Au Porphyry Deposits Research Articles

Relationships between high-K magmas and Cu–Au porphyry deposits in the Macquarie Arc, Australia

The Macquarie Arc is a long-lived volcanic arc terrane which began in the latest Cambrian and continued until the early Silurian. Early Silurian high-K suites associated with Cu–Au porphyry mineralization are typically considered shoshonitic; however, comparisons with well-studied examples of shoshonites in continental arcs and associated settings suggest few Macquarie Arc rocks would classify as shoshonitic. Notably, the Au-rich Cadia Intrusive Complex, which is marginally shoshonitic, and most high-K to ‘shoshonitic’ Macquarie Arc rocks are essentially high-K calc-alkaline, analogous to ‘primitive’ shoshonites generated within intraoceanic arc settings. Arc magmatism in the Macquarie Arc is consistently primitive and mantle derived prior to an increase in siliciclastic input and crustal thickening during the Silurian Benambran Orogeny, consistent with prior isotopic studies. Magma source regions can thus be interpreted as varying spatially and temporally due to variable enrichment from slab and mantle derived components, reflecting a maturing and potentially migrating arc system.

The deleterious role of gangue mineralogy in copper extraction: A case study of poor recovery in leaching low-grade Cu ores

In a series of four extended industrial column leach experiments spanning 542 days, aimed at examining Cu-recovery from low-grade sulfide ores, all instances yielded suboptimal Cu recovery rates (<27%). Through a nuanced analysis integrating time-resolved chemical data on leachate with detailed mineralogical investigations, we highlight that the dissolution of aluminosilicate gangue minerals is a pivotal factor constraining Cu recovery. The source materials, extracted from a low-grade segment of a Cu–Au porphyry deposit (∼0.32–0.36 wt% Cu), underwent diverse leaching conditions employing various lixiviants, curing phases, redox conditions, and particle size distributions. Notably, poor Cu recovery is closely tied to gangue mineralogy. The ammonia leach column experienced limited Cu leaching due to an inert gangue shielding chalcopyrite from the lixiviant. In acid columns, swift precipitation of silica-rich phases impeded effective Cu leaching at multiple scales, from large-scale-column to molecular levels. Identifying gangue mineralogy and considering their impact during acid leaching are crucial for formulating effective lixiviant strategies for Cu recovery from low-grade primary sulfide ores. The curing phase, requiring high acid concentrations (∼50 g/L H2SO4), emerges as a critical aspect for promoting effective gangue dissolution while preventing refractory silica-rich phase precipitation.

A cell-based association data integration method for locating potential drilling targets: A case study at the Dalli Cu–Au porphyry deposit in Iran

The presence of various spatial exploration proxies (predictor patterns) as vectors to distal porphyry mineralization in surrounding areas of a known outcropping mineralized porphyry center at the Dalli Cu–Au deposit in Iran provided evidence for the occurrence of similar porphyry mineralization in the subsurface. Factor analysis was applied to soil geochemical data from the Dalli South Hill area to define favorable geochemical variables and to create surface multi-element anomaly maps reflecting the presence of porphyry Cu–Au mineralization in the subsurface. Reduction-to-pole and analytical signal filters were applied to ground total magnetic field data to define high magnetic signatures that could be associated with potassic alteration and porphyry mineralization in the subsurface. Then, a data-driven cell based association (CBA) data integration method was applied to the favorable spatial proxies to define their association with Cu–Au mineralization and locate subtle high potential targets (cells) for drilling. The conventional CBA algorithms did not consider weights for the spatial proxies, and by using the ascendant hierarchical clustering (AHC) algorithm the results were classified into a binary map to show favorable and unfavorable cells with scores of 1 and 0, respectively. Predicting high potential targets for high-priced drilling, just based on using the binary map, could be very risky. Therefore, an innovative CBA data integration approach was employed to assign proper weights between 0 and 1 to the spatial proxies by calculating the importance of the presence of an exploration proxy in a cell using training (labeled) trenching geochemical data. A final continuous predictor map was generated and it successfully mapped the known outcropping porphyry center and its subsurface extension to the northeast as a high prospectivity class. This map also identified a narrow NE-trending high potential target (a few cells) of 200 m long in the southern part of the known porphyry center for drilling.

