Barren Intrusions Research Articles

Petrogenetic relationship of the variably Ni−Cu−platinum-group element (PGE)-mineralized intrusions of the Thunder Bay North Intrusive Complex, Midcontinent Rift, Canada: Implications for mineral exploration in mafic−ultramafic complexes

The Thunder Bay North Intrusive Complex (TBNIC) of Canada comprises six variably Ni−Cu−platinum-group element (PGE)-mineralized mafic−ultramafic intrusions that intruded the Archean Quetico Subprovince. These are the Current, Escape, Lone Island, Greenwich, and 025 intrusions, with the former three aligned along an east−west-trending intrusion termed the East−West Corridor. These intrusions have a consistent igneous stratigraphy comprising, from bottom to top, peridotite, gabbros, and quartz gabbro. Only the Current and Escape intrusions are known to host sulfide mineralization in the peridotite. Despite the economic significance of the TBNIC, little work has been done to characterize its petrogenetic evolution and the causes of the apparent variable fertility. Our new ages for Escape (1107.6 ± 0.9 Ma) and Greenwich (1105.7 ± 0.9 Ma), combined with previously determined ages for Current (1106.6 ± 1.6 Ma) and Lone Island (1106.3 ± 2.1 Ma), demonstrate that the TBNIC was emplaced as part of the ca. 1.1 Ga Midcontinent Rift event. The similarities in their rare earth element geochemistry and consistently low Th/La−Sri values indicate that the intrusions crystallized from genetically related magmas that did not assimilate significant amounts of crustal material. Accordingly, their systematically negative εNdt values and Nb anomalies likely reflect that their parent magmas were largely sourced from the subdcontinental lithospheric mantle and not the Keweenaw Plume. Given their genetic relationship, the variable fertility of the intrusions cannot be related to processes that occurred in their mantle sources. Since the gabbros have systematically depleted Cu/Zr−Cu/Pd ratios and the peridotites have enriched ratios, the apparent variable fertility is likely related to the timing and location of sulfide segregation and accumulation, with the barren gabbros preserving magmas that lost sulfide liquid during transport to shallow crustal levels, and the mineralized peridotites recording accumulation of sulfide liquid. This interpretation has important implications for exploration as it implies that sulfide mineralization in the apparently barren intrusions may be located at depth in undiscovered ultramafic cumulates.

Quantifying the Criteria Used to Identify Zircons from Ore-Bearing and Barren Systems in Porphyry Copper Exploration

Abstract Zircon is a common mineral in igneous rocks, which is resistant to both chemical weathering and physical abrasion. Its chemistry can potentially be used to distinguish ore-forming porphyry magmas from barren magma systems. This study compiles >23,000 zircon analyses from >30 porphyry deposits, barren intrusions, and rivers to determine the principal geochemical characteristics of fertile zircons using predictive modeling, and compares them with traditional geochemical thresholds. The results show that the Eu/Eu* and Dy/Yb ratios, P content, and the curvature at the end of rare earth element (REE) patterns (λ3) are the most diagnostic characteristics of fertile zircons. The use of geochemical thresholds, as Boolean conditions, reach their maximum performance for Eu/Eu* and Dy/Yb (sensitivity [sens] = 0.73, specificity [spec] = 0.90), but it is outperformed by the random forest model (sens = 0.91, spec = 0.93) in the testing set. Explanatory analysis of the models shows that the fertility signal in zircons becomes stronger as the porphyry system evolves and is accompanied by an overall decrease in the middle to light REE and P content, characteristics that are absent in barren zircons. We attribute the observed difference in λ3 to the co-crystallization of other accessory phases, suggesting that the changes in the zircon Ce anomaly is controlled by the depletion of light and middle REE. The low P content in fertile zircons is caused by extensive crystallization of apatite. Fertile zircons have an excess of (REE + Y)3+, which we attribute to charge-balance by H+ in hydrous magmas. Simple machine learning algorithms outperform the traditional geochemical discriminators in their predictions and provide insights into characteristics that have not previously been considered for evaluating porphyry copper fertility using zircon geochemistry. We propose simplified methods that can be easily incorporated into exploration workflows.

Magmatic–Hydrothermal Evolution at the Barren Wushan Pluton, Southeast China: Insight into Controls on Mineralization Potential

