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 Revsegg and Kvitenut Allochthons, Scandinavian Caledonides: origins and evolutions in the Caledonian Wilson cycle

The Caledonian Orogen preserves the record of a complete Wilson cycle from rifting to continent-continent collision and orogenic collapse. The Revsegg Allochthon, the uppermost tectonostratigraphic unit of the Hardangervidda-Ryfylke Nappe Complex of the southern Scandinavian Caledonides, is an understudied example illustrating key temporal and tectonic stages in a Wilson cycle. It overlies 1600-1500 Ma gneisses of the Kvitenut Allochthon that were deformed, metamorphosed and juxtaposed onto the Dyrskard Allochthon at 1000 Ma. The Revsegg Allochthon consists of leucosome-bearing mica-schists with meta-sandstone, and amphibolite and granitoids lenses. The timing of sedimentation of metasedimentary rocks is constrained to the period 780 - 495 Ma, but its association with a 495 Ma bimodal mafic and felsic intrusive suite suggests concurrent sedimentation in a Cambrian extensional setting. The Revsegg Allochthon underwent peak metamorphism at 480-470 Ma, followed by several metamorphic stages from 460 to 440 Ma, probably at an active margin outboard of Baltica, as postulated for the eclogite-bearing Jæren Nappe to the southeast. The Revsegg Allochthon was thrust onto the Kvitenut-Dyrskard duplex from 437 Ma to 434 Ma during an early Scandian phase also recognized in the Seve Nappe. Syn-deformational pegmatites, emplaced at 428 Ma represent the final stage in the nappe stack development. Thematic collection: This article is part of the Caledonian Wilson cycle collection available at: https://www.lyellcollection.org/cc/caledonian-wilson-cycle

Re-Evaluation of the First Metamorphic P-T Path Using QuiG Barometry and Equilibrium Thermodynamics

Quartz in garnet (“QuiG”) barometry is a relatively new technique that uses physical properties of minerals to estimate the pressure of garnet nucleation and growth history independent of chemical equilibrium. QuiG barometry was used to determine pressures of garnet growth and compared to thermodynamically calculated P-T conditions for two samples (FH-1M and Z3H) from the Lower Shieferhülle (Formation), Tauern Window, Austria. FH-1M was the first sample for which a P-T path was calculated through inversion of chemical zoning in garnet (Selverstone et al., 1984). Mineral Assemblage Diagrams (MADs) and geothermobarometric techniques were used to determine P-T conditions for garnet nucleation and peak metamorphism. No MAD reproduced either the results of Selvserstone et al. (1984) or petrologic observations such as mineral assemblages and likely P-T conditions as determined using independent thermobarometers. Thermobarometrically calculated rim conditions were consistent between our study and previous work in the Lower Schieferhülle. However, without appropriate inclusion assemblages and compositions, the accuracy of calculated core P-T conditions could not be independently assessed using thermobarometry for either rock. QuiG isomekes from both samples are broadly consistent with growth of garnet during exhumation with heating as originally proposed by Selverstone et al. (1984). However, the QuiG isomekes for Z3H suggest that 90% or more of the Z3H garnet grew over small changes in pressure and temperature or along a QuiG isomeke (heating with a slight increase in pressure). These results support the accuracy of prior P-T paths and their tectonic interpretations. However, inconsistencies between QuiG barometry vs. thermodynamic calculations remain unresolved.

The Garies Wollastonite Deposit, Namaqualand, South Africa: High-Temperature Metamorphism of a Low-δ18O Skarn?

ABSTRACT The Garies wollastonite deposit is located in the Bushmanland terrane of the Namaqualand Metamorphic Province and is part of a discontinuous calc-silicate unit bounded by granulite facies gneiss that experienced peak metamorphic temperatures above 800 °C. In bulk, the deposit is dominated by wollastonite, but varied proportions of garnet, diopside, quartz, calcite, and vesuvianite are also present. Mineral chemistry variations across the deposit are minor, and the absence of inclusions indicates textural and chemical equilibrium. The wollastonite-bearing rocks have unusually low mineral δ18O values: –0.6 to +2.2‰ for garnet, –0.2 to +2. 6‰ for clinopyroxene, and –0.2 to +0.4‰ for wollastonite. Calcite δ18O values range from 6.8 to 11. 8‰ and δ13C values from –6.4 to –3.2‰. Calcite δ18O values are unusually low for calc-silicate rocks, but Δcalcite-garnet values from 3 to 12‰ indicate O-isotope disequilibrium between calcite and the silicate minerals. Garnet-biotite metapelitic and diopside gneisses have unexpectedly low δ18O values (&lt;7‰). The approach to O-isotope equilibrium displayed by coexisting silicate minerals, and low mineral δ18O values in calc-silicate and metapelite and metapsammite gneisses, is consistent with low δ18O values being acquired before peak metamorphism. Low δ18O values in the minerals of the calc-silicate rocks require interaction with external fluid at high water/rock ratio. We suggest that the deposit represents a metamorphosed skarn that developed at the contact between the original carbonate rocks and intruding felsic magmas.

