Nicholas M.S. Rock

Can lamprophyres resolve the genetic controversy over mesothermal gold deposits

Associations between calc-alkaline lamprophyres and mesothermal gold deposits (Archean to Tertiary) in which the lamprophyres are coeval (as well as cospatial) with mineralization are increasingly recognized worldwide. Our suggested hypothesis regards lamprophyres as transporting agents for Au from Au-rich sources in the deep mantle, which then undergo extensive crustal interactions, generating felsic magmas or releasing their Au into metamorphic-hydrothermal systems. Such a model not only may reconcile the conflict between the extant magmatic and metamorphic models for mesothermal deposits but also may resolve the inconclusive relations between gold deposits and felsic (porphyry-granitoid) intrusions, because lamprophyres may act as parents to both. The gold-lamprophyre association implies that exceptionally deep-seated magmatism repeatedly accompanied gold mineralization in postcollisional orogenic, island-arc, oblique subduction, or graben environments. This has important implications for the late-stage evolution of mineralized Archean greenstone belts in particular, and for gold deposit genesis in general.

The International Mineralogical Association (IMA/CNMMN) pyroxene nomenclature scheme: Computerization and its consequences

SummaryA FORTRAN-77 program is documented, PXTAB, which automates the 1988 IMA/ CNMMN pyroxene nomenclature. PXTAB calculates formula units to 4 cations/6 oxygens by reallocation of Fe2+/3+ (pIus MD2+/3+ and Ti3+14+ if necessary), checks for certain analytical errors, names pyroxenes, and tabulates results in batches. K2O, P2O5 and CO. can be calculated out as K-feldspar/biotite, apatite and calcite impurities respectively. PXTAB attempts to computerize the IMA report as it stands, but some rules given in the report are incomplete, inexact and often ambiguous, so that certain assumptions have to be made in programming. The lack of defined compositional boundaries for certain rare (e.g., Mn-rich and Zr-rich) pyroxenes is one problematical omission. The report also inexplicably names pyroxenes which lie outside its own stated compositional limits. Further problems related to adjectival modifiers may particularly concern petrologists: (a) many modifiers remain formally undefined, so that there is no way of consistently distinguishing, say, ‘subcalcic augite’ from ‘augite’; (b) other modifiers are applied inconsistently in the report; (c) there is imbalance among modifiers: for example, aluminian applies to many more pyroxenes than titanian, even though the defined limits (>0,1 PFU) are the same. Overall, the IMA pyroxene nomenclature is at variance with its 1978 amphibole nomenclature in both principle and practice, posing problems for nomenclatural consistency.ZusammenfassungPXTAB berechnet Mineralformeln auf der Basis von 4 Kationen bzw. 6 Sauerstoffen unter Neuverteilung von Fe2+/3+ (wenn nötig auch von Mn2+/3+ und Ti3+/4+) kon trolliert mögliche durch die Analytik bedingte Fehler, benennt die Pyroxene und gibt die Resultate in Stapelform aus.K2O, P2O5 und CO2 können rechnerisch als Kalifeldspat/Biotit-, Apatit- und Calcitver unreinigungen angegeben werden.Es wurde zwar versucht, PXTAB so zu programmieren, daß es mit den IMA Nomenklatur-Richtlinien in Einklang steht, aber einige der angegebene Regeln sind unvollständig, ungenau und oftmals mehrdeutig. Daher mußten für das Programm bestimmte Annahmen gemacht werden. Zum Beispiel fehlen Abgrenzungen der Zusammensetzung seltner, etwa Mn- und Zr-reicher, Pyroxene.Der Bericht benennt außerdem, unerklärlicherweise, Pyroxene, die nicht den definierten Zusammensetzungen entsprechen. Weitere Probleme, in Zusammenhang mit den adjektivischen Ergänzungen, betreffen insbesondere Petrologen: (a) viele ergänzende Adjektiva werden formal nicht definiert, sodaß es keine Möglichkeit gibt, sagen wir z.B. “subcaleic augite” von “augite” zu unterscheiden; (b) andere Adjektiva werden inkonsequent verwendet; (c) es besteht eine Unausgewogenheit zwischen den Ergänzungen. so wird z.B. “aluminian” für weit mehr Pyroxene verwendent, als “titanian”, obwohl die definierten Grenzen (> d.1 PFU) dieselben sind.Die IMA Pyroxennomenklatur ist daher, zusammenfassend festgestellt, in vielerlei Hinsicht von der 1978 eingeführten Amphibolnomenklatur verschieden. Für die nomenklatorische Richtigkeit ergeben sich daher sowohl prinzipielle, wie auch praktische Probleme.

