Breccia Cement Research Articles

Diagenetic and hydrothermal events revealed by an Ediacaran dolomite breccia from the Araras-Alto Paraguai basin, southern Amazon Craton

The establishment of extensive carbonate platforms characterized the Ediacaran period during CaCO3 oversaturation events and the exceptional creation of accommodation space associated with the aftermath of the Marinoan glaciation (∼635 Ma). Carbonate deposits in the Araras Group record the early Ediacaran period, stage 1, in the Amazon Craton. The Serra do Quilombo Formation is an intermediate unit characterized by heavily fractured and faulted dolomites, as well as the presence of dolomite-cemented breccias (CB) overlying thick limestone packages associated with diagenetic and hydrothermal modifications. These features are commonly attributed to structurally controlled hydrothermal dolomitization (HTD) and are of significant economic importance as hydrocarbon reservoirs and Mississippi Valley-type (MVT) lead-zinc mineralization hosts. This study aims to unravel the origin of cemented breccias and the diagenetic/burial processes within the unit, focusing on dolomitization processes. Dolomite samples were analyzed using petrographic, scanning electron microscopy, microprobe, micro-Raman, cathodoluminescence, and isotopic analyses (δ13C, δ18O, 87Sr/86Sr) to unravel their burial history. CBs are sub-vertical to sub-horizontal bodies with complex geometries, generally cutting bedding at high angles, indicating hydrofracturing processes related to vertical flows of hydrothermal fluids (hydraulic breccia) and present the typical cockade texture of expansion breccias in dilatational faults. The substitutive matrix RD1 is the main constituent of the Serra Quilombo Formation, its low correlation between δ13C and δ18O (R2 = 0.009), the well-preserved fabric, and the similarity with the isotopic values (C and Sr) documented for Ediacaran carbonates, suggest that the syndepositional dolomitization. The first generation of dolomite cement (DC1) and the last phase of dolomitic cementation (saddle dolomite - SD) occur, filling pores, CBs, and fractures. The cockade texture of the breccias evidences a low precipitation rate or a pause in precipitation between DC1 and SD. Concurrently, DC1 has isotopic signals of δ18O = −4.34 ± 1.32‰ (n = 18) and 87Sr/86Sr = 0.708831 (n = 2), while SD has values of δ18O = −9.57 ± 2.51‰ (n = 15) and 87Sr/86Sr = 0.711464 (n = 3). The large isotopic fractionation between DC1 and SD suggests different dolomitizing fluids. This relationship shows an increase in 87Sr in the fluid as the temperature increases; moreover, the enrichment in 87Sr of the fluid is explained by the interaction of this fluid with rocks from the crystalline basement. Thus, the main conduit for the ascent of this radiogenic fluid would be faults with deep roots spatially close to tectonically active zones. Lastly, the presence of tectonic stylolites cutting cemented breccias and rotated zebra-like strata-bound structures suggests that brecciation occurred before the installation of fragile post-Ordovician transtensional structures, preceding the establishment of Paleozoic Basins on the South American Platform.

New insights into the genesis of the Sipingshan gold deposit in eastern Heilongjiang, north-eastern China: Evidence from geology, U-Pb and Rb-Sr geochronology, geochemistry, and H[sbnd]O[sbnd]S-Pb isotopes

The Sipingshan gold deposit is intermediately sized and located in the eastern Wandashan Terrane. It has attracted much attention for its uncommon genetic type of hot spring. Gold orebodies mainly occur in the upper and lower siliceous rocks, siliceous cemented breccia is found between the upper and lower siliceous rocks, and silicified rhyolite porphyry is observed below the lower siliceous rocks. Two main mineralisation periods (i.e. exhalative sedimentation and hydrothermal superimposition) have been identified at Sipingshan based on mineral paragenesis and crosscutting vein relationships. In this study, analyses of whole-rock geochemical compositions, pyrite Rb–Sr age, zircon U–Pb–Hf contents, and S–PbOH isotopes were conducted to determine the Au mineralisation time and ore genesis of this deposit. The ore-bearing siliceous rock revealed weighted mean 206Pb/238U ages of 121.8 ± 1.1 Ma and 117.6 ± 1.0 Ma, representing the mineralisation age of the exhalative sedimentation period. The Rb–Sr isochron age of pyrites (106 ± 3 Ma) was consistent with the Au–Cu mineralisation age in the Wandashan area and represented magmatic–hydrothermal mineralisation. The siliceous rocks were characterised by high SiO2 (97.92–99.18 wt%), low TiO2 (0.01–0.03 wt%), low Al2O3 (0.22–0.55 wt%), negative δCe (0.78–1.00), positive δEu (1.12–4.04), and low Sb, As, Cs, Cd and Li contents, indicating the genesis of hydrothermal sediments. The OH isotope results suggested that the ore-forming fluids were mainly derived from magmatic water and mixed with meteoric water in the later stage. The δ34S values of pyrites from the siliceous rocks (–25.8 to –8‰) indicated a biogenic sulfur-dominated source, corresponding to the exhalative sedimentation period. The δ34S values of pyrites (9.8 to 29.7 ‰ and –1.9 to –4.7 ‰) formed in the magmatic–hydrothermal period indicated that sulfur was most likely derived from sedimentary rock and magmatic sulfur sources. The Pb isotope data also implied a mixture of upper crust and mantle sources. Therefore, we propose that the Sipingshan gold deposit is a continental exhalative sedimentation-type gold deposit superimposed by later magmatic–hydrothermal fluids, and formed during the subduction of the Paleo-Pacific Plate during the Early Cretaceous.

