Normative Minerals Research Articles

WebNORM: a web application for calculating normative mineralogy

Normative mineralogy is a valuable tool for interpreting the mineralogical composition of rocks based on their bulk rock geochemical data. Despite the development of various computational tools for normative mineralogy calculations, the accessibility and continued usage of many existing tools is limited by the use of older languages, licensing constraints, and restricted access to the source code. In this paper, we introduce a Python-based implementation of the normative mineralogy algorithm, which is accessible through a user-friendly web application webNORM, or as part of the pyrolite Python package. The algorithm uses major, minor, and trace element oxides as input and returns up to 31 normative minerals. It offers two methods for estimating the Fe2O3:FeO ratio. The web application offers a convenient interface for users without programming experience, while the pyrolite integration allows more advanced users to leverage the Python ecosystem for end-to-end geochemical data analysis. Comparison between our implementation of the normative calculation and previous implementations shows that our version is accurate and reliable. webNORM offers the geoscience community an accessible solution for calculating normative mineralogy. The algorithm and web application are open-source, and we encourage contributions and adaptations by the community for specific needs and improvements.

Quantifying Hydrothermal Alteration: A Review of Methods

Hydrothermal alteration is proximal to many base and precious metal deposits, and its products can provide insights into the characteristics of hydrothermal systems. To be useful to exploration geologists and researchers, however, alteration needs to be typified and quantified. Alteration type informs on mineralising style (e.g., have we found a porphyry or a volcanogenic massive sulphide deposit?), while quantification of its intensity helps position a sample within the system (e.g., how close are we to the main economic deposit?). Numerous methods—all having their specific advantages and disadvantages—are dedicated to the characterisation of alteration. As alteration is a process that induces chemical and mineralogical changes in rocks, it can be studied using petrological (e.g., mineral recognition in thin sections, mineral chemistry), mineralogical (e.g., alteration indices that use normative minerals), and chemical (e.g., mass balance calculations) approaches. This short review provides an overview of the methods useful to researchers and that are also applicable in an exploration context.

Mantle metasomatism and magma formation in continental lithosphere: Data on xenoliths in alkali basalts from the Makhtesh Ramon, Negev desert, Israel

Mantle xenoliths (lherzolites, clinopyroxene dunites, wehrlites, and clinopyroxenites) in the Early Cretaceous volcanic rocks of Makhtesh Ramon (alkali olivine basalts, basanites, and nephelinites) represent metasomatized mantle, which served as a source of basaltic melts. The xenoliths bear signs of partial melting and previous metasomatic transformations. The latter include the replacement of orthopyroxene by clinopyroxene in the lherzolites and, respectively, the wide development of wehrlites and olivine clinopyoroxenites. Metasomatic alteration of the peridotites is accompanied by a sharp decrease in Mg, Cr, and Ni, and increase of Ti, Al, Ca contents and 3+Fe/2+Fe ratio, as well as the growth of trace V, Sc, Zr, Nb, and Y contents. The compositional features of the rocks such as the growth of 3+Fe/2+Fe and the wide development of Ti-magnetite in combination with the complete absence of sulfides indicate the high oxygen fugacity during metasomatism and the low sulfur concentration, which is a distinctive signature of fluid mode during formation of the Makhtesh Ramon alkali basaltic magma. Partial melting of peridotites and clinopyroxenites is accompanied by the formation of basanite or alkali basaltic melt. Clino- and orthopyroxenes are subjected to melting. The crystallization products of melt preserved in the mantle rock are localized in the interstices and consist mainly of fine-grained clinopyroxene, which together with Ti-magnetite, ilmenite, amphibole, rhenite, feldspar, and nepheline, is cemented by glass corresponding to quartz–orthopyroxene, olivine–orthopyroxene, quartz–feldspar, or nepheline–feldspar mixtures of the corresponding normative minerals. The mineral assemblages of xenoliths correspond to high temperatures. The high-Al and high-Ti clinopyroxene, calcium olivine, feldspar, and feldspathoids, amphibole, Ti-magnetite, and ilmenite are formed at 900–1000°. The study of melt and fluid inclusions in minerals from xenoliths indicate liquidus temperatures of 1200–1250°C, solidus temperatures of 1000–1100°C, and pressure of 5.9–9.5 kbar. Based on the amphibole–plagioclase barometer, amphibole and coexisting plagioclase were crystallized in clinopyroxenites at 6.5–7.0 kbar.

