Fertile Magmas Research Articles

Constraints of upper crust magmatic processes on the formation of Saindak porphyry Cu deposit: Insights from mineral geochemistry

Upper crustal magmatic processes play a crucial role in linking the magmatic and hydrothermal systems of porphyry deposits, ultimately controlling the generation of ore-forming fluids. Understanding these processes within a magma chamber is essential for deciphering the formation of porphyry deposits. The Saindak deposit, a large porphyry Cu system in the western Chagai belt, features coeval ore-forming and barren magma pulses, providing key insights into the characterization of porphyry systems. In this study, detailed petrographic observations and geochemical analyses of amphibole, plagioclase, and apatite were conducted to unravel the magmatic evolution at Saindak. Based on textural relationships and crystallization sequences, three distinct populations of amphibole in the ore-forming magma were identified. These amphiboles record the entire fractional crystallization process of magma that initially emplaced at a depth of ∼5 km to the connected magma system at ∼2 km. The study of amphibole and apatite reveals that the fertile magma underwent large-scale fluid exsolution in an open system. The simultaneous and rapid depletion of Cu, Cl, S, and H2O in the magma indicates that Cl and S were highly partitioned into the exsolved fluid phase. These elements formed complexes, which effectively concentrated metals in the fluid. Conversely, the barren magma was characterized by either low metal content or limited fluid exsolution within the magma chamber. Overall, we reconstruct the upper crustal magmatic processes at Saindak and conclude that efficient extraction of Cu through fluid exsolution is the key to porphyry deposit formation.

Magma mixing formed mineralized Middle–Late Triassic granitoids in the Qimantagh Metallogenic Belt, NW China: A case study from the Hutouya skarn deposit

Although a number of petrographic observations and isotopic data suggest that magma mixing is common in the genesis of granitoids, it is still controversial whether mantle-derived magmas are involved in their formation. Zircon Hf and O isotopes can place robust constraints on the origin of granitoids, particularly deciphering the contributions of mantle- versus crust-derived magmas. In this study, we carried out a systematic examination of in-situ zircon Hf–O isotopes, as well as whole-rock major and trace elements, for mineralized Middle–Late Triassic syenogranite and diorite in the Hutouya deposit, NW China. Our results show that the parental magmas for syenegranite and diorite both underwent significant crystal fractionation, and belong to I-type and A-type granitoid magmas, respectively. The Hutouya syenogranite and diorite are very homogeneous in terms of zircon Hf–O isotopes, with εHf(t) and δ18O values ranging from − 3.15 to 1.55 and 5.97 ‰ to 7.77 ‰, respectively. These lines of evidence collectively emphasize that both mantle- and crust-derived magmas contribute to the formation of mineralized granitoids from the Hutouya deposit, although the latter seemingly plays a predominant role compared to the former. Combined with published studies, we suggest that magma mixing may be the key to forming fertile magmas related to the Cu polymetallic mineralization in the Hutouya deposit.

Using compositions of zircon to reveal fertile magmas for the formation of porphyry deposits in the Loei and Truong Son fold belts, northern Laos

The Loei and Truong Son fold belts are well known as being the most geologically important and highly mineralized magmatic arc-related terranes in mainland southeast Asia. Numerous studies have examined the geology, geochemistry, and geochronology but only a few metallogenic and detailed deposit characterizations have been undertaken. This study of zircon geochemical analyses uses laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) to provide U-Pb ages and magmatic fertility of felsic to intermediate volcanic and plutonic rocks from ten prospects in the Loei and Truong Son fold belts, in the northwest Laos region. The geochemical and geochronological analyses suggest at least three episodes of magmatism occurred: Silurian-Devonian (434–411 Ma), Early to Middle Permian (299–277 Ma), and Late Permian to Middle Triassic (253–243 Ma). The variable ranges of ɛHf values (−12 to + 12) suggest that mantle-derived and crustal contamination signatures are related to the history of subduction and arc magmatism in this region. Key trace elements and ratios associated with oxygen fugacity of magmas (e.g., Eu/Eu*, Dy/Yb, Ce/Nd, ΔFMQ) imply that the Phu Kham and the west of Vientiane along the Mekong River are likely to be fertile for porphyry copper deposits. In contrast, the Pha Gnai and other suites are less fertile. Using zircon as a fertility indication can be a valuable tool to distinguish between fertile and barren magmatic suites in this area.

