High Nugget Effect Research Articles

The historical Corrente gold deposit, Minas Gerais, Brazil: Geological characteristics and a reconnaissance study of nugget effect

Corrente is a historical underground working in Minas Gerais, Brazil, mined for gold in the nineteenth century, for which little geological information is available. Retrieval of unpublished exploration reports from the 1980s has enabled the recognition of a 7-km-long linear cluster of historical underground and open-cast workings, here referred to as the Cata Branca–Corrente goldfield. Gold production ceased at Cata Branca, the most economically relevant deposit of the goldfield, in 1844. Both Corrente and Cata Branca are auriferous quartz-lode deposits. Geological characteristics of the Corrente deposit are presented, as well as reconnaissance whole-rock analyses for Au using a variety of analytical methods, including conventional fire assay, atomic absorption spectrometry, instrumental neutron activation analysis and laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) on pressed-powder pellets (PPPs). Corrente is characterised as a gold-only deposit. The Corrente auriferous quartz lode, hosted in the Archaean greenstone belt sequence of the Rio das Velhas Supergroup, is extensively oxidised and has conspicuous wall-rock alteration to fine-grained muscovite. The resulting sericite phyllite shows overprint by low-temperature kaolinitisation, likely the product of wall-rock hydration mediated by acidic water derived from sulfide oxidation at depth, below the modern weathering zone. The Corrente gold deposit has a high nugget effect. Time-resolved results of LA–ICP–MS on PPPs indicate the presence of particulate gold even in barren kaolinitised wall-rock phyllite.

Addressing Geological Challenges in Mineral Resource Estimation: A Comparative Study of Deep Learning and Traditional Techniques

Spatial prediction of orebody characteristics can often be challenging given the commonly complex geological structure of mineral deposits. For example, a high nugget effect can strongly impact variogram modelling. Geological complexity can be caused by the presence of structural geological discontinuities combined with numerous lithotypes, which may lead to underperformance of grade estimation with traditional kriging. Deep learning algorithms can be a practical alternative in addressing these issues since, in the neural network, calculation of experimental variograms is not necessary and nonlinearity can be captured globally by learning the underlying interrelationships present in the dataset. Five different methods are used to estimate an unsampled 2D dataset. The methods include the machine learning techniques Support Vector Regression (SVR) and Multi-Layer Perceptron (MLP) neural network; the conventional geostatistical methods Simple Kriging (SK) and Nearest Neighbourhood (NN); and a deep learning technique, Convolutional Neural Network (CNN). A comparison of geologic features such as discontinuities, faults, and domain boundaries present in the results from the different methods shows that the CNN technique leads in terms of capturing the inherent geological characteristics of given data and possesses high potential to outperform other techniques for various datasets. The CNN model learns from training images and captures important features of each training image based on thousands of calculations and analyses and has good ability to define the borders of domains and to construct its discontinuities.

NUGGET EFFECT INFLUENCE ON SPATIAL VARIABILITY OF AGRICULTURAL DATA

Spatial variability description of soil chemical properties by thematic maps depends substantially on suitable geostatistical models. One of the parameters composing a geostatistical model is nugget effect. This study aimed to evaluate the simultaneous influence of nugget effect and sampling design on geostatistical model estimation and estimation of soil chemical properties at unsampled sites, considering simulated data. Our results will be used as scientific basis for spatial variability analyses of soil chemical properties in agricultural areas. Given the simulation results and agricultural data, we concluded that the high nugget effect values obtained here reduced spatial estimation efficiency. Moreover, a systematic sampling design promoted the least accurate estimates of geostatistical model and at non-sampled sites. Despite that, these nugget effect estimates should be kept in the analysis. However, further studies will be needed to investigate which factors are responsible for such high nugget effect values.

