New radiogenic (40Ar/39Ar) and stable (oxygen, hydrogen, and sulfur) isotope analyses of metamorphic and metasomatic minerals constrain the age, metasomatic evolution, and genesis of the Eloise Cu-Au deposit (3.1 Mt @ 5.5% Cu, 1.4 g/t Au, and 16 g/t Ag). Biotite from a pre- to syn-D2-stage vein has an 40Ar/39Ar age of 1555 ± 4 Ma which is interpreted to coincide with a regional metamorphic event synchronous with D2. Six later stages of alteration and mineralization are recognized, all of which postdate peak metamorphism and D2. Stages I to III are volumetrically the most significant and comprise early albitization (stage I), quartz-hornblende-biotite veins and alteration (stage II), and Cu-Au mineralization (stage III). Stages IV, V, and VI are localized vein events and postdate the main Cu-Au mineralization. Stage II vein and alteration hornblendes have 40Ar/39Ar ages of 1530 ± 3 Ma. Biotite from the same stage has an 40Ar/39Ar age of 1521 ± 3 Ma. Muscovite from a postore shear zone has an 40Ar/39Ar age of 1514 ± 3 Ma. These results provide a maximum (ca. 1530 Ma) and minimum (ca. 1514 Ma) age for the mineralizing event. However, the intimate relationship between stage II mafic-silicate veins and alteration and the stage III Cu-Au event combined with fluid inclusion results which indicate that a cooling, evolving high-salinity fluid was responsible for both stages suggests that the older age is likely to be closer to the age of mineralization. The stage II biotite age of ca. 1521 Ma is interpreted to record thermal resetting during postore ductile deformation. The results suggest that micas in the Cloncurry district are more susceptible to later thermal resetting than amphiboles which may have significant implications for deposits which have only been dated by micas.
Quartz from stages II, III, and IV have δ 18Oquartz values ranging between 10.1 and 11.9 per mil. Stage II biotite has a lower δ 18O composition (5.6‰) than biotite from a pre-D2vein (6.9‰), but both have identical δ D compositions (–84‰). Stage II hornblende has lower δ 18O (6.8 and 7.4‰) and δ D (–88 and –90‰) values than does stage III actinolite (8.0 and –84‰, respectively). The δ 34S values for chalcopyrite, pyrrhotite, and pyrite fall in a narrow range between 0.0 and 2.3 per mil. A distinct trend can be recognized with δ 34S values becoming progressively greater from south to north, thus reflecting the zoned alteration system (magnetite-pyrite-rich in the south through to pyrrhotite-rich in the north). The change in mineralogy and sulfur isotope values may be due to cooling and sulfidation processes which resulted in changes in oxygen and sulfur fugacities. The oxygen, hydrogen, and sulfur isotope data, combined with high-temperature and high-salinity fluid inclusion data, indicate a predominantly magmatic origin for the ore forming fluids. The deposit is interpreted to have formed from magmatic hydrothermal fluids which were tapped by deep-seated crustal structures.
The Lightning Creek Cu-Au prospect is hosted by a cogenetic suite of plutonic, I-type granitoids. The dominant rock type is a porphyritic quartz monzodiorite that is intruded by more fractionated rocks, including monzogranite and alkali feldspar granite. A series of flat-lying sills are interpreted to be late-stage differentiates, based on their timing, mineralogy, and chemistry.
In parts of the prospect there is pervasive sodic-calcic alteration (pyroxene after amphibole, albite after K feldspar and oligoclase) of the plutonic rocks. This alteration predates sill emplacement and is unrelated to veining or fracturing of any kind. The presence of small amounts of carbonate in the altered rocks suggests that the fluids were CO2 bearing. Quartz and feldspar separates from these altered rocks have oxygen isotope compositions similar to those from fresh quartz-monzodiorite, suggesting that the fluids were hot and of magmatic composition. Sodium and Ca were added and K, Fe, Cl, and Cu were stripped during what is interpreted as an autometasomatic event.
