Pearce Element Ratio Research Articles

Polymagmatic Glaciovolcanism: Cracked Mountain Tuya, Canadian Cascades

Monogenetic volcanoes are characterized as having no temporal break in eruptive activity and are often assumed to have a simple (singular) magmatic plumbing system. However, recent studies on monogenetic systems have started to recover evidence of complexities within the magma-crustal dynamics. Here we investigate Cracked Mountain (CM), a 401 ± 38 ka glaciovolcanic basaltic landform in southwest, British Columbia, Canada. The volcano covers an area of ∼1.5 km2, has an eruptive volume of ∼0.18 km3, and comprises lapilli tuff, breccia, peperite, pillow and sheet lava, and dykes with no erosional surfaces present between the stratigraphic successions. The paleomagnetic signature of all volcanic lithofacies records a single-pole direction and, in conjunction with stratigraphic evidence, implies a monogenetic eruption. We establish that the Cracked Mountain volcano was fed by two separate crustally-stored magmas (i.e., polymagmatic), each characterized by a unique phenocryst assemblage indicative of different pre-eruptive storage conditions. The first mineral assemblage is an olivine-and-plagioclase phyric (OP) suite, and the second is an olivine-plagioclase-and-augite phyric (OPA) suite. The major-element geochemical compositions of the two petrographic suites vary slightly, with OPA samples higher in SiO2 and total-alkali contents than OP. The two magmas have similar rare earth (REE) trace element signatures, suggesting the same mantle source. We use thermodynamic modeling (Rhyolite-MELTS) to show that the OP suite derives from magma stored at depths <6 km (< 2 kbar) and temperatures of 1240–1155°C. In contrast, the OPA magmas crystallized at depths between 7–9 km (∼2–2.5 kbar) at 1,250–1,150°C prior to eruption. Both magmas are shown to be nearly “dry” having less than 0.5 H2O wt% in their respective systems. We use Pearce Element Ratios (PER) to show that the chemical variations within and between the two CM magmas are controlled solely by the crystal fractionation of two phenocryst assemblages that underwent syn-eruptive mixing. This study concludes that the polymagmatic plumbing system at Cracked Mountain shows similar complexities to other global investigations of monogenetic volcanoes. Lastly, we propose a causal link between the crustal dynamics of magma systems and the impact of crustal loading and unloading during cycles of glaciation.

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

Lithogeochemistry of Intrusive Rocks in the Halo Porphyry Copper-Molybdenum Prospect, Northeast Cambodia

The Halo copper-molybdenum prospect is a porphyry system in Ratanakiri province, northeastern part of Cambodia. There is only one research was carried out on this prospect about geological mapping and short wave infrared (SWIR) spectroscopy on alteration mineral identification. The purpose of this research is to confirm the deposit type from previous Angkor Gold’s report and find the centre of porphyry deposit based on characteristic of intrusive rocks at surface and subsurface, characteristics of the intrusive rocks and alteration lithogeochemistry of intrusive and volcanic rocks by using Pearce Element Ratio (PER) analysis. PER analysis was used to examine the nature and extend of the alteration halos in the porphyry Halo copper-molybdenum prospect. The intrusive rocks and volcanic rocks in Halo, range from diorite to granite (quartz feldspar porphyry) in composition as well as dacite to trachyandesite (andesite porphyry) in composition, respectively. They were formed in a subduction-related tectonic setting, likely a volcanic arc. Trace elements spider diagrams were normalized to primitive mantle display strong enrichment in large-ion lithophile elements such as Rb, Ba and K and depletion in some high-field strength elements such as Nb and Ti, suggesting magmas generated in a subduction-related tectonic setting. PER analysis indicates a moderate to high degree of sericite alteration of dacites, quartz feldspar porphyries, andesite porphyries and granodiorite porphyries. Moreover, three alterations in the Halo prospect such as potassic (secondary K-feldspar and biotite), phyllic (sericite) and propylitic (epidote) alteration were identified, which are consistent with petrography analysis. The potassic alteration zone is vectoring center of the hydrothermal system which may represent the locus of mineralization. Therefore the geochemical signature of potassic alteration within the quartz feldspar porphyry, andesite porphyry and granodiorite porphyry with high grad of copper ranges up to 2670 ppm and molybdenum ranges up to 5297 ppm represents a character for further exploration in the Halo porphyry copper-molybdenum prospect.

