number of sample assays were below detection limit for gold. Arsenic, however, has a positive correlation with gold, and the two generations of arsenopyrite identified in the gold quartz veins are consistent with different mineralizing events, which gave rise to different gold grades. Performing the multifractal analysis has shown problems arising in the subdivision of the area with boxes of constant side length and in the uncertainty the edge effects produce in the experimental estimation of the mass exponent. However, it was possible to closely fit a multifractal spectrum to the data with enrichment factors in the range 2.4–2.6 and constant K1 = 1.3. Such parameters may give some information on the magnitude of the concentration efficiency and heterogeneity of the distribution of arsenic in the mineralized structures. In a simple test with estimated points using ordinary lognormal kriging, the fitted multifractal model showed the magnitude of smoothing in estimated data. Therefore, it is concluded that multifractal models may be useful in the stochastic simulation of geochemical distributions.

Experimental Determination of Detection Limits for Performing Neutron Activation Analysis for Gold in the Field

Measurements are presented of gold concentration in rock/soil samples by delayed neutron activation analysis using a device and method that are potentially field portable. The device consists of a polyethylene moderator and 252Cf as the source of neutrons for activating the samples and a high-purity germanium detector to measure the 412-keV gamma-ray emissions from activated gold. This information is used to extract the gold concentration in the sample. Two types of samples were investigated: (a) pure SiO2 doped with a known amount of gold chloride and (b) U.S. Geological Survey standards. The former types were used to evaluate optimum device performance and to calibrate the device and method. The latter types were used to show typical system performance for the intended application (field exploration for gold deposits). It was found that the device was capable of determining gold concentrations to ~10 ppb with a turnaround time (the sum of irradiation, decay, and counting times) of ~10 days. For samples where the gold concentration was much higher (i.e., gold ore), turnaround times are ~2 days and could be shortened further by sacrificing accuracy (e.g., lessening irradiation, decay, and counting times) or by augmenting source strength.

Multidata set analysis for gold-deposit exploration criteria: application in the Catalonian Coastal Ranges (NE Spain)

The method developed in the European Union `Midas' project for the exploration of gold deposits was applied in the Catalonian Coastal Ranges of NE Spain. The method is based on the combined use of a multidata set compiled from tools that permit the identification of a series of regional characteristics and which are defined as signatures. These signatures are in turn contrasted with the attributes that characterize known gold deposits which allow for the identification of possible gold mineralization. In the present study, the superimposed multidata sets have helped to identify a series of signatures which are characteristic attributes of both turbidite-hosted and granite-associated type deposits.