The Chain of Processes Forming Porphyry Copper Deposits—An Invited Paper

Abstract Porphyry-related mineral deposits are giant geochemical anomalies in the Earth’s crust with orders-of-magnitude differences in the content and proportion of the three main ore metals Cu, Au, and Mo. Deposit formation a few kilometers below surface is the product of a chain of geologic processes operating at different scales in space and time. This paper explores each process in this chain with regard to optimizing the chances of forming these rare anomalies. On the lithosphere scale, deposits with distinct metal ratios occur in provinces that formed during brief times of change in plate motions. Similar metal ratios of several deposits in such provinces compared with global rock reservoirs suggest preceding enrichment of Au or Mo in lithospheric regions giving rise to distinct ore provinces. The largest Cu-dominated deposits and provinces are traditionally explained by selective removal of Au during generation or subsequent evolution of mantle magmas, but the possibility of selective Cu pre-enrichment of lithosphere regions by long-term subduction cannot be dismissed, even though its mechanism remains speculative. Evolution of hydrous basaltic melts to fertile magmas forming porphyry Cu deposits requires fractionation toward more H2O-rich magmas in the lower crust, as shown by their adakite-like trace element composition. The prevailing interpretation that this fractionation leads to significant loss of chalcophile ore metals by saturation and removal of magmatic sulfide might be inverted to a metal enrichment step, if the saturating sulfides are physically entrained with the melt fraction of rapidly ascending magmas. Ascent of fertile magma delivers a large mass of H2O-rich ore fluid to the upper crust, along points of weakness in an overall compressive stress regime, within a limited duration as required by mass and heat balance constraints. Two mechanisms of rapid magma ascent are in debate: (1) wholesale emplacement of highly fractionated and volatile-rich granitic melt into a massive transcrustal channelway, from which fluids are exsolved by decompression starting in the lower crust, or (2) partly fractionated magmas filling a large upper crustal magma chamber, from which fluids are expelled by cooling and crystallization. Transfer of ore-forming components to a hydrothermal ore fluid is optimized if the first saturating fluid is dense and Cl rich. This can be achieved by fluid saturation at high pressure, or after a moderately H2O rich intermediate-composition melt further crystallizes in an upper crustal reservoir before reaching fluid saturation. In either case, metals and S (needed for later hydrothermal sulfide precipitation) are transferred to the fluid together, no matter whether ore components are extracted from the silicate melt or liberated to the ore fluid by decomposition of magmatic sulfides. Production and physical focusing of fluids in a crystallizing upper crustal magma chamber are controlled by the rate of heat loss to surrounding rocks. Fluid focusing, requiring large-scale lateral flow, spontaneously occurs in mushy magma because high water content and intermediate melt/crystal ratio support a network of interconnected tubes at the scale of mineral grains. Calculated cooling times of such fluid-producing magma reservoirs agree with the duration of hydrothermal ore formation measured by high-precision zircon geochronology, and both relate to the size of ore deposits. Ore mineral precipitation requires controlled flow of S- and metal-rich fluids through a vein network, as shown by fluid inclusion studies. The degree of hydrothermal metal enrichment is optimized by the balance between fluid advection and the efficiency of cooling of the magmatic fluid plume by heat loss to convecting meteoric water. The depth of fluid production below surface controls the pressure-temperature (P-T) evolution along the upflow path of magmatic fluids. Different evolution paths controlling density, salinity, and phase state of fluids contribute to selective metal precipitation: porphyry Au deposits can form at shallow subvolcanic levels from extremely saline brine or salt melt; high-grade Au-Cu coprecipitation from coexisting and possibly rehomogenizing brine and vapor is most efficient at a depth of a few kilometers; whereas fluids cooling at greater depth tend to precipitate Cu ± Mo but transport Au selectively to shallower epithermal levels. Exhumation and secondary oxidation and enrichment by groundwater finally determine the economics of a deposit, as well as the global potential of undiscovered metal resources available for future mining.

Subduction-related mafic-silicic magmatism in the Nubian Shield: Tectono-magmatic implications of ensialic island arc association at Central Eastern Desert, Egypt.

For the Wadi El Mayet, the metagabbro-granodiorite-tonalite intrusive complex (MIC) occupies the eastern sector of famous wadi Mubarak–Dabr complex which considered as the largest intrusion in the Egyptian Eastern Desert. The basement associations in the area include volcano-sedimentary rocks, metavolcanics, and ultramafics. The whole successions were intruded by MIC, and granites. The (MIC) comprises multifarious gabbroic varieties namely: olivine, pyroxene, hornblende, uralitized gabbros and rare amphibolites, while silicic portion includes tonalite and granodiorite. Microprobe analytical chemistry of gabbros and granites refers that: the amphiboles are calcic magnesio hornblende formed at low pressure conditions. Chlorites have corundophillite composition. Plagioclase range between labradorite, andesine and oligoclase. Biotite is mainly siderophylite. Muscovite minerals are mainly cheladonite. Geothermo-barometers calculations reveal crystallization temperatures of 550 °C and 860 °C for granites and metagabbros respectively. The (MIC) cover a wide silica range (44.5–74.5 wt %). It has transitional tholeiitic to calc-alkaline magmas. The gabbroic rocks show distinct fecundity with LILE (e.g. Rb, Ba, U and Th), but show observed shortage in most HFSE (e.g. Zr, Nb, Ta). The REE patterns show enrichment in light REE and posse variably positive Eu. It is suggested that, the gabbroic units were likely generated by fractional crystallization of mafic magmas produced through partial melting of metasomatized mantle within island-arc environments. Tonalite - granodiorite (TG) suite show calc-alkaline magmas of I-type settings. They also exhibit enrichment in Ba, Zr, Hf, Rb, U and Th, while P, Nb, Ta and Sr are depleted. Such reductions are harmonious with fractionation of K-feldspar phases. The (TG) show enrichment of (LREE) compare to (HREE) this proposes either the existence of persistent garnet or LREE fertile magma sources. The (TG) is believed to be formed through dehydration melting processes at lower crust settings. Based on the field, geochemistry and structure elements, the investigated mafic and silicic suites are not linked to a single magma origin. The (MIC) can be comparable with intrusions formed in ensialic island-arcs settings. At subduction zone integration between mantle (basaltic suites) and silicic magmas from lower crust forming the early stages of (MIC) crystallization. The whole association was suffered by insignificant fractionation processes and addition of crustal materials during the final stages of (MIC) emplacement.

