Carbonaceous Sedimentary Rocks Research Articles

Sulphide petrology and ore genesis of the stratabound Sheep Creek sediment-hosted Zn–Pb–Ag–Sn prospect, and U–Pb zircon constraints on the timing of magmatism in the northern Alaska Range

The Sheep Creek prospect is a stratabound Zn–Pb–Ag–Sn massive sulfide occurrence in the Bonnifield mining district, northern Alaska Range. The prospect is within a quartz–sericite–graphite–chlorite schist unit associated with Devonian carbonaceous and siliceous metasedimentary rocks. Volcanogenic massive sulfide (VMS) deposits in the district are hosted in felsic metavolcanic rocks (362 ± 2 Ma) associated with siliciclastic and carbonaceous sedimentary rocks that overlie the stratigraphic sequence hosting the Sheep Creek prospect. Felsic metaigneous rocks in underlying units are 372 ± 4 to 366 ± 4 Ma. Sheep Creek is atypical of the other sulfide deposits in the district in (1) having Sn grades up to 1.2%; (2) being contained in fine-grained, quartz-rich rocks and quartz–pebble conglomerate that likely originated as chert and chert-clast sediment, respectively; and (3) showing minimal evidence of volcanic components in the host rocks. Comparison of immobile trace-element proportions for graphitic and siliceous rocks from the Sheep Creek area with those for argillite associated with the Bonnifield VMS deposits indicates a continental volcanic-arc provenance for the former and a within-plate and passive margin provenance for the latter. In contrast to previously published interpretations, our data analysis supports a clastic-dominated (CD) rather than a VMS affinity for the Sheep Creek prospect. In our model, Zn–Pb–Ag–Sn mineralization formed by syngenetic or early diagenetic processes on or beneath the seafloor, possibly in the shallow-water environment of an outer continental shelf setting. Potential analogues are the Paleozoic CD deposits in the Canadian Selwyn Basin outboard of the Laurentian continental margin.

Genesis of the Shibaogou Mo–Pb–Zn deposit in the Luanchuan ore district, China: Constraints from geochronology, fluid inclusion, and H–O–S–Pb isotopes

The Shibaogou deposit is located in the Luanchuan ore district within the East Qinling orogenic belt (EQOB), central China, which is a newly discovered Mo–Pb–Zn skarn deposit. The skarn and Mo–Pb–Zn ore bodies are mostly hosted in the contact zones between the Shibaogou porphyritic granite and carbonaceous sedimentary rocks from the Luanchuan and Guandaokou sets. A study combined of geochronology, fluid inclusion (FI), and stable isotopes was performed to constrain the mineralization age, source of ore materials, and the origin and evolution of the ore-forming fluids and their relationship with the subduction of the Paleo-Pacific Plate. The mineralization process includes skarn and quartz–sulfide episodes, which has four stages: skarn (I), quartz–molybdenite (II), quartz–galena–sphalerite (III), and quartz–calcite (IV). Molybdenite Re-Os dating suggests that the deposit was formed in the Late Jurassic (147.4 ± 7.2 Ma). Reportedly, there are five primary types of fluid inclusions: L-type, V-type, H-type, S-type, and C-type. In the skarn stage, coexisting H-type (35.58 wt%–46.05 wt% NaCl equiv.) and low-salinity V-type (0.35 wt%–5.7 wt% NaCl equiv.) fluid inclusions show similar homogenization temperatures, which suggests that fluid boiling occurred at 513–550°C and 580–650 bar (2.19–2.45 km). In the quartz–molybdenite stage, the homogenization temperatures of L-type, V-type, minor H-type, and S-type fluid inclusions indicate continued fluid boiling at 324–387°C and 180–250 bar (0.49–0.94 km). In the quartz–galena–sphalerite stage, a fewer number of coexisting V-type and L-type fluid inclusions in quartz shows different salinities with similar homogenization temperatures, indicating that they are trapped at 303–347°C and <150 bar in the boiling process (<0.56 km hydrostatic depth). The minor primary L-type fluid inclusions that have lower salinities of 0.88 wt%–11.34 wt% NaCl equiv were observed in quartz and calcite in the quartz–calcite stage; in addition, their homogenization temperatures are 103–247°C (typical post-ore conditions). This study found that the ore-forming fluids at the Shibaogou deposit were dominantly magmatic water at the early stage, with input of atmospheric water during fluid evolution, with δ18Ofluid values from −1.168‰ to 8.997‰ and δ18Dfluid values from −106.5‰ to −79.9‰, based on the O and H isotope data from garnet, quartz, and calcite. Furthermore, the S isotopic compositions were measured ranging from 0.8‰ to 14.7‰, and it demonstrated that the ore-forming fluid was mainly derived from magmatic sources. The relatively homogeneous Pb isotopic compositions are similar to those of Shibaogou granite porphyry, which demonstrated that the ore-forming materials were mainly derived from magmatic sources. Molybdenite was precipitated as a result of fluid–rock interactions and fluid boiling, and the galena and sphalerite were precipitated as a result of the decreasing temperature. The subduction of the Paleo-Pacific plate has a critical impact on the complex evolution of ore formation in the Shibaogou skarn deposit in EQOB.