Quartz veins and silver-containing crushing zones in the exocontact of Solur granite stock (Kularsky district, Yakutia)

The purpose of the work is a investigation of quartz varieties present in quartz veins and crushing zones in order to establish the formation conditions and the possibilities of using quartz varieties to study the distribution of mineralization during exploration. The work was stimulated by the discovery of gold and gold-silver mineralization in adjacent areas and the possibility of using some new methods for studying quartz. Research methods. Samples for the study were taken during the mineralogical mapping of the studied area in the course of work to assess the prospects for its ore potential. The characterization of quartz from veins and crushing zones was carried out on the basis of its field descriptions and laboratory microscopic study, as well as measurements of X-ray luminescence (RL) of vein quartz in the region of 350–550 nm. Fluid inclusions in quartz and sulfur isotope composition in sulfides from quartz veins were also studied. Results and conclusions. The granite stock, surrounded by a zone of contact hornfels with cordierite and andalusite, lies among terrigenous sedimentary rocks that have undergone metagenesis (anhimetamorphism). Veins of glassy coarse-grained quartz and recrystallized (granular) quartz are located directly in the granites, partially recrystallized gray quartz – among the contact cornea, and veins of deformed milky-white quartz - in terrigenous sedimentary rocks outside the contact halo. As the transition from veins located among granites to veins located outside the granite massif, the intensity of quartz RL decreases sharply. The nature of the RL indicates the probable presence in the crystal lattice of quartz from veins in granites of their own defects – vacancies of oxygen or silicon, and in gray and milkywhite quartz – cation-compensated [AlO4 /M+]-centers. Sulfide mineralization in quartz veins is mainly represented by arsenopyrite with an isotopic composition of sulfur close to the meteorite standard. In addition to quartz veins, later crushing zones with hydrothermal injection breccias are also common in the area. In breccia cement and in breccia-intersecting veins, early non-split hypidiomorphic quartz and later split comb-shaped and chalcedony quartz are present. In micro-cryptogrаined chalcedony quartz with grain size up to 0.05 mm contaminated with inclusions of finely crushed rock particles and kaolinite grains, the inclusions of the pyrargyrite – one of the main silver-containing minerals on a number of gold-silver deposits of the region – were found. Based on the known vertical zonality of epithermal gold-silver deposits with the confinement of kaolinite-containing ores and chalcedony quartz to the upper horizons of the deposits, and due to the presence of micro-cryptograined chalcedony quartz containing kaolinite and pyrargyrite in the considered crushing zones, it can be suggested that these crushing zones may be confined to the upper parts of the silver mineralization development zone.

Breccia and vein mineralization of the Balatoc Diatreme, Acupan gold deposit, Baguio Mineral District: An example of a diatreme-hosted epithermal deposit in the Philippines

The Acupan epithermal gold deposit is one of the Philippines’ largest gold camps, having produced over 200 t Au in the last century with average grades of ∼ 6 g/t Au mostly from well-studied vein orebodies hosted by the Virac Granodiorite. However, vein and breccia mineralization (90/99 veins and GW orebodies, respectively) currently being mined in the northeastern portion of the Acupan deposit is hosted by the less-studied Balatoc Diatreme. This study demonstrates the role of the diatreme as a precursor to epithermal mineralization at Acupan, providing structural control in focusing the fluid flow along the diatreme margin. Mineralization in the Balatoc Diatreme formed across five mineralization stages. Stage I, the main gold mineralization stage, is characterized by gray quartz with pyrite + marcasite + arsenopyrite + electrum + sphalerite ± chalcopyrite. Stage II is typified by white quartz associated with pyrite + electrum + chalcopyrite. Stage III, a newly recognized stage for the Acupan gold deposit, is composed of clear quartz hosting pyrite + stibnite + chalcopyrite + galena + sphalerite + electrum (±pyrite + marcasite + arsenopyrite). Stage IV and Stage V are associated with calcite and gypsum, respectively, hosting trace amounts of pyrite and sphalerite.Fluid inclusions hosted by Stage I and Stage II vein quartz revealed homogenization temperatures (Th) ranging from 220 to 230 °C and 280 to 290 °C, respectively. The fluid inclusions in Stage III quartz breccia cement recorded bimodal homogenization temperatures, 230 to 240 °C and 280 to 290 °C. Fluid inclusion and textural evidence from Stage I quartz and Stage II quartz suggest boiling conditions during ore formation of veins transecting the Balatoc Diatreme. Fluid mixing, on the other hand, is proposed for the formation of the base-metal rich Stage III mineralization of the GW orebodies quartz cement. Pyrite and sphalerite δ34S values (0.8 to 1.5 ‰) from Stage I to Stage IV veins and breccias indicate reduced ore-forming conditions in an H2S-dominated system. Meanwhile, the negative δ34S values (−1.6 to −1.5‰) measured from the pyrite of Stage V breccia, first reported in this study, imply partitioning of the heavier isotopes to the gypsum sulfate in an oxidizing environment and suggest possible spatial variations of sulfur isotope signatures across the Acupan epithermal vein system. This study emphasizes the significance of fluid mixing in diatreme-hosted epithermal deposits such as Acupan. The diatreme possibly provided pathways between the contrasting magmatic-hydrothermal and meteoric environments, resulting in ore precipitation.

Geology and Geochronology of the Miocene Rio Blanco Porphyry Cu-Mo Deposit, Northern Peru

