Alkali Alteration Research Articles

Distribution and mineralogical features of thorite in the Bayan Obo deposit: Implications for hydrothermal metasomatic Th re-enrichment

Thorite, as a principally Th-bearing independent mineral, is an important indicator for Th mineralization. However, its genesis and significance for Th enrichment processes are still unclear. Herein, we address this issue based on systematic mineralogical observation and major element analyses of thorites with related monazite and bastnäsite from different type ores in the Bayan Obo deposit. The thorites are mainly hosted in biotite-type and riebeckite-type ores with minor distribution in aegirine-type and banded-type ores, but are scarce in dolomite-type ores. The thorites in each type ores principally contain Th, O and Si with variable LREE and Fe concentrations. They commonly occur as vein (or veinlet) or aggregation crosscutting, cavity-infilling or around REE, Fe and Nb minerals, and some of these minerals exist as relict or inclusion within the thorites. In addition, late-stage huanghoite and pyrite are intergrown with the thorites or appear as inclusions within the thorites. These petrographic characteristics suggest that thorite formation was posterior to REE, Fe and Nb mineralization and was related to Paleozoic hydrothermal mineralization events. Barite and alkali alteration minerals (e.g., biotite and riebeckite) show a closely spatial relationship with the thorites, which indicates that hydrothermal fluids for thorite formation were enriched in alkalis (K+ and Na+) and sulfate (SO42-). Major element analyses of monazite and bastnäsite exhibit variable ThO2 concentration (unaltered region: 7.13 ∼ 14.34 wt% for monazite, 0.64 ∼ 1.33 wt% for bastnäsite; altered region: 2.50 ∼ 5.41 wt% for monazite, 0.17 ∼ 0.53 wt% for bastnäsite), which indicates that later-formed monazite and bastnäsite underwent Th leaching by chemical-coupled substitutions including: (1) 2REE3+ = Th4+ + Ca2+; and (2) P5+ + REE3+ = Si4+ + Th4+. In combination with relevant published data, our study suggests that later-stage Th-rich monazite and bastnäsite in the Bayan Obo deposit experienced mineral dissolution and Th leaching by metasomatism of hydrothermal fluids, which induced Th re-enrichment in form of thorite precipitation. On the other hand, as intrinsically high Th content of the thorites, this study also evaluates grain size and Th abundance of the thorites within different type ores, and proposes that biotite-type and riebeckite-type are targeted type ores for thorite extraction and Th collection.

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Tieshan complex body (TCB) is located in Zhenghe County on the border of Zhejiang and Fujian, covering an area of about 5.7 km2. Tieshan complex body (TCB) is the most developed metasomatised area in the southeast of China, which was misunderstood as a syenite intrusion.Much Study has carried on at Shizigang in the southwest of the TCB. The alkali metasomatites are composed of feldspar, pyroxene, amphibole, biotite and melanite et al. Potassic feldspars in Complex body have four kinds of forms and can be separated into three stages: (1) one in facial alkali alteration and one in alkali alteration with melanites, (2) one in light alkali feldspar veins, and (3) one in dark alkali feldspar veins and.Pyroxenes had three kinds of forms: some in breccias, some in feldspar and some accompanied with amphiboles. Amphiboles had two kinds of forms: in breccias and scatter in feldspars in the veins, its mineral compositions are composed of basaltic hornblende, ferrous pargasite, actinolite and ferroactinolite, all belong to calcic amphibole subgroup. Biotites are scattered in breccias and in the veins or which is the product of metasomatised pyroxenes. The compositions of biotites show low-Ti and Fe, but high-Mg, which were the thermal products of a higher oxygen fugacity. Melanites occurred as layers and belts, which were metasomatised by feldspars and epidotes and amphiboles and carbonates and sericites and diopsides, their ingredients are solid solution series between andradite and Grossular and reflect the melanites were formed in the relative oxidative, alkaline environment.X-ray powder diffraction and infrared spectrum analyses of K-feldspar from early alkali feldspar provide degrees of ordering of 0.71–0.80, structural parameters of 1.27–1.61, triclinicities of 0.23–0.29, T1 (o) + T1 (m) of 0.86–0.90 degrees of ordering of 0.67–0.88 from late K-feldspar, its structural parameters of 1.10–1.91, triclinicities of 0.30–0.70, T1 (o) + T1(m) of 0.84–0.93. Both two kinds of alkali feldspars are microcline, most adularia. Their structure temperatures vary from 200 to 370 °C which are the highest temperatures at which K-feldspars were formed. Thus, it is concluded the K-feldspars in Tieshan is originated from thermal fluid, and the fluid was potassium-rich and a little rich in sodium at the later stage.The 39Ar-40Ar age of feldspars shows 118.68 ± 7.70 Ma and 114.61 ± 6.74 Ma, which represent the ages of early stage facial alkali alteration and late stage dark alkali feldspar veins respectively. It is obvious that an original granites were formed at about 250 to 255 Ma, but all the rocks were considered to be strongly metasomatised by alkali fluid later, most occurred during 108–126 Ma in Yanshanism. Metasomatism of the TCB reflected that complex history of the region. The metasomatised rocks include fenite and amphibole feldsparite. Fenite, especially apatite-rich fenite is associated with REE mineralization, which is helpful to understand the mechanism of a large scale of rare earth deposits in the region.

