Potassic Igneous Rocks Research Articles

Late Palaeozoic potassic igneous rocks of the Molo‐Sarychat pluton in the eastern Kyrgyz Tien Shan: Geochemistry, U–Pb zircon geochronology and implications for related skarn‐porphyry Mo‐W‐Cu‐Au mineralization

Late Palaeozoic potassic igneous rocks of the Molo‐Sarychat pluton in the eastern Kyrgyz Tien Shan: Geochemistry, U–Pb zircon geochronology and implications for related skarn‐porphyry Mo‐W‐Cu‐Au mineralization

Late Paleozoic potassic igneous rocks of the Adyrtor intrusions in the eastern Kyrgyz Tien Shan: geochemistry, isotope U-Pb zircon geochronology, and related W-Mo(-Cu-Au) mineralization

ABSTRACT The Adyrtor multiphase pluton is situated in the easternmost part of the ‘Main Structural Line of Tien Shan’, or the ‘Nikolaev Line’, and intrudes the Paleoproterozoic metamorphic sequence representing the oldest (1.8–2.5 Ga) continental basement fragment in the Tien Shan orogenic belt. The intrusions comprise mafic to intermediate (monzodiorite, monzonite, quartz syenite) and silicic (quartz monzonite, monzogranite, and leucogranite-alaskite) rocks. Geochemical signatures of the igneous rocks correspond to high-K calc-alkaline to shoshonitic series intrusions, with a strong A-type granite affinity, emplaced in post-collisional setting. Based on the isotopic U-Pb data, quartz syenite from the Adyrtor intrusions crystallized at 330.7 ± 4.3 Ma, and quartz monzonite – at 329.5 ± 5.8 Ma. Although the ages identified correspond to those of the subduction-related magmatism in the western part of the Middle Tien Shan, geochemical characteristics of the rocks support rather post-collisional setting of the plutons, principally due to a ‘scissor-like’ closure of the Turkestan paleoocean, starting from the east. Magmatic evolution included a generation of shoshonitic magma by low-degree partial melting of the metasomatically-enriched upper mantle, followed by amphibole fractionation in a deep (lower crustal ?) magma chamber. Then, under the influence of mantle-supplied fluids and heat, granitic magmas in the crustal protolith could have been generated, with further mixing/mingling of the mantle-derived mafic (shoshonitic) magma and mantle-induced crustal granitic magma, followed by the magma fractionation and emplacement at shallower crustal levels. Assimilation of an old continental crust is also envisioned, consistent with the presence of the xenocrystic zircons dated at ca. 1.6 Ga to 2.5 Ga and the A-type granitoid affinity of the igneous rocks. The associated skarn-porphyry W-Mo(-Cu-Au) mineralization is consistent with post-collisional setting of the productive intrusions. This mineralization complements the group of similar deposits and occurrences associated with high-K calc-alkaline to shoshonitic series intrusions in the Middle Tien Shan and globally. The high endowment in W and Mo can be explained by the fertilization of subduction-modified subcontinental lithospheric mantle in these metals, with its further involvement in the magma generation in post-collisional setting. Also, a stronger W-Mo mineralization can reflect a greater role of a crustal protolith in the magma generation, as revealed by the distinct A-type affinity of the igneous rocks.

Central Java regional geochemistry: influence of environmental geology and mineral occurrences

Stream sediments geochemical surveys were undertaken in Central Java, Indonesia as a part of the Geological Agency’s geochemical mapping project with support from several universities. A total of 1583 stream sediment samples were collected from Central Java and analyzed using X-Ray Fluoresces for 29 elements. Geochemical maps were processed by the GIS software. Based on the overlay between geological and geochemical maps, there are several elemental anomalies, for example, the ultramafic formation from Kebumen showing distinctive Chromium (Cr), Cobalt (Co), and Magnesium (Mg) anomalies. Meanwhile, the potassic igneous rocks from Muria Mountain have Cerium (Ce), Lanthanum (La), and Zirconium (Zr) anomalies. Elemental anomalies related to mineral resources and mining areas such as Arsenic (As), Copper (Cu), and Lead (Pb) are found in Trenggalek, Pacitan, and Kebumen; while Barium (Ba), Iron (Fe), and Manganese (Mn) are scattered along the southern coast of Central Java. Anthropogenic elements (Chlorine (Cl), Ce, and As) spread along the north coast of Central Java probably because of pollution by industrial, farming, and urban areas. Heavy metals pollutions that were identified previously in the Bengawan Solo River flow can be seen on this geochemical map which can convincingly delineate elements anomalies due to geological and environmental conditions.