Spatial Analysis of Salt Heterogeneity in a Central Californian Grape Field

Grape is one of the major crops produced in California; however, moderate salt concentrations in the soil pore space can affect its growth. Therefore, it is important that the root zone salinity be rigorously monitored and maintained below the threshold limit for grape. Spatial analysis of salt heterogeneity can facilitate site-specific management for the crop. We conducted an electromagnetic salinity survey followed by semivariance modeling and kriging analyses in order to quantify the spatial variability and structure of salt distributions in a central Californian grape field. Soils in the study area had silty clayey texture and the salinity across the root zone remained largely elevated with random variations. Around 78 and 19 % of the salinity values remained in the 3-4 and 4-4.5 dS m-1 ranges, respectively. The 3.5-4 dS m-1 level represented one of the most abundant concentrations across the areas of salt build up in the grape field. Based on our variogram and crossvalidation analyses, the Gaussian model fitted best to the salinity data (r2 = 0.996, RSS = 0.0003). The salt heterogeneity was characterized by high structural variance (99 %) and low nugget effect (C0 = 0.014). The kriging analysis demonstrated a large spatial dependence of salinity that extended beyond a 1000 m radius. The isotropic nature of the spatial correlation structure produced an omnidirectional variogram. The overall salinity levels across the field surpassed the salt tolerance threshold for grape and the spatial analyses identified sensitive zones and their structures that require precise adaptive management.

Retraction Note to: Variographic analysis of chemical and piezometric data from the sandstone aquifer of Ain Oussera, Algeria

The perturbation of piezometric level and the bad water quality in Ain Oussera plain represents a serious problem for local authorities and water resources managers. The spatial distribution of piezometric level and chemical data was analyzed by applying geostatistical methods, which provide an indication of the uncertainty of the estimation using the computer software VARIOWIN 2.2 (Pannatier 1996). In this paper, the evolution of the piezometric level and hydrochemical data by referring to other research campaigns was studied. Two campaigns were chosen; September 2001 and May 2008 for pizometric level and 1994 and 2004 for chemical data. In the beginning, elementary statistics has been carried out to understand the statistical distribution and performed an analysis of variance for comparison between two campaigns. Geostatistics for modeling and cartography were applied. The results showed that piezometric levels admit spherical model that the direction of groundwater flow is from southwest to northeast. High nugget effect characterizes piezometric data and chemical data, linked to the presence of microregions due to excessive pumping and local evaporitics formations. Anisotropy is observed in the northeast-southeast direction from which a spatial continuity in this direction can be deduced.

Spatial variation in the solute transport attributes of adjacent Typic Haplusteps, Mollic Ustifluvents, and Lithic Ustipsamments

Spatial variation in water flow and solute transport are important considerations when assessing field-scale remediation options, and for managing water and chemicals during agricultural production. Here, we evaluated the relationship between laboratory-measured solute transport variables pore-water velocity (v), dispersivity (λ), and retardation coefficient (R) and land use, soil type, and other independently measured soil properties in adjacent Typic Haplusteps, Mollic Ustifluvents, and Lithic Ustipsamments. We also characterized the spatial variation in v, R, coefficient of hydrodynamic dispersion (D), and λ. We obtained 100 geo-referenced, undisturbed soil columns (8.4cm diameter and of varying length) from the topsoil at sites with varying land uses, topographies, and soil types, and synchronized disturbed soil samples to analyze the basic soil properties. We conducted miscible displacement tests on all 100 columns, and predicted the retardation coefficient (R) and D using the computer program CXTFIT, and measured v. All four variables, except R, had strongly right-skewed and kurtotic distributions, and were highly variable. Log λ (logarithm to base 10) was significantly correlated with land use, with the mean of λ being significantly greater in grasslands, followed by orchards and croplands. By contrast, v was significantly correlated with soil type, with the mean being significantly greater in Lithic Ustipsamments. Log v (logarithm to base 10) was also significantly correlated with the soil CaCO3 content, log organic matter content, pH, cation exchange capacity, and log electrical conductivity, whereas neither v nor λ was correlated with soil texture measures. An analysis of the spatial structure of solute transport variables using semivariograms showed that R and λ had considerably high nugget effect. Log-transformed semivariograms performed well using ordinary point kriging for v and D. However, λ and R exhibited a very weak spatial dependency that could not be interpolated using ordinary kriging at the current sampling resolution. Therefore, this should be considered when designing future sampling programs to analyze spatial structure of R and λ.