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Tieshan complex body (TCB) is located in Zhenghe County on the border of Zhejiang and Fujian, covering an area of about 5.7 km2. Tieshan complex body (TCB) is the most developed metasomatised area in the southeast of China, which was misunderstood as a syenite intrusion.Much Study has carried on at Shizigang in the southwest of the TCB. The alkali metasomatites are composed of feldspar, pyroxene, amphibole, biotite and melanite et al. Potassic feldspars in Complex body have four kinds of forms and can be separated into three stages: (1) one in facial alkali alteration and one in alkali alteration with melanites, (2) one in light alkali feldspar veins, and (3) one in dark alkali feldspar veins and.Pyroxenes had three kinds of forms: some in breccias, some in feldspar and some accompanied with amphiboles. Amphiboles had two kinds of forms: in breccias and scatter in feldspars in the veins, its mineral compositions are composed of basaltic hornblende, ferrous pargasite, actinolite and ferroactinolite, all belong to calcic amphibole subgroup. Biotites are scattered in breccias and in the veins or which is the product of metasomatised pyroxenes. The compositions of biotites show low-Ti and Fe, but high-Mg, which were the thermal products of a higher oxygen fugacity. Melanites occurred as layers and belts, which were metasomatised by feldspars and epidotes and amphiboles and carbonates and sericites and diopsides, their ingredients are solid solution series between andradite and Grossular and reflect the melanites were formed in the relative oxidative, alkaline environment.X-ray powder diffraction and infrared spectrum analyses of K-feldspar from early alkali feldspar provide degrees of ordering of 0.71–0.80, structural parameters of 1.27–1.61, triclinicities of 0.23–0.29, T1 (o) + T1 (m) of 0.86–0.90 degrees of ordering of 0.67–0.88 from late K-feldspar, its structural parameters of 1.10–1.91, triclinicities of 0.30–0.70, T1 (o) + T1(m) of 0.84–0.93. Both two kinds of alkali feldspars are microcline, most adularia. Their structure temperatures vary from 200 to 370 °C which are the highest temperatures at which K-feldspars were formed. Thus, it is concluded the K-feldspars in Tieshan is originated from thermal fluid, and the fluid was potassium-rich and a little rich in sodium at the later stage.The 39Ar-40Ar age of feldspars shows 118.68 ± 7.70 Ma and 114.61 ± 6.74 Ma, which represent the ages of early stage facial alkali alteration and late stage dark alkali feldspar veins respectively. It is obvious that an original granites were formed at about 250 to 255 Ma, but all the rocks were considered to be strongly metasomatised by alkali fluid later, most occurred during 108–126 Ma in Yanshanism. Metasomatism of the TCB reflected that complex history of the region. The metasomatised rocks include fenite and amphibole feldsparite. Fenite, especially apatite-rich fenite is associated with REE mineralization, which is helpful to understand the mechanism of a large scale of rare earth deposits in the region.

Factors controlling the generation and diversity of giant carbonatite-related rare earth element deposits: Insights from the Mianning–Dechang belt