Eocene–Oligocene Crustal Thickening-Collapse of the Eastern Tibetan Plateau: Evidence from the Potassic Granitoids in SW China

Abstract The eastern Tibetan Plateau is a key part of the eastern India–Asia collisional zone, a region that records multiple overprinting tectonic and magmatic events. This study presents new geochronological, geochemical, and Sr–Nd–Hf–O isotopic data for Cenozoic potassic granitoids in eastern Tibet, southwestern China, which recorded the tectonic evolution of the eastern Tibetan Plateau. These potassic granitoids are formed between 37.6 and 32.9 Ma and are geochemically subdivided into the following: Group 1, adakite-like granites; Group 2, syenites; and Group 3, low-εNdt granitoids. The Group 1 samples are similar to high-silica adakites in that they have variable SiO2 contents (63.31–73.62 wt.%) and high Sr/Y and La/Yb ratios. These samples have εNdt and εHft values that range from −5.8 to −0.6 and from −4.3 to +5.2, respectively, with δ18O values of 6.78‰–7.36‰. The Group 2 samples are syenites, contain 56.36–63.86 wt.% SiO2 and high concentrations of Y and Yb, and have εNdt values from −8.4 to −2.4, εHft values from −6.1 to +1.1, and δ18O values of 6.37‰–6.89‰. The Group 3 samples have a narrow range of SiO2 concentrations (62.27–64.59 wt.%), high Sr/Y and La/Yb ratios, δ18O values of 6.31‰–6.82‰, and low εNdt and εHft values (−12.6 to −10.9 and −11.4 to −6.6, respectively) that are similar to the values obtained for the contemporaneous Yao’an lamprophyres. These data indicate that the Group 1 samples are formed from magmas sourced from a heterogeneous and thickened region of the lower crust containing an enriched mantle component. Group 2 magmas were most likely derived from contemporaneous mafic melts sourced from an ancient region of the lithospheric mantle previously modified by the incorporation of recycled components. The Group 3 samples have distinct Sr–Nd–Hf isotopic compositions that are indicative of derivation from magmas generated by the fractional crystallization of lower crustal melts sourced from ancient enriched mantle of the Yangtze Block. Combining these new data with the results of previous research suggests that the Cenozoic potassic igneous rocks of eastern Tibet were formed as a result of the thinning of the lithospheric mantle and an associated crustal collapse event, potentially representing a regional late Eocene to early Oligocene transition from compression to transtension in the eastern Tibetan Plateau. These potassic igneous rocks are contemporaneous with or are younger than igneous rocks in the Qiangtang Block, suggesting that the magmatic response to the India–Asia collisional event was initiated in the central Tibetan Plateau before propagating towards the eastern margin of this region.

Porphyry Cu–Au ± Mo mineralization hosted by potassic igneous rocks: implications from the giant Peschanka porphyry deposit, Baimka Trend (North East Siberia, Russia)

Abstract Porphyry Cu–Au ± Mo mineralization at Peschanka is hosted by monzodiorite and monzonite intrusions with high-K calc-alkaline to shoshonitic compositions and dated at about 144.1 ± 1.5 Ma, using U/Pb zircon ages. The Cretaceous intrusions are emplaced in a melange of Cretaceous island arcs, a tectonic setting comparable with other world-class porphyry Cu–Au deposits, such as Oyu Tolgoi, Mongolia and Pebble, Alaska. Abundant primary magnetite contents of the Peschanka intrusions, as well as numerous gypsum and anhydrite veins, reflect the high oxidation states of their parental magmas. This mineralogical interpretation is confirmed by high whole-rock Fe 2 O 3 /FeO ratios and high V/Sc ratios of the rocks of up to 1.27 and up to 21.9, respectively. The whole-rock Eu/Eu* ratios of the Peschanka intrusions are ≥1 which is also typical for potassic igneous rocks with high oxidation states. Abundant amphibole and biotite phenocrysts of the intrusions as well as their high whole-rock Sr/Y ratios of up to 225 document significantly high H 2 O contents of the high-K magmas. Peschanka contains a resource of >9.5 Mt of copper at an average grade of 0.43 wt% and 16.5 Moz of gold at a high average grade of 0.23 g/t and thus represents one of the largest undeveloped greenfield copper projects worldwide. The vicinity of Peschanka still offers significant brownfield exploration potential. The hypogene vein-related and disseminated Cu–Au ± Mo sulfide mineralization at Peschanka is structurally controlled by significant NE-trending strike-slips that acted as the conduits for the hydrothermal fluids. The central part of the orebody consists of high-grade north–south-trending sheeted quartz–bornite veining with unusually high vein densities. The highest Cu and Au grades are directly correlated with high vein densities. Peschanka is defined by distinct hydrothermal alteration zones including potassic, phyllic, propylitic and argillic assemblages, but a distinct lack of advanced argillic alteration. The mineralization itself is also zoned ranging from a central Mo–Cpy–Bn sulfide assemblage to a peripheral Py–Mt-dominated zone (‘pyrite-shell’). Late-stage polymetallic assemblages overprint and surround the main stockwork zone.

