The gravity method in groundwater exploration in crystalline rocks: a study in the peninsular granitic region of Hyderabad, India

The application of variations in the earth's gravity in groundwater exploration on a regional scale, especially in sedimentary basins, metamorphic terrains, valley fills, and for buried alluvial channels, is well established. However, its use in hard crystalline rocks is little known. In granite, for example, the upper weathered layer is a potential primary aquifer, and the underlying fractured rock can form a secondary aquifer. Fracturing and weathering increases the porosity of a rock, thereby reducing the bulk density. Changes in gravity anomalies of 0.1–0.7 mGal for granites, due to weathering or variations in lithology, can be detected. To test the use of gravity as a groundwater exploration tool for crystalline rocks, a gravity survey of the peninsular shield granites underlying Osmania University Campus, Hyderabad, India, was undertaken. At the site, gravity anomalies reflect variations in the lithology and in the thickness of weathered zones. These anomalies also define the position of intrusives and lineaments. Areas of more deeply weathered granite that contain wells of higher groundwater yield are represented by negative gravity values. In the weathered zone, well yield has an inverse relation to the magnitudes of residual gravity. The study confirms the feasibility of gravity as a tool for groundwater exploration in crystalline rocks. Electronic Publication     

Ground penetrating radar study for hydrogeological conditions related with mining activity

In the Bicholime, Goa, there is an iron-ore mine surrounded by villages and small towns. Exploitation of such an enormous amount of iron ore may disturb the local groundwater table, thereby causing groundwater depletion. However, these effects are dependent mainly on the geological formations in the region and the mining method. The effects of Bicholime mining on the surrounding groundwater resources and subsurface strata conditions are the main objective of this study. For this purpose, ground penetrating radar surveys were carried out to study the possible effects of mining on the hydrological regime of this open-pit mine, and the subsurface conditions in the Bicholime opencast iron-ore mine, Goa. The results of these surveys show that laterite layers exist from the surface to depths varying from 5 to 22 m at all study locations in the mining area, and in most places clay layers exist, except for the GPR section along the profiles DD1 and EE1 in the village at depths of 8-22 m. Due to the presence of impervious clay layers, mining and dewatering in the leasehold areas will most likely not affect the surrounding hydrological regime of this mining area. It is concluded that no hydrological connection exists between the open pits and the surrounding villages, so that sufficient groundwater is available in the village area, in spite of the enormous extraction of iron ore from the underground.     

Collapse hazard assessment in evaporitic materials from ground penetrating radar: a case study

Evaporitic materials have been studied by means of ground penetrating radar (GPR) in order to evaluate the collapse hazard. The obtained 200 MHz GPR profiles show a low signal-noise ratio over the first 3 m depth, where well-defined and continuous reflectors can be observed. Between 3 and 4.5 m depth, the signal to noise ratio decreases due to attenuation of the electromagnetic (EM) waves. As a result, reflectors located deeper than 3 m become more discontinuous and poorly defined. GPR profiles show trails of continuous and subhorizontal reflectors, with a slightly undulated and irregular geometry. Although some of these reflectors laterally vanish or seem to disappear, sudden interruptions or hyperbolic reflections that could be originated by the occurrence of cavities have not been detected. These reflectors have been interpreted as corresponding to several evaporitic layers (gypsum) that constitute the main lithology in the area. Clear interruptions of reflectors have only been observed in some GPR profiles, and they could be attributed to small (1–2 m long) subvertical faults, with only a few centimetres offset. These faults may be generated by the accommodation of the evaporitic layers to local collapses affecting deeper materials.     

Ground penetrating radar and 2D electric resistivity studies for tracing hydrocarbon leakage site,close to Abha City: a case study

This work presents the application of the ground-penetrating radar (GPR) method and electric resistivity tomography (ERT) technique in outlining a zone of contamination due to the light non-aqueous phase liquid (LNAPL) plume underground in the area of an impacted fuel station, close to Abha City. The GPR has been performed using SIR3000 unit with the 100 and 400 MHz antennas. The main objective of the GPR survey was to evaluate the lateral extension of contamination. The complex GPR signature of the plume was well characterized. Low reflectivity zone corresponds to hydrocarbon vapor phase in the vadose zone. Enhanced reflections are associated with free and residual products in the fractured saturated zone directly above the water table. An electric resistivity tomography (ERT) survey was performed on four profiles within the site to investigate the vertical and horizontal extent of the contamination plume and to define the bottom of the landfill. The 2D electric profiles show the presence of low-resistivity (4O to 37 Ω m) anomalies that refers to the presence of accumulated hydrocarbons. From the interpretation of the GPR and ERT profile, it was possible to locate the top and bottom of the contamination plume of the waste disposal site. The radar signal penetrated deep enough and enabled the identification of a second reflector at approximately 10-m deep, interpreted as the hard basement surface which causes the strong amplitude reflection in the GPR profile. The results of GPR and ERT showed good agreement.     