Abstract The Rio Blanco porphyry Cu-Mo deposit occurs at the north end of the Miocene metallogenic belt of northern Peru. It has a thick supergene enrichment blanket; while normal for hyperarid Chile, this is unusual in mountainous, cloud forest terrain. Rio Blanco is hosted by the Portachuela batholith. Zircon U-Pb dating shows that the youngest part of the batholith was emplaced at 12.43 ± 0.13 Ma. The deposit formed during three magmatic-hydrothermal cycles. Cycle 1, by far the most important, occurred at 11.50 ± 0.17 to 10.92 ± 0.14 Ma (zircon U-Pb). Two intermineralization intrusions caused early potassic and propylitic alteration. This was then overprinted by a blanket of quartz-sericite, grading down into sericite-chlorite alteration. Cycle 1 was finally cut by a quartz-sericite–cemented breccia, which contains the highest-grade hypogene Cu-Mo grades. A cycle 1 molybdenite-bearing vein has a molybdenite Re-Os model age of 11.43 ± 0.16 Ma. Molybdenite Re-Os dating of the quartz-sericite–cemented breccias shows brecciation occurred at 11.28 ± 0.24 to 11.11 ± 0.18 Ma. Cycle 2 was restricted to the east side, where narrow porphyritic dacite 1 dikes (dated by zircon U-Pb at 10.62 ± 0.16 Ma) show biotite alteration and economic copper. Cycle 3, at 10.02 ± 0.12 to 9.06 ± 0.09 Ma (zircon U-Pb), was triggered by a swarm of NE-striking quartz-plagioclase porphyry and porphyritic dacite 2 dikes. Alteration was milder, and this cycle did not introduce economic copper. Nonmineralized pebble dikes cut the system, emanating from a major diatreme, about 3 × 1.3 km in size, on the north side of the deposit. The magmatic-hydrothermal history spanned about 2.5 m.y., with economic mineralization over about 1.48 m.y. However, metals were mostly introduced during cycle 1, which lasted approximately 0.58 m.y. Our work shows that while multiple magmatic-hydrothermal cycles produced Rio Blanco, sufficient metals were introduced to form a giant porphyry deposit within a single magmatic-hydrothermal cycle.

The microbial controls on the deposition of Pb‐Zn minerals in carbonate‐hosted Tunisian ore deposits

AbstractThe Ain Allega, Mjar Hannech and Sidi Driss Pb‐Zn‐Ba‐Sr‐As‐(Hg) deposits in northern Tunisia are hosted in a post‐nappe anticline with a core of a Triassic evaporite diapir affected by the NE–SW‐trending Ghardimaou‐Cap Serrat and E‐W lineament. The ore minerals show different styles; particularly, impregnation in dolomite, cement of breccias, replacement ore and open space filling in the dissolution cavities and fractures. Ore minerals include sphalerite, galena, marcasite, pyrite and cinnabar. Principal gangue minerals are composed of barite, celestine, calcite, dolomite and quartz. The orebodies are hosted by Triassic carbonate rocks (Ain Allega ore deposits), Cretaceous carbonate rocks (Mjar Hannech ore deposits) and by the volcanoclastic layers (Sidi Driss ore deposits). These host rocks show hydrothermal alteration, dissolution and brecciation. Sphalerite in the carbonate‐hosted Zn‐Pb deposits in Tunisia occurs as nano‐size microglobular blebs and peloids and forms finely‐laminated bands with wavy and even ripped up features. Peloids are present in collapse breccias of karst cavities. Peloids (5–100 μm) and bacterial filaments (1 μm long) locally form the common micro‐texture of the sulfide mineralization. The core and the shell of peloids are composed of different minerals; there are 1–3 shells and each shell is 1–1.5 μm thick, commonly with coarse crystals. These textures probably represent fossil microbial mats as in‐situ sulfate‐reducing bacteria products and are similar to nano‐textures observed in bacterial biofilms. Electron microscopy, combined with major element changes (Zn, Pb, As, Fe, and Cd) and sulfur isotope values suggest that sphalerite nano‐textures and nano‐spheres are associated with fossilized bacterial‐mat. The δ34S values of sphalerite (−43.9 to +22.38‰) show that the microbial production of H2S controlled sphalerite precipitation. The biogenic nano‐ to macro‐textured sphalerite and sulfur isotope data suggest that microbes had an important role in the genesis of carbonate‐hosted Zn‐Pb deposits in Tunisia during the Miocene.

Geochemical and microtextural properties of veins in a potential high-level radioactive waste disposal site

In this study, geochemical and petrographic characteristics of veins from the Permian Boda Claystone Formation (Mecsek Mts., Hungary) are presented. Understanding the connection between these properties is vital in the reconstruction of paleofluid history and tectonic evolution of this potential high-level radioactive waste disposal site. Each of the observed four vein generations consists of several mineral phases and has complex evolution, which can be attributed to multiple diagenetic and tectonic processes. Syntaxial and antitaxial veins have been observed suggesting oscillatory advective and diffusive material transport mechanisms; however, veins associated with mobile hydrofractures have also been detected. The parent fluids with a predominant temperature of 100–150 °C and variable salinity (3.5–13.7% wNaCleq) originated potentially by mixing of connate brine with freshwater released by smectite-to-illite transition and isotope exchange with sedimentary minerals, which may have caused the observed δ18O values (−1.12 to 4.67‰, V-SMOW). In contrast to the most vein-filling mineral phases, a breccia cement phase precipitated from a low-temperature (<50 °C) and salinity (0.0–0.4% wNaCleq) meteoric fluid. In the pores of this breccia veins, late-stage fluid migration is verified. Therefore, these veins are of paramount importance for studying the isolation properties of the rock body.

Syn‐rift hydrothermal circulation in the Mesozoic carbonates of the western Adriatic continental palaeomargin (Western Southalpine Domain, NW Italy)

