Brian Townley

Gold grain morphology and composition as an exploration tool: application to gold exploration in covered areas

The results of research in the use of Au grain morphological and compositional properties applied in primary Au ore exploration are presented here. Two different and independent topics are discussed: (1) morphological characteristics of Au grains from active stream sediments for use as a distance-to-source indicator; (2) compositional signature of Au grains from various deposit types for use as a discrimination tool for source type and present deposit erosion level determination. The purpose of this study is to improve and integrate these two approaches as an exploration tool for Andean covered areas. Au grain morphology for over 1500 nuggets recovered from 60 active stream sediment samples in the Coastal Cordillera of Central Chile shows morphological variations (general shape, outline, surface, primary crystal imprints, associated minerals, flatness index) characteristic of three distance ranges (0–50 m; 50–300 m; >300 m) from source. Comparison with results from other similar studies of Au morphology characteristics in different climatic and/or sedimentological environments (arid, semi-arid, wet, lateritic, fluvial, fluvio-glacial and glacial) resulted in the determination of the recommended parameters (outline, surface, associated minerals, flatness index) to be used as distance-to-source indicator, independent of the climatic and/or sedimentological environment. Au grain morphological characteristics may assist in location of target but are not indicators of source type. Study of Au composition via electron microprobe analysis of Au grain cores from epithermal, Au-rich porphyry and Au-rich porphyry Cu systems indicated Au–Ag–Cu contents to be the best discrimination tool for these different types of Au-bearing deposits. In addition, such analysis of grains recovered at different vertical levels from the Cerro Casale Au-rich porphyry provides evidence that the Au compositional signature for a single type of deposit can also aid in the determination of vertical position. This may provide an estimate of the current level of erosion and remaining potential of the source. Some limitations of the proposed techniques are: (1) Au liberated from rock fragments already distant from source would be common in cordilleran and glacial environments, although this would be a detectable feature; (2) these techniques are applicable only for coarse-Au sources; (3) estimate of erosion level of liberated Au is limited to the case here presented.

Geochemical background, baseline and origin of contaminants from sediments in the mining-impacted Altiplano and Eastern Cordillera of Oruro, Bolivia

Oruro, located in the Bolivian Altiplano, has been subjected to intense mining and smelting activities since Colonial times (17th Century), yet the current geochemical composition of sediments and trace element behavior is practically unknown. A collection of 91 sediment samples retrieved from five sedimentary cores from Lake Uru Uru (Altiplano) and Cala Cala Lagoon (Eastern Cordillera) subjected to a total digestion technique, with a compilation of a pre-existing database of trace element concentrations in soils and lacustrine sediments obtained from the Oruro Pilot Project (PPO), facilitated the proposal of geochemical backgrounds and a present-time baseline for As, Cd, Cu, Pb, Sb and Zn in sediments from this area. Results obtained by statistics and geographical information system (GIS) analyses showed that the natural geochemical backgrounds of As and Sb are significantly enhanced in comparison to the Upper Continental Crust (UCC) concentrations, world background levels, and those of industrial sites and historical mining sites. The use of a local enrichment factor (EF) normalized by the mean concentration within the Cala Cala lagoon (CCLAC) demonstrated that using UCC concentrations to calculate EFs (EFUCC) is inadequate for this highly mineralized environment and therefore is not supported. Regarding metals and metalloids, the strong multiplicity of sources in this environment makes it difficult to discriminate between natural and anthropogenic inputs into this endorheic drainage basin, although it is suggested that surficial soils are probably impacted by airborne particulates dispersed from the Vinto (Sb-Sn) smelter, while Lake Uru Uru is influenced by mining activities, particularly drainage waste of the San José and Huanuni mines. As a final contribution, a geochemical background and a present-time baseline for Bolivian highlands sediments are provided, which will be helpful for the improvement of environmental legislation and for the future interpretation of geochemistry data in contamination and/or pollution studies in the altiplanic area.

Copper geochemistry in salt from evaporite soils, Coastal Range of the Atacama Desert, northern Chile: an exploration tool for blind Cu deposits

The evaporite soil (1.5 to 15 m thick) in the Coastal Range of northern Chile is a serious obstacle to mineral exploration. We conducted a Cu geochemical study in salt beds (mainly gypsum and anhydrite) from this soil. The sampling was distributed over mineralized and barren rocks, in salts of evaporitic soils from hills, slopes, gullies, plains and alluvium-filled valleys. The samples of salt from hills and slopes reflect very well the presence of blind ore deposits, and the magnitude of the Cu anomalies (<200 to >1000 ppm Cu) is independent of the depth at which the mineralization is located. Similar geochemical response has been found in salt from evaporitic soils that cover gullies, plains and alluvium-filled valleys. However, in the last case the magnitude of the Cu anomalies (<20 to >200 ppm Cu) depends on the thickness of the gravels and the depth of leached bedrock that covers the blind deposits. Although the Cu anomalies are spiky, their contrast with the background Cu population is remarkable. Sampling of salt from evaporitic soils appears to be a valuable tool for exploration of buried Cu deposits in the exceedingly arid Coastal Range of the Atacama Desert.