Mercury isotopes reflect variable metal sources as a function of paleo-depositional setting in the Ediacaran-Cambrian Ocean, South China

A broad spectrum of marine sediments, including metalliferous black shales, stratiform barite, phosphorite and sapropelite (combustible shale) was widely deposited during the Ediacaran-Cambrian transition on the Yangtze Platform. The source of metals in the metalliferous black shales and the trigger for ocean dynamics during this critical period remain controversial. We analyzed Hg isotopes in late Ediacaran to Early Cambrian carbonaceous sedimentary rocks from the basinal Sansui section, South China, and observed Hg concentrations of 102–103 ppb, which is high but within the global range of black shales. The late Ediacaran carbonaceous chert shows near-zero Δ199Hg values (−0.05–0.09‰), which can be interpreted as input of terrestrial Hg or volcanic Hg. However, the transgressive near-coastal setting supports the terrestrial origin. The Early Cambrian V rich black shales display positive Δ199Hg values (0.03–0.18‰), which are similar to those observed in polymetallic Ni-Mo-rich sulfidic black shales (0.10–0.22‰) and phosphorites (0.13–0.24‰) in the stratigraphically equivalent Maoshi and Zhijin sections, respectively. These positive Δ199Hg values suggest that Hg was of dominantly seawater origin in a restricted basin setting. The variable Δ199Hg values of the rock spectrum reflect the dynamic interplay of marine and terrestrial metal sources as a function of paleo-depositional setting within the transgressive–regressive system on the continental margin of the Ediacaran-Cambrian Yangtze platform.

Impacts of active tectonics on geogenic arsenic enrichment in groundwater in the Hetao Plain, Inner Mongolia

High arsenic (As) groundwater has become a serious environmental problem threatening the health of 300 thousand people in the Hetao Plain (HP) of Inner Mongolia, China, where the microscopic mechanisms of As releasing in groundwater have been widely studied. However, the macro-behaviors of As under the large-scale geological conditions, particularly the impact of geogenic arsenic enrichment in groundwater by active tectonics is still uncertain. Here, we combine in-situ measurements of As concentration in groundwater and ore bodies, the locations of ore bodies, the active faults patterns and chronology, and the paleo-earthquake age sequences to explore the transport processes of As from source to groundwater controlled by the geological forces. Results show that the spatial pattern of groundwater As concentration exhibits large heterogeneity in the HP, particularly high in two areas: the Hangjin Rear Banner and Wuyuan area. Langshan, western part of the Western Yinshan Mountains, is more enriched in As-laden ore bodies and black rocks (dark carbonaceous sedimentary rocks and their metamorphic rocks). In the south of HP, reducing groundwater does not contain high As contents because the sediment (i.e., Yellow River alluvial deposit) itself bears little As. The larger vertical uplifting rate made a larger erosion rate of Langshan during the 50 ka B.P to 20 ka B.P, causing higher As background values in sediments. Differential uplifts, piedmont active faults, and earthquakes act as the internal dynamics, whereas the external effects of weathering, transport, and deposition play important roles in driving As-bearing bedrock debris to the foreland basin. The younger with more frequent seismotectonic events renew the As-bearing bedrock debris in sediments and the organic matter reactivity, causing higher As released to groundwater. This study can guide future identification of the geogenic high As groundwater.

Genesis and hydrothermal evolution of the Zhazigou skarn W (Mo) deposit, East Qinling, China: Constraints from fluid inclusions and H–O–S–Pb isotopes

The Zhazigou is a typical redox-intermediate skarn W (Mo) deposit, located in the Luanchuan ore district within the East Qinling orogenic belt, central China. Skarn and W (Mo) orebodies are hosted mainly in the contact zones between the Yuku porphyritic granite and carbonaceous sedimentary rocks of the Luanchuan and Guandaokou groups. This paper presents detailed fluid inclusion (FI) and isotope data that are used to reconstruct the hydrothermal processes and explore the role of CO2 during scheelite precipitation at the Zhazigou. Four mineralization and alteration stages are recognized: anhydrous skarn (I), hydrous skarn (II), quartz–sulfide (III), and carbonate (IV). Systematic analysis of the FIs elucidates the evolution of hydrothermal fluids through these stages. Four primary types of FIs are recognized: two-phase liquid-rich inclusions (L-type), two-phase vapor-rich inclusions (V-type), halite-bearing (hypersaline) inclusions (H-type), and two- or three-phase CO2-bearing inclusions (C-type). In stage I, coexisting H-type (36.19–46.37 wt% NaCl equiv.) and low salinity V-type FIs have similar homogenization temperatures, indicating that fluid boiling during the formation of anhydrous skarn minerals occurred at 500–550 °C and 550–700 bar (2.0–2.5 km lithostatic depth). In stage II, coexisting V- and L-type FIs in quartz and scheelite within quartz–scheelite ± actinolite ± tremolite veins have different salinities but similar homogenization temperatures, indicating they were also trapped in the boiling process at 350–380 °C and ~240 bar (1.8–2.8 km hydrostatic depth). In stage III, L-, V-, and minor H-type FIs have homogenization temperatures indicating continued fluid boiling at 300–340 °C and <100–200 bar (1.1–1.8 km hydrostatic depth). In stage IV, only minor primary L-type FIs were observed in quartz and calcite, which have lower salinities (0.88–11.34 wt% NaCl equiv.) and homogenization temperatures of 150–200 °C, reflecting typical post-ore conditions. The O and H isotope data from garnet, scheelite, and quartz indicate that the ore-forming fluids at the Zhazigou deposit have a dominantly magmatic water signature, with δ18Ofluid values from 7.9 to 1.8 ‰ and δ18Dfluid values from −99.1 to −84.4 ‰. The S isotopic compositions, which range from 1.8 to 5.6 ‰, and the relatively homogeneous Pb isotopic compositions, demonstrate that the ore-forming materials were dominantly derived from magmatic sources with only a minor contribution from sedimentary sources. Scheelite was precipitated as a result of fluid–rock interactions and/or fluid boiling, and the former released abundant Ca2+ by altering calcium-rich minerals (e.g., andradite, diopside, hedenbergite, and plagioclase) into the mineralizing fluids. Abundant C-type FIs occur in quartz within the hydrous skarn stage, providing evidence of CO2-rich fluids that would have decrease the pH and generated relatively acidic conditions to help precipitate scheelite.