Unification of Isocon and Pearce Element Ratio Techniques in the Quantification of Material Transfer

Unification of Isocon and Pearce Element Ratio Techniques in the Quantification of Material Transfer

Mesoproterozoic rift sedimentation, fluid events and uranium prospectivity in the Cariewerloo Basin, Gawler Craton, South Australia

ABSTRACTThe Cariewerloo Basin formed in the Mesoproterozoic following assembly of the Gawler Craton, South Australia, and was filled by arenaceous redbeds of the Pandurra Formation. While previous regional-scale work reveals a basin with similar size and sedimentary fill to the Proterozoic Athabasca and Kombolgie basins that host unconformity-related uranium deposits, few details of the Cariewerloo Basin are known. In this study, stratigraphy, petrography, lithogeochemistry, stable isotope geochemistry and 40Ar/39Ar geochronology are integrated to clarify the depositional history of the Pandurra Formation, and to assess fluid events in the basin that could be linked to the formation of uranium deposits. In the study area, the Pandurra Formation was deposited in two eastward-thickening packages that terminate at faulted basement uplifts, interpreted as half-grabens that formed in a continental rift system as the eastern Gawler Craton underwent extension. Deposition occurred between 1575 Ma (latest Hiltaba Suite age) and ca 1490 Ma, the 40Ar/39Ar age of diagenetic illite in the basal Pandurra. Diagenesis involving fluids having δ18O and δ2H values between –2.1 and 3.6‰, and between –66 and –8‰, respectively, occurred at around 150°C. Protracted diagenesis preferentially occurred in the upper Pandurra Formation based on petrography and Pearce Element Ratios that show complete replacement of detrital lithic and feldspathic grains by diagenetic phyllosilicates, and younger 40Ar/39Ar ages between ca 1330 and 1200 Ma that record fluid events later into basin history. Conversely, the basal Pandurra Formation shows better preservation of detrital grains, and older 40Ar/39Ar ages around 1450 Ma that suggest these strata became closed to fluid flow earlier in basin history. Although, based on O-isotope ratios, fluid–rock interaction did not occur in the Cariewerloo Basin to the same extent as that in the Athabasca or Kombolgie basins, it is possible that a uranium deposit formed where the upper Pandurra Formation was in contact with metasedimentary basement units outside the present basin margins.

Alteration and lithogeochemistry in the Masara gold District, Eastern Mindanao, Philippines, as tools for exploration targeting

The intermediate to low sulfidation epithermal gold deposit in Masara, Compostela Valley, Eastern Mindanao, Philippines, is associated with a diorite porphyry from the Late Miocene Lamingag Intrusive Complex. Detailed mineralogical investigation of the host rocks in the deposit reveals five major alteration zones and at least two mineralizing events. A potassic alteration zone characterized by stockwork magnetite veins, secondary biotite and magnetite and an early-stage chlorite-sericite alteration composed of quartz+chlorite+illite+sericite+pyrite±biotite±magnetite±calcite assemblages are interpreted to be linked with an earlier porphyry copper system in the area. These were overprinted by late-stage chlorite-sericite alteration consisting of quartz+chlorite+illite+sericite+pyrite±adularia±magnetite±epidote and sericite assemblages which generally contain quartz+illite+sericite+pyrite. These alteration overprints are inferred to be related to a younger low to intermediate-sulfidation epithermal gold mineralization which involved near-neutral fluids. Advanced argillic alteration comprising quartz+kaolinite+magnetite+dickite±illite±calcite was also observed in the area. Isocon analysis shows that alteration zones associated with epithermal mineralization are generally characterized by enrichment in SiO2 and K2O and depletion of CaO and Na2O as well asadditions of Au, Cu, Zn and Pb. Pearce Element Ratio (PER) analysis indicates high degrees of sericitization in the altered diorite porphyry, tuff and quartz diorite units. At least three major alteration centers are defined based on the spatial distribution of the Alteration Index of surface samples calculated from the PER analysis. The results of this study support the use of whole-rock geochemistry asa complement to traditional techniques in exploration geochemistry.

