Geochemical Prospecting Techniques Research Articles

Geochemical modeling and low-frequency geoelectrical methods to evaluate the impact of an open dump in arid and deltaic environments

In Mexico, open dumps that are maintained by the municipality but provide no covering of waste are not uncommon. Further, disposal at these sites is often performed by burning. The aim of this study was to determine the leachate plume from an open dump located in a depositional deltaic environment, with arid climate, low rainfall and where the water table is about 2 m below the surface. The methodology comprised analysis of groundwater monitoring wells and geophysical and geochemical prospecting techniques. The 3D geoelectric interpretation shows a typical area of these depositional environments with low resistive values (10–20 Ω-m) associated with the presence of sands and clays interbedded. However, there is a very low resistivity zone associated with the dump’s impact which reaches values below 5 Ω-m, and it is located where the disposal and burning of wastes occurred. Another zone with values above 16 Ω-m appears as a limit for the advance of the saline. This is interpreted as a sandy lenses area. These sandy lenses with higher porosity transport aquifer’s water. Thus the dump is in direct contact by this conduct with clean groundwater. Piper diagrams constructed with the chemical data analysis reveal that the groundwater in the area corresponds to the chlorinated and/or sulfated sodium type, showing the impact caused by the dump. The monitoring well (NP8, on-site) with the highest dissolved solids content behaves anomalously and belongs to the more conductive zone according to the geoelectric profiles. The subsoil geochemical behavior is influenced by the seasonal water table variations provoking the dissolution of burned and unburned wastes, but the effects of slow flows in the direction of the regional flow are not discarded. Although the most impacted area within the dump is characterized to a depth of 10 m, there is a slow flow in the direction of the regional flow that has been moving pollutants out of the dumpsite during its almost 20 years of operation. The results of this study provide valuable information for the authorities to carry out an appropriate restoration project.

Geologic remote sensing for geothermal exploration: A review

This paper is a comprehensive review of the potential for remote sensing in exploring for geothermal resources. Temperature gradients in the earth crust are typically 25–30°C per kilometer depth, however in active volcanic areas situated in subduction or rift zones gradients of up to 150°C per kilometer depth can be reached. In such volcanic areas, meteoric water in permeable and porous rocks is heated and hot water is trapped to form a geothermal reservoir. At the Earth's surface hot springs and fumaroles are evidence of hot geothermal water. In low enthalpy systems the heat can be used for heating/cooling and drying while in high enthalpy systems energy is generated using hot water or steam. In this paper we review the potential of remote sensing in the exploration for geothermal resources. We embark from the traditional suite of geophysical and geochemical prospecting techniques to arrive at parameters at the Earth surface that can be measured by earth observing satellites. Next, we summarize direct and indirect detection of geothermal potential using alteration mineralogy, temperature anomalies and heat fluxes, geobotanical anomalies and Earth surface deformation. A section of this paper is dedicated to published remote sensing studies illustrating the principles of mapping: surface deformation, gaseous emissions, mineral mapping, heat flux measurements, temperature mapping and geobotany. In a case study from the La Pacana caldera (Chili) geothermal field we illustrate the cross cutting relationships between various surface manifestations of geothermal reservoirs and how remotely sensed indicators can contribute to exploration. We conclude that although remote sensing of geothermal systems has not reached full maturity, there is great potential for integrating these surface measurements in a exploration framework. A number of recommendations for future research result from our analysis of geothermal systems and the present contributions of remote sensing to studying these systems. These are grouped along a number of question lines: ‘how reproducible are remote sensing products’, ‘can long term monitoring of geothermal systems be achieved’ and ‘do surface manifestations link to subsurface features’?