Base and precious metals geochemistry of rock units of the mainland Aysén region, Chilean Patagonia

Abstract In order to evaluate the applicability of regional rock geochemistry as an aid in mineral exploration, over 1000 rock chip samples of the Aysen region, Chile, were taken during the period 1993–1995. All samples were analyzed at commercial laboratories for 30 elements by induced coupled plasma atomic emission spectrometry (ICP–AES), and in addition, Au was determined by atomic absorption spectrometry (AAS). Rock samples were classified into two broad groups: (1) unaltered unmineralized rocks; and (2) altered and mineralized rocks. The geologic–tectonic setting of the area is a segment of the active continental margin of South America where, during the Late Jurassic and Cretaceous, a magmatic arc developed accompanied by an easterly marine back-arc basin. The basement is formed of Paleozoic metamorphic rocks that are interpreted as sedimentary wedges accreted to the Gondwana continent. The back-arc basin was filled by the end of the Mesozoic, and Tertiary volcanic and terrestrial sedimentary rocks that represent local basins and within-plate volcanism lie uncomformably on older units. The main mineralization in the region is coeval with Late Jurassic–Cretaceous magmatism and this is consistent with the geochemical data presented in this paper. Younger rocks show low geochemical values, suggesting that the change of tectonic regime by the end of the Mesozoic resulted in limited mineralizing processes during the Tertiary in the region. The regional rock geochemistry shows that unaltered unmineralized rock units of this region are well within global mean ranges for similar rock types, excepting As, which exhibits a conspicuous positive anomaly for most rock types. Altered and mineralized rock geochemistry and statistical treatment of data suggest potential for polymetallic mineralization in the region, the most prospective rock units being the volcanic Mesozoic rocks.

Soil gas geochemical exploration in covered terrains of northern Chile: data processing techniques and interpretation of contrast anomalies

The Campanani, Casualidad and Inca de Oro prospects, owned by Codelco-Chile, are extensive areas covered by post-mineralization deposits. To assist the detection of potential exploration targets, we carried out basic soil gas sampling campaigns by means of the Ore Hound GOCC® passive collectors, which were buried on a regular grid at a depth of 30–40 cm. After 90–120 days, samples were retrieved, sealed, and adsorbent material was submitted to an acid elution and analysed by ICP-MS for a set of 72 elements. In this study, databases have been analysed and modelled by statistical and geostatistical methodologies, mainly by factorial kriging analysis (FK). In all cases, a systematic analytical error was detected, discriminated and filtered, allowing the definition of contrast anomalies at different scales of spatial variation, and the interpretation of evidence of secondary dispersion processes by which present time surface contrast anomalies have been developed. The interpretation of contrast anomalies and element associations allowed the recognition of structural influences as pathways for gaseous circulation at the Campanani and Casualidad prospects. Likewise, lithological controls were detected at the Campanani and Inca de Oro prospects. At the Casualidad Prospect, a Co-Sn-Cu association describes a prospective zone c. 2 km north of the Casualidad deposit. At the Inca de Oro prospect, a marked influence of groundwater flow and/or surface drainage has been recognized in the configuration of contrast anomalies.

Change of support using non-additive variables with Gibbs Sampler: Application to metallurgical recovery of sulphide ores

Abstract Flotation tests at laboratory scale describe the metallurgical behavior of the minerals that will be processed in the operational plant. This material is generally composed of ore and gangue minerals. These tests are usually scarce, expensive and sampled in large supports. This research proposes a methodology for the geostatistical modelling of metallurgical recovery, covering the change of support problems through additive auxiliary variables. The methodology consists of simulating these auxiliary variables using a Gibbs Sampler in order to infer the behavior of samples with smaller supports. This allows downscaling a large sample measurement into smaller ones, reproducing the variability at different scales considering the physical restrictions of additivity balance of the metallurgical recovery process. As a consequence, it is possible to apply conventional multivariate geostatistical tools to data at different supports, such as multivariable exploratory analysis, calculation of cross-variograms, multivariate estimations, among others. The methodology was tested using a drillhole database from an ore deposit, modelling recovery at a smaller support than that of the metallurgical tests. The support allowed for the use of the geochemical database, to consistently model the metal content in the feed and in the concentrate, in order to obtain a valid recovery model. Results show that downscaling the composite size reduces smoothing in the final model.