porphyry-Cu Deposits Research Articles

Trace Element Distribution in Magnetite Separates of Varying Origin: Genetic and Exploration Significance

Magnetite is a widespread mineral, as disseminated or massive ore. Representative magnetite samples separated from various geotectonic settings and rock-types, such as calc-alkaline and ophiolitic rocks, porphyry-Cu deposit, skarn-type, ultramafic lavas, black coastal sands, and metamorphosed Fe–Ni-laterites deposits, were investigated using SEM/EDS and ICP-MS analysis. The aim of this study was to establish potential relationships between composition, physico/chemical conditions, magnetite origin, and exploration for ore deposits. Trace elements, hosted either in the magnetite structure or as inclusions and co-existing mineral, revealed differences between magnetite separates of magmatic and hydrothermal origin, and hydrothermal magnetite separates associated with calc-alkaline rocks and ophiolites. First data on magnetite separates from coastal sands of Kos Island indicate elevated rare earth elements (REEs), Ti, and V contents, linked probably back to an andesitic volcanic source, while magnetite separated from metamorphosed small Fe–Ni-laterites occurrences is REE-depleted compared to large laterite deposits. Although porphyry-Cu deposits have a common origin in a supra-subduction environment, platinum-group elements (PGEs) have not been found in many porphyry-Cu deposits. The trace element content and the presence of abundant magnetite separates provide valuable evidence for discrimination between porphyry-Cu–Au–Pd–Pt and those lacking precious metals. Thus, despite the potential re-distribution of trace elements, including REE and PGE in magnetite-bearing deposits, they may provide valuable evidence for their origin and exploration.

The behavior of Fe and S isotopes in porphyry copper systems: Constraints from the Tongshankou Cu-Mo deposit, Eastern China

Iron isotopes have been widely applied to trace mineralization processes of magmatic-hydrothermal ore systems, yet the application of Fe isotopes in porphyry Cu deposits (PCDs) is still restricted by the lack of understanding of its behaviors during hydrothermal alteration and ore formation processes. In this study, Fe isotope systematics in whole-rocks and Fe-bearing minerals coupled with S isotope compositions of Fe-sulfides are investigated for the early Cretaceous Tongshankou porphyry Cu-Mo deposit from Eastern China. The results show that altered ore-hosting granodiorite porphyries display a small δ56Fe variation (0.04‰ to 0.17‰) with an average of 0.10 ± 0.08‰ (2SD, n = 10), indistinguishable from the values for global igneous rocks. The δ56Fe of magnetite (0.22‰) and biotite (-0.08‰ to 0.12‰) from the altered porphyries are also within the δ56Fe ranges of those from global igneous rocks. These results indicate that hydrothermal fluids have negligible effect on Fe isotope compositions of the porphyry host rocks, likely due to the insignificant contributions of fluid-derived iron relative to the total Fe budget of the host rocks.The Fe-sulfides in the porphyry system record geochemical and iron isotopic characteristics of the ore-forming fluids. Specifically, chalcopyrites show large Fe isotope variations with δ56Fe ranging from −0.60‰ to 0.61‰. The calculated Fe isotope compositions of ore-forming fluids based on the chalcopyrite δ56Fe are thus not uniform and fall in the range from −0.69 ± 0.18‰ to 0.52 ± 0.18‰ (2SD). On the other hand, pyrites exhibit a δ56Fe variation between −0.48‰ and 0.40‰ and show distinct Fe isotopic characteristics for disseminated and vein-type pyrite. The δ56Fe for disseminated pyrites are consistently higher than whole-rock values, ranging from 0.14‰ to 0.40‰, likely reflecting equilibrium Fe isotopic exchange between pyrite and dissolved Fe in fluid. By contrast, δ56Fe for pyrites in veins are highly variable (−0.48‰ to 0.34‰) and show an inverse correlation with the chalcopyrite δ56Fe. These observations, in concert with S isotope data, have been interpreted to reflect pyrite formation through the FeS pathway, whereas the wide range in δ56Fe for pyrite may record isotopic change from initial FeS-fluid equilibrium towards pyrite-fluid equilibrium due to different extent of reaction upon precipitation. Additionally, when compared to sulfides formed by non-magmatic, low-temperature processes, the sulfides in our study exhibit a relatively narrow range of δ56Fe and δ34S, implying a high-temperature magmatic origin for both elements. Overall, our study demonstrates that the coupling of Fe and S isotopes could be a useful tool for tracking the processes of sulfide mineralization in porphyry-related deposits.