Role of groundwater protection planning in groundwater governance: A case study from North India

In the present case study, impact of urbanization and industrial development on the shallow groundwater regime of Saharanpur town of Uttar Pradesh in India is examined with the aim of planning groundwater protection for better governance. The hazardous physicochemical and bacteriological parameters and heavy metals detected in the shallow aquifer include harmful pathogens like fecal coliforms, heavy metals like cadmium, chromium, nitrates and sulphates. An assessment of ground water vulnerability using the well known DRASTIC method has confirmed that the shallow groundwater in some central and southern localities of Saharanpur town fall in the medium risk zones. Further, using field data of 32 electrical resistivity soundings, the protective capacity of the unconfined aquifer is assessed in terms of a ‘total longitudinal conductance’ of the semi-pervious to impervious sediments overlying the unconfined aquifer. However, some areas aligned along a northwest-southeast and in the western parts of the town seem to have relatively higher protective capacity against infiltrating waste pollutants. A ground water protection planning map prepared by combining the DRASTIC map and the ‘potentially hazardous pollutants’ map has brought out the need to install eleven new groundwater quality monitoring wells in the town at locations near the line sources and point sources of pollution. This approach can be readily employed by the decision makers in framing sound guidelines for groundwater protection and governance.     

Methodology for mapping shallow groundwater quality in urbanized areas: A case study from Lithuania

Cities have a negative impact on the quality of shallow groundwater. Many of Lithuania's urban residents drink water from dug wells. Moreover, polluted shallow groundwater contaminates deeper aquifers of fresh drinking water. Therefore, this situation should be controlled and managed, as far as possible. In order to evaluate the quality of shallow groundwater in an urban area and to create an optimal monitoring system, an original methodology for groundwater mapping has been proposed. It resembles the GIS (geographical information system) technologies. The set of maps, laid one over another, consists of the following: (1) urbanization map, (2) geological-hydrogeological map, (3) groundwater chemistry map, (4) resulting groundwater chemistry factorial analysis map, and (5) pollution and pollutant transport map. The data obtained from studies on dug and geotechnical wells have been used for compilation of the maps. The system for shallow groundwater monitoring in the city with an area of 70 sq km and a population of 140,000 is proposed to consist of about 30 monitoring wells and several dug wells.     

Mineral exploration in regolith-dominated terrains: Global considerations and challenges

Mineral exploration through transported cover is becoming one of the fundamental challenges for the exploration industry in this century. The sharp increase in demand for commodities driven by a growing population and technology-based society is coupled with the decrease in world-class ore deposit discoveries in the last three decades. This is setting the stage for an unprecedented scenario, that is, ensuring that the market supply for critical metals (Ni, Co, Au, PGE, etc.) is satisfied. This situation is becoming the driving force for a restructuring of mineral exploration paradigms. New technologies and methodologies are being developed; and, as a consequence, regions that were considered to be unfavourable for ore deposit exploration are now being reconsidered. Among these areas are vast regions of Regolith-Dominated Terrains (RTD) with basement rock suites buried under thick transported cover and/or deeply weathered profiles.This Special Issue presents several studies on mineral exploration in RTDs in Brazil, China and Australia, and addresses some of the issues that hinder exploration in these problematic terrains, in the hope of reaching a broad audience, and encouraging researchers from a wide spectrum of disciplines to contribute to this exciting and inspiring exercise.     

Ground‐penetrating radar and sedimentological analysis of Holocene floodplains: Insight from the Tuross valley,New South Wales

Ground‐penetrating radar (GPR) has been used on an array of floodplain types on the lower Tuross River, in southeastern New South Wales, as part of an investigation into controls on channel‐floodplain relationships. Ground‐penetrating radar transects from two floodplains are presented, along with sedimentological detail from trenches dug along the profiles at key locations. Sedimentological investigations showed that 100 MHz antenna gave an approximation of overall bedding trends in the upper 3 m when automatic gain control processing was used. Spreading and exponential compensation processing provided insight into textural changes associated with increased silt content distal of the levee crest. One trench showed that thinning beds were responsible for onlapping reflectors. Signal attenuation at ～4 m depth below the raised floodplain surface resulted from a >50 cm‐thick bed of sandy clay. The close integration of GPR and sedimentological data produced an excellent dataset, that enabled form‐process associations and floodplain evolution to be established for these sandy floodplains. However, accurate subsurface assessment and interpretation must stem from carefully combined GPR and sedimentological datasets.     

