EVALUATION OF DUPLICATE SAMPLES REGIONAL SEDIMENT SURVEYS , BRITISH COLUMBIA
INTRODUCTION The ability to discriminate real trends related to geological and geochemical causes from those that result from spurious factors such as sampling and analytical errors is of paramount importance in the suI:cess of geochemical data interpretation. Since the estimate of reproducibility (precision) allows us t o quantify the amount of variation due t o sampling and laboratory analysis it is an integral part of the evaluetion of geochemical data and should be conducted prior to carrying out any detailed interpreation. As part of our continuing study of rapid but thorough evaluation procedures for multi-element stream-sediment data (for example, Matysek. et a/., 1981) we designed a systematic computer-oriented method of evaluating the quality of geochemical survey data based on field-site duplicates. It irlcorporates a bias test, an analysis of variance technique, and the Thompson and Howarth (1976) approach to qua'ytifying precision. This detailed procedure utilizes the type and quality of data incorporated in various regional geochemical programs undertaken by the British Columbia Ministry of Energy, Mines and Petloleum Resources (the Ministry), but can be adapted easily for other programs, GENERAL METHODOLOGY steps: In brief, our general approach t o evaluation of the quality of geochemical data sets involves the following (1) Extraction of a t least 50 independent, field-site duplicate pairs from a geochemical data set f cmr use in subsequent data analysis. (2) Determination of the degree of systematic bias between duplicate pairs based upon the number of cases in which the first observation is greater than the duplicate. (3) Evaluation of the duplicated geochemical data set in terms of metal variability a t the regional, between-sample site and a t the local, within-sample site, sampling, and analvtical levels by a two-factor analysis of variance technique. (4) Quantification of the within-sample site variability by estimating precision utilizing the Thsmpson and Howarth (19761 method. D A T A Data obtained from the three most recent regional geochemical surveys undertaken Iby the Ministry (1981 I were evaluated using this method. The base data consist o f analyses of stream sediments and water samples collected a t an average density of one sample per 17.3 square kilometres over NTS map-sheet areas 92H, 921, and 92J. Silt samples were field dried and the minus 80-mesh (177 microns) fraction retained for subsequent analyses. The samples were analyzed for zinc, copper, lead, nickel, cobalt, silver, manganese, iron, molybdenum, arsenic, and antimony by the atomic absorption …
A new approach to the estimation of analytical precision
Abstract Methods used for the estimation of analytical precision commonly suffer from two deficiencies which give rise to misleading results. These are: (1) the methods take no account of changes in absolute or relative error over the concentration range, and (2) they tend for other reasons to produce optimistically-biassed results. The difficulties can be avoided by the correct use of duplicate determinations. One method presented allows precision parameters to be estimated. The other gives rise to a simple control chart for use in geochemical analysis.
Igneous Rock Associations 23. The Bushveld Complex, South Africa: New Insights and Paradigms
SUMMARYThe Bushveld Complex has continued to serve as the basis for study into the fundamental nature of petrological processes for layered intrusion formation and for oxide and sulphide hosted Platinum Group Element (PGE)–Cu–Ni ore deposits. These studies have included discoveries in terms of the physical extent of Bushveld magmatism, both laterally and internally. Lateral variations in the mafic to ultramafic Rustenburg Layered Suite of the Northern Lobe of the complex have also revealed petrologically distinctive Upper Critical Zone equivalent rocks (the so-called Flatreef) with enhanced contamination and mineralization traits that reflect a transition between Eastern and Western Lobe equivalent stratigraphy and Platreef-style complexity. Traditional magma mixing models have been re-examined in light of radiogenic isotopic evidence for crustal involvement early in the chromite precipitation or formation process, combined with evidence for associated heterogeneous fluid contents, cryptic layering profiles, and textural evidence. A wide variety of alternative ore-genesis models have been proposed as a consequence. The fundamental mechanics of magma chamber processes and the existence of the magma chamber as an entity have been called into question through various lines of evidence which have promoted the concept of progressive emplacement of the complex as a stack of not-necessarily-quite-sequentially intruded sills (with or without significant quantities of transported phenocrysts), emplaced into variably crystallized and compacted crystal-liquid mush mixtures, modified by compaction-driven late magmatic fluid (silicate and aqueous) activity. Alternatively, petrological and geochemical observations have been used to discount these interpretations in favour of more conventional cooling and gravity-driven accumulation of silicate and ore minerals in a large, liquid-dominated system.RÉSUMÉLe complexe de Bushveld a demeuré à la base d’études sur la nature fondamentale des processus pétrologiques de formation d’intrusions litées et des gîtes des éléments du groupe platine (ÉGP)-Cu-Ni hébergés dans les oxydes et les sulfures. Ces études ont comporté des découvertes sur l’étendue physique, à la fois latérale et interne, du magmatisme de Bushveld. Les variations latérales de la suite stratifiée et mafique à ultramafique Rustenburg du lobe nord du complexe ont également révélé des roches équivalentes pétrologiquement distinctes de la zone critique supérieure (le communément désigné Flatreef) avec des traits de contamination et de minéralisation accrus qui reflètent une transition entre la stratigraphie équivalente des lobes est et ouest et la complexité de type Platreef. Les modèles traditionnels de mélanges magmatiques ont été réexaminés à la lumière de preuves isotopiques radiogéniques indiquant une implication de la croûte au début du processus de précipitation ou de formation de la chromite, combinées à des preuves de contenu fluide hétérogène associé, de profils de litage cryptique et de preuves texturales. Ainsi, une grande variété de modèles alternatifs de genèse de minerai a été proposée. La mécanique fondamentale des processus de la chambre magmatique et l'existence de la chambre magmatique en tant qu'entité ont été remises en question au moyen de divers éléments de preuve qui ont mis en avant le concept de mise en place progressive du complexe sous forme d'un empilement non-nécessairement séquentiel de sills injectés (avec ou sans quantités significatives de phénocristaux transportés) mis en place dans des mélanges de bouillie cristaux/liquide à cristallisation et compaction variable, modifiés par une activité tardive de fluide magmatique (silicaté et aqueux) induite par la compaction. Alternativement, des observations pétrologiques et géochimiques ont été utilisées pour écarter ces interprétations en faveur d'un processus plus conventionnel de refroidissement et d’accumulation de minérais silicatés et minéralisés induite par la gravité dans un vaste système à dominance liquide.