Intrusive Breccia Research Articles

Role of volatiles in intrusion emplacement and sulfide deposition in the supergiant Norilsk-Talnakh Ni-Cu-PGE ore deposits

Abstract The Norilsk-Talnakh orebodies in Siberia are some of the largest examples on Earth of magmatic Ni–Cu–platinum group element (PGE) deposits, formed by segregation of immiscible sulfide melts from silicate magmas. They show distinctive features attributable to degassing of a magmatic vapor phase during ore formation, including: vesiculation of the host intrusions, widespread intrusion breccias, and extensive hydrofracturing, skarns, and metasomatic replacement in the country rocks. Much of the magmatic sulfide was generated by assimilation of anhydrite and carbonaceous material, leading to injection of a suspension of fine sulfide droplets attached to gas bubbles into propagating tube-like host sills (“chonoliths”). Catastrophic vapor phase exsolution associated with a drop in magma overpressure at the transition from vertical to horizontal magma flow enabled explosive propagation of chonoliths, rapid “harvesting” and gravity deposition of the characteristic coarse sulfide globules that form much of the ore, and extensive magmatic fluid interaction with country rocks.

Chapter 8 Scottish mineral Geological Conservation Review sites – Replacement minerals

This is, the eighth and final paper of a series that describes the Scottish mineral Geological Conservation Review (GCR) sites within both mainland Scotland and the associated Isles. It features locations where the mineralogy of the original rock unit has been either largely or completely replaced as a consequence of their interaction with either fluids or gaseous components derived from external sources. At both Lunda Wick and the Queyhouse Quarry, Unst, deformation of ultrabasic rocks located within or close to Cambrian-age obduction-related shear zones has resulted in the formation of new mineral species including talc, actinolite, chlorite and biotite. Within the former mine site at Craignure, copper and nickel mineralisation is a consequence of the metasomatic alteration of metabasic igneous intrusions within the youngest part of the Dalradian Supergroup sequence resulting in the formation of pyrrhotite, pentlandite and chalcopyrite. The Lecht Mine site involves Dalradian and younger rocks, where iron and manganese oxides, derived from the metamorphic host rocks, have replaced both clasts and matrix within Devonian-age intrusive breccia pipes. The predominant replacement mineral within this setting is cryptomelane, but also includes ramsdellite, pyrolusite, chalcophanite, lithiophorite, woodruffite and caconexite. These replacement minerals also contain enhanced concentrations of barium, zinc, thallium, molybdenum and silver.

Igneous breccia system of the Shand porphyry Cu-Mo deposit, Northern Mongolia

The Shand Cu-Mo deposit is located in the Orkhon-Selenge depression, Northern Mongolia. It lies near the Erdenetiin Ovoo porphyry Cu-Mo deposit, which together define one of Northern Mongolian’s most economically significant metallogenic belts. In the Shand Cu-Mo (Au?) deposit, several ore related breccia types are associated with the porphyritic granodiorite intrusions, and they contain pre-, synand post-mineralized porphyry stocks, magmatic-hydrothermal and intrusive breccia. There are genetically at least two type of hydrothermal breccias have recognized in Shand deposit, i.e. magmatic-hydrothermal breccia and intrusive breccia. Magmatichydrothermal breccia is presented spatially associated with intrusions but extending sub vertical which characterized by angular fragments/clasts supported or infilled by minerals commonly indicative of high temperature and salinity (e.g. tourmaline, feldspar), silicas, carbonates and sulphides (Cu, Mo, (Au)) matrix derived from hydrothermal fluids precipitation. May grade downwards into cupolas of intrusive with or without intrusive breccia and pegmatite where occur at approximately deep from 250-1300 m depth. Intrusive breccia is mostly occurred in contact between margin intrusions at shallow depth which is mainly composed by granodiorite porphyry, granodiorite and dacite. Our drillhole relogging and petrographical observations are granodiorite hosted breccia and granodiorite porphyry hosted breccia. Here, we present an integrated study involving detailed drillhole logging, and petrographical observations to elucidate the genetic relationship and evolution of the Shand deposit for magmatic-hydrothermal breccia and intrusive breccia. Also, we propose that the magmatic breccia types indicate emplacement of igneous rocks from initially dacitic magma composition.

Recognizing subsurface breccias in Archean terranes: Implications for district scale metallogeny