As a key source of strategic commodities for advanced technologies, carbonatite-related (carbonatite–nordmarkite complex) rare earth element (REE) deposits are globally important resources. Such deposits have different ore types, scales, ore-formation structures, and evolutionary processes. However, the causes of these variations have not been well studied, which prevents us from understanding the formation of such deposits. The factors that lead to the formation of large carbonatite-related REE deposits have also not been carefully documented. Fortunately, the Cenozoic Mianning–Dechang (MD) REE belt in eastern Tibet contains several medium- and large-scale carbonatite-related REE deposits, including brecciated, weathered, and disseminated ores, providing an ideal opportunity to investigate these factors. Current and this studies show that the deposits in the MD belt have similar formation stages, crystallization sequences, sources, and REE mineralization conditions. Previous studies have shown that the REE mineralization occurred alongside the formation of large-scale overprinted gangue minerals, such as barite, fluorite, and calcite during the lowest-temperature hydrothermal stage (<350 °C), and the REE ores all formed within limited ranges of temperature (247–442 °C) and pressure (< 500 bars). This implies that all the REE minerals formed under the same conditions, that this mineral assemblage (barite + fluorite + calcite + bastnäsite) has higher ore grades (<13%), and that F−, SO42−, and CO2 are important for transporting and precipitating REE. Detailed geological mapping suggests that the variation in ore type was caused by local ore-controlling structures, rather than being due to different depths of formation. Bastnäsite has similar initial 87Sr/86Sr (0.7059–0.7080), 143Nd/144Nd (0.5123–0.5124), 207Pb/204Pb (15.594–15.648), and 208Pb/204Pb (38.422–71.488) ratios along the whole belt, implying that all the deposits have the same source and the REE were sourced directly from a mixture of crustal and mantle material. The C–O isotopic compositions of the carbonatites (δ18OV-SMOW = 6.4‰ to 10.5‰, δ13CV-PDB = − 3.9‰ to − 8.5‰) and bastnäsite (δ18OV-SMOW = 9.5‰–16.3‰, δ13CV-PDB = − 4.7‰ to − 9.3‰) suggest that low-temperature alteration occurred along with REE mineralization. Development of pegmatitic stage and aegirine–augite and arfvedsonite fenitization alteration may promote the formation of large-scale REE deposits. The giant Maoniuping and Dalucao deposits both had pegmatitic stages that involved the formation of coarse-grained barite, fluorite, and calcite with REE contents of up to 6305 ppm, suggesting the pegmatitic stage suggest another REE enrichment event after the magmatic stage, and large-scale REE mineralization. The intense alkali alteration that forms arfvedsonite and aegirine–augite in carbonatite–nordmarkite complexes was only found in the Maoniuping deposit, whereas biotitation was found in the Dalucao and Lizhuang deposits. This suggests that the Maoniuping deposit experienced more intense alteration, which may have led to more effective REE migration from the wallrock into the ore-forming fluids. The mantle source, the evolution of the magma, the presence of a pegmatitic stage, the composition of the fluids, and the high degree of alkaline alteration could have affected the scale of the deposits, in addition to the large volume of carbonatite–nordmarkite complexes and multiple phases of tectonism.

Multiphase carbonatite-related magmatic and metasomatic processes in the genesis of the ore-hosting dolomite in the giant Bayan Obo REE-Nb-Fe deposit

The origin of dolomite that hosts the Bayan Obo REE-Nb-Fe deposit (57.4 Mt.@6% REE2O3, 2.16 Mt.@0.13% Nb2O5, and >1500 Mt.@35% iron oxides) has been controversial for decades, but it is integral to understanding of the genesis of this giant deposit. In this study, based on the textures and in situ major and trace element composition of its carbonates, the dolomite was proved to be initially generated from magnesio-ferro‑carbonatite melts. It subsequently experienced magmatic-hydrothermal alteration and recrystallization in a low strain environment, caused by calcio‑carbonatitic fluids, with formation of finer-grained dolomite, interstitial calcite and increasing amounts of associated fluorocarbonates. Available stable isotope analyses indicate that the recrystallized ore-hosting dolomite has higher δ13C and δ18O ratios compared to its igneous coarse-grained precursor. Rayleigh fractionation during the recrystallization process, rather than crustal contamination, played a major role in the highly-variable stable isotope composition of carbonates in the dolomite. Low-T alteration increased variability with apparently random increases in δ18O within carbonates. The REE, Ba and Sr were added simultaneously with the elevated (La/Yb)cn from magnesio-ferro‑carbonatite melts to calcio‑carbonatitic fluids, and to carbonatite-derived aqueous fluids, through which extensive fluorine metasomatism and alkali alteration overlapped the recrystallization of the ore-hosting dolomite. Therefore, the multi-stage REE mineralization at Bayan Obo is closely related to metasomatism by calcio‑carbonatitic fluids of previously-emplaced intrusive magnesio-ferro‑carbonatite bodies during late evolution of the Bayan Obo carbonatite complex. Then, the ore-hosting dolomitic carbonatite was subjected to compressive tectonics during a Paleozoic subduction event, and suffered intense, largely brittle, deformation, which partially obscured the earlier recrystallization process. The complex, multi-stage evolution of the ore-hosting dolomite is responsible for the uniqueness, high grade and giant size of the Bayan Obo deposit, the world's largest single REE resource with million tonnes of REE oxides.