Abstract A-type granites typically exhibit enrichment and mineralization of critical metals such as molybdenum and tin, essential for emerging technologies. However, the key factors influencing their mineralization potential remain elusive. The scarcity of studies on barren systems impedes the understanding of this question. Here, a detailed melt and fluid inclusion study was conducted on the barren Wushan pluton to reconstruct its magmatic evolution and magmatic–hydrothermal transition and explore the factors controlling the metallogenic potential of Mo and Sn in A-type granites. The Wushan pluton displays apparent lithological zoning consisting of two major phases, i.e., medium-grained seriate to porphyritic alkali feldspar granite and fine-grained porphyritic granite. Miarolitic cavities are widely developed in each lithofacies. The silicate melt inclusions from two granitic phases are rhyolitic, with moderate F contents (0.06–0.53 wt %) and depleted H2O contents (2.0–3.5 wt %). Melt inclusions show a wide range of incompatible element contents, such as Cs (9–1977 μg/g) and Rb (268–2601 μg/g), suggesting that Wushan has undergone a high degree of magma evolution. Mo behaves incompatibly in the magmatic evolution, and its content is enriched with the increasing degree of fractional crystallization, but remains constant after the Cs content exceeds 50 μg/g. Rayleigh fractionation model suggests that a large amount of Mo is extracted from fluid exsolution, which restrains Mo from further enrichment. In contrast, Sn behaves as a mildly incompatible element during the entire magmatic evolution history. The contents of Sn increase slowly compared to the trend of Mo, and the maximum contents reach ~30 μg/g in the highly evolved melts. The separation and crystallization of Sn-bearing minerals such as biotite, magnetite, and titanite inhibit the enrichment of Sn. Intermediate-density (ID-type) fluid inclusions hosted in the miarolitic quartz, representing the initial fluid exsolving from magma, display high Mo but low Sn concentrations. Constrained from two assemblages of coexisting ID-type fluid and melt inclusions, the fluid/melt partition coefficients of metals are obtained, with DMo, fluid/melt at 16–19, while DSn, fluid/melt is only about 1. The comparison between Mo-mineralized and barren intrusions worldwide shows that the metal contents in melts and fluids are not fundamentally different. The mineralized intrusions are characterized by the lower melt viscosity and the development of apophyses, both of which facilitate the extraction of metals and fluids from large magma chambers, followed by their concentration into a small rock volume. Consequently, it appears that physical and structural conditions rather than chemical compositions play a crucial role in the Mo mineralization process. Enrichment of Sn in melts is necessary but not decisive for Sn mineralization, whereas Sn enrichment in the initial exsolving fluid determines the Sn mineralization potential of a given granitic system. Compared to Sn enrichment in source melting and fractional crystallization which commonly enhance final Sn fertility in the highly evolved melts, the efficiency of Sn partitioning between melt and fluid plays a fundamental role in converting melt fertility into Sn-enriched fluids and thereby high mineralization potential of the magmatic–hydrothermal system. Our findings suggest a prospect for Mo exploration in the coastal A-type granite belt in South China, while the potential for Sn mineralization is expected to be limited.

Metasomatized mantle facilitates the genesis of magmatic nickel–copper sulfide deposits in orogenic belts: A copper isotope perspective

The role of metasomatized mantle in the genesis of magmatic Ni–Cu sulfide deposits in orogenic belts remains vigorously debated. The greatest challenge in clarifying uncertainties is how to track the behavior of metals during metasomatic processes in the mantle. Copper isotopes can shed new light on this enigmatic topic as the transfer of Cu during metasomatic processes can significantly fractionate Cu isotopes. In this regard, we have characterized the Cu isotope and trace-element compositions of fertile and barren mafic–ultramafic intrusions, and coeval basalts in the Eastern Tianshan, Central Asian Orogenic Belt (CAOB). Fertile mafic–ultramafic intrusions are characterized by elevated and highly variable Ba/Nb (40–756, average of 172) and Ba/La (16–219, average of 48) ratios, and exhibit δ65Cu values (−0.97 ‰ to +0.57 ‰) that vary over 1.54 ‰. In contrast, barren intrusions and coeval Permian basalts are characterized by relatively low and invariable Ba/Nb (2–95, average of 29) and Ba/La (0.9–26, average of 8) ratios, and have δ65Cu values that are comparatively less variable (−0.06 ‰ to +0.49 ‰ for δ65Cu) and more similar to bulk silicate Earth (BSE, δ65Cu = 0.06 ± 0.20 ‰). The difference in Cu isotope compositions of fertile and barren intrusions, alongside the variability in Ba enrichment, suggests that a high degree of mantle metasomatism promotes the fertilization of primary magmas in metals and, thus, represents one of the first stages in the development of magmatic Ni–Cu sulfide deposits in convergent margins.

Controls on metal fertility of dioritic intrusions in the Laiwu district, North China Craton: Insights from whole-rock geochemistry and mineral compositions

