Magmatic-hydrothermal Activity Research Articles

Geology and genesis of auriferous porphyritic monzogranite and its correlation with the Qiyugou porphyry-breccia system in East Qinling, Central China

The Qiyugou deposit in the Qinling Orogenic Belt includes breccia pipe hosted gold mineralization and newly discovered porphyry gold mineralization, whereas the genetic relationship between porphyry stocks, stockwork-disseminated mineralization and breccia pipe-hosted mineralization remains obscure. Here we present the results from an integrated field investigation, petrography, whole-rock geochemistry, zircon U-Pb geochronology, trace elements and Hf-O isotopes on a suite of porphyritic monzogranite from the newly discovered auriferous ‘191 pluton’, to elucidate the petrogenesis, magma source, timing of magmatic-hydrothermal event and the possible genetic link with the Qiyugou porphyry-breccia system. The porphyritic monzogranite shows varying degree of hydrothermal alterations including potassic alteration, phyllic alteration, silicification, sericitization and chloritization, resulting in the high loss on ignition values (LOI = 2.88–6.64 wt%) and linear relationships between LOI and large ion lithophile elements (LILEs) which suggest element mobility in fluids. The fluid immobile high field strength elements (HFSEs) and zircon trace elements indicate that the porphyritic monzogranite is typical I-type granite. Zircon U-Pb dating of the porphyritic monzogranites yield weighted mean ages of 136.4 ± 1.1 Ma, 134.6 ± 1.8 Ma and 135.2 ± 1.1 Ma, representing the emplacement age of the auriferous ‘191 pluton’. Zircon Hf-O isotopes show εHf(t) values from −25.6 to −20.0 with crustal model ages ranging from 2807 to 2458 Ma, and δ18O values from 4.75‰ to 5.66‰, indicating that the parental magma involved mixing between the partially melted Taihua Group lithologies and mantle-derived component. The available geochronological data constrain the timing and duration of magmatic-hydrothermal activities in the Qiyugou deposit, including pre-mineralization 158.7–150.1 Ma granitic magmatism, syn-mineralization 136.4–128.6 Ma porphyry magmatism and Au mineralization, and post-ore 128.9–124.7 Ma granitic dyke intrusion. The duration of the magmatic-hydrothermal activity in the Qiyugou deposit ranges between 136.4 Ma and 128.6 Ma, when the breccia pipes and granitoid plutons formed through multiple pulses of magma emplacement and fluid exsolution from a fertile magma chamber. The 136.4–131.3 Ma porphyry magmatism and gold mineralization in the ‘191 pluton’ represent an early stage magmatic-hydrothermal activity in the Qiyugou porphyry-breccia system. We predict that the deeper levels of the breccia pipes could be potential target for porphyry gold ore bodies in the Qiyugou deposit.

Timing of magmatic-hydrothermal activity in the Variscan Orogenic Belt: LA-ICP-MS U–Pb geochronology of skarn-related garnet from the Schwarzenberg District, Erzgebirge

Here, we present in situ U–Pb laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) ages of andradite-grossular garnet from four magmatic-hydrothermal polymetallic skarn prospects in the Schwarzenberg District, Erzgebirge (Germany), located in the internal zone of the Variscan Orogenic Belt. Within the geochronological framework of igneous rocks and hydrothermal mineralization in the Erzgebirge, the obtained garnet ages define three distinct episodes of Variscan skarn formation: (I) early late-collisional mineralization (338–331 Ma) recording the onset of magmatic-hydrothermal fluid flow shortly after the peak metamorphic event, (II) late-collisional mineralization (~ 327–310 Ma) related to the emplacement of large peraluminous granites following large-scale extension caused by orogenic collapse and (III) post-collisional mineralization (~ 310–295 Ma) contemporaneous with widespread volcanism associated with Permian crustal reorganization. Our results demonstrate that the formation of skarns in the Schwarzenberg District occurred episodically in all sub-stages of the Variscan orogenic cycle over a time range of at least 40 Ma. This observation is consistent with the age range of available geochronological data related to magmatic-hydrothermal ore deposits from other internal zones of the Variscan Orogenic Belt in central and western Europe. In analogy to the time–space relationship of major porphyry-Cu belts in South America, the congruent magmatic-hydrothermal evolution in the internal zones and the distinctly later (by ~ 30 Ma) occurrence of magmatic-hydrothermal ore deposits in the external zones of the Variscan Orogenic Belt may be interpreted as a function of their tectonic position relative to the Variscan collisional front.

Magmatic-Hydrothermal Processes of Vein-Type Haman-Gunbuk-Daejang Copper Deposits in the Gyeongnam Metallogenic Belt in South Korea

Haman, Gunbuk, and Daejang deposits are neighboring vein-type hydrothermal Cu deposits located in the SE part of the Korean Peninsula. These three deposits are formed by magmatic-hydrothermal activity associated with a series of Cretaceous granodioritic intrusions of the Jindong Granitoids, which have created a series of veins and alterations in a hornfelsed shale formation. The copper deposits have common veining and alteration features: 1) a pervasive chlorite-epidote alteration, cut by 2) Cu-Pb-Zn-bearing quartz veins with a tourmaline-biotite alteration, and 3) the latest barren calcite veins. Chalcopyrite, pyrite, and pyrrhotite are common ore minerals in the three deposits. Whereas magnetite is a dominant mineral in the Haman and Gunbuk deposits, no magnetite is present, but sphalerite and galena are abundant in the Daejang deposit. Ore-bearing quartz veins have three types of fluid inclusions: 1) liquid-rich, 2) vapor-rich, and 3) brine inclusions. Hydrothermal temperatures obtained from the brine inclusion assemblages are about 340–600, 250–500, and 320–460°C in the Haman, Gunbuk, and Daejang deposits, respectively. The maximum temperatures (from 460 to 600°C) recorded in the fluid inclusions of the three deposits are higher than those of the Cu ore precipitating temperature of typical porphyry-like deposits (from 300 to 400°C). Raman spectroscopy of vapor inclusions showed the presence of CO2 and CH4 in the three deposits, which indicates relatively reduced hydrothermal conditions as compared with typical porphyry deposits. The Rb/Sr ratios and Cs concentrations of brine inclusions suggest that the Daejang deposit was formed by a later and more fractionated magma than the Haman and Gunbuk deposits, and the Daejang deposit has lower Fe/Mn ratios in brine inclusions than the Haman and Gunbuk deposits, which indicates contrasting redox conditions in hydrothermal fluids possibly caused by an interaction with a hosting shale formation. In brines, concentrations of base metals do not change significantly with temperature, which suggests that significant ore mineralization precipitation is unlikely below current exposure levels, especially at the Haman deposit. Ore and alteration mineral petrography and fluid inclusions suggest that the Haman deposit was formed near the top of the deep intrusion center, whereas the Gunbuk deposit was formed at a shallower intrusion periphery. The Daejang deposit was formed later at a shallow depth by relatively fractionated magma.