Rhenium abundance in molybdenites: a case study on vein-type Cu-Mo-Au mineralisation in the Qarachilar area, Sungun porphyry Cu and Siah Kamar porphyry Mo deposits, NW Iran

The vein-type Cu-Mo-Au mineralisation in Qarachilar, the Sungun porphyry Cu deposit (PCD) and the Siah Kamar porphyry Mo deposit (PMD) are all located at the northwestern end of the Neo Tethys-related Urumieh–Dokhtar volcano-plutonic belt of Iran. Re contents of molybdenite samples from the Qarachilar, Sungun and Siah Kamar deposits are about 112.67–462 ppm, 53.24–252.29 ppm, and 10.44–41.05 ppm, respectively. Re contents of the first two deposits fall in the range of PCDs, while those of the latter are lower and correspond to PMDs. The relatively high Re content of the Qarachilar and Sungun molybdenites can be explained considering the low abundance of molybdenite, a mantle-dominated source for ore materials and the incorporation of oxidized and acidic hydrothermal fluids with high f Cl. The high abundance of molybdenite in the Siah Kamar PMD has resulted in volume dilution of Re. Furthermore, occurrence of the main ore within the potassic alteration zone and, hence, the alkaline nature of the responsible fluids in this zone have also affected the Re content of molybdenites. Variations of the Re content in different veins/veinlets showed a negative relationship with the formation temperature of these veins, and a positive/negative relationship with the acidity/alkalinity of the hydrothermal fluids, while the grain size of molybdenites showed a positive relationship.

Interpreting element addition and depletion at the Ann Mason porphyry-Cu deposit, Nevada, using mapped mass balance patterns

Elemental mass changes were calculated for 154 samples from four rock types around the Ann Mason porphyry-Cu-(MoAu) deposit, Yerington district, Nevada. The purpose of these calculations was to investigate addition and depletion patterns in two-dimensional (2-D) map space. Mass changes in Ca, Mg, Na, P, Cu, Mo, As, Sb, Sr, and Pb from samples in the pre- to syn-mineralisation Yerington batholith vary systematically with lateral and vertical distance from the Ann Mason deposit. At the core of the Ann Mason deposit, Cu and Mo were strongly added (>10,000% relative to average protolith values), with restricted lateral mass addition halos around the deposit. Copper is depleted in wall rocks surrounding the deposit. Calcium and Sr were removed (up to 98% depletion) from parts of the deposit centre due to the destruction of calcic-plagioclase and other calcium-bearing minerals during potassic alteration. Calcium (up to 1570%) and Sr (up to 315%) were added outside the deposit and were concentrated around 4 km from the deposit centre where epidote alteration is most intense. Arsenic and Sb were removed from the deposit centre (up to 99% depletion) where silicate-mineral alteration assemblages dominate. Antimony shows an addition halo (ranging from 25% up to 1900%) that extends from the deposit centre 500 m laterally away, and correlates spatially with the increased abundance of pyrite that typically surrounds porphyry deposits. Epidote-altered samples of Shamrock monzonite from outside the Casting Copper skarn show addition of Ca (up to 196%), Sb (up to 1100%), and depletion of Na (up to −98%). Samples from the central part of the post-mineralisation Shamrock monzonite record lower mass changes in Mg, Ca, P, and Mo, compared to other parts of the batholith. Some samples from the northern contact of the Shamrock batholith exhibit addition of Ca (up to 554%), Sr (up to 504%), and depletion in Na (up to −98%). These samples are coincident with strong epidote alteration.Our results provide a rock-normalised geochemical map that reflects quantified element addition and depletion patterns, which is useful for interrogating geological processes at a regional scale. Although interpreting relative element addition and depletion in map space is possible using conventional immobile element ratio approaches, mass balance calculations provide the benefit of quantified results.