Breccias are common in ancient and modern volcanic terranes, where they form at and below surface through volcanic, hydrovolcanic, magmatic, or tectonic mechanisms. They are critical for volcanic reconstruction as stratigraphic markers and indicators of geodynamic change and since they can be associated with mineralization their genesis is also important from an exploration perspective. Their origin can be difficult to ascertain in ancient terranes that have undergone polyphase deformation and associated metamorphism. The Joliet Breccia is a subeconomic Cu-Ag prospect within the Neoarchean Rouyn-Noranda mining district, in the Abitbi greenstone belt, Canada. It was previously interpreted as a phreatic breccia formed on the seafloor. This study presents new data indicating that the Joliet Breccia is a subsurface magmatic-hydrothermal breccia. Specifically, the recognition of gradational contacts with host rocks and between internal breccia domains, lack of sedimentary features and the spatial and temporal association with a tonalite intrusion strongly supports this interpretation. The angular, poorly sorted, lithic clasts derived from the immediate host rocks, hydrothermal cement, complete absence of a rock flour matrix, and presence of a radial and concentric fracture pattern extending into the host rocks, further support a subsurface magmatic-hydrothermal emplacement mechanism.A new TIMS U-Pb zircon age of 2698.0 ± 0.9 Ma (2σ) for a tonalite block within the breccia constrains the maximum age of brecciation. Four LA-ICP-MS U-Pb dates on hydrothermal monazite (2693.7 ± 1.0/8.9 Ma; 2696.9 ± 0.45/8.9 Ma; 2698.7 ± 1.4/8.9 Ma; 2701.2 ± 0.33/8.9 Ma; 2 s/2ssys) found in the cement of the Breccia indicate brecciation and mineralization occurred shortly after the emplacement of the tonalite. Similarly, the ca. 2697 Ma St. Jude intrusive breccia indicates a localized ca. 2699–2695 Ma magmatic-hydrothermal event superimposed on ca. 2704–2701 Ma strata. These breccias are temporally correlative with the youngest units of the Blake River Group strata and associated ca. 2698–2695 Ma volcanogenic massive sulfide (VMS) deposits, which suggests an indirect genetic link between these two hydrothermal systems. The VMS deposits may represent the near surface, distal manifestations of a deeper magmatic-hydrothermal system akin to the porphyry-intermediate sulfidation epithermal continuum in modern subaerial volcanic arcs.

First Finding of the Orbicular Gabbroids in the Ukrainian Antarctic Station Area (Wilhelm Archipelago, West Antarctica)

A new occurrence of igneous rocks with an orbicular structure was discovered by the authors in West Antarctica. The place of finding is Hovgaard Island in the Wilhelm Archipelago located near the Graham Coast of the Antarctic Peninsula. Until now, not a single manifestation of these rare rocks was known in this region. Usually orbicular rocks are formed under the condition of local "coincidence" of many geological and petrogenetic factors. The study of the geological position, texture features and mineral composition of the orbicular rocks on Hovgaard Island was carried out in order to create their primary petrographic description. It was found that orbiculites are exposed in a small area, about 200 m2, in the field of amphibolized gabbroids and their intrusive breccias. The occurrence and textures of the orbiculites indicate that their crystallization occurred at the hypabyssal depth. Probably, this occurrence is a marginal facies of a small gabbroid intrusion, some parts of which are exposed on the adjacent coastal areas of Hovgaard Island. The studies performed have shown that the orbiculites of Hovgaard Island belong to the rarest petrographic representatives of these rocks namely orbicular gabbroids. In their petrographic feature, they differ markedly from the famous French napoleonites (corsites) exposed on the Corsica Island. The mineral composition of the orbicules is represented by calcium plagioclase (An88-97), hornblende (#Mg = 0.77-0.81), clinopyroxene (Wo48-50En43-47Fs5-8), spinel (Sp62-72Hrc14-20Mt12-17), actinolite, phlogopite, chlorite, magnetite and apatite. The interorbicular matrix has a gabbroid composition and a porphyritic texture. It differs from orbicules in somewhat less calcium plagioclase and less magnesian hornblende, as well as in the absence of spinel. In view of the rarity of orbicular gabbroids and the specificity of the described manifestation, it is proposed that the orbicular gabbro on Hovgaard Island be considered as a new petrographic variety of the gabbroid family. The name "hovgaardite" is recommended for the name of this variety of orbicular gabbro.

Rapid Formation of Porphyry and Skarn Copper-Gold Mineralization in a Postsubduction Environment: Re-Os and U-Pb Geochronology of the Ok Tedi Mine, Papua New Guinea