Archean cratons, diamond and platinum: Evidence for coupled long-lived crust-mantle systems

Diamondiferous intrusions and magmatic Pt-Pd deposits are both concentrated on, or adjacent to, the oldest cratons, those with >3.0 Ga high-grade gneiss terranes and/or greenstone belts. Given the old age (>3.0 Ga) of peridotitic inclusions both in diamonds and in kimberlites, diamonds presumably grew in mantle depleted in basaltic major elements near the base of thickened lithosphere and below early sialic nuclei. Most genetic models for magmatic Pt-Pd deposits require a Pt-Pd–enriched, high–Mg-Si melt generated from analogously depleted mantle. The depleted mantle was most likely formed by removal of basaltic melts that contributed to Archean intracratonic greenstone belts. Extensive melting also decreased density and increased rigidity beneath ancient cratons, favoring stabilization and preservation of thick Archean continental lithosphere. Overall, these considerations suggest that localized, thick sialic crust and rigid lithosphere developed before 3.0 Ga, forming enduring, coupled crust-mantle systems below ancient sialic nuclei, the sites of selectively preserved greenstone belts. The data confirm that Early Archean terranes are highly prospective for post-Archean magmatic ore deposits.

Criteria for the recognition of metamorphosed or altered lamprophyres: A case study from the Archaean of Kambalda, Western Australia

Abstract The ability to distinguish ancient lamprophyres from other mafic and ultramafic intrusive rock types has become increasingly important. Lamprophyres have been used as marker horizons in the mapping of several Precambrian terranes, they have been recognized for decades as possible indicators of gold mineralization, and their presence has implications for the tectonic interpretation of ancient greenstone belts. Whilst fresh, undeformed lamprophyres are readily recognized, metamorphosed, altered and/or deformed examples are more problematical. The intrusive ages of Kambalda lamprophyres are constrained by available field and geochronological evidence, including direct dating of one kersantite, to the period 2684-2660 Ma. These rocks therefore belong to the worlds oldest group of alkaline rocks and lamprophyres so far recognized, which also includes contemporaneous examples from Canada, Zimbabwe and, probably, Tanzania. Despite their extreme age and low- to medium-grade regional metamorphic history, these rocks still preserve characteristic lamprophyric textures, chemistry and mineralogy. Only where metamorphism has been overprinted by gold-related alteration and deformation do the lamprophyres cease to be recognizable petrographically. However, their unique chemistry persists and, despite losses of the most mobile elements from presumed magmatic compositions, altered and deformed specimens still retain far higher contents of Sr, K, Ba, Th, Nb, P, Zr and Ce than basalts of the local greenstone succession. Even lamprophyres which have been converted to biotite-plagioclase-carbonate-pyrite schists retain this distinctive chemical signature. During both alteration and metamorphism, ‘mobile’ elements (K, Rb, Sr, Ba) display far larger changes than archetypal ‘immobile’ elements (Ce, Nb, Zr, P, Ti), clearly sanctioning traditional preference for the latter in assessing altered rocks. However, the surprising loss of Nb during biotitization of hornblende indicates that negative Nb anomalies, conventionally regarded as indicators of subduction-related magmatism, can also be induced purely by alteration processes.