Application of support vector machine for the separation of mineralised zones in the Takht-e-Gonbad porphyry deposit, SE Iran

Classification of mineralised zones is an important factor for the analysis of economic deposits. In this paper, the support vector machine (SVM), a supervised learning algorithm, based on subsurface data is proposed for classification of mineralised zones in the Takht-e-Gonbad porphyry Cu-deposit (SE Iran). The effects of the input features are evaluated via calculating the accuracy rates on the SVM performance. Ultimately, the SVM model, is developed based on input features namely lithology, alteration, mineralisation, the level and, radial basis function (RBF) as a kernel function. Moreover, the optimal amount of parameters λ and C, using n-fold cross-validation method, are calculated at level 0.001 and 0.01 respectively. The accuracy of this model is 0.931 for classification of mineralised zones in the Takht-e-Gonbad porphyry deposit. The results of the study confirm the efficiency of SVM method for classification the mineralised zones.

Discrimination of Sericite Phyllic and Quartz-Rich Phyllic Alterations by Using a Combination of ASTER TIR and SWIR Data to Explore Porphyry Cu Deposits Hosted by Granitoids, Kerman Copper Belt, Iran

Various mapping methods such as Spectral Angle Mapper, band ratio, and Principal Component Analysis were applied to the Advanced Spaceborne Thermal Emission and Reflection Radiometer data to highlight various hydrothermal alteration minerals (e.g., sericite, kaolinite, epidote and quartz) related to Cu mineralization in the Southern Jabal–Barez Range (SJBR), Kerman province, Iran. The integration of sericite and quartz images with Boolean logic in a geographic information system environment discriminated quartz-rich phyllic hydrothermal alteration from sericitic or sericite phyllic alteration. The mapped hydrothermally altered areas were sampled randomly and studied in the laboratory (microscopic studies, spectral and X-ray diffraction analyses) for data validation. Accuracy assessment of the resulting hydrothermal alteration maps was conducted by using a confusion matrix. The overall accuracy was estimated to be 79.69%. The hydrothermal alteration zones enhanced through this research (i.e., sericite phyllic, argillic, and quartz-rich phyllic) are in good agreement with the known locations of porphyry Cu deposits (PCDs) in the SJBR. Propylitic alteration, as reported and documented from many PCDs, is absent or only poorly developed in the SJBR. The quartz-rich phyllic alteration occurs around the more common sericitic alteration. These findings indicate that the integration of sericite and quartz images is useful for mapping quartz rich-phyllic hydrothermal alteration around PCDs hosted by granitic rocks.

Application of concentration gradient coefficients in mining geochemistry: A comparison of copper mineralization in Iran and Canada

In this work, the concentration gradient (CG) analysis of local-scale exploration for Porphyry-Cu deposits is applied in two zones using the G(Vz) index (CG(Zn*Pb)/CG(Cu*Mo)). The first zone is covered by a 1:2000 map of the Sungun and Astamal areas in NW Iran and the second one in the Inza area in British Columbia, Canada. The rock samples are taken from Sungun and Astamal and the soil samples are taken from Inza. The Inza samples are analyzed for Cu, Pb, Zn, and Mo elements by the atomic absorption method, while the rock samples of Astamal and Sungun are analyzed for Cu, Pb, Zn, Mo, Ag, As, and Sb elements. The indices of gradient geochemical zonality (G(Vz)) of multi-elements around the mineral deposits and their spatial associations with particular geological, geochemical, and structural factors are the critical aspects that must be considered in mineral exploration. The values for the G(Vz) indices allow a distinction between the sub-ore and supra-ore anomalies, which are associated with Zone Dispersed Mineralization (ZDM) and Blind Mineralization (BM), respectively. For a comparative identification of BM and ZDM, a supra-ore (Pb*Zn) anomaly, a sub-ore (Cu*Mo) anomaly, and Vz maps are used in place of the mining geochemistry representing the supra-ore gradient anomaly, sub-ore gradient anomaly and G(Vz) map. The G(Vz) model outperforms the Vz model. The introduced technique allows for a computational distinction between the BM and ZDM ore mineralizations without exploration drilling. Prior to writing this paper, the blind porphyry-Cu mineralization was intersected at depth through borehole exploration in a highly prospective zone delineated by the G(Vz) model. The results obtained confirm the usefulness of the G(Vz) modeling for local-scale targeting of blind mineral deposits.