Fenitization in the giant Bayan Obo REE-Nb-Fe deposit: Implication for REE mineralization

Fenitization, a unique type of alkali metasomatism related to carbonatite or alkaline igneous rocks, is widely distributed around the Mesoproterozoic carbonatite dykes in the Bayan Obo giant REE-Nb-Fe deposit. Meanwhile, represented by alkali silicates (sodic amphibole or aegirine) enriched REE-(Fe) ores, there is extensive alkali alteration in ores at Bayan Obo, whose similarity to fenitization has been proposed. Fenitization associated with carbonatite is an appropriate indicator of REE mineralizing process. Alkali silicates (sodic amphibole, aegirine and phlogopite) from the East and Main open pits, and fenite around the Wu dyke at Bayan Obo were collected for EMPA and laser ablation ICPMS analyses. Aegirine and phlogopite from the open pits and fenite around the Wu dyke contain similar major components. Sodic amphiboles from those two localities contain comparable trace element composition but with different Cao, MgO and F contents. Alkali silicates from fenites around the Wu dyke show systematic enrichment of Zr, Hf and Ti over alkali silicates from the open pits. Considering different wallrocks and other factors influencing the composition, combined with previous results of geochronology, mineralogy and fluid inclusion, the alkali alteration in the open pits is essentially fenitization in ore-hosting dolomite. A two-stage fenitization process is supported by the zonal texture of sodic amphiboles from Wu carbonatite dyke. The early stage fenitization was Mg, Na, K, F rich and Fe, La, Ce depleted, distributed in alkali silicates from the banded REE-Fe ores and in the core of sodic amphiboles from the Wu dyke. The late stage was Fe, LREE rich and Mg, Na, K, F depleted, distributed in the massive REE-(Fe) ores and the rim of sodic amphiboles from the Wu dyke. The initial alkali fluids derived from carbonatite commonly exhibits LREE-rich patterns. Therefore, the LREE-depletion and low La/Nd ratio of the early stage silicates indicates intense LREE-bearing mineral precipitation and the REE mineralization gradually weakened in the late stage when alkali fluids kept the initial REE patterns unchanged.

Metasomatic Alteration Control of Petrophysical Properties in the Great Bear Magmatic Zone (Northwest Territories, Canada)

The Great Bear magmatic zone in the Northwest Territories of Canada contains large iron-oxide alkali alteration systems noted for having high potential for iron oxide-apatite, iron oxide-copper-gold, and affiliated ore deposits. Physical properties (density, magnetic, and electrical) were measured on 824 rocks samples selected to represent the range of metasomatic alteration types in the region. Mineralogical and geochemical classification of the prograde iron oxide and alkali alteration facies reveals large variations in physical properties through the evolution of the metasomatic systems. In particular, deep and early sodic alteration produced rocks having low densities and low magnetic susceptibilities. During calcium and iron precipitation, rocks gain extremely high density and susceptibility, due to crystallization of amphibole and especially magnetite. Subsequent hightemperature, potassic- and iron-altered rocks are marked by cocrystallization of magnetite with K-feldspar or biotite, and as the transition from magnetite to hematite takes place, K-feldspar crystallizes instead of iron oxides leading ultimately to potassic felsites having low densities and susceptibilities. Subsequent cooler, shallower, and more oxidized potassic-iron alteration produced high densities but moderate susceptibilities owing to crystallization of hematite. Understanding these major variations in physical properties of rocks enables detailed geophysical mapping of alteration zones, which builds and improves the regional context for integrated mineral exploration vectors to potential ore deposits.

Tectonomagmatic Evolution of the Southern Great Bear Magmatic Zone (Northwest Territories, Canada): Implications for the Genesis of Iron Oxide-Alkali–Altered Hydrothermal Systems