Importance of good sampling practice throughout the gold mine value chain

The mining industry routinely collects samples to assist with decision making, whether for exploration, resource estimation, grade control, or plant design and balances. Poorly designed sampling protocols can result in elevated project risk by increasing variability. Critically, such variability produces both financial and intangible losses. Sample collection, preparation and assay or test work protocols that are optimised to suit the ore type, together with QAQC systems will reduce variability. Many gold deposits display a high natural variability, where the in situ variability can be enhanced by poor sampling practice to yield a high-nugget effect. In this case, specialised protocols are often required. Reporting codes require the Competent Person to consider the quality and implication of sampling programmes. Despite its importance, sampling often does not receive the attention it deserves. In this paper, the importance of good sampling practice is exemplified through a series of case studies, which show the many sampling issues that frequently go unrecognised or unaddressed, resulting in poor decisions and financial loss.

A test of the appropriateness of the LUC technique in high-nugget Birimian-style gold deposits

'=:17<7 The localized uniform conditioning (LUC) technique was proposed by Marat Abzalov in 2006. The technique converts conventional uniform conditioning (UC) grade-tonnage curves into single grade values attached to each smallest mining unit (SMU). This is achieved by ranking the SMUs within a panel in increasing order of their grade (based on the direct kriging of SMUs). This ranking is then used to localize the conventional UC grade-tonnage curves for each panel by dividing them into classes and computing their mean grades, which are assigned to the SMUs. The quality of this localization process will depend heavily on the validity of the grade patterns predicted by the direct kriging of the SMUs. Abzalov noted that where the distribution of data available for the direct kriging of the SMU is characterized by strong short-range variability, the advantages of using the LUC approach may be more limited. Consequently, a study was undertaken to determine how valid the predicted grade patterns of a typical Birimian-style gold deposit (with high nugget effect and strong short-range variability) might be expected to be. This was determined by comparing the direct SMU kriging ranking (based on sparse data) with the grade control model ranking (based on close-spaced data and the best available estimate of the deposit). The results showed a satisfactory correlation between these rankings and it was concluded that, although the grade patterns predicted by the direct kriging of the SMUs may be less meaningful for deposits exhibiting strong short-range continuity, there is nevertheless a convincing relationship with the actual (or best available) rankings. Therefore, the LUC technique is still considered to be useful for this style of deposit.

Spatial variability of heavy metals in the coastal soils under long-term reclamation

The coastal plain of Cixi City, China, has experienced over 1000 years of reclamation. With the rapid development of agriculture and industry after reclamation, successive inputs into agricultural soils have drastically modified the soil environment. To determine the spatial distribution of heavy metals and to evaluate the influence of anthropogenic activities, a total of 329 top soil samples were taken along a transect on the coastal plain. The samples collected across 11 sea dikes, were selected by a nested sampling methodology. Total Cu, Fe, Mn, Ni, Pb, and Zn concentrations, as well as their diethylenetriamine penta-acetic acid (DTPA) extractable (available) concentrations were determined. Results indicated that except for Zn concentrations, there was neither heavy metals pollution nor mineral deficiency in the soils. Heavy metals exhibited considerable spatial variability, obvious spatial dependence, and close relationships on the reclaimed land. For most metals, the reclamation history was the main influencing factor. Metals concentrations generally showed discontinuities around the position of sea dikes, and the longer reclamation histories tended to have higher metals concentrations than the recently reclaimed sectors. As for Cu and Zn total concentrations, stochastic factors, like industrial waste discharge, fertilization and pesticide application, probably led to the high nugget effect and altered this relationship. The 6th and 10th zones generally had the highest total metals concentrations, due to the concentration of household appliance manufacturers in these reclaimed areas. The first two zones were characterized by high available metals concentrations, probably due to the alternant flooding and emergence, low pH values and high organic matter contents in these paddy field soils. From the 3rd to 7th zones with the same land use history and soil type, metals concentrations, especially available concentrations, showed homogeneity. The nested sampling method adopted demonstrated that the 500-m interval was enough to capture the spatial variation of the metals. These results were useful in evaluating the variation in the environment quality of the soils under long-term reclamation and to formulate plans for future reclamation projects.

