Ore-forming Magmas Research Articles

Multiscale 3-D imaging of the crustal electrical structure beneath the Caosiyao porphyry Mo deposit, North China

SUMMARY We proposed a stepwise procedure to obtain a multiscale 3-D resistivity model beneath the Caosiyao porphyry Mo deposit. Due to the joint inversion of data collected from 302 audio-frequency magnetotelluric and 33 broad-band magnetotelluric sites, this model can simultaneously combine both the shallow resolution and detection depth. The near-surface part of the model reveals the high-resolution structures. Specifically, a high-resistivity body (>500 Ω m) is interpreted as the late Jurassic porphyry intruding into the metamorphic basement. Two NE-trending high-conductivity (<100 Ω m) belts are interpreted as fault zones. The one in the southeast extends deeply (>15 km) is suggested to be a crust-scale weak zone and conduit for transporting ore-forming magmas and fluids. The other one is shallow (<2 km), but coincides with outcropped orebodies and alteration zones, so it may have played an important role in the hydrothermal alteration and mineralization. A predominant high-conductivity block (<30 Ω m) occurs within the lower part (∼15–30 km) of the model and connects with Caosiyao deposit through the inferred crust-scale fault. Its enhanced conductivity may result from the abundant fluids. We determined it had been a mid-lower crustal magma chamber providing space for the mixing of ore-forming magmas and fluids, as well as the element exchange during the mineralization. Combined with a previous magnetotelluric imaging, we suggested the heats and fluids needed for the mineralization might have been transported from a giant upper-mantle basaltic magma reservoir as indicated by a conductor beneath the Datong volcanic area through a lithosphere-scale shear zone. Summarily, the multiscale resistivity model provides us a chance to depict the multiscale metallogenic system for the Caosiyao porphyry Mo deposit from the aspect of electrical resistivity.

Apatite halogens and Sr-O and zircon Hf-O isotopes: Recycled volatiles in Jurassic porphyry ore systems in southern Tibet

In-situ analysis of minerals, such as zircon hafnium (Hf) and oxygen (O) isotopes, has been pivotal in investigating magmatic evolution and related ore systems. In order to better constrain the petrogenesis of Cu-porphyries and related ore-forming processes in southern Tibet, we used a combined application of both zircon by Hf-O isotopes and apatite by developing geochemical indicators involving Sr-O isotopes and volatile (chlorine and sulfur) concentrations. Apatite has several advantages over zircon: it carries ore-related volatiles, is present in less-evolved magmas and is sensitive to magmatic processes. Data on major- and trace elements and Sr-O isotopes in gem-like apatite can supplement information from zircon Hf-O isotopes. We have applied these techniques to granitoids from the Jurassic arc in southern Tibet to illustrate their use in tracing volatile evolution and related genesis of porphyry systems. We demonstrate that (1) robust apatite Sr-O isotopes combined with zircon Hf-O isotopes can record recycled oceanic components in the sources of Gangdese Jurassic porphyries; (2) volatile-enriched magmas metasomatised hydrous mantle wedge to form the Jurassic mineralised rocks; (3) the ore-forming magma with higher contents of water, Cl and S, favoured the transfer of metals and volatiles to the upper crust and to form the Jurassic porphyry Cu-Au deposits.

Magmatic evolution and formation of the giant Jiama porphyry-skarn deposit in southern Tibet

The post-collisional Gangdese metallogenic belt with over 45 Mt Cu has attracted worldwide attention. We report the whole-rock geochemistry, zircon SIMS U–Pb geochronology, and in situ zircon Hf–O isotopic compositions of the Jiama deposit in the Gangdese belt. The intermediate to the felsic intrusive rocks are characterized by high SiO2 (64.8 to 78.8 wt. %) and Al2O3 (12.1 to 16.3 wt. %) contents, high Sr/Y ratios (43–74), showing the geochemical characteristics of adakites. Zircons from four intrusives yield U–Pb ages of 16.19 ± 0.20 Ma, 15.67 ± 0.32 Ma, 15.44 ± 0.16 Ma, and 14.90 ± 0.16 Ma, respectively, which brackets magmatic event related to mineralization in 1.3 Myr (16.19 to 14.90 Ma). Magmatic zircons have intermediate δ18O (+5.21 to + 7.18‰), and depleted Hf isotopic values (εHf(t) = 1.8 to 9.4). In combination of whole rock, plagioclase, and zircon geochemistry, we suggest the main ore-forming magma was derived from the remelting of the newly formed lower crust. During the whole journey of magma ascending from lower crust to the surface, no significant injection or mixing of mafic melt from different source has been observed. This is supported by three lines of evidence: 1) There is no clear reverse zonation in plagioclase, 2) the major and trace element compositions of continuous lithologies from diorite to granite shows no significant mixing, 3) there is no significant change of the zircon O and Hf isotope among different intrusions. Our study suggests that the formation of the giant porphyry deposits such as Jiama in post-collisional setting likely form in a short-term (<1.5 Myr) without significant magma mixing during magmatic evolution from diorite to granite.