The Great Bear magmatic zone in northwestern Canada is a Paleoproterozoic volcano-plutonic belt of high K, calc-alkaline to shoshonitic affinity interpreted as a continental arc that formed between 1.87 and 1.85 Ga following the short-lived Calderian orogeny. Tectonomagmatic evolution of this magmatic zone favored the formation of multiple iron oxide and alkali alteration systems within a time frame of 10 m.y., as constrained geochronologically within error between 1875 and 1865 Ma. This illustrates a temporal and genetic relationship between shoshonitic to high K, calc-alkaline continental arc magmatism and the formation of iron oxide-rich deposits and alkali alteration associated with iron oxide-copper-gold (IOCG) mineralization. Early rhyolitic magmatism formed the basal sequence of the Lou assemblage under a compressive (or transpressive) tectonic regime. Between 1872 and 1867 Ma, an apparent higher magmatic production is linked to a marked change in composition of the volcanic and plutonic rocks, from rhyolite to intermediate/felsic and locally mafic. Compositional homogeneity of the intrusive and volcanic rocks of the Mazenod and Bea assemblages, termination of ductile to brittle-ductile deformation along the main deformation zones, and transition to widespread brittle fracturing and breccia formation are interpreted to reflect a change in the regional stress regime, from compressional/transpressional to extensional/transtensional. This change in stress regimes is associated with iron oxide-rich mineralization that initially formed the NICO Au-Bi-Co deposit, followed by IOCG mineralization in the NICO and Sue-Dianne deposits as well as albitite-hosted U mineralization in the Southern Breccia.

Fault reconstructions using aeromagnetic data in the Great Bear magmatic zone, Northwest Territories, Canada

The Great Bear magmatic zone, located in Wopmay orogen, is a 1.875–1.84 Ga belt, 450 km long by 100 km wide of volcanic and allied plutonic rocks interpreted as a Paleoproterozoic magmatic arc. The belt, which contains economically important mineralization, was folded and subsequently cut by a swarm of northeast-striking transcurrent faults, which are part of a regional conjugate fault system interpreted to result from terminal collision of the Nahanni – Fort Simpson terrane. Fault reconstructions based on the interpretation of aeromagnetic data and geological maps provide first-order models of deformation mechanisms associated with, and the configuration of the Great Bear magmatic zone prior to, its dissection by northeast-striking transcurrent faults. The models show that vertical axis block rotation (plane strain) of ∼4.5° can explain fault offsets in the south, but that greater rotation is required to explain many of the displacements in the north. However, offsets on transcurrent faults that border the Camsell River district are greater than can be explained by vertical axis block rotation model alone and may include a component of Mesoproterozoic contractional deformation associated with the Racklan–Forward orogeny. Following reconstruction, iron oxide alkali alteration and associated mineralization, which pre-date transcurrent faulting, form a pair of northerly trending zones on the east and west margins of the belt. We suggest that these zones, whose exposure is related to broad synclinal folding of some of the oldest rocks in the Great Bear magmatic zone, are where iron oxide copper–gold (IOCG)-targeted exploration efforts should be focused on these areas in both outcrop and subcrop.

Multiple methods for regional- to mine-scale targeting, Pataz gold field, northern Peru

Gold production in the Pataz district, northern Peru, is derived from mesothermal veins hosted by the Pataz batholith and basement-hosted epithermal and carbonate–base metal veins. At the regional scale, processing of Advanced Spaceborne Thermal Emission and Reflection Radiometer data can be used to delineate district-scale argillic alteration. One such area extends for tens of kilometres NNW of Vijus in the Maranon Valley. At the southern end of this area, basement-hosted quartz–carbonate–sulfide veins in faults support artisanal gold-mining operations. SEM analyses show that the alteration envelopes around these faults are dominated by illitic clays. These artisanal gold workings highlight the economic potential of the largely unexplored parts of the district-scale argillic alteration zone, further north. At the district scale, paleostress modelling maps areas of low minimum stress during Carboniferous ENE–WSW shortening, based on a new 1:25 000 geological map of the Pataz district. The resulting distribution of low minimum stress is used to predict sites of rock fracture under high fluid pressure, and consequent vein formation. These areas of low minimum stress occupy 11% of the modelled area but contain 50% of the known veins in the Pataz district. Some areas of low minimum stress contain no known veins, and where these are poorly explored or poorly exposed, they are proposed as potential targets for gold exploration. In combination with SEM microanalysis, ASD hyperspectral reflectance analysis of drill core samples shows that visible proximal sericitic alteration around batholith-hosted auriferous veins is predominantly phengitic illite. Automated software interpretation of ASD reflectance spectra using The Spectral Assistant shows that sericite in cryptic alteration distal from auriferous veins varies from mainly illite adjacent to the phengitic illite zone, to more distal muscovite. Reactivation of faults and mineralised vein contacts during the largely Cenozoic Andean orogeny produced chlorite alteration that locally overprints proximal phengitic illite alteration. ASD spectrometry identifies relict phengitic illite in some chloritic alteration zones and thus indicates proximity to mineralised veins at the deposit scale. Elevated pathfinder element concentrations within proximal phengitic illite alteration zones around batholith-hosted veins do not extend more than a few metres beyond the visible alteration envelope. The alkali alteration index [(Rb + Cs)/Th]N is elevated above background levels for up to 15 m beyond the visible sericite alteration zone in one of two holes investigated. In the other hole, both [(Rb + Cs)/Th]N and 3K/Al can be used as a lode-scale vector to gold-bearing veins within broad intersections of visible sericite alteration.