Alkali-Hydrothermal Treatment of K-Rich Igneous Rocks for Their Direct Use as Potassic Fertilizers

Due to the increasing demand for conventional sources of potassium (K) and their inaccessibility by African countries, K-rich igneous rocks are increasingly studied as potential alternative sources. In this study, six potassic igneous rocks (syenites and trachytes) from the Tamazeght, Jbel Boho, Ait Saoun, and El Glo’a regions (Morocco) were sampled and characterized. Then they were hydrothermally treated to enhance their K release for potential use as potassic fertilizers. The raw materials are mainly formed by microcline (up to 74%), orthoclase (20–68%), albite (36–57%), biotite-muscovite (15–23%), and titanite, calcite, hematite, and apatite as accessory minerals. These samples were crushed and milled to reach a particle size <150 µm and mixed with 4 N NaOH solution in an autoclave. The liquid/solid (L/S) ratio was about 44 mL/50 g. The powders were allowed to react with the solution at 170 °C for 7 h. For all tests, NaOH reacted completely with the powders and no liquid was observed after the treatment. X-ray diffraction (XRD), thermal gravimetric analysis (TGA), infrared spectroscopy (IRTF), and scanning electron microscopy (SEM-EDS) were carried out on treated samples to characterize the mineralogical and structural changes due to the alkali-hydrothermal treatment. Indeed, the treated samples revealed the presence of sodic neoformed phases such as thermonatrite, sodalite, analcime, and cancrinite. The treated material was leached for a week using deionized water and the elements released were measured using inductively coupled plasma–atomic emission spectroscopy (ICP-AES). The hydrothermal process showed a strong effect on structure breakdown as well as on the release of K and other nutrients such as P, Fe, Si, Mg, and Ca. Therefore, the alkali-hydrothermal treatment allowed the release of 50.5 wt% K. Moreover, the release of Mg, Ca, Fe, P, K, and Si were significantly increased. Mg, Ca, Fe, P, K, and Si release within raw materials was about (0.5–3.6), (3.5–31.4), (0.01–0.4), (0.01–0.3), (20–55), and (4.6–8) mg/kg, respectively, whereas treated samples showed a higher release of these elements. Quantitatively, Mg, Ca, Fe, P, K, and Si releases were about (10–11.8), (60–70), (7–20), (1.2–15), (218–1278), and (1119–2759) mg/kg, respectively. Consequently, the treated igneous rocks (syenite and trachyte) could be directly used as potassic fertilizers that would also be a source of other nutrients.

Petrogenesis and tectonic implications of Early Cretaceous shoshonitic syenites in the northern Wuyi Mt Range, Southeast China

Potassic igneous rocks, such as shoshonitic rocks, are an important constituent of young volcanic-intrusive suites in varied tectonic settings. The genesis of these rocks is related to tectonic environments and is thus important for tectonic reconstruction of ancient terranes. Here we report zircon U–Pb and Hf isotopes and whole-rock geochemical compositions for shoshonitic quartz syenites from Madi and Chashan in the northern Wuyi Mt Range, southeast China. LA-ICP-MS zircon U-Pb dating shows that both the Madi and Chashan quartz syenites were formed at 134.8 Ma and 135.0 Ma, respectively. They also have similar geochemical compositions and are characterized by high K2O contents and enriched in LILEs and LREEs but depleted in HFSEs, suggesting a shoshonitic affinity. Elemental and isotopic compositions indicate that they were likely formed by partial melting of metasomatised lithospheric mantle with the presence of phlogopite and potassic amphibole. It is also suggested that silicate melts derived from subducted sediments played a dominant role in mantle metasomatism. An integration of our new results with previous ones allows us to propose a modified tectonic model about the evolution of SE China in which a continental arc related to the subduction of the Palaeo-Pacific plate during the Middle Jurassic was gradually changed to southeastward-migrating, extension-related magmatic arc and back-arc basins formed in a continental arc-rifting setting as a consequence of slab rollback since the Early Cretaceous.