Geochemical characterization of adsorbed light gaseous alkanes in near surface soils of the eastern Ganga basin for hydrocarbon prospecting

This study aims to assess the hydrocarbon potential of Ganga basin utilizing the near surface geochemical prospecting techniques. It is based on the concept that the light gaseous hydrocarbons from the oil and gas reservoirs reach the surface through micro seepage, gets adsorbed to soil matrix and leave their sig� natures in soils and sediments, which can be quantified. The study showed an increased occurrence of meth� ane (C1), ethane (C2) and propane (C3) in the soil samples. The concentrations of light gaseous hydrocarbons determined by Gas Chromatograph ranged (in ppb) as follows, C1: 0-519, C2: 0-7 and C3: 0-2. The carbon isotopic (VPDB) values of methane varied between -52.2 to -27.1‰, indicating thermogenic origin of the desorbed hydrocarbons. High concentrations of hydrocarbon were found to be characteristic of the Muzaf� farpur region and the Gandak depression in the basin, signifying the migration of light hydrocarbon gases from subsurface to the surface and the area's potential for hydrocarbon resources.

Early years in the Geochemical Prospecting Research Centre, Imperial College of Science and Technology, London: exploration geochemistry in Zambia in the late 1950s; a personal recollection

ABSTRACT Professor John S. Webb of the Department of Mining Geology at the Royal School of Mines, Imperial College, London, took his first PhD student (Tony Millman) to Africa in 1949 to try some of the just-emerging geochemical prospecting techniques. In 1955 I joined the newly-formed Geochemical Prospecting Research Centre in the Department as a PhD student. I too was despatched to Africa to do geochemical prospecting research in the Zambian Copperbelt. This paper is a much-abbreviated account of a detailed exploration geochemical drainage and soil investigation of the dispersion of Cu from the copper-cobalt ore bodies using the relatively primitive analytical techniques of the period. In addition to some science, an attempt is made to give a flavour of the time and place through anecdotes and a description of the way of life. The paper ends with a short account from a current PhD student at Adelaide University of some contemporary exploration geochemical work in Africa and northern Australia.

Molecular Geomicrobiology.: J. F. BANFIELD, J. CERVINISILVA, AND K. M. NEALSON, EDITORS. 294 Pp. Mineralogical Society of America, Reviews in Mineralogy and Geochemistry. Volume 59. 2005. ISBN: 093995071-5. US$40.00.

The field of geomicrobiology is rapidly expanding as both geoscientists and biologists have come to recognize the importance of both disciplines for understanding nature’s complexities. Bacteria and other microorganisms are extremely important in their direct and indirect effects on geochemical cycling of metals and organic matter throughout Earth history. Indeed, classically trained microbiologists are currently being hired by geoscience departments, which will ensure future beneficial collaborations in the field of geomicrobiology. Southam and Saunders (2005) reviewed much of the recent geomicrobiology literature in Economic Geology , trying to highlight important advances that might impact ore-forming processes and, perhaps, even geochemical prospecting techniques. No doubt, bacteria are directly responsible for forming a number of types of low-temperature ore deposits such as roll-front uranium deposits and Superior-type iron formations. In addition, bacteria have produced reduced sulfur for some types of higher-temperature deposits such as sediment-hosted Zn-Pb deposits. Bacteria have even been found as fossils in sulfide minerals. Thus, there are many aspects of bacterial metabolism that warrant understanding by economic geologists. Furthermore, as we explore for more deeply buried mineral deposits below the water table, exploration geochemists are going to have to factor in the effects of microbial processes in controlling dispersion of metals away from ores …

Proposed method of hydrogeochemical exploration for salt deposits using Cl [sbnd]Br ratios, Eastern Province, Kingdom of Saudi Arabia

Despite the value of the salt (NaCl) and brine used by the chemical industry, geochemical prospecting techniques are not customarily employed in the search for these raw materials. In this study, Br geochemistry is used as the basis for a proposed hydrogeochemical prospecting technique that was designed to search for shallow halite beds in the Eastern Province of the Kingdom of Saudi Arabia. Near-surface brine samples were collected at Sabkhah Jayb Uwayyid, both directly above and distant from a buried salt bed. Brine samples collected both directly above and offset to the north-west of the salt bed had Cl Br ratios > 8000. The regional background Cl Br ratio of fresh nonmarime ground water is ≈300. The large range in Cl Br ratios and the association of high Cl Br ratios with the buried salt body suggest that the ratio can be useful in hydrogeochemical prospecting for sibakh-associated, shallow salt bodies.