Geochemistry of Archean shoshonitic lamprophyres from the Yilgarn Block, Western Australia: Au abundance and association with gold mineralization

Spatial and temporal associations between Archean mesothermal gold deposits, shoshonitic minor intrusions (e.g. lamprophyre dikes), and crustal-scale fault systems are well recognized features of some Archean terranes. It has been proposed that the association may be due to a combination of genetic factors, including intrinsic Au enrichment of shoshonitic magmas, and tectono-structural factors arising from crustal-scale orogenic activity in the Late Archean. To determine the nature of the association in the highly mineralized Archean Yilgarn Block, the major, trace and precious metal geochemistry of a suite of 49 lamprophyres and related microdiorite porphyries, covering a range of alteration states and proximities to gold mineralization, were investigated. The lamprophyres exhibit rock fabrics indicative of partial to extensive metamorphic recrystallizatio, range from primitive to more evolved compositions (MgO ∼9 to <5 wt%) and have geochemical signatures typical of Phanerozoic subduction-related magmas. Variable mobile lithophile element (K, Rb, Ba, Sr) concentrations and anomalously high δO signatures of the lamprophyres reflect their interaction with hydrothermal ± metamorphic fluids. Lamprophyres emplaced in proximity to gold deposits are commonly affected by carbonation, have enhanced S and Au contents and have Au/Pd ratios that exceed primitive mantle values by up to several orders of magnitude. In contrast, lamprophyres emplaced in locations remote from gold mineralization tend to be depleted in S and Au and have low Au/Pd ratios. High Au contents were mostly acquired by interaction with Au-mineralizing fluids, whereas very low Au contents are the result of fluid leaching in lamprophyres remote from gold deposits. However, some lamprophyres of high F content display small intrinsic enrichments in Au of ≈2 to 3 times typical igneous rock abundances. The F, S and CO contents of the Yilgarn lamprophyres can effectively discriminate mineralized lamprophyres from non-mineralized samples. This study shows that shoshonitic lamprophyres are unlikely to have contributed significant Au or other components to Yilgarn mesothermal gold deposits.

Late Caledonian lamprophyre dyke swarms of South-Eastern Scotland

SummaryCale-alkaline lamprophyre (minette) dykes in the eastern Southern Uplands of Scotland form part of a swarm nearly parallel to the inferred Iapetus Suture, stretching from the Ards Peninsula of Northern Ireland to St. Abbs Head in the east. The dykes are clustered close to several small granitoid bosses, but appear to be younger than the plutons and their associated porphyrite-porphyry dykes. Mica- (minette and kersantitic-minette) and hornblende-lamprophyres are present further west near Hawick where no intermediate-acid plutons or dykes occur. The lamprophyres have enrichments in LILE and LREE and relative depletions of HFS elements typical of subduction-related ultrapotassic magmas. These incompatible element enrichments are present in rocks with high Mg number and Ni and Cr contents, which combined with experimental constraints, their fine-grained nature and presence of chilled margins, imply a near-primary status for the least evolved varieties. High values of LREE, LILE, La/Nb, La/Yb, s Sr and low ɛ Nd imply derivation from a previously metasomatised source. The minettes were probably derived from a source containing garnet and phlogopite, and the hornblende varieties from a shallower source in the stability field of amphibole. The minettes of the eastern Southern Uplands have not provided a parental component to the 410 Ma. granitoids which were derived from a more depleted source. The similarity of the lamprophyres to those in the Lake District south of the Iapetus Suture is taken to indicate underthrusting of Lake District lithosphere beneath the Southern Uplands. Emplacement of lamprophyre dyke swarms is likely to be structurally controlled, and the presence of the main swarm in the Southern Uplands may indicate the sub-surface trace of the Iapetus Suture.ZusammenfassungKalk-alkalische Lamprophyrgänge (Minette) im östlichen Teil der südlichen schottischen Uplands sind Teil eines Gangscharensystems, das fast parallel zu der vermuteten lapetus-Suturzone, die sich von der Halbinsel von Ards in Nordirland bis nach St. Abbs Head im Osten erstreckt, verläuft. Die Gänge konzentrieren sich um kleinere Granitoidkörper, scheinen aber jünger zu sein als die Plutone und ihre assoziierten Porphyrit-Porphyry Dikes. Glimmer- (Minette und kersantitische Minette) sowie Hornblende-Lamprophyre kommen westlich von Hawick vor, wo keine intermediären bis sauren Plutone oder Dikes anzutreffen sind. Die Lamprophyre sind an LIL- und LRE-Elementen an- bzw. an HFS-Elementen abgereichert, was typisch für subduktionsbezogene ultrapotassische Magmen ist. Diese Anreicherung an inkompatiblen Elementen ist in Gesteinen mit hohen Gehalten von Mg, Ni und Cr beobachtbar, was in Verbindung mit experimentellen Untersuchungen, der feinkörnigen Natur der Gesteine und dem Auftreten von gefritteten Intrusionskontakten einen nahezu primären Zustand der am wenigsten entwickelten Varietäten impliziert. Hohe LREE-, LILE-, La/Nb-, La/Yb-, ɛSr- und niedrige ɛNd-Werte belegen die Herkunft aus einer ursprünglich metasomatschen Quelle. Die Minetten stammen vermutlich aus einer granat- und phlogopitführenden, die hornblendeführenden Gesteine aus einer seichteren im Stabilitätsbereich von Amphibol liegenden Quelle. Die Minetten lieferten keinen Beitrag zu den 410 Ma Granitoiden, die aus einer stärker verarmten Quelle stammen müssen. Die Ähnlichkeit der schottischen Lamprophyre zu jenen im Lake District südlich der lapetus-Sutur zeigt ein Unterschieben der Lake Distric Lithospäre unter die südlichen Uplands an. Die Platznahme der Lamprophyrgänge ist sehr wahrscheinlich strukturell kontrolliert und die Anwesenheit der Gangsysteme könnte den Verlauf der Iapetus-Sutur in der Tiefe nachzeichnen.