D. Müller and D.I. Groves (2019) Potassic igneous rocks and associated gold-copper mineralization, fifth edition. Mineral resource reviews, Springer, 398 p

D. Müller and D.I. Groves (2019) Potassic igneous rocks and associated gold-copper mineralization, fifth edition. Mineral resource reviews, Springer, 398 p

Potassic Igneous Rocks and Associated Gold-Copper Mineralization, Fourth Edition (D. Müller and D.I. Groves)

Potassic Igneous Rocks and Associated Gold-Copper Mineralization, Fourth Edition. Daniel Muller, David I. Groves. 2016. Springer International Publishing, Pp. 311. ISBN 978-3-319-23051-1. Price: ebook, $139; hardcover, $179. The fourth edition of this comprehensive textbook, which succeeds those published in 1995, 1997, and 2000, very nicely summarizes the geochemical and petrological characteristics of potassic igneous rock complexes and the different tectonic settings in which they occur. The authors provide an overview and a classification of these rocks and they outline the geochemical differences between barren and mineralized potassic igneous complexes. Owing to the common association of potassic igneous rocks with many gold- and copper-rich ore deposits, this book will be of interest not only to research scientists but also to those exploring for major deposits in young and ancient terranes. In fact, there was a clear attempt by the authors to provide a good mix of theoretical discussions based on experimental work, with case studies that illustrate field and applied research. This newly updated and enlarged edition was clearly an opportunity to improve the “look” because compared to the earlier black-and-white version, it’s bigger (8- × 10-inch pages). In addition, it has a two-column layout and contains numerous new color photographs of gold and copper ores from major deposits hosted by potassic igneous rocks. Whereas the previous versions included all references compiled in one section at the end of the …

Shonkinites from Salem, southern India: Implications for Cryogenian alkaline magmatism in rift-related setting

Alkaline and potassic igneous rocks, although minor components of the continental crust, provide important constraints on magma processes and tectonic settings. Here we report petrology, mineral chemistry, whole rock geochemistry, zircon U–Pb and Lu–Hf data on shonkinites from the Salem Block in the Southern Granulite Terrane of India. The shonkinite is composed mainly of porphyritic clinopyroxene, K-feldspar and olivine, and intrudes into the surrounding ultramafic rocks composed of wehrlite (clinopyroxene and olivine) and dunite (mostly olivine with minor spinel). The geochemical data on shonkinites from Salem show alkaline and ultrapotassic features with marked enrichment in incompatible elements, and high K2O and K2O/Na2O ratios suggesting magma derivation from metasomatized lithospheric mantle. Their HFSE depletion relative to LILE compositions, coupled with positive Nb anomalies suggests continental rift setting. The zircon grains in shonkinite show typical magmatic crystallization features and their LA-ICP-MS U–Pb data yield weighted mean age of 818±6.3Ma (MSWD=0.32). The zircon εHf values range between −11.1 and −12.7, consistent with magma derivation from an enriched mantle, with depleted mantle model ages (TDM) in the range of between 2406 and 2513Ma, indicating the involvement of Neoarchean–Paleoproterozoic subduction-related components. Together with similar-aged alkaline plutons with comparable geochemical features, we envisage a major alkaline magmatic event associated with rifting during Cryogenian in the northern part of the Southern Granulite Terrane.