Using geochemical proxies to model nuggety gold deposits: An example from Sunrise Dam, Western Australia

Gold distribution in vein-hosted hydrothermal ore deposits is commonly nuggety (i.e. occurs as very localised concentrations of gold). In these cases samples for gold assay from diamond drill core may be too small to model the underlying heterogeneity of gold distribution and result in poorly constrained ore body models and underestimated gold resources. Hence, it is common practice to use more spatially continuous proxies for mineralisation to help define the boundaries of mineralised regions. We present a method for automating the use of geochemical proxies for nuggety gold ore bodies.Sunrise Dam Gold Mine, in Western Australia, is a world-class gold deposit with a very high nugget effect. Multi-element geochemical data has been collected at this site in order to improve prediction of mineralised regions. Suitable proxy elements have been selected from this data set, in particular, those that are spatially related to gold mineralisation but do not display nuggety distribution, such as Sb, Rb and Cr.We applied a probabilistic approach to the problem of quantifying the relationship between gold assay values and geochemical elements. It is shown that a kernel density estimator and Bayes conditional probability can provide an effective method for calculating the probability of a sample having elevated gold content and that this measure will be more spatially continuous than gold assay values if the appropriate geochemical proxies are selected. Using conditional probability and suitable cut-off values, we reclassified approximately 27% of samples as mineralised which returned low Au assay results. When plotted on drill holes conditional probability values provided a much more spatially continuous guide to mineralised regions than Au assay values alone.

Anthropogenic pollution and variability of manganese in alluvial sediments of the Yellow River, Ningxia, northwest China

Heavy metal pollution of sediments is a global concern and can be a serious problem in heavily industrialized parts of the world. Pollution by manganese is particularly common due to its ubiquitous natural occurrence, ease of mobilization, and extensive association with industry. In Ningxia, China, manganese pollution of Yellow River alluvial sediments was assessed by comparing manganese concentrations in 35 sediment samples with background values derived from similar sediments obtained at sites considered remote from potential sources of contamination. Natural background values of manganese were found to range from 192 to 323 mg/kg for surface sediments, and from 220 to 325 and 283 to 394 mg/kg for subsurface sediments at depths of 45–50 and 95–100 cm, respectively. In the study area, manganese content ranged from 565 to 1,363 mg/kg, indicating anthropogenic pollution extending to a depth of at least 1 m in the study area. All 35 samples were found to exceed the threshold effect concentration (TEC) of 460 mg/kg, below which adverse effects on sediment-dwelling organisms are not expected to occur, and one sample (T12) was found to exceed the probable effect concentration (PEC) of 1,100 mg/kg. PEC defines the threshold above which adverse effects are likely to be observed. Variogram analysis of the surface sediment manganese data revealed adherence to a Gaussian model, and ordinary kriging was used to generate a manganese distribution map. Analysis of the high nugget effect ratio indicates high, small-scale variations that are consistent with potential emissions from an adjacent electrolytic manganese plant.

Approaches to reporting grade uncertainty in high nugget gold veins

High nugget effect gold veins are generally considered to be one of the most challenging of deposit types to evaluate and exploit. To potential investors and mining companies, they are viewed as high risk because of the associated uncertainties in the grade estimate and general paucity of reserves at production start-up. Despite the well-known risks, these deposits are often high grade and have a grade upside which makes them potentially attractive. The reporting of resource grade is of importance and perhaps even more so is the reporting of the associated uncertainty attached to the grade estimate. The quoting of the estimated grade within a grade range is recommended to achieve more complete and useful disclosure. The definition of the grade range can be somewhat subjective, ranging from opinion-based through to data-based estimates and geostatistical conditional simulation. Whichever approach is used, the over-riding issue must be the provision of enough data of appropriate quality and full disclosure to the public of relevant uncertainties.