Several large high-grade iron skarn deposits in the Laiwu district of Shandong Province (North China Craton) are developed along the contact zone between the Early Cretaceous Kuangshan diorite pluton and Middle Ordovician dolomitic limestone. In sharp contrast, no iron skarn mineralization has been recognized in association with other contemporaneous dioritic intrusions (Jiaoyu, Jinniushan, Tietonggou) in this district. The reason for this contrast remains elusive. Here we present a comparative study of the ore-related Kuangshan pluton and its barren equivalents using whole-rock and mineral (zircon, apatite) geochemistry combined with existing Sr-Nd isotope data to provide new insights into the critical controls on iron skarn mineralization and shed light on future mineral exploration. Geochemical and Sr-Nd isotope data suggest that both the ore-related and barren intrusions were derived from a common enriched lithospheric mantle source following variable degrees of fractional crystallization and crustal assimilation. Modeling based on whole-rock Sr-Nd isotopes implies that the magmas responsible for the Kuangshan and Jinniushan diorite plutons assimilated significant quantities of the Ordovician evaporite-bearing carbonates. Both the fertile and barren diorites show high whole-rock Sr/Y ratios, high zircon Eu/Eu* and Eu/Eu*/Y ratios, and low zircon Dy/Yb ratios. These data, combined with high magmatic water contents (4.4−8.0 wt%) calculated from amphibole compositions, indicate that those four plutons crystallized from hydrous magmas. High whole-rock Fe2O3/FeO, and high ΔFMQ values (mostly >1; fayalite-magnetite-quartz oxygen buffers) estimated from zircon, apatite, and amphibole suggest that the magmas from which the diorite intrusions crystallized were all oxidized. Using available partitioning coefficients for Cl between apatite and magma, the parental melts of the Kuangshan and Jinniushan diorite are estimated to have had higher Cl contents (0.15−0.77 wt%) than the Jiaoyu and Tietonggou diorites (0.06−0.34 wt%). Two types of apatite from the intrusions studied are recognized based on their paragenetic relations: Type 1 apatite (Ap1) is an early-formed phase enclosed in euhedral amphibole, plagioclase, and biotite, whereas type 2 (Ap2) is a late-formed variety hosted in, or intergrown with, anhedral K-feldspar (± anhedral amphibole ± subhedral to anhedral quartz). In the fertile Kuangshan diorite, Cl and OH contents of apatite decrease, but Sr contents increase from Ap1 to Ap2. However, those parameters show reverse trends or remain constant from Ap1 to Ap2 in barren diorites. Such distinctive apatite geochemical signatures are best interpreted as reflecting more efficient fluid exsolution from the parental magma of the Kuangshan diorite than from the barren intrusions. This view is supported by the presence of abundant fluid inclusions in apatite and quartz exclusively from the mineralized intrusion, indicating its crystallization from volatile oversaturated melts. The results presented here indicate that elevated magma Cl contents and efficient separation of magmatic hydrothermal fluid from cooling magmas have been the critical factors controlling iron skarn mineralization in the Laiwu district. Mass balance constraints on Cl budget indicate that a minimum of 44−60 km3 of magma was required to form the Fe deposits associated with the Kuangshan pluton. Our preliminary results suggest that decreasing OH and Cl contents from early to late apatite in intermediate rocks can be used as an indicator for efficient fluid exsolution and consequently metal fertility of those rocks.

Syn-collisional extension and Ni-Cu sulfide-bearing mafic magma emplacement along the Irtysh Shear Zone in the Central Asian Orogenic Belt

The Central Asian Orogenic Belt was characterized by a long-lived accretionary history from Neoproterozoic to Paleozoic, followed by a collisional phase of orogeny in the latest Paleozoic with the closure of the Paleo-Asian Ocean. However, Ni-Cu sulfide mineralization associated with mantle-derived mafic magmas in the Central Asian Orogenic Belt was relatively short-lived. Thus, the coherence between the protracted accretionary and collisional history of the Central Asian Orogenic Belt and the transient Ni-Cu sulfide-bearing mafic magmatism is difficult to understand. We investigated both Ni-Cu sulfide-bearing and sulfide-barren mafic intrusions along a major suture zone of the Irtysh Shear Zone within the Central Asian Orogenic Belt to understand the origin of short-lived Ni-Cu sulfide mineralization in the context of the collision of two arc systems of the Chinese Altai and the East Junggar regions. Zircon U-Pb dating results for the Ni-Cu sulfide-bearing mafic intrusions show that they were only emplaced briefly from 290 Ma to 280 Ma after subduction terminated (ca. 312 Ma), substantially earlier than Ni-Cu sulfide-barren mafic intrusions (ca. 274−271 Ma). Zircons from all Ni-Cu sulfide-bearing/barren mafic intrusions yield arithmetic mean δ18O values ranging from 6.0 to 7.4‰, higher than the typical mantle value (∼5.3‰), which indicates their derivation from a metasomatized mantle with inherited subduction signatures. The δ11B values of the rocks from these intrusions range from −17.2‰ to −2.3‰, significantly lower than those of typical volcanic arcs (up to +18‰), which suggests that the metasomatic components in the mantle source were dominated by sediment melts rather than fluids. The pressure calculated using amphibole thermobarometry shows that the parental magmas of the Ni-Cu sulfide-bearing mafic intrusions were emplaced shallower than those of Ni-Cu sulfide-barren mafic intrusions. This is supported by the fact that the country rocks of the Ni-Cu sulfide-bearing mafic intrusions are shallow crustal lithologies of sedimentary rocks, whereas the country rocks of the barren intrusions are relatively deeper crustal lithologies of high-grade metamorphic rocks and granites. Olivine from the Ni-Cu sulfide-barren intrusions has forsterite (Fo) contents ranging from 51.1 mol% to 76.2 mol% and Ni from 196 ppm to 1312 ppm. These values are much lower than those of the Ni-Cu sulfide-bearing intrusions (Fo = 72.6−79.4 mol%; Ni = 1022−1925 ppm), which indicates that the parental magmas of the barren intrusions may have experienced extensive olivine crystallization. Notably, the emplacement of the Ni-Cu sulfide-bearing mafic intrusions (290−280 Ma) coincidently overlapped with a transient period of orogen-parallel extension (ca. 295−280 Ma) in response to the collision of the Chinese Altai and East Junggar. Syn-collisional extension may lead to the rapid ascent and emplacement of mantle-derived mafic magmas at a shallower crustal level, preventing significant loss of Ni from olivine crystallization at depths, and thus providing metal-rich mafic magmas for potential Ni-Cu sulfide mineralization. Our results highlight that syn-collisional extension is an essential geodynamic mechanism that controls the emplacement of Ni-Cu sulfide-bearing mafic intrusions in orogenic belts.

