The Regional Significance of a Rare High-Pressure Kyanite + K-Feldspar Assemblage as Preserved in the Goochland Terrane, Virginia, USA

Nephelinites from Burko volcano (Tanzania) record the phase relations among perovskite, magnetite, titanite and andradite in evolved alkaline and silica-undersaturated systems

The Santa María and Antares Zn-Pb(-Ag) skarn deposits in the Velardeña Mining District are located in central–NW Mexico. They lie 470 m apart along the contact between Oligocene felsic intrusions and Cretaceous limestones, and were developed during prograde, retrograde, post-ore (Santa María), and late stages. Firstly, the prograde stage was formed by fluids at ~ 600 °C and 15 wt% NaCl equiv., and consists of garnet + wollastonite ± clinopyroxene and biotite ± K-feldspar assemblages. Secondly, the retrograde/ore stage was formed by fluids at 300–500 °C with salinities of 20–30 wt% CaCl2 (Santa María) and > 40 wt% NaCl equiv. (Antares). It comprises assemblages of chlorite, amphibole, epidote, calcite, scapolite, quartz, sericite, adularia, fluorite, and muscovite associated with sphalerite, pyrite, galena, pyrrhotite, arsenopyrite, chalcopyrite, and Pb-Bi-Sb sulfosalts. Thirdly, the post-ore stage was formed by fluids at ~ 400 °C and 20–30 wt.% CaCl2 and comprises poorly mineralized calcite veins. Fourthly, the late stage was formed by fluids at < 300 °C and 20–30 wt.% CaCl2 (Santa María) and ~ 15 wt% NaCl equiv. (Antares), and crystallized tetrahedrite-group minerals and pyrite + marcasite. δ18Ofluid between ~ 14‰ and 23‰ at Santa María and between ~ 12‰ and 17‰ at Antares show a less-modified magmatic affinity for mineralizing fluids at Antares; δ13Cfluid between 0‰ and –6‰ register recycling of sedimentary C. Moreover, sulfides with δ34SVCDT between –3‰ and 2‰ reveal a magmatic source for S. Altogether, these data suggest that, at Santa María, magmatic-derived fluids actively interacted with the wall rocks, whereas at Antares the fluid-rock interaction was milder. In both deposits, metal deposition was triggered by the cooling and neutralization of ore-bearing fluids with carbonate rocks. Our 40Ar/39Ar dates for adularia of ca. 37.5 Ma place the deposits within the Eocene–early Miocene metallogenetic epoch of central–NW Mexico, during which other world-class skarn-epithermal systems were emplaced (e.g., Concepción del Oro and Mazapil-Peñasquito).

Metasomatized asthenospheric mantle contributing to the generation of Cu-Mo deposits within an intracontinental setting: A case study of the ∼128 Ma Wangjiazhuang Cu-Mo deposit, eastern North China Craton

This study provides the first evidence for the occurrence of ultrahigh-temperature (UHT) granulite-facies metamorphism in the Yenisei Ridge (Angara–Kan block). UHT metamorphism is documented in Fe-Al-rich metapelites on the basis of the garnet–hypersthene–sillimanite–cordierite–plagioclase–biotite–spinel–quartz–K-feldspar assemblage. Microtextural relationships and compositional data for paragneisses of the Kan complex attest to three distinct metamorphic episodes: (M1) pre-peak prograde (820⎯900°C/5.5–7 kbar), (M2) peak UHT (920–1000°C/7–9 kbar), and (M3) post-peak retrograde (770⎯900°C/5.5–7.5 kbar). The observed counterclockwise P–T evolution at a high geothermal gradient (dT/dP = 100–200°C/kbar) suggests that UHT metamorphic assemblages were formed in an overall extensional tectonic setting accompanied by underplating of mantle-derived mafic magmas, which may be sourced from ~1750 Ma giant radiating dike swarms linked to the Vilyuy mantle plume as part of the Trans-Siberian LIP. The broad synchroneity of UHT metamorphism (1744 ± 26 Ma; monazite–zircon isochron age) and rift-related endogenic activity in the region can provide an additional line of evidence for the two-stage evolution of granulite-facies metamorphism in the Angara–Kan block. The Aldan–Stanovoy, Anabar, and Baikal basement inliers of high-grade metamorphic rocks within the Siberian craton record two Paleoproterozoic peaks (1.9 and 1.75 Ga) of granulite-facies metamorphism. The synchronous sequence of tectonothermal events at the periphery of the large Precambrian Laurentian, Baltica, and Siberian cratons provide convincing evidence for their spatial proximity over a wide time interval, which is consistent with the most recent paleomagnetic reconstructions of the Proterozoic supercontinent Nuna.