Abstract The Ok Tedi copper-gold mine in Western Province, Papua New Guinea, is situated in the western part of the Ok Tedi Complex where monzodiorite to quartz monzonite intrusions are associated with porphyry- and skarn-style copper-gold mineralization. The Pleistocene age of the intrusive rocks and mineralization provides an opportunity to study the longevity of the magmatic and hydrothermal evolution at Ok Tedi through U-Pb dating of zircon and high-precision Re-Os dating of molybdenite. Six main phases of intrusive rocks can be recognized within the mine area, with the sequence of intrusion indicated by contact relationships. Each has been dated by the SHRIMP U-Pb technique with correction for Th-U disequilibrium based on the U and Th content of each sample. In order of intrusion from oldest to youngest these include: Sydney Monzodiorite (1.368 ± 0.045 Ma), Warsaw Monzodiorite (1.269 ± 0.039 Ma), Kalgoorlie Monzodiorite (1.261 ± 0.050 Ma), Ningi Quartz Monzonite Porphyry (QMP)(1.229 ± 0.051 Ma), Bonn Quartz Monzonite (1.219 ± 0.040 Ma), and Fubilan QMP (1.213 ± 0.049 Ma). The intrusions are alkaline, high K to shoshonitic rocks with high Sr/Y ratios typical of Cu-fertile arc magmas. Chondrite-normalized REE patterns have minor or no negative Eu anomalies and downward sloping to listric-shaped HREE patterns typical of arc magmas in which high water contents supress plagioclase fractionation in favor of an evolution by hornblende ± garnet ± titanite fractionation. Cu-Au mineralization at Ok Tedi can be divided into four main stages based on crosscutting relationships: (1) skarn-endoskarn and associated vein-style mineralization in the Darai Limestone, Ieru siltstone, and Sydney Monzodiorite; (2) porphyry-style veins and breccias within the Ningi QMP and older intrusions, and at Siltstone Ridge: (3) porphyry-style veins and breccias in the Fubilan QMP and older intrusions: and (4) skarn-style mineralization in the lower part of the Darai Limestone along the Taranaki thrust. High-precision Re-Os dating of molybdenite has enabled a chronology to be established for the first three stages. Molybdenite from a quartz-mushketovite-epidote-carbonate-pyrite-chalcopyrite-molybdenite vein in clinopyroxene- and garnet-altered Sydney Monzodiorite has an age of 1.3206 ± 0.0020 Ma, and this dates the formation of the Gold Coast and Berlin skarns. Molybdenite from a quartz-pyrite-chalcopyrite-molybdenite vein in the sericite-altered Sydney Monzodiorite yields an age of 1.3166 ± 0.0043 Ma, and a quartz-pyrite-chalcopyrite-molybdenite vein with K-feldspar alteration selvages hosted in Ieru siltstone beneath the Gold Coast skarn has an age of 1.3031 ± 0.0015 Ma. Samples of molybdenite from quartz-sulfide veins from Siltstone Ridge have ages of 1.2116 ± 0.0029 and 1.2078 ± 0.0031 Ma. Molybdenite from a quartz-K-feldspar-pyrite-molybdenite vein, which overprints propylitic alteration in the Sydney Monzodiorite, has an age of 1.2120 ± 0.0024 Ma. These samples date porphyry-style mineralization in and around the Ningi QMP and at Siltstone Ridge. A sample of molybdenite from the matrix of hydrothermal intrusive breccia in the Fubilan QMP has an age of 1.2146 ± 0.0020 Ma, similar to the age of the adjacent Siltstone Ridge mineralization, and is interpreted to have been mechanically incorporated into the breccia during its formation. Several samples have been dated from the Fubilan porphyry system, including molybdenite from the matrix of a hydrothermal intrusive breccia (1.1648 ± 0.0020 Ma) and three samples from veins which postdate the breccias: a vuggy quartz-sulfide vein (1.1532 ± 0.0027 Ma), chalcopyrite-pyrite-molybdenite vein (1.1446 ± 0.0028 Ma), and duplicate analyses of a molybdenite-only vein (1.1326 ± 0.0034 and 1.1297 ± 0.0026 Ma) in agreement at 2σ. Molybdenite from a quartz-K-feldspar-biotite-magnetite-pyrite-chalcopyrite-molybdenite vein in endoskarn-altered Sydney Monzodiorite (beneath the Gold Coast skarn) has an age of 1.1459 ± 0.0012 Ma, and a similar vein without magnetite hosted in Warsaw Monzodiorite has an age of 1.1438 ± 0.0042 Ma, both within error of the chalcopyrite-pyrite-molybdenite vein in Fubilan QMP. Intrusive rocks in the Ok Tedi mine were emplaced over a period of approximately 200,000 years, with Cu-Au mineralization formed in discrete episodes of much shorter duration. The Gold Coast skarn and associated porphyry-style veins in Sydney Monzodiorite and Ieru siltstone formed in 14,000 to 21,000 years (n = 3), the Siltstone Ridge porphyry system in 2,000 to 12,000 years (n = 4), and the Fubilan porphyry system in 31,000 to 40,000 years (n = 6). The Taranaki skarn has not been dated in the mine area due to a lack of molybdenite, but geologic relationships indicate it is younger than the Fubilan QMP.