Early Precambrian high-grade metamorphosed terrigenous rocks of granulite-gneiss terranes of the Sharyzhalgai uplift (southwestern Siberian craton)

We present results of geochemical and Sm-Nd isotope studies of high-grade metaterrigenous rocks of the Kitoi and northwestern Irkut terranes of the Sharyzhalgai uplift on the Siberian Platform in comparison with paragneisses of the southeastern Irkut terrane. The metasedimentary rocks of the first region are high-alumina garnet-sillimanite-cordierite-bearing paragneisses; their protoliths were mostly mudstones and pelitic mudstones by major-element composition. The low-alumina biotite gneisses of the Kitoi terrane formed, most likely, from magmatic protoliths similar in petrochemical features to intraplate volcanics. The major factor controlling the composition of the studied high-alumina paragneisses is precipitation of most of incompatible trace elements in the clay fraction of sediments, as evidenced from the positive correlation between trace-element and Al2O3 contents. The Cr and Ni contents, showing a positive correlation with MgO and no correlation with Al2O3, are an indicator of the contribution of the mafic-source material to the formation of high-alumina rocks. The contribution of a mafic source-derived material to the formation of terrigenous rocks increases in passing from Kitoi to northwestern Irkut terrane. The high-alumina and garnet-biotite paragneisses of the southeastern Irkut terrane are similar in trace-element patterns to the analogous rocks of the Kitoi terrane and northwestern part of the Irkut terrane but show higher Th contents and a distinct negative Eu anomaly related to the change in the composition of the felsic source. The participation of felsic potassic igneous rocks in the formation of the southeastern terrigenous sediments is consistent with their deposition after the Neoarchean collision processes (metamorphism and granite magmatism), whereas sedimentation in the Kitoi and northwestern Irkut terranes preceded them. The Sm-Nd isotope characteristics indicate that the latter sediments formed mostly as a result of the erosion of the Paleo-Mesoarchean crust, whereas the metasediments of the southeastern Irkut terrane formed with the participation of Paleoproterozoic juvenile rocks. Thus, the variations in the trace-element and isotope compositions of the high-grade metamorphosed terrigenous rocks reflect recycling and growth of the continental crust of the Sharyzhalgai uplift during the Neoarchean-Pa- leoproterozoic transition.

CHEMICAL SUBSTITUTIONS IN OXIDIZED TOURMALINE IN GRANITE-RELATED MINERALIZED HYDROTHERMAL SYSTEMS, WESTERN TURKEY

Tourmaline is the most common borosilicate mineral in granite-related magmatic-hydrothermal ore deposits in the Aegean region of western Turkey. Hydrothermal deposits there include the mesothermal argentiferous Pb–Zn–Cu deposit at Kadikalesi (Bodrum–Mugla) and the porphyry Au deposit at Kisladag (Esme–Usak), with related potassic igneous rocks and accompanying Tertiary strata of western Anatolia. At Kadikalesi, mesothermal vein-type mineralization is linked to monzodioritic intrusions in a subvolcanic suite, which is overlain by volcanic rocks ranging in composition from calc-alkaline to potassic. In the non-brecciated hydrothermal systems, tourmaline occurs as subhedral to anhedral crystals in monzodiorite or as an essential mineral in veins and their reaction zones. At Kisladag, magmatic-hydrothermal brecciation associated with multiphase intrusions of latite porphyry is related to the Kisladag caldera. Tourmaline is an abundant and widespread alteration-induced mineral throughout the Kisladag porphyry system. Electron-microprobe analyses of the tourmaline crystals collected from both hydrothermal deposits show significant chemical variation within the range of buergerite compositions. The main substitution mechanism involves the CaFe 2+ [Na −1 (Al,Fe 3+ ) −1 ] exchange vector. Compositions deviate from ideal schorl–dravite along a trend that closely approximates the uvite exchange-vector. Mossbauer spectroscopy shows the presence of significant Fe 3+ in all the tourmaline crystals, suggesting a high oxidation state for the ore-forming fluids, and for related hydrothermal systems associated with emplacement of subvolcanic intrusions.

Mineral chemistry of barium- and titanium-bearing biotites in calc-alkaline volcanic rocks from the Mezitler area (Balikesir-Dursunbey), western Turkey.