AbstractEvidence of hydrothermal activity is reported for the Mesozoic pre‐ and syn‐rift successions of the western Adriatic palaeomargin of the Alpine Tethys, preserved in the Western Southalpine Domain (NW Italy). The products of hydrothermal processes are represented by vein and breccia cements, as well as dolomitization and silicification of the host rocks. In the eastern part of the study area, interpreted as part of the necking zone of the continental margin, Middle Triassic dolostones and Lower Jurassic sediments are crossed by veins and hydrofracturing breccias cemented by saddle dolomite. The precipitation of dolomite cements occurred within the stratigraphic succession close to the sediment–water interface. Despite the shallow burial depth, fluid inclusion microthermometry and clumped isotopes show that hydrothermal fluids were relatively hot (80–150°C). In the western part of the study area, interpreted as part of the hyperextended distal zone, a polyphase history of host‐rock fracturing is recorded, with at least two generations of veins cemented by calcite, dolomite and quartz. Vein opening and cementation occurred at shallow burial depth around the time of deposition of the syn‐rift clastic succession. Fluid inclusion microthermometry on both quartz and dolomite cements indicates a fluid temperature of 90–130°C, again pointing to hydrothermal fluids. Both in Fenera‐Sostegno and Montalto Dora areas, O, C and Sr isotope values, coupled with fluid inclusion and clumped isotope data, indicate that hydrothermal fluids derived from seawater interacted with crustal rocks during hydrothermal circulation. Stratigraphic and petrographic evidence, and U–Pb dating of dolomitized clasts within syn‐rift sediments, document that hydrothermal fluids circulated through sediments from the latest Triassic to the Toarcian, corresponding to the entire syn‐rift evolution of the western portion of the Adriatic palaeomargin. The documented hydrothermal processes are temporally correlated with regional‐scale thermal events that took place in the same time interval at deeper crustal levels.

Mineralogy of hydrothermal breccia cement of Humpa Leu East porphyry copper-gold prospect, Sumbawa Island, Indonesia

Indonesia is a country that has several world-class copper-gold deposits, particularly in eastern Sunda arc. The Hu’u complex has several prospects in the surface as lithocap of extensive epithermal style alteration, but some were detected associated with porphyry beneath the surface. The study focuses on hydrothermal breccia cement as a factor influencing the porphyry system in the Hu’u district. The methods used is mineralogical analysis with petrography and μ-XRF elemental mapping on hydrothermal breccia samples. The Hu’u district is interpreted as a paleo-volcano; a member of the Old Volcanics Rocks Formation. The Humpa Leu East lithology consists of pre-volcanics unit (lava and pyroclastics), diorite, andesite-μ-diorite, and tonalitic intrusion at the depth. Hydrothermal alteration evolved from tonalite body to outward, consist of potassic, inner propyllitic and overprinted by phyllic and advanced argillic. Several phases of hydrothermal activities occur in this system, including the hydrothermal breccia phase associated with complex fluids. The hydrothermal cement of Humpa Leu East porphyry at least have three phases of mineralogical assemblages and possibly influencing the mineralization. The mineralogical assemblage of hydrothermal cement in HLE consists of quartz-feldspar-plagioclase-biotite as a high-temperature phase; then followed by epidote-sericite-chlorites-anhydrite-carbonates in medium temperature; there are aluminum-rich clay minerals interpreted as gibbsite. Mineralization occurs in three phases including chalcopyrite-magnetite, bornite-chalcopyrite and chalcopyrite-sphalerite phases. The occurrences of chalcopyrite at all phases indicate the stability of intermediate sulfidation proses in Humpa Leu and as a possible factor to answer the abundant copper in the Hu’u hydrothermal fluid system

The late Miocene Öksüt high sulfidation epithermal Au-Cu deposit, Central Anatolia, Turkey: Geology, geochronology, and geochemistry

The Öksüt high sulfidation (HS) epithermal Au (-Cu) deposit (35.32 million tonnes of ore at 1.22 g/t Au) is located in the Central Anatolian Volcanic Province (CAVP) at the south-eastern segment of the Central Anatolian Crystalline Complex (CACC). The deposit is hosted by large hydrothermal breccia bodies and hornblende-rich basaltic andesite porphyry of the Develidag Volcanic Complex (DVC), which were pervasively altered to form silicification and argillic assemblages. The host rocks and mineralization are covered by post-mineral pyroxene-rich basaltic andesite of DVC. The volcanic rocks have a medium-K calc-alkaline affinity similar to regional volcanic rocks of the DVC. The gold and copper mineralization is mostly associated with quartz and primary sulfides hosted by silicification and argillic alteration particularly quartz-alunite cemented breccias and subsequently enriched by supergene oxidation processes causing destabilization of the primary sulfides.The LA-ICP-MS zircon U-Pb geochronology indicates a range of 206Pb/238U ages between 5.4 and 6.0 Ma with an average mean age of autocrystic grains of 5.73 ± 0.06 Ma for the host hornblende-rich basaltic andesite porphyry and 5.67 ± 0.07 Ma for the post-mineralization basaltic andesite cover. The high-precision CA-ID-TIMS analysis of the youngest grains from both rock types defines ages 5.778 ± 0.013 Ma and 5.700 ± 0.019, respectively, which bracket the ore mineralization between 5.791 and 5.681 Ma, including the error uncertainty. Consequently, the Öksüt gold mineralization took place in a very narrow lifetime of 78 ± 32 Ka. This is the youngest age for an economic gold deposit in Turkey.

Carbonates at the supergiant Olympic Dam Cu-U-Au-Ag deposit, South Australia. Part 1: Distribution, textures, associations and stable isotope (C, O) signatures

