Metamorphism, Geochronology and Stratigraphy of an Amphibolite-Facies Greenstone-Hosted Gold Deposit: Plutonic Gold Mine, Marymia Inlier, Western Australia

<p>A significant proportion of the world's Au occurs in the metamorphosed mafic rocks of Archaean greenstone belts. In such deposits, the original stratigraphy and its possible role in localising Au mineralisation can be difficult to discern due to a lack of distinctive marker units and the mineralogically and texturally monotonous nature of the metabasaltic host rocks. Understanding the effects of metamorphism, deformation, and alteration on these largely uniform host rocks, which may have experienced multiple generations of Au mineralisation, is essential for finding and extracting Au from within those deposits, and for discovering new greenstone-hosted Au deposits. This study examines the effects of primary stratigraphy on Au mineralisation, the conditions and possible controls on metamorphism, and the timing of Au mineralising events at Plutonic Gold Mine (Plutonic), Plutonic Well Greenstone Belt (PWGB), Marymia Inlier, Western Australia. Questions that remain unresolved in over 20 years of mining can now be addressed utilising advances in portable X-Ray fluorescence (pXRF), thermodynamic modelling of mineral activities and geochronological techniques. The stratigraphy of the Au-mineralised amphibolite-facies metabasalts that comprise the mine sequence at Plutonic has been examined using pXRF techniques. The results illustrate a geochemical stratigraphy in which individual lava flows can be identified on the basis of element concentrations. The most evolved basalts are at the structural base of the succession, and the least evolved at the top of the sequence. This confirms previous geochemical interpretations and textural evidence that the sequence is overturned, and demonstrates for the first time that the presented section does not involve significant structural repetition. In conjunction with Au assay data, the pXRF data reveal that Au typically occurs along basalt flow boundaries. The elemental concentration data clearly demonstrate stratigraphic control on Au mineralisation that is not readily apparent at the macroscopic level. Results of P–T pseudosection calculations in the NCFMASHTOS (Na₂O-CaO-FeO-MgO-Al₂O₃-SiO₂-H₂O-TiO₂-O-SO₂) system are presented for two typical metabasaltic rocks from the Plutonic. Those results, together with changes in mineral compositions and mineral assemblages observed in the rocks, are used to argue that a previously-unrecognised steep pressure increase (from ~ 3–4 kbar at ~ 500 °C to ≥ 8 kbar at ~ 600 °C) accompanied metamorphism to peak temperatures. Existing models for the early evolution of the PWGB involve nappe stacking supported by relatively cold strong crust, with little overall change in thickness and with peak metamorphism at temperatures similar to those reported here, but with pressures of ~ 4 kbar. Prior to this study the main episode of Au mineralisation in the PWGB was interpreted to either have accompanied or shortly followed the attainment of peak metamorphic conditions in the late Archaean at ~ 2650 Ma. New Pb-isotope results reveal that the majority of Au-associated sulphides at Plutonic are Proterozoic in age, at ~ 2200 Ma, suggesting that Au-mineralisation may have been widespread in the inlier and associated cratonic areas at that time. Later Au-mineralising events have also been constrained at ~ 1830 Ma, and at 1730–1660 Ma. Rb-Sr data from a biotite from Plutonic possibly indicates that the metamorphism was followed by a protracted period of slow cooling. Alternatively, the biotite data may reflect some combination of resetting, probably related to metasomatic events associated with Au mineralisation at ~ 2200 Ma, or with the Capricorn Orogeny at ~ 1830 Ma, and cooling. A further metasomatic event at ~ 1720 Ma is dated by both U-Pb dating of zircon overgrowths, and a new ²⁰⁷Pb-²⁰⁶Pb age from a hydrothermal sphene in chlorite-carbonate vein of 1725 ± 26 Ma. This metasomatic event was probably associated with Au mineralisation, as the Pb-isotope ages for the final Au-mineralising event range from 1730–1660 Ma.</p>

Metamorphism, Geochronology and Stratigraphy of an Amphibolite-Facies Greenstone-Hosted Gold Deposit: Plutonic Gold Mine, Marymia Inlier, Western Australia