A FORTRAN Program for tabulating and naming amphibole analyses according to the International Mineralogical Association Scheme

SummaryAmphibole analyses input to program AMPHTAB (optionally with H2O, F, Cl, minor elements and one or both of Fe2O3, FeO), are output in conventional column tables, with formula units, complete IMA names, and extensive messages indicating the quality of each analysis. AMPHTAB accepts data-files ranging from a single amphibole analysis to (in principle) unlimited batched data-e.g., multiple analyses from each of numerous probe sections. Options are provided to reallocate total Fe in probe data between Fe3 and Fe2, to allow for common analytical problems (e.g., low H2O values), and to enable the number of oxygens in the formula unit to be either preset or assigned automatically. Results for 57 probe analyses are compared with an unpublished BASIC program, which uses a different Fe-reallocation method. Three-quarters of the IMA names resulting from the two programs are essentially identical, despite somewhat different estimated formula units. The remainder are borderline cases, where incidental changes in formula units effect disproportionate differences in name.ZusammenfassungIn das Programm eingegebene Amphibolanalysen (wahlweise mit H2O, F, Cl, Spurenelementen und Fe2O3, und/oder FeO), werden in Form herkömmlicher Spaltentabellen mit den Formeleinheiten, vollständigen IMA-Namen und ausführlichen Bemerkungen bezüglich der Analysenqualität ausgegeben. Das AMPHTAB-Programm verarbeitet Datensätze von einer einzigen Amphibolanalyse bis zu (im Prinzip) unbegrenzten Datenblöcken; z. B. Mehrfachanalysen eines Probenabschnittes von mehreren. Es sind die Möglichkeiten vorgesehen, das Gesamteisen in einer Analyse in die Fe3 und Fe2-Anteile umzurechnen, allgemeine analytische Probleme (z. B. niedrige H2O-Gehalte) zu lösen und die Anzahl der Sauerstoffatome in der Formeleinheit entweder vorzugeben oder automatisch setzen zu lassen. Die Resultate von 57 Mikrosonden-Analysen sind mit einem unveröffentlichten BASICProgramm, das eine andere Methode zur Aufteilung des Fe-Gehaltes benutzt, verglichen worden. Drei Viertel der von den beiden Programmen vorgeschlagenen Mineralnamen sind identisch, trotz einiger unterschiedlich berechneter Formeleinheiten. Die übrigen 25% sind Grenzfälle, bei denen zufällige Veränderungen in den Formeleinheiten unverhältnismäßige Unterschiede in der Benennung bewirkten.