The anisotropic properties of granites – effects of tectonic emplacement mode on potential crystalline host rocks for nuclear waste deposits in Germany

Abstract. For permanent nuclear waste disposal sites, crystalline rocks, especially granitic/granodioritic batholiths, are considered an appropriate host rock. Principally, three types of granitic plutons occur in the extra-alpine crystalline basement of Germany that were consolidated during the late Paleozoic Variscan orogeny of Central Europe: (i) Pre-Variscan voluminous granodiorites that are hardly affected by the subsequent continent–continent collision; (ii) voluminous granites in various tectonic settings intruded during the late orogenic stage of the Variscides; (iii) post-orogenic granites related to vast Permian intracontinental extension. Thus, in terms of the syn-intrusive tectonic setting and post-intrusive processes there are significant differences. Although it can be expected that different tectonic environments caused significant differences in the material properties, for Germany, however, there is no systematic study regarding the fabric of such plutonites. In order to find the most suitable “granite” we investigate the primary anisotropy of granites evolved during the emplacement and crystallization of the melt. For this we sample rocks of all three principal types and various syn-intrusive tectonic settings, i.e., compression, extension, strike-slip, transtension, and transpression. By means of combined measurements of the “Anisotropy of the Magnetic Susceptibility” and the “Shape Preferred Orientation” we characterize the syn-intrusive flow pattern, i.e., the magmatic foliation and lineation. The Crystallographic Preferred Orientation is analyzed by a combination of neutron time-of-flight experiments and electron backscatter diffraction measurements at the Frank Laboratory of Neutron Physics at JINR, Dubna, Russia, and the TU Bergakademie Freiberg respectively. Furthermore, special attention is given to the systematic mapping of annealed microcracks evolved during late magmatic fluid escape and/or post-crystallization hydrothermal activity. In a second step we compare the primary anisotropy with the post-magmatic fracture pattern of the particular granites. Those fractures constitute probable fluid pathways and, thus, the first-order risk for a potential permanent nuclear waste disposal. All datasets are organized in a Geological Information System allowing for a complete traceability of the different investigation steps. The results of this study will serve as a basis for a future detailed exploration.

Interpretation of Recent Unrest Events (Bradyseism) at Campi Flegrei, Napoli (Italy): Comparison of Models Based on Cyclical Hydrothermal Events versus Shallow Magmatic Intrusive Events

Several recent models that have been put forth to explain bradyseism at Campi Flegrei (CF), Italy, are discussed. Data obtained during long-term monitoring of the CF volcanic district has led to the development of a model based on lithological-structural and stratigraphic features that produce anisotropic and heterogeneous permeability features showing large variations both horizontally and vertically; these data are inconsistent with a model in which bradyseism is driven exclusively by shallow magmatic intrusions. CF bradyseism events are driven by cyclical magmatic-hydrothermal activity. Bradyseism events are characterized by cyclical, constant invariant signals repeating over time, such as area deformation along with a spatially well-defined seismogenic volume. These similarities have been defined as “bradyseism signatures” that allow us to relate the bradyseism with impending eruption precursors. Bradyseism is governed by an impermeable shallow layer (B-layer), which is the cap of an anticlinal geological structure culminating at Pozzuoli, where maximum uplift is recorded. This B-layer acts as a throttling valve between the upper aquifer and the deeper hydrothermal system that experiences short (1-102 yr) timescale fluctuations between lithostatic/hydrostatic pressure. The hydrothermal system also communicates episodically with a cooling and quasi-steady-state long timescale (103-104 yr) magmatic system enclosed by an impermeable carapace (A layer). Connectivity between hydrostatic and lithostatic reservoirs is episodically turned on and off causing alternatively subsidence (when the systems are connected) or uplift (when the systems are disconnected), depending on whether permeability by fractures is established or not. Earthquake swarms are the manifestation of hydrofracturing which allows fluid expansion; this same process promotes silica precipitation that seals cracks and serves to isolate the two reservoirs. Faults and fractures promote outgassing and reduce the vertical uplift rate depending on fluid pressure gradients and spatial and temporal variations in the permeability field. The miniuplift episodes also show “bradyseism signatures” and are well explained in the context of the short timescale process.

Magmatic hydrothermal fluid genesis of zeolite in the Paleogene Kongdian Formation of Zaoyuan oilfield, Bohai Bay Basin, China

Electronic probe, fluid inclusion homogenization temperature, Raman spectroscopy and laser ablation inductively coupled plasma mass spectrometry were utilized to identify the hydrothermal fluid-rock interactions in the second member of the Paleogene Kongdian Formation of Zaoyuan oilfield in Bohai Bay Basin (Kong 2 Member for short) of Well Z56 to find out the relationship between zeolite and hydrothermal fluid. The experimental results show that: (1) Pyrobitumen coexists with hydrothermal fluid characteristic minerals such as chlorite, barite, chalcopyrite, pyrite, natrolite and analcime in mudstone fractures. (2) The temperatures calculated from laser Raman spectrum of pyrobitumen, from the chlorite geothermometer and from measured homogenization temperature of natrolite inclusions are 324–354 °C, 124–166 °C and 89–196 °C, respectively; although vary widely, all the temperatures are obviously higher than the normal geothermal temperature. (3) The positive Eu anomaly of chlorite and barite, and the similar distribution pattern in rare earth elements between natrolite and basalt indicate they are from magmatic hydrothermal fluid. Moreover, drilling data shows that the Kong 2 Member in Well Z56 has several sets of basalt interlayers, suggesting there was geologic base of magmatic hydrothermal fluid activity. The magmatic hydrothermal fluid-rock interaction may be one of the reasons for the abnormal enrichment of zeolite in Kong 2 Member of the Cangdong Sag.