Geochemical Features of Potentially Ore-Bearing Mafic Intrusions at the Eastern Norilsk Region and Their Relationships with Lavas (NW Siberian Traps Province)

The problem of the world-class PGE-Cu-Ni Norilsk deposits’ origin has attracted geologists for several decades. The main goal of this study is to determine the specific features of ore-bearing intrusions in comparison with thousands of similar barren intrusions widespread within the Siberian igneous province, and to establish their genesis. As a result of statistical processing of previously published isotope-geochemical data and obtained by the authors, systematic differences were found in the distribution of the isotopic ratio of Nd in ore-bearing and barren intrusions, as well as in volcanic rocks at the Norilsk region. Thus, ore-bearing rocks in ten deposits (Talnakh, Kharayelakh, Norilsk 1, South-Maslovsky, North-Maslovsky, Norilsk 2, Chernogorsky, Zub-Mrksheydersky, Pyasino-Vologochansky, Imangdinsky), different in Ni and PGE reserves, show a very narrow range of Nd isotopic ratio, ԐNd(T) = 1.0 ± 1.0 (2σ, N = 139), whereas barren and volcanic rocks are characterized by a rather wide ԐNd(T) range, from −10 to +7 units (N = 256). Furthermore, ore-bearing intrusions are characterized by reduced and compact variations of the La/Lu ratio due to lower concentrations of light lanthanides. For the first time the authors studied two new intrusions penetrated by MD-48 and MD-60 boreholes drilled by Norislkgeologia LLT at the eastern part of the Mikchangda area. Their economic values are still unclear and should be estimated using geochemical methods. Both intrusions lie in the Devonian rocks, have similar thickness and mineral composition, but differ in textural and structural features, which indicate a rapid crystallization of the MD-48 intrusion. According to the contents of the major oxides, the rocks in MD-48 and MD-60 are identical, but they differ in U/Nb, La/Sm, and Gd/Yb ratios. It is important that the rocks in the MD-60 borehole are characterized by ԐNd(T) = 1.0 ± 0.6 (2σ) and fall into the range of ore-bearing intrusions, whereas the rocks in MD-48 have ԐNd(T) 2.4 ± 0.9, and, thus, are outside of ore-bearing intrusions. Therefore, ԐNd(T) values can be used as a local criterion for the estimation of economic potential of mafic intrusions, which is demonstrated for the Mikachangda area.

Geochronology and geochemistry of zircons from fertile and barren intrusions in the Sangan mining area (NE Iran): Implications for tectonic setting and mineral exploration

The Sangan mining area is the largest Cenozoic skarn iron ore district in the far eastern part of the Alborz Magmatic Arc in Iran. In this study, we evaluate the zircon composition as an important exploration tool and pathfinder for skarn mineralization to elucidate distinctive zircon signatures diagnostic of metallogenic fertility of the parent magma. Granitoid intrusions in the Sangan mining area are subdivided into the fertile Sarnowsar intrusion composed of syenite, syenogranite, and granite and the barren Sarkhar and Bermani intrusions composited of monzogranite and syenogranite. The Eocene Sarnowsar intrusion was produced by the flare-up magmatism and is an oxidized I-type granitoid suite. Fourteen iron ore bodies occur along the contact of this intrusion with the carbonate country rocks. In the barren Sarkhar and Bermani intrusions, monzogranites have zircon U-Pb ages of 41.7 ± 0.6 Ma and 41.9 ± 0.3 Ma, whereas syenogranites have ages of 37.8 ± 0.3 Ma, 37.9 ± 1.7 Ma, and 37.4 ± 1.8 Ma. These ages span the same time interval as the Eocene flare-up occurrences throughout Iran.

Geochronology, petrogenesis and metallogenic implications of granitoids in the Xiaotuergen Cu deposit, Northern Chinese Altai Orogen, NW China: Constraints from zircon, apatite and whole-rock geochemistry