深穿透地球化学勘查技术对隐伏稀有金属矿的勘查指示：以甲基卡X03号锂矿脉为例

花岗伟晶岩型锂矿是锂资源的重要矿产类型,目前随着勘查程度的提高,发现地表出露的伟晶岩型锂矿资源的几率越来越小,下一步的勘查重点应放到找寻隐伏型锂矿资源。近年来,深穿透地球化学勘查手段在隐伏矿勘查中发挥了重要作用。其中土壤微细粒分离测量技术能有效探测深部异常信息,已在多种景观覆盖区进行了试验,并取得了显著的效果。本文以四川甲基卡X03号隐伏锂矿脉为研究对象,利用土壤微细粒分离测量技术,开展深穿透地球化学勘查技术对隐伏锂矿脉探测的试验研究,并分析了成矿及伴生元素组合特征,结果表明：Li、Be、Rb、Cs、Sn、Nb、Ta、Bi、Pb、U等元素具有较好的相关性,富集程度高,异常明显,异常峰值显著;其中,Li-Be-Bi-Cs-Rb-Pb-U元素组合与研究区X03号矿脉成矿元素组合具有良好的耦合性;利用土壤微细粒分离测量技术取得的元素高异常范围与已知隐伏矿体具有较好的对应关系,可以将上述元素作为该区域寻找锂矿的指示元素。以上结果证实了深穿透地球化学勘查技术能有效指示深部隐伏锂矿体,可以将深穿透地球化学勘查技术应用于西部高寒山区景观覆盖区地球化学找矿勘查中。     

Combined ground-penetrating radar (GPR) and electrical resistivity applications exploring groundwater potential zones in granitic terrain

Frequent failures of monsoons have forced to opt the groundwater as the only source of irrigation in non-command areas. Groundwater exploration in granitic terrain of dry land agriculture has been a major concern for farmers and water resource authorities. The hydrogeological complexities and lack of understanding of the aquifer systems have resulted in the failure of a majority of the borehole drillings in India. Hence, a combination of geophysical tools comprising ground-penetrating radar (GPR), multielectrode resistivity imaging (MERI), and vertical electrical sounding (VES) has been employed for pinpointing the groundwater potential zones in dry land agricultural of granitic terrain in India. Results obtained and verified with each other led to the detection of a saturated fracture within the environs. In GPR scanning, a 40-MHz antenna is used with specifications of 5 dielectric constant, 600 scans/nS, and 40 m depth. The anomalies acquired on GPR scans at various depths are confirmed with low-resistivity ranges of 27–50?Ω m at 23 and 27 m depths obtained from the MERI. Further, drilling with a down-the-hole hammer was carried out at two recommended sites down to 50–70 m depth, which were complimentary of VES results. The integrated geophysical anomalies have good agreement with the drilling lithologs validating the MERI and GPR data. The yields of these bore wells varied from 83 to 130 l/min. This approach is possible and can be replicated by water resource authorities in thrust areas of dry land environs of hard rock terrain around the world.     

迭代法在现代地下水年龄估算中的应用——以银川平原为例

为克服应用氚同位素方法确定现代地下水年龄过程中存在的,需确定地下水系统输入项的历史背景浓度问题,本文根据地下水同位素数学物理方程,以全混模型为例,利用不同时期地下水同位素的测试结果,通过数学迭代运算,推导出计算现代地下水年龄的计算公式。该方法不仅可以很好地解决地下水系统输入项氚背景浓度恢复时间序列较长的问题,而且对于提高地下水的定年精度具有十分重要的意义。该方法应用到银川平原取得良好的效果,研究表明银川平原浅层地下水的周转时间为30 a。     

Application of remote-sensing data to groundwater exploration: A case study of the Cross River State, southeastern Nigeria