Low-Sulfide Platinum Group Element Ores of the Norilsk-Talnakh Camp

Abstract Low-sulfide platinum group element (PGE) mineralization of the Norilsk-type intrusions is located within the Upper Gabbroic Series, which comprises rocks heterogeneous in texture and composition. The highest grade of 10 to 50 g/t PGEs is confined primarily to chromitiferous taxitic gabbrodolerite, which forms irregular lens- and vein-like bodies that interfinger with contact gabbrodolerite, intrusion breccia, leucogabbro, and gabbrodolerite variably enriched in olivine, from olivine free up to picritic compositions. The abundant amygdules and pegmatoidal textures in Upper Gabbroic Series taxitic rocks, as well as the high enrichment of halogen in minerals (e.g., ≤4.6 wt % Cl in apatite), indicate a higher volatile content of the local magma compared to the magma that precipitated the Main Series. The observed diversity in spinel compositions, which evolve from chromite to Cr magnetite as well as toward hercynite, titanomagnetite, and ulvöspinel, is also indicative of crystallization from a fluid-saturated mush that subsequently reacted, to varying degrees, with contaminated trapped melt and immiscible fluid. The high PGE/S ratio is a primary feature of this mineralization style, albeit the ratio partly increased during sulfide replacement and resorption. The PGE tenor of bulk sulfides calculated as ΣPGE (g/t) in 100% sulfides exceeds 160 and may reach up to 1,400 to 2,500 in low-S ores (0.2–3 wt % S), whereas the value does not exceed 42 in the Talnakh disseminated ore and ranges from 35 to 120 in the Norilsk disseminated ore (1–10 wt % S). Several PGE peaks in the vertical sections correlate well with Cu, Ni, S, and Cr peaks, as well as with observed elevated proportion of amygdules. Low-sulfide ores are composed of two primary sulfide assemblages of pyrrhotite + pentlandite + chalcopyrite and pentlandite + pyrrhotite. The primary sulfides are depleted in the heavier 34S isotope relative to sulfides of the corresponded main orebodies (e.g., mean δ34S = 8.9‰ versus δ34S = 12.3‰, respectively, in the Kharaelakh intrusion). A secondary pyrite + millerite + chalcopyrite assemblage has isotope composition enriched in 34S by 2 to 6‰ δ34S with respect to primary sulfides. The directly measured PGE content in sulfides (e.g., 11–2,274 g/t Pd in pentlandite and 0.10–33.3 g/t Rh in pyrrhotite) is within the range of the typical Norilsk-type magmatic sulfide compositions. The textural setting and diversity of platinum group minerals (PGMs) favor the hypothesis of fluid-controlled crystallization. However, the distinct PGM assemblages in Norilsk 1 and Talnakh-Kharaelakh low-sulfide ores are comparable with those of the corresponding presumably magmatic disseminated and massive orebodies. The most remarkable characteristic is the widespread Pt-Fe alloys in Norilsk 1 and their absence in Talnakh-Kharaelakh, which is interpreted to reflect better preservation of the high-temperature PGMs in Norilsk 1 in contrast to their substantial replacement in more oxidized fluid-enriched environments in Talnakh-Kharaelakh.

Low-Sulfide Platinum–Palladium Deposits of the Paleoproterozoic Fedorova–Pana Layered Complex, Kola Region, Russia

Several deposits of low-sulfide Pt–Pd ores have been discovered in recent decades in the Paleoproterozoic Fedorova–Pana Layered Complex located in the Kola Region (Murmansk Oblast) of Russia. The deposits are divided into two types: reef-style, associated with the layered central portions of intrusions, and contact-style, localized in the lower parts of intrusions near the contact with the Archean basement. The Kievey and the North Kamennik deposits represent the first ore type and are confined to the North PGE Reef located 600–800 m above the base of the West Pana Intrusion. The reef is associated with a horizon of cyclically interlayered orthopyroxenite, gabbronorite and anorthosite. The average contents of Au, Pt and Pd in the Kievey ore are 0.15, 0.53 and 3.32 ppm, respectively. The North Kamennik deposit has similar contents of noble metals. The Fedorova Tundra deposit belongs to the second ore type and has been explored in two sites in the lower part of the Fedorova intrusion. Mineralization is mainly associated mainly with taxitic or varied-textured gabbronorites, forming a matrix of intrusive breccia with fragments of barren orthopyroxenite. The ores contain an average of 0.08 ppm Au, 0.29 ppm Pt and 1.20 ppm Pd. In terms of PGE resources, the Fedorova Tundra is the largest deposit in Europe, hosting more than 300 tons of noble metals.

Geochemistry and Sr-Nd isotopic compositions of Permian ultramafic lamprophyres in the Reelfoot Rift – Rough Creek Graben, southern Illinois and northwestern Kentucky

Permian dikes, sills, and diatremes in southern Illinois and northwestern Kentucky (the Omaha, Wildcat Hills, Cottage Grove, Will Scarlet, Williams, Grant, and Clay Lick intrusions) share similar geochemistry and are classified as ultramafic lamprophyres. Major element compositions are 30–35 wt% SiO2, 6–7% Al2O3, 12–14% FeOt, 16–19% MgO, 3–5% TiO2, 11–16% CaO, 0.1–0.7% Na2O, 1.2–2.7% K2O, and 0.4–1.3% P2O5. The Grant Intrusive Breccia is an exception, with lower SiO2, Al2O3, FeOt, MgO, TiO2, and higher CaO. Typically, these rocks are fine grained, with phlogopite, serpentinized olivine (~Fo88), diopside, perovskite, Fe-Ti-spinel, apatite, and calcite. Blocky and lath-shaped pseudomorphs in some samples probably represent melilite, which would make the rocks alnöites. The Grant and Williams diatremes contain sedimentary and igneous clasts (including amphibole megacrysts) within a carbonate-rich matrix. The Grant exhibits pelletal lapilli and is characterized as a lamprophyre‑carbonatite tuffisite. Trace element patterns exhibit enrichment of LREE, strong REE fractionation, and relative depletions of K, Sr, Zr, and Hf, closely matching those of the mela-aillikites of Aillik Bay, Labrador. The Grant Intrusive exhibits even greater REE enrichment and notable peaks at Nb, La, and Ce. Geochemical characteristics, including distributions of 143Nd/144Nd and 87Sr/86Sr, are consistent with near-primary melts from a metasomatized peridotite source containing phlogopite-rich veins. Derivation of the lamprophyres from carbonate-rich parental melts similar to the Grant Intrusive could be achieved by separation of carbonatite. A narrow range of initial 87Sr/86Sr (0.70301–0.70449), and initial εNd (3.7–5.1), suggests a uniform mantle source close to Bulk Earth. T-depleted mantle model ages range from 540 to 625 Ma, and might correlate with timing of enrichment of a lithospheric mantle source during the breakup of Rodinia.