Geochronology, Fluid Inclusions and Isotopic Characteristics of the Dongjun Pb‐Zn‐Ag Deposit, Inner Mongolia, NE China

AbstractThe Dongjun Pb‐Zn‐Ag deposit in the northern part of the Great Xing'an Range (NE China) consists of quartz‐sulfide vein‐type and breccia‐type mineralization, related to granite porphyry. Hydrothermal alteration is well‐developed and includes potassic‐silicic‐sericitic alteration, phyllic alteration and propylitic alteration. Three stages of mineralization are recognized on the basis of field evidence and petrographic observation, demarcated by assemblages of quartz‐pyrite‐arsenopyrite (early stage), quartz‐polymetallic sulfide (intermediate stage) and quartz‐carbonate‐pyrite (late stage). Zircon LA‐ICP‐MS U‐Pb dating indicates that the granite porphyry was emplaced at 146.7 ± 1.2 Ma (Late Jurassic). Microthermometry and laser Raman spectroscopy shows that ore minerals were deposited in conditions of intermediate temperatures (175–359°C), low salinity (0.5–9.3 wt% NaCl eqv.) and low density (0.60–0.91 g/cm3). Ore‐forming fluids were derived largely from magmatic hydrothermal processes, with late‐stage addition of meteoric water, belonging to a H2O‐NaCl‐CO2 ± CH4 system. The δ34SV‐CDT values range from 0.75‰ to 4.70‰. The 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb values of the ore minerals are in the ranges of 18.240–18.371, 15.542–15.570, and 38.100–38.178, respectively. Data for the S and Pb isotopic systems indicate that the ore‐forming metals and sulfur were derived from Mesozoic magma. Based on the geological characteristics and geochemical signatures documented in this study, we conclude that the Dongjun deposit is a mesothermal magmatic hydrothermal vein‐type Pb‐Zn‐Ag deposit controlled by fractures and related to granite porphyry, in response to Late Jurassic tectonic–magmatic–hydrothermal activity. We further conclude that fluid immiscibility, fluid mixing and fluid‐rock interactions were the dominant mechanisms for deposition of the ore‐forming materials.

Helvine-danalite mineralogy of the Dulong Sn-Zn polymetallic deposit in southeast Yunnan, China

Abstract Southeastern (SE) Yunnan is a major Sn polymetallic province of China, with the Dulong large Sn-Zn polymetallic deposit (in the Laojunshan orefield) being one of the most representative deposits. Our recent work had first identified helvine-group minerals in this deposit. These minerals mainly occur in massive sphalerite ores, and coexist with sphalerite, pyrrhotite, biotite, talc, cassiterite, and fluorite. Raman spectroscopic, X-ray diffraction (XRD), scanning electron microscopic (SEM), and electron probe microanalysis (EPMA) analyses indicate that these helvine-group minerals are oscillatory-zoned helvine-danalite. Both the helvine and danalite zones are mixed with varying proportion of the other helvine-group end-member, and our studies indicate that the oscillatory zoning was formed mainly by periodic fluctuations of the fluid physicochemical conditions (notably fS2 and fO2), but less related to the variation of the fluid Mn, Fe, and Zn contents. The helvine zone was likely formed in a higher fS2 but lower fO2 environment than the danalite zone. In this study, we present the first LA-ICP-MS in situ trace element data for the helvine-danalite minerals from Dulong, and the results indicate that the helvine has considerably high contents and a wide range of trace elements. The helvine is rich in Ca, Al, Sc, and Y, while the danalite is rich in Sn and P (reaching thousands of parts per million). Such trace element enrichments are likely controlled by their respective ionic size and chalcophile behavior. Meanwhile, the fO2 and fS2 conditions during the zoning formation may have also influenced the trace element distributions: trace elements may have mainly entered the helvine-group minerals by substituting into the M-sites in M4[BeSiO4]3S, for instance Al, Sc, and Y substitute for Mn, and Sn and Mg for Fe and Zn. It is noteworthy that the helvine and danalite zones are all HREE-enriched and have distinct negative Eu anomalies. This may be related to the high fluid F-Y-P contents during the mineral formation. High-F-Y fluids can readily incorporate HREEs into helvine-group minerals, and phosphates incorporate HREEs more readily in alkali fluids. Europium occurs as Eu2+ in the fluid, causing the negative Eu anomalies observed. We have also identified grains of cassiterite in the helvine-group minerals and its coexisting sphalerite. U-Pb dating on these cassiterite grains yielded 86.5 ± 1.6 Ma, coeval with the reported sulfide mineralization age. This indicates that both the Be and Sn-Zn polymetallic mineralization occurred in the Cretaceous, and may have been products of the Late Yanshanian Laojunshan magmatic-hydrothermal activity. Considering the close relations with many W(-Be) deposits nearby (e.g., Nanyangtian, Saxi, and Maka), the Laojunshan orefield may also have substantial Be mineralization potential.