The supergiant Olympic Dam Cu-U-Au-Ag deposit in South Australia is a type example of the iron-oxide copper–gold (IOCG) deposit family. Hosted entirely within heterogeneous breccia in 1.59 Ga granite, the deposit contains a volumetrically substantial and mineralogically diverse component of carbonate minerals. Carbonate minerals are always associated with ore minerals (sulfides, uraninite), implying a genetic relationship and providing an opportunity to use gangue carbonates to better understand ore formation. This study provides the first detailed and comprehensive petrographic and chemical/isotopic study of Olympic Dam carbonates, with a particular emphasis on petrography and texture, and an attempt is made to relate carbonate formation to local and regional events that have affected Olympic Dam.Based on a set of 196 carbonate-bearing samples, carbonate minerals are observed in all lithologies present at Olympic Dam. Carbonates occur as cement in breccia and conglomerates, as breccia clasts, in veins crosscutting ore-rich breccia and other rock types, in pores and ooids, and in the form of laminated carbonate. Siderite and siderite-rhodochrosite-magnesite solid solution are by far the most common carbonate types, whereas calcite, dolomite-ankerite solid solution and REE-fluorocarbonates are locally abundant. Single carbonate grains typically show compositional zones (simple or oscillatory) and replacement textures (including mutual replacement of carbonates with other carbonates and with hematite) are common. In the absence of consistent, deposit-wide paragenetic relationships, the carbonates were placed in seven associations based on host rock, mineralogy and texture: (1) coarse-grained calcite veins in weakly brecciated granite and rhyolite, (2) carbonates in strongly brecciated granite, (3) carbonate veins in bedded clastic facies, (4) carbonates in mafic and ultramafic igneous rocks, (5) massive barite-fluorite-dominated veins with minor carbonate, (6) laminated siderite, and (7) carbonate matrix in conglomerate-breccia-sandstone above the unconformity. Some of these associations can be related to regional tectonic events based on local context and relationships with dated assemblages. δ13C (−6.5‰ to −2‰) values for the carbonates show a relatively limited range whereas δ18O is more variable (+9.4‰ to + 20.9‰). C-O isotopic compositions for the various carbonate associations tend to overlap, suggestive of mixed fluid sources, recycling of older carbonate and perhaps other fractionation processes. The C-O isotope data overlap the compositional fields of several major carbon–oxygen reservoirs (magmatic, sedimentary) and carbon sources in local granite, felsic volcanics, older BIF and sedimentary rocks are all possible at different stages of carbonate deposition.

Hydrothermal Precipitation of Sanidine (Adularia) Having Full Al,Si Structural Disorder and Specular Hematite at Maunakea Volcano (Hawai'i) and at Gale Crater (Mars)

AbstractHydrothermal high sanidine and specular hematite are found within ferric‐rich and gray‐colored cemented basaltic breccia occurring within horizontal, weathering‐resistant strata exposed in an erosional gully of the Pu'u Poliahu cinder cone in the summit region of Maunakea volcano (Hawai'i). The cone was extensively altered by hydrothermal, acid‐sulfate fluids at temperatures up to ~400 °C, and, within strata, plagioclase was removed by dissolution from progenitor Hawaiitic basalt, and sanidine and hematite were precipitated. Fe2O3T concentration and Fe3+/∑Fe redox state are ~12 wt.% and ~0.4 for progenitor basalt and 46–60 wt.% and ~1.0 for cemented breccias, respectively, implying open‐system alteration and oxic precipitation. Hydrothermal high sanidine (adularia) is characterized by full Al,Si structural disorder and monoclinic unit‐cell (Rietveld refinement): a = 8.563(19) Å, b = 13.040(6) Å, c = 7.169(4) Å, β = 116.02(10)°, and V = 719.4(19) Å3. Hematite (structure confirmed by Rietveld refinement) is the predominant Fe‐bearing phase detected. Coarse size fractions of powdered hematite‐rich breccia (500–1000 μm) are dark and spectrally neutral at visible wavelengths, confirming specular hematite, and SEM images show platy to polyhedral hematite morphologies with longest dimensions &gt;10 μm. Smectite and 10‐Å phyllosilicate, both chemically dominated by Mg as octahedral cation, are additional diagenetic hydrothermal alteration products. By analogy and as a working hypothesis, high sanidine (Kimberly formation) and specular hematite (Mt. Sharp group at Hartmann's Valley and Vera Rubin ridge) at Gale crater are interpreted as diagenetic alteration products of Martian basaltic material by hydrothermal processes.

Structural Controls on Shallow Cenozoic Fluid Flow in the Otago Schist, New Zealand

The Otago Schist in the South Island of New Zealand represents an exhumed Mesozoic accretionary prism. Two coastal areas (Akatore Creek and Bruce Rocks) south of Dunedin preserve structural and geochemical evidence for the development of postmetamorphic hydrothermal systems that involved widespread fluid-rock reaction at shallow crustal depths. The Jurassic to Triassic pumpellyite-actinolite (Akatore Creek) to upper greenschist facies (Bruce Rocks) metamorphic fabrics were crosscut by sets of regionally extensive Cretaceous exhumation joints. Many of the joints were subsequently reactivated to form networks of small-displacement (&lt;metres) strike-slip faults containing cemented fault breccias and veins composed of hydrothermal calcite, siderite, and ankerite. Paleostress analysis performed on infrequent fault slickenlines indicates an overall strike-slip paleostress regime and a paleo-σ1 orientation (azimuth 094°) similar to the contemporary σ1 orientation in Otago and Canterbury (azimuth c. 110°-120°). High δ18O values in vein calcite (δ18OVPDB=21 to 28‰), together with the predominance of Type I calcite twins, suggest that vein formation occurred at low temperatures (&lt;200°C) in the shallow crust and was associated with strongly channelized fluid flow along the joint and fault networks. Mass-balance calculations performed on samples from carbonate alteration zones show that significant mobilisation of elements occurred during fluid flow and fluid-rock reaction. Whole-rock and in situ carbonate 87Sr/86Sr data indicate varying degrees of interaction between the hydrothermal fluids and the host rock schists. Fluids were likely derived from the breakdown of metamorphic Ca-rich mineral phases with low 87Rb in the host schists (e.g., epidote or calcite), as well as more radiogenic components such as mica. Overall, the field and geochemical data suggest that shallow fluid flow in the field areas was channelized along foliation surfaces, exhumation joints, and networks of brittle faults, and that these structures controlled the distribution of fluid-rock reactions and hydrothermal veins. The brittle fault networks and associated hydrothermal systems are interpreted to have formed after the onset of Early Miocene compression in the South Island and may represent the manifestation of fracturing and fluid flow associated with reverse reactivation of regional-scale faults such as the nearby Akatore Fault.