In situ U-Pb geochronology, elemental and Nd isotopic compositions of titanite from the Mesozoic porphyry Mo deposits, NE China

Northeastern (NE) China is one of the most important regions for Mo resources in China with more than 80 porphyry/skarn Mo deposits, most of which were formed in the Mesozoic. Geology, geochemistry and geochronology of individual deposits have been studied and a large amount of data was accumulated, but the factors controlling large-scale Mo mineralization in this region is still debated. To better understand the factors controlling the Mo mineralization, three typical large to giant Mesozoic porphyry Mo deposits located in NE China, namely the Daheishan (DHS), Fu’anpu (FA) and Luming (LM) deposits have been selected. In this paper, U-Pb age, elemental and Nd isotopic compositions of titanite from these deposits have been analyzed to reveal their chemical and isotopic compositions, which provide insights into the nature of source magmas and unravel the physical and chemical conditions of Mo mineralization. In-situ titanite Nd isotopic compositions from all the ore-related granites indicate that they originate from a juvenile crustal source. Titanite occurring as euhedral grains in DHS, FA and LM ore-related granites are characterized by high Th/U and LREE/HREE ratios, as well as low Lu/Hf and 147Sm/144Nd ratios, indicate a magmatic origin. In the LM-barren granite, titanite occurs as magmatic and hydrothermally altered type. The euhedral envelope-shaped magmatic titanite has the same characteristics as the titanite in DHS, FA and LM ore-related granites, with higher Th/U and LREE/HREE ratios and lower147Sm/144Nd ratios. In contrast, hydrothermally altered titanite in the LM-barren granite has lower Th/U and LREE/HREE ratios, as well as higher Lu/Hf and 147Sm/144Nd ratios, distinct Eu anomaly and tetrad effect than the magmatic titanite. In-situ magmatic titanite U-Pb dating of DHS, FA, LM ore-related granite yields crystallization ages of ca.168 Ma, ca.166 Ma and ca.178 Ma, respectively, which are consistent with the mineralization ages within error. Hydrothermally altered titanite in the LM-barren granite yields a crystallization age of 177.9 ± 2.3 Ma (MSWD = 2.3), which is indistinguishable from the LM-ore granites and molybdenite Re-Os age, implying that the formation of the Luming porphyry Mo deposit was coeval with the wall-rock alteration. Relatively homogeneous and consistent Nd isotopic compositions of magmatic titanite from LM-ore granite and hydrothermally altered titanite from LM-barren granite suggest that the hydrothermal fluids have the same origin with the granite and could be directly exsolved from the ore-forming magma. Trace element characteristics of magmatic titanite from these porphyry Mo deposits show that the magma of ore-related granite has higher oxygen fugacity and water contents than those of the LM-barren granite. Hydrothermally altered titanite in the LM-barren granite shows higher F contents than that in the magmatic titanite. The results from this study show that high Mo content in the source region is a prerequisite for the formation of porphyry Mo deposits but the specific magma composition (e.g., with abnormally high Mo contents) may be not critical. Other factors such as the higher F, oxygen fugacity and water contents are also important for Mo mineralization, which could lead to the magma evolution and accumulation of metal elements.

Sulfide Ore Formation of the Kalatongke Ni-Cu Deposit as Illustrated by Sulfide Textures

Abstract The Kalatongke magmatic Ni-Cu deposit features high Ni-Cu grades compared with other Ni-Cu deposits in the Central Asian orogenic belt. The sulfides, mainly hosted by olivine norite and gabbronorite, are characterized by high Cu/Ni ratios. There is wide variety of textural relationships in the mineralized rocks, including globular, sulfide matrix, emulsion, disseminated, net-textured, and semimassive to massive textures. Quantitative textural measurements reveal that more than 65 vol % of the total sulfide volume in disseminated ore (defined as containing 4–10 vol % sulfide) and more than 90 vol % of the total sulfide volume in net-textured ore (typically 16 vol % sulfide) are hosted in few interconnected networks with equivalent sphere diameters (ESDs) larger than 6 mm. This illustrates that sulfide coalescence is a critical process. The remaining sulfide blebs define two groups of log-linear particle size distribution (PSD), i.e., a finer group (ESD of 0.080–&amp;lt;0.529 mm) and a coarser group (ESD of 0.529–4.084 mm). The PSD of the finer group differs slightly among different types of mineralization. The origin of this group of sulfides is attributed to sulfide nucleation simultaneously with crystallization of the olivine-orthopyroxene-plagioclase phases from the ore-forming magma in the current magma chamber. The PSD of the coarser group from net-textured ores is parallel to that from the disseminated ore but has lower intercept values. Modeling results show that aggregation of ~40–70% sulfide blebs of different grain size from the disseminated ore into the networks can generate the coarser group sulfide PSD of net-textured ore. In addition, monosulfide solution (MSS, pyrrhotite component)-enriched sulfide globules are commonly located close to sulfide matrix ore breccias and emulsion-textured ores, rimming the net-textured and massive orebodies. These globules are different from the sulfide component that was in equilibrium with the mafic magma, suggesting they were formed by mechanical remobilization of a cumulus MSS-enriched component from a previously segregated and partially crystallized sulfide pool. These observations, combined with the ubiquitous chilled margin xenoliths in the high-grade ores, suggest that the Kalatongke deposit was the result of voluminous magma flow through the current location accompanied by sulfide reworking and percolation. All these textural characteristics could be explained by emplacement within a laterally propagating bladed dike. Moreover, the deformation recorded in the emulsion-textured and massive ores suggests the fault system remains active during magma solidification, driving downward migration of sulfide from the sulfide pool at magmatic temperature to form vein-type massive ore. We suggest that the Kalatongke deposit formed by magma pulses injecting into the current location and that the tectonic movement remains active during the solidification of the intrusion. The syntectonic emplacement model, which may be a common feature in the Central Asian orogenic belt Ni-Cu deposits, indicates that the fault systems beneath the intrusion are of great exploration interest for high-grade ores.

