Pearce Element Ratio Analysis Research Articles

Molar element ratio analysis of lithogeochemical data: a toolbox for use in mineral exploration and mining

Molar element ratio analysis of element concentrations consists of four basic tools that provide substantial insight into the lithogeochemistry (and mineralogy) of rocks under examination. These tools consist of: (1) conserved element ratio analysis; (2) Pearce element ratio analysis; (3) general element ratio analysis; and (4) lithogeochemical mineral mode analysis. Conserved element ratio analysis is useful in creating a chemostratigraphic model for the host rocks to mineral deposits, whereas Pearce element ratio analysis and general element ratio analysis are primarily used to identify mineralogical and metasomatic controls on rock compositions and to investigate and quantify the extent of the material transfers that formed the host rocks and mineralization. Lithogeochemical mineral mode analysis converts element concentrations into mineral concentrations using a matrix-based change-of-basis operation, allowing lithogeochemical data to be interpreted in terms of mineral modes. It can be used to provide proper names to rocks, an important activity for an exploration geologist because of the implications that rock names have on genetic processes and mineral deposit models. This paper provides a review of the theoretical foundations of each of these four tools and then illustrates how these techniques have been used in a variety of exploration applications to assist in the search for, evaluation and planning of, and the mining of mineral deposits. Examples include the evaluation of total digestion lithogeochemical datasets from mineral deposits hosted by igneous and sedimentary rocks and formed by hydrothermal and igneous processes. In addition, this paper illustrates a more recent geometallurgical application of these methods, whereby the mineral proportions determined by lithogeochemical mineral mode analysis are used to predict rock properties and obtain the ore body knowledge critical for resource evaluation, mine planning, mining and mine remediation. Thematic collection: This article is part of the Exploration 17 collection available at: https://www.lyellcollection.org/cc/exploration-17

Kimberlites: Magmas or mixtures?

Although the presence of xenocrystic olivine is widely recognized in kimberlite, there is little consensus about its contribution to the existing estimates for the composition of kimberlite magma. Whole rock geochemistry is critical to the debate regarding the composition of kimberlite magma, however, it has received little attention as an indicator of diamond grade due to conventional thought that diamonds are xenocrysts unrelated to their host kimberlite. The Foxtrot kimberlite Field in Northern Québec is comprised of at least three distinct kimberlite intrusions exhibiting variation in both diamond grade and geochemistry making it an ideal suite with which to test a possible correlation between diamond grade and whole rock composition. Olivine is ubiquitous (30 to 70%) in the Foxtrot kimberlites and exhibits a restricted composition that overlaps that of olivine in harzburgite xenoliths suggesting that the majority of olivine is xenocrystic. Carbonate is also abundant (8 to 35%) in the Foxtrot kimberlites and exhibits magmatic textures requiring that carbon be considered in any petrogenetic model for the Foxtrot kimberlites. Pearce element ratio analysis assuming P as a conserved element indicates that much of the major element variation in the Foxtrot kimberlites can be explained by variable amounts of olivine and orthopyroxene in proportions (~ 80/20), similar to that of cratonic mantle xenoliths. The xenocrystic nature of olivine requires that the contribution of mantle harzburgite must be removed to constrain the composition of the magma. The calculated magma composition that results from the mathematical removal of olivine and orthopyroxene (80/20) from the whole rock compositions is significantly poorer in MgO (15 wt.%) and silica (~ 24 wt.%), but CO 2 rich (~ 17 wt.%) compared to previous estimates for kimberlite magma. The Foxtrot kimberlites are best modelled as mixtures of harzburgite mantle and a relatively carbonate-rich magma. According to this model the correlation between whole rock composition and diamond grade reflects the fact that the diamonds are also xenocrystic and sourced in the harzburgite component.