Geological setting of the Moorowie Formation, lower Cambrian Hawker Group, Mt Chambers Gorge, eastern Flinders Ranges, South Australia

Geological mapping in 1970 of lower Cambrian outcrop in the eastern Flinders Ranges of South Australia included the description and naming of the Moorowie Formation, representing the uppermost Hawker Group. The mapping is supported by 885 m of measured sections. An early Cambrian regressive-marine shelf-margin succession is described, from the massive Wilkawillina Limestone (base), the grey laminated limestones, syndepositional slump-induced intraformational folds and breccias, collapse talus, and graded sediment gravity flow deposits of the Mernmerna Formation, terrace-edge attrition megabreccias and reefs of the Moorowie Formation, passing up to the red beds of the Billy Creek Formation (top). Rapid changes in sedimentary facies are attributed to basement block movements and diapiric influence on sedimentation with abrupt vertical relief and paleoslope indicated by syndepositional slumping and platform margin collapse. Tuffs in the Mernmerna Formation record contemporaneous volcanism. Massive archaeocyathan limestones, ooid grainstones, peloid limestones, reef-sourced megabreccias and red shales define five members in the overlying Moorowie Formation, signalling shallow-marine regressive conditions and the development of a biologically diverse carbonate platform seaward of evaporitic lagoons and supratidal sabkhas. The megabreccias of the Moorowie Formation formed as thin semi-autochthonous debris aprons or shallow tidal-channel infills resulting from gradual and persistent wave attrition and repeated collapse of a carbonate terrace that was vigorously reworked by tidal currents. Shale interbeds within the Moorowie Formation represent lightly channelised shallow-marine ramp deposits, with adjacent mud flats, as easterly equivalents of the Oraparinna Shale, the earliest of which formed the substrate to the attrition megabreccias. Emergent evaporite diapirs near Mt John and Mt Frome are the probable sources of coarse siliciclastics within the carbonates. Some of the siliciclastics were likely transported onto the carbonate platform by sandstorms or migrating dunes. Late in the Cambro-Ordovician Delamerian Orogeny, earlier salt diapirs at depth were compressed and reactivated as highly mobile evaporite-rubble breccias and intruded as small plugs and dykes into fissures in a lithified and folded cover.The intrusive breccias include metasediment and metabasic xenoclasts attributable to the Callanna Group, while diapir-related faults also host minor copper, lead and barite mineralisation. Documentation of this unique record contributes to wider investigations of the Ediacaran-to-early-Cambrian succession of the Flinders Ranges sector of the Arrowie Basin, adding to its global heritage values, effective management and appreciation by the wider community.

Nb–Ta–Ti Oxides in Topaz Granites of the Geyersberg Granite Stock (Erzgebirge Mts., Germany)

Oxide minerals (Nb–Ta-rich rutile, columbite-group minerals and W-bearing ixiolite) represent the most common host for Nb, Ta and Ti in high-F, high-P2O5 Li-mica granites and related rocks from the Geyersberg granite stock in the Krušné Hory/Erzgebirge Mts. batholith. This body forms a pipe like granite stock composed of fine- to middle-grained, porphyritic to equigranular high-F, high-P2O5 Li-mica granites, which contain up to 6 vol. % of topaz. Intrusive breccia’s on the NW margin of the granite stock are composed of mica schists and muscovite gneiss fragments enclosed in fine-grained aplitic and also topaz- and Li-mica-bearing granite. Columbite group minerals occur usually as euhedral to subhedral grains that display irregular or patched zoning. These minerals are represented by columbite-(Fe) with Mn/(Mn + Fe) ratio ranging from 0.07 to 0.15. The rare Fe-rich W-bearing ixiolite occurs as small needle-like crystals. The ixiolite is Fe-rich with relatively low Mn/(Mn + Fe) and Ta/(Ta + Nb) values (0.10–0.15 and 0.06–0.20, respectively). Owing to the high W content (19.8–34.9 wt. % WO3, 0.11–0.20 apfu), the sum of Nb + Ta in the ixiolite does not exceed 0.43 apfu. The Ti content is 1.7–5.7 wt. % TiO2 and Sn content is relatively low (0.3–4.1 wt. % SnO2).