The Coulon deposit: quantifying alteration in volcanogenic massive sulphide systems modified by amphibolite-facies metamorphism

The Coulon deposit is a volcanogenic massive sulphide (VMS) system in the James Bay area, Superior craton, Quebec, that was metamorphosed to amphibolite-facies conditions. The chemistry and mineralogy of the VMS-related alteration halo proximal to the mineralized sulphide lenses are investigated, using samples collected in the field and 5583 chemical analyses provided by Osisko Ltd. Alteration is quantified using mass balance and normative calculations, and the application and performance of these methods in an exploration context are investigated. In VMS systems, altered rocks proximal to the ore zones are characterized by multi-element metasomatism, which is best quantified by mass balance methods that have been successfully applied in the study area. However, mass balance calculations necessitate the documentation of a precursor, which is not always possible in an exploration context; therefore, an alternative method (i.e., alteration indices) was also evaluated. In most VMS systems, proximal alteration is characterized by chlorite (chloritization), muscovite (sericitization), and quartz (silicification), while at the Coulon deposit, altered rocks contain mostly cordierite, biotite, sillimanite, and quartz. Alteration indices were calculated using observed and normative minerals, and provide satisfactory results similar to those obtained with mass balance calculations. Using these results, recommendations are made to estimate the intensity of alteration in the core shack using the proportions of observed minerals. Alteration indices are sensitive to the composition of precursors; and because of high-grade metamorphism, chloritization and sericitization are not precisely quantified. Recognizing these limitations is essential to successful quantification of alteration in areas metamorphosed to high-grade conditions.

Quantifying hydrothermal alteration with normative minerals and other chemical tools at the Beattie Syenite, Abitibi greenstone belt, Canada

Methods to quantify hydrothermal alteration are used to: document alteration halos; to comprehend hydrothermal processes; and to prospect for mineralisation formed by such processes. The chemical methods available are numerous and each has specific advantages, limitations and fields of application. This study focuses on the hydrothermal process at Beattie, a Neoarchean gold deposit hosted by the Beattie Syenite within the Abitibi Greenstone Belt, Superior Province, Canada. To quantify alteration, it was necessary to use the following chemical methods: (1) mass-balance calculations; (2) normative minerals and related alteration indices; and (3) Pearce Element Ratios (PER) diagrams. In the study area, silicification and carbonatisation are satisfactorily quantified by alteration indices, while alkali metasomatism was best estimated by mass-balance calculations and PER diagrams. Combining these methods, the following alteration types have been documented: K-feldspar alteration, silicification, and Na-Ca-leaching are intense and proximal to gold mineralisation; carbonatisation is widespread and intense; and sericitisation and chloritisation are minor to absent. It is proposed that, at the Beattie Syenite, the formation of white mica and chlorite and the mobilisation of alkali and silica are consequences of the predominant process related to carbonatisation.

CONSONORM_LG: New Normative Minerals and Alteration Indexes for Low-Grade Metamorphic Rocks

The CONSONORM\_LG method provides a standardized solution for approximating metamorphic parageneses, as well as indexes for estimating chemical and mineralogical changes caused by hydrothermal alteration. CONSONORM\_LG is designed for rocks dominated by silicates, Fe-Ti oxides, and/or carbonates, and it approximates the main parageneses of greenschist and lower amphibolite-grade metamorphic rocks for three sets of temperature and pressure conditions (350°C and 2.5 kbars, 450°C and 2.5 kbars, 575°C and 2 kbars). For each model, the norm calculates the main paragenesis using an ACFMNK tetrahedron, a convenient way of representing a large number of silicate assemblages. In addition to silicate minerals, CONSONORM_LG calculates Fe-Ti oxides and other accessory minerals from minor elements, as well as sulfides from analyzed S or from analyzed metals, and carbonates from analyzed CO2 or from normative CO2 estimated from loss on ignition (LOI). CONSONORM_LG also calculates several alteration indexes to estimate Fe-Mg (e.g., chloritization), Ca (e.g., propylitic alteration), Na and K acid alterations (e.g., sericitization, phyllic alterations), and Al gain (e.g., argillization). Carbonatation indexes are also calculated using the amount of normative minerals formed by this type of alteration, i.e., carbonates, chlorite, and muscovite. The normative calculation is validated using published whole-rock analyses and petrographic descriptions. Alteration indexes are validated using several natural samples of alteration halos around base metal and gold deposits.