CONTROLS ON THE METAL ENDOWMENT OF PORPHYRY Mo DEPOSITS: INSIGHTS FROM THE LUMING PORPHYRY Mo DEPOSIT, NORTHEASTERN CHINA

AbstractProcesses controlling the metal endowment of arc-related porphyry Mo deposits are not well understood. Located in northeastern China, the arc-related Luming porphyry Mo deposit has a proven reserve of 0.75 Mt Mo at an average grade of 0.092 wt % and is characterized by multiple pulses of alteration and mineralization. These features make this deposit an ideal location to investigate the role of multiple pulses of magmatism and fluid release in the evolution and formation of an arc-related porphyry Mo deposit. Molybdenum mineralization at Luming is typically observed as a series of molybdenite-bearing veins hosted within a composite intrusive complex, referred to as the Luming Intrusive Suite. Crosscutting relationships between intrusive units and offset veins indicate that the Luming Intrusive Suite is composed of five major, successive granitic intrusions: the premineralization plutonic biotite monzogranite and monzogranite units, and the synmineralization stock- and dike-like porphyritic monzogranite, granite porphyry, and syenogranite units. Each synmineralization unit is associated with similar vein sequences that comply with the general form of early EB-type biotite veins, through A-type quartz ± biotite and B-type quartz-molybdenite veins, to late D-type quartz-molybdenite ± pyrite ± chalcopyrite, molybdenite, quartz-pyrite ± calcite, and calcite ± clays veins. The intensity and volume of alteration and mineralization within a given synmineralization unit decrease from early- through inter- to late-mineralization units. Although minor Mo mineralization is associated with potassic alteration along B-type veins, the majority of the ore is associated with D-type quartz-molybdenite-pyrite and molybdenite veins rimmed by sericite-chlorite-pyrite alteration, which are primarily hosted in the two premineralization units.A combination of laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) zircon U-Pb and hydrothermal biotite 40Ar/39Ar studies, together with available isotope dilution-inductively coupled plasma-mass spectrometry (ID-ICP-MS) molybdenite Re-Os data, has resulted in a substantial reappraisal of the timing of magmatism and its association with molybdenite mineralization at Luming. The volumetrically dominant premineralization intrusive units have indistinguishable zircon U-Pb weighted mean 206Pb/238U ages ranging from 187.5 ± 2.8 to 186.5 ± 3.6 Ma (2σ), whereas the synmineralization units yield weighted mean 206Pb/238U ages from 178.6 ± 2.2 to 175.6 ± 3.0 Ma (2σ). The zircon U-Pb weighted mean 206Pb/238U ages of the synmineralization units are indistinguishable from the mean molybdenite Re-Os model (178.1 ± 2.7; 2σ) and hydrothermal biotite 40Ar/39Ar plateau (174.7 ± 1.1 Ma; 2σ) ages within uncertainty, confirming a genetic link with mineralization. Melt inclusion data show that the synmineralization intrusions were Mo poor, with Mo concentrations <4 ppm.The data presented here suggest that molybdenite mineralization at Luming was most likely accomplished through three discrete magmatic-hydrothermal events during assembly of the Mo-poor synmineralization intrusive complex. The giant Luming deposit appears to be related to multiple pulses of magmatic-hydrothermal activities, resulting in the superposition of temporally distinct mineralization events. Our results suggest that pulsed release of ore-forming magmas and fluids, which are channeled along focusing structures like small porphyry fingers within a focused area, from a large magma chamber at depth may play a major role in the formation of large to giant porphyry Mo deposits of both the arc-related and Climax types. This conclusion is in line with field observations of a number of large to giant porphyry Mo deposits, which commonly show reversals in magmatic-hydrothermal evolutionary trend and are associated with multiple pulses of small stocks and dikes that are separate in time and space.

Synsedimentary to Diagenetic Cu±Co Mineralization in Mesoproterozoic Pyritic Shale Driven by Magmatic-Hydrothermal Activity on the Edge of the Great Falls Tectonic Zone–Black Butte, Helena Embayment, Belt-Purcell Basin, USA: Evidence from Sulfide Re-Os Isotope Geochemistry

Abstract The ca. 1,500 to 1,325 Ma Mesoproterozoic Belt-Purcell Basin is an exceptionally preserved archive of Mesoproterozoic Earth and its paleoenvironmental conditions. The Belt-Purcell Basin is also host to world-class base metal sediment-hosted mineralization produced in a variety of settings from the rift stage of basin evolution through the subsequent influence of East Kootenay and Grenvillian orogenies. The mineral potential of this basin has not been fully realized yet. New rhenium-osmium (Re-Os) data presented here for chalcopyrite, pyrite, and black shale contribute to refine a robust genetic model for the origin of the Black Butte copper±cobalt±silver (Cu±Co±Ag) deposit hosted by the ca. >1,475 Ma Newland Formation in the Helena Embayment of the Belt-Purcell Basin in Montana, USA. Chalcopyrite Re-Os data yield an isochron age (1,488±34 Ma, unradiogenic initial 187Os/188Os composition Osi-chalcopyrite=0.13±0.11) that overlaps with the geological age of the Newland Formation. Further, the Re-Os data of synsedimentary to diagenetic massive pyrite yield evidence of resetting with an isochron age (1,358±42 Ma) coincident with the timing of the East Kootenay orogeny. The unradiogenic Osi-chalcopyrite at ca. 1,488 Ma (0.13±0.11) argues for derivation of Os from a magmatic source with a 187Os/188Os isotopic composition inherited from the upper mantle in the Mesoproterozoic (Osmantle 1,475 Ma=0.12±0.02). The unradiogenic Osi-chalcopyrite also suggests limited contamination from a continental crustal source. This source of Os and our new sulfur isotopic signatures of chalcopyrite (–4.1 to +2.1‰-VCDT) implies a dominantly magmatic source for metals. We integrate our new results and previously published geological and geochemical evidence to conceptualize a genetic model in which Cu and metals were largely contributed by moderate-temperature, reduced magmatic-hydrothermal fluids carrying reduced sulfur species with a magmatic origin and flowing as highly metalliferous fluids within the shale sequence. A subsidiary derivation of metals during thermally forced shale diagenesis is possible. Chalcopyrite mineralization replaced locally massive synsedimentary to diagenetic pyrite units close to the sediment-water interface, i.e., an ideal locus where magmatic-hydrothermal fluids could cool and the solubility of chalcopyrite would fall. We suggest that Cu mineralization was coeval with the timing of an enhanced thermal gradient in the Helena Embayment triggered until ca. 1,455 Ma by tholeiitic dike swarm that intruded into Archean basement rocks and intersected the NE-SW-trending Great Falls Tectonic Zone.