RETRACTED ARTICLE: Variographic analysis of chemical and piezometric data from the sandstone aquifer of Ain Oussera, Algeria

The perturbation of piezometric level and the bad water quality in Ain Oussera plain represents a serious problem for local authorities and water resources managers. The spatial distribution of piezometric level and chemical data was analyzed by applying geostatistical methods, which provide an indication of the uncertainty of the estimation using the computer software VARIOWIN 2.2 (Pannatier 1996). In this paper, the evolution of the piezometric level and hydrochemical data by referring to other research campaigns was studied. Two campaigns were chosen; September 2001 and May 2008 for pizometric level and 1994 and 2004 for chemical data. In the beginning, elementary statistics has been carried out to understand the statistical distribution and performed an analysis of variance for comparison between two campaigns. Geostatistics for modeling and cartography were applied. The results showed that piezometric levels admit spherical model that the direction of groundwater flow is from southwest to northeast. High nugget effect characterizes piezometric data and chemical data, linked to the presence of microregions due to excessive pumping and local evaporitics formations. Anisotropy is observed in the northeast-southeast direction from which a spatial continuity in this direction can be deduced.

Use of trans-Gaussian kriging for national soil geochemical mapping in Ireland

Geochemical mapping requires a good understanding of geochemical variables and proper use of mathematical and mapping techniques. In this study, GIS mapping for the geochemical variables in Irish soils was achieved using trans-Gaussian kriging following investigation of probability features, outlier identification, optimal power transformation by the Box–Cox method and spatial structure modelling. Relatively high nugget effect values for the variogram models were observed for most variables. The assumption of spatial autocorrelation was seriously breached by two variables, available P and available K, and spatial interpolation was not performed for these. Two or even three nested theoretical models were needed to model variogram structures for most variables reflecting their variation at different spatial scales. The spatial distribution patterns for most elements illustrated the influences of underlying bedrock and soil type, while some of the heavy metals (e.g. Pb, Zn and Cu) were also affected by mining activities and urban sprawl. The available nutrients (P, K and Mg) appeared to be most affected by agricultural activities.

Gold particle clustering: a new consideration in sampling applications

Recent research indicates that an enhanced coarse gold problem may be present in some gold deposits. This can be due to the presence of gold particle clusters that give the effect of individual coarse-gold particles and leads to a high constitution heterogeneity (CH). These clusters, which may be distributed on the centimetre-scale or greater, give rise to high assay variability in field samples. However, once a sub-sample is pulverised, the effect of the clusters may be removed if the individual gold particles making up the clusters are liberated. Thus any resulting pulp will tend to have a low variability, unless true coarse-gold particles exist. Clusters are readily recognised in coarse gold deposits where they accentuate existing high nugget effect and CH. Clusters may however also occur in fine gold deposits where they produce an apparent coarse gold style. This is signaled by an unexpectedly high nugget effect from variography and poor correlation between field sample duplicates. The Gy sampling equation is applied to model coarse and fine gold deposits with clustered and non-clustered gold grains. Case studies illustrating observed effects of clustering are presented. If clustered particles exist, then proper protocol design at the field and early laboratory stage is paramount. The sample characterisation stage should include an assessment of possible gold particle clustering, in addition to the standard descriptions of the gold particle size distribution.