Interrogation and 3D visualisation of multiple multi-element data sets collected at the Ranger 1 No. 3 uranium mine, in the Northern Territory of Australia, show a distinct and large-scale chemical zonation around the ore body. A central zone of Mg alteration, dominated by extensive clinochlore alteration, overprints a biotite–muscovite–K-feldspar assemblage which shows increasing loss of Na, Ba and Ca moving towards the ore body. Manipulation of pre-existing geochemical data and integration of new data collected from targeted ‘niche’ samples make it possible to recognise chemical architecture within the system and identify potential fluid conduits. New trace element and rare earth element (REE) data show strong fractionation associated with the zoned alteration around the deposit and with fault planes that intersect and bound the deposit. Within the most altered portion of the system, isocon analysis indicates addition of elements including Mg, S, Cu, Au and Ni and removal of elements including Ca, K, Ba and Na within a zone of damage associated with ore precipitation. In the more distal parts of the system, processes of alteration and replacement associated with the mineralising system can be recognised. REE element data show enrichment in HREE centred about a characteristic peak in Dy in the high-grade ore zone while LREEs are enriched in the outermost portions of the system. The patterns recognised in 3D in zoning of geochemical groups and contoured S, K and Mg abundance and the observed REE patterns suggest a fluid flow regime in which fluids were predominately migrating upwards during ore deposition within the core of the ore system.

The ~3240 Ma Panorama volcanic-hosted massive sulfide (VHMS) district is unusual for its high degree of exposure and low degree of postdepositional modification. In addition to typical seafloor VHMS deposits, this district contains greisen- and vein-hosted Mo-Cu-Zn-Sn mineral occurrences that are contemporaneous with VHMS orebodies and are hosted by the Strelley granite complex, which also drove VHMS circulation. Hence the Panorama district is a natural laboratory to investigate the role of magmatic-hydrothermal fluids in VHMS hydrothermal systems. Regional and proximal high-temperature alteration zones in volcanic rocks underlying the VHMS deposits are dominated by chlorite-quartz ± albite assemblages, with lesser low-temperature sericite-quartz ± K-feldspar assemblages. These assemblages are typical of VHMS hydrothermal systems. In contrast, the alteration assemblages associated with granite-hosted greisens and veins include quartz-topaz-muscovite-fluorite and quartz-muscovite (sericite)-chlorite-ankerite. These vein systems generally do not extend into the overlying volcanic pile. Fluid inclusion and stable isotope studies suggest that the greisens were produced by high-temperature (~590°C), high-salinity (38–56 wt % NaCl equiv) fluids with high densities (>1.3 g/cm3) and high δ 18O (9.3 ± 0.6 ‰ ). These fluids are compatible with the measured characteristics of magmatic fluids evolved from the Strelley granite complex. In contrast, fluids in the volcanic pile (including the VHMS ore-forming fluids) were of lower temperature (90°–270°C), lower salinity (5.0–11.2 wt % NaCl equiv), with lower densities (0.88–1.01 g/cm3) and lower δ 18O (−0.8 ± 2.6 ‰ ). These fluids are compatible with evolved Paleoarchean seawater. Fluids that formed the quartz-chalcopyrite-sphalerite-cassiterite veins, which are present within the granite complex near the contact with the volcanic pile, were intermediate in temperature and isotopic composition between the greisen and volcanic pile fluids (T = 240°–315°C; δ 18O = 4.3 ± 1.5 ‰ ) and are interpreted to indicate mixing between the two end-member fluids. Evidence of mixing between evolved seawater and magmatic-hydrothermal fluid within the granite complex, together with the lack of evidence for a magmatic component in fluids from the volcanic pile, suggest partitioning of magmatic-hydrothermal from evolved seawater hydrothermal systems in the Panorama VHMS system. This separation is interpreted to result from either the swamping of a relatively small magmatic-hydro-thermal system by evolved seawater or density contrasts precluding movement of magmatic-hydrothermal fluids into the volcanic pile. Variability in the salinity of fluids in the volcanic pile, combined with evidence for mixing of low- and high-salinity fluids in the massive sulfide lens, is interpreted to indicate that phase separation occurred within the Panorama hydrothermal system. Although we consider this phase separation to have most likely occurred at depth within the system, as has been documented in modern VHMS systems, the data do not allow the location of the inferred phase separation to be determined.

