Geochemical Exploration Surveys Research Articles

Pb-Nd-Sr Isotope Geochemistry of Metapelites from the Iberian Pyrite Belt and Its Relevance to Provenance Analysis and Mineral Exploration Surveys

Abstract The Iberian Pyrite Belt is a world-class metallogenic district developed at the Devonian-Carboniferous boundary in the Iberian Variscides that currently has seven active mines: Neves Corvo (Cu-Zn-Sn) and Aljustrel (Cu-Zn) in Portugal, and Riotinto (Cu), Las Cruces (Cu), Aguas Teñidas (Cu-Zn-Pb), Sotiel-Coronada (Cu-Zn-Pb), and La Magdalena (Cu-Zn-Pb) in Spain. The Iberian Pyrite Belt massive sulfide ores are usually hosted in the lower sections of the volcano-sedimentary complex (late Famennian to late Visean), but they also occur in the uppermost levels of the phyllite-quartzite group at the Neves Corvo deposit, stratigraphically below the volcano-sedimentary complex. A Pb-Nd-Sr isotope dataset was obtained for 98 Iberian Pyrite Belt metapelite samples (from Givetian to upper Visean), representing several phyllite-quartzite group and volcano-sedimentary complex sections that include the footwall and hanging-wall domains of ore horizons at the Neves Corvo, Aljustrel, and Lousal mines. The combination of whole-rock Nd and Sr isotopes with Th/Sc ratios shows that the siliciclastic components of Iberian Pyrite Belt metapelites are derived from older quartz-feldspathic basement rocks (–11 ≤ εNdinitial ≤ –8 and (87Sr/86Sr)i up to 0.727). The younger volcano-sedimentary complex metapelites (upper Tournaisian) often comprise volcanic-derived constituents with a juvenile isotopic signature, shifting the εNdi up to +0.2. The Pb isotope data confirm that the phyllite-quartzite group and volcano-sedimentary complex successions are crustal reservoirs for metals found in the deposits. In Neves Corvo, where there is more significant Sn- and Cu-rich mineralization, the higher (206Pb/204Pb)i and (207Pb/204Pb)i values displayed by phyllite-quartzite group and lower volcano-sedimentary complex metapelites (up to 15.66 and 18.33, respectively) suggest additional contributions to the metal budget from a deeper and more radiogenic source. The proximity to Iberian Pyrite Belt massive sulfide ore systems hosted in metapelite successions is observed when (207Pb/204Pb)i >15.60 and Fe2O3/TiO2 or (Cu+Zn+Pb)/Sc >10. These are important criteria that should be considered in geochemical exploration surveys designed for the Iberian Pyrite Belt.

Ứng dựng nghiên cứu địa hóa trầm tích nông trong tìm kiếm thăm dò dầu khí trên biển và thềm lục địa Việt Nam

Nghiên cứu địa hóa bề mặt đã được ứng dụng trong tìm kiếm thăm dò dầu khí hơn 100 năm qua và là một công cụ hữu dụng để giảm thiểu chi phí, giảm thiểu rủi ro trong công tác tìm kiếm thăm dò dầu khí hiện nay. Một số nghiên cứu khảo sát địa hóa trầm tích nông được các nhà thầu thực hiện tại các lô hợp đồng dầu khí trên biển và thềm lục địa Việt Nam đều có kết quả tốt phục vụ cho công tác định hướng tìm kiếm thăm dò.
 Bài báo giới thiệu kết quả ứng dụng nghiên cứu địa hóa trầm tích nông trong tìm kiếm thăm dò dầu khí trên biển và thềm lục địa Việt Nam.

Characterizing and Indexing Regolith Materials Using Geochemistry towards Hidden Mineral Anomaly Delineation: A Case Study of Savannah Region of NW Ghana

As the discovery rate of world-class gold deposits continues to decline, increased attention is focused on geochemical exploration methods designed for regolith-dominated terrains. This involves mapping the regolith and classifying the mapping units into different regolith classes on the basis of weathering and geomorphic histories. The challenges of identifying some regolith mapping units in the field by novice in regolith mapping require the characterization and indexing of regolith from major element geochemistry in the regolith profiles. XRF analytical method was used to measure the weight % of the major oxides in regolith samples. The metal weight % of Mg, K and Al were calculated from their oxides and were normalized relative to immobile Al calculated from its oxide. The plot of Mg/Al and K/Al identified the regolith of the study area to consist of 137 transported clays, 4 ferruginous sediments or ferricrete, 2 lateritic duricrust and 4 saprolites. Plot of Mg/Al and K/Al highlighted the compositional variability of the regolith samples and refute the notion of the homogeneity of all the sampled materials in the area. The study thus recognized Mg/Al versus K/Al plots to be used in supporting field identification of regolith mapping units particularly in complex regolith terrains of savannah regions of Ghana and in similar areas where geochemical exploration surveys are being carried out under cover.