CONSONORM_HG: a new method of norm calculation for mid‐ to high‐grade metamorphic rocks

AbstractThe CONSONORM_HG norm provides a standardized solution to approximate metamorphic assemblages for mid‐ to high‐grade metamorphic rocks. Other applications for this norm include: (i) the standardized classification of metamorphic rocks; (ii) the interpretation of hydrothermal alteration using indices calculated from normative minerals; (iii) the estimation of the carbonate content of rocks, even when CO2 has not been analysed. CONSONORM_HG is designed for rocks dominated by silicates, Fe–Ti oxides or carbonates, and it approximates the main mineral assemblages of amphibolite, granulite, greenschist and blueschist facies metamorphic rocks. The norm calculates silicate assemblages using one of the three Al–Ca–NaK–FeMg tetrahedra, a convenient way of representing a large number of silicate assemblages, for each of the 17 P–T conditions modelled and based on the Fe–Mg–Mn composition of the sample. In addition to silicate minerals, CONSONORM_HG calculates Fe–Ti oxides and other accessory minerals from minor elements, sulphides from analysed sulphur and/or from analysed metals and also carbonates from analysed CO2 or from normative CO2 estimated from the loss on ignition. CONSONORM_HG also integrates many reactions to address silicon deficits, as well as quartz–carbonate reactions to approximate natural assemblages better. The normative calculation is validated using published whole‐rock analyses and petrographic descriptions and its various applications are discussed.

Petrogenesis of Early Cretaceous alkaline dolerite dykes in the Dabie Orogen, China: Constraints on the timing of lithospheric thinning

The Dabie Orogen in central China has long been proposed to transform from Triassic compression related to northward deep subduction to Late Jurassic to Early Cretaceous extension involving marked crustal thinning and doming processes. Zircon U–Pb ages, mineral chemistry, and whole-rock geochemistry of alkaline dykes intruding Early Cretaceous intermediate–mafic–ultramafic complexes are presented herein to provide evidence for the timing of lithospheric thinning and extension. These dykes of dominantly dolerites with minor monzodiorites have zircon U–Pb ages of 128.3–129.7 Ma and are characterized by Sr–Nd isotopic ratios around the enriched mantle I. The dolerites can be divided into two groups, with the first group having lower incompatible elements but more enriched Sr–Nd isotopes than the second group. Equilibrium partial melting and isotopic mixing modeling show that the dolerites were first generated from partial melting of a heterogeneous enriched mantle at different degrees and then experienced fractional crystallization and crustal assimilation. Systematic normative minerals, trace elements and isotopic contrasts of Early Cretaceous gabbros (125–130 Ma), dolerite dykes of this study, and Early Cretaceous mafic dykes (111–115 Ma) indicate that the source regions for the dolerites are deeper than those for the host gabbros. Therefore, the lithospheric mantle thickening probably continued until the formation of the dolerites, and the lithospheric mantle thinning occurred significantly later than previously proposed. The thickness of the lithosphere was more than the depth range for the source region of alkaline basalts at 125–130 Ma but changed to less than that for the source region of tholeiitic basalts before 115 Ma, suggesting a minimum decrease of ~ 20 km within ~ 10 m.y. This rate of thinning can be explained only by delamination of voluminous crustal root with the underlying lithospheric mantle between 125 and 115 Ma.