Summary statistics in geochemistry: A study of the performance of robust estimates

Numerical data summaries in many geochemical papers rely on arithmetic means, with or without standard deviations. Yet the mean is the worst average (estimate of location) for those extremely common geochemical data sets which are non-normally distributed or include outliers. The widely used geometric mean, although allowing for skewed distributions, is equally susceptible to outliers. The superior performance of 19 “robust” estimates of location (simple median, plus various combined, adaptive, trimmed, and skipped,L, M, andW estimates) is illustrated using real geochemical data sets varying in sources of error (pure analytical error to multicomponent geological variability), modality (unimodal to polymodal), size (20 to >2000 data values), and continuity (continuous to truncated in either or both tails). The arithmetic mean tends to overestimate location of many geochemical data sets because of positive skew and large outliers; robust estimates yield consistent smaller averages, although some (e.g., Hampels and Andrews) do perform better than others (e.g., Shorth mean, dominant cluster mode). Recommended values for international standard rocks, and for such important geochemical concepts as “average chondrite,” can be reproduced far more simply via robust estimation on complete interlaboratory data sets than via the rather complicated and subjective methods (e.g., “laboratory ratings”) so far used in the literature. Robust estimates also seem generally less affected by truncation than the mean; for example, if values below machine detection limits are alternatively treated as missing values or as real values of zero, similar averages are obtained. The standard (and mean) deviations yield consistently larger values of scale for many geochemical data sets than the hinge width (interquartile range) or median absolute deviation from the median. Therefore, summaries of geochemical data should always include at least the simple median and hinge width, to complement the often misleading mean and standard deviation.

Relationships between calc-alkaline acidic and basic (mantle-derived) magmas in Late Archaean composite dykes, Kambalda Goldfield, Western Australia

Abstract Composite dykes from the North Dome area, Kambalda, comprise basic spessartite (hornblende-plagioclase lamprophyre, ≈56% SiO 2 ), with mg numbers, Cr and Ni contents typical of primitive mantle-derived rocks, and more acidic plagioclase-phyric quartz-andesite (≈ 66% SiO 2 ), together with felsic net-veins and hornblendic patches suggesting local differentiation. The spessartites can be regarded chemically as volatile-, LILE-and incompatible element-enriched versions of siliceous high-magnesian basalts from the local greenstone sequence. A Pb-Pb isochron on one dyke yields an age of 2636±27 Ma, making this the oldest composite dyke so far dated. Field and geochemical evidence, including identical Pb initial-ratios for the constituent rock-types, virtually indistinguishable REE spectra, and tight linear trends on plots such as Y versus Ti and Ce/Cr versus Ti/Cr, suggests that the two magmas (lamprophyric and acidic) were genetically related and were emplaced contemporaneously into the same fracture. Quantitative least-squares modelling of whole-rock and mineral compositions suggests that the acidic components resulted mainly from fractionation of hornblende from parental spessartite magmas, together with a possible contribution from mixing of the same spessartite magma with crustally-derived acidic (e.g., trondjhemitic) magma; the latter is represented by separate dykes and stocks in the Kambalda area. Close similarities between the Kambalda composite dykes and Archaean-Tertiary composite dykes in other parts of the world, imply a persistent role for mantle-derived magmas such as lamprophyres in the genesis of the dominant intermediate-acidic (e.g., granitoid) magmas in many calc-alkaline provinces.

REGRES: a FORTRAN-77 program to calculate nonparametric and :20structural” parametric solutions to bivariate regression equations

Abstract REGRES allows a range of regression equations to be calculated for paired sets of data values in which both variables are subject to error (i.e. neither is the “independent” variable). Nonparametric regressions, based on medians of all possible pairwise slopes and intercepts, are treated in detail. Estimated slopes and intercepts are output, along with confidence limits, Spearman and Kendall rank correlation coefficients. Outliers can be rejected with user-determined stringency. Parametric regressions can be calculated for any value of λ (the ratio of the variances of the random errors for y and x )—including: (1) major axis ( λ = 1); (2) reduced major axis ( λ = variance of y /variance of x ); (3) Y on X λ = infinity; or (4) X on Y ( λ = 0) solutions. Pearson linear correlation coefficients also are output. REGRES provides an alternative to conventional isochron assessment techniques where bivariate normal errors cannot be assumed, or weighting methods are inappropriate.