Relationship Between Hydrothermal Alteration Index and Geological Attributes at Chuquicamata Cu–Mo Porphyry Deposit, Chile

AbstractThe distribution of mineral zones, hydrothermal alteration assemblages and ore associations are important geological attributes of the Chuquicamata Porphyry Cu–Mo deposit. The development and morphology of these attributes were influenced by syn‐mineralization structures with later modification of post‐mineralization tectonism to yield the present day form. In this work we evaluate the elemental mass transfer index due to hydrothermalism by using Pearce Element Ratios, using a database of 1110 samples consisting of elemental concentrations by chemical analysis. We study the relationship of the aforementioned index with the geological attributes in the porphyry as well as evaluate the type of information that the index provides according to its distribution, magnitude and variability. The magnitudes of the alteration index are organized into deciles, each of which represents a source of information, or macrostate. Within these, the geological attribute, or microstate, has a probability to participate that depends on the possible categorization of a particular attributes. The amount of information or uncertainty that each geological attribute provides to each decile of the distribution of the alteration index is determined by measuring the value of entropy. The results indicate that this distribution is controlled by the geometric–kinematic properties of syn‐mineralization structures that generate the primary pattern related to potassic alteration that develops in the hanging wall of the East Shear Zone. Subsequently the shape of the potassic alteration is modified by the kinematic effect imposed by the Americana and Estanques Blancos faults, and this broadly defines the geometry of the Qz–Mo event and the shape of the phyllic alteration. In addition, the sinistral‐inverse shear of the West Fault Zone allows the construction of an enriched blanket and also generates the drag fold that is characteristically observed in the south wall of the mine. All of the above events combine to yield the final geometry of the geological attributes that controls the shape and magnitude of the alteration index in the porphyry. The magnitude of the alteration index is mainly driven by the potassic and phyllic alteration stages. Potassic alteration is dominant in the first four deciles, while phyllic alteration is dominant in the last six deciles. Supergene mineralization attributes contribute discreetly to the average magnitude of the alteration index for each decile. The distribution of the probability of participation of the geological attributes in the distribution of the alteration index suggests that in the potassic alteration, biotite and green‐gray sericite facies contribute to strengthen the index in an associated manner, while in the phyllic alteration, the Qz–Ser textural type is the most significant.

Red Sea rift-related Quseir basalts, central Eastern Desert, Egypt: Petrogenesis and tectonic processes

Mineral and whole-rock chemistry of Red Sea rift-related Tertiary basalts from south Quseir city, central Eastern Desert of Egypt is presented to investigate their petrogenesis and relationship to tectonic processes. The south Quseir basalts (SQB) are classified as high-Ti (TiO2 >2 wt.%) subalkaline transitional lava emplaced in an anorogenic tectonic setting. Their Mg# varies from 48 to 53 indicating the evolved nature of the SQB. Pearce element ratios suggest that the SQB magmas evolved via fractional crystallization of olivine + clinopyroxene ± plagioclase, but the absence of Eu anomalies argues against significant plagioclase fractionation. Clinopyroxene compositions provide evidence for polybaric fractionation of the parental mafic magmas. Estimated temperatures of crystallization are 1015 to 1207 °C for clinopyroxene and 1076 to 1155 °C for plagioclase. These values are interpreted to result from early stage crystallization of clinopyroxene followed by concurrent crystallization of clinopyroxene and plagioclase. The incompatible trace element signatures of the SQB (La/Ba = 0.08–0.10 and La/Nb = 0.89–1.04) are comparable to those of ocean island basalts (OIB) generated from an asthenospheric mantle source unaffected by subduction components. Modeling calculations indicate that the SQB primary magmas were derived from ~4–5% partial melting of a garnet-bearing lherzolite mantle source. The NE Egyptian basaltic volcanism is spatially and temporally related to Red Sea rifting and to the local E–W striking faults, confirming a relationship to tectonic activity. Our results suggest that the extensional regime associated with Red Sea rifting controlled the generation of the Egyptian basalts, likely as a result of passive upwelling of asthenospheric mantle.