The Productora Cu-Au-Mo Deposit, Chile: A Mesozoic Magmatic-Hydrothermal Breccia Complex with Both Porphyry and Iron Oxide Cu-Au Affinities

Abstract The Productora Cu-Au-Mo deposit is hosted by a Cretaceous hydrothermal breccia complex in the Coastal Cordillera of northern Chile. The current resource, which includes the neighboring Alice Cu-Mo porphyry deposit, is estimated at 236.6 Mt grading 0.48% Cu, 0.10 g/t Au, and 135 ppm Mo. Local wall rocks consist of a thick sequence of broadly coeval rhyolite to rhyodacite lapilli tuffs (128.7 ± 1.3 Ma; U-Pbzircon) and two major intrusions: the Cachiyuyito tonalite and Ruta Cinco granodiorite batholith (92.0 ± 1.0 Ma; U-Pbzircon). Previous studies at Productora concluded the deposit had strong affinities with the iron oxide copper-gold (IOCG) clan and likened the deposit to Candelaria. Based on new information, we document the deposit geology in detail and propose a new genetic model and alternative classification as a magmatic-hydrothermal breccia complex with closer affinities to porphyry systems. Hydrothermal and tectonic breccias, veins, and alteration assemblages at Productora define five paragenetic stages: stage 1 quartz-pyrite–cemented breccias associated with muscovite alteration, stage 2 chaotic matrix-supported tectonic-hydrothermal breccia with kaolinite-muscovite-pyrite alteration, stage 3 tourmaline-pyrite-chalcopyrite ± magnetite ± biotite-cemented breccias and associated K-feldspar ± albite alteration, stage 4 chalcopyrite ± pyrite ± muscovite, illite, epidote, and chlorite veins, and stage 5 calcite veins. The Productora hydrothermal system crosscuts earlier-formed sodic-calcic alteration and magnetite-apatite mineralization associated with the Cachiyuyito stock. Main-stage mineralization at Productora was associated with formation of the stage 3 hydrothermal breccia. Chalcopyrite is the dominant hypogene Cu mineral and occurs predominantly as breccia cement and synbreccia veins with pyrite. The Alice Cu-Mo porphyry deposit is characterized by disseminated chalcopyrite and quartz-pyrite-chalcopyrite ± molybdenite vein stockworks hosted by a granodiorite porphyry stock. Alice is spatially associated with the Silica Ridge lithocap, which is characterized by massive, fine-grained, quartz-altered rock above domains of alunite, pyrophyllite, and dickite. Rhenium-Os dating of molybdenite indicates that main-stage mineralization at Productora occurred at 130.1 ± 0.6 Ma, and at 124.1 ± 0.6 Ma in the Alice porphyry. Chalcopyrite and pyrite from Productora have δ34Ssulfide values from –8.5 to +2.2‰, consistent with a magmatic sulfur source and fluids evolving under oxidizing conditions. No significant input from evaporite- or seawater-sourced fluids was detected. Stage 3 tourmalines have average initial Sr of 0.70397, consistent with an igneous-derived Sr source. The Productora magmatic-hydrothermal breccia complex formed as a result of explosive volatile fluid release from a hydrous intrusive complex. Metal-bearing fluids were of magmatic affinity and evolved under oxidizing conditions. Despite sharing many similarities with the Andean IOCG clan (strong structural control, regional sodic-calcic alteration, locally anomalous U), fluid evolution at the Productora Cu-Au-Mo deposit is more consistent with that of a porphyry-related magmatic hydrothermal breccia (sulfur-rich, acid alteration assemblages and relatively low magnetite contents, &amp;lt;5 vol %). The Productora camp is an excellent example of the close spatial association of Mesozoic magnetite-apatite, porphyry, and magmatic-hydrothermal breccia mineralization styles, a relationship seen throughout the Coastal Cordillera of northern Chile.

Genesis and age of the Toudaoliuhe breccia-type gold deposit in the Jiapigou mining district of Jilin Province, China: Constraints from fluid inclusions, H–O–S–Pb isotopes, and sulfide Rb–Sr dating

The Jiapigou mining district (JMD) in the northeastern part of the North China Block mainly contains quartz vein- and altered rock-type gold deposits that have been prospected and mined for over 200 years. Recently, a breccia-type gold deposit, known as Toudaoliuhe, was discovered in the JMD, that has attracted attention for prospecting and exploration. In this paper, we report the ore geology, gold-bearing sulfide (i.e., pyrite, galena, and sphalerite) Rb–Sr age, fluid inclusions (FIs), and H–O–S–Pb–Sr isotope data of the Toudaoliuhe gold deposit to determine its genesis and ore-forming mechanisms. Gold orebodies mainly occur in the breccia cements, as well as in several quartz veins controlled by NW-trending brittle faults. There were three hydrothermal stages: the early (quartz–pyrite), main (quartz–polymetallic sulfide), and late (quartz–carbonate) stages. Gold mineralization occurred in the main hydrothermal stage. Three types of FIs were identified: CO2–H2O–NaCl (C-type), H2O–NaCl (W-type), and pure CO2 (PC-type). The FIs in quartz from the early hydrothermal stage are predominantly of the C- and W-types (with traces of PC-type), and have homogenization temperatures of 299.8–340.0 °C and salinities of 6.5–14.8 wt% NaCl equivalent (E). The FIs in the quartz and sphalerite forming the main hydrothermal stage are also predominantly of the C- and W-types (with trace PC type), and have homogenization temperatures of 169.1–298.7 °C and salinities of 5.7–16.5 wt% E. The FIs in the quartz and calcite from the late hydrothermal stage were solely of the W-type, and have homogenization temperatures of 126.0–210.2 °C and salinities of 2.1–11.4 wt% E. The ore-forming fluids are characterized by moderate temperature and low salinity, suggesting a CO2–H2O–NaCl system. The H–O–S–Pb–Sr isotopic results suggest that the ore-forming fluids were dominated by magmatic water, and that the ore-forming materials were mainly extracted from the Middle Jurassic magmatic reservoir (177–163 Ma), sourced mainly from the lower crust with trace mantle components. Fluid boiling was the dominant mechanism for gold and associated sulfide precipitation. The Rb–Sr isochron age of the gold-bearing sulfides at 177.7 ± 1.7 Ma indicates that gold mineralization occurred in the early Middle Jurassic. We therefore proposed that the Toudaoliuhe is a mesothermal gold deposit that formed in an extensional setting related to the subduction of the Paleo-Pacific Plate beneath the Eurasian continent.