Genesis of the Gangcha gold deposit, West Qinling Orogen, China: Constraints from Rb-Sr geochronology, in-situ sulfur isotopes and trace element geochemistry of pyrite

The Gangcha gold deposit, in the western zone of the Xiahe-Hezuo region within the West Qinling orogen of Central China, is a newly discovered medium-scale epithermal gold deposit with 19 tons of identified gold reserves. Mainly hosted in the Permian metasedimentary rocks and Triassic altered volcaniclastic rocks, and gold mineralization is closely associated with pyrite. Here we present results from field observation and geochemical studies of pyrite using electron probe microanalysis (EPMA), in-situ laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) and femtosecond laser ablation coupled multi-collector inductively coupled plasma mass spectrometry (fsLA-MC-ICP-MS) analyses. Based on the results, we identified five types of pyrite in the ores and wall rocks (namely Py1, Py2, Py3, Py4 and Py5). Among these, the pre-ore Py1, which is enriched in As, shows markedly negative δ34S values of −30.99 to −17.70‰ that are assumed to be the reflection of abiotic thermochemical reduction of sulfate (TSR) related to extremely oxidized magmas that have assimilated carbonaceous sedimentary rocks. Py2, occurring in the pyrite veins, displays internal rhythmic As- and Au-enriched growth zoning, and the δ34S values of Py2 progressively increase from the cores to the outer rims that are interpreted to result from mixing between the pyrite-forming fluid and magmatic vapor that invaded the main hydrothermal system episodically. The intermediate ore stage pyrite (Py3), represented by the quartz-pyrite (±arsenopyrite) veins, is enriched in As, Sb and Au. The comparable δ34S values of Py3 (−4.59 to + 7.05‰, average + 4.46‰) and Py4 (−5.66 to + 6.50‰, average + 1.80‰) could indicate magmatic sulfur. Compared with pyrites of the earlier stages, the late ore stage pyrite (Py5) is distinctly more enriched in δ34S, and is interpreted to have formed by mixing between a magmatic fluid and meteoric water. Additionally, some of the pyrites (Py5) from Stage V, occurring as the rim of pyrite grain, are interpreted to have formed by replacement via a dissolution-reprecipitation process of the earlier-formed Py4. The Rb-Sr data of pyrites from veins of different mineralization stages yield a mean isochron age of 225.3 ± 2.6 Ma, interpreted as the age of gold mineralization. Based on a combination of geochronological and geochemical date of pyrite, we propose that the ore-forming fluids of the Gangcha gold deposit are closely associated with coeval magmatic activity during the region collisional orogenic event.

Mineralogical-geochemical features of black shales from the frame of the Yayu igneous complex, Polar Urals

The results of study of mineralogical-geochemical features of black shales from the frame of the Yayu igneous complex (Polar Urals) are presented on the basis of modern analytical methods in order to identify possible mobilization, redeposition and concentration of ore material during intensive hydrothermal alteration. The high contents of Li, Be, Ba, Ti, Mn, Zr, Rb, Sc, Cs, P, Nb, Ta, Th, and REE are revealed. Rare metal and rare-earth minerals and a silver mineral (most likely, hessite) are found, the presence of alluvial gold in alluvium is established. The metallogenic features of carbonaceous sedimentary rocks afected by hydrothermal processes at the contact zone with igneous rocks are determined. The conclusion is made on a possible fnding of primary gold related to hydrothermal mineralisation in black shale of the region.

The large Bakyrchik orogenic gold deposit, eastern Kazakhstan: Geology, mineralization, fluid inclusion, and stable isotope characteristics

The large (>13 Moz Au, averaging 5–6 g/t Au) Bakyrchik Au deposit in eastern Kazakhstan is part of a district-scale mineralized zone, situated in an accretionary terrane. The deposit is localized within a shear zone in carbonaceous sedimentary rocks, which crosscuts the stratigraphy and comprises subconcordant dikes of high-K calc-alkaline (to shoshonitic?) rocks (lamprophyre to monzogabbro to monzodiorite). The tectonized/cataclased carbonaceous rocks contain numerous banded (bedded?) pyrite clasts and bands and abundant disseminated auriferous sulfides (pyrite and acicular arsenopyrite); the latter overprint pyritic clasts and occur proximal to (although mostly outside of) the concordant quartz veins and associated quartz-cemented hydrothermal breccias. Late granodiorite-plagiogranite dikes postdate auriferous sulfides and associated quartz veins/hydrothermal breccia but are overprinted by the late quartz veins containing free native gold and some Cu-Zn-Pb sulfides. This mineralization, however, is minor at the deposit.The most abundant stylolitic quartz veins accompanied by auriferous sulfide mineralization were formed from a homogenous and then boiling aqueous-carbonic fluid at temperatures of >315–300 to 265–240 °C and pressures of 0.8 to 1.2 kbar. Boiling was a dominant process during the formation of hydrothermal breccias accompanied by most intense auriferous sulfide (pyrite, arsenopyrite) mineralization at the temperatures of 250–230 °C and pressures of ~1.1 kbar. The late quartz veins were also formed from a homogenous and then boiling aqueous-carbonic fluid at temperatures of >350–315 to 235–225 °C and pressures of 1.1 to 1.3 kbar. The stable isotope data support a variable – from sedimentary to magma-derived - source of sulfur (δ34S = −20 to ~0‰), with a dominantly sedimentary to metamorphic source of carbon (δ13C = −27.4 to −22.5‰).The deposit is polygenetic and exhibits combined synsedimentary-exhalative, metamorphic to intrusion-related, magmatic-hydrothermal signatures, with strong structural control and close association with various dikes. Most of auriferous sulfide mineralization occurred after or nearly coeval to the intrusion of mafic to intermediate dikes, with a strong positive correlation of Au with As, Ssulf, Ag, Sb, and Hg. Homogenized δ34S values of auriferous pyrite and arsenopyrite (from −2.4 to −0.5‰), close to the meteorite standard, indicate dominantly magma-derived sulfur. Late quartz veins with free native gold and minor Cu-Zn-Pb sulfides are more distinctly related to granodiorite-plagiogranite intrusions.