Barium- and titanium-bearing biotites from Miocene volcanic rocks of Mezitler area, eastern Balikesir, western Turkey are studied. The chemical composition of volcanic rocks range from andesite to rhyodacite. The iron-enrichment index of micas (average I.E. = 0.40) is intermediate between annite and phlogopite. The biotite phenocrysts contain up to 1.72 wt.% BaO and 5.90 wt.% TiO2, with the average formulae (K0.807Na0.131Ca0.036Ba0.027) (Mg1.404Fe2+0.800 Fe3+0.131 Ti0.303Al0.056Mn0.023) (Si2.832Al1.167)O10[(OH)1.976Cl0.024]. The BaO content of electron-microprobe micas is positively correlated with the Al2O3, TiO2, and FeO contents, and with the I.E., and is negatively correlated with the SiO2, K2O, and MgO contents. Ba- and Ti-rich micas are generally found in potassic igneous rocks, in subalkaline and alkaline gabbroic rocks and in contact metamorphic rocks, whereas Ba- and Ti-bearing micas in this study occur in calc-alkaline volcanic rocks that hosted manganese-oxide and barite deposits. Most of the phenocrysts analyzed have deficiencies in their octahedral and partly interlayer sites. Deficiencies in the octahedral sites may arise from the Ti-vacancy and partly the Ti-tschermakite substitution. On the other hand, deficiencies in the interlayer-site are due to the replacement of K by Ba. The substitution mechanism in the Mezitler micas is characterized by Ba + 2Ti + 3Al = (K + Na + Ca) + 3(Mg + Fe + Mn) + 3Si, with an excellent correlation coefficient. In terms of aluminum and titanium contents, micas from the Mezitler area lie on a similar trend parallel to that for metasomatic phlogopites from Canary Island xenoliths, which overlap the field for micas from the Ilha da Trindade xenolith, South Atlantic. Biotite compositions from the Mezitler area fall between the quartz-fayalite-magnetite (QFM) and nickel-nickel oxide (NNO) oxygen fugacity buffers. All these show that Mezitler micas with low to moderate Ba- and Ti-contents may be formed from magmas in a subduction-enriched sub-continental lithospheric mantle environment.

Potassic Igneous Rocks and associated Gold–Copper Mineralization (3rd Ed.)

Daniel Muller & David I. Groves , Springer Verlag 2000; xiii+252 pp. ISBN 3-540-66371-1. As a research student working on lamprophyres with Nick Rock, I well remember the discussion initiated by Nick and David Groves that lamprophyres and similar potassic rocks are intimately associated with gold mineralization. A growing body of evidence now supports the idea of a close link between potassic magmatism and Au mineralization. The book comprises ten chapters , the first two of which take the reader through the history, nomenclature and classification of potassic igneous rocks (PIR’s), the latter being particularly useful in sorting out the previous plethora of terms. They …

Potassic igneous rocks from the vicinity of epithermal gold mineralization, Lihir Island, Papua New Guinea

Many world-class porphyry copper–gold and epithermal gold deposits worldwide are hosted by volatile-rich and oxidized alkaline rocks. This study investigates potassic igneous rocks from the vicinity of epithermal gold mineralization at Lihir Island, Papua New Guinea. The island consists of five Pliocene–Pleistocene stratovolcanoes, one of which hosts Ladolam, one of the largest epithermal gold deposits discovered to date. Petrographically, the rocks range from porphyritic trachybasalts, trachyandesites and latites to rare phonolites and olivine–clinopyroxene cumulates. In some places, these rocks are cut by monzodiorite stocks. According to Al-in-hornblende barometry, the main crystallization of these rocks occurred close to the surface. Titanium-in-hornblende thermometry as well as olivine–spinel geothermometry and oxygen barometry indicate temperatures of 787–965°C at elevated oxygen fugacities ( f O 2 ) of 1.4–4.8 log units above that of the FMQ buffer. Although previous studies have suggested high f O 2 of alkaline rocks associated with copper–gold mineralization based on abundant primary magnetite contents, this is the first direct determination of the f O 2 of such rocks. High f O 2 of parental melts commonly delays the early crystallization of magmatic sulphides; this is important because metals such as Au and Cu preferentially partition into sulphide phases resulting in their depletion in the melt during increasing fractionation. Geochemically, the rocks range from primitive to relatively evolved compositions, as reflected by their SiO 2 (45.8–55.0 wt.%) and MgO (1.4–15.3 wt.%) contents and variable concentrations of mantle-compatible elements (130–328 ppm V, 1–186 ppm Ni). Their high K 2O content (up to 4.7 wt.%), high average K 2O/Na 2O ratios (0.8) and high average Ce/Yb ratios (14) are typical of high-K igneous rocks transitional to shoshonites. Although these rocks formed by decompression melting related to back-arc rifting in the Manus Basin, the high LILE, low LREE and very low HFSE concentrations are typical of potassic igneous rocks from oceanic (island) arc settings. The reason for this remarkable composition is the partial melting of subduction-modified lithospheric mantle, which developed in a stalled subduction zone. Mica phenocrysts in the rocks reveal unusually high halogen concentrations. Magmatic phlogopites contain high F (up to 5.6 wt.%) and elevated Cl contents (<0.08 wt.%). Hydrothermal biotites from rocks that display potassic alteration have low F (<0.08 wt.%), but very high Cl concentrations (up to 0.15 wt.%). It is suggested that chloride complexing largely controlled the abundances of Au and Cu in the aqueous fluids responsible for the hydrothermal gold mineralization at Ladolam.