Fine scale spatial pattern of Pacific sardine (Sardinops sagax) and northern anchovy (Engraulis mordax) eggs

AbstractPacific sardine (Sardinops sagax) and northern anchovy (Engraulis mordax) eggs exhibited different spatial structure on the scale of 0.75–2.5 km in two egg patches sampled in the Southern California Bight in April 2000. Plankton samples were collected at 4‐min intervals with a Continuous Underway Fish Egg Sampler (CUFES) on 5 × 5 km grids centered on surface drifters. Variograms were calculated for sardine and anchovy eggs in Lagrangian coordinates, using abundances of individual developmental stages grouped into daily cohorts. Model variograms for sardine eggs have a low nugget effect, about 10% of the total variance, indicating high autocorrelation between adjacent samples. In contrast, model variograms for anchovy eggs have a high nugget effect of 50–100%, indicating that most of the variance at the scales sampled is spatially unstructured. The difference between observed spatial patterns of sardine and anchovy eggs on this scale may reflect the behavior of the spawning adults: larger, faster, more abundant fish may organize into larger schools with greater structure and mobility that create smoother egg distributions. Size and mobility vary with population size in clupeoids. The current high abundance of sardines and low abundance of anchovy off California agree with the greater autocorrelation of sardine egg samples and the observed tendency for locations of anchovy spawning to be more persistent on the temporal scale of days to weeks. Thus the spatial pattern of eggs and the persistence of spawning areas are suggested to depend on species, population size and age structure, spawning intensity and characteristic physical scales of the spawning habitat.

Grade and geological continuity in high-nugget effect gold–quartz reefs: implications for resource estimation and reporting

High-nugget effect gold-quartz reefs are one of the most challenging styles of mineralisation to evaluate and exploit. The estimation of mineral resources, and subsequent ore reserves, forms a critical base for exploitation. The understanding and application of grade and geological continuity issues during estimation is of importance. Most reporting codes make specific references to grade and geological continuity issues. Within the high nugget environment, confidence in the tonnage estimate is variable depending upon geological continuity, though it is usually higher than the confidence in the grade estimate. Estimation of grade is problematic as a result of the erratic and low continuity of gold values and gold-bearing domains. Detailed geological interpretation supported by 3D modelling to produce either 'conceptual' or 'visual' continuity models is critical. The inherent risk associated with the estimation of mineral resources and ore reserves and subsequent classification can be reduced by a strong understanding of geological and grade continuity.The level of investment required to acquire this information is generally substantial, and will almost certainly involve underground development supported by close-spaced diamond drilling and bulk sampling/trial mining.

Author's Reply to "Comments on Classification and Reporting of Mineral Resources for High-Nugget Effect Gold Vein Deposits," by M. Vallee

There are two main issues stemming from Vallee’s (2002) comments on Dominy et al. (2001b). First, he raises the importance of quality assurance/quality control (QA/QC) during resource estimation programs, and second, he indicates that the resolution and understanding of continuity (grade and geological) issues are paramount in the classification of resources. In particular, continuity is critical at the boundary between the Inferred and Indicated Mineral Resource categories. The original contribution agrees with the importance of these points, and the principal author offers the following discussion and clarification. Further comment is also provided on the reporting of grade uncertainty through grade ranges. It is worth remembering that vein-gold deposits (e.g., shear-zone, lode-gold, mesothermal types etc.), particularly those containing coarse gold particles (e.g., >100 μm), generally show large variations in grade and an erratic grade distribution. In most cases, diamond drilling is a good measure of geological continuity (depending on drill spacing), but a poor measure of grade and its distribution. High-nugget effect deposits rank among the most difficult deposit types in terms of producing an accurate and precise resource estimate, and clearly represent a distinct deposit type. ### Quality Assurance/Quality Control The issue of quality assurance (QA) and quality control (QC) during exploration, estimation/evaluation and exploitation has been at the forefront of industry perceptions since the Bre-X affair in the late 1990s. The need for QA/QC programs is paramount in any mining/minerals project (Vallee, 1998a, 1998b, 2002). Quality is a critical issue at all stages of a minerals project. For example, QA/QC of analytical and test data, and the corroboration of geological data within the resource/reserve estimation process must now be considered mandatory. Indeed, the Canadian National Instrument 43–101 requires mandatory QA/QC programs (CSA, 2001). The 1999 JORC Code does not prescribe QA/QC programs but, for example, QA/QC for sampling/assaying is …