Relation between shallow sediment bitumens and deeper reservoired hydrocarbons, offshore Santa Maria basin, California, U.S.A.

Detailed molecular and isotopic analysis of near surface liquid hydrocarbon seepage is useful not only in determining the presence of petroleum hydrocarbons but also to indicate the type and quality of deeper seated hydrocarbons. Near surface sediment bitumens collected over known production in the offshore Santa Maria basin were shown to be similar to reservoired petroleum in the adjacent onshore. Triterpane, sterane and aromatized sterane compositional similarities between oils and near surface sediment bitumens suggest a common source in the Monterey Formation. An empirical model based on the relation between oil composition (pristane/phytane and bisnorhopane/hopane ratios) and API gravity was applied to the near surface bitumens and accurately predicted the API gravity of the nearest known production. The improvement and refinement of geochemical prospecting techniques has greatly enhanced their use and reliability as exploration tools.

Surface geochemical methods used for oil and gas prospecting — a review

The majority of the world's oil and gas deposits have been discovered by drilling in the vicinity of natural petroleum seeps, and to date the most successful geochemical prospecting methods still rely upon the surface detection of hydrocarbons. Gas chromatographic techniques are now commonly used in the analysis of hydrocarbon gases for prospecting both onshore (analysis of soils and rocks) and offshore (analysis of near-bottom waters and sediments). Detection of helium fluxes has been partially successful as a geochemical prospecting technique. Many indirect techniques such as the determination of isotope and metal-leaching anomalies in surface rocks and the measurement of radon fluxes have not been widely used. Onshore geochemical prospecting appears to have more problems associated with it than offshore prospecting due to the more complex migration mechanism of near-surface waters containing dissolved gases. No onshore prospecting studies have been published which thoroughly consider this factor and the success of onshore prospecting remains equivocal. In offshore prospecting “sniffers” have been used to detect hydrocarbon anomalies in near-bottom waters, and coring equipment has been used for the detection of hydrocarbons in near-surface sediments. Success is claimed using these techniques. Geochemical prospecting methods are complementary to the widely used geophysical methods. Geochemical methods can provide direct evidence for the presence of petroleum accumulations and are relatively cheap and rapid. Failures in prospecting to date are attributable to the simplistic manner in which data have been interpreted; insufficient attention has been paid to the hydrological and geological factors which modify the upward migration of indicator species to the surface. As oil and gas deposits become more difficult to locate, greater attention should be paid to geochemical prospecting techniques, especially as a regional exploration tool.

Trend-surface analysis in geochemical prospecting data, Arouca-Castro Daire region, northern Portugal

The mineral potential of the Arouca-Castro Daire area in the north of Portugal was studied, using geochemical prospecting techniques, with the particular objective of discovering new occurrences of Sb which, in general, are associated with Au. Regional geochemical prospecting of the area was carried out by means of systematic sampling of stream sediments and subsequent analysis for Sb. Detailed geochemical studies made later in an anomalous area and including soil sampling, confirmed its potential interest in terms of possible Sb mineralization. The geochemical data obtained were treated by univariate statistical-mathematical techniques (mainly computer-orientated), which proved to be of great importance in terms of prospecting in making a reliable interpretation of the results obtained. Particular emphasis is given to the application of trend-surface and moving-average analysis in the interpretation of the geochemical data previously obtained.