Mineralogy and U–Pb geochronology of REE fluorocarbonates from the Gansha Obo REE deposit, northwest China

The Gansha Obo deposit is a recently discovered rare earth element (REE) deposit in northwestern China, with REE2O3 reserves of 0.6 Mt at 1.39–1.65 wt%. It is hosted in the Early Cretaceous Gansha Obo alkaline–carbonatite igneous complex. In this study, we present a detailed investigation of the carbonate mineral assemblage in the economic carbonatite veins in the Gansha Obo REE deposit, including the mineral textures, geochemical compositions, and U–Pb ages. The mineral textures and compositions show that bastnäsite is the primary ore mineral, while parisite and synchysite are both secondary. In situ U–Pb dating of bastnäsite and synchysite yielded ages of 141.8 ± 4.3 Ma (n = 41; MSWD = 0.53) and 53.3 ± 4.4 Ma (n = 85; MSWD = 1.7), respectively. This significant interval of ages cannot be explained by continuous magmatic–hydrothermal activity. The bastnäsite U–Pb age reflects the timing of REE mineralization in the deposit. In contrast, the synchysite U–Pb age records a later tectono-thermal event that might have been related to Eocene collision between the Indian and Eurasian plates. Our results show that REE carbonate U–Pb dating is a powerful tool for understanding the precipitation of REE minerals and, potentially, for identifying regional tectono-thermal events.

Geochemistry of magnetite from the Hongniu–Hongshan Cu skarn deposit in Yunnan Province, SW China

The Hongniu–Hongshan Cu skarn deposit is located in the central part of the Zhongdian porphyry and skarn Cu belt in southwestern China. We investigated the petrography and trace-element chemistry of magnetite in the deposit to examine the nature and origin of the ore-forming fluids. Three types of magnetite were identified in porphyritic granite and skarn in the Hongniu–Hongshan deposit. Mt-type1 crystallized from magmas, is typical magmatic magnetite. Mt-type2 formed later than type1, but earlier than the syn-ore stage, and was related to magmatic–hydrothermal activity. Mt-type3 formed during the early syn-ore stage, is hydrothermal skarn magnetite. The magnetites have variable K, Mg, Al, Ti, V, Cr, Mn, Co, Ni, Cu, Pb, Zn, Ga, W, Sn, and Ge contents. The high contents of Ti, V and Cr in the Mt-type1 is largely controlled by its magmatic origin. For the type 2 magnetites, the high Al, Ti, and Ga contents were controlled by hydrothermal fluid composition and temperature, and V, Sn, and Cr were controlled by fO2 values. The enrichment of Mg, Mn, and Zn in the Mt-type3 was largely due to fluid–rock interactions. Co-precipitating mineral phases appear to have exerted less control on magnetite compositions in the deposit. Some of the Mt-type2 had experienced coupled dissolution and reprecipitation (CDR) processes. The infiltration of saline hydrothermal fluids is considered to enhance CDR processes in the skarn deposit. We suggest that the crystallization of magnetite in the porphyritic granite reduced the sulfate content of the magma, which resulted in Cu and Au being complexed more readily into fluids. The precipitation of Mt-type3 might have resulted in a change from an oxidized to a more reduced environment, thereby creating conditions favorable for the precipitation of Cu sulfides.

Imaging the shallow crustal velocity structure of the Qingchengzi ore field on the Liaodong Peninsula, China, with a short-period dense array using ambient noise tomography

The Qingchengzi ore field, which is located in the middle of the Liaodong Peninsula, has a large number of Au, Ag, and Pb-Zn deposits owing to Mesozoic tectonic and magmatic-hydrothermal activities. To reveal the relationship between shallow crustal velocity and gold mineralization and provide constraints on deep ore prospecting (> 1.5 km), ambient noise tomography consisting of an experimental short-period dense array with 334 seismographs surrounding the study region, is adopted. Shallow crustal velocities at depths less than 5.0 km are obtained by a direct inversion method of Rayleigh wave phase velocity dispersion at a period of 0.5–3.0 s. We obtain the following results. (1) The Jianshanzi fault (JSZF) is well imaged at a depth of 5.0 km. Because ore-forming fluids and materials originated from adjacent concealed granite intrusions characterized by low-velocity anomalies, we infer that the JSZF contributed to mineralization. (2) The Baiyun gold alteration belt in the northern ore field has a high velocity and cuts through the surrounding low-velocity area, extending downward to approximately 0.9 km. This high-velocity belt may be a superposition of the marble of the Dashiqiao Formation and mineralization-alteration, which suggests that the depth of the belt may be of favorable prospecting potential. (3) Based on the obvious low-velocity intrusive anomalies beneath the southern Xiaotongjiapuzi gold belt, we deduce that the mineralization of the Xiaotongjiapuzi gold belt is controlled by unexposed granite intrusions and thus has high mineralization potential at great depths. (4) A geologically reasonable model of the distribution of possible concealed granite intrusions related to deep mineralization and two prospective mineralization areas is presented.

Depositional age and genesis of the host strata of the Shuangjianzishan Ag–Pb–Zn deposit: Implications for the Late Carboniferous magmatic–hydrothermal activities and tectonic evolution of the eastern central Asia orogenic belt

The Shuangjianzishan Ag–Pb–Zn deposit is hosted by an over 2030-m-thick succession of Late Carboniferous slate-dominated deep-water sediments. Three tuff layers interbedded with slates yield zircon U–Pb ages of 314.4 ± 2.5 Ma, 315.9 ± 3.6 Ma, and 317.9 ± 2.3 Ma. Using Monte-Carlo simulations, we constrain the maximum time span between the bottom tuff and the top tuff to no longer than 6.9 Myr at a 2σ (95.3%) confidence level. On the basis of this time span, the long-term accumulation rate of the Shuangjianzishan slates is 294 m/Myr. This accumulation rate is faster than any subduction-related accumulation rates by at least 30%; it is similar to that of fine deposits in a rift setting. The variably low δ18O of the zircons from the interbedded tuffs also suggest that a high-temperature hydrothermal alteration occurred during the generation of the low δ18O parental magma. Magmatic–hydrothermal events related to a rifting event are the most likely mechanism that caused the low δ18O signature. The chemical compositions of the slates suggest that their source rocks were very immature, probably similar to contemporary intermediate to acidic volcanic rocks. On the basis of down-well logging data, the dipping angle data are plotted with the stratigraphic depth of each stratum. The stable dipping angle of the slates also supports a sedimentation processes in an active rifting environment.