The Xiaotuergen Cu deposit is the first Cu deposit described in the Northern Chinese Altai Orogen, Xinjiang, NW China. The multiple phases of granitic intrusions occur in the Xiaotuergen region, including the intrusions of granodiorite porphyry, granite porphyry, and biotite monzogranite. Among these types of intrusions, Cu mineralization is considered to be closely related to the early Hercynian granodiorite porphyry. However, the precise emplacement ages, petrogenesis, redox states, and tectonic setting of the three Xiaotuergen granitoids and the relationship between these granitoids and mineralization was unclear. To resolve these issues, zircon U–Pb dates, zircon in situ trace element concentrations, Hf isotopic compositions, apatite in situ major element concentrations, and whole-rock geochemistry were determined in this study. Three different intrusive phases are recognized within the district; in order of deceasing age, these phases are biotite monzogranite, granodiorite porphyry, and granite porphyry, which have the ages of 400.2 ± 1.6 Ma to 399.6 ± 2.4 Ma, 390.3 ± 1.6 Ma to 377.1 ± 1.6 Ma, 381.0 ± 2.0 Ma to 378.7 ± 1.2 Ma, respectively, suggesting that they were emplaced during the Early-Middle Devonian. Geochemical data show that these intrusions are metaluminous to weakly peraluminous (A/CNK = 0.91–1.28), calc-alkalic to shoshonitic I-type granitoids. The two Harker plot trends reveal derivation from two magma sources (biotite monzogranite vs granodiorite porphyry and granite porphyry) that experienced different evolutionary processes. Multiple indicators, including the zircon Ce4+/Ce3+ ratio and oxygen fugacity (logfO2) values, apatite Mn oxybarometer, and whole-rock Fe2O3/FeO ratio, consistently indicate that the parental magma of the fertile granodiorite porphyry was more oxidized than that of the two barren intrusions (biotite monzogranite and granite porphyry). Furthermore, apatite grains from the fertile granodiorite porphyry have higher Cl contents and Cl/F ratios than those of the ore-barren biotite monzogranite and granite porphyry, which may have resulted from the higher water content of the parental magma and increased lithospheric mantle-derived fluid involvement in their generation. Thus, higher Cl/F ratios of apatite can be used to discriminate fertile granitoid intrusions that host Cu mineralization in the Northern Altai orogen. Zircon Hf isotopic compositions show widely ranging and mostly positive εHf (t) values (−4.2 to +11.4, mainly concentrated between +2 and +8). These isotope characteristics in combination with young zircon TDMC (Hf) ages (655–1,659 Ma; mainly concentrated between 850 Ma and 1250 Ma) indicate that the Xiaotuergen granitoids were most likely derived from the partial melting of juvenile lower crust, with some interaction of mantle materials from the underlying mantle wedge. The three Xiaotuergen granitoids were enriched in light rare earth elements and large ion lithophile elements, depleted in heavy rare earth elements, and showed negative Nb, Ta, Sr, P, and Ti anomalies, displaying typical arc geochemical affinities. Combined with the regional tectonic evolution, we propose that the Xiaotuergen granitoids were generated in a continental arc setting associated with subduction processes, which were the products of syn-accretion orogenic processes.

Trace elements in apatite from Gejiu Sn polymetallic district: Implications for petrogenesis, metallogenesis and exploration

The world-class Gejiu tin polymetallic district is located in southeastern Yunnan Province. Yet, the largest deposits are concentrated in the Masong and Laoka granites in the eastern area of Gejiu. To investigate the factors controlling Sn mineralization among intrusions in the Gejiu area, we collected apatite from eight different intrusions (including gabbro, monzonite, and granite) and determined the fertility indices of Sn deposits through the chemical composition of apatite. Analysis of the redox-sensitive elements in apatite showed that the ore-bearing granites have high Mn and low S content, high Y/∑REE and low La/Sm ratios, extreme negative Eu anomalies, and weak Ce positive anomalies compared to the barren intrusions, indicating that the formation environment of the ore-bearing granites was more reducing. The apatites in ore-bearing intrusions have lower Sr and higher Y contents, which suggests that these intrusions have a higher degree of fractionation than barren intrusions. The apatites in the Masong and Laoka granites have a high F content, which reflects the high F content of magma that contributed to the enrichment of Sn. The decreasing (Sm/Yb)N ratios with stable Sr content imply exsolution of Cl-rich fluids or crystallization of monazite and allanite. The exsolution of Cl-rich fluids can increase Sn solubility in the melt and promote the extraction of Cu from basalt. The apatites in the ore-bearing intrusions have high Li, Mn, Fe, and F contents and extremely low (Eu/Eu*)N ratios (<0.05). These results show that apatite can be used to distinguish Sn deposits from other deposit types and guide the Sn exploration.

Nickel in olivine as an exploration indicator for magmatic Ni-Cu sulfide deposits: A data review and re-evaluation