Genesis of the Zaozigou gold deposit, West Qinling orogen, China: Constraints from sulfide trace element and stable isotope geochemistry

The West Qinling orogen, central China is endowed with dozens of world-class sediment-hosted gold deposits, most of which have formed from metamorphic fluids released from Paleozoic sedimentary rocks during the Triassic collisional orogeny. Here we present trace element and sulfur isotope data of gold-bearing pyrite and arsenopyrite, along with oxygen and hydrogen isotopes of ore-related sericite, to show that the Early Triassic Zaozigou gold deposit (142 t) in the Xiahe-Hezuo district may have contrasting origin. Gold mineralization at Zaozigou is dominated by fine-grained pyrite and arsenopyrite disseminations (Py1 and Asp1) in hydrothermally altered Triassic flysch and, less significantly, in Early Triassic granitoid dike swarm. The disseminated ores are overprinted by quartz-sulfide-ankerite-sulfide vein/veinlets, which contain less abundant, coarser pyrite and arsenopyrite (Py2 and Asp2) and are less economically significant. Gold occurs as invisible gold in fine-grained arsenian pyrite and arsenopyrite of the disseminated ores, with subordinate native gold grains locally observed in the quartz-sulfide-ankerite vein/veinlets. The two types of pyrite (Py1 vs. Py2) and arsenopyrite (Asp1 vs. Asp2) are consistently enriched in As, Au, Co, Ni, Mn, Cu, Zn, Ag, Sb, W, Pb, Te, and Bi and have well comparable δ34S values ranging from −12.0‰ to −5.5‰. Grain-scale variation has been observed for Au and other trace elements both in Py1 and Py2, likely reflecting changes in physio-chemical conditions such as fO2, fS2 and temperature due to fluid-rock interactions and/or fluid mixing. The δ34S values are broadly consistent with sulfur isotopic compositions of many Early Triassic ilmenite-series intrusions in the Xiahe-Hezuo district, and thus suggest a magmatic sulfur derivation from the reduced magmas that have assimilated carbonaceous sedimentary rocks. The δD and δ18O values of ore-fluids calculated from isotopic compositions of ore-related sericite range from −56 to −76‰ and 10.1 to 12.4‰, respectively, indicating a magmatic fluids exsolved from the above-mentioned ilmenite-series magma or a mixture of such as fluid with a metamorphic derived component. Synthesis of geochronological and geochemical data throughout the Xiahe-Hezuo district led us to conclude that the Zaozigou gold deposits can be considered as a shallow-seated, distal product of a reduced intrusion-related gold system.

Sediment‐Hosted Gold Deposits in Southeast Asia

AbstractSediment‐hosted gold (SHG) deposits are becoming more widely recognized in the Southeast Asian region. They are characterized by very fine‐grained gold, typically occurring in stratabound jasperoid replacement and structurally controlled orebodies in calcareous or carbonaceous sedimentary rocks, which are commonly distal to porphyry copper–gold deposits. Gold mineralization is associated with anomalous As‐Sb‐Hg‐Tl and is normally concentrated in the arsenic‐rich rims of pyrite and arsenopyrite grains (commonly refractory). The examples discussed in this paper are located in Southeast Asian countries, and all occur in island or continental arc terrains. SHG deposits have many features comparable to the Carlin deposits in Nevada; however, they differ in tectonic setting, their shallow level of emplacement, fluid composition, and probable metal source. As such, the Southeast Asian examples described in this compilation may be classified as “Carlin‐like” but not “Carlin‐type.” Furthermore, they appear to represent a subclass of low‐temperature epithermal deposits, with most occurrences showing late‐stage colloform quartz vein events. Indeed, they are better described as sediment‐hosted epithermal gold deposits.Several examples are multimillion ounce deposits, such as Sepon, Laos, with its endowment of more than 4 million ounces of gold. Mesel, Sulawesi, and Bau (Jugan and Tai Parit), Sarawak, also possessed multimillion ounce resources prior to mining. Two centuries of Chinese alluvial and eluvial mining at Buduk in northwest Kalimantan has produced a significant unrecorded amount of gold. Structurally controlled feeder zones frequently contain higher‐grade gold mineralization (e.g. Mesel), and constitute valid exploration targets. More recently discovered deposits, such as those in Myanmar (Gegalaw, Taungzaw, and Katsena) and the Philippines (Hijo and Las Cogon), remain to be thoroughly explored. Most of the deposits (Sepon, Mesel, Abong, Sihayo, Jugan, Tai Parit, Hijo, and Lascogon) can be directly linked spatially and genetically to gold–copper porphyry intrusive centers. It is postulated that localities with no obvious connection to known porphyry deposits (Buduk, Ban Bak, Kyaukpahto, Gegalaw, Taungzaw, and Katsena) may represent district‐scale exploration targets for porphyry copper–gold deposits. In terms of discovery techniques, of the 13 SHG deposits discussed here, several were sites of historical mining (Mesel, Bau, and Kyaukpahto). The majority, however, are virgin deposits discovered in the past 25 years by intuitive area selection followed by systematic regional geochemical exploration programs and routine geological inspections. At the project scale, electrical resistivity techniques, such as controlled‐source audio frequency magnetotellurics (CSAMT) and induced polarization (IP), have proven to be useful for mapping blind jasperoid bodies and associated feeder zones and silicified structures. Traditional field mapping and stream float sampling in tropical regions are also effective exploration tools. Deposits that have significant jasperoid development may have conspicuous positive topographic relief, depending on erosion level.