Trace-element systematics and isotopic characteristics of sphalerite-pyrite from volcanogenic massive sulfide deposits of Betul belt, central Indian Tectonic Zone: Insight of ore genesis to exploration

• Co/Ni ratio of pyrite characterized submarine-magmatic hydrothermal ore process. • High-temperature VMS system > 300 °C formed from multiple sulfur sources. • Geochemical trends in sphalerite and pyrite by LA-ICP-MS suggest VMS-type deposits in the Betul belt. • PCA on pyrite and sphalerite helps in the reconstruction of the metallogeny and exploration prospectivity. • Chalcopyrite disease in sphalerite reflect high-temperature ore fluid and metamorphism . The Paleoproterozoic volcanogenic massive sulfide (VMS) deposits in the Betul belt (BB) (ca.1.7 Ga) in central India are well-known for their low-grade Zn-Cu-Pb sulfide mineralization. The physicochemical conditions of ore formation, size, grade, extent, and nature of mineralization are the ongoing debates and concerns to the exploration agencies for the last two decades. Field examination, drill core analysis, petrography, and scanning electron microscopic studies reveal the occurrence of chalcopyrite, sphalerite, pyrite, galena with minor pyrrhotite in the form of dissemination, stringers, and semi-massive sulfide veins. We present the minor-trace element contents in sphalerite and pyrite from drill core samples of four deposits (Banskhapa, Jangaldehri, Biskhan, and Bhuyari), analyzed by electron probe microanalyzer (EPMA) and in-situ laser ablation inductively coupled plasma mass spectroscopy (LA-ICP-MS) for determining ore-forming temperature, ore genesis and exploration prospectivity. The EPMA and LA-ICP-MS studies of sphalerite display a slightly high concentration of trace elements like Mn, In with a mean value of 3533 ppm and 33 ppm, respectively. On the other hand, Ga, Ge, and Ag content are low in abundance along with a low concentration of Bi, Pb, and Sb in all four deposits. Pyrites show low Ni and high Co/Ni (mostly > 1) ratios and variable Ti, Se, As, and Mn. Multivariate statistical analysis, especially the principal component analysis (PCA) of these trace elements, defines the geochemical variations among the Betul belt deposits. The ore-forming temperature is estimated using sphalerite thermometry (374 °C to 402 °C) and pyrite thermometry (225 °C to 484 °C). The geochemical discrepancies occur due to magmatic-hydrothermal activities caused by the sub-volcanic intrusions, followed by recrystallization and remobilization of the sub-microscopic inclusions during subsequent metamorphism. Additionally, sulfur isotope analysis provides insights into the sulfur source variations, metals, and mineralization. Carbon and Oxygen isotopes are used to understand the physicochemical conditions of the ore-forming environment. Sphalerite, pyrite, and chalcopyrite from Banskhapa deposit suggest a broad range of δ 34 S values (7.27–7.79‰, 8.37–8.61‰, and 6.58–6.72‰ respectively, n = 7), which advocate sulfur mostly derived by thermochemical reduction (TSR) of seawater sulfate with some input of magmatic sulfur leaching from the igneous basement. Carbon and oxygen isotope studies of carbonates from Jangaldehri and Bhuyari deposits (δ 13 C: −6.0 to −13.45‰ with an average value of −9.51‰, n = 7; δ 18 O: 7.32‰ to 20.58‰ with an average value of 15.65‰, n = 7) indicate that the carbonates formed from a hydrothermal fluid (seawater with an insignificant magmatic input), the mixing of hydrothermal fluid with ingressing seawater, or both in the mineralized zone. These processes collectively led to Zn-Cu and Zn-Cu-Pb mineralization in the Betul belt. High temperature, strong isotopic and trace element geochemical variations in sphalerites (Ag-Mn, In/Cd-Mn, In/Cd-Fe, Mn-In/Ge, Fe-In/Ge, Fe-In) and pyrite (Ni-Co, As-Cu, Ag-Au, and Se-Te) attest to the mineralization as being volcanogenic massive sulfide (VMS) deposits in Betul belt and differentiated from other genetic types like Mississippi Valley-type (MVT), sedimentary-exhalative (SEDEX), skarn, porphyry, and epithermal hydrothermal deposits.

The influence of magmatic-hydrothermal activities on porosity and permeability of sandstone reservoirs in the Linxing area, Ordos Basin, Northern China

The eastern margin of the Ordos Basin in the Linxing area has great potential for exploration of natural gas. However, the intrusion of the Zijinshan igneous pluton and associated hydrothermal activities have affected the sandstone reservoirs. Through the data analyses of rock thin sections, cathodoluminescence, X-ray diffraction, vitrinite reflectance, fluid inclusions, and reservoir physical property, this paper aims to clarify the influences of magmatic-hydrothermal activity and to explore the response characteristics of reservoir porosity and permeability. From the outer zone to the inner zone, the closer to the pluton, the closer the contact between the reservoir clastic particles, which confirms the compression caused by magma intrusion. The differences in feldspar dissolution and alteration, siliceous cementation, clay mineral content, vitrinite reflectivity, homogenization temperature and salinity of fluid inclusions, confirmed that magmatic-hydrothermal activity affected fluid properties and evolution of strata temperature. Combined with the vertical distribution characteristics of feldspar dissolution-alteration and the abnormal conversion of clay minerals, it is considered that the influence of hydrothermal fluid was limited to the reservoir under the thick mudstone of the Xiashihezi Formation. Magma intrusion causes the development of reservoir fractures, thus the permeability of the reservoir decreases from the inner zone to the outer zone, and the porosity presents the distribution pattern of “high-low-slightly high”. The reservoir porosity and permeability genetic model was established to provide references about favorable areas for oil and gas exploration and development in the Linxing area and areas with similar geological history.