Evaluating Sn mineralization potential at the Haobugao skarn Zn–Pb deposit (NE China) using whole-rock and zircon geochemistry

The metal association in a skarn deposit is largely controlled by the magma redox state and fractionation degree. The Haobugao skarn deposit in northeastern China contains significant Zn–Pb sulfides, with subordinate by-products Cu and Fe ores. Recent explorations also found minor cassiterite occurring locally in the deposit, inspiring the speculation of Sn mineralization. To better evaluate the Sn mineralization potential at Haobugao, the geochemical compositions of the ore-related biotite granite porphyry and its magmatic zircons have been investigated to quantify the oxidation state and the degree of magmatic evolution, which provide constraints on the capability of the Haobugao intrusion to produce economic Sn mineralization.The causative biotite granite porphyry (~140 Ma) of the Haobugao skarn is a typical I-type granite that belongs to high-K calc-alkaline series. It contains 68.88–72.27 wt% SiO2 and 0.52–1.38 wt% MgO, and has a Rb/Sr ratio range of 0.82–2.07 and a Nb/Ta ratio range of 8.69–22.49, indicating that it is moderately fractionated. The zircon Ce and Eu anomalies have been used as proxies for magma redox state, and the results (CeN/CeN* = 36–213, EuN/EuN* = 0.17–0.52) suggest that the ore-forming magmas are relatively oxidized, with an estimated ΔFMQ range of −0.4 to +3.5 (+1.0 in average). This is also consistent with the high whole-rock Fe2O3/(FeO + Fe2O3) mass ratios (0.54–0.76) and the occurrence of abundant ferric garnet in the skarn (i.e., andradite). These results collectively show that the ore-related granitic intrusion at Haobugao is generally more oxidized but less evolved when compared with other intrusions responsible for typical skarn Sn mineralization worldwide. Therefore, it is unlikely for the Haobugao intrusion to produce Sn mineralization of economic significance. The present study demonstrates that the whole-rock and zircon geochemistry data are useful in evaluating the mineralization potential of metals like Sn in a given mineral deposit or prospect, which may help guide exploration.

Genesis of High-Mg Adakites in the Southeastern Margin of North China Craton: Geochemical and U-Pb Geochronological Perspectives

In the eastern North China Craton (NCC), Mesozoic tectonics was dominated by the Paleo-Pacific subduction rollback and the Tanlu crustal-scale fault movement. The regional transtension had generated extensive adakitic magmatism, some Cu-Au ore-forming but others not. To establish the geodynamic setting and any metallogenic link for the adakites from the southeastern NCC margin, we analyzed the ore-barren adakitic rocks from underground mines in the Huaibei-Linhuan coalfield (where surface igneous outcrops are scarce), and compared their ages and geochemistry with other mineralized and ore-barren adakites across Eastern China. Zircon U-Pb dating reveals two magmatic episodes in the Huaibei-Linhuan coalfield: 1) early-Early Cretaceous (ca. 130–129 Ma) (quartz-)diorite and granodiorite, and 2) late-Early Cretaceous (ca. 115.8 and 105.8 Ma) microgabbro and dolerite. Whole-rock geochemistry indicates that the (quartz-)diorite and granodiorite are high-Mg adakitic, featured by low K2O/Na2O (avg. 0.33), high Sr/La (avg. 44.3), and lack of correlation between SiO2 (fractionation index) and Sr/Y (avg. 56.55) and MREE/HREE (avg. 1.09), resembling typical adakites derived from oceanic-slab partial melting. Geochronological correlation with the regional tectonic events suggests that the slab-melting may have been caused by the Paleo-Pacific subduction rollback. Further extension and crustal thinning in the late-Early Cretaceous along the southern Tanlu fault may have formed the gabbro-dolerite in the coalfield. Geochemical comparison suggests that parental magma of the Huaibei-Linhuan adakites may have had similar water content [similar zircon 10,000*(Eu/Eu*)/Y and Eu/Eu* ratios] to typical porphyry Cu-Au ore-forming magmas, yet the former may have been considerably more reduced (lower zircon Ce/Nd and whole-rock V/Sc ratios). We considered that the assimilation of Carboniferous-Permian coal seams in the area may have further lowered the magma fO2 and thus its potential to form Cu-Au mineralization.

Increasing sulfur and chlorine contents in ore-forming magmas: The key to Pulang porphyry Cu-Au formation, SW China