Mantle Plume‐Lithosphere Interactions Beneath the Emeishan Large Igneous Province

AbstractThe formation of large igneous provinces (LIPs) has been widely believed to be linked to mantle plume activity. However, how the plume modifies the overlying lithosphere, particularly its compositional structure, remains uncertain. Here, we characterize the deep thermochemical structure beneath the Emeishan LIP (ELIP), which is a well‐known Permian plume‐related LIP in China, by taking a multi‐observable probabilistic inversion. Our results find a clear correlation between the lithospheric composition with the ELIP's concentric zones. We infer that the fertile feature of the lithospheric mantle in the ELIP's inner zone was caused by the plume‐derived fertile magmas which infiltrated into and chemically refertilized the ambient depleted lithosphere. This plume‐modified lithospheric compositional structure is likely to be preserved after the plume event, while the present lithospheric thermal structure has been mainly influenced by the subsequent thermal‐tectonic activity. Our results improve our understanding of the physicochemical interactions between the lithosphere and ancient plume.

Volatile-rich and oxidized ore-forming magmas of the Zijinshan porphyry Cu deposit, China: A mineral composition perspective

The Zijinshan orefield (Fujian Province) is a representative Cu-Au-Mo-Ag porphyry-epithermal mineralization system in eastern China. It contains both Cu ore-bearing and ore-barren granitoids, which provide an opportunity to study the unique properties of fertile magmas. This study focuses on chemistry of minerals such as zircon, apatite, amphibole, and plagioclase in ore-related (zircon U–Pb ages of 103–101 Ma) and ore-barren (zircon U–Pb ages of 157–156 Ma and ∼105 Ma) granitoids to constrain the characteristics of mineralized magmas. The results of whole-rock geochemistry and zircon Hf isotopes suggest that both ore-barren and ore-related granitoids were derived from partial melting of the lower crust, with the ore-related magma being influenced by mantle materials. The analysis of mineral chemistry reveals that the Zijinshan Cu ore-bearing porphyry magmas have high oxidation states (ΔFMQ +0.23–+2.25), high water contents (4.4–4.9 wt%), and shallow emplacement depths (1.9–4.0 km). These magmatic conditions are favorable for fluid exsolution and mineralization, similar to the global Cu-fertile porphyry magmas, but distinct from regional pre-mineralization ore-barren igneous rocks. Apatites from the ore-related granitoids have significantly lower Cl contents (0.03–0.83 wt%) and higher F/Cl ratios (1.52–377) compared to coeval ore-barren monzogranites (Cl = 0.14–2.42 wt%; F/Cl = 0.07–2.82). Additionally, the apatites’ volatile budgets, including XOH, XCl and XF values, and their covariation relationship, in the ore-related intrusions are consistent with apatite crystallization in H2O-saturated conditions. This suggests that extensive fluid exsolution from the parent magma in the chamber plays a crucial role in the formation of porphyry Cu deposits from a low-Cl magma. These low-Cl-apatite intrusions have the potential to form porphyry deposits. Together, we propose that the high oxygen fugacity, high water contents, and fluid exsolution of the parent magma may have triggered the Zijinshan porphyry mineralization.

Local controls on magma fertility in the Mio-Pliocene metallogenic belt of central Chile: Exploration implications