Structures and Implications for Fluid Migration in the Jiadi Carlin‐Type Gold Deposit, Guizhou Province, Southwest China

AbstractThe Jiadi gold deposit is a newly discovered Carlin‐type gold deposit in the Guizhou Province, Southwest China. This deposit is structurally controlled by a shallow fold–fault superimposed system along the Lianhuashan trend. Field geological investigations, structural analysis, and mathematical research are conducted to study its structures and hydrothermal fluid flow process. Geological investigations (i.e. sections, stope, and drill hole) indicate that the structures are dominated by NE‐trending folds, NWW‐trending folds, and faults. A trend‐surface analysis of the low interformational fracture zone suggests that the overall distribution of the Lianhuashan trend is controlled by the NE‐trending Lianhuashan anticline. Nearly all primary Carlin‐type gold deposits are distributed along the southeastern flank where the fold axis changes from NE to EW. Gold orebodies are hosted by the interformational fracture zones and primarily situated at the transitions from the high‐value areas to the low‐value areas of the interformational fracture zones. A stress analysis on the hydrothermal veins indicates that they are of tenso‐shear properties. The ore‐forming elements (Au, As, Sb, Hg, and Tl) of the hydrothermal veins from the interformational fracture zones and intrusive breccia body present strong positive anomalies compared with those from the adjacent wall rocks. According to the development patterns of the ore‐forming elements and hydrothermal veins, a migration process of the ore‐forming fluids is proposed: the deeply sourced ore‐forming fluids migrated vertically to shallow crust along an intrusive breccia body, subsequently flowed horizontally along the interformational fracture zones accompanied by gold precipitation in the early stage, and finally migrated outward along steep micro‐fractures during following stages.

A new look on Imperial Porphyry: a famous ancient dimension stone from the Eastern Desert of Egypt—petrogenesis and cultural relevance

Imperial Porphyry, a famous dimension stone of spectacular purple color, was quarried in the Mons Porphyrites area north of Jabal Dokhan in the Eastern Desert of Egypt, from the beginning of the first until the middle of the fifth century AD. During this period, the valuable material was processed as decorative stone and was used for objects of art, reserved exclusively for the Imperial court of the Roman Empire. Later on, only antique spoils of smaller or bigger size have been re-used for these purposes. The Imperial Porphyry is a porphyritic rock of trachyandesitic to dacitic composition that occurs in the uppermost levels of shallow subvolcanic sill-like intrusions, forming a member of the Dokhan Volcanic Suite. Its purple color is mainly due to dispersed flakes of hematite, resulting from hydrothermal alteration of a dark green Common Porphyry of similar composition, underlying the Imperial Porphyry. Both, the Common Porphyry and the purple Imperial Porphyry’, are extensively exposed in the Roman quarries. Contacts between Common and Imperial Porphyry are irregular and gradational. In both rock types, intrusive breccias are frequent, indicating a complex intrusion history. U–Th–Pb zircon geochronology on two samples of Imperial Porphyry and one sample of the Common Porphyry yielded an age range of 609–600 Ma, thus confirming earlier results of radiometric dating. Geochemical evidence indicates that both the Imperial and the Common Porphyry are of medium- to high-K calc-alkaline affinity. The magmas have formed by partial melting of a subduction-modified upper mantle. The subsequent intrusion took place within a highly extended terrane (HET).

Textural development in sulfide-matrix ore breccias in the Aguablanca Ni-Cu deposit, Spain, revealed by X-ray fluorescence microscopy

The intrusion-hosted Ni-Cu sulfide deposit at Aguablanca in south-western Spain contains a high proportion of ores in the form of sulfide-matrix ore breccias. These are polymict, comprising autoliths and xenoliths in sulfide-rich and/or sulfide-poor matrices. Inclusion lithologies include calc-silicate skarn rocks from the adjacent marbles, ultramafic and mafic cumulates, and remelted and recrystallized mafic rocks containing spinifex-like textures.Breccia textures have been investigated at mm to cm scale using desk-top and synchrotron-based microbeam XRF mapping which reveal a number of distinctive common features: disaggregation of inclusions into adjacent sulfide along original silicate grain boundaries; complex reverse and oscillatory zoning in Cr content of clinopyroxene grains within sulfide and inside inclusions; narrow reaction rims between country rock clasts and enclosing silicates; and preferential disposition of pyroxene crystals within pyrrhotite-pentlandite aggregates (original MSS) relative to inclusion-poor chalcopyrite.The observed range of textures is explained by a model of percolation of molten sulfide through a pre-existing silicate-matrix intrusion breccia, preferentially displacing a cotectic or eutectic plagioclase-pyroxene melt. The process is analogous to that believed to have formed interspinifex ore in komatiite-hosted deposits, and also to that responsible for superficially similar sulfide matrix ore breccias at Voisey’s Bay. The preserved range of textures is interpreted as being due to late stage gravity-driven percolation of sulfide liquid from above into a pre-existing partially molten intrusion breccia. This intrusion breccia itself may have been emplaced into the neck of the Aguablanca stock, in the waning stages of magma flow.

A low-δ18O intrusive breccia from Koegel Fontein, South Africa: Remobilisation of basement that was hydrothermally altered during global glaciation?