AbstractNickel contents of olivine have been widely used as petrogenetic indicators and as fertility indicators for magmatic sulfide potential of mafic-ultramafic intrusions, on the assumption that olivines crystallized from magmas that had equilibrated with sulfide liquid should be relatively depleted in Ni compared with a sulfide-free baseline. This has given rise to a large accumulation of data that is brought together here, along with data on volcanic olivines, to critically evaluate the effectiveness of the approach. We identify multiple sources of variance in Ni content of olivine at a given Fo content, including variability in mantle melt composition due to depth, water content (and possibly source), subsequent fractional crystallization with and without sulfide, recharge and magma mixing, batch equilibration between olivine and sulfide at variable silicate-sulfide ratio (R), and olivine/liquid ratio; and subsequent equilibration during trapped liquid crystallization in orthocumulates. Baselines for Ni in olivine in relation to Fo content are somewhat lower in orogenic belt settings relative to intrusions in continental large igneous provinces (LIPs). This is probably related to differences in initial parent magma compositions, with plume magmas generally forming deeper and at higher temperatures. No clear, universal discrimination is evident in Ni in olivine between ore-bearing, weakly mineralized, and barren intrusions, even when tectonic setting is taken into account. However, sulfide-related signals can be identified at the intrusion scale in many cases. Low-R factor and low-tenor sulfides are associated with low-Ni olivines in several examples, and these cases stand out clearly. Anomalously high-Ni olivines are a feature of some mineralized intrusions, in part due to trapped liquid reaction effects. However, in some cases, this mechanism cannot account for the magnitude of enrichment. In these cases, enrichment may be due to re-entrainment of “primitive” Ni-rich sulfide by a more evolved Fe-rich magma, driving the olivine to become Ni-enriched due to Fe-Ni exchange reaction between sulfide and olivine during transport. An extreme case of this process may account for ultra-Ni enriched olivine at Kevitsa (Finland), but more subtle signals elsewhere could be positive indicators. A lack of clear mineralized/barren distinction in specific groups of related intrusions, e.g., the deposits of NW China or the Kotalahti Belt in Finland, may well be due to “false negatives” where undiscovered mineralization exists in specific intrusions or in their feeder systems, or may also be due to a multiplicity of confounding factors. Wide variability of both Fo and Ni between related intrusions at regional scale may be a useful regional prospectivity indicator, more than an intrusion-scale discriminant, and is certainly informative as a petrogenetic indicator. In general, the use of Ni-olivine as a fertility tool is more likely to generate false negatives than false positives, but both are possible, and the technique should be used as part of a broader weight-of-evidence approach.

Comparative analysis of exploration potential within the Urumieh Dokhtar Magmatic Arc, Iran, with a detailed example from mineral chemistry of the Marshenan intrusion

The Early Miocene Marshenan intrusion, in the central part of the Urumieh-Dokhtar magmatic arc (UDMA), central Iran, includes granodiorite-granite and diorite; it is a classic example of a CuAu barren intrusive system. Zircon U-Pb dating indicates that the diorite display an age of 20.32 ± 0.36 Ma, coeval with granodiorite rocks (20.5 ± 0.8 Ma). These rocks are composed of feldspar, quartz, amphibole, biotite, titanite, and magnetite. In the granodiorites, the plagioclase composition ranges from oligoclase to bytownite, the amphiboles are magnesio-hornblende and biotites is Mg rich, related to calc-alkaline orogenic suites in the region. Plagioclase phenocrysts exhibits oscillatory zoning and marked changes in the abundance of elements, such as Ba, Sr, and Fe, suggesting magma mixing/mingling may have had a role in generating these parental magmas. The average calculated P–T conditions of the granodiorite and diorite rocks are about 730 °C and 2.1 kbar and 733 °C and 1.7 kbar, respectively, corresponding to near solidus conditions equal to emplacement depths of ~7 to 8 km. Magmatic H2O contents and ƒO2 calculated from crystallized amphiboles indicate that the Marshenan granodiorite had initial magmatic H2O contents ~5 wt% and relatively high ƒO2 (ΔNNO; ave. 1.3) and the diorite had initial magmatic H2O contents ~4.7 wt% and relatively high ƒO2 (ΔNNO; ave. 1.5), both consistent with the presence of phases, such as hornblende, biotite, magnetite, and titanite. In comparison with other intrusions in the UDMA, the Marshenan intrusion formed via the same physico-chemical mechanisms like other barren intrusions, whereas the fertile intrusions exhibit slightly higher temperatures, pressures, ƒO2, and H2O values than the barren intrusions. These compiled data suggest that, in spite of the high magmatic H2O and ƒO2 contents, the Marshenan and other barren intrusions in the UDMA will not produce porphyry Cu mineralization, unlike the giant Kerman deposit, probably due to magma source, magmatic evolution processes, timing of volatile exsolution, pre-existing crustal-scale fractures, coeval volcanism, and extended duration of volatile saturated crystallization to subsolidus conditions.

Apatite and zircon compositions for Miocene mineralizing and barren intrusions in the Gangdese porphyry copper belt of southern Tibet: Implication for ore control

Gangdese porphyry copper belt in southern Tibet of China contains many Miocene fertile and barren adakite-like intrusions. Their magmatic oxidation state and volatile concentrations were poorly constrained. Here, we present apatite and zircon composition data for the Miocene fertile intrusions in the Qulong, Jiru, and Zhunuo porphyry copper deposits, and the coeval barren porphyries at Yare, Mayum, and Lasa in the Gangdese belt. Magmatic zircons of intrusions from the six locations share similar ΔFMQ (FMQ represents the reference buffer fayalite + magnetite + quartz) values, indicating both fertile and barren rocks have similar magmatic oxidation state. Variations of apatite Cl concentrations and characteristics of S- and Cl-rich core to abruptly S- and Cl-poor rim at Qulong, Jiru, Zhunuo, Yare, and Mayum are indicative of fluid exsolution from vapor-saturated magma. In contrast, fluid exsolution was not recognized from the available apatite composition data at Lasa which are characterized by constant or increasing apatite Cl concentrations with decreasing F/Cl ratios from core to rim. Some apatites at Lasa have high SO3 concentrations at the core compared with the rim, which would be attributed to early crystallization from S-rich magma followed by crystallization of sulfate minerals in a vapor-undersaturated magma. Some apatites from the fertile intrusions at Qulong, Jiru, and Zhunuo have lower Cl concentrations than those from the barren rocks at Yare, Mayum, and Lasa, which is consistent with the observation from the calculated melt Cl concentration. In addition, the SO3 concentrations at the core of apatites from all the fertile rocks are overlapping with those from all the barren rocks. Therefore, enrichment of Cl and S alone in melts may not be critical to forming economic porphyry copper mineralization in the Gangdese belt. Cautions must be taken when using compositions of detrital zircon and apatite grains to evaluate metallogenic potential during regional geochemical surveys.