Understanding what produces and how to detect fertile magmas, capable of forming large, economic porphyry-type deposits, is not only a major scientific quest, but it is also relevant economically, as it could lead to better exploration models and higher success rates in mineral exploration. In this work, we use new and previous geochemical and geochronological data from the Mio-Pliocene metallogenic belt of Central Chile to assess the existing tools to detect fertile magmas, study their spatiotemporal variations at different scales and discuss the geological and exploration implications of our findings.Our results suggest that the geochemical ratios currently used to detect fertile magmas are effective, as there is a good correlation between fertile signatures and the presence of known porphyry Cu deposits and prospects, with few false positives, which might be explained by the exhumation and erosion of most of the metal-rich part of the hydrothermal systems.Regarding the genesis of fertile magmas, the spatiotemporal variations we document here are not satisfactorily explained by the processes commonly brought forward to produce this type of intrusions, such as crustal thickening and subduction of sediments, crustal fragments or oceanic fracture zones. These processes operate at regional- or continental-scales in entire arc segments, while magma fertility shows strong contrasts in several coeval intrusions separated only by a few kilometers or tens of kilometers, emplaced within the same arc segment, under the same tectonic regime, crustal thickness and subduction parameters. This implies that there is a strong local control on the production of fertile magmas.Our results support the notion that the most important variable affecting the generation of fertile magmas in subduction margins, is magma residence time at different crustal levels. This variable has a strong local control, as it will depend not only on the overall crustal thickness or the regional tectonic regime, but also on the local orientation of structural pathways controlling magma ascent through the crust, relative to the predominant stress tensor. If magma pathways are oriented at higher angles relative to the maximum stress, magma residence times will be longer, allowing the fractionation of abundant hornblende (giving the residual magma its characteristic fertile or “adakite-like” signature) and the accumulation of magmatic sulfides which can be remobilized by later magmatic pulses; becoming progressively more oxidized due to intra-crustal processes; and concentrating large amounts of incompatible volatiles and metals in the residual melts. This can explain the strong variability observed in magma fertility in nearby coeval intrusions, and also in cross-cutting intrusions separated by very short time periods, which might have followed different ascent pathways.Our results also suggest that the Teno-Maule segment, in the southernmost part of the Mio-Pliocene metallogenic belt, shows striking differences in magma fertility patterns compared with the rest of the belt. There, fertile magmas are scarce and, contrary to the northern part of the belt, there is no evidence of an enhanced magma fertility during the late Miocene – early Pliocene.

Petrogenesis and metallogenic implications of Eocene–Oligocene magmatism in the Yulong porphyry copper belt, eastern Tibet: a review and analysis of geochronological, geochemical, and Sr–Nd–Pb–Hf isotopic data

The Eocene Yulong porphyry copper belt (YPCB) in eastern Tibet contains more than 10 Mt of Cu metal, associated with subvolcanic to volcanic porphyry systems intruding Cenozoic sedimentary basins. Geochemical data analysis shows that all samples belonged to high-K calc-alkaline and shoshonite series with metaluminous–peraluminous characteristics, light rare earth elements and large ion lithophile elements enrichment, high field strength elements depletion, and share geochemical similarities to adakitic rocks. We have established a spatiotemporal evolution framework through geochronological data analysis, combined with calculated bulk zirconium saturation temperature, calculated crustal thickness and geophysical evidence. We identified three magmatic processes triggered by asthenospheric upwelling and thermal erosion of thickened crust: (1) ultrapotassic magma derived from metasomatized mantle; (2) coeval ultrapotassic melt that promoted the partial melting of eroded lower crust and the generation of high-K adakitic felsic melt; high-K and Mg# adakitic fertile magma was formed by interaction between the eroded sulfide-rich juvenile lower crust-derived melt and mantle peridotite; (3) high-K adakitic fertile magma was derived from sulfide-rich juvenile lower crust, triggered by injecting hydrous ultrapotassic magma into thinned lower crust. The difference between deposit size in the north/south section of Yulong belt may be caused by the temperature of magmatic source and the volume of coeval potassic–ultrapotassic magmatism. Our analysis of the YPCB suggests that magmatism after 175 Ma within Intermontane arc complex of the Canadian Cordillera has high potential for porphyry deposits.

Determining the impact of magma water contents on porphyry Cu fertility: Constraints from hydrous and nominally anhydrous mineral analyses

Most large-scale porphyry-style mineralization is genetically associated with oxidized, sulfur-rich, and hydrous magmas, the sources of which were enriched in mantle-derived or juvenile crust−derived materials. Geochemical indices for these features can distinguish fertile magmas and barren magmas. However, fertile magmas that generate different-sized porphyry deposits are usually geochemically indistinguishable, and the factors controlling the size of porphyry mineralization are poorly understood. Here, we used zircon H2O contents and the compositions of apatite hosted in zircon, phenocrysts, and groundmass to compare the water contents and volatile evolution of fertile magmas in the Yulong ore belt, Tibet, which generated giant-, large-, and medium-sized porphyry deposits. We found that these porphyries have comparable major- and trace-element and zircon Hf-O isotope compositions, with high oxygen fugacity and sulfur contents. A negative correlation between [see PDF for equation] versus [see PDF for equation] of apatites suggests that the magmas consistently achieved H2O saturation before zircon crystallization, and zircon hydroxyl contents reveal that the water contents of the melt positively correlate with the size of the deposits, consistent with larger deposits achieving H2O saturation at deeper crustal levels. The deeper H2O saturation also caused larger amounts of Cl extraction, as indicated by the trends of increasing [see PDF for equation] and decreasing [see PDF for equation] with larger deposit sizes. These data sets can be reconciled by magmas forming larger deposits having higher H2O contents and suggest that the H2O content is a key control on their fertility. Our study highlights the utility of integrated hydrous and nominally anhydrous mineral analyses for constraining enigmatic magmatic volatile processes in magmatic ore-forming systems.

