Biotite Monzogranite Porphyry Research Articles

Platinum-Group Element Geochemistry of Igneous Rocks in the Chongjiang Cu–Mo–Au Deposit, Southern Tibet: Implications for the Formation of Post-Collisional Porphyry Cu Deposits

Abstract The timing and extent of sulfide saturation have been suggested as controlling factors in the formation of economically significant porphyry Cu deposits in subduction zone settings. However, details on the sulfide saturation history in post-collisional porphyry systems remain ambiguous. Accordingly, we have characterized the whole-rock geochemistry, including platinum-group elements (PGE), of igneous intrusions in the post-collisional Chongjiang porphyry Cu–Mo–Au deposit (southern Tibet) and utilize this data in conjunction with zircon U–Pb geochronological results and sulfide chemistry to assess the timing of sulfide saturation, the nature and amount of magmatic sulfide produced. The Chongjiang intrusions (monzogranite, biotite monzogranite porphyry, granodiorite, dacite porphyry, and quartz diorite porphyry) and mafic microgranular enclaves (MMEs) have zircon U–Pb ages of 14.2 to 12.8 Ma. Covariations in whole-rock major and trace elements among the Chongjiang intrusions and MMEs, together with similarities in their Sr–Nd and zircon Hf isotope compositions, indicate that they are co-magmatic and crystallized from a juvenile lower crustal melt that mixed with mafic melt derived from the lithospheric mantle; this hybrid melt subsequently evolved via fractional crystallization. Trace-element ratios in zircon and temperature − ∆FMQ estimates of the different intrusions suggest that they all crystallized from oxidized (average ∆FMQ = 1.9–2.6) and water-rich magmas. Palladium contents and Pd/Pt ratios in the Chongjiang igneous intrusions increase with decreasing MgO up to 3.9 wt % MgO, after which they abruptly decrease. The initial increase in Pd/Pt ratios likely results from the fractionation of a Pt-rich mineral (e.g. Pt–Fe alloy). The decrease in Pd contents and Pd/Pt ratios at 3.9 wt % MgO likely results from sulfide saturation during magma evolution, but prior to volatile exsolution, which occurred at approximately 1.4 to 2.4 wt % MgO. The presence of magmatic sulfide inclusions in amphibole and magnetite in samples with 3.9 wt % MgO, and the geochemical compositions of sulfide inclusions suggest that they represented trapped sulfide liquid and intermediate solid solution. Results of Monte Carlo simulations demonstrate that 0.003 to 0.009 wt % magmatic sulfide is required to have fractionated from the magma to explain the decrease in Pd contents at 3.9 wt % MgO. Highly chalcophile elements, such as Pd, will be sequestered by the magmatic sulfide that saturates at depth, decreasing their concentrations in the residual silicate melt, whereas concentrations of the less chalcophile elements, such as Cu, Mo, and even Au, will not be as significantly affected. Consequently, sufficient concentrations of Cu–Mo–Au will remain in the residual melt and, upon reaching volatile saturation, can be transported by the vapor phase to form porphyry Cu–Mo–Au deposits. In the case of the Chongjiang deposit, sulfide saturation was likely triggered by the high pressures and/or depletion of FeO caused by the thick (~70 km) crust beneath the Gangdese belt. This contribution presents evidence of sulfide saturation in post-collisional magmatic systems, and demonstrates that the amount of magmatic sulfide produced is a critical factor in controlling the formation of post-collisional porphyry Cu deposits.