Source and possible tectonic driver for Jurassic–Cretaceous gold deposits in the West Qinling Orogen, China

The West Qinling Orogen (WQO) in Central China Orogenic Belt contains numerous metasedimentary rock-hosted gold deposits (>2000 t Au), which mainly formed during two pulses: one previously recognized in the Late Triassic to Early Jurassic (T3–J1) and one only recently identified in the Late Jurassic to Early Cretaceous (J3–K1). Few studies have focused on the origin and geotectonic setting of the J3–K1 gold deposits.Textural relationships, LA-ICP-MS trace element and sulfur isotope compositions of pyrites in hydrothermally altered T3 dykes within the J3–K1 Daqiao deposit were used to constrain relative timing relationships between mineralization and pyrite growth in the dykes, and to characterize the source of ore fluid. These results are integrated with an overview of the regional geodynamic setting, to advance understanding of the tectonic driver for J3–K1 hydrothermal gold systems. Pyrite in breccia- and dyke-hosted gold ores at Daqiao have similar chemical and isotopic compositions and are considered to be representative of J3–K1 gold deposits in WQO. Co/Ni and sulfur isotope ratios suggest that ore fluids were derived from underlying Paleozoic Ni- and Se-rich carbonaceous sedimentary rocks. The geochemical data do not support the involvement of magmatic fluids. However, in the EQO (East Qinling Orogen), J3–K1 deposits are genetically related to magmatism. Gold mineralization in WQO is contemporaneous with magmatic deposits in the EQO and both are mainly controlled by NE- and EW-trending structures produced by changes in plate motion of the Paleo-Pacific plate as it was subducted beneath the Eurasian continent. We therefore infer that the J3–K1 structural regime facilitated the ascent of magma in the EQO and metamorphic fluids in the WQO with consequent differences in the character of contemporaneous ore deposits. If this is correct, then the far-field effects of subduction along the eastern margin of NE Asia extended 1000's of km into the continental interior.

Mineralogical Distribution of Germanium, Gallium and Indium at the Mt Carlton High-Sulfidation Epithermal Deposit, NE Australia, and Comparison with Similar Deposits Worldwide

Germanium, gallium and indium are in high demand due to their growing usage in high-tech and green-tech applications. However, the mineralogy and the mechanisms of concentration of these critical elements in different types of hydrothermal ore deposits remain poorly constrained. We investigated the mineralogical distribution of Ge, Ga and In at the Mt Carlton high-sulfidation epithermal deposit in NE Australia, using electron probe microanalysis and laser ablation inductively-coupled plasma mass spectrometry. Parageneses from which selected minerals were analyzed include: Stage 1 acid sulfate alteration (alunite), Stage 2A high-sulfidation enargite mineralization (enargite, argyrodite, sphalerite, pyrite, barite), Stage 2B intermediate-sulfidation sphalerite mineralization (sphalerite, pyrite, galena) and Stage 3 hydrothermal void fill (dickite). Moderate to locally high concentrations of Ga were measured in Stage 1 alunite (up to 339 ppm) and in Stage 3 dickite (up to 150 ppm). The Stage 2A ores show enrichment in Ge, which is primarily associated with argyrodite (up to 6.95 wt % Ge) and Ge-bearing enargite (up to 2189 ppm Ge). Co-existing sphalerite has comparatively low Ge content (up to 143 ppm), while Ga (up to 1181 ppm) and In (up to 571 ppm) are higher. Sphalerite in Stage 2B contains up to 611 ppm Ge, 2829 ppm Ga and 2169 ppm In, and locally exhibits fine colloform bands of an uncharacterized Zn-In mineral with compositions close to CuZn2(In,Ga)S4. Barite, pyrite and galena which occur in association with Stage 2 mineralization were found to play negligible roles as carriers of Ge, Ga and In at Mt Carlton. Analyzed reference samples of enargite from seven similar deposits worldwide have average Ge concentrations ranging from 12 to 717 ppm (maximum 2679 ppm). The deposits from which samples showed high enrichment in critical elements in this study are all hosted in stratigraphic sequences that locally contain carbonaceous sedimentary rocks. In addition to magmatic-hydrothermal processes, such rocks could potentially be important for the concentration of critical elements in high-sulfidation epithermal deposits.