Geology, mineralogy, ore paragenesis, and molybdenite Re-Os geochronology of Sn-W (-Mo) mineralization in Padatgyaung and Dawei, Myanmar: Implications for timing of mineralization and tectonic setting

• Magmatic-hydrothermal activity occurred as several discrete events in the Western Granite Province of SE Asia. • The Sn-W (-Mo) mineralization constrained to Cretaceous to Paleocene age. • Extensive granite-associated Sn-W (-Mo) mineralization events in the Western Granite Province occurred during ca. 75 Ma and 60 Ma. The Sn-W (-Mo) deposits of Myanmar are mostly located in the Western Granite Province that is well known for its world-class Sn-W (-Mo) deposits. Previous studies have constrained the age of the granitic intrusions of the province and the timing of mineralization for a few deposits, but most of the mineralization ages are not well established. In this study, new molybdenite Re-Os dating of two Sn-W-(Mo) regions, Padatgyaung and Dawei, together with their geological setting and mineral paragenesis are carried out to constrain the timing of ore formation and geodynamic setting. In the Padatgyaung region, two weighted average Re-Os model ages of 64.23 ± 0.29 Ma (MSWD = 0.49, 2σ) and of 60.54 ± 0.45 Ma (MSWD = 1.3, 2σ) from vein molybdenites are considerably younger than molybdenite from tin mineralized greisen which has a weighted Re-Os model age of 68.5 ± 2.7 Ma (MSWD = 0.14, 2σ). This demonstrates that the vein-type W-Mo mineralization formed after tin mineralized greisenization. Combining our new age data with previous geochronological data, the Re-Os model age of 63.09 ± 0.17 Ma from the Wagone quartz vein suggests that the Sn-W(-Mo) mineralization in the Dawei region took place at around 70–60 Ma (Late Cretaceous to Paleocene). This study indicates the presence of a significant and discrete granite-related Sn-W(-Mo) mineralization with an age of 75–60 Ma in the Western Granite Province, although the overall age range of Sn-W mineralization in the belt spans from 120 to around 40 Ma emplaced during normal subduction and roll-back of the Neo-Tethyan oceanic crust.

On Archean craton growth and stabilisation: Insights from lithospheric resistivity structure of the Superior Province

The nature of lithospheric evolution and style of the driving tectonic processes during the growth and stabilisation of continental crust in the Archean remain enigmatic. The hotter, rheologically weak Archean crust would be unable to support thick orogens. Thus, gravitational collapse likely occurred when continental fragments became overthickened and/or when far-field stresses were relieved during the terminal stages of orogenesis. 3D magnetotelluric resistivity models of the Archean Superior Province, reveal the presence of low resistivity zones in the mid-lower crust that reflect a protracted history of magmatic-hydrothermal activity contemporaneous with construction and collapse. These include sub-vertical zones of low resistivity in the mid-upper crust, inferred to represent corridors of paleo-fluid flow along crustal-scale structural networks developed in response to terrane amalgamations. Subsequent orogenic collapse resulted in widespread lateral flow within the lower crust accommodated by sub-horizontal shear zones and included magmatic refertilisation. Thus, the preserved low-resistivity anomalies in the mid-lower crust represent an amalgamation of magmatic-hydrothermal and deformational processes that occurred during construction, peak orogenesis, and collapse in the Archean.

LA-ICPMS trace elements of pyrite from the super-large Dulong Sn-Zn polymetallic deposit, southeastern Yunnan, China

滇东南马关都龙是一个以锡锌为主，共-伴生铟、铜、铅、钨、铁、银等多种元素的锡锌多金属超大型矿床。虽然前人从矿物学、矿床地球化学、年代学等不同角度开展了较多的研究，该矿床锡锌多金属矿化为燕山晚期岩浆热液活动的产物已是不争的事实，但关于该矿床是否存在热水沉积作用及其与锡锌多金属成矿作用的关系依然存在较大争议。本文选取都龙矿区广泛存在的黄铁矿作为主要研究对象，在矿相学基础上利用LA-ICPMS对不同阶段黄铁矿的微量元素组成开展了系统的研究。野外及显微鉴定结果表明，矿区存在四种类型（期次）的黄铁矿，即：鲕状黄铁矿Py1；穿切或交代Py1的细脉状黄铁矿Py2；与闪锌矿等硫化物共生的自形黄铁矿Py3；包裹早期黄铁矿或闪锌矿等硫化物的他形黄铁矿Py4。LA-ICPMS分析结果表明，该矿床黄铁矿中富集多种微量元素，其中Co、Ni、As、Ge等元素以类质同象的形式存在黄铁矿晶格中，而其余元素多以显微矿物包体形式赋存于黄铁矿中。上述四期黄铁矿微量元素组成存在较大差别，Py1相对富集Zn和As，而其余微量元素含量较低，Co与Ni含量较低，Co/Ni比值远低于1.00，其微量元素组成与典型沉积作用形成黄铁矿基本一致；Py2与Py1具有相似的微量元素组成特征，其Co/Ni比值接近Py1变化范围；Py3和Py4除富集Zn、As外，Mn、Co、Ni、Cu、Sb、Pb、Bi元素含量也相对较高，其Co/Ni比值相对较高，多大于1，与典型岩浆热液型黄铁矿微量元素组成相似，而与沉积型黄铁矿差异明显。结合各阶段黄铁矿产出地质特征，对比不同类型黄铁矿微量元素组成，本研究认为：Py1鲕状黄铁矿为热水沉积作用形成；Py2为Py1变质改造形成的细脉状黄铁矿，其微量元素继承了Py1；Py3为岩浆热液活动形成的自形黄铁矿；Py4为岩浆热液活动晚期形成的他形黄铁矿，Ag和Bi组成作为区分不同成因类型黄铁矿的化学指标的潜力。矿区早期沉积作用形成鲕状黄铁矿过程可能为后期成矿作用提供了部分硫源及少量Zn等成矿物质，海西-印支期区域变质改造作用对矿区成矿作用影响不大，而燕山晚期岩浆热液活动才是矿区锡多金属大规模成矿作用的主导因素。;The Dulong Sn-Zn polymetallic deposit, located in the southeastern Yunnan Province, is a super-large Sn-Zn polymetallic deposit with by-products of In, Cu, Pb, W, Fe, Ag, etc. Many studies on mineralogy, ore deposit geochemistry and chronology have been carried out on this deposit, confirming a genetical relationship of its Sn-Zn mineralization to the Late Yanshanian magmatic activity. However, whether the deposit underwent exhalative sedimentary Sn-Zn mineralization is still controversial. Based on detailed petrographic observation, the LA-ICPMS trace element compositions of pyrite that widely occur in the Dulong deposit are investigated systematically in this paper. Field and microscopic observations show that four types of pyrite have been identified in Dulong, including the oolitic pyrite (Py1), fine vein pyrite (Py2) crosscutting and/or replacing Py1, euhedral pyrite (Py3) coexisting with sphalerite and other sulfides, anhedral pyrite (Py4) enclosing early generated pyrite and sphalerite. LA-ICPMS analysis results suggest that pyrite is enriched in Co, Ni, As and Ge, which are mainly incorporated into the pyrite lattice in the form of isomorphism. Other elements occur mostly as mineral micro-inclusions in pyrite. The trace element compositions of the four types' pyrite exhibit significant difference from each other: Py1 is enriched in Zn and As but depleted in Co, Ni and Cu with low Co/Ni ratios (<1.0), which share similar features with typical sedimentary pyrite; Py2 is similar in its trace element compositions and Co/Ni ratios to those in Py1; while Py3 and Py4 have elevated Zn, As, Mn, Co, Ni, Cu, Sb, Pb and Bi contents with high Co/Ni ratios (mostly >1.0), which is consistent with that of euhedral pyrite formed by magmatic-hydrothermal activity rather than sedimentary pyrite. Comparing with the trace element compositions of pyrite in different types, we consider that Py1 is of sedimentary origin, Py2 that inherits the trace element features of Py1 is formed by metamorphic reworking of Py1, while Py3 and Py4 are formed by late magmatic-hydrothermal activity. Ag and Bi in pyrite display potential probobilities as a geochemical indicator to distinguish pyrite of different genetic types. Oolitic pyrite formed in the depositional diagenesis stage may provide some sulfur and a small amount of Zn for the later Sn-Zn mineralization. The regional metamorphic reworking in the Hercynian to Indosinian displays a limited effect on the Sn-Zn mineralization, while Late Yanshanian magmatic-hydrothermal activity is the controlling factor for the formation of the super-large scale Dulong Sn-Zn polymetallic deposit.