MÜLLER, D. &amp; GROVES, D. I. 2000. Potassic Igneous Rocks and Associated Gold–Copper Mineralization, 3rd ed. xiii+252 pp. Berlin, Heidelberg, New York, London, Paris, Tokyo, Hong Kong: Springer-Verlag. Price DM 129.00, Ös 942.00, SFr 117.50, £44.50, US $79.95 (hard covers). ISBN 3 540 66371 1.

MÜLLER, D. &amp; GROVES, D. I. 2000. Potassic Igneous Rocks and Associated Gold–Copper Mineralization, 3rd ed. xiii+252 pp. Berlin, Heidelberg, New York, London, Paris, Tokyo, Hong Kong: Springer-Verlag. Price DM 129.00, Ös 942.00, SFr 117.50, £44.50, US $79.95 (hard covers). ISBN 3 540 66371 1. - Volume 138 Issue 2

The shoshonite porphyry Cu-Au association at Bajo de la Alumbrera, Catamarca Province, Argentina: a reply

Bajo de la Alumbrera forms an example of a series of world class copper-gold deposits that are hosted by potassic igneous rocks. The Bajo de la Alumbrera porphyry copper-gold deposit is situated in the Catamarca Province of Northwest-Argentina and belongs to the ten largest copper mines in the world; it is also one of the largest gold producers in South America.

Potassic and Sodic Igneous Rocks from Eastern Paraguay: their Origin from the Lithospheric Mantle and Genetic Relationships with the Associated Parana Flood Tholeiites

Eastern Paraguay represents the westernmost fringe of Early Cretaceous Parana flood tholeiites (Serra Geral Formation SGF). Besides the SGF, eastern Paraguay has been the site of alkaline magmatism since Mesozoic times: (1) Late Permian-Early Triassic sodic intrusions and lavas: (2) Early Cretaceous potassic igneous rocks and very scarce sodic lavas (3) Late Cretaceous–Oligocene sodic lavas. Two distinct magmatic events are dominant in the Asuncion–Sapucai graben (ASU) eastern Paraguay: (1) wide-spread potassic magmatism and SGF tholeiites (Early Cretaceous) (2) Asuncion sodic magmatism (Late Cretaceous–Oligocene). The potassic rocks form a compositional continuum from moderately to strongly potassic. Two potassic suites are proposed, i.e. basanite to phonolite and alkali basalt to trachyte and their intrusive analogues. The sodic rocks include ankaratites, nephelinites and phonolites. Two similar but distinct parental magmas have been inferred for the potassic suites, both characterized by strongly fractionated REE and negative ‘Ta–Nb–Ti anomalies’. Slight positive Ta and Nb anomalies distinguish the sodic rocks. Sr–Nd isotope data confirm the distinction of the potassic rocks, enriched in radiogenic Sr and low in radiogenic Nd, from the sodic rocks, close to bulk Earth. Crustal contamination does not appear to have been significant in the generation of the investigated rocks, supported by δ 18O data. The source of potassic rocks is constrained by high LILE, LREE, Th, U and K, relative to a primitive mantle composition, and a garnet peridotite is favoured as a possible mantle source for the investigated rocks. The close association of potassic and sodic rock suites in the ASU demands that their parental magmas derived from a heterogeneous subcontinental mantle, variously enriched in incompatible elements. Significant H2O, CO2 and F are expected in the mantle source(s) for the occurrence of coeval carbonatites. Any genetic hypothesis based on a ‘mantle plume’ system is constrained by strong lithospheric mantle characteristics. This does not preclude that thermal perturbations from the asthenosphere may have trggered magmatic activity in the lithospheric mantle. Model ages indicate that two distinct metasomatic events may have occurred during Late and Early–Middle Proterozoic as precursor to the genesis of tholeiitic and alkaline magmatism in the Parana basin. These metasomatic processes were chemically distinct, indicated by the strong differences in Ti, LILE and HFSE concentrations found in both alkaline provinces (e.g. potassic rocks from ASU vs Alto Paranaiba Igneous Province) and Parana tholeiites (low vs high Ti). Ingeneral, the relationships between the alkaline rocksfrom southeastern Brazil, i.e. AltoParanaiba, Ponta Grossa Arch, Serra do Mar, Lages and the flood basalts of the Parana Basin, support a common origin in the lithospheric mantle. Sr–Nd isotope and other geochemical data indicate that a significant role was played by a mantle component depleted in incompatible elements and with high Sm/Nd ratio. This component (N-MORB type) would be represented by the depleted portions of a mantle which was variously metasomatized during Proterozoic times. The isotopic and geochemical features of the modernTristan da Cunha plume are distinctly different from the component depleted in incompatible elements, and its contribution is not apparent in the compositions of the Parana tholeiites.