Unravelling regolith material types using Mg/Al and K/Al plot to support field regolith identification in the savannah regions of NW Ghana, West Africa

The XRF analytical method was used to measure the weight % of the major oxides in regolith samples. The metal weight % of Mg, K and Al were calculated from their oxides and were normalised relative to immobile Al calculated from its oxide. The plot of Mg/Al and K/Al identified the regolith of the study area to consist of 137 transported clays, 4 ferruginous sediments or ferricrete, 2 lateritic duricrust and 4 saprolites. Surface regolith that had undergone secondary transformation and shows compositional overlaps were 4 transported clays with Fe-oxide impregnation may be referred to as nodular laterite and 5 ferruginous saprolites. The variable regolith materials features identified from the 154 samples enabled the characterisation and identification of the different sample materials because an overprint of bedrock geochemistry is reflected in the regolith. Plot of Mg/Al and K/Al highlighted the compositional variability of the regolith samples and refute the notion of the homogeneity of all the sampled materials in the area. The study thus recognized Mg/Al versus K/Al plots to be used in supporting field identification of regolith mapping units particularly in complex regolith terrains of savannah regions of Ghana and in similar areas where geochemical exploration surveys are being carried out under cover.

Assessment of the potential occurrence of acid rock drainage through a geochemical stream sediment survey

During large constructions of roads or structures, unexpected acid rock drainage (ARD) can be caused by local mineralization containing sulfides in the geology. The potential of ARD occurrence of a certain area sometimes must be assessed before initiation of any engineering earth works. However, it is difficult to assess the entire area through collecting rock samples and predicting the potential by laboratory tests, such as the acid–base accounting method. In this study, a new prediction protocol using a geochemical exploration survey technique of stream sediment is proposed. Sediment samples were collected at the case study area where a large development is expected in the future, and the contents of some major and heavy metal elements were compared according to the major geologies of the sampling points. The modified geoaccumulation indices (Igeo) of Fe, Pb and As could indicate a possible zone of pyrophyllite mineralization, which may cause the occurrence of ARD at the study area. Using the enrichment index of the three elements relative to the median values of the area, a high potential zone of ARD could be designated, which was in agreement with the laboratory ARD prediction tests of the rock samples. In the other areas with different mineralization processes, other metallic elements can be selected as indicators of the ARD potential. Likewise, the potential of the occurrence of ARD at an area can be assessed by evaluating the geochemical distributions and drawing the indicator elements for ARD through a stream sediment survey.

Geochemical associations and their spatial patterns of variation in soil data from the Marrancos gold–tungsten deposit: a pilot analysis

ABSTRACT This paper discusses the results of a soil geochemical exploration survey in the environs of the Marrancos gold–tungsten deposit, dealing with 144 samples of topsoil, analysed for 53 chemical elements. At this initial stage of the project the main objectives were: (1) to identify associations between chemical elements; (2) to estimate spatial patterns of variation for such associations in the surficial materials. The associations were established using principal component analysis (PCA). Variography was used to obtain the theoretical model of spatial continuity of each variable. These models were used in the interpolation method (kriging) selected to estimate surficial dispersion patterns. The results of PCA and PCA mapping indicate distinct distribution patterns for the metals related to the primary mineralogical or geochemical association, as identified for this deposit. Based on the results, six of the 53 chemical elements studied were selected to estimate spatial patterns of dispersion in the topsoil. This approach allows the recognition of the geochemical signature of the deposit, and the selection of several target areas for follow-up work. Several sites were selected for water sampling (surficial and groundwater), and the soil samples to be used in partial extractions are now defined.