Metal source and hydrothermal evolution of the Jiaoxi quartz vein-type tungsten deposit (Tibet): Insights from textural and compositional variations of wolframite and scheelite

Abstract We evaluate the controlling factors of hydrothermal wolframite and scheelite precipitation in the quartz vein-type Jiaoxi tungsten deposit situated in the western part of the Lhasa terrane (Tibet, China) using texture, major and trace element mineral geochemistry, and sulfur stable isotope geochemistry. Pyrite and chalcopyrite that are intergrown with Fe-enriched wolframite and siderite, have distinct in situ S isotope compositions (δ34SV-CDT) of −31.38 to +1.77‰, and +2.07 to +2.30‰, respectively. Major and trace element contents and in situ S isotope compositions of pyrite and chalcopyrite indicate that the hydrothermal evolution involved fluid-fluid mixing and greisenization. We report evidence for an early magmatic fluid, which is characterized by the enrichment of W, Mn, Zr, Ti, Sc, and Sn and depletion of Fe. This magmatic fluid was diluted by meteoric water and interacted with biotite monzogranite porphyry to leach Fe, Mg, and Zn into the system to form wolframites with variable Fe/(Fe+Mn) ratios ranging between 0.06–0.84. The late Fe-enriched magmatic fluid released from the muscovite granite mixed with meteoric water that leached minor Fe and S from shale to form late shale-hosted wolframite with a Fe/(Fe+Mn) mass ratio of >0.75 and coeval siderite and sulfides. This study highlights that multiple Fe sources were present in the system, including muscovite granite-released Fe through fluid exsolution, biotite monzogranite porphyry-released Fe during greisenization, and minor Fe released from the shale as a result of meteoric water leaching.

Halogens and trace elements of apatite from Late Mesozoic and Cenozoic porphyry Cu-Mo-Au deposits in SE Tibet, China: Constraints on magmatic fertility and granitoid petrogenesis

Abstract Tethyan orogenesis and Indian–Eurasia collision facilitate the development of abundant Late Mesozoic and Cenozoic porphyries and associated Cu ± Mo ± Au deposits in the SE Tibetan plateau. Typical deposits including Beiya with the largest Au reserve of 350 t, Tongchanggou with the largest Mo reserve of 142.5 Mt and Pulang with the largest Cu reserve of 804 Mt were mostly concerned by economic geologists. Porphyries occurred in these three typical deposits were classified as Au-fertile (Wandongshan monzogranite porphyry in Beiya), Mo-fertile (granodiorite porphyry in Tongchanggou), Cu-fertile (quartz monzogranite porphyry in Pulang) and ore-barren (Baliancun biotite monzogranite porphyry in Beiya) systems. In this study, apatite geochemistry from four porphyry systems with distinctive metal endowments was investigated to further constrain magmatic fertility indicators and granitoid petrogenesis. All the studied apatite grains belong to magmatic fluorapatite and are generally wrapped within plagioclase and biotite phenocrysts, suggesting that apatite occurs as early-crystallization phase during crystal fractionation. Apatite grains with porous texture, irregular patchy zoning and secondary rare earth mineral inclusions were eliminated to make the minimal effect of hydrothermal alteration. The F and Cl contents in melt are also estimated based on analyzed F–Cl contents and corresponding F–Cl partition coefficients between apatite and melt under different P–T conditions in this study. Apatite from the Wandongshan (WDS) Au-fertile monzogranite porphyry (MP) has the highest Sr content (average of 1530 ppm) and (F/Cl)melt ratios (44 to 125), indicating that the WDS MP experienced the highest degree of melt fractionation. Additionally, the WDS Au-fertile porphyry is more oxidized than ore-barren, Mo-fertile and Cu-fertile porphyries, evidenced by its highest apatite δEu (0.56–0.83) and lowest δCe (0.98–1.05) values. Rare earth elements (REEs), Y and Sr in apatite were found to be most effective for discriminating magma types. All the apatite grains from typical Late Mesozoic and Cenozoic porphyries in SE Tibet show relatively high Sr/Y (0.5–23.6) and δEu (0.41–0.83) values, pointing to an adakitic affinity of apatite-bearing rock, which is consistent with the whole-rock geochemistry. These porphyries formed under distinctive tectonic settings show distinguishable Clmelt and (F/Cl)melt ratios. The Pulang Cu-fertile quartz monzonite porphyry formed under a subduction setting has relatively high Clmelt contents of 100–1000 ppm and low (F/Cl)melt ratios of 2–22, suggesting derivation of the parental magma from a metasomatized hydrous mantle wedge. Whereas, the WDS Au-fertile MP and Bailiancun ore-barren biotite monzonite porphyry generated under intracontinental setting show low Clmelt contents of 23–180 ppm and variable (F/Cl)melt ratios of 12–125, because F-bearing minerals were decomposing at greater depths under higher pressures during the collision.