The giant Pulang porphyry Cu-Au district (446.8 million tonnes at 0.52% Cu and 0.18 g/t Au) is located in the southern Yidun terrane of the Sanjiang (Three-river) region, southwest China. It has six phases of porphyry intrusions: premineralized fine-grained quartz diorite (FQD) and coarse-grained quartz diorite porphyry (CQD), the main synmineralized quartz monzonite porphyry (QMP), and late synmineralized granodiorite porphyry (GP), diorite porphyry (DP), and aplite. New zircon U-Pb ages for the premineralized FQD and late synmineralized GP, DP, and aplite are 226.3 ± 2.6 Ma (MSWD = 1.9), 216.4 ± 1.0 Ma (MSWD = 2.0), 216.0 ± 1.1 Ma (MSWD = 1.7), and 216.0 ± 1.5 Ma (MSWD = 1.7), respectively. All Pulang porphyry rocks have high whole-rock Sr/Y ratios (37.8–184) with minor or no negative Eu anomalies (0.73–1.04), and are characterized by high large-ion-lithophile-element (LILE: Th, U, Ba, Rb) abundances and low high-field-strength-element (HFSE: Nb, Ta, Ti, Zr, Hf).Zircons from premineralized FQD and synmineralized GP, DP, and aplite rocks have consistently high EuN/EuN* ratios (mostly > 0.4). The magmatic oxygen fugacity calculated from zircon compositions, show that the FQD (ΔFMQ = 1.59 ± 0.43), GP (ΔFMQ = 1.52 ± 0.18), DP (ΔFMQ = 1.52 ± 0.30), and aplite (ΔFMQ = 1.75 ± 0.60) were all derived from moderately oxidized magmas. Magmatic water contents estimated from amphibole compositions and the extensive presence of amphibole phenocrysts suggest that H2O contents are >4 wt%. Together with the published results for premineralized CQD and main synmineralized QMP, it is concluded the causative magmas for the six porphyry phases at Pulang were all relatively oxidized and hydrous.The difference between premineralized FQD and CQD and synmineralized QMP, GP, and DP are reflected in magmatic chlorine and sulfur contents. To minimize the effect of hydrothermal alteration, the apatite microphenocrysts enclosed by least altered phenocrysts were selected for analyses. The Cl contents of magmatic apatite microphenocrysts from the synmineralized QMP (avg = 0.340 ± 0.192, n = 26), GP (avg = 0.317 ± 0.151, n = 21), and DP (avg = 0.179 ± 0.112, n = 18) are significantly higher than those from the premineralized FQD (avg = 0.046 ± 0.032, n = 9) and CQD (avg = 0.043 ± 0.034, n = 13). Similarly, the SO3 contents of igneous apatite microphenocrysts from the synmineralized QMP (avg = 0.259 ± 0.156, n = 25), GP (avg = 0.203 ± 0.163, n = 20), and DP (avg = 0.306 ± 0.173, n = 18) are higher than those from the premineralized FQD (avg = 0.181 ± 0.136, n = 10) and CQD (avg = 0.085 ± 0.068, n = 6). The S-Cl contents of synmineralized melts estimated from apatite S-Cl compositions are higher than those of premineralized magmas, suggesting that the elevated magmatic S-Cl contents in the synmineralized porphyries might have been important for Cu-Au mineralization. This was likely due to S-Cl rich mafic magma recharge, supported by the presence of abundant mafic magmatic enclaves in the synmineralized QMP.

Tectono-magmatic controls on decratonic gold deposits

Magmatic-hydrothermal gold–copper deposits in post-subduction settings represent essential targets for mineral exploration, but controls on their formation remain controversial. The early Cretaceous lode Au districts that formed during lithosphere destruction of the North China Craton provide an ideal opportunity to better understand the key tectono-magmatic factors responsible for the genesis of Au-rich deposits in post-subduction settings. Here, we present a LA-ICP-MS study of silicate melt inclusions and sulfide inclusions from ore-related mafic to intermediate rocks in the central Taihangshan Au district in the interior of the North China Craton to constrain the content and evolution of magmatic ore metals ± volatiles. The results, combined with numerical modeling, suggest that the ore-related magmas contained only a few ng/g Au, which is similar to the Au content of non-mineralization-related mafic to intermediate magmas worldwide. The low Au content of the lode Au-related magmas suggest that large volumes of magmas had to accumulate in the middle to lower crust through trans-lithospheric fault systems to produce the lode Au deposits. It is further suggested that the lode Au-related magmas were alkali-rich, hydrous, oxidized and relatively rich in sulfur and chlorine (mafic melt inclusions contain 0.14‒0.24 wt% S and 0.1‒0.2 wt% Cl). These properties are considered critical for the generation of auriferous ore fluids. By comparing the tectono-magmatic setting of the giant Jiaodong Au province (~ 4000 t Au) with the central Taihangshan district (~ 150 t Au), we propose that the much larger total Au tonnage of the Jiaodong district results from the accumulation of a much larger volume of ore-forming magmas at deep crustal levels, induced by a stronger degree of lithosphere modification. In addition, given that the composition of lode Au-related magmas is similar to that of porphyry Cu–Au-related magmas, the lack of giant, early Cretaceous porphyry Cu–Au deposits in the North China Craton suggests that strong extensional settings favor the formation of lode Au deposits instead of porphyry Cu–Au deposits. The present study, therefore, has general implications for the genesis of Au-rich deposits in strongly extensional settings.