The sills display considerable textural and mineralogical complexity and evolved from equigranular, quartzofeldspathic rocks (aplites), with magmatic chemistry, to unusual Fe-rich rocks (albite-magnetite-quartz) that exhibit a range of bizarre spherulitic textures. Some of the albite and magnetite in the sills is secondary. Albite forms pseudomorphs after K feldspar (Na-Fe ± Ca alteration) along sill margins and within sills, at the contacts between different textural zones. Halos of disseminated magnetite + clinopyroxene (Fe-Ca ± Na alteration) are developed adjacent to early magnetite veins.
Fluid inclusion studies indicate that these rocks crystallized at temperatures in excess of 500°C and at pressures in excess of 1.5 kbar. The range of spherulitic textures is taken to indicate crystallization under hydrous conditions with the episodic release of a fluid phase. This magmatic fluid phase was dominated by H2O, CO2, and chlorine and underwent phase separation into a CO2-rich vapor and a hypersaline brine (33–55 wt % NaCl equiv). The hypersaline fluid was enriched in Fe (~10 wt %) and Cu (~1 wt %, PIXE analysis), in addition to Na, K, and Ca. Where this fluid was retained within Fe-rich portions of the sills, it caused Ca-Fe ± Na alteration (pyroxene-albite ± magnetite growth at the expense of quartz). Where the fluid was expelled from the sills, it produced quartz-magnetite ± clinopyroxene ± albite veins (broadly coeval with the early magnetite veins). Although rich in Cu, these granitoid-derived magmatic fluids did not generate significant Cu(-Au) mineralization, perhaps because of the high temperatures involved and/or a lack of reduced sulfur in the fluids or host rock. However, the amount of iron present is estimated (from the aeromagnetic anomaly) to be in excess of 2,000 million tonnes (Mt).
A later generation of calcite ± chlorite ± pyrite ± chalcopyrite veins contain traces of Cu-Au mineralization. Fluid inclusion and stable isotope work indicate that these veins probably crystallized from cooler (<200°C), more dilute (15–28 wt % NaCl equiv) fluids, perhaps generated by the admixture of a meteoric component.
The conclusions reached in this study have implications for understanding the genesis of Fe oxide Cu-Au deposits and related sodic-calcic alteration. The study indicates the potential for CO2-rich granitoid magmas to evolve hypersaline, Fe- and Cu-rich fluids capable of causing intense magnetite veining and Cu(-Au?) mineralization. Autometasomatic sodic-calcic alteration of the granitoids may be an important precursor to mineralization, contributing Fe, K, Cu, and Cl to the magmatic fluids.
U-Pb isotope dating of zircons from a granophyric dike complex which crosscuts the Kiirunavaara magnetite-apatite body demonstrates that the ore was emplaced before 1.880 + or - 0.003 Ga. Whole-rock Sm-Nd isotope compositions for five out of six samples from the footwall and hanging-wall complexes lie on a 1.89 + or - 0.09-Ga isochron. The initial ratio of 0.50991 + or - 0.00007 corresponds to epsilon Nd (T) = -5.6 and suggests a substantial late Archean component in the igneous rocks.Since the ore cannot have been emplaced after the granophyre nor before the host rocks, ore formation is limited to the period between 1.90 and 1.88 Ga. However, the granophyre remains at present in the form of a crosscutting dike, largely unaffected by the ductile and brittle deformation that has affected both the ore and host rocks. This implies that ore formation, igneous activity, and deformation were all confined to a short time interval during this period and suggests that the Kiruna magnetite-apatite ores were generated during orogenesis.This association of orogenesis, emplacement temperatures up to 600 degrees C, abundant evidence of hydraulic processes during ore formation, and extensive porphyries is reminiscent of other types of intrusion-related, high-temperature, ore-forming processes.
The field of rock dating, known as geochronology, is very important within the Earth Sciences. The ability to measure accurately the age of a rock allows a geologist to build up a clear picture of a rock's history. Not only can this provide the rock's age, but it can also give information about the pressure and temperature (PT) history and the timing of episodes of deformation.