The application of rapid potential field methods for the targeting of IOCG mineralisation based on physical property data, Great Bear magmatic zone, Canada

Abstract New techniques for the integration of gravity and aeromagnetic data through rock physical property relationships are tested for the rapid location of potential iron oxide–copper–gold (IOCG) exploration targets across the Great Bear magmatic zone (GBMZ). These techniques are based on the recognition of coincident or near-offset magnetic and gravity anomalies associated with IOCG deposits, resulting from their intense and combined iron-oxide and alkali alteration. In the GBMZ, the NICO (Au–Co–Bi–Cu) deposit is an atypical magnetite-group IOCG hosted within an intensely hydrothermally-altered metasedimentary sequence. Prospectivity maps for the NICO area are derived from the integration of high-resolution aeromagnetic and surface gravity data with physical property measurements of magnetic susceptibility and density. Method 1 combines the pseudo-gravity (derived from the aeromagnetic data) and Bouguer gravity data to determine the locations of superimposed high anomalies. Method 2 estimates the apparent susceptibility and density subsurface distribution based on vertical prism models of the magnetic and gravity data. The apparent susceptibility data are transformed to apparent density using physical property relationships. The two apparent density datasets are then combined to determine the location of coincident magnetic and gravity derived anomalies. The results of these methods are supported by, and compared with, similar prospectivity maps generated through weights of evidence techniques. At NICO, the prospectivity maps accurately locate the deposit and nearby occurrences. Despite the lower resolution of the regional gravity data, application of the techniques across the GBMZ is successful at locating mineralised IOCG systems including the magnetite-group systems at Grouard, Fab, DeVries and Cole Lake, the magnetite to hematite-group systems of the Port Radium-Echo Bay district, and has highlighted possible targets for future exploration.

Hydrothermal alteration in metasedimentary rock-hosted orogenic gold deposits, Reefton goldfield, South Island, New Zealand

Orogenic or mesothermal quartz lodes in lower Palaeozoic Greenland Group metasedimentary rocks of the Reefton area have produced 67 tonnes (t) of gold prior to 1951, and recent exploration has identified new gold resources in several deposits, including the largest past producers, Blackwater and Globe-Progress. The metasedimentary rocks consist of alternating sandstone and mudstone beds that were metamorphosed to lower greenschist facies prior to being hydrothermally altered adjacent to the quartz lodes. The sandstones are feldspathic litharenites averaging Q65–F10–R25, with detrital grains of quartz, rock fragments, muscovite, and plagioclase and biotite that were altered to albite and chlorite, respectively, during metamorphism. Accessory minerals are graphite, apatite, zircon, tourmaline and titanite. Hydrothermal alteration of the sandstones has developed a mineral assemblage of K-mica, carbonate (dolomite, ankerite, ferroan magnesite and magnesian siderite), chlorite, pyrite and arsenopyrite. The abundance of hydrothermal chlorite is greater at Blackwater than at the other prospects studied. Hydrothermal alteration associated with the quartz lodes is marked by bleaching, magnesian siderite spots, disseminated arsenopyrite and pyrite and thin carbonate, quartz and sulphide veins. These trends are accompanied by increasing concentrations of S, As and Sb and decreasing Na, and a decrease of Fe and Mg in K-mica. The alkali alteration indices 3K/Al (representing K-mica) and Na/Al (representing albite) generally show antipathetic trends, with 3K/Al increasing near the lodes and Na/Al decreasing. These trends reflect the replacement of albite by K-mica. Carbonate alteration indices CO2/(Ca + Mg +Fe) and CO2/[Ca + Mg + Fe –0.5(S + As)] quantify the abundance of hydrothermal carbonates, but they show variable correlation with the lodes. They increase the width of the alteration halo in the hanging wall of the lodes at the Globe-Progress and General Gordon prospects, but the peak values are as far as 150 m from the lodes. By contrast, peak values of the carbonate alteration indices are within 10 and 2 m of the lodes, respectively, at the Merrijigs and Blackwater deposits. Data show that for deposits with wide hydrothermal alteration halos, such as at the Globe-Progress and General Gordon prospects, the use of a suite of geochemical indicators can assist exploration by indicating trends in hydrothermal alteration that provide vectors to mineralisation. They also increase the size of the exploration target. By contrast, the alteration halo of the Blackwater deposit is restricted to within less than 5 m of the quartz lode and, therefore, the geochemical indicators are of more limited assistance to exploration.