Origin of high Ba-Sr granitoids at Chigou in central China and implications for Cu mineralization: Insights from whole-rock geochemistry, zircon U–Pb dating, Lu–Hf isotopes and molybdenite Re–Os systematics

The Chigou copper deposit is situated in the southern part of the Qinling orogenic belt, central China, where quartz diorite and quartz diorite porphyry are associated with porphyry-type copper mineralization. Copper generally occurs as veins, veinlets, and disseminations within-host porphyries. The zircon dates tightly constrain the timing of host porphyries to ca. 149–146 Ma, in agreement with the age of porphyry copper mineralization (146.1 ± 2.8 Ma) constrained by using molybdenite Re–Os dating. The calculated zircon ΔFMQ values for quartz diorite are −9.4 to +9.9 (averaging +4.3). The Chigou granitoids show similar geochemical compositions to high-Barium-Strontium (high Ba-Sr) granites, with high Ba (2256–3717 ppm) and Sr (737–1345 ppm), low Rb (86.2–129 ppm), U (1.9–4.7 ppm), Th (16.2–31.2 ppm), and Nb (11.4–19.1 ppm) concentrations, as well as lack of negative Eu anomalies. The concentrated εHf(t) values (−3.0 to +1.4), moderate Mg# values (41–52), and high (Hf/Sm)PMN and La/Sm ratios, but low Ba/Th, U/Th, and (Ta/La)PMN ratios indicate that ore-forming magmas derived from an enriched lithospheric mantle that had been metasomatized by subducted sediment-derived melts with a minor crustal contribution. Combined with previous studies, we suggest that the Chigou deposit formed in a post-collisional tectonic setting during the Late Jurassic.

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 &amp;lt;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.

Difference in the nature of ore-forming magma between the Mesozoic porphyry Cu-Mo and Mo deposits in NE China: Records from apatite and zircon geochemistry

Porphyry Cu-Mo and Mo deposits are the most important source of Mo in the world, but factors controlling their difference remain enigmatic. In recent years, many Mesozoic porphyry Cu-Mo and Mo deposits have been discovered in NE China, which provides a good opportunity to explore the difference between the Cu-Mo and Mo mineralization. In this region, porphyry Cu-Mo deposits (Wunugetushan, Jinchanggou) are distributed along the subduction zone, while the porphyry Mo deposits (Luming, Fu'anpu, Daheishan) are far away from the subduction zone. On the basis of whole rock geochemical studies, we present a comprehensive investigation on the apatite and zircon from Mesozoic typical porphyry Cu-Mo deposits and Mo deposits, in order to reveal the nature of parent magma constraints on Cu-Mo and Mo mineralization. Whole-rock geochemistry, in-situ apatite Sr-Nd and zircon Hf isotopic compositions from these granites in porphyry Cu-Mo and Mo deposits indicate that these granites originated from a juvenile crustal source. Apatites from the porphyry Cu-Mo mineralized granites show relatively high Cl and low F contents, accompanied by enrichment of fluid-mobile elements Ba and Sr, indicating that the magmatic source of porphyry Cu-Mo deposits were modified by subducted slab-derived fluids. In comparison, apatites from the porphyry Mo mineralized granites have high F and low Cl contents with correspondingly high F/Cl ratios, suggesting that these granites mainly formed in juvenile crustal dehydration self-metasomatism under the intra-continental environment. Additionally, apatites from porphyry Cu-Mo mineralized granites have higher OH contents than those from porphyry Mo mineralized granites, indicating that the original melt of porphyry Cu-Mo system was more hydrous, consistent with their higher (Ce/Nd)/Y and lower Dy/Yb ratios of zircon. It is here proposed that the formation of porphyry Cu-Mo deposits in NE China are related to partial melting of the juvenile crust, which metasomatized by slab-derived Cl-enriched fluids with carry significant amounts of Cu, whereas the porphyry Mo deposits are related to partial melting of juvenile crustal material with released F-enriched fluids by the dehydration of hydrous minerals.

The source, enrichment and precipitation of ore-forming elements for porphyry Mo deposit: Evidences from melt inclusions, biotite and fluorite in Dasuji deposit, China

The Dasuji ore deposit is a large-scale porphyry Mo deposit at the northern margin of the North China Craton, China. Four stages of hydrothermal veins were recognized in the deposit, from early to late: K-feldspar–quartz veins (stage 1), quartz–molybdenite veins (stage 2), quartz–carbonate–pyrite–galena–sphalerite veins and fluorite veins (stage 3), and carbonate–quartz ± fluorite veins (stage 4). The melt inclusions in the earliest K-feldspar–quartz veins retain information about the ore-forming magma. Melt inclusions in quartz were studied in detail to investigate the magmatic evolution. In chondrite- and primitive-mantle-normalized trace element diagrams, all the inclusions have negative Ba, Sr, and Eu, and positive Pb anomalies. The inclusions have uniform (La/Sm)N ratios, and Sr concentrations decrease with decreasing Ba content, indicating the magma experienced significant fractionation, particularly of plagioclase and biotite. With increasing SiO2, Mo concentrations gradually increased, whereas Cu concentrations first increased and then decreased, suggesting that magmatic evolution was beneficial for Mo, but not Cu, enrichment. This indicates that highly fractionated magmas are conducive to the formation of porphyry Mo ore deposits rather than Cu ore deposits. The high F contents in biotite (up to 4.03 wt%) and pervasive occurrence of fluorite are indicative of a high-F magmatic–hydrothermal system. The disappearance of magnetite means the decrease of oxygen fugacity during stage 2, which promotes molybdenite precipitation. The contents of trace elements in fluorite before the stage 4 changed little, indicating that the fluid system was closed prior to this stage. The highly fractionated granitic magmatic system with high oxygen fugacity and high fluorine content potentially contain new porphyry Mo deposit.