Techniques of geochemical exploration in the southeast Piedmont of the United States

The Piedmont of the southeastern United States is a region that is peculiarly well suited to applications of geochemical prospecting techniques, although skill and care are required in interpretation of results. The Piedmont is a region of low to moderate relief and sluggish drainage covered by a thick blanket of residual soil that effectively obscures conventional geologic observations. Bedrock consists of igneous and metamorphic sequences that are derived from late Precambrian to early Paleozoic eugeosynclinal volcanic and sedimentary rocks. Contained ore deposits include a variety of types typical of such an environment, and while past production has not been large, the geologic potential of the region is promising for discovery of hidden deposits. Geochemical prospecting techniques have proven to be valuable in this environment. The techniques described in this paper are the result of about 40 man-years of exploration in the Piedmont spanning about 18 years of elapsed time. Techniques were initially developed by trial and error and were then considerably improved by study of the physiographic and geochemical conditions prevalent in the region. Stream sediment techniques have not proven to be universally effective because of well-leached character of the residual soils, shallow water table and ever present problems of contamination due to population density. Geochemical sampling by analysis for total metal of residual soils, when properly interpreted, has proved to be an effective and valuable prospecting aid. Soil prospecting techniques have been used successfully for evaluation of a variety of geologic types of exploration targets. Soil sampling techniques have been used in evaluating base-metal-bearing massive sulfide targets. Several prospects are known in the Piedmont of massive sulfide deposits in eugeosynclinal volcano-sedimentary strata that are similar to other Appalachian deposits. Geochemical sampling has effectively delineated known target areas at such prospects, although no new deposits have been found. Soil sampling has also been effective in delineating and evaluating disseminated Cu-Mo or “porphyry” targets. Several such occurrences in mid-Paleozoic quartz monzonite intrusions have been discovered by geochemical prospecting, although none have proved to be commercial. Gold deposits have also been explored and evaluated by soil sampling. Problems of behavior of gold in the residual soils have not been entirely solved, but direct analysis of such soils for gold appears to effectively delineate auriferous areas. Disseminated or “no-see-um” gold deposits, if they exist in the Piedmont, should readily be detected by soil analyses. Soil sampling has also been proved effective in evaluating nickeliferous areas for both sulfide and laterite ore potential in areas of ultramafic rocks.

Geochemical prospecting for porphyry copper-type mineralization — A review

During the past 25 years, geochemical prospecting techniques have been applied in the search for porphyry copper-type mineralization in many parts of the world. This review lists several important porphyry copper deposits which were discovered, or their potential indicated, by the application of geochemical exploration methods. In addition, a discussion is presented of selected case histories and research illustrating the effectiveness of the use of stream sediments, soils, water, rock, vegetation and other sample media for porphyry copper exploration in a wide variety of environments. Continuing studies with SO 2, the halogens, rhenium, mercury, precious metals, the composition of fluid inclusions and the major elements associated with alteration are expected to lead to the identification of additional new techniques which will be of value in future exploration. Gamma-ray spectrometry and infrared spectroradiometry show considerable promise for airborne or satellite survey. Other remote sensing and atmogeochemical techniques which may be applicable to the reconnaissance for porphyry copper-type mineralization are being studied or are in the development stage. Experience to date clearly indicates that geochemical methods have application at all stages in the exploration for and development of porphyry copper-type deposits. Anticipated future developments are expected to increase and improve this application.

Geochemical prospecting at Cobalt, Ontario

A program of geochemical prospecting was carried out in the summer of 1952 near Cobalt, Ontario. The area in which this work was done lies in the Precambrian shield of Canada, about 320 miles north of Toronto. Much of the area is covered by glacial deposits. The known ore bodies are predominantly thin veins of silver, and nickel and cobalt arsenides found as mesothermal fissure fillings in vertical faults and joints of the area. The cobalt concentration in samples of glacial material, collected on a horizontal and vertical grid over an area of proven mineralization at depth, was determined in order to test the applicability of geochemical prospecting techniques in locating ore bodies in similar situations. A cobalt anomaly in the overburden was located; subsequent drilling proved up mineralization in non-commercial quantities in the bedrock. It is concluded that the present technique is applicable in locating ore bodies where mineralization extends to the bedrock surface in geologic settings similar to those in the Cobalt, Ontario district.