Alkali alteration styles and mechanisms, and their implications for a ‘brine factory’ source of base metals in the rift‐related McArthur group, Australia

McArthur Group alkali metasomatism occurred in several phases, none of which fit a classic model of alkali‐mineral zonation towards an evaporitic basin centre as previously proposed. They comprise: (i) trough‐margin alteration along the Emu Fault Zone, formed by gravity‐driven meteoric‐evaporitic brines sourced from adjacent shelves, and focused up to 3.5 km out into the adjacent deeper water clastic succession of the Barney Creek Formation; these formed zoned albite‐microcline assemblages, variably overprinted by B‐bearing illite; (ii) ore‐related alteration at the HYC Zn‐Pb‐Ag deposit, which overprinted trough‐margin alteration in places, adding disseminated ankerite, adularia and base‐metal sulfides to some feldspathised beds; (iii) early diagenetic alkali metasomatism that was associated with descending/advecting brines sourced from overlying evaporite sequences, and transmitted through porous clastic sediments (e.g. parts of the Lynott Formation, Mara Dolomite and Tatoolla Sandstone); and (iv) extensive inversion‐related K‐feldspar metasomatism of volcaniclastic beds towards the basin centre. The descending/advecting brine model (iii) has implications for sedimentary Zn‐Pb ore genesis in the basin, because it offers a mechanism to efficiently deliver batches of saline fluid into the deep diagenetic environment, resulting in alkali alteration and metal leaching, and the production of ideal Zn‐Pb‐transporting ore fluids. A variation in the flux of saline diagenetic water, caused by eustatically and tectonically driven aridity cycles in sedimentary facies, could have imposed strong episodicity on the formation of related Zn‐Pb deposits. This speculative model is relevant to metals exploration on the premise that each ‘brine factory’ potentially gave rise to a related cycle of Zn‐Pb ore deposition at the same or an overlying stratigraphic level.

Hostrocks to the stratabound iron-formation-hosted Starra gold-copper deposit, Australia

This paper documents the timing, geochemistry and possible origin of sodium enrichment associated with Proterozoic iron formation-hosted copper-gold ore lenses at the Starra deposit, Australia. The ore lenses are immediately underlain by variably sheared albitemagnetite-hematite-pyrite-bearing rocks, and overlain by mainly unaltered, less-sheared, meta-sediments. Evidence indicating that albite was abundant prior to deformation, and also prior to the main hydrothermal addition of Fe, S, Cu and Au, includes (1) regional albite development at this stratigraphic level; (2) alteration of albite by ore-related sericite; and (3) inclusion of unfoliated inclusions of albite and hematite in ore-related pyrite. Nevertheless, albite also developed around dolerites and within hangingwall shears during metamorphism and deformation. “Least altered” albite footwall rocks have SiO2 (67.31–73.13 wt.%), Al2O3(9.89–12.51 wt.%), total alkali elements (6.40–9.43 wt.%) and HFS elements (e.g., Zr = 179–275 ppm; Ti/Zr = 7.6–14.1) comparable with felsic volcanics, but have high relative Na2O contents (5.08–5.81 wt. %), and variable to high Na2O/K2O ratios (0.05–68.37). An episode of alkali alteration by high-Na fluids prior to the ore-related alteration is postulated to account for sodic compositions, after chemical consideration of the alternatives (1) arkosic sediments; (2) trondhjemitic volcaniclastics; and (3) Na-metasomatism of Staveley Formation sediments. During the most intense ore-alteration, isocon analysis indicates a density increase of ∼ 23% compared to the “least altered” albitised hostrocks. Cu, Fe, Au, Sn, W, Zn, K, Ba and Rb were enriched during the mineralising process, whereas Mn, Ca, Si, Nb and Zr were lost, and Al, Ti and Y were not changed (and were used to define the isocon). The loss of some high field strength elements during this alteration indicates that igneous rock classification diagrams based on immobile element ratios should be used with caution for albitemuscovite magnetite-pyrite assemblages associated with iron formation-hosted copper-gold deposits. Zircon was partially soluble, whereas titanium oxides were probably stable in the saline, high temperature (260°–380 °C), acid (pH = 3.9–6.0) fluids envisaged for ore-related alteration.