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.

An integrated lithogeochemical approach to detecting and interpreting cryptic alteration around the Elura Zn-Pb-Ag deposit, New South Wales, Australia

ABSTRACT Hydrothermal alteration around the sediment-hosted Elura Zn-Pb-Ag sulphide deposit has produced detectable and systematic chemical changes that are also reflected in subtle mineralogical features (cryptic alteration). Iron carbonate development accompanied by potassic alteration, the destruction of albite and the absence of chlorite are the dominant mineral alteration effects in the surrounding turbidites. Key elements enriched in the primary dispersion zone are Zn, Pb, Ag, As, Rb, Tl, and particularly Sb. Sodium is strongly depleted in the alteration system. Cryptic alteration and primary dispersion can be detected up to 350 m from the orebody below the weathering front. Pearce Element Ratio (PER) and General Element Ratio (GER) techniques more clearly identify and quantify this alteration and could assist in vectoring towards high intensity alteration adjacent to ore during exploration drilling. These data assessment techniques avoid the problems of closure when comparing element contents for altered and unaltered samples and allow modelling of the mineralogical controls on chemical variation. There are important differences in the alteration effects observed in the fine and coarse fractions of the enclosing turbidite units. For vectoring purposes, the best lithotype to sample is the shale component.

Advances in understanding the Kombolgie Subgroup and unconformity-related uranium deposits in the Alligator Rivers Uranium Field and how to explore for them using lithogeochemical principles

The Alligator Rivers Uranium Field (ARUF) includes the mined and unmined Jabiluka, Ranger, Koongarra and Nabarlek unconformity-related uranium deposits and several small prospects including the newly discovered King River prospect. Uranium mineralisation is hosted by a variety of metamorphosed Nimbuwah Domain lithologies that are unconformably overlain by the Kombolgie Subgroup, a basin package of unmetamorphosed arenites and mafic volcanics. All of the uranium deposits and prospects preserve an identical alteration assemblage that is subdivided into a distal and proximal alteration zone. The distal alteration zone comprises an assemblage of sericite and chlorite that replace albite and amphibole. In some cases, this alteration can be traced >1000 m from the proximal alteration zone that is dominated by uraninite, hematite, chlorite and sericite. Uranium precipitated in the basement as uraninite at 1680 Ma at around 200°C from a fluid having δ18Ofluid values of 3.0±2.8‰ and δDfluid values of −28±13‰ VSMOW reflecting an evolved marine source. These geochemical properties are indistinguishable from those recorded by diagenetic illite and chlorite that were collected from the Kombolgie Subgroup sandstones across the ARUF. The illite and chlorite formed in diagenetic aquifers, and where these aquifers intersected favourable basement rocks, such as those containing graphite or other reductants, U was precipitated as uraninite. Therefore, it is proposed that the Kombolgie Subgroup is the source for fluids that formed the deposits. A post-ore alteration assemblage dominated by chlorite, but also comprising quartz±dolomite±sulfide veins cut the uranium mineralisation at all deposits and has historically been recorded as part of the syn-ore mineralisation event. However, these minerals formed anywhere between 1500 to 630 Ma from fluids that have distinctly lower δ18Ofluid values around 1.5‰ and lower δDfluid values around −45‰ reflecting a meteoric water origin. Despite unconformity-related uranium deposits having a large alteration halo, they remain difficult to find. The subtle alteration of albite to sericite several hundred metres from mineralisation occurs in isolation of any increase in trace elements such as U and radiogenic Pb and can be difficult or impossible to identify in hand specimen. Whole rock geochemical data indicate that Pearce Element Ratio (PER) analysis and General Element Ratio (GER) analysis may vector into this subtle alteration because it does not rely on an increase in trace elements to identify proximity to ore. PER and GER plots, Al/Ti vs (2Ca + Na + K)/Ti, Na/Al vs (Na + K)/Al, K/Al vs (Na + K)/Al and (Fe + Mg)/Al vs (Na + K)/Al provide a visual guide that readily distinguish unaltered from altered samples. A plot of (Na + K)/Al and (Fe + Mg)/Al on the x-axis against the concentration of trace elements on the y-axis reveals that U, Pb, Mo, Cu, B, Br, Ce, Y, Li, Ni, V and Nd are associated with the most intensely altered samples. The lithogeochemical vectors should aid explorers searching for uranium mineralisation in a prospective basin environment, but exploration must first focus on the characteristics of the basin to assess its mineralisation potential. A holistic model that describes the evolution of the Kombolgie Subgroup from deposition through diagenesis to formation of the uranium deposits in the underlying basement rocks is presented and has application to other basins that are considered prospective for unconformity-related uranium deposits. The model outlines that explorers will need to consider the thickness of the sedimentary pile, its lithological composition relative to depositional setting, the depth to which the sediments were buried during diagenesis and the degree of diagenesis achieved, which may be time dependant, before deciding on the prospectivity of the basin.