Comparison of petrological and geochemical characteristics of three different types of Eocene copper-gold mineralization in eastern Iran

The Maherabad, Qaleh Zari, and Hired deposits in eastern Iran are porphyry (Cu-Au)-, Iron oxide copper–gold (IOCG)-, and reduced intrusion-related gold system (RIRGS)-type mineralization, respectively. They are located within the Lut block magmatic assemblage, which is the main metallogenic province in eastern Iran. The mineral assemblages and paragenesis of the deposits indicate that highest log fO2 and log fS2 contents belong to the Qaleh Zari deposit. In contrast, the lowest values of oxygen and sulphur fugacity are related to the RIRGS system in the Hired deposit. The Maherabad mineralization has moderate log fO2 and log fS2 contents relative to the Qaleh Zari and Hired deposits.Geochemically, these three deposits are related to different types of granitoids, which occurred within the short time interval from 37 to 41 Ma. The Maherabad and Qaleh Zari intrusions are related to oxidized magnetite-series granitoids, whereas the mineralization at Hired deposit is related to the reduced ilmenite-series granitoids. Magnetite-series intrusive rocks at Hired are barren. Intrusive bodies in the three studied sites are metaluminous, high-K calc-alkaline to shoshonitic, and formed in a volcanic arc setting. However, there are distinct differences in the radiogenic isotopic composition, content of some elements and elemental ratios reflecting the effect of different sources in the formation of intrusions. The reduced granite of the Hired deposit has the lowest Eu/Eu* of <0.7, P + Sr < 1000 ppm, and Sr/Y < 20 associated with (La /Yb)N < 12. Crustal contamination played an important role in magma evolution (87Sr/86Sri = 0.70627 and −εNd(t) = -2.60). P + Sr and Sr/Y contents in the productive oxidant magnetite-series intrusive rocks, such as the Maherabad and Qaleh Zari deposits, are more than 1500 ppm and 35, respectively. On this basis, petrological characteristic, geochemical indicator elements and mineral assemblage and paragenesis can be used for discrimination of productive and barren intrusions in the three different deposit types studied in this research. This can be used as an exploration criteria for the hydrothermal deposits associated with different magmatic intrusions in some areas.

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.

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.

Correlation of the Permian-Triassic Ore-Bearing Intrusions of the Norilsk Region with the Volcanic Sequence of the Siberian Traps Based on the Paleomagnetic Data

Abstract We present detailed paleomagnetic data from the Permian-Triassic Cu-Ni-Pt–bearing intrusions of the Norilsk region (northwestern Siberian platform). Based on the analysis of geomagnetic secular variations recorded in the intrusions, we correlate the Norilsk ore-bearing intrusions with the Siberian Traps volcanic sequences in the region. The similarity of paleomagnetic directions of the ore-bearing intrusions to those of the Morongovsky and Mokulaevsky volcanic formations suggests that the emplacement of these lavas and intrusions occurred during the same stage of the magmatic activity. Furthermore, some weakly mineralized intrusions and barren nondifferentiated sills demonstrate paleomagnetic directions close to those of the ore-bearing intrusions. This implies that ore-bearing, weakly mineralized, and barren intrusions were emplaced during the same stage of magmatic activity. On the other hand, the paleomagnetic directions of different ore-bearing intrusions (Norilsk 1, Talnakh, and Kharaelakh), while close, are statistically different. Thus, the formation of these three intrusions and their satellites corresponds to discrete magmatic events within the same stage of magmatic activity. Based on the paleomagnetic data, the emplacement duration of individual intrusions can be estimated as &amp;lt;10 k.y. Our correlation scheme is consistent with some genetic models for the ore-bearing intrusions. The analysis of geomagnetic secular variations can be used as an independent tool for the investigation of other Cu-Ni and platinum group element-bearing magmatic complexes.

Protracted Magmatism and Mineralized Hydrothermal Activity at the Gibraltar Porphyry Copper-Molybdenum Deposit, British Columbia