Oxidized sulfur-rich arc magmas formed porphyry Cu deposits by 1.88 Ga

Most known porphyry Cu deposits formed in the Phanerozoic and are exclusively associated with moderately oxidized, sulfur-rich, hydrous arc-related magmas derived from partial melting of the asthenospheric mantle metasomatized by slab-derived fluids. Yet, whether similar metallogenic processes also operated in the Precambrian remains obscure. Here we address the issue by investigating the origin, fO2, and S contents of calc-alkaline plutonic rocks associated with the Haib porphyry Cu deposit in the Paleoproterozoic Richtersveld Magmatic Arc (southern Namibia), an interpreted mature island-arc setting. We show that the ca. 1886–1881 Ma ore-forming magmas, originated from a mantle-dominated source with minor crustal contributions, were relatively oxidized (1‒2 log units above the fayalite-magnetite-quartz redox buffer) and sulfur-rich. These results indicate that moderately oxidized, sulfur-rich arc magma associated with porphyry Cu mineralization already existed in the late Paleoproterozoic, probably as a result of recycling of sulfate-rich seawater or sediments from the subducted oceanic lithosphere at that time.

Constraints on the Formation of the Giant Daheishan Porphyry Mo Deposit (NE China) from Whole-Rock and Accessory Mineral Geochemistry

AbstractThere are more than 80 porphyry (or skarn) Mo deposits in northeastern China with Jurassic or Cretaceous ages. These are thought to have formed mainly in a continental arc setting related to the subduction of the Paleo-Pacific oceanic plate in the Jurassic and subsequent slab rollback in the early Cretaceous. The Jurassic Daheishan porphyry Mo deposit is one of the largest Mo deposits in NE China, which contains 1·09 Mt Mo with an average Mo grade of 0·07 %. To better understand the factors that could have controlled Mo mineralization at Daheishan, and potentially in other similar porphyry Mo deposits in NE China, the geochemical and isotopic compositions of the ore-related granite porphyry and biotite granodiorite, and the magmatic accessory minerals apatite, titanite and zircon from the Daheishan intrusions, were investigated so as to evaluate the potential roles that magma oxidation states, water contents, sulfur and metal concentrations could have played in the formation of the deposit. Magmatic apatite and titanite from the causative intrusions show similar εNd(t) values from –1·1 to 1·4, corresponding to TDM2 ages ranging from 1040 to 840 Ma, which could be accounted for by a mixing model through the interaction of mantle-derived basaltic melts with the Precambrian lower crust. The Ce and Eu anomalies of the magmatic accessory minerals have been used as proxies for magma redox state, and the results suggest that the ore-forming magmas are highly oxidized, with an estimated ΔFMQ range of +1·8 to +4·1 (+2·7 on average). This is also consistent with the high whole-rock Fe2O3/FeO ratios (1·3–26·4). The Daheishan intrusions display negligible Eu anomalies (Eu/Eu* = 0·7–1·1) and have relatively high Sr/Y ratios (40–94) with adakitic signatures; they also have relatively high Sr/Y ratios in apatite and titanite. These suggest that the fractionation of amphibole rather than plagioclase is dominant during the crystallization of the ore-related magmas, which further indicates a high magmatic water content (e.g. &amp;gt;5 wt%). The magmatic sulfur concentrations were calculated using available partitioning models for apatite from granitoids, and the results (9–125 ppm) are indistinguishable from those for other mineralized, subeconomic and barren intrusions. Furthermore, Monte Carlo modelling has been conducted to simulate the magmatic processes associated with the formation of the Daheishan Mo deposit, and the result reveals that a magma volume of ∼280 km3 with ∼10 ppm Mo was required to form the Mo ores containing 1·09 Mt Mo in Daheishan. The present study suggests that a relatively large volume of parental magmas with high oxygen fugacities and high water contents is essential for the generation of a giant porphyry Mo deposit such as Daheishan, whereas a specific magma composition (e.g. with unusually high Mo and/or S concentrations) might be less critical.

Reduction of oxidized sulfur in the formation of the Grasberg porphyry copper-gold deposit, Papua, Indonesia

The reduction of oxidized sulfur is essential in porphyry copper deposits whose mineralization predominantly comprises copper sulfides, whereas their source magmas are oxidized with most of their sulfur as SO42− and with exsolved fluids having SO2>>H2S. To estimate the redox state of sulfur, we examined drill cores that intersect potassic and unaltered intrusive rocks in the deeper levels of the Grasberg porphyry copper-gold deposit. Magmatic oxybarometers such as the amphibole-titanite-magnetite-quartz assemblage, anhydrite, and amphibole consistently show that Grasberg ore-forming magmas were oxidized with fO2 > FMQ+3. Initial hydrothermal events formed sulfide-free, anhydrite-rich K-feldspar, and biotite alteration, followed by successive vein stages of (1) magnetite, (2) biotite, (3) quartz, (4) anhydrite-chalcopyrite, (5) chalcopyrite ± sericite selvages, and (6) pyrite-chalcopyrite-quartz + sericite selvages. The δ34S values of sulfide-sulfate mineral pairs indicate SO2-derived SO42− and H2S in SO42−/H2S molar proportions of ~ 4:1 to ~ 3:1 at > 550 °C. The hydrothermal fluid then likely followed a rock-buffered trajectory and became more reduced at < 550 °C. Hydrothermal biotite that replaces igneous amphibole and biotite has a phlogopitic composition, suggesting that Fe2+ was liberated from igneous mafic minerals and oxidized by reaction with SO42− to form magnetite, resulting in sulfide formation by the simplified reaction 12FeO + SO42− + 2H+ → 4Fe3O4 + H2S.