Geochemical insights into the role of metasomatic hornblendite in generating alkali basalts

Experimental petrology suggested the role of hornblendite in generating alkali basalt. This mechanism is confirmed by an integrated study of major-trace elements and radiogenic isotopes for Mesozoic alkali basalts from the Qinling orogen in China. The alkali basalts have high contents of MgO (4.8–11.1 wt %, Mg# = 47–69), Na2O + K2O (2.9–5.4 wt %), TiO2 (2.0–3.1 wt %) but low content of SiO2 (41.4–49.6 wt %), which are generally silica-undersaturated with normative minerals of nepheline and olivine. They exhibit OIB-like trace element distribution patterns, with enrichment of LILE and LREE but no depletion of HFSE relative to the primitive mantle. They also show relatively depleted Sr-Nd-Hf isotope compositions, with low initial 87Sr/86Sr ratios of 0.7028–0.7058, positive εNd(t) values of 4.0–9.8 and εHf(t) values of 8.8–13.5 for whole-rock, and positive εHf(t) values of 5.2–16.4 for zircon. Such element and isotope features indicate their origination from the juvenile subcontinental lithospheric mantle (SCLM) source with involvement of crustal components. The alkali basalts generally have high K2O/Na2O ratios, and high K2O and TiO2 contents, suggesting their derivation from partial melting of hornblendite-rich mantle lithology. They also exhibit variable K/La and Ti/La ratios that are correlated with (La/Yb)N ratios, indicating a geochemical heterogeneity of the SCLM source. Taken together, all the above geochemical features can be accounted for by partial melting of a hornblendite-rich SCLM source. The hornblendite would be generated by reaction of the juvenile SCLM wedge peridotite with hydrous felsic melts derived from subducted Palaeotethyan oceanic crust at the slab-mantle interface in the subduction channel. Therefore, orogenic alkali basalts record recycling of the subducted fossil oceanic crust, and the metasomatic hornblendite is an important lithology in local SCLM domains above fossil subduction channels.

Predicting Archaean volcanogenic massive sulphide deposit potential from lithogeochemistry: application to the Abitibi Greenstone Belt

Archaean greenstone belts are prime exploration targets for volcanogenic massive sulphide deposits and the use of lithogeochemistry plays an important role in detecting and mapping hydrothermal alteration zones and in recognizing the massive sulphide potential of volcanic assemblages. This study reviews the state of existing lithogeochemical databases and the use of new geochemical analyses for the identification of alteration associated with massive sulphide deposits. This paper also provides a new methodology for the prediction of missing values in geochemical compositions. Through the application of statistical methods, combined sets of lithogeochemical data can be used to create predictive maps of prospective massive sulphide successions. Predictive maps are created through the use of multivariate statistical methods, lithogeochemical alteration indices and normative minerals from which areas of multi-element enrichment and depletion are defined. These indices, when combined in an exploration programme, are potentially useful for massive sulphide discovery. Additionally, the use of rare earth elements can be used to characterize differences in the various supracrustal assemblages and provide a better regional understanding of potential metal endowment. Supplementary material: Provides details on the datasets selected in this study and is available at http://www.geolsoc.org.uk/SUP18712

Litoquímica e química mineral da Formação Quarenta Ilhas no Distrito Mineiro de Pitinga (AM)

The Quarenta Ilhas Formation consists of basic to intermediate subvolcanic rocks, with tholeitic affinity and ages around 1.78Ga. The magmatic mineralogy is composed by olivine, pseudomorfosed by ferrous-saponite, plagioclase varying from labradorita to oligoclase (An65Ab35 a An20Ab80), Fe-Ti oxides (Ilm87-95 and Usp22-37), diopside to augite (Wo44-35En44-21Fs38-16), with ferro-hornblende (as coronitic texture), alkali-feldspar and quartz in the most differentiated rocks. Petrographic and geochemical data point to an evolution through fractional crystallization from diabase/gabbro to quartz diorite, with fractionation of olivine+ilmenita, magnetite+plagioclase+augite, and crystallization of ferro-hornblende, alkali-feldspar and quartz in the later stages. The affinity with tholeiitic subalkaline magma is indicated by features of major elements, normative minerals (olivine+hypersthene and quartz+hypersthene) and high Y/Nb and La/Yb ratios. High Cs, Rb, Ba and K, and low Nb, Ta and Zr, suggest a lithospheric mantle source, enriched in LILE and depleted by subduction. The REE pattern is LREE enriched faced to HREE, that suggest a source of garnet lherzolitic composition. The emplacement of the Quarenta Ilhas subvolcanic bodies follow the orogenetic activity of the Transamazonian Cycle (~2.0Ga), the post-collisional Uatumã magmatismo (~1.88Ga) and the anorogenic magmatism of the Madeira Suite (~1.83Ga), in a fully intracratonic setting