Abstract The Gibraltar Cu-Mo deposit, with a total tonnage of 3.2 million tons (Mt) Cu, is located in the Canadian Cordillera and hosted by the Late Triassic Granite Mountain batholith. The batholith formed through multiple intrusions of tonalitic rocks over a period of ~25 m.y. beginning at 229.2 ± 4.4 Ma in the Quesnel island arc before the accretion of the arc to the North American continent. Late in its evolution, Cu fertile magmas intruded in the center of the batholith, during at least three events from 218.9 ± 3.1 to 205.8 ± 2.1 Ma. The fertile magmas were hotter and more mafic than older barren magmas. They generated magmatic-hydrothermal activity, forming potassic alteration and white mica alteration, and produced Cu mineralization as chalcopyrite-quartz veinlets and disseminated chalcopyrite. Zircon in the Cu-bearing tonalite intrusion (218.9 ± 3.1 Ma) shows high Ce4+/Ce3+ (681 ± 286 [2σ], n =15) compared to those from older barren intrusions (129 ± 56 [2σ], n = 118). Oxidation conditions for parental magmas are calculated using the compositions of zircon and amphibole. The magmas for Cu-bearing intrusions have an average of fayalite-magnetite-quartz buffer (FMQ) +1.7 ± 0.7 (2σ, n = 73), whereas those for older barren intrusions have slightly lower fO2 (avg FMQ +1.3 ± 0.5 [2σ], n = 108), although the values are overlapping for the two. The bulk rocks of Cu-bearing tonalite intrusions in the Granite Mountain batholith have low Sr/Y ratios (&amp;lt;22) independent of the degrees of alteration. The low ratios are also reflected by low Sr/Y in zircon, suggesting that the low Sr/Y ratios of bulk rocks represent those of unaltered rocks. The values are low compared to those associated with many other porphyry Cu deposits globally. The data suggest that igneous rocks elsewhere with low Sr/Y in bulk rocks may have a potential to host economic Cu deposits. Ratios of Ce/Nd and Ce/Ce* (=Ce/((NdN)2SmN)) in zircon are positively correlated with the Ce4+/Ce3+ in zircon from the Granite Mountain batholith. Since the former two ratios can be obtained solely from zircon composition, these ratios from detrital zircon may be useful in evaluating the occurrences of oxidized intrusions in regional mineral exploration.

Using magmatic biotite chemistry to differentiate barren and mineralized Silurian–Devonian granitoids of New Brunswick, Canada

The geochemistry of biotite crystals from thirty fertile and barren Silurian–Devonian granitoids of New Brunswick, Canada, was studied in situ using electron microprobe and laser ablation inductively coupled plasma-mass spectrometry to investigate the suitability of biotite geochemistry as a diagnostic fertility index among these intrusions. The Fe2+/(Fe2+ + Mg2+) ratio of biotite varies as a function of intrusion metal affinity, increasing from Cu–Mo-related (mean of 0.56 ± 0.12), to Mo-related (mean of 0.69 ± 0.06) to Sn–W-related (mean of 0.77 ± 0.16), with barren granitoids lying between Cu–Mo and Mo types (mean of 0.66 ± 0.06). The results show a distinctive geochemical contrast between mineralized and barren samples. Compatible elements (Ti, Mg, Co, Ni, V, Cr, Ba, and Sr) decrease from barren to Cu–Mo, Mo, and Sn–W granitoids, whereas incompatible elements (Mn, Zn, Sn, W, Rb, Cs, and Li) show the opposite trend. These two trends might indicate higher degree of fractionation indicated by biotite chemistry in Sn–W-related granites. Furthermore, barren intrusions have the lowest water content (1–3 wt.% H2O), whereas Sn–W and Cu–Mo-related intrusions have between 3 and 6 wt.% H2O. Mo-bearing intrusions have a limited range of H2O contents (4–4.5 wt.%). A high degree of halogen enrichment related to degree of fractional crystallization results in enrichment of incompatible elements in the magmas associated with Sn–W mineralization and is reflected by the geochemical characteristics of biotite from these systems. New metallogenic classifications are introduced using ternary V–Na–Li (ppm) and Sn + W (ppm) versus Ga (ppm) to differentiate barren and mineralized granitic systems in New Brunswick.

In situ major and trace element compositions of apatite from the Yangla skarn Cu deposit, southwest China: Implications for petrogenesis and mineralization

The Yangla copper deposit is a rare Triassic intrusion-related Cu skarn system in the central part of the Jinshajiang tectonic belt, central Sanjiang orogenic belt, southwest China. Major- and trace-element compositions of apatites from ores, fertile calc-alkaline intrusions and its mafic microgranular enclaves, and barren calc-alkaline intrusions from the Yangla skarn copper deposit were measured by electron probe microanalysis and laser ablation–inductively coupled plasma–mass spectrometry. Results indicate that apatites grown within ores, fertile intrusions and its enclaves show similar features, but are different from that grown within barren intrusions. Apatite from fertile intrusions has higher Ca and correspondingly lower total trace-element contents, which partition onto the Ca sites, relative to that from barren intrusions. Apatites from the barren intrusions have high F and low Cl contents, and correspondingly high F/Cl ratios, which may have resulted from the assimilation of sediment. Apatites from fertile intrusions show relatively high Cl and low F contents, which may reflect crystallization from magmas with high water contents. Redox-sensitive elements in apatite can be used as indicators of the oxidation state of magmas. Apatites from the barren intrusion have lower V and As contents, higher Mn, Fe, and U contents, and more pronounced negative Eu anomalies than those from fertile intrusion, indicating that the barren magma was more reduced, consistent with its lower whole-rock Fe2O3/FeO and zircon Ce4+/Ce3+ ratios. These findings demonstrate that apatites are effective in distinguishing fertile from barren intrusions and can be used as a robust tool in mineral exploration.