Volatile evolution of magmas associated with the Bairong deposit, Tibet, and implications for porphyry Cu-Mo mineralization

Volatiles are considered to play an important role in porphyry Cu mineralization, and high apatite S and Cl contents are often used to distinguish fertile magma systems from barren ones in the subduction zone. However, magmatic volatile evolution and its implications in post-collisional porphyry deposits remain enigmatic. To address this issue, we conducted zircon U-Pb dating and Hf isotope, and zircon and apatite chemical compositions of multi-stage magmatic rocks from the Bairong post-collisional porphyry Cu-Mo deposit in the Gangdese belt, Tibet. The magmatic rocks included pre-mineralized porphyritic monzogranite and biotite monzogranite, syn-mineralized monzogranite porphyry, and post-mineralized dacite porphyry that formed in 14.1–12.7 Ma, ca. 12.1 Ma, and 11.8–11.6 Ma, respectively. All these intrusions have depleted zircon Hf isotope (εHf(t) = 1.7–8.0, 3.7–9.0, 3.3–9.7 for pre-, syn-, and post-mineralized intrusions, respectively) together with the research results of contemporaneous magmatic rocks in the adjacent area, suggesting that the multi-stage intrusions at Bairong are derived from a cogenetic magma chamber in post-collisional extension setting. All the intrusions have similar zircon CeN/CeN*, EuN/EuN* ratios with average ΔFMQ values of 2.1 – 2.6, together with the high apatite EuN/EuN* ratios and low Ga concentrations, implying that their magmas are all oxidized. These intrusions have zircons (10,000 × EuN/EuN*) /Y all > 1 and Ce/Nd/Y mostly > 0.01, and obtain low average titanium-in-zircon temperatures of 659 to 714 °C, which indicate their magmas are hydrous. Apatite Cl contents decrease, combined with the drop of apatite XCl/XOH and the surge of XF/XCl ratios from pre- to syn-mineralized intrusions, possibly indicating that fluid exsolved from magma at the syn-mineralization stage. The apatite SO3 contents in these different-stage intrusions, however, are almost unchanged. Combined with previous studies, the buffering effect of magmatic anhydrite maintains the stability of magma S content, which together with fluid exsolution leads to the S-Cl decoupling. Consequently, we conclude that the oxidized and hydrous magma with effective fluid exsolution is a critical control on the formation of Bairong porphyry Cu-Mo mineralization. Considering the wide variations of apatite Cl contents and the occurrences of S-Cl decoupling, simple application of high apatite SO3 and Cl contents to distinguish the ore-forming and barren systems in the post-collisional porphyry system should be cautious.

Tracing the effects of fO2, pressure and H2O on the ore-forming magmas: Perspective from zircon REE composition

Efforts have been devoted to explore if there are independent zircon trace element ratios that could be proxy of oxidation state and magmatic water content, and can distinguish between mineralization and barren granites. In this study, I evaluated the difference of the zircon elemental signatures between fertile magmatic suites (which host porphyry Cu-Mo, Mo, Cu-Au, Cu, Cu-Mo-Au, Mo-Cu, and Au deposits) and infertile I-type granites. The “potential” magmatic suites, which means that they are supposed to develop porphyry deposits based on the previously proposed ore-forming criteria (e.g., zircon Eu/Eu* > 0.5) and yet no associated mineralization has been found, were also included in this comparison. Most of the trace elemental signatures cannot differentiate the I-type granite zircons from fertile zircons and “potential” zircons, except for the Eu anomaly (∼0.5 < Eu/Eu* < ∼0.8 for ore-related and “potential” suites; ∼0.2 < Eu/Eu* < ∼0.5 for I-type granites) and middle/heavy rare earth element variation diagrams (MREE and HREE; e.g., Eu/Gd versus Gd/Yb, Lu/Yb versus Gd/Yb, Sm/Ho versus Sm/Lu, and Yb/Lu versus Dy/Lu). The behavior of zircon REE possibly indicate a more MREE-depleted signature for the ore-forming and “potential” suites. Taken together, zircon REE features documented in this study highlight the fractional crystallization of hornblende and garnet, and the delay of plagioclase crystallization, revealing the combined effect of high overlying pressure, high fO2 and high H2O (or H+) concentration on the differentiation of fertile magmas.

Rapid transition to fertile magma and promotion of porphyry mineralization: A case study from the Don Javier deposit