Litoquímica e química mineral da Formação Quarenta Ilhas no Distrito Mineiro de Pitinga (AM)

Lithochemistry and mineral chemistry of Quarenta Ilhas Formation in the Pitinga Min- ing District, Pitinga (AM), Brazil. The Quarenta Ilhas Formation consists of basic to intermediate subvo- lcanic rocks, with tholeitic affinity and ages around 1.78Ga. The magmatic mineralogy is composed by olivine, pseudomorfosed by ferrous-saponite, plagioclase varying from labradorita to oligoclase (An 65 Ab 35 a An 20 Ab 80 ), Fe-Ti oxides (Ilm 87-95 and Usp 22-37 ), diopside to augite (Wo 44-35 En 44-21 Fs 38-16 ), with ferro-hornblende (as coronitic texture), alkali-feldspar and quartz in the most differentiated rocks. Petrographic and geochemical data point to an evolution through fractional crystallization from diabase/gabbro to quartz diorite, with fractionation of olivine+ilmenita, magnetite+plagioclase+augite, and crystallization of ferro-hornblende, alkali-feldspar and quartz in the later stages. The affinity with tholeiitic subalkaline magma is indicated by features of major ele - ments, normative minerals (olivine+hypersthene and quartz+hypersthene) and high Y/Nb and La/Yb ratios. High Cs, Rb, Ba and K, and low Nb, Ta and Zr, suggest a lithospheric mantle source, enriched in LILE and depleted by subduction. The REE pattern is LREE enriched faced to HREE, that suggest a source of garnet lherzolitic composition. The emplacement of the Quarenta Ilhas subvolcanic bodies follow the orogenetic activity of the Transamazonian Cycle (~2.0Ga), the post-collisional Uatuma magmatismo (~1.88Ga) and the anorogenic magmatism of the Madeira Suite (~1.83Ga), in a fully intracratonic setting.

Origin of layered phosphorites and geochemical-mineralogical peculiarities of their accumulation

the phosphorites and the source of phosphorus have been raised and disputed. The similarity of formation conditions of the layered phosphorites led investiga� tors to the supposition that the geological conditions of sedimentation in aggregate were one of the main factors of their arising [10]. We made an attempt to prove or deny this point of view with the help of reveal� ing the differences and similarities of phosphate accu� mulation basins based on comparison of sedimentary rocks seria not only according to their chemical but also mineral composition in the form of normative minerals calculated by one method for all objects. The work was carried out with the help of the com� puter program MINLITH [11], which allowed calcu� lating the mineral composition of sedimentary rocks from their chemical analyses with a high degree of approximation to the real mineral compositions. This program allows establishing independent criteria of their comparison in phosphorites from different basins. We used published data from the largest phos�

Sr, Nd and Pb isotopic and geochemical constraints for the origin of magmas in Popocatépetl volcano (central Mexico) and their relationship with the adjacent volcanic fields

We present new geochemical data (major- and trace-elements, as well as Sr and Nd isotopic compositions) of volcanic rocks erupted from Popocatépetl volcano during the volcanic event from December 2000 to January 2001. These data along with an exhaustive compilation of geochemical and Sr, Nd, and Pb isotope data reported for Popocatépetl rocks and nearby volcanic areas are used to examine the origin and geochemical evolution of the magmas in the central Mexican volcanic belt (CMVB). During this period of volcanic eruptions Popocatépetl produced ash columns as high as 7 km. Pyroclastic flows and lahars were observed after the completion of the activity. Samples of banded pumice and a bomb fragment transported by the lahar were chemically analysed for this work. Rocks show an andesitic composition with 58.5–61.7 wt.% SiO2 and 5.9–4.0 wt.% MgO. Contents of large ion lithophile elements (LILE), rare-earth elements (REE) and Zr are nearly constant through the compositional range. No significant Eu anomaly is present, but the samples show Nb-anomaly relative to LILE and high-field strength elements (HFSE). Nd- and Sr-isotopic compositions of these samples range from 143Nd/144Nd = 0.51291 to 0.51287 and 87Sr/86Sr = 0.70399 to 0.70422. Comparison of Popocatépetl products with volcanic rocks from the nearby areas shows that the magmas in CMVB were generated in a heterogeneously veined-mantle source enriched in LILE, HFSE, and REE. Additional crustal assimilation as well as fractional crystallization could account for the great chemical variability of rocks in the CMVB. Statistical comparison of the geochemical compositions of the volcanic products ejected from 1994 to 2000 to those ejected during the 2001 event shows that most geochemical parameters (major- and trace-elements, normative minerals, Sr and Nd isotopic composition, as well some elemental ratios) present no statistically significant differences. Statistically significant differences in the mean only were computed for the major-elements SiO2, FeO, MgO, CaO, and K2O, as well as for the rare-earth elements Nd, Sm, Eu, Gd, Dy, Ho, Tm, and Yb.