Vertical geochemical zonation in the Masjed Daghi porphyry copper-gold deposit, northwestern Iran: implications for exploration of blind mineral deposits

The geochemical zonation method allows distinction between sub-ore and supra-ore primary halos, prediction of erosion level of mineralization, and exploration of blind mineral deposits. In this study, the geochemical zonation of the Masjed Daghi porphyry-Cu deposit, located in the NW of Iran, was investigated to estimate the level of erosion of this deposit based on the geochemical data obtained from 46 drill cores. We used vertical zonation indices (Pb.Zn/Cu.Mo) and (Pb.Zn/Cu.Ag) as geochemical indicators in exploration of blind porphyry-Cu deposits. The values of Vz 1 varied in different parts of the study area. Its values are considerably higher in the eastern and southeastern parts of the area (7-100), and lower in the northwestern parts of the area (0.03–1), indicating the least amount of erosion in the east and SE in comparison with the northwestern parts of study area. Vertical variations of the Pb.Zn/Cu.Mo index show a significant decrease with increased depth; for example, the level decreases to 0.001 at a depth of 580 metres in the NW of the area, indicating a sub-ore zone of halo. With respect to the present level of erosion, high values of Vz indices imply the presence of sub-cropping to blind porphyry-Cu deposits in this area.

Discrimination between mineralized and unmineralized alteration zones using primary geochemical haloes in the Darreh-Zar porphyry copper deposit in Kerman, southeastern Iran

Primary geochemical haloes were studied at the Darreh-Zar porphyry Cu-deposit, southern Iran. In terms of geochemical signatures, high K2O/Na2O enrichment, HREEs and HFSE's depletion in the potassic alteration, high (La/Sm)cn, (La/Yb)cn and (Gd/Yb)cn ratios in mineralized sericitic and potassic zones and notable depletion in the REEs content in argillic alteration is recognized. Further, Mg, Li, Sc, P enrichment and W depletion can serve to separate potassic alteration from the other altered zones, while (Eu/Eu*)cn and (Ce/Ce*)cn don't show pronounced changes in different alteration zones. The coupled positive Tl, Se, S, Rb, Co, Cs, Mo, K and negative Te, Ta, Ti, Sr, Rb, As, Bi, Ga, Hf, In, Mn, Zn and Zr anomalies can be adequately used in discriminating between the mineralized zones (potassic, chlorite-sericite and sericite alterations) and the barren (propylitic zone). The behavior of the trace elements on isocon diagrams reveal that HFSEs are depleted in mineralized altered zones and display variations in the amounts in the barren facies.Zonality index in the axial direction from drill holes 146 to 124 estimates the zonality sequence as Pb-Zn-Ag-Cu-Pb-Zn in the surface horizons. The calculated zonality in five drill holes and six levels indicates that the level of 550 m at the DH 117 in the central part of the area has the highest value (0.76) for Cu. The zonality sequence from the surface to the depth is variable and can be demonstrated as follow: DH 146: Pb-Zn-Cu-Mo-Ag; DH 137: Zn-Cu-Mo-Pb-Ag; DH 117: Ag-Zn-Pb-Mo-Cu; DH: 121: Cu-Mo-Zn-Ag-Pb; DH 136: Pb-Ag-Zn-Cu-Mo; DH 124: Zn-Mo-Cu-Pb-Ag. Available data of the enrichment factors shows different enrichment for copper and molybdenum (i.e. EF > 10), selenium and silver (i.e. EF > 5), tin and LREEs (i.e. 1 < EF < 5).

Recognition of an enhanced multi-element geochemical signature of porphyry copper deposits for vectoring into mineralized zones and delimiting exploration targets in Jiroft area, SE Iran

Delimiting exploration targets using geochemical exploration data can be a challenging issue when different geochemical signatures represent the same deposit-type sought. In this regard, fuzzy operators have been used to integrate different geochemical evidence layers into a single model for generating target areas. In this paper, a GIS-based expected value function was adapted to integrate different geochemical evidence layers into a stronger geochemical signature for delimiting exploration targets. Then, the expected value function and fuzzy operators were compared. The comparison demonstrated that the former is more efficient than the later for generating a stronger geochemical evidence layer. The higher efficiency of the expected value function is because it simultaneously uses the value of all input variables and their relative importance in the process of integration. The proposed approach was evaluated by using a lithogeochemical data set for prospecting porphyry-Cu deposits in Jiroft area, Kerman province, southeast of Iran, as a case study.