Multiphase intrusion at the giant Pulang porphyry Cu-Au deposit in western Yunnan (Southwestern China): comparison between ore-causative and barren intrusions

Porphyry Cu-Au deposits (PCDs) have commonly multiphase porphyry intrusions, yet only one (or a few) of the intrusive phase is/are ore-causative. Understanding the criteria to form ore-causative porphyry is important for both ore deposit research and PCD exploration. At the giant Pulang PCD in western Yunnan, there are three generations of porphyries, i.e., (from oldest to youngest) the quartz diorite porphyry (QDP), quartz monzonite porphyry (QMP) and granite porphyry (GP), and only the QMP has major mineralization. Based on ore deposit geology, and zircon U–Pb–Hf–O isotope systematics, we suggested that the QMP is spatially and temporally ore-related. Oxygen fugacity of the QMP and GP magmas is estimated to be higher than that of the QDP. In addition, Hf–O isotope compositions of the QMP and GP are close to those of the average mantle. We suggested that with the continuous Late Triassic subduction of the Garze–Litang ocean basin, the subducted-slab dehydration and the addition of mantle-wedge materials to the ore-forming magmas increased gradually. The younger magmas (e.g., QMP and GP) are thus more oxidized, and have higher water and ore-material contents, giving them higher ore-forming potential. The reason maybe that the late GP is barren may be due to the depletion of ore-forming materials by the main-stage mineralization during the QMP intrusion.

The petrogenesis of back-arc magmas, constrained by zircon O and Hf isotopes, in the Frontal Cordillera and Precordillera, Argentina

The Andean margin of the South American continent has been magmatically and tectonically active for over 330 million years. It is the type location where “Cordilleran-type” magmatism and orogenesis are manifest. In Argentina and Chile, between the latitudes of 28° and 40° S, magmatism related to the Gondwanan “Cordilleran-type” orogeny is reflected in a series of Carboniferous to Triassic intrusions. A comprehensive model exists for the petrogenesis of such magmas in Chile, however there is relatively little understanding of the nature and timing of Permo-Triassic magmatism in the Frontal Cordillera and Precordillera in Argentina. To address this, we present a new dataset of in situ zircon U–Pb, O and Hf isotopes from 15 felsic intrusions from Argentina. Zircon geochronology shows that magmatism in this region commenced at ca. 285 Ma and continued until ca. 250 Ma. Zircon O and Hf isotopes suggest that the oldest Permian magmas were derived from young supracrustal sources, with elevated δ18O (~ 8.5 to 7.5‰) and negative eHf values (~ − 1 to − 3 eHf). The emplacement of these magmas was coeval with the formation of mantle-derived magmas characterised by mantle-like δ18O (~ 6.0 to 5.5‰) and moderately positive eHf values (~ 4 to 1 eHf). As magmatism continued, transitional isotope signatures became predominant as melts of these disparate sources interacted and hybridised. It is proposed that under a compressional regime, mantle-derived magmas were halted in the lower continental crust, where they exchanged heat and volatiles with an older fertile lithosphere to generate melts from supracrustal sources. A shift in the stress regime at ca. 285 Ma permitted both crustally derived and juvenile mantle-derived magmas to exploit newly formed conduits to rise into the upper crust. A regional compilation of zircon O and Hf isotopes from felsic igneous rocks reveals a coherent secular trend over ~ 100 million years, where the oldest magmatism exhibits a dominant supracrustal component and younger magmas progressively (over 50 Ma) transition towards juvenile mantle-like isotopic compositions. This new dataset from Argentina fills a significant gap in the previous regional models between 285 and 250 Ma and documents the isotopic response of magmas produced in back-arc regions to a transition between compression and extensional/neutral stress regimes. These results give insight into the generation of new, or recycling of, continental crust in a back-arc setting and how the transition from compression to extension is imperative for ore-forming magmas to reach the upper crust.

Geochemistry of zircon and apatite from the Mo ore-forming granites in the Dabie Mo belt, East China: Implications for petrogenesis and mineralization

The Dabie molybdenum (Mo) belt in east China is an important Mo belt in both China and the world. All of the Mo deposits in the Dabie area are porphyry (-skarn) type. The ore-related granite samples from the Qian’echong, Tangjiaping, and Doupo Mo deposits in the Mo belt have been collected for zircon and apatite trace elements analysis. Additionally, the Doupo ore-related granite has also been chosen for zircon U-Pb dating. LA-ICP-MS zircon U-Pb dating of the ore-related granite in the Doupo Mo deposit yields a crystallization age of 127 ± 1 Ma, which is consistent with the molybdenite Re-Os age of the deposit. The chondrite-normalized patterns of all zircons are characterized by positive Ce anomalies with variable negative Eu anomalies. The ore-related granites in the Qian’echong and Tangjiaping Mo deposits show relatively high magmatic oxygen fugacity (ΔFMQ ≥ +1.5), whereas the Doupo ore-related granite has a relatively low magmatic oxygen fugacity (ΔFMQ − 0.3), which might have caused it to have relatively small Mo reserve compared to the Qian’echong and Tangjiaping deposits. All of the apatites from the three Mo deposits are characterized by LREE-enriched and HREE-depleted chondrite-normalized patterns, with various degrees of Eu depletion and Ce enrichment. Not only redox condition but also other factors could have influenced the Eu/Eu* or/and Ce/Ce* of apatite in this study, and the anomalies of Eu and Ce of apatites from the ore-related granites in this study could not be used to evaluate the magma redox states. The geochemical characteristics of apatite indicate that the ore-forming magmas of the Mo deposits in this study are oxidized and metaluminous I or A type magmas. During the Early Cretaceous regional extension and asthenospheric upwelling in the Dabie orogen, partial melting of the crustal rocks generated oxidized Mo-bearing magma. Relatively oxidized condition prevents Mo from incorporating into sulfides or silicates in the magma, and finally leads to Mo precipitation from the ore-forming fluids, which released from the magma.