Comments on "Classification and Reporting of Mineral Resources for High-Nugget Effect Gold Vein Deposits," by S.C. Dominy, P.R. Stephenson and A.E. Annels

Research Article| January 01, 2002 Comments on “Classification and Reporting of Mineral Resources for High-Nugget Effect Gold Vein Deposits,” by S.C. Dominy, P.R. Stephenson and A.E. Annels MARCEL VALLÉE MARCEL VALLÉE Géoconseil Marcel Vallée inc., Sainte-Foy, Québec, Canada G1X 3J9 Search for other works by this author on: GSW Google Scholar Exploration and Mining Geology (2002) 11 (1-4): 113–117. https://doi.org/10.2113/11.1-4.113 Article history first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation MARCEL VALLÉE; Comments on “Classification and Reporting of Mineral Resources for High-Nugget Effect Gold Vein Deposits,” by S.C. Dominy, P.R. Stephenson and A.E. Annels. Exploration and Mining Geology 2002;; 11 (1-4): 113–117. doi: https://doi.org/10.2113/11.1-4.113 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyExploration and Mining Geology Search Advanced Search This text consists of two parts, comments on “Classification and Reporting of Mineral Resources for High-Nugget Effect Gold Vein Deposits,” by S.C. Dominy, P.R. Stephenson and A.E. Annels, Volume 10, No. 3, pp. 215-233, and related comments on some features of the JORC Code. This paper is, in many respects, a very elaborate and valuable account of the problems encountered in the estimation and classification of high-nugget gold deposits. However, in some key areas, it appears to fall short of its objective to help “resolve satisfactorily most issues” and “contribute to improved estimation and classification” (abstract). One must realize that... You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

The Occurrence of High-grade Gold Pockets in Quartz Reefs at the Gwynfynydd Mine, Wales, United Kingdom: A Geological Explanation for the Nugget Effect

Gold-bearing quartz reefs commonly show extremely erratic and unpredictable grade variation, although gross geological continuity may be good. This type of variation is often described as being nuggety or having a high nugget effect and can be measured quantitatively using the semi-variogram. Understanding of geological features such as reef texture and structure will provide improved models for the interpretation of assay data, drill core descriptions, etc. In this paper, a case study from the quartz-hosted gold occurrences in the Gwynfynydd mine, United Kingdom, is described. The work provides a starting point for models of lateral variation and demonstrates the types of structural and textural features that may be sought to give clues to the prediction of gold distribution in similar deposits. Gold deposition at Gwynfynydd is primarily associated with the reef footwall. The host veins form a discrete group of structures that may have been emplaced early in the sequence of quartz veins forming the reef. This potentially exposes them to a maximum number of dissection events during repeat dilations of the lode. Gold was deposited in the local porosity at extreme concentrations, equivalent to kg/t grades, when the host fissure had become filled with a porous framework of crystalline quartz and sulfides. Distribution of porosity varied laterally along the vein as the result of the formation of clump-like growths of minerals from the vein walls. The growth style is relevant to the distribution of gold although it may not be relevant to the occurrence of gold in a particular vein. The preservation of pores with some connectivity late in the vein fill sequence may be important in permitting continued but slow fluid flows. This facilitates effective fluid reaction with wall rock-derived methane, thus changing the relative rates of gold and quartz deposition in favor of gold. The textural studies explain the first-order control of nugget distribution at Gwynfynydd but do not allow for prediction and optimization of the resource estimation process. This is principally because of the very low geological continuity and poor predictability of the high-grade pockets. Nonetheless, the work does provide a clear geological explanation for the erratic nature of the gold. It indicates textures and structures that can be used to determine geological continuity of the gold-bearing elements within the gross reef envelope.