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.

Lithogeochemistry of the Collahuasi porphyry Cu–Mo and epithermal Cu–Ag (–Au) cluster, northern Chile: Pearce element ratio vectors to ore

Abstract Lithogeochemical vectors to ore-bodies are obscured by the variety of lithological units and the alteration events that have affected them. In the magmatic-hydrothermal context, Pearce element ratio (PER) analysis can be a cost-effective lithogeochemical technique that identifies and eliminates non-hydrothermal sources of variation, thus permitting the definition of material transfer related only to hydrothermal alteration. Dedicated PER diagrams for mafic and felsic lithologies in the world-class porphyry Cu–Mo and epithermal Cu–Ag (–Au) cluster of the Collahuasi district, I Region, northern Chile, effectively model background variability and discriminate between fresh or propylitically altered rocks and those with a hydrolytic alteration overprint. Furthermore, PER diagrams allow for the definition of an alteration index (AI) that quantifies the degree of metasomatic exchange during hydrolytic alteration of a particular rock. Plots of different PER values against the AI show that potassium enrichment during hydrolytic alteration was followed by potassium, calcium and sodium depletion as a result of the destruction of feldspars during sericitic and later argillic alteration. Although PER plots record no mass transfer processes involving major elements during propylitic alteration, the integrated AI for both mafic and felsic units spatially defines the major alteration centres within the district.

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.

Geochemistry of the Bonito Lake stock, Lincoln County, New Mexico: Petrogenesis and hydrothermal alteration

The Bonito Lake stock is one of three stocks that intrude the Sierra Blanca Volcanics in the Nogal-Bonito mining district of New Mexico. The 26.6-Ma stock consists primarily of porphyritic syenite with subordinate quartz syenite and monzonite. SiO 2 concentrations range from 54 to 66 weight percent. Na 2 O exceeds K 2 O and combined alkalis average 9.4 weight percent. The rocks are alkaline and metaluminous and plot in the syenite and syenodiorite fields on an R 1 R 2 multicationic plot. The chondrite-normalized extended trace element diagram has a negative slope with Ba values 167 to 286 times chondritic values and Yb values eight to 14 times chondritic values. The chondrite-normalized REE plot has a negative slope with mild LREE enrichment. (La/Yb) N values range from 15 to 22 and the mean Eu/Eu* is 0.91. Clinopyroxene, zircon, and apatite fractionation, along with feldspar accumulation were the most important factors in controlling elemental abundances. Quasi Pearce Element Ratio (PER) analysis confirms the role of clinopyroxene fractionation, as well as indicates that an increase in K and a loss of Ca ± Na occurred during alteration. The quasi PER analysis also shows that the rocks are slightly metasomatized. Whole-rock δ 18 O values for the Bonito Lake stock range from 2.0 to 8.4 per mil. Oxygen isotope values for the more permeable surrounding volcanic rocks range from 1.0 to 7.8 per mil. The lower values of the volcanic rocks are reflective of meteoric-hydrothermal water interaction. These data help us better understand not only this stock, but also the Sierra Blanca igneous complex and the Lincoln County Porphyry Belt. This study also contributes to our understanding of the Nogal-Bonito mining district and the larger Rocky Mountain alkalic province.