Geology, geochronology and geochemistry of large Duobaoshan Cu–Mo–Au orefield in NE China: Magma genesis and regional tectonic implications

Duobaoshan is the largest porphyry-related Cu–Mo–Au orefield in northeastern (NE) Asia, and hosts a number of large-medium porphyry Cu (PCDs), epithermal Au and Fe–Cu skarn deposits. Formation ages of these deposits, from the oldest (Ordovician) to youngest (Jurassic), have spanned across over 300 ​Ma. No similar orefields of such size and geological complexity are found in NE Asia, which reflects its metallogenic uniqueness in forming and preserving porphyry-related deposits. In this study, we explore the actual number and timing of magmatic/mineralization phases, their respective magma genesis, fertility, and regional tectonic connection, together with the preservation of PCDs. We present new data on the magmatic/mineralization ages (LA–ICP–MS zircon U–Pb, pyrite and molybdenite Re–Os dating), whole-rock geochemistry, and zircon trace element compositions on four representative deposits in the Duobaoshan orefield, i.e., Duobaoshan PCD, Tongshan PCD, Sankuanggou Fe–Cu skarn, and Zhengguang epithermal Au deposits, and compiled published ones from these and other mineral occurrences in the orefield.In terms of geochronology, we have newly summarized seven magmatic phases in the orefield: (1) Middle–Late Cambrian (506–491 ​Ma), (2) Early and Middle Ordovician (485–471 ​Ma and ~462 ​Ma), (3) Late Ordovician (450–447 ​Ma), (4) Early Carboniferous and Late-Carboniferous to Early Permian (351–345 and 323–291 ​Ma), (5) Middle–Late Triassic (244–223 ​Ma), (6) Early–Middle and Late Jurassic (178–168 ​Ma and ~150 ​Ma), and (7) Early Cretaceous (~112 ​Ma). Three of these seven major magmatic phases were coeval with ore formation, including (1) Early Ordovician (485–473 ​Ma) porphyry-type Cu–Mo-(Au), (2) Early–Middle Triassic (246–229 ​Ma) porphyry-related epithermal Au-(Cu–Mo), and (3) Early Jurassic (177–173 ​Ma) Fe–Cu skarn mineralization. Some deposits in the orefield, notably Tongshan and Zhengguang, were likely formed by more than one mineralization events.In terms of geochemistry, ore-causative granitoids in the orefield exhibit adakite-like or adakite-normal arc transitional signatures, but those forming the porphyry-/epithermal-type Cu–Mo–Au mineralization are largely confined to the former. The varying but high Sr/Y, Sm/Yb and La/Yb ratios suggest that the ore-forming magmas were mainly crustal sourced and formed at different depths (clinopyroxene-/amphibole-/garnet-stability fields). The adakite-like suites may have formed by partial melting of the thickened lower crust at 35–40 ​km (for the Early Ordovician arc) and >40 ​km (for the Middle–Late Triassic arc) depths. The Early Jurassic Fe–Cu skarn ore-causative granitoids show an adakitic-normal arc transitional geochemical affinity. These granitoids were likely formed by partial melting of the juvenile lower crust (35–40 ​km depth), and subsequently modified by assimilation and fractional crystallization (AFC) processes.In light of the geological, geochronological and geochemical information, we proposed the following tectonometallogenic model for the Duobaoshan orefield. The Ordovician Duobaoshan may have been in a continental arc setting during the subduction of the Paleo-Asian Ocean, and formed the porphyry-related deposits at Duobaoshan, Tongshan and Zhengguang. Subduction may have ceased in the latest Ordovician, and the regional tectonics passed into long subsidence and extension till the latest Carboniferous. This extensional tectonic regime and the Silurian terrestrial-shallow marine sedimentation had likely buried and preserved the Ordovician Duobaoshan magmatic-hydrothermal system. The south-dipping Mongol-Okhotsk Ocean subduction from north of the orefield had generated the Middle–Late Triassic continental arc magmatism and the associated Tongshan PCD and Zhengguang epithermal Au mineralization (which superimposed on the Ordovician PCD system). The Middle Jurassic closure of Mongol-Okhotsk Ocean in the northwestern Amuria block (Erguna terrane), and the accompanying Siberia-Amuria collision, may have placed the Paleo-Pacific subduction system in NE China (including the orefield) under compression, and formed the granodiorite-tonalite and Fe–Cu skarn deposits at Sankuanggou and Xiaoduobaoshan. From the Middle Jurassic, the consecutive accretion of Paleo-Pacific arc terranes (e.g., Sikhote-Alin and Nadanhada) onto the NE Asian continental margin may have gradually distant the Duobaoshan orefield from the subduction front, and consequently arc-type magmatism and the related mineralization faded. The minor Late Jurassic and Cretaceous unmineralized magmatism in the orefield may have triggered mainly by the far-field extension led by the post-collisional (Siberia-Amuria) gravitational collapse and/or Paleo-Pacific backarc-basin opening.