Calculation of the CIPW norm: New formulas

A completely new set of formulas, based on matrix algebra, has been suggested for the calculation of the CIPW norm for igneous rocks to achieve highly consistent and accurate norms. The suggested sequence of derivation of the normative minerals greatly deviates from the sequence followed in the classical scheme. The formulas are presented in a form convenient for error-free implementation in computer programs. Accurate formulas along with the use of variable molecular weights for CaO and FeO; corrected formula weights for apatite, pyrite and fluorite; and suggested measures to avoid significant rounding off errors to achieve absolute match between the sum of the input weights of the oxides and the sum of the weights of the estimated normative minerals. Using an analogous procedure for determining the oxidation ratios of igneous rocks as used in the SINCLAS system of Verma et al (2002, 2003), the suggested calculation scheme exactly reproduces their results except for apatite for reasons explained in the text, but with a superior match between the totals for about 11,200 analyses representing rocks of a wide range of composition

The normative mineralogy of 10 soil profiles in Fennoscandia and north-western Russia

Abstract The mineralogical composition of soil horizons in different soil types of different ages was estimated by applying the NORMA software, which was developed originally for calculating the normative mineralogical composition of young podsols. Ten soil profiles from six sites in NW Russia, two in Finland, and one in NE Norway were sampled in 1999 as a part of the pilot phase of a large geochemical mapping project. Total element concentrations were determined from the 4 extraction. Extractable concentrations for Al, Ca, Cr, Fe, K, Mn, Mg, Na, P, S, Ti, Zr, and Ba were determined by ICP-MS or ICP-AES after aqua regia (a 1:3 mixture of strong HCl and HNO 3 ) extraction. Total C was determined using a thermal conductivity detector from a sample burned in an O 2 stream. The NORMA software was used to calculate the percentage of normative soluble minerals pyrite, apatite, titanite, calcite, biotite, chlorite, weathered albite, hydrous Al-silicate, goethite and soluble residue. The percentages of non-soluble normative minerals rutile, hornblende, K-feldspar, albite, anorthite, tremolite, wollastonite, kaolinite, magnetite, zircon, quartz, carbon (graphite), and non-soluble residue were calculated after soluble minerals. The calculated mineralogical composition of C-horizon samples in each profile reflected the known geological composition of the bedrock from which the soil parent material was derived during geological processes. Secondary minerals including goethite and hydrous Al-silicates, were detected in upper soil horizons reflecting the development of soils. Rather than age, the local bedrock geology together with the mineralogical composition and chemical properties of the parent material proved to be the controlling factor in the formation of secondary minerals. The results showed that the NORMA method can be used in defining the mineralogy of soil horizons in a large variety of soil types.

A method to estimate hydraulic conductivity from bulk geochemical compositions

A new method has been developed to estimate the hydraulic conductivity in unconsolidated aquifers. The method uses bulk geochemical compositions correlated with hydraulic conductivities measured by pumping tests. The concept is based on a general rule that hydraulic conductivity is principally controlled by grain-size distribution and particle shape,both of which relate to mineralogical composition. Using a MINLITH algorithm, normative mineralogical compositions can be derived from bulk geochemical compositions economically and expediently, and then correlated to the hydraulic conductivity determined by pumping tests in the field. In this study, 202 sediment samples from nine unconsolidated aquifers were analyzed by X-ray fluorescence. Although hydraulic conductivity does not show a definite relationship with geochemical compositions, it does demonstrate a linear logarithmic equation to the content of normative earthy minerals. However, linear regressed equations should not be applied to aquifers composed of medium to coarse sand and gravel sizes due to interference from lithic fragments. In addition, this equation tends to overestimate hydraulic conductivity possibly because the effect of compaction is ignored in this study.