Mineralogy and prospect of noble metals of gold-bearing ore breccias from ore fields of polymetallic deposits of Ore Altai

The object of the present study are the ore-bearing breccias of the worked-out lower-middle Devonian gold-silver barite-polymetallic Zmeinogorsky deposit belonging to the cognominal ore district and the conserved massive sulfide Chekmar deposit belonging to the Leninogorsk ore district. Both ore districts occupy an adjacent position within the northeastern depression zone of the Ore Altai megatrough. A brief structural and geological description of near-fault ore-bearing breccias, which developed within the peripheral parts of the deposits, is given. Analysis of petrographic composition of breccia fragments and their cementing material allowed to define barite-quartzite breccias of the Zmeinogorsky deposit and pyritized adularia-quartz metasomatites formed upon acidic igneous rocks and greenschists of the Chekmar deposit. Results of mineralogical studies with use of raster electron microscope show that there are ore minerals microinclusions in the detrital material and cement of breccias. Barite-quartzite breccias of Zmeinogorsk deposit contain barite, galena, Hg and Sb containing polymineral formations, tetrahedrite-boulangerite, bromargyrite, leucoxene. Microquartzites of these breccias mostly consist of quartz. Chekmar deposit ore breccias detrital material contains pyrite, barite, arsenopyrite, galena, sphalerite, zircon, titanite, leucoxene and REE minerals. Quartzitic compound of these breccias is cryptocrystalline and consists of quartz and adularia with admixtures of sericite and pyrite. Due to results obtained by use of SEM it was stated that barite, sulfides and sulfosalts of breccias contain noticeable amounts of dissipated Au, Pd, Pt, Ir. ICP-MS results confirm that studied ore breccias contain such noble metals as Au, Ag, Pd, Pt. Moreover, noble metals contents in Zmeinogorsk deposit breccias reach industrial standards while noble metals of Chekmar deposit ores can be extracted along with the processing of basic massive sulfide ores. Further study of ore breccias is important as these breccias can be considered as a noble metals containing subformation among polymetallic ores of Ore Altai.

Utilization of pre-existing competent and barren quartz veins as hosts to later orogenic gold ores at Huangjindong gold deposit, Jiangnan Orogen, southern China

Most, if not all, orogenic gold deposits form during the late stages of deformation. In some cases, there is barren pre-mineralization hydrothermal alteration which may be part of a single progressive hydrothermal event or a temporally distinct event relative to deposition of gold. At the Huangjindong orogenic gold deposit in the Jiangnan Orogen of southern China, there are two phases of vein generation, the earlier of which is largely barren. Structural analysis, petrography, mineralogy, and in situ laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) gold mapping of different generations of gold-bearing sulfides reveal the relationship between different phases of hydrothermal alteration and quartz veins. Pre-existing barren and brittle quartz veins, which are controlled by tight folds and thrust faults, provide favorable competent hosts for later overprinting by gold-bearing ore-bearing fluids that infiltrated during ongoing reverse-sinistral fault movement. These overpressured fluids caused hydrofracturing to complete brecciation of segments of the earlier quartz veins and associated pre-existing breccias, with deposition of auriferous quartz-sulfide veins and breccia cements or matrices within them. Although fluid inclusion data are non-definitive, fluid pressure fluctuations are interpreted to have resulted in phase separation within the ore fluid which was primarily responsible for destabilization of gold complexes and deposition of native gold with other invisible gold-bearing ore-related sulfides formed by sulfidation reactions, in hydrofractures and breccia matrices. Thus, competent barren quartz veins in fold/thrust belts may locally host superimposed gold mineralization and provide favorable targets for gold resources.

Alunite 40Ar/39Ar and Zircon U-Pb Constraints on the Magmatic-Hydrothermal History of the Zijinshan High-Sulfidation Epithermal Cu-Au Deposit and the Adjacent Luoboling Porphyry Cu-Mo Deposit, South China: Implications for Their Genetic Association