Late Triassic E-MORB-like basalts associated with porphyry Cu-deposits in the southern Yidun continental arc, eastern Tibet: Evidence of slab-tear during subduction?

It is generally believed that andesite–dacite–rhyolite suites and contemporary porphyry Cu deposits are related to subduction in active continental margin settings. However, it is still unclear which tectonic events result in the generation of porphyry Cu deposits and whether asthenospheric mantle material is involved in this process. Widespread andesitic–dacitic felsic intrusions associated with porphyry Cu deposits and rarer basalts have been identified in the Late Triassic southern Yidun arc (SYA) of eastern Tibet. However, few geochronological and geochemical data are available for these basalts, thereby hampering the development of geodynamic models for this magmatic event and the formation of related porphyry Cu deposits in the region. Here we present the first geochemical and SIMS (secondary ion mass spectrometry) zircon U–Pb data of Xiaxiaoliu basalts in the SYA. The age of the Late Triassic Xiaxiaoliu basalts (216.1±2.8Ma) is consistent with the timing of emplacement of voluminous porphyritic intrusions and the formation of Cu deposits within the SYA (peaking at 215–217Ma). The Xiaxiaoliu basalts have E-MORB-like trace element patterns that are free of negative Nb–Ta anomalies, and have high 143Nd/144Nd(t) values, suggesting they were sourced from asthenospheric mantle without any arc-type influence. These observations, combined with the fact that some Late Triassic mineralized porphyritic intrusions within the SYA have adakitic affinities, suggest that the basalts and other igneous rocks and associated porphyry Cu deposits within the SYA were produced by tearing of a westward-dipping slab, triggering the upwelling of asthenospheric mantle material during subduction of the Garze–Litang Ocean crust.

Multifractal interpolation and spectrum–area fractal modeling of stream sediment geochemical data: Implications for mapping exploration targets

The spectrum–area (S–A) fractal model is a powerful tool for decomposition of complex anomaly patterns of gridded geochemical data. Ordinary moving average interpolation techniques are commonly being used for gridding geochemical data; however, these methods suffer from two major drawbacks of (1) ignoring the locally high values and (2) smoothing the interpolated surface. Multifractal moving average interpolation methods have been developed to overcome the shortcomings of ordinary moving average methods. This study seeks to compare two sets of multifractal and ordinary gridded geochemical data using success rate curves and applies the S–A fractal model to decompose anomalous geochemical patterns. A set of stream sediment geochemical data in Ahar area, NW Iran, was used as a case study. Then, a mineralization-related multi-element geochemical signature was gridded by ordinary and multifractal approaches and considered for further analyses. The S–A fractal method was applied to decompose anomaly and background components of the resultant multi-element geochemical signature. Exploration targets were delimited and further evaluated using two bivariate statistical procedures of Student's t-value and normalized density index. The results revealed that (a) application of multifractal gridded data enhances the predicting ability of geochemical signatures, (b) application of S–A fractal model on multifractal gridded data allows for superior discrimination of geochemical anomalies, and (c) the multi-element geochemical anomalies in the Ahar area related to porphyry-Cu deposits were properly delineated through sequence application of multifractal interpolation and S–A fractal model.