Surface geochemical exploration for oil and gas: New life for an old technology

Editor's Note: The Geologic Column, which appears monthly in TLE , is (1) produced cooperatively by the SEG Interpretation Committee and the AAPG Geophysical Integration Committee and (2) coordinated by M. Ray Thomasson and Lee Lawyer. Surface geochemical exploration for petroleum is the search for chemically identifiable surface or near-surface occurrences of hydrocarbons, or hydrocarbon-induced changes, as clues to the location of oil and gas accumulations. It extends through a range of observations from clearly visible oil and gas seepage (macroseepage) at one extreme to identification of minute traces of hydrocarbons (microseepage) or hydrocarbon-induced changes at the other. Surface geochemical methods have been used since the 1930s, but the past decade has seen a renewed interest in geochemical exploration. This, together with developments in analytical and interpretive methods, has produced a new body of data and insights about geochemical exploration. Many of these developments are summarized in “Hydrocarbon Migration and Its Near-Surface Expression” ( AAPG Memoir 66 ). Geochemical surveys and research studies document that hydrocarbon microseepage from oil and gas accumulations (1) is common and widespread, (2) is predominantly vertical (with obvious exceptions in some geologic settings), and (3) is dynamic (responds quickly to changes in reservoir conditions). The principal objective of a geochemical exploration survey is to establish the presence and distribution of hydrocarbons in the area and, more importantly, to determine the probable hydrocarbon charge to specific exploration leads and prospects. For reconnaissance surveys, seeps and microseeps provide direct evidence that thermogenic hydrocarbons have been generated; that is, they document the presence of a working petroleum system and identify the portions of the basin that are most prospective. Additionally, the composition of these seeps can indicate whether a basin or play is oil-prone or gas-prone. If the objective is to evaluate individual exploration leads and prospects, the results of …

Abstract: Surface geochemical exploration surveys identify active petroleum systems in deepwater Angola

Abstract: Surface geochemical exploration surveys identify active petroleum systems in deepwater Angola

Abstract: Integrated Surface/Subsurface Geochemical Exploration Surveys Identify Active Petroleum System in Deepwater Angola

Abstract: Integrated Surface/Subsurface Geochemical Exploration Surveys Identify Active Petroleum System in Deepwater Angola

Enrichment of metals in the organic surface layer of natural soil: identification of contributions from different sources

The humus layer of natural soils is frequently used as a sampling medium in geochemical exploration surveys. This procedure has been questioned because the concentration level in the humus layer does not necessarily reflect the geochemistry in the underlying mineral material. Four sources of metals to the organic surface layer are identified: (1) bioturbation; (2) the ‘vascular pump’; (3) atmospheric input of marine origin; and (4) airborne pollution. Two methods to define and discuss the contributions from these sources are presented for a survey of Norwegian soils. The first method is based on calculation of ratios of concentrations between humus and undisturbed subsoil within regions of similar topography, climate and air pollution. Concentration ratios consistently below 0.1 and low inter-region variation, such as for Al and Fe, characterize elements supplied to the humus by mixing-in of mineral soil, while concentration ratios appreciably higher than 0.1 identify some additional mechanism. The latter is evident for elements such as Pb, Zn and Cd (air pollution), Sr and Mg (atmospheric input of sea-salt), and Ca (the ‘vascular pump’). The second method involves subtraction of the contributions from mineral matter, assuming that the ash content reflects the mass, and C-horizon soil the composition of the mineral matter in the humus layer. As mineral grains mixed into the humus layer are often fairly strongly weathered compared with the undisturbed mineral soil, this method will result in a certain over-compensation; nevertheless it leads to some interesting conclusions as is shown for Ba (the ‘vascular pump’), and Zn and V (long-range transport and local air pollution).

Evaluation of geochemical data acquired from regular grids

Geochemical data obtained during mineral exploration often are biased by systematic as well as random errors; these may result in failures when usual methods of evaluation are used. This is true particularly in soil surveys carried out in regions where a long history of prospecting and mining activity has occurred and/or where aerial chemical pollution is likely to have occurred. A satisfactory evaluation of geochemical data even in such an unfavorable case requires sampling on a relatively dense grid and utilization of all available knowledge of types of mineralization. The evaluation procedure proposed consists of five consecutive phases: (1). Dividing the area of interest into subareas of a relatively homogeneous geological nature. (2). Processing by multivariate methods (factor analysis, in particular) without consideration of geographic relations. (3). A preliminary interpretation and search for a geochemical explanation of factors. (4). Processing of individual factors in two-dimensional geographic space by directional and frequency linear filtering methods. (5). Final interpretation and construction of a geochemical model. The procedure is illustrated by an example from a geochemical exploration survey in the vicinity of Přibram (Middle Bohemia).