The Cretaceous Koegel Fontein igneous complex is situated on the west coast of South Africa, and has a high proportion of rocks with abnormally low δ18O values. The rocks with the lowest δ18O values (−5.2‰) belong to intrusive matrix-supported breccia pipes and dykes, containing a variety of clast types. The breccia rocks range in SiO2 from 44 to 68wt% and their whole-rock δ18O values vary between −5.2‰ and +1.8‰. The major and trace element composition of the breccia rocks is consistent with them containing variable proportions of clasts of Cretaceous intrusive rocks and basement gneiss and the matrix being fluidized material derived from the same source as the clasts. Based on the nature of the clasts contained in the breccia, it was emplaced just prior to intrusion of the main Rietpoort Granite at 134Ma. All components of the breccia have low δ18O value and, at least in the case of the gneiss clasts, this predates incorporation in the fluidized material.Although the early Cretaceous appears to have been a period of cold climate, it is unlikely that the δ18O values of ambient precipitation (~−10‰) would have been low enough to have generated the required 18O-depletion. The basement gneiss was probably ~2–3km below the Cretaceous surface, minimizing the possibility of interaction with isotopically unmodified meteoric water, and there is no evidence for foundered blocks of cover rocks in the breccia. There is, therefore, no evidence for downwards movement of material. We favour a model where basement gneiss interacted with extremely 18O-depleted fluid during crustal reworking at ~547Ma, a time of global glaciation. Low-δ18O metamorphic fluids produced by dehydration melting of 18O-depleted gneiss became trapped and, as the fluid pressure increased, failure of the seal resulted in explosive upwards movement of fluidized breccia. Migration was along pre-existing dykes, incorporating fragments of these dykes, as well as the country rock gneiss.

Orebody geometry, fluid and metal sources of the Omitiomire Cu deposit in the Ekuja Dome of the Damara Belt in Namibia

The Omitiomire Cu deposit (resource of 137 Mt at 0.54% Cu) in the Ekuja Dome of the Damara Belt in Namibia is hosted by an anastomosing, low-angle Pan-African (ca. 520 Ma) shear zone system developed around an older (ca. 1100–1060 Ma), late Mesoproterozoic intrusive breccia between a suite of mafic rocks (originally lava flows) and later tonalitic gneisses. High-grade ore shoots preferentially formed along contacts between tectonically interleaved biotite-epidote-quartz-chalcocite schists and felsic gneisses, and are directly related to an increase in the number and cumulative thickness of thin, contact-parallel mineralized shear zones. Alteration and mineralization are associated with elevated concentrations of K2O, Cr, Rb, S, and Cu and a loss of Na2O, CaO, and MgO. Oxygen isotope fractionation for quartz-biotite, quartz-feldspar, and quartz-amphibole mineral pairs support equilibrium temperatures of between 500 and 650 °C during the fluid/rock interaction. Mineral separates from amphibole-biotite gneisses and mineralized schists have similar ranges in δ18O values of about 1.2 to 2 ‰ relative to VSMOW. Coexisting minerals are arranged in an order of increasing δ18O values from biotite, to epidote, amphibole, and quartz, suggesting that the Omitiomire Shear Zone was a rock-dominated system. Similarly, H-isotope results for mineral separates from biotite-epidote schists and amphibole gneisses do not show any reversals for D/H fractionations, with δD values of between −48 and −82 ‰, typical of metamorphic-magmatic rocks. The homogeneous and low δ34S values (−6.1 to −4.7 ‰ CDT) are compatible with a local redistribution of sulfur from magmatic rocks and interaction with sulfur derived from metamorphic fluids of metasedimentary origin. The relatively low fluid/rock ratios and elevated Cu values (>1500 ppm) from unaltered amphibolite point to a local redistribution of an earlier (late Mesoproterozoic) Keweenaw-type Cu mineralization into later Pan-African shear zones during the exhumation of the Ekuja Dome. The timing, polyphase evolution, and tectonic setting of the Omitiomire deposit show remarkable similarities with the large Cu deposits of the Domes Region in the adjoining Lufilian Arc of northern Zambia. This suggests the presence of a much larger, regionally significant Cu province extending from central Namibia, through northern Botswana, and into Zambia.

Textural development in sulfide-matrix ore breccias in the Voisey's Bay Ni-Cu-Co deposit, Labrador, Canada

Magmatic Ni-Cu sulfide ores at Voisey’s Bay contain complex assemblages of extremely heterogeneous rocks. These range from polymict breccias, with rock fragments in sulfide-rich and/or sulfide-poor matrices, to heterogeneous “vari-textured” gabbros with rapid short range variations in grain size and content of hydrous phases. Rock fragment populations in the breccias include endogenous olivine gabbros (cumulate and non-cumulate) and cumulate peridotites along with extensively depleted plagioclase-hercynite gneisses interpreted as restites from extensive partial melting of country rock quartzo-feldspathic paragneisses. Using a combination of desk-top microbeam XRF mapping at cm scale and 3D X-ray tomography, we show that both sulfide-poor and sulfide-rich breccias comprise heterolithic assemblages of clasts within a matrix of olivine gabbro. This matrix is characterised by an interconnected 3D framework of plagioclase crystals, highly variable in grain size at mm to cm scale, with interstitial olivine and poikilitic clinopyroxene, and is texturally indistinguishable from clast-free olivine gabbro. Sulfide forms interconnected networks at cm to dm scale and possibly larger. Much of the plagioclase developed by outgrowth from the margins of paragneiss xenoliths when the porosity was occupied by silicate melt. The observed range of textures is explained by a model of percolation of molten sulfide through variably crystalline inter-clast matrix, displacing the silicate melt to leave the refractory plagioclase-olivine or in some cases plagioclase-only component, now entirely within a sulfide matrix. The process is analogous to that believed to have formed interspinifex ore in komatiite-hosted deposits. Biotite rims on plagioclase enclosed in sulfide are interpreted as the result of reaction between plagioclase, olivine and a hydrous component derived from the sulfide melt itself, with a possible component of migrating residual silicate melt wicking along sulfide-silicate contacts. This sulfide infiltration model offers an alternative to the current model for upward emplacement of a slurry of silicate melt, sulfide melt and breccia fragments as a late stage injection into the dyke-sill complex. The preserved range of textures is interpreted as being due to gravity-driven percolation of sulfide liquid through a pre-existing partially molten intrusion breccia. In this model, the breccia serves as a physical trap site, accumulating downward migrating sulfide liquid. However, the invariable close mutual association of sulfide and rock fragments at Voisey’s Bay implies a common derivation.