 The Cross River State, Nigeria, is underlain by the Precambrian-age crystalline basement complex and by rocks of Cretaceous to Tertiary age. The exploration for groundwater in this area requires a systematic technique in order to obtain optimum results, but the non-availability of funds and facilities has made it extremely difficult to carry out site investigations prior to the drilling of water wells. Therefore, the failure rate is as high as 80%. In order to delineate areas that are expected to be suitable for future groundwater development, black and white radar imagery and aerial photographs were used to define some hydrological and hydrogeological features in parts of the study area. Lineament and drainage patterns were analysed using length density and frequency. Lineament-length density ranges from 0.04–1.52; lineament frequency is 0.11–5.09; drainage-length density is 0.17–0.94, and the drainage frequency is 0.16–1.53. These range of values reflect the differences in the probability of groundwater potentials. Results were then used to delineate areas of high, medium, and low groundwater potential. Study results also indicate that correlations exist between lineament and drainage patterns, lithology, water temperature, water conductivity, well yield, transmissivity, longitudinal conductance, and the occurrence of groundwater. Received, August 1994 · Revised, March 1996, June 1996 · Accepted, October 1996     

Alternative water resources in granitic rock: a case study from Kinmen Island,Taiwan

Kinmen Island is a small, tectonically stable, granitic island that has been suffering from a scarcity of fresh water resources due to excessive annual evapotranspiration over annual precipitation. Recent studies further indicate that shallow (0–70 m) sedimentary aquifers, the major sources of groundwater supply, have already been over-exploited. Therefore, this preliminary study is to investigate the existence of exploitable water resources that can balance the shortage of fresh water on this island. Site characterization data are obtained from island-wide geophysical surveys as well as small-scale tests performed in a study area formed by three deep (maximum depth to 560 m) vertical boreholes installed in mid-east Kinmen northeast to Taiwu Mountain. Vertical fracture frequency data indicate that the rock body is fractured with a spatially correlated pattern, from which three major fracture zones (depths 0–70, 330–360, and below 450 m) can be identified. Geologic investigations indicate that the deepest fracture zone is caused by the large-scale, steeply dipping Taiwushan fault. This fault may have caused a laterally extensive low-resistivity zone, a potential fractured aquifer, near Taiwu Mountain. The middle fracture zone is induced by the Taiwushan fault and intersects the fault approximately 21 m southeast of the study area below a depth of 350 m. Slug testing results yield fracture transmissivity varying from 4.8 × 10⁻⁷ to 2.2 × 10⁻⁴ m²/s. Cross-hole tests have confirmed that hydraulic connectivity of the deeper rock body is controlled by the Taiwushan fault and the middle fracture zone. This connectivity may extend vertically to the sedimentary aquifers through high-angle joint sets. Despite the presence of a flow barrier formed by doleritic dike at about 300 m depth, the existence of fresh as well as meteoric water in the deeper rock body manifests that certain flow paths must exist through which the deeper fractured aquifers can be connected to the upper rock body. Therefore, groundwater stored within the Taiwushan fault and the associated low-resistivity zone can be considered as additional fresh water resources for future exploitation.     

Geochemical challenges of diverse regolith-covered terrains for mineral exploration in China

In recent years mineral exploration has concentrated on concealed deposits in regolith-covered terrains. In China, the regolith-covered landscapes mainly include desert windblown sand basins, desert peneplains, semi-arid grassland, loess plateaus, forestry land, alluvial plains and laterite terrains. These diverse regolith-covered areas represent geochemical challenges for mineral exploration in China. This paper provides an overview of recent progress on mechanisms of metal dispersion from the buried ore deposits through the transported cover to the surface and penetrating geochemical methods to detect the anomalies. Case studies show that, in arid and semi-arid desert sand-covered terrains, sampling of fine-fraction (− 120 mesh, < 0.125 mm) clay-rich horizon soil is cost-effective for regional geochemical surveys for sandstone-type uranium, gold, and base metal deposits. Fine-fraction sampling, selective-leaching and overburden drilling geochemical methods can effectively indicate the 210 gold ore body at Jinwozi goldfield. In alluvium-covered terrains, fine-grained soil sampling (− 200 mesh, < 0.074 mm) combined with selective leaching geochemistry shows clear ring-shaped anomalies of Cu and Ni over the Zhouan concealed Cu–Ni deposit. In laterite-covered terrains, the anomalies determined by the fine-fraction soils and selective leaching of absorbed metals on coatings of Fe–Mn oxides coincide well with the concealed deposit over the Yueyang ore deposits at the Zijin Au–Cu–Ag field. Nanoparticles of hexagonal crystals mainly native copper, gold and alloys of Cu–Fe, Cu–Fe–Mn, Cu–Ti, and Cu–Au were observed in gases, soils and ores using a transmission electron microscope (TEM). The findings imply that nanoparticles of gold and copper may migrate through the transported cover to the surface. Uranium is converted to uranyl ions [UO₂^2 +] under oxidizing conditions when migrating from ore bodies to the surface. The uranyl ions are absorbed on clay minerals, because clay layers have a net negative charge, which needs to be balanced by interlayer cations. Nanoparticles of Au and Cu and ion complexes of U are more readily absorbed onto fine fractions of soils containing clays, colloids, oxides and organic matters. Thus, fine-grained soils enriched with clays, oxides and colloids are useful media for regional geochemical surveys in regolith-covered terrains and in sedimentary basins. Fine-fraction soil sampling combined with selective leaching geochemistry is effective for finding concealed ore bodies in detailed surveys. Penetrating geochemistry at surface sampling provides cost-effective mineral exploration methods for delineation of regional and local targets in transported cover terrains.     