Geology, geochronology and geochemistry of the Miocene Jiaoxi quartz vein-type W deposit in the western part of the Lhasa Terrane, Tibet: Implications for ore genesis

The Jiaoxi deposit is the first Miocene quartz vein-type W deposit discovered in the western part of the Lhasa Terrane comprising with 39,000 metric tons of WO3. The mineralization occurs in wolframite-bearing hydrothermal tensional quartz veins and is associated with Miocene granites. The mineralization event can be divided into an early quartz-oxide stage succeeded by a sulfide stage, and a final fluorite-carbonate stage. Magmatic intrusions at the deposit include biotite monzogranite porphyry, biotite monzogranite, granite porphyry, and muscovite granite at depth and show crystallization ages ranging between 14.5 and 13.7 Ma. Whole-rock geochemistry shows that the intrusions are alkali-rich peraluminous granites that experienced strong fractional crystallization. The most evolved muscovite granite has high W, Nb and Cs contents and is considered to be the source for mineralization. During the late crystallization stage of the highly fractionated muscovite granite, exsolved hydrothermal fluids interacted with the muscovite granite magma and migrated along extensional fractures forming the Jiaoxi deposit. Zircon grains from the granites show uniform εHf (t) values with a wide range between −7.08 and +3.87. The granites have initial Sr isotopic compositions ranging between 0.7094 and 0.7124 and negative εNd (t) values ranging between −4.9 and −9.4. The Sr-Nd-Hf isotope data indicate that the granites were derived from partial melting of a crustal source with minor contributions from the lithospheric mantle. The 3He/4He ratios for wolframite and pyrite vary between 0.30 and 0.74 Ra (Ra = 1.4 × 10–6 for air), confirming that W was mainly sourced from the crust. Collectively, the regional geology, geochronology and geochemistry, indicate that the Miocene post-collisional peraluminous granites and associated W mineralization were triggered by the roll-back and gradual break-off of the northward-subducting Indian continental lithosphere slab. The less evolved biotite monzogranite porphyry, which contains ore-bearing veins and local greisen alteration, appears to be unrelated to W mineralization.

Geochemistry and zircon trace elements composition of the Miocene ore‐bearing biotite monzogranite porphyry in the Demingding porphyry Cu‐Mo deposit, Tibet: Petrogenesis and implication for magma fertility

Demingding is a poorly studied post‐collisional porphyry Cu‐Mo deposit, located in the eastern part of the Gangdese porphyry copper belt. In this study, we present LA‐ICP‐MS zircon U‐Pb dating, whole‐rock geochemistry, and zircon trace elemental data for the mineralization related biotite monzogranite porphyry in Demingding. Zircon U‐Pb dating shows that the weighted mean 206Pb/238U ages of the Miocene biotite monzogranite porphyry is 20.36 ± 0.46 Ma (MSWD = 2.6, n = 21). The Miocene biotite monzogranite porphyry is characterized by high SiO2, K2O, and Al2O3 contents and shows adakite‐like signatures. These rocks have high Sr/Y ratios, fractionated REE patterns with low HREE abundances, enrichment in LILE compared with HFSE, and have (87Sr/86Sr)i values of 0.7059 to 0.7062, εNd(t) values of −2.35 to −1.67 and (206Pb/204Pb)i ratios of 18.50–18.55. These features are similar to other Miocene adakite‐like intrusions in the Gangdese belt. We propose that the biotite monzogranite porphyry shares the same petrogenesis with other Miocene ore‐bearing adakite‐like intrusions in the Eastern Gangdese belt and was derived from remelting of the subduction‐modified, eclogitized Tibetan lower crust and mixing with hydrous mafic magmas from metasomatized Tibetan mantle. The zircons from biotite monzogranite porphyry have only minimal negative Eu anomalies (EuN/EuN* > 0.3), and exhibit large ratios of Ce4+/Ce3+(average value of 113), 10,000*(EuN/EuN*)/Y (3.94–8.14, >1), Ce/Nd (9–58, Average value of 30), (Ce/Nd)/Y (0.008–0.127, >0.003), and low value of Dy/Yb (0.15–0.24, <0.3), indicating that the biotite monzogranite porphyry is relatively hydrous and oxidized. The biotite monzogranite porphyry, therefore, is believed to be a relatively fertile intrusion with certain exploration potential.