Porphyry deposits of the Urals: Geological framework and metallogeny

Most of the Cu (±Mo,Au) porphyry and porphyry-related deposits of the Urals are located in the Tagil-Magnitogorsk, East-Uralian Volcanic and Trans-Uralian volcanic arc megaterranes. They are related to subduction zones of different ages:(1)Silurian westward subduction: Cu-porphyry deposits of the Birgilda-Tomino ore cluster (Birgilda, Tomino, and Kalinovskoe) and the Zeleny Dol Cu-porphyry deposit;(2)Devonian Magnitogorsk eastward subduction and the subsequent collision with the East European plate: deposits and occurrences are located in the Tagil (skarn-porphyry Gumeshevskoe etc.) and Magnitogorsk terranes (Cu-porphyry Salavat and Voznesenskoe, Mo-porphyry Verkhne-Uralskoe, Au-porphyry Yubileinoe etc.), and probably in the Alapaevsk-Techa terrane (occurrences of the Alapayevsk-Sukhoy Log cluster);(3)Late-Devonian to Carboniferous subduction: deposits located in the Trans-Uralian megaterrane. This includes Late-Devonian to Early Carboniferous Mikheevskoe Cu-porphyry and Tarutino Cu skarn-porphyry, Carboniferous deposits of the Alexandrov volcanic arc terrane (Bataly, Varvarinskoe) and Early Carboniferous deposits formed dew to eastward subduction under the Kazakh continent (Benkala, etc.).(4)Continent-continent collision in Late Carboniferous produced the Talitsa Mo-porphyry deposit located in the East Uralian megaterrane.Porphyry mineralization of the Magnitogorsk megaterrane shows an evolving relationship from gabbro-diorite and quartz diorite in the Middle Devonian (Gumeshevskoe, Salavat, Voznesenskoe) to granodiorite-plagiogranodiorite in the Late Devonian (Yubileinoe Au-porphyry) and finally to granodiorite in the Carboniferous (Talitsa Mo-porphyry) with a progressive increase in total REE, Rb and Sr contents. This corresponds to the evolution of the Magnitogorsk terrane from a volcanic arc which gave place to an arc-continent collision in the Famennian.

Application of fuzzy analytical hierarchy process (AHP) and prediction-area (P-A) plot for mineral prospectivity mapping: a case study from the Dananhu metallogenic belt, Xinjiang, NW China

This paper describes a GIS-based application of a fuzzy analytical hierarchy process (AHP) to map porphyry-Cu prospectivity in the Dananhu metallogenic belt, NW China. Based on a model of porphyry-Cu mineralization, evidential layers were derived from geological, geochemical, and geophysical data. These layers were subsequently assigned weights by implementing a fuzzy AHP method using knowledge from three experts in porphyry-Cu exploration. After obtaining normalized weights, different fuzzy operators were tried to combine the weighted evidential layers into potential maps, which were then compared and evaluated by predication-area (P-A) plots. Subsequently, a ternary map was generated by defuzzification of the optimum prospectivity map as selected by the P-A plot; this ternary map shows zones of high, moderate, and low favorability for porphyry-Cu deposits in the study area. To further evaluate the results, potential zones were analyzed for two-dimensional spatial domain. The results demonstrate that the fuzzy AHP method can be effectively applied to mineral prospectivity mapping in vaguely known areas.

Recognition of significant multi-element geochemical signatures of porphyry Cu deposits in Noghdouz area, NW Iran

Diversity and complexity of geological processes in different areas result in different element associations for prospecting a certain mineral deposit-type sought. In this regard, because element associations are affected by the diversity of geological characteristics of different areas, it is important to analyze and recognize significant geochemical signatures that represent the deposit-type sought. This paper aims to recognize significant multi-element geochemical signatures of porphyry-Cu deposits in the Noghdouz area, Iran, using stream sediment data. For this, we used factor analysis and two modeling methods of geochemical anomalies, sample catchment basin and contour map. Then, to recognize significant geochemical signatures of the deposit-type sought and evaluate the anomaly mapping methods, we adapted prediction-area (P-A) plot, normalized density, and success rate curve. By using these processes, we recognized the best geochemical signature of the deposit-type sought in the study area. The proposed methods in this paper can efficiently be used in other areas to recognize significant geochemical signatures of different types of mineral deposits.