Vein topology, structures, and distribution during the prograde formation of an Archean gold stockwork

The Archean Cheechoo stockwork gold deposit is hosted by a felsic intrusion of tonalitic-granodioritic composition and crosscutting pegmatite dikes in the Eeyou Istchee James Bay area of Quebec, Canada (Archean Superior craton). The evolution of the stockwork is characterized herein using field relationships, vein density, and connectivity measurements on drill core and outcrop zones. The statistical distribution of gold is used to highlight mechanisms of stockwork emplacement and gold mineralization and remobilization. Two statistical populations of gold concentration are present. Population A is represented by gold grades below 1 g/t with a lognormal cumulative frequency. It is widespread in the hydrothermally altered (albite and quartz) and mineralized facies of the pluton. It is controlled by the development of quartz-feldspar-diopside veins as shown by the similar lognormal distribution of grades and vein density and by the correspondence of grades with network connectivity. Diopside and actinolite porphyroblasts in deformed veins within sodic and calcsilicate alteration zones are evidence for auriferous vein emplacement prior to the amphibolite facies peak of metamorphism. Population B (&gt;1 g/t) is erratic and exhibits a strong nugget effect. It is present throughout the mineralized portion of the pluton and in pegmatites. This population is interpreted as the result of gold remobilization during prograde metamorphism and pegmatite emplacement following the metamorphic peak. The pegmatites are interpreted to have scavenged gold emplaced prior to peak metamorphism. These results show the isotropic behavior of the investigated stockwork during regional deformation and its development during the early stages of regional prograde metamorphism.

Diapiric relamination of the Orocopia Schist (southwestern U.S.) during low-angle subduction

The Orocopia Schist and related schists are sediments subducted during the Laramide orogeny and are thought to have been underplated as a laterally extensive layer at the base of the crust in the southwestern United States Cordillera. This concept is hard to reconcile with the existence of continental mantle lithosphere in southeastern California and western Arizona. Analytical solutions and numerical modeling suggest that the Orocopia Schist may have ascended through the mantle lithosphere as sediment diapirs or subsolidus crustal plumes to become emplaced in the middle to lower crust. Modeled time-temperature cooling paths are consistent with the exhumation history of the Orocopia Schist and explain an initial period of rapid cooling shortly after peak metamorphism. The Orocopia Schist represents a potential example of relaminated sediment observable at the surface.

Late Triassic orogenic assembly of the Tibetan Plateau: constraints from magmatism and metamorphism in the east Lhasa terrane

The early Mesozoic evolution of the Lhasa terrane, which represents a major component of the Himalayan-Tibetan orogen, remains highly controversial. In particular, geological units and events documented either side of the eastern Himalayan syntaxis (EHS) are poorly correlated. Here, we report new petrological, geochemical and geochronological data for co-genetic peraluminous S-type granites and metamorphic rocks (gneiss and schist) from the Motuo–Bomi–Chayu region of the eastern Lhasa terrane, located on the eastern flank of the EHS. Zircon U–Pb dating indicates that these units record both Late Triassic magmatic (216–206 Ma) and metamorphic (209–198 Ma) episodes. The granites were derived from a Paleoproterozoic crustal source with negative zircon εHf(t) values (–5.5 to –16.6) and TDM2 model ages of 1.51–1.99 Ga, and are interpreted to have formed by crustal anatexis of nearby metasediments during collisional orogeny and crustal thickening. The gneisses and schists experienced similar upper amphibolite-facies peak metamorphism and associated partial melting, followed by decompressional cooling and retrograde metamorphism. These rocks were buried to lower-crustal depths and then exhumated to the surface in a collisional orogenic setting during plate convergence. From comparison of these data to other metamorphic belts with similar grades and ages, and association of coeval granitic magmatism widespread in the central-east Lhasa terrane, we propose that the studied co-genetic magmatism and metamorphism in the Motuo–Bomi–Chayu region records Late Triassic accretion of the North Lhasa and South Lhasa terranes, which represents the first evidence of the Paleo-Tethys ocean (PTO) closure in this part of Asia. These data provide new constraints on the spatial and temporal evolution of the Paleo-Tethyan Wilson Cycle and provide a ‘missing link’ to correlate the geology and tectonic history of the Lhasa terrane continental crust on either side of the EHS.