The intrusion breccia in the valley of Roaring Brook, Giant Mountain, Adirondack Highlands, New York: A modern interpretation

An intrusion breccia exposed along Roaring Brook on the west face of Giant Mountain in the Adirondack Highlands (eastern USA) has been reinterpreted as a magma conduit. It lacks metasedimentary xenoliths, as previously proposed, and field, petrographic, and geochemical evidence point to an igneous origin for the vast majority of the enclaves. A mechanism involving magma mixing is proposed for the enclaves and explains their textural features (e.g., crescumulate and comb textures and mantled feldspars), compositional layering, and geochemical trends. Rock types ranging from charnockite to pyroxenite occur in the exposed section and document the variety of magmatic processes and compositional variation of magmas composing the Marcy Anorthosite massif. The bulk of the enclaves in the intrusion breccia can be explained by a batch of more felsic magma (charnockite-mangerite?) that intruded into a partially crystallized zone of anorthositic, gabbroic, and dioritic rock. This resulted in disruption, partial mixing and hybridization, and chilling of mafic magma within, and against, the more silicic melts.

페루 남동부 아뿌리막주 트라피체 동-몰리브데늄 광상의 지질 및 광화작용

Trapiche project corresponds to the advanced exploration stage which is thought to be a part of various porphyry copper deposits occurring in the margin of Andahuyalas-Yauri metallogenic belt. This deposit is genetically related to the monzonitic porphyry intrusion and Oligocene breccia pipe. Mineralization consists of primary sulfides such as pyrite, chalcopyrite, bornite, and molybdenite and secondary sulfides such as chalcocite, covellite and digenite. It occurs malachite, tenorite and cuprite as copper oxide. As a result of lixiviation or enrichment process, mineralization shows untypical zonation structure. Breccia and porphyry areas characterize the vertical zonation patterns. In the northern area, lixiviation zone, secondary enrichment zone, transitional zone and primary mineralized zone are distributed in northern area. In the western area of deposit, oxidation zone and mixed zones are narrowly occurred. Inferred resources of deposit is estimated to be 920 Mt @ 0.41% Cu with the cut-off grade of 0.15%.

Sulfide Immiscibility Induced by Wall-Rock Assimilation in a Fault-Guided Basaltic Feeder System, Franklin Large Igneous Province, Victoria Island (Arctic Canada)

The Southern Feeder Dike Complex is part of the Franklin Large Igneous Province (LIP), exposed in the Minto Inlier of Victoria Island in the Canadian Arctic. Previous field and geochemical studies on the Franklin LIP considered its igneous rocks to be prospective for Fe-Ni-Cu mineralization. The Southern Feeder Dike Complex comprises a series of NW-SE-trending gabbroic intrusions and sedimentary hosts. Field and textural relationships show that the Complex intrusions were emplaced contemporaneously with Neoproterozoic normal faulting. Faulted contact zones correspond to prominent first derivative magnetic lineaments. Gabbroic dikes have intrusive contacts against brecciated country rock, and diabasic microxenoliths in basaltic matrices indicate multiple intrusive/brecciation events. Intrusive breccias are commonly overprinted by hydrothermal greenschist facies assemblages, with calcite + pyrite veins filling open spaces between breccia fragments. Late dikes emplaced into these heterogeneous breccias contain disseminated globular and net-textured sulfides suggesting that sulfide immiscibility was triggered on a local scale by assimilation of local wall rock. This inference is supported by elevated δ34S values of sulfides in these dikes, consistent with assimilation of country rocks. Wall-rock assimilation would have been facilitated by fault-related brecciation and cataclasis, which would expose extensive xenolith surface areas to fresh magma. Gossanous and meter-scale semimassive sulfide showings associated with dikes and sills located upsection from the Southern Feeder Dike Complex suggest that immiscible sulfide liquids may have been flushed downstream (or upsection) during replenishment of composite dike systems. Fault-mediated melt ascent along northwest-southeast faults has been documented elsewhere in the Minto Inlier, providing equivalent opportunities for wall-rock assimilation and consequent triggering of sulfide immiscibility and sulfide melt redistribution. The evidence preserved in the Complex confirms the Fe-Ni-Cu potential of the Franklin LIP and informs current models of ore deposit formation in conduit-type magmatic plumbing systems.