Oceanic anoxic events, subduction style and molybdenum mineralization

Nearly half of the world's Mo resources are hosted in Cenozoic porphyries along the east Pacific margin. In contrast, there are essentially no large Mo porphyry deposits along the west Pacific. We propose that this uneven distribution is mainly due to Oceanic Anoxic Events (OAEs) and the different subduction regimes in the eastern and western Pacific margins. Molybdenum is rare in the Earth, with an abundance of 50 ppb in the primitive mantle, and 0.8 ppm in the continental crust. Pre-enrichment is needed for such a moderately incompatible element to form porphyry Mo deposits. The oxidation–reduction cycle on the Earth's surface is the major process that concentrates Mo. Molybdenum was more intensively oxidized under high erosion rates during alternating high-atmospheric-CO2 and high-oxygen periods immediately before OAEs, resulting in high Mo concentrations in surface water and subsequently higher Mo concentrations in organic-rich sediments during the OAEs. Large amounts of Mo-enriched sediments formed on the Pacific Ocean floor during at least 9 major OAEs since the Late Jurassic. Given that Mo comes from chemical weathering, far more Mo-enriched sediment was available in the east Pacific because of the erosion of the continental arc. In contrast, due to backarc extension, erosions in the Western Pacific margin was less extensive, resulting in less Mo in OAE sediments on the Pacific Ocean floor. Importantly, the Eastern Pacific plate has been mostly subducted, with only Cenozoic crust left. During plate subduction, large amount of the Mo-rich sediments in the Eastern Pacific have been taken down to the mantle wedge. Subsequently these sediments were metamorphosed and then Mo was transferred to porphyry deposits through partial melting. In contrast, only a small amount of the sediment in the Western Pacific has been subducted, with most of the Jurassic and Cretaceous oceanic crust well preserved.Molybdenum deposits of the Eastern Pacific margin can be classified into three types: porphyry-Cu-Mo, high-F porphyry and low-F porphyry-Mo deposits. Large and superlarge porphyry-Cu-Mo deposits are usually associated with ridge subductions, and thus formed through partial melting of subducted oceanic crust and metamorphosed Mo-rich sedimentary material due to subduction erosion. The high-F porphyry-Mo deposits (e.g., Climax-type) were likely formed by partial melting of subducted and thus metamorphosed Mo-enriched sediments during slab rollback (usually associated with A-type granite). Low-F porphyry-Mo deposits were formed through direct partial melting of metamorphosed Mo-enriched sediments entrained into the mantle wedge through subduction erosion at slightly shallower depths (usually associated with arc granite). Porphyry-Cu (Au) deposits in the southwest Pacific margin are all associated with subduction of backarc basins younger than the last OAE (55.8 Ma). Therefore, porphyry Cu (Au) deposits in the southwest Pacific margin have no economic levels of Mo. The oxygen fugacity of northwest Pacific is lower than ΔFMQ +2, which inhibits the formation of porphyry deposits.

Prospectivity modeling of porphyry-Cu deposits by identification and integration of efficient mono-elemental geochemical signatures

Dispersion pattern of geochemical elements in stream sediment data of a study area is affected by several factors, e.g., geological and geomorphological characteristics of the area. In this paper, we demonstrated recognition of efficient and inefficient mono-elemental geochemical signatures in a study area and exclusion of inefficient elements is worthwhile for increasing the probability of exploration success. For this, we adapted prediction-area (P-A) plot, normalized density and success-rate curve as tools that evaluate the ability of geochemical signatures in prediction of undiscovered mineral deposits and in delimiting exploration targets. After identification of efficient and inefficient elements, we combined efficient indicator elements to generate an effective prospectivity model. To illustrate the procedure we used a stream sediment data set for prospecting porphyry-Cu deposits in the Noghdouz area, Iran.

3D Inversion of Magnetic Data through Wavelet based Regularization Method

This study deals with the 3D recovering of magnetic susceptibility model by incorporating the sparsity-based constraints in the inversion algorithm. For this purpose, the area under prospect was divided into a large number of rectangular prisms in a mesh with unknown susceptibilities. Tikhonov cost functions with two sparsity functions were used to recover the smooth parts as well as the sharp boundaries of model parameters. A pre-selected basis namely wavelet can recover the region of smooth behaviour of susceptibility distribution while Haar or finite-difference (FD) domains yield a solution with rough boundaries. Therefore, a regularizer function which can benefit from the advantages of both wavelets and Haar/FD operators in representation of the 3D magnetic susceptibility distributionwas chosen as a candidate for modeling magnetic anomalies. The optimum wavelet and parameter β which controls the weight of the two sparsifying operators were also considered. The algorithm assumed that there was no remanent magnetization and observed that magnetometry data represent only induced magnetization effect. The proposed approach is applied to a noise-corrupted synthetic data in order to demonstrate its suitability for 3D inversion of magnetic data. On obtaining satisfactory results, a case study pertaining to the ground based measurement of magnetic anomaly over a porphyry-Cu deposit located in Kerman providence of Iran. Now Chun deposit was presented to be 3D inverted. The low susceptibility in the constructed model coincides with the known location of copper ore mineralization.