The Don Javier porphyry Cu–Mo deposit formed contemporaneously with the Incaic I orogeny (∼60 Ma) in southern Peru. The causative dacite porphyry is hosted by the Yarabamba Superunit, which is the youngest batholithic unit in the Toquepala arc. In this study, we conducted elemental and isotopic analyses on samples from the dacite porphyry and Yarabamba Superunit in an effort to clarify the origins and formation mechanism of the deposit. Zircon U–Pb dating shows that the young part of the Yarabamba Superunit was emplaced at 65.4 ± 0.7 to 63.5 ± 0.8 Ma, and the causative dacite porphyry was emplaced at Don Javier between 59.9 ± 0.4 and 59.2 ± 1.1 Ma. The Yarabamba Superunit and dacite porphyry have similar εHf(t) and δ18O isotope values, ranging from –4.6 to +1.9 and 5.1‰ to 6.4‰, respectively. The Yarabamba Superunit has initial Sr and Nd isotope values of 0.70533–0.70579 and 0.51242–0.51244, respectively. Isotopic data suggest that the Yarabamba Superunit and dacite porphyry evolved from an isotopically homogeneous magma reservoir with minor crustal assimilation. However, the dacite porphyry has whole-rock and zircon Eu/Eu* values of 0.9 and 0.34, respectively, higher than those of the Yarabamba Superunit (0.6 and 0.13).Additionally, the dacite porphyry has significantly higher apatite S content (0.07 wt%) and XCl values (0.21) than those of the Yarabamba Superunit (apatite S: 0.01 wt%; XCl values: 0.15). The calculated magmatic oxidation state shows that the dacite porphyry has a significantly higher magmatic oxygen fugacity (ΔFMQ +1.1) than the Yarabamba Superunit (ΔFMQ –0.5).Together, the data suggest that the magmas formed the Yarabamba Superunit and causative dacite porphyry are characteristics of distinct tectonic regimes. The magma of the Yarabamba Superunit was generated during normal subduction and underwent low-pressure, H2O-poor, plagioclase-dominated fractionation while the magma of the dacite porphyry was generated during arc compression and underwent high-pressure, H2O-rich, amphibole-(garnet) dominated fractionation, which led to a high oxidation state and volatile-rich magma that promoted porphyry mineralization at Don Javier. This rapid elevation of the magmatic oxidation state and volatile content occurred synchronously with the Incaic I orogeny (∼60 Ma), which indicates that this orogeny had a significant impact on the formation of the very large–super-giant porphyry ore systems in southern Peru.

Discovery of Yaozhuang Stock and Deep Ore Prospecting Implication for the Western Mangling Orefield in North Qinling Terrane, Central China

In the western Mangling orefield, the molybdenum (Mo) polymetallic deposits are closely related to the ore-bearing porphyry stocks (individual outcrop size: &amp;lt;1 km2). In this study, we have discovered several granitic stocks at Yaozhuang. Systematic petrologic, zircon U-Pb-Hf isotope and whole-rock geochemical studies show that both the granitic stocks of porphyritic granite (157 ± 2 Ma) and the intruding monzogranite dike (153 ± 1 Ma) were emplaced in the Late Jurassic. These granitic stocks are characterized by high SiO2(66.83–75.63 wt%), high K2O (4.15–5.05 wt%), high Al2O3(12.90–16.93 wt%), and low MgO (0.06–0.73 wt%) and are metaluminous to weakly peraluminous, being highly fractionated I-A-type transition granites. The content of the total rare Earth element (ΣREE) of the porphyritic granite (139.6–161.7 ppm) is lower than that of the monzogranite (151.4–253.6 ppm). The porphyritic granite has weakly negative Eu anomalies (Eu/Eu* = 0.77–0.93), whereas the monzogranite has weakly positive Eu anomalies (Eu/Eu* = 0.97–1.21) and are more enriched in light rare Earth elements. Both of them are enriched in large ion lithophile elements (LILEs, e.g., K, Rb, and Ba) but depleted in high-field-strength elements (HFSEs, e.g., Nb, Ta, Ti, Zr, and Hf). The zircon εHf(t) values of all the samples range from −16.1 to −6.9, and the two-stage model ages (tDM2) are 1.78–2.16 Ga. The magma may have originated from partial melting of the lower crust (more than 40 km in depth) caused by mantle-derived magma underwelling. The plutons and stocks were emplaced into the intersection of the early EW-trending faults and the late (Yanshanian) NE-trending faults. The fertile magma with high water content (H2O &amp;gt; 4%) and high oxygen fugacity (Delta FMQ &amp;gt; 1.5) indicates that the Yaozhuang area has significant potential for porphyry Mo polymetallic ore discovery.

Geology and factors controlling the formation of the newly discovered Beimulang porphyry Cu deposit in the western Gangdese, southern Tibet