Channel iron deposits (CID) consist of ferruginous chemical sediments, and are an unusual subset of Phanerozoic coidal ironstone deposits almost unique to the Pilbara, where they constitute around 19 per cent of the Pilbara's iron ore exports. These deposits occupy many of the ancestral
river channels of the Pilbara region, especially the Robe River palaeochannels, incising and meandering through the Precambrian banded iron formations (BIF) of the Hamersley Group and the underlying Fortescue Volcanics.
Plutons of the Naraku Batholith were emplaced into Proterozoic metasediments of the northern portion of the Eastern Fold Belt of the Mt Isa Inlier during two intrusive episodes approximately 200 million years apart. Structural relationships and geochronological data suggest that the older plutons (ca 1750 Ma) are contemporaneous with granites of the Wonga Batholith to the west. The Dipvale Granodiorite and the Levian Granite represent these older intrusive phases of the Naraku Batholith, and both contain an intense tectonic foliation, S1, which is interpreted to have formed during the north‐south shortening associated with D1 of the Isan Orogeny. The geometry of S1 form surfaces at the southern end of the Dipvale Granodiorite, and of the previously unrecognised sheeted contact, defines a macroscopic, steeply south‐southwest‐plunging antiform, which was produced by the regional D2 of the Isan Orogeny. S1 form surfaces in the Levian Granite define open F2 folds with wavelengths of several hundred metres. The structural age of emplacement of the Dipvale Granodiorite and the Levian Granite is interpreted to be pre‐ or syn‐ the regional D1. An intense foliation present in some of the younger (ca 1505 Ma) granites that comprise the bulk of the Naraku Batholith is interpreted to represent S3 of the Isan Orogeny. Foliations commonly have similar styles and orientations in both the pre‐D1 and younger plutons. This emphasises the simplicity with which regional fabrics can be, and probably have been, miscorrelated in the Eastern Fold Belt, and that the classification of granites in general on the basis of structural and geometric criteria alone is fraught with danger.
New radiogenic (40Ar/39Ar) and stable (oxygen, hydrogen, and sulfur) isotope analyses of metamorphic and
metasomatic minerals constrain the age, metasomatic evolution, and genesis of the Eloise Cu-Au deposit (3.1
Mt @ 5.5% Cu, 1.4 g/t Au, and 16 g/t Ag). Biotite from a pre- to syn-D2-stage vein has an 40Ar/39Ar age of 1555
± 4 Ma which is interpreted to coincide with a regional metamorphic event synchronous with D2. Six later stages of alteration and mineralization are recognized, all of which postdate peak metamorphism and D2. Stages I to III are volumetrically the most significant and comprise early albitization (stage I), quartz-hornblende-biotite veins and alteration (stage II), and Cu-Au mineralization (stage III). Stages IV, V, and VI are localized vein events and postdate the main Cu-Au mineralization. Stage II vein and alteration hornblendes have 40Ar/39Ar ages of 1530 ± 3 Ma. Biotite from the same stage has an 40Ar/39Ar age of 1521 ± 3 Ma. Muscovite from a postore shear zone has an 40Ar/39Ar age of 1514 ± 3 Ma. These results provide a maximum (ca. 1530 Ma) and minimum (ca. 1514 Ma) age for the mineralizing event. However, the intimate relationship between stage II
mafic-silicate veins and alteration and the stage III Cu-Au event combined with fluid inclusion results which indicate that a cooling, evolving high-salinity fluid was responsible for both stages suggests that the older age
is likely to be closer to the age of mineralization. The stage II biotite age of ca. 1521 Ma is interpreted to record
thermal resetting during postore ductile deformation. The results suggest that micas in the Cloncurry district
are more susceptible to later thermal resetting than amphiboles which may have significant implications for deposits which have only been dated by micas. Quartz from stages II, III, and IV have δ18Oquartz values ranging between 10.1 and 11.9 per mil. Stage II biotite has a lower δ18O composition (5.6‰) than biotite from a pre-D2 vein (6.9‰), but both have identical δD compositions (–84‰). Stage II hornblende has lower δ18O (6.8 and 7.4‰) and δD (–88 and –90‰) values than does stage III actinolite (8.0 and –84‰, respectively). The δ34S values for chalcopyrite, pyrrhotite, and pyrite fall in a narrow range between 0.0 and 2.3 per mil. A distinct trend can be recognized with δ34S values becoming progressively greater from south to north, thus reflecting the zoned alteration system (magnetitepyrite-rich in the south through to pyrrhotite-rich in the north). The change in mineralogy and sulfur isotope values may be due to cooling and sulfidation processes which resulted in changes in oxygen and sulfur fugacities. The oxygen, hydrogen, and sulfur isotope data, combined with high-temperature and high-salinity fluid inclusion data, indicate a predominantly magmatic origin for the ore forming fluids. The deposit is interpreted
to have formed from magmatic hydrothermal fluids which were tapped by deep-seated crustal structures.