MINLITH—an experience-based algorithm for estimating the likely mineralogical compositions of sedimentary rocks from bulk chemical analyses

The MINLITH algorithm is a tool for estimating the likely mineralogical compositions of sedimentary rocks, using information from bulk chemical analyses. It is an experience-based algorithm that represents compositions in terms of a simplified set of normative minerals. MINLITH has been designed to be applied principally to mature sedimentary rocks, but it can (with care) be applied also to immature sediments and to metasedimentary rocks; the compositions that MINLITH gives for metasedimentary rocks are approximations to the original (i.e. pre-metamorphic) mineralogical compositions. The experience base on which MINLITH is built is a collection of 600 reference samples of sedimentary rocks. The compositional regularities found in these samples have allowed empirical rules to be developed to predict how the oxides reported in a bulk chemical analysis should be partitioned among the minerals most likely to be present. The discrepancies between MINLITH-estimated compositions and physically determined modal compositions are relatively small for the most widespread types of mature sedimentary rocks; they are comparable in their magnitude to the discrepancies associated with other methods for estimating mineralogical compositions from bulk chemical analyses, and to the discrepancies associated with quantitative X-ray diffractometry. The MINLITH algorithm is of particular value: (1) for providing preliminary estimates of mineralogical composition, prior to precise modal analysis; (2) for identifying systematic compositional variation within suites of samples; (3) in generalised sample classification; (4) in the sedimentological interpretation of metasedimentary rocks.

Normative minerals and alteration indices developed for mineral exploration

NORMAT is a new technique for quantifying hydrothermal alteration in greenschist facies rocks using normative mineral ratios without precursor or mineralogical determination. Using five petrologic rules, the NORMAT calculations first compute normative alteration (paragonite, sericite, chlorite, pyrophyllite) and precursor minerals (albite, orthoclase, anorthite), as well as normative loss on ignition (LOI) and CO 2 contents. The results are then computed as alteration indices that quantify the overall alkali element depletion in terms of the paragonitization, sericitization, chloritization and pyrophillitization components. Equations for the alteration indices are ratios between the normative alteration and precursor minerals, thus providing a relative numerical measure of alteration intensity that is independent of protolith. Alteration by carbonatization is quantified by the IPAF index, which represents the position of the sample's analytical LOI value within a range bounded by CO 2-free normative LOI and maximum possible normative LOI-CO 2 content. This procedure also allows the CO 2 content to be estimated from LOI, which can then be used to calculate normative carbonate minerals. Alteration index (AI) values and normative minerals for 28 selected samples from the Val d'Or Formation illustrate the main properties of normative mineral ratio alteration indices.

Experimental determination of oxygen isotope fractionations between CO 2 vapor and soda-melilite melt

We report results of experiments constraining oxygen isotope fractionations between CO 2 vapor and Na-rich melilitic melt at 1 bar and 1250 and 1400°C. The fractionation factor constrained by bracketed experiments, 1000 .lnα CO2-Na melilitic melt, is 2.65±0.25 ‰ (±2σ; n=92) at 1250°C and 2.16±0.16 ‰ (2σ; n=16) at 1400°C. These values are independent of Na content over the range investigated (7.5 to 13.0 wt. % Na 2O). We combine these data with the known reduced partition function ratio of CO 2 to obtain an equation describing the reduced partition function ratio of Na-rich melilite melt as a function of temperature. We also fit previously measured CO 2-melt or -glass fractionations to obtain temperature-dependent reduced partition function ratios for all experimentally studied melts and glasses (including silica, rhyolite, albite, anorthite, Na-rich melilite, and basalt). The systematics of these data suggest that reduced partition function ratios of silicate melts can be approximated either by using the Garlick index (a measure of the polymerization of the melt) or by describing melts as mixtures of normative minerals or equivalent melt compositions. These systematics suggest oxygen isotope fractionation between basalt and olivine at 1300°C of approximately 0.4 to 0.5‰, consistent with most (but not all) basalt glass-olivine fractionations measured in terrestrial and lunar basalts.