Ar-Ar ages of gold deposits in the Song Hien domain (NE Vietnam): Tectonic settings and comparison with Golden Triangle in China in terms of a single metallogenic province

The Song Hien rift basin is an important metallogenic area in NE Vietnam. This domain consists mainly of Triassic sulfide-rich black shale beds, which play a role as a sedimentary host for various mineral systems such as antimony, mercury and gold-sulfide deposits. Most of gold deposits are hosted in carbonaceous sedimentary rocks, however some deposits, which have similar characteristics, are hosted in fine-grained mafic magmatic rocks. An Ar-Ar isotopic dating of hydrothermal sericite from the sedimentary hosted Bo Va and Khung Khoang gold deposits and intrusion hosted orogenic Hat Han gold deposit yields plateau ages of 184.8±2.1Ma, 211.63±2.3Ma, and 209.12±2.3Ma, respectively. The obtained Ar-Ar ages convincingly show that the orogenic gold deposits in the Song Hien domain were formed in Late Triassic to Early Jurassic, while the age of the Bo Va deposit is at least older than 184.8±2.1Ma. Loss of argon by volume diffusion, supported by previously reported mineralogical and isotopic features of the Bo Va deposit may suggest that the Jurassic-Cretaceous (Yanshanian) tectonothermal events overprinted some deposits in the Song Hien domain. Formation of gold deposits in the Song Hien domain is linked to the same tectonic event as the Carlin-like gold deposits in SW China and is associated with an extensional tectonic regime that followed continental collision between the Indochina and South China Blocks. The similarity in geology setting and mineral composition of gold deposits of the Song Hien domain and the Golden Triangle region, as well as timing and kinematics of deformation, magmatic features, and stratigraphic sequence and bulk architecture, lead to conclusion that NE Vietnam and SW China is a single metallogenic zone. The study of gold deposits in Vietnam will provide a new data on the metallogenic history of this important part of SE Asia.

A REVIEW OF THE APPLICATION OF MULTIPLE S ISOTOPES TO MAGMATIC Ni-Cu-PGE DEPOSITS AND THE SIGNIFICANCE OF SPATIALLY VARIABLE Δ33S VALUES

The determination of Δ 33 S values in conjunction with δ 34 S values provides an effective means of evaluating the importance of S derivation from Archean country rocks in magmatic Ni-Cu-PGE ore genesis, particularly where δ 34 S values of potential S source rocks are near 0‰ and similar to most S of mantle derivation. However, Δ 33 S values of sulfides in some Archean rocks may also be near 0‰; care should be taken in the evaluation of S derivation from Archean rocks based solely on this parameter. Although anomalous Δ 33 S values are found primarily in Archean rocks, rare occurrences of nonzero Δ 33 S values have been identified in sulfides from younger rocks, and hence knowledge of the multiple S isotope composition of potential contaminants is essential for an accurate assessment of S contamination of mantle-derived magmas. Restricted ranges in Δ 33 S values may be characteristic of particular Archean formations, and when used in conjunction with mixing calculations may fingerprint a distinct S source rock. Spatial variability in Δ 33 S values within different types of mineralized systems may be caused by the emplacement of isotopically distinct magma pulses, by locally variable mixing proportions, and the addition of secondary sulfide to the primary assemblage. In the case of conduit-type deposits in the Midcontinent rift system, three-component mixing calculations show that both high and low Δ 33 S values in some units that are similar to end-member values in the Archean country rocks must be a function of assimilation of primarily Archean S even though Proterozoic carbonaceous sedimentary country rocks contain in excess of 1 wt % S and are particularly viable S source rocks.

Geology, mineralogy, geochemistry and δ34S of sedimentary rock-hosted Au deposits in Song Hien structure, NE Vietnam

The Song Hien rift basin is considered to be one of the most important regions of gold mineralisation in North East Vietnam. A number of gold deposits in the Song Hien rift basin are hosted in Triassic and Devonian sedimentary formations of the basin. The largest among them are the Bo Va, Tham Riem and Khung Khoang deposits. The Bo Va deposit is hosted in carbonaceous sedimentary rocks of Triassic age, whereas the Tham Riem and Khung Khoang deposits are hosted in carbonaceous sedimentary rocks of Devonian ages. Based on the mineral composition of the ores, the deposits can be divided into to two types: (i) pyrite dominated and (ii) pyrite-arsenopyrite dominated. The Khung Khoang is of the first type and the Bo Va and Tham Riem deposits belong to the second type. The isotopic composition of pyrite and arsenopyrite in the Tham Riem deposit however, is close to that for the ores of the Bo Va deposit. The δ34S value for pyrite ranging from −3.7‰ to −7.4‰ and for arsenopyrite ranging from −3.2‰ to 7.4‰. The δ34S of pyrite in the ore from the Khung Khoang deposit however, has a much heavier isotopic composition of +18.9 to +20.2‰. A narrow range of the variation of sulfur isotopic composition of pyrite and arsenopyrite, the presence of visible gold as inclusions, the presence of chalcopyrite, sphalerite and other inclusions in arsenopyrite and pyrite, the large size of the grains of major ore minerals allow us to assume that the primary gold ores of the Bo Va and Tham Riem deposits underwent metamorphic transformations. The absence of arsenic, antimony, mercury and other characteristic elements in the ores of the Khung Khoang deposit, and substantially heavier isotopic composition of sulfur similar to the sulfur isotopic composition of marine sulfates in the Devonian, allow us to assume another source of the ore components, not connected with the Triassic sedimentary rocks of the Song Hien rift.