Kimberlites and aillikites as probes of the continental lithospheric mantle

Although the mantle xenoliths carried by kimberlites are the source of much of our information about the composition of the mantle beneath the continents, the compositions of kimberlites themselves have received little attention for the information they carry about the nature of the lithospheric mantle. This neglect in part reflects their common fragmental, contaminated, and hybrid nature, but also the pervasive view that Group-I kimberlites are sourced in the underlying asthenosphere. Insight into the nature of kimberlites and their relationship to the other alkaline ultramafic rocks, such as aillikites, olivine lamproites, and meimechites, can be obtained by comparing their major element compositions in a way that treats their carbonate content as a primary magmatic phase. Group-I kimberlites and aillikites contain significant magmatic carbonate and their compositions fall to the Si-poor side of the composition of olivine. Group-I kimberlite can be distinguished from aillikite on the basis of Fe content, but there appears to be a gradation between these two end-members. In contrast, olivine lamproites and meimechites contain relatively little primary magmatic carbonate and have compositions that are more Si-rich than olivine. Pearce element ratio analysis assuming P as a conserved element indicates that much of the major element variation in hypabyssal kimberlites can be explained by variable amounts of olivine and orthopyroxene in proportions (∼ 70/30) similar to that of cratonic mantle xenoliths. Much of the olivine is present as xenocrysts, but the orthopyroxene is occult and has presumably been assimilated. The fact that individual fields of alkaline ultramafic rocks are characterized by uniform Fe and Ti contents that can be mapped on a regional scale suggests that the major element composition of these unusual rocks, and Group-I kimberlites in particular, is a reflection of the continental lithospheric mantle with which they have interacted. The association of Fe-rich aillikitic magmas with zones of cratonic rifting, and the requirements of Fe–Mg partitioning indicate that they form deeper than Group-I kimberlites, near the intersection of the 1350 °C mantle adiabat with the CO 2 mantle solidus, at pressures of 8 +GPa. According to our working model, Group-I kimberlites are produced by the influx and reaction of carbonate-rich magmas with the highly magnesian harzburgites of the lithospheric mantle beneath continental cratons. In non-cratonic environments, these rising carbonate-rich magmas evolve into aillikites because of the lower Mg # of the asthenospheric mantle. They rarely reach the surface, however, but become the enriched component incorporated into higher-degree basaltic melts. An inverse correlation between diamond grade and kimberlite Fe and Ti contents may simply reflect the fact kimberlites with the lowest Ti contents have interacted with the most depleted and reduced harzburgites of the lithospheric mantle, where diamonds are most likely to be encountered.

Two lithogeochemical approaches to the identification of alteration patterns at the Elura Zn–Pb–Ag deposit, Cobar, New South Wales, Australia: use of Pearce Element Ratio analysis and Isocon analysis

Lithogeochemical approaches to the identification of ore-related alteration can be obscured by closure effects and/or pre-existing lithological variations. Two mass balance approaches that can address these problems are Pearce Element Ratio analysis (PER) and Isocon analysis (IA). Both approaches have been applied to sedimentary rocks hosting the Elura ore-body, north of Cobar, in central-west New South Wales, Australia. Elura is an epigenetic, zinc–lead–silver deposit, (45 Mt with 8.5% Zn, 5.3% Pb, and 69 ppm Ag), hosted within Devonian siltstone–sandstone turbidites of the Cobar Basin. The ability of PER and IA to identify alteration at Elura and to navigate towards ore within altered zones is evaluated. PER identified mineralogical controls on calcium, carbonate, potassium and aluminium. Molar Ca/Ti v. C/Ti plots segregate calcite-bearing background samples from iron–magnesium–carbonate altered rocks closer to ore. Molar K/Ti v. Al/Ti and K/Ti v. (Al–Na)/Ti diagrams show altered rock bulk compositions to be consistent with the development of muscovite, while background samples have an illite ± albite precursor. The precursor lithotype has a major influence on the intensity of alteration and resultant mineralogy of altered samples. IA shows that carbonate carbon, Ag, As, K, Pb, Rb, Sb, Tl and Zn are strongly added to host rocks, while Na is strongly leached. Other elements show variable trends towards ore, with lithotype strongly affecting the patterns. Samples have greater mass gains closer to ore. Selection of an appropriate parent rock is critical in gaining meaningful results in IA.

Geological and Mineralogical Characterization of Zeolites in Lacustrine Tuffs, Ngakuru, Taupo Volcanic Zone, New Zealand