Lithium-Bearing Hydrothermal Alteration Phyllosilicates Related to Portalet Fluorite Ore (Pyrenees, Huesca, Spain)

AbstractPhyllosilicate associations in hydrothermally altered fluorite ore bodies are: Li-chlorite ± pyrophyllite ± interstratified minerals ± muscovite +± kaolinite. Chlorites, the main alteration minerals, are dioctahedral, d060 = 1.489-1-490/~,, of Ia polytype. The structural formulae indicate substitution of AI for Si from 0.61-0.78 atoms. The total octahedral occupancy ranges from 4.52-4-71 atoms, with 0.49-0-69 Li atoms per half cell unit. This composition indicates that the chlorites are intermediate members of the donbassite-cookeite series proposed by Sudo (1978). The mineralogical associations and textural relations suggest that after intensive silicification which produced alkali alteration under acid conditions, pyrophyllite was produced at the expense of muscovite and then Li-bearing donbassite formed from the pyrophyllite. The Li needed for the formation of the chlorites could be genetically related to granitic batholiths which occur close to the fluorite ores.

Long-Term Evolution of Fluid-Rock Interactions in Magmatic Arcs: Evidence from the Ritter Range Pendant, Sierra Nevada, California, and Numerical Modeling

A record of > 100 million years of fluid flow, alteration, and metamorphism in the evolving Sierra Nevada magmatic are is preserved in metavolcanic rocks of the Ritter Range pendant and surrounding granitoids. The metavolcanic rocks consist of: (1) a lower section of mostly marine volcaniclastic rocks, lavas, and intercalated carbonate rocks that is Triassic to Jurassic in age, and (2) an upper section comprising a subaerial caldera-fill complex of mid-Cretaceous age. Late Cretaceous high-temperature contact metamorphism (∼2 kbar, >450–500°C) occurred after renewed normal faulting along the caldera-bounding fault system juxtaposed the two sections. The style and degree of alteration and δ18O values differ among the rocks of the upper and lower sections and the granitoids. Rocks of the lower section show pervasive lithologically controlled alkali alteration, local Mn and Mg enrichment, and oxidation. Some ash flow tuffs now contain up to 10% K2O by weight. The rocks of the upper section show lesser extents of alkali alteration. Granitoids that cut both sections are generally unaltered. Most metavolcanic rocks of the lower section have high δ18O values (+ 11 to + 16%; whole rock and quartz phenocrysts); however, lower-section rocks within the caldera-bounding fault system have low δ18O values of + 4 to +7‰. The metavolcanic rocks of the upper section also have low δ18O values of + 2 to + 7‰. Granitoids have δ18O values of + 7 to + 10‰, typical of unaltered Sierran granitoids. The lower section contains discontinuous veins of high-temperature (450–500°C) calc-silicate minerals. These veins are typically <5 m long, do not cross intrusive contacts, and postdate the pervasive alkali alteration. Late veins are typically > 10 m long, formed at temperatures of less than 450–500°C, and cross intrusive contacts. Veins have similar δ18O values to those of the local host rocks. The nature of the alteration and the high oxygen isotopic values of the rocks of the lower section indicate that these rocks interacted extensively with seawater at temperatures <300°C, probably in superposed marine hydrothermal systems associated with coeval volcanic centers. Metavolcanic rocks of the upper section evidently interacted with meteorie waters, probably in a hydrothermal system associated with the Cretaceous caldera; rocks of the lower section that were adjacent to the caldera were also affected by this alteration. The preservation of the signatures of these earlier events, the nature of the early veins, and results from numerical models of hydrothermal flow that include fluid production indicate that during prograde contact metamorphism, the rocks of the pendant primarily interacted with locally derived fluids. Fluid flow was predominantly upward and away from intrusive contacts and down-temperature. Permeabilities are estimated to have been between 0⋅1 and 1μD, which is that necessary for maintenance of lithostatic fluid pressures. In hydrothermal models with such permeabilities, large-scale circulation of meteoric fluids develops after prograde metamorphism ceases. The nature of the late veins in the Ritter Range pendant suggests that such a flow pattern evolved only after the pendant and granitoids had cooled below 450–500°C. The long-term history of alteration documented in the Ritter Range pendant is probably typical of wall rocks in most batholiths