Constraints of fluid inclusions and in-situ S-Pb isotopic compositions on the origin of the North Kostobe sediment-hosted gold deposit, eastern Kazakhstan

The North Kostobe gold deposit in the Kalba gold province, eastern Kazakhstan, is tectonically located in the Chara shear zone of the western Altaids. The Chara shear zone separates the Kazakhstan microcontinent and Siberia craton which collided in the late Carboniferous. In the North Kostobe deposit, Au mineralization is distributed along an E-W striking shear zone linked to the NW-SE trending regional faults, and is mainly concentrated in quartz-sulfide-carbonate veins or in disseminated sulfides hosted in carbonaceous metasedimentary rocks. Sulfide minerals are mainly pyrite and arsenopyrite, and were formed in three generations. The first generation is dominated by pyrite (py1) occurring as microcrystal aggregates, followed by the second generation including euhedral arsenopyrite and compact pyrite (py2) locally overgrowing early py1 grains as rims; third generation of pyrite (py3) is present in barren micro-fractures crosscutting early mineralized rocks. Native gold is present in cracks of brecciated arsenopyrite grains and as inclusions in py2. Investigations on fluid inclusions in auriferous quartz veins indicate that the ore-related fluids are CO2-bearing, with homogenization temperatures of 288°C and low salinity (1.42 to 8.03wt% NaCl equiv). The fluids have calculated δ18O and δD values ranging from 9.96 to 11.86‰ and from −75 to −97.1‰, respectively, suggesting that they were most likely metamorphic in origin.The first and second generations of ore sulfides have similar δ34S values (−6 to +2.6‰) which possibly indicating a common sulfur source as the hosting sedimentary rocks. In contrast, py3 grains have a huge range of δ34S values (−40 to +54.5‰), indicating a biogenic source for the sulfur. On the other hand, in-situ Pb isotopic compositions of different generations of sulfides show a similarly mixed lead source, possibly involved orogen, mantle and lower crust Pb reservoirs. We propose that the gold-bearing fluids of the North Kostobe gold deposit were most likely derived from gold-rich, pyritic carbonaceous sedimentary rocks through dehydration during the metamorphism that was related to the collision between the Kazakhstan microcontinent and Siberia craton in the late Carboniferous. It is thus concluded that the North Kostobe gold deposit shares many characteristics with typical orogenic gold deposits in terms of tectonic settings, mineralization styles, fluid compositions and source of fluids and gold. Our new findings thus have important implications for regional exploration in the Kalba gold province.

Re-Os systematics and age of pyrite associated with stratiform Zn-Pb mineralization in the Howards Pass district, Yukon and Northwest Territories, Canada

Stratiform Zn-Pb deposits hosted in unmetamorphosed carbonaceous and siliceous mudstones of the Ordovician to Silurian Duo Lake Formation define the Howards Pass district in Yukon Territory and Northwest Territories, western Canada. Collectively, the deposits are amongst the largest in the world, containing drill-indicated and inferred resources of 423 Mt at 4.84 % Zn and 1.59 % Pb. Sulphide textures include (a) fine-scale laminations of sphalerite, galena, and pyrite from <0.05 mm to 1 cm thick, interbedded with carbonaceous sedimentary rock; (b) layers of coarse sulphide that are structurally controlled by microfolds; and (c) veins that cut bedding and sulphide laminations. The finely interlaminated nature of sulphides with mudstone has been used as evidence for syngenetic mineralizing processes, whereas paleomagnetic data determined on coarse layered sulphides suggest a Middle Jurassic age of mineralization. Here, we present new rhenium-osmium (Re-Os) isotopic data for 12 pyrite separates obtained from 4 laminated sulphide-rich samples from the XY Central (XYC) and Don (DON) deposits and for 1 unmineralized organic-rich mudstone ∼20 m stratigraphically below the sulphide-bearing zone. Pyrite separates that lack mudstone inclusions (“pure”) from the XYC deposit contain 2.2 to 4.0 ppb Re and 93.4 to 123.4 ppt Os; pure pyrite from the DON deposit is significantly more enriched in Re and Os (34–37 ppb Re; 636.8–694.9 ppt Os). The 187Re/188Os values of pure pyrite separates from the XYC and DON deposits range from 137.6 to 197 and 182.1 to 201.4, respectively. Regression of all pure pyrite Re-Os data from both deposits yields an isochron age of 442 ± 14 Ma (MSWD = 7.4) and an initial 187Os/188Os (Osi) value of 0.71 ± 0.07. The Re-Os age indicates that the early phase of pyrite precipitation (and by inference, sphalerite and galena) occurred during the early Silurian, consistent with biostratigraphic ages of the host rocks. The Osi value of ∼0.8 for earliest Silurian seawater recorded from organic-rich shale in the basal Silurian Global Stratotype Section and Point (GSSP) at Dobs Linn, Scotland is very similar to that provided by the Howards Pass pyrite regression and hence suggests a hydrogenous (seawater) source of Os for the pyrite. Therefore, two possible sources of Os are (1) the Zn- and Pb-bearing hydrothermal fluid that leached Os from footwall sedimentary rocks, which were deposited in seawater, or (2) directly from seawater during precipitation of the pyrite, which suggests that the Os content of the hydrothermal fluid was minor relative to that of seawater.