AbstractMordenite and clinoptilolite have replaced glass shards and pumice in vitric tuffs that are the products of ash fall-out into lake basins of late Quaternary age in the Taupo Volcanic Zone. The vitric tuffs are intercalated with siltstone and diatomite and overlie pumice-rich, rhyolitic ignimbrite. A Zr/TiO2-Nb/Y immobile element ratio plot indicates that the vitric tuffs, like the ignimbrite, are of rhyolitic composition. X-ray diffraction and scanning electron microscopy studies indicate that the mordenite and clinoptilolite are accompanied by authigenic K-feldspar and Opal-CT. The zeolites and other authigenic minerals are very fine grained (<10 µm), with open meshes of acicular mordenite crystals that result in low densities (0.7–1.0 g cm−3) in mordenite-rich tuffs. From Pearce element ratio analysis of whole-rock chemical analyses, only Na and K appear to have been mobilized during alteration. The zeolite deposits are associated with sinter, hydrothermal eruption breccias and silicified fault breccias that represent surface or near-surface manifestations of geothermal activity. Plant material extracted from a sinter overlying one of the deposits has a 14C age of 8498±60 BP, which is interpreted to be the age of zeolite deposition for this deposit. Mordenite and clinoptilolite occur in the lower-T (60–110°C) parts of some active or recently active geothermal systems elsewhere in the Taupo Volcanic Zone. The main fluid in these systems is weakly saline (alkali-chloride) water heated by geothermal activity. The Ngakuru zeolite deposits are interpreted as the products of the reaction of vitric tuffs with this type of water in the near-surface part of recently active geothermal systems.

Experimental determination of aluminous clinopyroxene–melt partition coefficients for potassic liquids, with application to the evolution of the Roman province potassic magmas

We have experimentally determined the partition coefficients of rare earth elements (REE), HFSE (Ti, Zr, Hf, Nb, Ta) and other trace elements between clinopyroxene and potassic silicate melts at 0.1–200 MPa and 1042–1140°C. The major and trace element contents of clinopyroxene and liquid were determined by electron probe and ion microprobe, respectively. The liquidus clinopyroxenes are extremely rich in Al 2O 3 (up to 12%) and Fe 2O 3, which enables us to test suggestions that partition coefficients of HFSE and REE depend on the Al content of the clinopyroxene (e.g., Lundstrom, C.C., Shaw, H.F., Ryerson, F.J., Williams, Q., Gill, J., 1998. Crystal chemical control of clinopyroxene–melt partitioning in the Di–Ab–An system: implications for elemental fractionations in the depleted mantle. Geochim. Cosmochim. Acta 62, 2849–2862.). When compared with previous data for low-alumina clinopyroxene crystallised under similar P– T conditions, the increase in weight partition coefficient D i (=[ I] xtl/[ I] liq) for all the HFSE and REE is dramatic. For Ti and the REE, partition coefficients increase by about one order of magnitude as Al iv concentration of clinopyroxene increases from 0.02 to 0.5. For Nb, Ta and Zr, the effect is even more pronounced, a two order of magnitude increase being found to correspond to this increase in Al iv. We applied the data to model the fractionation of historic lavas from Vesuvius. Previous work using the major elements (Belkin, H.E., Kilburn, C.R.J., DeVivo, B., 1993. Sampling and major element chemistry of the recent (AD 1631–1944) Vesuvius activity. J. Volcanol. Geotherm. Res. 58, 273–290.) indicates a liquid line of descent from trachybasalt to leucite tephrite dominated by clinopyroxene crystallisation. The REE and HFSE concentrations in the lavas are consistent with 40–50% fractional crystallisation of clinopyroxene (accompanied by minor leucite) between the two magmatic endmembers, with partition coefficients for the REE appropriate for clinopyroxenes containing ≤7.5 wt.% Al 2O 3. Pyroxenes of the latter composition are frequent phenocrysts in the lavas and previous experimental work (Dolfi, D., Trigila, R., 1988. Chemical relations between clinopyroxenes and coexisting glasses obtained from melting experiments on alkaline basic lavas. Rend. Soc. Ital. Mineral. Petrol. 43, 1101–1110; Trigila, R., De Benedetti, A.A., 1993. Petrogenesis of Vesuvius historical lavas constrained by Pearce element ratio analysis and experimental phase equilibria. J. Volcanol. Geotherm. Res. 58, 315–343.) implies that they precipitate under very low pressure low water activity conditions, i.e. within the magma chamber at <200 MPa.