Lithogeochemistry and Hydrothermal Alteration at the Halfmile Lake South Deep Zone, a Volcanic-Hosted Massive Sulfide Deposit, Bathurst Mining Camp, New Brunswick

The Halfmile Lake South Deep zone, Bathurst Mining Camp, New Brunswick, was discovered by Noranda Inc. (Exploration) as a result of a 3-D seismic survey in 1998 and the subsequent drilling of ten diamond-drill cores. The deposit consists of massive, breccia, and stockwork Pb-Zn-Cu sulfide minerals, and is hosted by a volcano-sedimentary sequence belonging to the overturned Ordovician Tetagouche Group. Epiclastic rocks and interbedded fine-grained felsic pyroclastic rocks dominate the stratigraphic footwall. Locally, crystal-rich felsic tuffs and subordinate epiclastic rocks comprise the immediate stratigraphic hanging wall. The entire stratigraphic sequence was intruded by quartz-feldspar porphyritic intrusions, and cut by intermediate and basic dikes. The volcanic and sedimentary rocks can be discriminated geochemically using trace element ratios such as Zr/TiO2 and Nb/TiO2, despite intensive alteration and cleavage development. These ratios indicate that four protolith volcanic compositions exist: rhyolite, dacite, andesite, and basalt. The aphyric and feldspar- and quartzphyric volcanic rocks are dacitic and rhyolitic in composition; epiclastic rocks have trace element ratios consistent with dacitic compositions. Pearce element ratio diagrams, general element ratio diagrams, and petrographic observations demonstrate that the rhyolitic volcanic rocks exhibit evidence of albite, potassium feldspar, and quartz fractionation, dacitic volcanic rocks exhibit little evidence of any fractionation, and epiclastic sedimentary rocks exhibit evidence of quartz sorting only. Hydrothermal alteration is best represented by the presence of phengitic muscovite and daphnitic chlorite. Minor calcite occurs in the stratigraphic hanging wall, deep in the stratigraphic footwall, and in post-mineralization dikes, and thus is not interpreted to be part of the causative hydrothermal event. Element additions and losses during alteration have been used to determine net alteration reactions, and these have been used to identify alteration parameters that are independent of other forms of alteration and fractionation/sorting, and which can be used to guide exploration. These parameters include: a bulk hydrolysis measure, (2Ca+Na+K–2CO2)/Al; a muscovite alteration measure, K/Al; an albite destruction measure, Na/Al; a chlorite alteration measure, (Fe+Mg–S/2)/Al; a chlorite composition measure, (Fe–S/2)/Mg; a sulfidization measure, S/Ti; and a carbonatization measure, CO2/Ti. With the exception of the carbonatization measure, these alteration parameters define a distinct lateral and vertical zone of intense hydrothermal alteration in the stratigraphic footwall of the deposit, and demonstrate that the epiclastic rocks are predominantly chlorite altered, and the volcanic rocks are predominantly muscovite altered. Within the alteration halo, element additions of Fe and H, and element losses of Na are characteristic. Potassium was added to muscovite-altered rocks, but subsequently removed from chlorite-altered rocks. The results from this study demonstrate that metamorphism and deformation have not significantly obscured hydrothermal alteration signatures.

Geochemical approaches in the discrimination of synvolcanic intrusions as a guide for volcanogenic base metal exploration: an example from the Abitibi belt, Canada

Synvolcanic plutons are fundamental in the formation of volcanogenic massive sulphide deposits (VMS). Since the plutons are much larger than the actual deposits, they make a valuable guide in the location and identification of large areas for potential VMS deposits. However, in orogenic belts with a long history of magmatism, identification of target plutons is challenging. Using the Achaean Abitibi greenstone belt as an example, it is shown that synvolcanic intrusions may be identified using relatively inexpensive major element geochemistry. Two methods were developed and validated with known synvolcanic and syntectonic intrusions. The first method involves the chemical comparison of samples from the intrusion and from its surrounding volcanites. Using Al2O3 v. TiO2 diagrams, it is demonstrated that synvolcanic intrusions follow the same trend as their volcanic counterpart, whereas syntectonic intrusions define a restricted discordant compositional field. The second approach uses Pearce element ratio (PER) analysis to quantify the alteration recorded by the plutons. This work has led to the identification of nine new synvolcanic plutons that constitute the first priority targets for VMS exploration.