Direct and indirect associations between potassic igneous rocks, shoshonites and gold-copper deposits

Many epithermal to mesothermal and porphyry-style Au ± Cu deposits worldwide occur associated with, or are hosted by, potassic igneous rocks and shoshonites. The host rocks are generally characterized by high halogens such as Cl and F, high LILE and low HFSE. Mineralization genetically associated with potassic igneous rock suites appears to be restricted to three tectonic settings: late oceanic arcs, continental arcs and postcollisional arcs. Examples of associations in late oceanic arcs include the Pleistocene Ladolam gold deposit, Lihir island, Papua New Guinea, the Pliocene Emperor gold mine, Viti Levu, Fiji and the Ordovician Goonumbla porphyry cooper-gold deposit, N.S.W., Australia. Examples from continental arcs include many epithermal gold and porphyry copper-gold deposits in the Mesozoic to Cenozoic Chilean Andes. An example of the association in a postcollisional-arc is the Miocene Porgera gold deposit, Papua New Guinea. For those potassic igneous rocks interpreted to be genetically related to gold-copper deposits in late oceanic arcs, continental and postcollisional arcs, the Cl contents of their mica phenocrysts are anomalously high with respect to Cl contents of barren high-K rocks from the same tectonic settings. Mineralized potassic igneous rocks from late oceanic arcs appear to be typified by both anomalously high Cl and F in mica phenocrysts.

Parallels in the origin of the geochemical signatures of island arc volcanics and continental potassic igneous rocks: The role of residual titanates

The extent of control of Ti, Nb and Ta concentrations by residual titanate minerals in island arc and subcontinental mantle enrichment processes is assessed with reference to recent reviews of the geochemistry of Indonesian Sunda island arc volcanics and of continental ultrapotassic rocks, and a reinterpretation of experimental results on TiO 2 saturation levels in arc volcanic compositions. It is considered likely that the low-Ti-Nb signature of potassic eastern Sunda arc rocks originates in at least two distinct mantle sources, both of which contain residual titanates during partial melting. A low-Nb-Ta component, which is also characteristic of island arcs in which potassic magma types are absent, is probably due to melting of the rutile-bearing hybridized product of reaction between silicic partial melts of subducted oceanic crust and the peridotite mantle wedge immediately above the subduction zone. A low-Ti component containing moderate amounts of Nb is assigned to K-rich low-degree partial melts of peridotite which have solidified at shallower levels, creating a vein system with incompatible-element-enriched composition. The source of potassic arc volcanics is thus a composite of peridotite wedge plus hybridized mantle and K-rich veins. Five plausible models with two residual titanates are presented, in which the second residual titanate must be active either during melting of the peridotite wedge or K-rich veins, or during melting of peridotite at deeper levels producing the K-rich melt which later crystallised as veins. Titanate saturation in basic melt compositions is promoted by a combination of the effects of high pressure, low temperature assisted by high H 2O content, high f O 2 and high contents of incompatible elements. The effects of high pressures and high f O 2 of reducing TiO 2 contents of melts in equilibrium with titanate minerals have probably been underestimated in previous studies. Continental mantle enrichment events may be analogous to the K-rich sub-arc component due to comparably low heat flow enabling H 2O assisted low-degree melting of mantle material in conditions which do not occur in other tectonic environments. Continental ultrapotassic rocks show a gradation of HFSE abundances which overlap with island arc potassic rock characteristics, implying a continuum of varying environments and physicochemical conditions such as H 2O content, f O 2 and source mineralogy. These variables may also determine which, if any, titanate mineral is present in individual sub-continental regions.