The multiple granitic magmatism in the giant Huayangchuan uranium polymetallic ore district: Implications for tectonic evolution of the southern margin of North China Craton in the Qinling Orogen

The giant Huayangchuan U–Nb–Pb deposit in the Qinling Orogen is a newly verified carbonatite-hosted deposit in the southern margin of the North China Craton (NCC), central China. Granitic magmatism is extensively developed in the Huayangchuan deposit area, but their ages and petrogenesis are not well constrained. The exposed granitic rocks are mainly biotite monzogranite porphyry, granite pegmatite, granodiorite, and biotite monzogranite with zircon U–Pb ages of 1808 ± 10 Ma, 1807 ± 14 Ma, 233 ± 1.4 Ma, and 132 ± 0.6 Ma, respectively. The Paleoproterozoic biotite monzogranite porphyry belongs to shoshonite and metaluminous series, showing enrichment of LREE and LILE and depletion of HREE and HFSE, with high Zr + Y + Nb + Ce values and Ga/Al ratios, which are consistent with A-type granite. Whereas, the contemporaneous granite pegmatite dykes with weak mineralization are also cala-alkaline to shoshonite and peraluminous series, enriched in Rb, Ba, and LREE, and depleted in Nb, Ta, Ti and HREE. The shoshonite and weakly peraluminous Triassic granodiorite is slightly enriched in LREE with flat HREE patterns, enriched in Ba and Sr and depleted in Nb, Ta, P, and Ti, with similar geochemical characteristics to adakite-like rocks. The Early Cretaceous biotite monzogranite is characterized by LREE enrichment and flat HREE patterns, belonging to shoshonite and metaluminous to weakly peraluminous I-type granite, with U and LILE enrichment, and HFSE-depleted. The high initial 87Sr/86Sr ratios and enriched Nd (εNd(t) = −17.5 to −17.1) and Hf (εHf(t) = −33.2 to −14.6) isotopes reveal that the Huayangchuan granitic rocks are obviously sourced from crustal-derived magmas. Combined with regional geology and this study, we proposed that: (1) the Paleoproterozoic biotite monzogranite porphyry and granite pegmatite were generated from ancient lower crust during post-collisional extension setting; (2) the granodiorite was likely sourced from partial melting of thickened lower crust with pelagic sediments materials addition during the Triassic although the mineralization-related carbonatite having similar age with granodiorite may derive from mantle; and (3) the Early Cretaceous biotite monzogranite was mainly sourced from the partial melting of lower crust induced by the underplating of the mafic magma. We suggest that the first phase of magma occurred in the Huayangchuan district during the Paleoproterozoic following the amalgamation of the Eastern and Western blocks along the Trans-North China Orogen. Since the Mesozoic, the ongoing northward subduction of the Yangtze Craton (YZC) resulted in the crust thickening, developing the Triassic granitic magmatism and major mineralization associated with carbonatite. The widespread Cretaceous granite and deposit in the southern margin of NCC indicate that intracontinental extension and lithospheric thinning occurred in response to the tectonic regime transition from NS-trending to EW-trending subduction.