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.

Petrogenesis of calc-alkaline and shoshonitic post-collisional Oligocene volcanics of the Cover Series of the Sesia Zone, Western Italian Alps

High-K calc-alkaline and shoshonitic Oligocene volcanics of the Cover Series of the Sesia Zone occur within a volcano-sedimentary unit located in the internal part of the Sesia Zone in proximity of the Canavese Line. High-K calc-alkaline rocks are basaltic andesites to andesites (K 2 O from 2.0 to 2.6 wt%) with high alumina contents (19.2-20.3 wt% for basaltic andesites). These rocks are enriched in incompatible trace elements of low ionic potential (with Ba ranging from 920 to 1320 ppm, Rb/Sr = 0.11-0.17). High-field-strength elements (HFSE) are within the typical range of orogenic andesites, with Zr ranging from 150 to 180 ppm. Rare Earth Element patterns for high-K intermediate rocks are enriched in LREE (La/Yb ranging 15.2-16.0) and show a small Eu negative anomaly. Shoshonitic rocks are trachyandesites to trachydacites, with K 2 O ranging from 4.6 to 5.5 wt%. Alumina contents are lower than those of the high-K calc-alkaline rocks ranging from 14.8 to 16.8 wt%. When compared to calc-alkaline andesites, shoshonitic rocks show higher contents in incompatible elements (with Ba higher than 2500 ppm; Rb/Sr = 0.30-0.35; La/Yb = 26.2- 28.8; Zr = 380-450 ppm). Thermobarometric data indicate that andesitic s.l. lavas of the Cover Series of the Sesia Zone started to crystallize their phenocrystic phases at pressures between 3.6-4.8 kbar, for temperatures ranging from 1,080 to 1,150 °C, for water contents of at least 2-2.7 wt%. Shoshonitic lavas crystallized at 2-2.6 kbar for temperatures of about 900 °C, and approaching water-saturated conditions. REE patterns and spiderdiagrams, together with Pearce element ratios, indicate comagmatism between the lavas and the rocks of the Valle del Cervo Pluton. In particular there is a close genetic link between the shoshonitic and the high-K calc-alkaline volcanic rocks. Geochemical modelling shows that both suites may derive from a common “parental calc-alkaline basalt ” with variable enrichments in terms of incompatible elements and transition metals. This basalt has suffered a different degree of fractionation in the genesis of balsaltic andesites (with F = 0.29) and monzonites of the Valle del Cervo Pluton (F = 0.19). High-K calc-alkaline andesites may be obtained by fractional crystallization of basaltic andesites, whereas shoshonitic lavas result from fractional crystallization of a monzonitic parent. This suggests that these lavas represent a differentiated top of a magma chamber, whose remnants are now represented by the Valle del Cervo Pluton. This is also supported by comparison of the Sr-Nd isotopic signatures of both intrusive and volcanic rock suites (the latter ranging from 0.7094 to 0.71175 in measured 87 Sr/ 86 Sr, and 0.51120-0.51228 in measured 143 Nd/ 144 Nd). Calculated isotopic parameters (??Sm/Nd = 0.46-0.52, ??Sm/Nd = 0.23 to 0.26 calculated at 2 b.y., and present ??Nd = –7 to –8.5) indicate that parental basaltic magmas to the high-K calc-alkaline and shoshonitic rocks were derived by low degree of partial melting (5-7%) of a moderately enriched “eclogitic” metasomatised mantle-source (i.e., garnet pyroxenite). This is consistent with the presence of ultrapotassic dykes in this sector of Western Alps (showing high values in radiogenic Sr-Nd isotope ratios). Finally, we point out that the effusive and intrusive rocks of the investigated area represent a volcano-plutonic complex within the “Tertiary Periadriatic Province”.

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.