The porphyry Cu deposits in the Gangdese metallogenic belt were mainly formed in the eastern portion. The Beimulang is a newly discovered deposit formed in post-collisional setting in the western part of the belt and adjacent to the super-large Zhunuo porphyry Cu deposit. There are three epochs of magmatism at Beimulang, including pre-mineralization quartz porphyry (PQP; 49.7 ± 0.4 Ma), inter-mineralization monzogranite porphyry (IMGP; 14.8 ± 0.2 Ma), monzogranite (IMG; 14.1 ± 0.2 Ma), and late-mineralization granite porphyry (LGP; 11.7 ± 0.1 Ma). The various degrees of alteration and sulfide occurred at the IMG(P) and LGP indicate that there are at least two episodes of fluid exsolution events associated with different intrusions spanning ∼ 3 m.y. at Beimulang, which are unlike other Gangdese porphyry Cu deposits showing single fluid exsolution around only one porphyry at Miocene (e.g., Qulong, Zhunuo, Dabu). The PQP is characterized by high SiO2 (75.6–81.4 wt%) and K2O (2.1–4.3 wt%) concentrations, high Nb/Ta (8.4–11.3) ratios, and low Mg# (22–43), Ni (1.5–2.7 ppm) contents. They have high (87Sr/86Sr)i (0.705098–0.707767) ratios and low εNd(t) (-4.9 to −1.9) values. These geochemical characteristics indicate that they were probably derived from a juvenile basaltic lower crust during the ongoing Indian-Asian collision. The IMGP and IMG show similar petrography and geochemical characteristics, such as high SiO2 (65.0–69.8 wt%), K2O (3.6–4.3 wt%), Sr (550–809 ppm) contents, high Sr/Y (79–89) ratios, and low Mg# (43–48), Th/Yb, and Nb/Yb values, combined with high (87Sr/86Sr)i (0.707254–0.708017) and low εNd(t) (-7.9 to −6.0), implying a similar source probably originated in the subduction-modified lower crust metasomatized by Neo-Tethyan oceanic slab-derived fluids. The LGP displays trends produced by fractionation of magma with a composition similar to the IMG(P), but wall rock assimilation plays a significant role during magmatic evolution. Zircons from the IMG(P) show higher Eu/Eu* (>0.3), 10000*(Eu/Eu*)/Y (>4), and (Ce/Nd)/Y (>0.04) ratios than those of the PQP, indicating the fertile magmas were more oxidized and hydrous. The high magmatic oxidation state for the IMG(P) is also supported by using the zircon (ΔFMQ = +4.1 to + 6.1), and amphibole (ΔFMQ =+2.6 to + 3.4) oxybarometers. Their high magmatic water contents are supported by evaluations using amphibole (∼3.5 wt%) and plagioclase (∼6.7 wt%) compositions, respectively. However, the LGP has higher magmatic oxygen fugacity (Eu/Eu* >0.3, ΔFMQ = +4.2 to + 7.3) caused probably by auto-oxidation of the magmas, but lower magmatic water contents (Sr/Y = 31 ± 15; V/Sc = 6 ± 4) caused presumably by previous fluid exsolution and/or fractionation of hydrous silicate minerals during magma ascent than those of the IMG(P). Therefore, the highly hydrous and oxidized magmas with juvenile material are favorable to form porphyry Cu deposits in Tibetan orogen. On the contrary, magmatic evolution can probably reduce the water content of magma chamber, which is not beneficial to porphyry Cu mineralization.

Machine learning for geochemical exploration: classifying metallogenic fertility in arc magmas and insights into porphyry copper deposit formation

A current mineral exploration focus is the development of tools to identify magmatic districts predisposed to host porphyry copper deposits. In this paper, we train and test four, common, supervised machine learning algorithms: logistic regression, support vector machines, artificial neural networks (ANN) and Random Forest to classify metallogenic ‘fertility’ in arc magmas based on whole-rock geochemistry. We outline pre-processing steps that can be used to mitigate against the undesirable characteristics of geochemical data (high multicollinearity, sparsity, missing values, class imbalance and compositional data effects) and therefore produce more meaningful results. We evaluate the classification accuracy of each supervised machine learning technique using a tenfold cross-validation technique and by testing the models on deposits unseen during the training process. This yields 81–83% accuracy for all classifiers, and receiver operating characteristic (ROC) curves have mean area under curve (AUC) scores of 87–89% indicating the probability of ranking a ‘fertile’ rock higher than an ‘unfertile’ rock. By contrast, bivariate classification schemes show much lower performance, demonstrating the value of classifying geochemical data in high dimension space. Principal component analysis suggests that porphyry-fertile magmas fractionate deep in the arc crust, and that calc-alkaline magmas associated with Cu-rich porphyries evolve deeper in the crust than more alkaline magmas linked with Au-rich porphyries. Feature analysis of the machine learning classifiers suggests that the most important parameters associated with fertile magmas are low Mn, high Al, high Sr, high K and listric REE patterns. These signatures further highlight the association of porphyry Cu deposits with hydrous arc magmas that undergo amphibole fractionation in the deep arc crust.

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.

Biotite compositions and geochemistry of porphyry-related systems from the central Urumieh Dokhtar Magmatic Belt, western Yazd, Iran: Insights into mineralization potential