Extensional tectonic setting of the Triassic metallogenic event in the Xiaoqinling goldfield: Evidence from &lt;sup&gt;40&lt;/sup&gt;Ar-&lt;sup&gt;39&lt;/sup&gt;Ar chronology of the tectonite

小秦岭金矿田位于华北克拉通南缘，同时也是秦岭复合型造山带的北缘组成部分。这里是我国第二大黄金产地，大规模金的成矿作用形成于早白垩世岩石圈大规模减薄的区域伸展构造背景。此外，越来越多的同位素年代学数据显示区内还存在三叠纪的成矿事件，发育钼、铅、铀、铌等与岩浆热液活动密切相关的多金属矿床，主要分布在小秦岭的南北边缘。目前，对于三叠纪多金属成矿作用的构造背景研究尚属薄弱，还没有直接对有关的构造变形开展过专题研究。为此，我们对小秦岭金矿田东端边缘的构造带进行构造观察和测量，并采用⁴⁰Ar-³⁹Ar年代学方法厘定变形时限，揭示三叠纪成矿事件的构造背景，为深入认识区内三叠纪多金属成矿作用特征和规律提供构造证据。研究表明，小秦岭金矿田三叠纪成矿事件发生于晚三叠世的伸展构造背景，可以划分为两个阶段，早期（221~216Ma）是在碰撞向后碰撞转折阶段的重力滑脱构造环境中，发育火成碳酸岩型钼铅、铀铌铅多金属矿床；晚期（214~203Ma）是在后碰撞阶段由俯冲板片断离或岩石圈拆沉作用等深部过程导致的区域伸展构造环境中，发育石英脉型和斑岩型钼矿床。小秦岭中生代晚三叠世和早白垩世两期成矿事件都是在伸展构造背景中的构造-岩浆-流体活动的产物。;The Xiaoqinling goldfield is located in the southern margin of the North China craton, which is also a marginal part of the Qinling complex orogeny. This goldfield is the second largest gold producer in China, where the large-scale gold mineralization developed in the extensional tectonic setting due to regional lithospheric thinning in Early Cretaceous. In addition, a growing number of isotope chronological data indicates that there also developed metallogenic events in Triassic here, which resulted in the formation of polymetallic deposits, such as molybdenum, lead, uranium, and niobium, etc. The Triassic deposits are closely related to magmatic hydrothermal activities, which are mainly distributed in northern and southern margins of the Xiaoqinling goldfield. However, there is little study on the tectonic setting of the Triassic polymetallic mineralization so far, especially the work on tectonic deformation is almost absent. Therefore, we have carried out the observation and measurement on the structure developed within the tectonic belt occurred in the eastern end of Xiaoqinling goldfield, and then employed the ⁴⁰Ar-³⁹Ar chronology method to determine the time limit of deformation, to reveal the tectonic setting of Triassic metallogenic event, providing tectonic evidence for further understanding characteristics and patterns of Triassic polymetallic mineralization in the Xiaoqinling goldfield. Our study shows that the metallogenic event occurred in the extensional tectonic setting in Late Triassic, which can be divided further into two stages. The early stage (221~216Ma) is of the transition stage from collision to post-collision, developing gravitational decollement tectonics, and carbonatite-type molybdenum, lead, uranium, and niobium polymetallic deposits are well developed in this extensional tectonic setting. The late stage (214~203Ma) is of post-collision, developing regional extensional tectonics caused by the break off of subducting slab or lithospheric delamination, and quartz vein-type and porphyry molybdenum deposits are well developed. The two metallogenic events in Late Triassic and Early Cretaceous in the Xiaoqinling goldfield are the products of the tectonic deformation-magmatic activity-fluid flow in extensional tectonic setting.