In-situ X-ray absorption study of Iron(II) speciation in brines up to supercritical conditions

Aluminous sapphirine granulites from the Eastern Ghats Belt (India): Phase relations and relevance to counterclockwise P-T history

The Nelson Batholith is a ca. 1,800 km 2 Jurassic intrusive body in southeastern British Columbia surrounded by a contact aureole, 0.7–1.8 km wide, developed in graphitic argillaceous rocks that show only minor variations in bulk composition. Contrasting prograde sequences of mineral assemblages are developed around the aureole in a regular pattern, reflecting different pressures of contact metamorphism. The following assemblages are seen going from lower to higher pressure (all assemblages contain muscovite + biotite + quartz ± Mn-rich garnet): (1) cordierite-only assemblages, (2) mix of cordierite-only and cordierite + andalusite assemblages, locally with cordierite + K-feldspar and andalusite + K-feldspar assemblages at higher grade, (3) andalusite-only assemblages, with sillimanite + andalusite assemblages and locally sillimanite + K-feldspar assemblages at higher grade, (4) staurolite-only assemblages, (5) staurolite ± andalusite assemblages, with sillimanite-bearing and locally sillimanite + K-feldspar assemblages at higher grade. The higher-pressure sequences with staurolite ± andalusite are restricted to the aureole surrounding the east half of the batholith, whereas the lower-pressure cordierite ± andalusite are restricted to the aureole surrounding the west half of the batholith and its northern and southern tips. The sequences of mineral assemblages correspond closely to the facies series of Pattison & Tracy (1991) and are interpreted to represent a series of approximately isobaric metamorphic field-gradients below the Al2SiO5 triple point, providing an excellent opportunity to evaluate thermodynamically calculated low-pressure phase equilibria in the metapelitic system. The total difference in pressure represented by the contrasting assemblages is about 1.0 kbar, showing that they are a sensitive measure of small differences in pressure within the stability field of andalusite. Thermobarometry results from the aureole are moderately consistent with the mineral assemblage constraints, but carry pressure uncertainties larger than the total range of pressure represented by the aureole's assemblages. Pressures of the intrusive rocks derived from hornblende barometry are scattered, and many do not agree with the pressure constraints from the aureole. The mineral-assemblage constraints indicate down-to-the-west post-metamorphic tilting of the batholith and aureole of about 10°, interpreted to be due to a combination of eastward thrusting of the Nelson Batholith over crustal-scale ramps during Cretaceous–Paleocene shortening and Eocene east-side-down normal motion on the Slocan Lake – Champion Lakes fault system that forms the western boundary of the batholith.