<p>A significant proportion of the world's Au occurs in the metamorphosed mafic rocks of Archaean greenstone belts. In such deposits, the original stratigraphy and its possible role in localising Au mineralisation can be difficult to discern due to a lack of distinctive marker units and the mineralogically and texturally monotonous nature of the metabasaltic host rocks. Understanding the effects of metamorphism, deformation, and alteration on these largely uniform host rocks, which may have experienced multiple generations of Au mineralisation, is essential for finding and extracting Au from within those deposits, and for discovering new greenstone-hosted Au deposits. This study examines the effects of primary stratigraphy on Au mineralisation, the conditions and possible controls on metamorphism, and the timing of Au mineralising events at Plutonic Gold Mine (Plutonic), Plutonic Well Greenstone Belt (PWGB), Marymia Inlier, Western Australia. Questions that remain unresolved in over 20 years of mining can now be addressed utilising advances in portable X-Ray fluorescence (pXRF), thermodynamic modelling of mineral activities and geochronological techniques. The stratigraphy of the Au-mineralised amphibolite-facies metabasalts that comprise the mine sequence at Plutonic has been examined using pXRF techniques. The results illustrate a geochemical stratigraphy in which individual lava flows can be identified on the basis of element concentrations. The most evolved basalts are at the structural base of the succession, and the least evolved at the top of the sequence. This confirms previous geochemical interpretations and textural evidence that the sequence is overturned, and demonstrates for the first time that the presented section does not involve significant structural repetition. In conjunction with Au assay data, the pXRF data reveal that Au typically occurs along basalt flow boundaries. The elemental concentration data clearly demonstrate stratigraphic control on Au mineralisation that is not readily apparent at the macroscopic level. Results of P–T pseudosection calculations in the NCFMASHTOS (Na₂O-CaO-FeO-MgO-Al₂O₃-SiO₂-H₂O-TiO₂-O-SO₂) system are presented for two typical metabasaltic rocks from the Plutonic. Those results, together with changes in mineral compositions and mineral assemblages observed in the rocks, are used to argue that a previously-unrecognised steep pressure increase (from ~ 3–4 kbar at ~ 500 °C to ≥ 8 kbar at ~ 600 °C) accompanied metamorphism to peak temperatures. Existing models for the early evolution of the PWGB involve nappe stacking supported by relatively cold strong crust, with little overall change in thickness and with peak metamorphism at temperatures similar to those reported here, but with pressures of ~ 4 kbar. Prior to this study the main episode of Au mineralisation in the PWGB was interpreted to either have accompanied or shortly followed the attainment of peak metamorphic conditions in the late Archaean at ~ 2650 Ma. New Pb-isotope results reveal that the majority of Au-associated sulphides at Plutonic are Proterozoic in age, at ~ 2200 Ma, suggesting that Au-mineralisation may have been widespread in the inlier and associated cratonic areas at that time. Later Au-mineralising events have also been constrained at ~ 1830 Ma, and at 1730–1660 Ma. Rb-Sr data from a biotite from Plutonic possibly indicates that the metamorphism was followed by a protracted period of slow cooling. Alternatively, the biotite data may reflect some combination of resetting, probably related to metasomatic events associated with Au mineralisation at ~ 2200 Ma, or with the Capricorn Orogeny at ~ 1830 Ma, and cooling. A further metasomatic event at ~ 1720 Ma is dated by both U-Pb dating of zircon overgrowths, and a new ²⁰⁷Pb-²⁰⁶Pb age from a hydrothermal sphene in chlorite-carbonate vein of 1725 ± 26 Ma. This metasomatic event was probably associated with Au mineralisation, as the Pb-isotope ages for the final Au-mineralising event range from 1730–1660 Ma.</p>

The Petyayan-Vara Carbonatite-Hosted Rare Earth Deposit (Vuoriyarvi, NW Russia): Mineralogy and Geochemistry