Geology, geochronology, geochemical characteristics and origin of Baomai porphyry Cu (Mo) deposit, Yulong Belt, Tibet

The Baomai porphyry Cu-Mo deposit is a newly discovered deposit located in the northern section of the Yulong porphyry Cu belt, eastern Tibet. It is associated with Eocene biotite monzogranite intrusions that were emplaced into Paleoproterozoic Ningduo Group and Middle-Upper Triassic sedimentary rocks. A biotite monzogranite porphyry with a zircon U-Pb age of 42.7 Ma was intruded by several post-ore diorite porphyry dikes (ca. 37.8 Ma). Alteration mineral assemblages consistent with argillic, phyllic, and hornfels were identified in ore-bearing porphyries and host rocks at Baomai. Argillic and phyllic alteration assemblages are spatially related to high-grade Cu and Mo mineralization, whereas hornfels alteration is accompanied by weak Cu mineralization. Molybdenite from the mineralized porphyry returned a Re-Os isochron age of 42.6 Ma, and is considered to represent the age of porphyry-style mineralization at the deposit. Ore-bearing porphyries in Baomai are high-K (K2O/Na2O >1), alkaline-rich intrusions rich in LREE, LILE (e.g., Rb, U, K), depleted in HFSE (e.g., Nb, Ta, P and Ti) and contain absent or slightly negative Eu anomalies and no significant Ce anomalies. Slightly high 87Sr/86Sr ratios (0.7064–0.7076), low εNd(t) values (−2.5 to −4.6), uniform (206Pb/204Pb)i ratios (18.70–18.75) and positive εHf(t) values (0–5.3) of ore-bearing porphyries at Baomai indicate that they share the same or similar petrogenesis to other mineralized porphyries within the Yulong porphyry Cu belt. The porphyries were likely generated from partial melting of phlogopite-bearing amphibolite or amphibolite eclogite with a low degree of crystal fractionation, and related to enrichment mantle (EMII) transformed or mixed by older subducted slab-derived fluid. Actually, there are no significant three stages of ore-bearing magmatic activities in Yulong porphyry Copper belt (Hou et al., 2003b), Eocene intrusions (37–43 Ma) and regional strike slip faulting were all important factors in controlling the formation of these Cu-polymetallic deposits during the Eocene India-Tibetan collision.

Constraints on the origin of adakites and porphyry Cu-Mo mineralization in Chongjiang, Southern Gangdese, the Tibetan Plateau

Chongjiang is a low-grade porphyry Cu deposit, located in the Gangdese belt, south Tibet. The petrogenesis and geodynamic settings of the Miocene intrusions associated with the deposit remain controversial. This study presents new results on in situ zircon Hf-O isotopic compositions and U-Pb ages, whole rock major and trace elements, and Sr-Nd isotopes for the adakitic intrusions from Chongjiang deposit. The ore-bearing biotite monzogranite porphyry has adakitic characteristics, with enriched large-ion-lithophile elements (LILE) and light rare earth elements (LREE), and depleted in high-field-strength elements (HFSE), P and Ti. LA-ICP-MS zircon U-Pb dating indicates that the ore-bearing and barren adakites were emplaced at 14.9±0.3Ma and 12.9±0.3Ma, respectively. The porphyry is characterized by relatively high initial 87Sr/86Sr ratios (0.7059 to 0.7066), and negative whole-rock εNd(t) values (−3.8 to −2.6). Zircon δ18O is slightly higher than mantle values (5.0 to 7.2‰), with varied εHf(t) (−1.0 to 7.6). Most of the in situ zircon Hf-O isotopic data plot in a binary mixing trend between MORB and lower continental crust-derived melts. These results indicate contributions from mixing of a mantle-like source (e.g., slab melts) with continental crust. Interestingly, most of the samples plot in the field defined by Dabie adakites (representing partial melting of the lower continental crust), with several samples near/in the circum-Pacific adakite field (representing partial melting of subducted oceanic slabs), which seemingly indicates that Chongjiang adakites mostly formed through partial melting of lower continental crust, with a small amount derived from oceanic slab melts. These may be plausibly explained by plagioclase retention in the thickened Tibetan continental crust, which lowers Sr contents in the magmas during crustal assimilation. Such a model is supported by other adakite discrimination diagrams, which all point towards slab melting. Crustal contamination can compellingly explain the low grade of the Chongjiang deposit. Considering the temporal-spatial distribution of porphyry Cu deposits, geochemical characteristics and high oxygen fugacity, we propose that the subducting Ninetyeast Ridge probably played a critical role in controlling the formation of Miocene adakites and porphyry copper deposits in the eastern Gangdese belt.