Fault control on Cu mineralization in the Kerman porphyry copper belt, SE Iran: A fractal analysis

The relationship between faults and location of the Porphyry Cu Deposits (PCDs) has been evaluated using the fractal analysis method. In Kerman Porphyry Copper Belt (KPCB), SE Iran, the PCDs are hosted in Miocene porphyry stocks and surrounding volcanic rocks. In KPCB, the trends of the main drainages transecting the porphyry copper deposits, generally allow the distinction between high grade (>0.2% Cu) from low grade (<0.2% Cu) deposits. Comparing the fractal dimension between faults and drainages revealed that the major drainage trend has a high correlation (>82%) with the main fault trends. This result shows that the distribution of the PCDs is controlled by faults. A more accurate assessment of the fractal dimension between spatial distribution of the PCD and fault intersection points (as a point features) shows a variation in coincidence (from 0 to >70%) between these features. Based on this coincidence, KPCB can be subdivided to three subzones. The high correlation subzone shows the maximum values of coincidence between faults and spatial distribution of the PCDs that occurred in Dehaj (~72%), Sarduiyeh, Khaneh Khatun, Pariz and Bardsir (~71%) areas. In these areas, the spatial distribution of the major PCDs in KPCB confirms this coincidence. The medium correlation subzone shows a variation in the coincidence (40–60%) and in the low correlation subzone, coincidence is less than 40%. Our findings indicated that the intermediate and minimum values of coincidence are related to regions with poor/lack of mineralization. This study shows that the high-medium coincidence subzones can be considered as targets in regional exploration programs. Detailed structural analysis in target zones can be a useful tool to indicate the location of the potential Cu mineralization.

A genetic linkage between subduction- and collision-related porphyry Cu deposits in continental collision zones

The genesis of continental collision-related porphyry Cu deposits (PCDs) remains controversial. The most common hypothesis links their genesis with magmas derived from subduction-modified arc lithosphere. However, it is unclear whether a genetic linkage exists between collision- and subduction-related PCDs. Here, we studied Jurassic subduction-related Cu-Au and Miocene collision-related Cu-Mo porphyry deposits in south Tibet. The Jurassic PCDs occur only in the western segment of the Jurassic arc, which has depleted mantle-like isotopic compositions [e.g., ( 87 Sr/ 86 Sr) i = 0.7041–0.7048; e Nd(t) as high as 7.5, and e Hf(t) as high as 18]. By contrast, no Jurassic PCDs have been found in the eastern arc segment, which is isotopically less juvenile [e.g., ( 87 Sr/ 86 Sr) i = 0.7041–0.7063, e Nd(t) < 4.5, and e Hf(t) ≤ 12]. These results imply that incorporation of crustal components during underplating of Jurassic magma induced copper accumulation as sulfides at the base of the eastern Jurassic arc, inhibiting PCD formation at this time. Miocene PCDs are spatially confined to the Jurassic arc, and the giant Miocene PCDs cluster in its eastern segment where no Jurassic PCDs occur. This suggests that the arc segment barren for subduction-related PCDs could be fertile for collision-related PCDs. Miocene ore-forming porphyries have young Hf model ages and Sr-Nd-Hf isotopic compositions overlapping with those of the Jurassic rocks in the eastern segment, whereas contemporaneous barren porphyries outside the Jurassic arc have abundant zircon inheritance and crustlike Sr-Nd-Hf isotopic compositions. These data suggest that remelting of the lower crustal sulfide-bearing Cu-rich Jurassic cumulates, triggered by Cenozoic crustal thickening and/or subsequent slab break-off, led to the formation of giant Miocene PCDs. The spatial overlap and complementary metal endowment between subduction- and collision-related magmas may be used to evaluate the mineral potential for such deposits in other orogenic belts.