AbstractThe large Zijinshan high-sulfidation epithermal Cu-Au deposit, together with the adjacent Luoboling porphyry Cu-Mo deposit, constitutes a major porphyry-epithermal ore district in South China. Current debate centers on whether the Zijinshan and the adjacent Luoboling deposits are cogenetic or represent separate ore-forming events, which is a question of importance for exploration in the district. In this contribution, the magmatichydrothermal history of the relationship between Zijinshan and Luoboling is reconstructed based on new alunite 40Ar/39Ar ages from Zijinshan and zircon U-Pb ages of ore-related intrusions from both deposits. This study has been complemented by S isotope analysis on the dated alunite to assess their origin.Three types of coexisting alunite and sulfide assemblages exist at Zijinshan, namely, (1) alunite-quartz-covellite cemented breccias; (2) alunite-digenite veins and (3) banded alunite-pyrite veins. Their field occurrences and S isotope features suggest a magmatic-hydrothermal origin for alunite-quartz-covellite cemented breccias and alunite-digenite veins, whereas the origin of alunite-pyrite veins is likely to be related to magmatic steam. Given the intimate textural coexistence between sulfides and alunite, four undisturbed 40Ar/39Ar plateau ages obtained from alunite-quartz-covellite cemented breccia and alunite-digenite vein-type alunite define the timing of Zijinshan high-sulfidation mineralization from 102.86 ± 0.61 to 101.19 ± 0.60 Ma. These agree with bracketing zircon U-Pb ages of pre-ore dacite porphyry at 104.8 ± 0.9 Ma and 104.7 ± 0.5 Ma and a zircon U-Pb age of a post-ore granite porphyry dike at 99.5 ± 0.7 Ma. Combined with their field occurrences, the four alunite ages may imply episodic hydrothermal pulses and a possible time span of over 500 k.y. for the overall high-sulfidation mineralization. Two alunite samples from alunite-pyrite veins yield a slightly disturbed 40Ar/39Ar plateau age at 101.67 ± 0.61 Ma and an apparently undisturbed age at 99.91 ± 0.59 Ma, probably reflecting partial or complete thermal resetting related to the coeval granite porphyry dikes. At Luoboling, zircon U-Pb analysis yields an age of 133.6 ± 1.1 Ma for a dark, ore vein-bearing quartz-diorite porphyry sample and confirms the petrographic observation that they are xenoliths of early wall rocks for the porphyry mineralization. A granodiorite porphyry sample with abundant A-veins is interpreted as an intermineral porphyry phase, dated at 106.5 ± 1.4 Ma. This age is interpreted as the upper limit for porphyry Cu-Mo mineralization and agrees well with previously reported molybdenite Re-Os and biotite 40Ar/39Ar ages, which collectively suggest porphyry Cu-Mo mineralization and potassic alteration at Luoboling were formed within the interval between ca. 106 and 104 Ma.Taken together, the age of the Zijinshan high-sulfidation mineralization is substantially younger than the porphyry mineralization at Luoboling, and we conclude that there is no direct genetic link between the two deposits. However, the Zijinshan Cu-Au deposit is temporally associated with the deep porphyritic granodiorite, which gives zircon U-Pb ages from 102.1 ± 0.8 to 101.8 ± 0.5 Ma, overlapping with the alunite 40Ar/39Ar ages for the high-sulfidation mineralization. This finding has important implications for the ongoing exploration for the two mineralization types in the ore district and elsewhere.

Au and Te Minerals in Seafloor Massive Sulphides from Semyenov-2 Hydrothermal Field, Mid-Atlantic Ridge

The Semyenov-2 hydrothermal field located at 13°31′N of the Mid-Atlantic Ridge (MAR) is associated with an oceanic core complex (OCC) and hosted by peridotites and basalts with minor amounts of gabbro and plagiogranites. Seafloor massive sulphides (SMS) are represented by chimneys with zonality, massive sulphides without zonality and sulphide breccia cemented by opal and aragonite. The mean value of Au (20.6 ppm) and Te (40 ppm) is much higher than average for the MAR SMS deposits (3.2 ppm and 8.0 ppm, respectively). Generally, these high concentrations reflect the presence of a wide diversity of Au and Te minerals associated with major mineral paragenesis: primary native gold, melonite (NiTe2) and tellurobismuthite (Bi2Te3) are related to high-temperature chalcopyrite (~350 °C); electrum (AuAg)1, hessite (Ag2Te) and altaite (PbTe) are related to medium- and low-temperature Zn-sulphide and opal assemblages (260–230 °C). Calaverite (AuTe2) and Te-rich “fahlore” Cu12(Sb,As,Te)4S13 are texturally related to the chalcopyrite-bornite-covellite. Enrichment of Au in sulphide breccia with opal and aragonite cement is driven by the re-deposition and the process of hydrothermal reworking of sulphide. The low-temperature fluid mobilizes gold from primary sulphide, along with Au and Te minerals. As a result, the secondary gold re-precipitate in cement of sulphide breccia. An additional contribution of Au enrichment is the presence of aragonite in the Cu-Zn breccia where the maximal Au content (188 ppm) is reached.

Chemical and boron isotope compositions of tourmaline in the Hadamiao porphyry gold deposit, Inner Mongolia, China

The Hadamiao porphyry gold deposit in Inner Mongolia, China, is located in the northern margin of the North China Craton (NCC). Tourmaline is a common mineral in hydrothermal veins and granitic rocks contemporaneous with the mineralized granite porphyry in this deposit. The tourmaline can be divided into six types, including magmatic tourmaline (Tur-M1) in granite porphyry dikes, magmatic-hydrothermal transitional tourmaline (Tur-M2) in nodular granitic rocks, and hydrothermal tourmalines in early tourmaline veins (Tur-V1), gold-bearing quartz-tourmaline-chalcopyrite veins (Tur-V2), breccia cements (Tur-V3) and late carbonate-quartz veins (Tur-V4). The gold mineralization is closely related to the occurrence of Tur-V2.Almost all the tourmalines from the Hadamiao deposit belong to the alkali group and show a schorlitic–dravitic composition, except for some Tur-M1 belonging to uvite. The concentrations of most elements in different tourmaline types overlap. Tur-M1 has higher Ca, W, Th, U, Pb, Mn, and Zr contents than other types, and the hydrothermal tourmalines (V1–V4) show slightly higher Ba, Cu and Y contents than Tur-M1 and Tur-M2. The concentration of Au in Tur-M1 and Tur-M2 can reach up to 80 ppb and is higher than that in Tur-V1; the Au content in Tur-V2 is an order of magnitude higher than that in other stages of tourmaline. Systematic contrasts in major and trace element compositions between magmatic and hydrothermal tourmalines are attributed to the assimilation of the country rock by the magmatic-hydrothermal system.The six types of tourmaline exhibit variable δ11B values ranging from −8.7‰ to +21.2‰. Tur-M1 shows the highest δ11B values of +15.1‰ to +21.2‰, and Tur-V2 yields the lowest values ranging from−8.7‰ to −5.4‰. The boron probably came from two sources with different isotopic compositions, one with high positive δ11B values and the other with negative δ11B values. Magmatic hydrothermal fluids could have provided isotopically light boron for hydrothermal tourmalines, whereas the heavy boron in magmatic tourmaline was probably derived from marine carbonates that are widespread in the study area. The magma related to the mineralization and the one containing magmatic tourmaline may have experienced different degrees of interaction with wall rocks, resulting in the diversity of δ11B values in Hadamiao tourmaline.