Nature and origin of Triassic igneous activity in the Western Qinling Orogen: the Wenquan composite pluton example

ABSTRACTThe Western Qinling has been acknowledged to witness superimposed orogeny including north subduction of Paleotethys ocean and collision between North China and South China blocks; however, the precise timing constraints on transition of tectonic regime are remaining enigmatic. The Wenquan composite batholith comprising five phases and mafic enclaves is an ideal example to unlock this puzzle. The host granitoids are felsic, metaluminous to peraluminous, and high-K calc-alkaline to shoshonitic suite with I-type affinity. The mafic enclaves, however, are intermediate, and high-K calc-alkaline to shoshonitic. Zircon ages of multiple phases indicate an episodic growth lasting nearly 30 million years ranging from 238, 228, 218 to 208 Ma, consistent to Triassic igneous activity recording a transition regime from a subduction setting to a syn-collision setting and a post-collision setting in Western Qinling. Lead isotopes of whole-rock and K-feldspar at Wenquan and Lu-Hf isotopes of zircons separated from biotite monzogranite porphyry, porphyritic monzogranite, monzogranite porphyry, and hosted mafic enclaves suggest that the heat and the hot mafic melt initiated by the break-off of the northward subducting South China block lithosphere triggered partial melting of the Mesoproterozoic subcontinental lithospheric mantle to produce mafic magmas, and the underplated mafic magmas caused partial melting of the shallow subducted Mesoproterozoic lower crust generating granitic magmas at Wenquan. Combined our field observations and petrology study with a holistic review on previous geochronological and geochemical data of Triassic granitoids throughout the Western Qinling, we in this contribution proposed that the Triassic igneous activity in the Western Qinling corresponding to superimposed orogeny evolved from the northward subduction of Palaeotethys ocean (250–235 Ma) through syn-collision (228–215 Ma) to post-collision (215–185 Ma) between the North China and South China blocks.

Rare earth element geochemistry of the molybdenum-bearing granitoids in the Jinduicheng-Huanglongpu district, Shaanxi Province, northwest China

Located in the Luonan county, Shaanxi Province, northwest China, Jinduicheng, Shijiawan and Huanglongpu molybdenum deposits constitute the most important molybdenum mineralized district in China. Among these three deposits, the Jinduicheng and Shijiawan molybdenum deposits are connected spatially and genetically with granitoid porphyry (124 ± 6 Ma, K-Ar biotite), and consist of disseminated-veinlet ores. Geochemical studies of rare earth elements (REE) furnish further evidence for understanding the rock- and ore-forming processes of these two porphyry molybdenum deposits and their related granitoid rocks. The REE distribution in molybdenum ore, granitoids and their Middle Proterozoic meta-volcanic wall rocks is discussed. The similarities between the REE signatures of the Shijiawan molybdenum-bearing monzogranite porphyry and the neighbouring Laoneushan monzogranite (130 ± 5 Ma, K-Ar biotite) show that they were produced at the same evolutional stage of granitoid magma derived mainly from crustal anatexis. The Shijiawan biotite monzogranite porphyry may be an apophysis of the Laoneushan granitoid batholith. Compared to the Shijiawan monzogranite porphyry, the Jinduicheng molybdenum-bearing granite porphyry is characterized by a high content of HREE, and depletion in LREE. The unique REE patterns indicates that the molybdenum-bearing granite porphyry was formed by thermogravitation diffusion of a granitoid magma. The slight depletion of REE abundance in the altered granitoid porphyry and meta-volcanic wall rocks shows that leaching of REE occurred during breakdown of the primary mineral assemblage, and crystallization of secondary minerals. The high REE content of molybdenum ore represented re-deposition of the mobilized molybdenum and REE.