Till and bedrock geochemistry in Sn exploration: A case study

The youngest intrusive phases of the rapakivi granites are known as Sn granites. The endocontact and exocontact zones of the Viipuri rapakivi massif and its satellites have been considered on theoretical grounds as critical areas for Sn deposits. The area between the Viipuri rapakivi massif and its satellite, the Ahvenisto massif, was selected for prospecting as one Sn critical area.Heavy-mineral geochemistry was used in the reconnaissance phase. Samples (10 1) from the surficial part of the till deposits were taken from a sparse sampling grid. Heavy minerals of the samples were concentrated by washing on a riffle box and by subsequent heavy-liquid separation. The non-magnetic heavy fraction was analyzed by XRF. Quite a large and coherent glaciogenic Sn anomaly was found, the head of which terminated in the southern part of the Ahvenisto rapakivi massif.The source of the anomaly was sought by taking basal till and bedrock samples with a tractor-mounted percussion drill. The fine fraction of till was analyzed by AAS and the bedrock samples by XRF and AAS. Mainly on the basis of Sn and Cu anomalies in basal till and bedrock 10 holes were diamond drilled into bedrock. Several topaz-bearing greisen bodies with uneconomic Sn content were penetrated. The results demonstrate the soundness of the theory and the methods used.     

Monazite behaviour and age significance in poly-metamorphic high-grade terrains: A case study from the western Musgrave Block, central Australia

Integrated, in situ textural, chemical and electron microprobe age analysis of monazite grains in a migmatitic metapelitic gneiss from the western Musgrave Block, central Australia has identified evidence for multiple events of growth and recrystallisation during poly-metamorphism in the Mesoproterozoic. Garnet + sillimanite-bearing metapelite underwent partial melting and segregation to palaeosome and leucosome during metamorphism between 1330 and 1296 Ma, with monazite grains in leucosome recording crystallisation at 1300 Ma. Monazite breakdown during melting is inferred to have occurred in the palaeosome. During a subsequent granulite facies event at 1200 Ma, deformation and metamorphism of leucosome and palaeosome resulted in partial disturbance of ages and potential minor growth on 1300 Ma monazite in leucosome. Growth of new, high-Y (+HREE) monazite in palaeosome domains occurred during garnet breakdown in the presence of sillimanite to cordierite and spinel, as a result of post-peak isothermal decompression. Diffusive enrichment of resorbed garnet rims in Y + HREE suggests garnet breakdown occurred slower than volume diffusion of REE. Monazite in both palaeosome and leucosome were subsequently partially to penetratively recrystallised during a retrogression event that is suggested to have occurred at 1150–1130 Ma. The intensity of recrystallisation and disturbance of ages appears linked to proximity to retrogressed garnet porphyroblasts and their occurrence in the relatively reactive or ‘fertile’ local environments provided by the palaeosome/mesosome volumes, which caused localised changes in retrogressive fluids towards compositions more aggressive to monazite. Like reaction textures, it is apparent that domainal equilibrium and reaction may control or at least strongly influence monazite REE and U–Th–Pb chemistry and hence ages.     