Cu isotope variations between conduit and sheet-style Ni–Cu–PGE sulfide mineralization in the Midcontinent Rift System, North America

Disseminated Ni-Cu-PGE sulfide mineralization in sheet-like intrusions of the Duluth Complex are characterized by elevated bulk rock Cu/Ni ratios, typically in the range of 3 to 8. In order to evaluate whether the addition of crustal Cu to parental magmas may have contributed to the high Cu/Ni ratios, Cu isotope ratios were measured from sulfide minerals in the disseminated mineralization found within the sheet-style Partridge River and South Kawishiwi Intrusions of the Duluth Complex, as well as sulfide mineralization in geometrically distinct conduit-style deposits in the Eagle Intrusion and Tamarack Igneous Complex. These intrusions were emplaced during early stages of rift evolution (~1110–1105Ma) from picritic parental magmas, with associated mineralization characterized by lower Cu/Ni ratios. In addition, Cu isotope ratios were determined from sulfidic and carbonaceous sedimentary country rocks to the intrusions. δ65Cu values of disseminated sulfide mineralization in intrusions of the Duluth Complex range from −0.85 to 0.45‰ whereas values from the Eagle Intrusion and the Tamarack Igneous Complex range from 0.69 to 1.84‰. δ65Cu values of the sedimentary rocks range from −0.33 to 1.18, with two samples showing higher values of 2.65 and 3.12‰. The δ65Cu values of the Duluth Complex mineralization is consistent with the assimilation of significant amounts of sedimentary Cu only if the δ65Cu value of the parental mantle-derived magma was lower than −0.25‰, a value less than that proposed for the mantle based on analyses of terrestrial mafic igneous rocks. The distinct difference in δ65Cu values between the sheet-style and conduit-style mineralization suggests that either mantle sources of the parental magmas in the rift differed, or that Cu isotope fractionation accompanied magmatic processes (i.e., variable degrees of melting and sulfide retention, sulfide liquid fractionation) that were responsible for the relative enrichment of Cu in the sheet-style intrusions of the Duluth Complex.

From passive margin to volcano–sedimentary forearc: The Tonian to Cryogenian evolution of the Anosyen Domain of southeastern Madagascar

The Anosyen Domain of southeastern Madagascar represents a part of a vast sequence of predominantly siliciclastic and carbonaceous sedimentary and felsic volcanic rocks that can be traced from Madagascar through southern India, Sri Lanka, and into East Antarctica. We present new age and geochemical data from the Iakora and Horombe groups: units that represent the sedimentary and volcanogenic components of the Anosyen Domain. Our data demonstrate that the siliciclastic sedimentary rocks of the Iakora Group contain predominantly 2400–1600Ma aged detrital zircon with a modal age peak at approximately 1850Ma. This age spectrum is comparable to those reported from the majority of the other sedimentary rocks exposed in Madagascar. The felsic volcanic rocks of the Horombe Group are of both metaluminous and peraluminous composition and have a similar chemistry and the same age as the Imorona–Itsindro Suite (840–700Ma), a voluminous suite of continental arc-related granitoids that intrude central and northern Madagascar.Taken together we interpret the strata of the Anosyen Domain to have been deposited in an expansive continent margin basin that existed on the western ocean-facing margin of the Indo-Antarctic Craton. We infer this basin records both passive margin and forearc phases of deposition and, on the basis of the regional similarities in detrital zircon spectra and cross-cutting relationships, suggest the older passive margin phase of deposition occurred after c. 980Ma. The younger forearc phase of deposition occurred in the Cryogenian synchronous with the development of the Imorona–Itsindro magmatic arc (840–700Ma). This suite reflects east-dipping subduction beneath the western margin of the Indo-Antarctic Craton and defines a linear belt of intrusions that, similar to the forearc sediments of the Anosyen Domain, can be traced from East Antarctica through Madagascar and potentially into East Africa, regions we take to have been continuous within the pre-Gondwana Indo-Antarctic plate. The volcanic and plutonic rocks of the Malani Igneous Suite (780–730Ma) may represent the coeval backarc basin.

Analysis of single oil-bearing fluid inclusions in mid-Proterozoic sandstones (Roper Group, Australia)

Hydrocarbons and organic biomarkers extracted from black shales and other carbonaceous sedimentary rocks are valuable sources of information on the biodiversity and environment of early Earth. However, many Precambrian hydrocarbons including biomarkers are suspected of being younger contamination. An alternative approach is to study biomarkers trapped in oil-bearing fluid inclusions by bulk crushing samples and subsequently analysing the extracted hydrocarbons with gas chromatography–mass spectrometry. However, this method does not constrain the hydrocarbons to one particular oil inclusion, which means that if several different generations of oil inclusions are present in the sample, a mix of the content from these oil inclusions will be analysed. In addition, samples with few and/or small inclusions are often below the detection limit. Recently, we showed that it is possible to detect organic biomarkers in single oil-bearing fluid inclusions using time-of-flight secondary ion mass spectrometry (ToF-SIMS). In the present study, single fluid inclusion analysis has been performed on Proterozoic samples for the first time. Four individual oil-bearing fluid inclusions, found in 1430Ma sandstone from the Roper Superbasin in Northern Australia, were analysed with ToF-SIMS. The ToF-SIMS spectra of the oil in the different inclusions are very similar to each other and are consistent with the presence of n-alkanes/branched alkanes, monocyclic alkanes, bicyclic alkanes, aromatic hydrocarbons, and tetracyclic and pentacyclic hydrocarbons. These results are in agreement with those obtained from bulk crushing of inclusions trapped in the same samples.The capability to analyse the hydrocarbon and biomarker composition of single oil-bearing fluid inclusions is a major breakthrough, as it opens up a way of obtaining molecular compositional data on ancient oils without the ambiguity of the origin of these hydrocarbons. Additionally, this finding suggests that it will be possible to analyse minute oil samples beyond the capability of established techniques. This may allow the biomarker record of the biosphere, as preserved in fluid inclusions, to be extended further back in time, and hence makes it possible to more accurately trace the early evolution of life on Earth, and search for life on other planets or moons.