Revised tectonostratigraphy and structural evolution of the Köli Nappe Complex, Central Caledonides in Nordland, Norway

The nappe stack in the Røssvatnet-Hattfjelldal region in the Central Norwegian Caledonides consists of seven nappes formed at the boundary between tectonostratigraphically upper and uppermost Caledonian levels. The rocks of all nappes share a polyphase tectono-metamorphic evolution that is younger than the 491±10 Ma depositional (volcanic) age of parts of the succession. Early deformation stages characterized by centimetre- to kilometre-scale folding and intense shearing accompanied by greenschist- to amphibolite-facies peak metamorphism are correlated with the Early Ordovician Taconian accretionary orogeny along the Laurentian margin. The Taconian structures are cut by the Krutfjellet gabbro and diorite, which yield U-Pb zircon ages of 446±5 and 444±4 Ma, respectively. Large-scale nappe stacking and folding postdating emplacement of the gabbro was related to collision of Laurentia with Baltica (Scandian orogeny) and followed by late-/postorogenic extension. The revised tectonostratigraphy assigns the structurally higher nappes to the Uppermost Allochthon while the lower nappes are correlated with the Middle Köli Nappe Complex (Upper Allochthon). The boundary between them is marked by an imbricate zone. Taconian deformation was probably much more penetrative and widespread than hitherto thought; thus, parts of the nappe stack were likely assembled prior to Scandian collision.Supplementary material:https://doi.org/10.6084/m9.figshare.c.5357255

Deciphering the structural and metamorphic history of the Balsfjord Series in the Upper Allochthon of the Scandinavian Caledonides in northern Norway

&lt;p&gt;Deciphering the structural and metamorphic history of the Balsfjord Series in the Upper Allochthon of the Scandinavian Caledonides in northern Norway&lt;/p&gt;&lt;p&gt;H&amp;#246;pfl Stephan&lt;sup&gt;1&lt;/sup&gt;, Konop&amp;#225;sek Ji&amp;#345;&amp;#237;&lt;sup&gt;1&lt;/sup&gt;, St&amp;#252;nitz Holger&lt;sup&gt;1,2&lt;/sup&gt; Bergh G., Steffen&lt;sup&gt;1&lt;/sup&gt;&lt;/p&gt;&lt;p&gt;UiT Norges arktiske universitet, Institutt for geovitenskap, [email protected]&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;&lt;sup&gt;1&lt;/sup&gt;Department of Geosciences, UiT The Arctic University of Norway, Troms&amp;#248; 9037, Norway&lt;/p&gt;&lt;p&gt;&lt;sup&gt;2&lt;/sup&gt;Institut des Sciences de la Terre (ISTO), Universit&amp;#233; d&amp;#8217;Orleans, Orleans 45100, France&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;The Balsfjord Series is located in the central part of Troms&amp;#8211;Finnmark County, northern Norway, and is part of the upper allochthon of the Scandinavian Caledonides. It consists of an Ordovician&amp;#8211;Silurian metsedimentary sequence lying on top of the mostly gabbroic Lyngen Magmatic Complex (LMC). The unit exhibits an inverted metamorphic gradient, where the metamorphic conditions increase from the base to the top, from very low grade in the southeast to medium grade in the west and northwest. The Balsfjord Series is sandwiched between two high-grade units, the Nakkedal + Troms&amp;#248; Nappe Complex in the hanging wall and the Nordmannvik Nappe as the top part of the Reisa Nappe Complex (RNC) in the footwall. The Nakkedal + Troms&amp;#248; Nappe Complex features metamorphic peak ages of ca. 455&amp;#8211;450 Ma and the Nordmannvik Nappe of ca. 430 Ma. The peak metamorphism of the Balsfjord Series has never been dated and the role of the inverted metamorphic gradient is not yet understood. One of the main motivations in this project is to resolve the Caledonian deformation history in the Balsfjord Series, ideally leading to a regional tectonic model explaining the tectonostratigraphic and metamorphic relationships between the abovementioned units.&lt;/p&gt;&lt;p&gt;The Balsfjord Series features two main discernible folding phases. The earlier phase displays tight to isoclinal folds with flat lying axial surfaces parallel to the penetrative foliation. Observed fold axes are parallel with the stretching lineation. These folds are best preserved in the northwestern, upper part of the unit and are syn-metamorphic in certain areas, as they fold original bedding (transposed foliation). A later folding phase is represented by mainly open folds with inclined to steep axial surfaces. Their fold axes are gently plunging with a predominant NE&amp;#8211;SW orientation. We interpret these two folding events to be genetically related but slightly diachronous. The earlier folding phase with flat lying axial surfaces was likely generated during nappe thrusting and peak metamorphism of the Balsfjord Series. The subsequent open folding with inclined to steep axial surfaces is explained as a result of continued shearing and shortening of the weaker metapelitic Balsfjord Series against the more rigid gabbroic part of the LMC during the late stages of the Caledonian nappe thrusting.&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&lt;/p&gt;&lt;p&gt;Observed thrust kinematics and penetrative retrogression at the bottom of the Nakkedal + Troms&amp;#248; Nappe Complex suggest that its final exhumation took place during prograde metamorphism of the underlying Balsfjord Series. The ongoing dating of the prograde metamorphism in the Balsfjord series will provide important information about a possible continuity between the timing of peak metamorphism in the Nakkedal + Troms&amp;#248; Nappe Complex, the Balsfjord series and the underlying RNC.&lt;/p&gt;