The Bajo de la Alumbrera porphyry Cu-Au deposit, Argentina, is in the eastern Andes, near the north edge of a region of reverse fault-bound basement uplifts that overlie a low-angle segment of the subduction zone. Alumbrera, now above the transition from steep to flat subduction, formed at ~7 Ma in the Farallon Negro volcanic field, which was active as volcanism was waning regionally above the flattening subduction zone. Reconstruction of volcanic structure suggests that the top of the exposed orebody was emplaced beneath about 2.5 km of andesite and dacite but not directly beneath the vent of a stratovolcano. Production plus remaining resources are 605 million metric (Mt) tons of ore that averages 0.54 percent Cu and 0.64 g/t Au. The deposit is centered on a closely spaced cluster of small felsic porphyry stocks and dikes, emplaced into andesites during seven phases of intrusion. Dikes of several phases define a radial pattern. Most of the porphyries are very similar to one another, with phenocrysts of plagioclase, hornblende, biotite, and quartz, in a matrix of fine-grained quartz, K-feldspar, and minor plagioclase, biotite, and magnetite. Individual porphyries are distinguished mainly on the basis of intrusive contact relationships. Highest Cu-Au grades are associated with abundant quartz veins, secondary K-feldspar, ±magnetite, ±biotite, ±anhydrite, in the earliest porphyry (P2), and adjacent andesite. P2-related mineralization is truncated by porphyries of the second phase of ore-related intrusions (Early P3 and Quartz-eye porphyry), which contain similar but generally less intense mineralization and alteration. Porphyries of the next phase (Late P3) truncate mineralization associated with earlier phases and are weakly mineralized with Cu-Au, sparse quartz veins, and secondary biotite. The still later Northwest porphyries truncate most Cu-Au, quartz veins, and potassic alteration, and themselves contain only traces of such mineralization and partially biotitized hornblende. Postmineral porphyries, the youngest, truncate all such mineralization and alteration, and none of their hornblende is biotitized. Los Amarillos porphyry and igneous breccia, along the western periphery of the porphyry cluster, is between P2 and Early P3 in age but shows little relationship to mineralization. Zones of secondary K-feldspar associated with the earlier porphyries are surrounded by a larger zone of secondary biotite. All significant Cu-Au lies within these potassic zones. The biotite zone is surrounded by epidote-chlorite alteration lacking significant sulfides. Like potassic alteration, epidote-chlorite alteration is also truncated by Postmineral porphyries. Strong feldspar destructive alteration, consisting mostly of veinlet-controlled sericite-quartz-pyrite, is younger than all secondary K-feldspar, biotite, and epidote-chlorite and occurs in a shell in the outer parts of the biotite zone. Weaker feldspar destructive alteration occurs inside and outside this shell. Pyrite veins with sericite-quartz-pyrite alteration cut Postmineral porphyries. In the earliest secondary K-feldspar assemblage, which is usually barren of Cu sulfides, biotite is altered to magnetite plus K-feldspar. Most Cu sulfides are associated with slightly later K-feldspar-biotite ± magnetite assemblages. Where feldspars and biotite are not overprinted by later feldspar destructive or chloritic alteration, Cu minerals are bornite and chalcopyrite, coexisting with magnetite. Barren as well as Cu sulfide-bearing assemblages are associated with early veinlets, including A-type quartz, which are truncated by the next later porphyry. Deposition of Cu-Au during or between emplacement of closely related porphyries suggests high temperatures and magmatic fluids, and the assemblage bornite-chalcopyrite-magnetite indicates a relatively low sulfidation state, and along with the assemblage K-feldspar-biotite ± magnetite ± anhydrite a relatively high oxidation state. Cu-Au distribution is not related to feldspar destructive zones nor to the interface between sericitic and potassic zones. Much Cu-Au mineralization, however, has been overprinted by late alteration, resulting in partial destruction of feldspars, chloritization of mafics, and sulfidation of bornite-chalcopyrite-magnetite to chalcopyrite-pyrite ± relict magnetite. This probably took place at significantly lower temperature. A low-grade core zone consists in large part of barren K-feldspar-magnetite alteration and quartz veins in Early P3 porphyry, and in part consists of later barren porphyry, so is mostly younger than the Cu-Au deposited with P2 porphyry. The youngest features at Alumbrera include small postore normal faults, gypsum veins due to hydration and mobilization of anhydrite, local dissolution of gypsum veins, and locally developed thin zones of near-surface oxidation, leaching, and secondary enrichment.

Abstract Occurrence of fine-grained, fingerprint-like symplectic intergrowths of cordierite, K-feldspar and quartz formed due to the retrogressive, garnet breakdown reactions following decompression during uplift, is reported here from metapelites of Usilampatti area. These metapelites consisting of cordierite, garnet, hypersthene, biotite, hercynitic spinel, sillimanite, K-feldspar assemblage were metamorphosed at 6.5-7.5 Kb and 750-800°C. The symplectites were formed at 600-670°C and 5 Kb pressure. The textural and mineralogical evolutions in these metapelites indicate a rapid decompressional P-T evolution. Petrological and geochemica1 data suggest that these metasediments were derived mostly from an evolved basement source of granodiorite-granite composition with a minor basic component.

Abstract The Settupalle igneous complex is characterised by syenitic variants with distinct mineralogic assemblages. The variable perthitic patterns, nature of plagioclase, fayalite-quartz assemblage in Settupalle syenites clearly suggest that the Settupalle igneous complex has evolved under fluctuating pressure and temperature conditions. The mineral chemistry of K-feldspar and plagioclase suggests that K-feldspar assemblage in these rocks indicates their subsolvus nature. The probable crystallization sequence of rocks is fayalite clinopyroxene syenite (FC-Syenite) - fayalite quartz syenite (FQ-Syenite)-hornblende syenite - quartz syenite - nepheline syenite in that order.