Primary colloform textures preserved in ore deposits can be a useful tool in understanding changing conditions of ore formation due to the sequential development of the colloform layers. However, the growth controls that influence formation of these textures are poorly understood. To try to address this problem, samples from two ore deposits, Greens Creek in Alaska and Ezuri in Japan, have been systematically analyzed for grain size and shape, crystal preferred orientation (CPO), sulfur isotope composition, and trace element content. Grain size and shape varies between layers of equant, ~20 μm crystals to acicular and elongate crystals up to several millimeters in length. Electron backscatter diffraction (EBSD) reveals that both samples have an initial random orientation of crystals with CPO in subsequent layers developed either about <100>, <110>, or <111> crystallographic axes. Despite similarity in texture, the sulfur isotope results from Greens Creek colloforms have a very negative, open-system bacteriogenic δ34S between -40 and -32‰, whereas the Ezuri colloform has a positive δ34S of ~+5‰, typical of hydrothermal sulfur in Kuroko ores. Trace element results indicate variability in As, Sb, and Cu distribution. Whereas trace element variability at Greens Creek appears to be related to changes in δ34S, with a heavier signature correlating with sequestration of Sb in outer layers, overall the detailed analyses reveal that in both Greens Creek and Ezuri, there is no systematic correlation between sulfur source or trace element sequestration and CPO. This suggests that the abrupt changes in CPO recorded appear most likely to be influenced by changes in degree of supersaturation.
Hyperspectral analysis at seven gold deposits within the eastern Yilgarn Craton of Western Australia has revealed significant occurrences of previously unrecognised clinozoisite, and spatial relationships between the distribution of clinozoisite, epidote and gold deposits. Here we report the development of an index to allow the systematic spectral mapping of the epidote–clinozoisite solid solution. The combination of the wavelength position and depth of the 1550 nm absorption was used to characterise the solid- solution series spectrally. The spectral responses from CSIRO HyChips™, fitted with an Analytical Spectral Devices (ASD) FieldSpec-3 spectrometer, and a SisuCHEMA™ spectral-imaging camera were calibrated against electron microbe analyses of epidote–clinozoisite. The spectral-imaging camera helped resolve correlations for samples with complex paragenetic histories. Textural studies found genetic links between epidote and Mg-chlorite, and between clinozoisite and Fe-chlorite, with each mineral combination part of separate, diagnostic hydrothermal assemblages. Spectra from epidote–clinozoisite-dominated veins showed that shifts in the 2250 nm absorption correlate with epidote–clinozoisite composition and not with chlorite composition, and that coexisting amphibole phases have a closer compositional tie than chlorite in the given samples. The genetic affiliation, yet compositional discordance, between coexisting epidote–clinozoisite and chlorite suggests that the compositional spectral index associated with each are wholly independent, but in combination are diagnostic for the mapping of separate hydrothermal assemblages. Of the newly defined compositional relationships, vein-hosted clinozoisite was found to be a proxy for pre-existing structurally-controlled hydrothermal tschermakite. A comparison of spectral and stable isotopic characteristics from diamond drill hole CD5026, St Ives mining camp, shows correlations between the epidote–clinozoisite spectral index and δ13C of carbonate and δ34S of sulfide. Such correlations imply a redox control on the distribution of clinozoisite and epidote, and mean that the spectral logging of epidote–clinozoisite transitions can serve as a proxy for mapping paleoredox gradients.
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