The Vuoriyarvi Devonian carbonatite–ijolite–pyroxenite–olivinite complex comprises several carbonatite fields: Neske Vara, Tukhta-Vara, and Petyayan-Vara. The most common carbonatites in the Tukhta-Vara and Neske-Vara fields are calciocarbonatites, which host several P, Fe, Nb, and Ta deposits. This paper focuses on the Petyayan-Vara field, in which the primary magmatic carbonatites are magnesian. The least altered magnesiocarbonatites are composed of dolomite with burbankite and are rich in REE (up to 2.0 wt. %), Sr (up to 1.2 wt. %), and Ba (up to 0.8 wt. %). These carbonatites underwent several stages of metasomatism. Each metasomatic event produced a new rock type with specific mineralization. The introduction of K, Si, Al, Fe, Ti, and Nb by a F-rich fluid (or fluid-saturated melt) resulted in the formation of high-Ti magnesiocarbonatites and silicocarbonatites, composed of dolomite, microcline, Ti-rich phlogopite, and Fe–Ti oxides. Alteration by a phosphate–fluoride fluid caused the crystallization of apatite in the carbonatites. A sulfate-rich Ba–Sr–rare-earth elements (REE) fluid (probably brine-melt) promoted the massive precipitation of ancylite and baryte and, to a lesser extent, strontianite, bastnäsite, and synchysite. Varieties of carbonatite that contain the highest concentrations of REE are ancylite-dominant. The influence of sulfate-rich Ba-Sr-REE fluid on the apatite-bearing rocks resulted in the dissolution and reprecipitation of apatite in situ. The newly formed apatite generation is rich in HREE, Sr, and S. During late-stage transformations, breccias of magnesiocarbonatites with quartz-bastnäsite matrixes were formed. Simultaneously, strontianite, quartz, calcite, monazite, HREE-rich thorite, and Fe-hydroxides were deposited. Breccias with quartz-bastnäsite matrix are poorer in REE (up to 4.5 wt. % total REE) than the ancylite-dominant rocks (up to 11 wt. % total REE).

HFSE‐REE Transfer Mechanisms During Metasomatism of a Late Miocene Peraluminous Granite Intruding a Carbonate Host (Campiglia Marittima, Tuscany)

The different generations of calc‐silicate assemblages formed during sequential metasomatic events make the Campiglia Marittima magmatic–hydrothermal system a prominent case study to investigate the mobility of rare earth element (REE) and other trace elements. These mineralogical assemblages also provide information about the nature and source of metasomatizing fluids. Petrographic and geochemical investigations of granite, endoskarn, and exoskarn bodies provide evidence for the contribution of metasomatizing fluids from an external source. The granitic pluton underwent intense metasomatism during post‐magmatic fluid–rock interaction processes. The system was initially affected by a metasomatic event characterized by circulation of K‐rich and Ca(‐Mg)‐rich fluids. A potassic metasomatic event led to the complete replacement of magmatic biotite, plagioclase, and ilmenite, promoting major element mobilization and crystallization of K‐feldspar, phlogopite, chlorite, titanite, and rutile. The process resulted in significant gain of K, Rb, Ba, and Sr, accompanied by loss of Fe and Na, with metals such as Cu, Zn, Sn, W, and Tl showing significant mobility. Concurrently, the increasing fluid acidity, due to interaction with Ca‐rich fluids, resulted in a diffuse Ca‐metasomatism. During this stage, a wide variety of calc‐silicates formed (diopside, titanite, vesuvianite, garnet, and allanite), throughout the granite body, along granite joints, and at the carbonate–granite contact. In the following stage, Ca‐F‐rich fluids triggered the acidic metasomatism of accessory minerals and the mobilization of high-field-strength elements (HFSE) and REE. This stage is characterized by the exchange of major elements (Ti, Ca, Fe, Al) with HFSE and REE in the forming metasomatic minerals (i.e., titanite, vesuvianite) and the crystallization of HFSE‐REE minerals. Moreover, the observed textural disequilibrium of newly formed minerals (pseudomorphs, patchy zoning, dissolution/reprecipitation textures) suggests the evolution of metasomatizing fluids towards more acidic conditions at lower temperatures. In summary, the selective mobilization of chemical components was related to a shift in fluid composition, pH, and temperature. This study emphasizes the importance of relating field studies and petrographic observations to detailed mineral compositions, leading to the construction of litho‐geochemical models for element mobilization in crustal magmatic‐hydrothermal settings.