Geochemical dispersion patterns associated with the Lake Yindarlgooda sulphide mineralization, Western Australia

Geochemical investigations have been carried out in the Lake Yindarlgooda area, 40 km east of Kalgoorlie, Western Australia, where sulphide deposits and black shales occur in a sequence of metavolcanics and metasediments of the Eastern Goldfields greenstone belt. The studies were made in order to understand the processes of element migration and the formation of dispersion patterns in semi-arid regions having partly lateritic weathering conditions. Outcrop exposure in the vicinity of the mineralized zones is limited to the siliceous caprock and gossans. Mineralogical data, and geochemical data on the distribution of Cu, Ni, Co, Zn, Pb and other elements, have been obtained from unweathered and weathered rocks, and different horizons of the overlying soil in the area surrounding the mineralized zones of Lake Yindarlgooda, Scotia and Ringlock. The results indicate, in principle, two different kinds of weathering environment: (1) playa lakes, with the development of solonchaks; and (2) Tertiary peneplains outside the playa lakes, with lateritic soils, solonized brown soils or desert loams. The trace element distribution patterns in the solonchaks are determined only by the initial element content of the alluvial sediments. Therefore, sampling of solonchaks generally cannot be recommended for geochemical exploration surveys, but sampling of weathered bedrock below the solonchaks is highly recommended in the playa lake environment. By sampling of the bedrock, the non-outcropping continuation of the Lake Yindarlgooda sulphide body has been precisely delineated. In areas of the Tertiary peneplains outside the playa lakes, lateritic soils, solonized brown soils or desert loams cover the weathered bedrock. Comparative geochemical investigations in these areas in the vicinity of the nickel sulphide deposits at Scotia and Ringlock have shown that the migration of elements in solution is restricted, a result of the very low water table. In this area the main process of dispersion is mechanical. Therefore, the secondary dispersion haloes of Scotia and Ringlock are narrow, restricted to a few tens of metres, and sampling at closer intervals is needed to locate the orebodies.

Groundwater in Andesitic Areas in Japan: ABSTRACT

The hydrogeologic and geohydrologic studies of volcanoes and volcanic islands in Japan are carried on by the author and his collaborators by field geologic, geophysical and geochemical exploration survey methods, test drilling, and aquifer tests. The main purpose of these studies is to find groundwater resources at an altitude higher than the spring zone of volcanoes. Stress is laid on two points: (1) to establish a simulated grid drilling program that deals with the groundwater valley, and (2) to clarify relation between End_Page 1462------------------------------ the occurrence of springs and character of volcanic rock. End_of_Article - Last_Page 1463------------

Secondary dispersion patterns of fluoride in the osor area, Province of Gerona, Spain

The distribution of fluoride in spring and stream waters, stream sediments and soils of the Osor district, Province of Gerona, northeastern Spain, has been studied. Fluoride was determined by use of a fluoride-sensitive electrode. This potentiometric method has been found useful for geochemical exploration for fluorite because of its simplicity and high sensitivity. Investigations of the fluoride contents of natural waters and stream sediments resulted in the discovery of new fluorite mineralizations. Secondary dispersion patterns of soils have been studied. Different analytical procedures to extract the fluoride have been used. The application of the methods in geochemical exploration surveys is discussed.

Interpretation of a rock geochemical exploration survey in Cyprus - statistical and graphical techniques

A rock geochemistry orientation survey in Cyprus showed that anomalous dispersion patterns for Cu, Zn, Ni, and Co could be reliably measured in pillow lavas for a maximum distance of only 20 m from a sulphide deposit. Frequency distributions of the elements showed systematic (though small) differences between background areas and groups of samples from traverses extending more than 2 km from mineralization. It is concluded that the variation in concentration of individual elements alone is inadequate as a basis for an exploration procedure. A very successful exploration technique using the combined inter-variation of all the elements measured was developed for these unpromising data. Two interpretative methods are described - a computer calculation of characteristic discriminant functions for each of several pre-determined groups (e.g., anomalous and background) and a graphical derivation of what is termed “determinative functions” which allows the classification of individual samples as background or anomalous. Both of these techniques allow the recognition of an anomalous zone more than 2 km from the mineralization; either technique may be useful for exploration geochemistry in regions of weak anomalies.