Biogeochemical sampling for mineral exploration in arid terrains: Tanami Gold Province,Australia

Vegetation sampling is an effective mineral exploration technique in areas of transported cover in the Tanami Gold Province where other techniques have been of limited success. This research tests the ability of plants to show signatures of mineralisation as well as the optimum scale of sampling for first-pass mineral exploration surveys. The semi-arid Tanami Gold Province in northern Australia encompasses nearly 160,000 km² and is a highly prospective yet under-explored region due to transported cover masking mineralisation. Various dominant plant species were sampled along transects across four sites of Au mineralisation in the Tanami (Larranganni, Hyperion, Coyote and Titania). Snappy gum (Eucalyptus brevifolia) gave a geobotanical (plant distribution) and lithological (S and Zn) signature of an underlying geological structure known to host Au mineralisation at the Coyote Prospect. Soft spinifex (Triodia pungens) provided an Au, As, ± Zn, ± S, and ± Ce expression of buried Au mineralisation at Coyote and Titania. At the Hyperion prospect, mineralisation is located at the contact between granite and dolerite, and biogeochemical signatures from snappy gum and dogwood (Acacia coriacea) show elevated ± Au, Ce, S and Zn that corresponded to the contact. Biogeochemistry is able to determine the location of mineralisation at each site, except at Hyperion where the sample density was too low. It is able to identify underlying substrate differences, however, background knowledge relating to regolith, geology, hydrology and geophysics are important in aiding the interpretation of the elemental data. It was found that having too few samples (at Hyperion) there was insufficient useful information for mineral exploration. Grid coverage of an area (at Titania) provided information on mineralisation and groundwater dispersion plumes; however, in this case a single transect over the same area would have been sufficient for a first-pass mineral exploration survey.     

Exploration of underground strata conditions for a traffic bypass tunnel using ground penetrating radar system – a case study

A ground-penetrating radar (GPR) survey was carried out in connection with the construction of a traffic bypass tunnel between Saraikale Khan and Dr. Zakir Hussain Marg in Delhi. Five boreholes were drilled in this area to correlate and confirm the GPR signatures with borehole data. This Note covers the correlation of the borehole data with GPR signature to explore the real subsurface strata conditions for the construction of the bypass tunnel.     

A numerical scheme for groundwater development in a watershed basin of basement terrain: a case study from India

The development of watershed basins to increase groundwater recharge potential is becoming a major issue in India due to an acute shortage of groundwater resources, resulting from the marked expansion of land-use activities and the explosive growth in population. It is necessary to study the regional characteristics in order to identify potential artificial groundwater recharge zones. A combination of morphometric analysis coupled with hydrogeological information is used to prepare a generalized scenario for watershed development plans. A numerical scheme is, thus, proposed for the relative evaluation of surface rock-permeability in relation to morphometry (stream order, stream length, drainage density, channel maintenance, overland flow, basin shape, etc.). An attempt is made, from the morphometrical studies of the Varaha watershed of the Precambrian Eastern Ghats basement terrain in Eastern India, to illustrate how the numerical scheme is helpful as a tool in watershed development planning programs. This method involves the designation of various recharge-related measures, based upon the relative ranking of surface-material permeability after comparison with the hydrogeological conditions of sub-basins of the river basin. The scheme can also help to pin-point areas of study on a local scale, and thus facilitate developmental programs to augment groundwater recharge.     

The petrogenetic characterization of intermediate and silicic charnockites in high-grade terrains: a case study from southern India

Large charnockite massifs occur in some of the Precambrian high-grade terrains like the southern Indian granulite terrain. The Cardamom Hill charnockite massif from the Madurai Block, southern India, consists of an intermediate type and silicic type, with the intermediate type showing similarities to high-Ba−Sr granitoids with low K₂O/Na₂O ratios and the silicic type showing similarities to high-Ba–Sr granitoids with high K₂O/Na₂O ratios. Within the constraints imposed by near basaltic composition of the most mafic samples and their relatively high concentrations of both compatible and incompatible elements, comparison with recent experimental studies on various source compositions, and trace- and rare-earth-element modeling, the distinctive features of the intermediate charnockites can be best explained in terms of assimilation–fractional crystallization (AFC) models involving interaction between a mantle-derived basaltic magma and lower crustal materials. Silicic charnockites on the other hand are high temperature melts of moderately hydrous basaltic magmas. A two-stage model which involves an initial partial melting of hydrous basaltic magma and later fractionation explains the geochemical features of the silicic charnockites, with the fractionation stage most probably an open system AFC. It is suggested that for massifs showing spatial association of intermediate and silicic charnockites, a model taking into account their compositional difference in terms of the effect of variations in the conditions (e.g., temperature, water fugacity) that prevailed, can account for plausible petrogenetic scenarios.