Orthopyroxene-enrichment in the lherzolite-websterite xenolith suite from Paleogene alkali basalts of the Poiana Ruscă Mountains (Romania)

In this paper we present the petrography and geochemistry of a recently collected lherzolite-websterite xenolith series and of clinopyroxene xenocrysts, hosted in Upper Cretaceous–Paleogene basanites of Poiana Ruscă (Romania), whose xenoliths show notable orthopyroxene-enrichment. In the series a slightly deformed porphyroclastic-equigranular textured series could represent the early mantle characteristics, and in many cases notable orthopyroxene growth and poikilitic texture formation was observed. The most abundant mantle lithology, Type A xenoliths have high Al and Na-contents but low mg# of the pyroxenes and low cr# of spinel suggesting a low degree (< 10 %) of mafic melt removal. They are also generally poor in overall REE-s (rare earth elements) and have flat REY (rare earth elements+ Y) patterns with slight LREE-depletion. The geochemistry of the Type A xenoliths and calculated melt composition in equilibrium with the xenolith clinopyroxenes suggests that the percolating melt causing the poikilitization can be linked to a mafic, Al-Na-rich, volatile-poor melt and show similarity with the Late Cretaceous–Paleogene (66–72 Ma) subduction-related andesitic magmatism of Poiana Ruscă. Type B xenoliths, with their slightly different chemistry, suggest that, after the ancient depletion, the mantle went through a slight metasomatic event. A subsequent passage of mafic melts in the mantle, with similar compositions to the older andesitic magmatism of Poiana Ruscă, is recorded in the pyroxenites (Fe-rich xenoliths), whereas the megacrysts seem to be cogenetic with the host basanite. The Poiana Ruscă xenoliths differ from the orthopyroxene-enriched mantle xenoliths described previously from the Carpathian-Pannonian Region and from the Dacia block.

Petrogenesis and tectonic implications of Late Mesozoic granites in the NE Yangtze Block, China: further insights from the Jiuhuashan–Qingyang complex

The Jiuhuashan–Qingyang complex is one of the Mesozoic granite complexes in the NE Yangzte Block, China. New petrographical and petrochemical data show that the complex comprises a dominant granodiorite–monzogranite, the Qingyang body, which was intruded by the Jiuhuashan granite body. The two are characterized by distinct mineral components and trace element patterns. Compared to the Qingyang granodiorite and monzogranite, the Jiuhuashan granite is enriched in Rb, Th, U, Nb, Ta, Hf, Yb and Lu, and depleted in Ba, Sr, Nd, Sm, Eu, Gd and Ti, which are ascribable to the separation of plagioclase and biotite, and crystallization of thorite and fergusonite during the magmatism. New LA-ICPMS zircon U–Pb dating suggests that the crystallization age of the Qingyang body is 139–133 Ma, and the Jiuhuashan granite followed at 127 Ma. Moreover, the new zircon U–Pb dates reveal that Archaean materials were involved in the formation of these magmas, and that a sodium-rich metasomatic event occurred at about 100 Ma. The CHIME monazite and zircon ages studied for the Jiuhuashan body agree well with the LA-ICPMS zircon ages. Integrating this information with previous studies for granites in the NE Yangtze Block and in the coastal area of SE China, we believe that all of these Late Mesozoic granites were produced under the tectonic regime of palaeo-Pacific plate subduction towards the SE China continent in a NW direction, but the granites in the NE Yangtze Block are basically derived by crustal melting with limited mixing of juvenile material during the magma generation.

The geochemistry and structural significance of a set of Middle Precambrian diabase dikes from the Highland Range, southwestern Montana

The Middle Precambrian diabase dikes of the Highland Range in southwestern Montana are moderate- to high-TiO2 continental tholeiites and are related along a differentiation trend involving strong iron enrichment. Postmagmatic metamorphism and K, Rb, and Sr metasomatism have altered the chemical composition of the igneous rocks of some of the samples. The metamorphic assemblage in the diabase dikes belongs to the low-pressure calcic plagioclase – actinolite hornfels facies, and we suggest that thermal effects associated with the intrusion of the Boulder Batholith are responsible for the metamorphic overprints in these rocks.Combined chemical data from the diabase dikes in the Highland Range, the Ruby Range, and the Tobacco Root Mountains produce smooth differentiation trends for most major oxides and trace elements, and we conclude that one magma was responsible for the dikes in the three ranges. Discrepancies in Rb–Sr age dates obtained for the dikes in the Tobacco Root Mountains can be explained if a Rb, Sr, and K metasomatic event like the one observed in the Highland Range had occurred in the Tobacco Root Mountains as well.Structurally, the diabase dikes in the Highland Range intruded into both east–west- and northwest-trending fractures at the same time. All dikes dip steeply to the north or northeast and are believed to have intruded into tensionally opened fractures related to the opening of the Belt Basin.