Geochemical evolution of Jurassic diorites from the Bristol Lake region, California, USA, and the role of assimilation

Late Jurassic dioritic plutons from the Bristol Lake region of the eastern Mojave Desert share several geochemical attributes with high-alumina basalts, continental hawaiite basalts, and high-K are andesites including: high K2O concentrations; high Al2O3 (16–19 weight %); elevated Zr/TiO2; LREE (light-rare-earth-element) enrichment (La/YbCN=6.3–13.3); and high Nb. Pearce element ratio analysis supported by petrographic relations demonstrates that P, Hf, and Zr were conserved during differentiation. Abundances of conserved elements suggest that dioritic plutons from neighboring ranges were derived from similar parental melts. In the most voluminous suite, correlated variations in elemental concentrations and (87Sr/86Sr)i indicate differentiation by fractional crystallization of hornblende and plagioclase combined with assimilation of a component characterized by abundant radiogenic Sr. Levenberg-Marquardt and Monte Carlo techniques were used to obtain optimal solutions to non-linear inverse models for fractional crystallization-assimilation processes. Results show that the assimilated material was chemically analogous to lower crustal mafic granulites and that the mass ratio of contaminant to parental magma was on the order of 0.1. Lack of enrichment in 18O with differentiation is consistent with the model results. Elemental concentrations and O, Sr, and Nd isotopic data point to a hydrous REE-enriched subcontinental lithospheric source similar to that which produced some Cenozoic continental hawaiites from the southern Cordillera. Isotopic compositions of associated granitoids suggest that partial melting of this subcontinental lithosphere may have been an important process in the development of the Late Jurassic plutonic arc of the eastern Mojave Desert.

The petrology of Poás volcano lavas: basalt-andesite relationship and their petrogenesis within the magmatic arc of Costa Rica

Basalt, andesite and dacite from Poás volcano exhibit a distinct petrogenetic evolution. The basalts (Al 2O 3 = 16–17.4%, K 2 O = 1.43–1.61%) are hypersthene-normative tholeiites with phenocrysts of labradorite, olivine, and two pyroxenes. Orthopyroxene (En 67-56) is generally rimmed by clinopyroxene (Fs 10–18 Wo 39–48). In all basalts and in a few andesites, pyroxene phenocrysts have Al- and Ti-rich salite rims that surround cores of orthopyroxene or augite having lower values. The Al z values (% of tetrahedral site occupied by Al) for these augites are above 6%, thus defining on the Al z vs. TiO 2 diagram an intermediate trend between arc-related cumulates and rift-related tholeiites and ophiolite-layered cumulates. Moreover, the presence of olivines with coronae of wormy intergrowths of orthopyroxene and Ti-magnetite (similar to those occurring in metagabbros of the granulite facies and characteristic of Arenal gabbroic xenoliths) together with anorthite megacrysts (An 94-91) suggest a possible contamination origin for these basalts. Sr-isotope ratios for the basalts are between 0.703816 and 0.703849 whereas andesites and dacites have lower ratios (0.703676–0.703715): these relationships together with the mineral compositions suggest that the basalts may have formed from basaltic andesites by contamination with olivine gabbros and anorthosites of the lower crust. Pearce element-ratio analysis confirms that the basalts had a different origin from the rest of the suite and that the dacite may be derived by crystal fractionation of the less silicic basaltic andesite with removal mainly of clinopyroxene and plagioclase.

Endmember unmixing of compositional data

Manipulation of the n-endmember mixing equations, appropriate to compositional data, leads to a general method (1) to test hypotheses about endmember compositions and (2) to recover the concentrations of all compositional components in an endmember when the concentrations of n − 1 components are known or measured in that endmember. Additionally, Pearce element-ratio analysis is shown to be a special (and not efficiently posed) two-endmember case of n-endmember unmixing. Information available on Pearce element-ratio diagrams as values of the slopes of lines is contained on Harker diagrams as values of points.

The validity of pearce element ratio analysis in petrology: an example from the Uwekahuna laccolith, Hawaii

Chemical variation diagrams formed from ratios with a common denominator exhibit induced correlations which can obscure the effects of true compositional variations. Nevertheless, important conclusions have been obtained using Pearce element ratio (and isochron) diagrams, even though these diagrams are formed from axes ratios with a common denominator. In this paper, we consider two approaches to mitigate the effects of induced correlation, and in doing so, demonstrate the validity of geochemical analysis using diagrams formed from ratios with a common denominator. First, we propagate analytical error onto Pearce element ratio diagrams to distinguish chemical variations associated with analytical error from chemical variations related to geological processes. Second, we consider an alternative diagram formed from ratios with different denominators which does not exhibit any induced correlation. We demonstrate both theoretically and by example that results produced using this diagram are identical to those using diagrams with a common denominator. These two lines of evidence confirm that Pearce element ratio (and isochron) diagrams are useful and valid tools in the analysis of geochemical variations.