Commonly biotite occurs as a minor rock-forming mineral in a wide range of felsic to intermediate intrusive rocks. It can record the physicochemical conditions of the parent magma due to its complex crystal structure that allows multiple site substitutions of a wide range of elements. Geochemical studies of whole rocks and biotite (using EPMA and LA-ICP-MS methods) with petrographic analysis of biotite provide distinction between barren and fertile granitoids in western Yazd. Fertile suites are characterized by Si- and Mg-rich (3.47–4.35, ave. 3.75 a.p.f.u.) and total REE-poor biotite. In contrast, Fe (2.66–3.15, ave. 3.08 a.p.f.u.), Al, Ti, and REEs (in particular HREE+Y) increase in barren rocks. On the basis of biotite geobarometry, the fertile rocks (Darezereshk Porphyry Copper Deposit (PCD)) solidified at low pressure (ca. 0.6 kb, i.e., subvolcanic); in contrast, the barren rocks (Galvic and Shirkouh granodiorites) crystallized at higher pressures (1.1 to 3.6 kb). The temperature of the fertile granitoids is inferred to be about 750 °C, whereas T's of 625–675 °C for barren granitoids. The fertile suites have redox characteristic near the Ni-NiO buffer, whereas the barren suites are more reduced, near the quartz-fayalite-magnetite (QFM) buffer. Therefore, biotite major-element compositions, fertile magma bodies are emplaced at higher temperature and redox to lower pressures (subvolcanically) than the barren intrusive bodies. The whole-rock geochemical signatures show that both barren and fertile suites are represented by peraluminous to metaluminous granite to diorite that have calc-alkaline characteristics with an I-type volcanic arc (VAG) tectonic setting. However, fertile suites originated from the mantle-crust mixed magma exhibiting calc-alkaline affinity, whereas the barren suites are derived predominantly from recycling of continental crust.

ZIRCON TRACE ELEMENT GEOCHEMISTRY AS AN INDICATOR OF MAGMA FERTILITY IN IRON OXIDE COPPER-GOLD PROVINCES

Abstract Extrusive and intrusive felsic magmas occur throughout the evolution of silicic-dominated large igneous province magmatism that is temporally related to numerous economically significant iron oxide copper-gold (IOCG) deposits in southern Australia. We investigate zircon trace element signatures of the felsic magmas to assess whether zircon composition can be related to fertility of the volcanic and intrusive suites within IOCG-hosted mineral provinces. Consistent with zircon forming in oxidizing magmatic conditions, the rare earth element (REE) patterns of zircon sourced from both extrusive and intrusive magmatic rocks are characterized by light REE depletions and a range of positive Ce and negative Eu anomalies. The timing of the major phase of IOCG mineralization overlaps with the early part of the first phase of Lower Gawler Range Volcanics magmatism (1593.6–1590.4 Ma) and older intrusive magmatism of the Hiltaba Suite (1593.06–1590.50 Ma). Zircon in these mineralization-related intrusives and extrusives is distinguished from zircon in younger, mineralization-absent rocks by higher Eu/Eu*, Ce/Ce*, and Ti values and separate magma evolution paths with respect to Hf. These zircon characteristics correspond to lower degrees of fractionation and/or crustal assimilation, more oxidizing magmatic conditions, and higher magmatic temperatures, respectively, in magmas coeval with mineralization. In this respect, we consider higher oxidation state, lower degrees of fractionation, and higher magmatic temperatures to be features of fertile magmas in southern Australian IOCG terrains. Similar zircon REE characteristics are shared between magmas associated with southern Australian IOCG and iron oxide-apatite (IOA) rhyolites from the St. Francois Mountains, Missouri, namely high Ce/Ce* and high Dy/Yb, indicative of oxidized and dry magmas, respectively. The dry and more fractionated nature of the IOCG- and IOA-associated magmas contrasts with the hydrous and unfractionated nature of fertile porphyry Cu deposit magmas. As indicated by high Ce/Ce* ratios, the oxidized nature is considered a key element in magma fertility in IOCG-IOA terrains. In both IOCG and IOA terrains, the trace element compositions of zircon are able to broadly differentiate fertile from nonfertile magmatic rocks.

Zircon Petrochronology of the Kışladaǧ Porphyry Au Deposit (Turkey)

Abstract Porphyry deposits typically occur in subduction-related arcs but have more recently also been described in postsubduction, collisional to extensional back-arc settings. These different tectonic environments not only might imply different genetic processes but also seem to result in different metal endowments (e.g., Au rich versus Cu rich). It is therefore relevant, also for exploration purposes, to understand the magmatic processes involved in porphyry formation in these different tectonic environments. This study focuses on the Kışladaǧ porphyry Au (17.4 Moz) deposit in western Anatolia, which is centered on a series of porphyritic monzonite stocks of high-K calc-alkaline to shoshonitic affinity and formed in a continental rifting environment. With 17.4 Moz of Au, Kışladaǧ is of global metallogenic importance and hence a good example for studying the genetic processes associated with porphyry deposits in extensional back-arc settings. We herein combine a comprehensive set of new zircon textural observations, in situ zircon trace element and Hf isotope data, and previously published zircon geochronology to study the magmatic processes associated with porphyry deposit formation at Kışladaǧ. We show that mafic rejuvenation of a slowly crystallizing (between ~15.8 and 14.9 Ma) magma reservoir below Kışladaǧ immediately preceded porphyry deposit formation. Zircon trace elements and geochronology suggest a longer and deeper evolution for the early fertile magmas compared to the later infertile magmas. Magma evolution at Kışladaǧ was accompanied by crustal wall-rock assimilation. Whole-rock Nd and Sr radiogenic isotopes show that increasing asthenosphere-derived melt input under accelerated regional extension caused a loss in fertility of the system over time.