Formation of Cu–Au porphyry deposits: hydraulic quartz veins, magmatic processes and constraints from chlorine

Copper–gold porphyry deposits are the world’s main source of copper and a significant source of gold. They consist of vein networks and their surrounding alteration zones. Commonly the deposits are centred on narrow intrusions (stocks), but calling these deposits ‘porphyries’ is unjustified because the name carries little descriptive or genetic value. Extensional veins were formed by hydraulic fracturing of the stocks, at depths where open spaces could not be maintained and where fluid pressure approaches lithostatic pressure. The post-crystallisation timing of the veins is important because it indicates that the host stocks could not have been the direct sources of either metals or ore-forming fluids. In the traditional magmatic model, precursor batholiths, lying at depth, are inferred to be the sources of the Cu and Au in the overlying host stocks. In this model, the batholiths are assumed to have crystallised and produced the mineralising aqueous fluids, Cu and Au. However, in many porphyry deposits, the concept of metal and fluid supply from deeper batholiths is problematic. Neither Cu nor Au is strongly enriched during the crystallisation of silicate magmas, and although hypersaline fluids are a characteristic of Cu–Au porphyry deposits globally, the source of the Cl remains unconstrained. There is little evidence that silicate magmas can release such Cl-rich fluids, and it remains unexplained how elevated levels of Cl may be achieved in a silicate magma. Therefore, the starting assumption that these deposits formed predominantly from magmatic sources and processes is questioned. This study has selectively focused on the roles of rheology, rock mechanics, vein control, metal-enrichment processes and the sources of Cl. Non-magmatic processes may be enough to facilitate strong partitioning of Cu and Au into high-temperature, oxidising, high-salinity, hydrothermal fluids to form Cu–Au porphyry deposits.

Platinum-Group Element Geochemistry and Magma Evolution of the Mount Hagen (Papua New Guinea) Magmatic System

Abstract Mount Hagen is a mid-Pleistocene stratovolcano located in the Papua New Guinea Highlands. It was selected for this study because of (1) its close location to several giant Cu–Au porphyry deposits in the region; (2) its high-K calc-alkaline character (absarokite–shoshonite) and (3) its wide compositional range (2 to 12 wt % MgO). Whole-rock geochemistry, petrography and QEMSCAN observations are consistent with fractional crystallisation of the evolving magma being controlled by the phases now represented by the megacrysts in the lavas: olivine and clinopyroxene at high MgO and hornblende and plagioclase at low MgO. However, the role of plagioclase is enigmatic. Although it is an abundant megacryst phase in the low-MgO samples, plagioclase has little or no influence on fractional crystallisation, suggesting that it may be an antecryst. The appearance of plagioclase megacrysts does not slow the rate of increase in Al2O3 or Sr with decreasing sample MgO and the low-MgO samples do not have Eu anomalies. At ~5.5 wt % MgO, there are significant changes in the system that are inconsistent with simple fractional crystallisation. These changes include hornblende and plagioclase replacing olivine and clinopyroxene as the principal megacryst phases, a sharp rise in platinum-group elements (PGE) concentrations, the brief reappearance of high Fo olivine megacrysts and reversed zoning in the plagioclase megacrysts. These changes are interpreted to have resulted from a new pulse of magma entering the system, with higher water, MgO, SiO2 and PGE concentrations than the original parent magma. Scatter in the PGE data is attributed to the presence of micronuggets in all samples, including the most mafic samples. We conclude that the magma system became sulphide saturated during an early stage in its fractionation history, probably before leaving the crust to deep crustal magma chamber, and then became undersaturated as a consequence of decreasing pressure as magma ascended into a mid-crustal magma chamber. The early saturation episode had a pronounced influence on PGE concentration but had little affect Cu and Au, due to their lower partition coefficient into sulphides. The magma became sulphide saturated again in the crustal magma chamber at ~8.5 wt % MgO, this time affecting all chalcophile elements. Given the early episodes of sulphide saturation, depletion in Au in the Mount Hagen magma system and the absence of a known porphyry system, it is unlikely that Mount Hagen produced economic porphyry mineralisation.

Mineral and whole-rock chemistry of the Chating porphyry Cu--Au deposit related intrusions in the Middle-Lower Yangtze River Belt, Eastern China: Implications for magma evolution and mineralization

The Chating deposit is the largest Cu--Au porphyry deposit in the Xuancheng district of the Middle-Lower Yangtze River Metallogenic Belt. It is associated with three intrusive phases, an amphibole diorite porphyry, an ore-bearing quartz diorite porphyry and a late diorite porphyry. The amphibole diorite porphyry and quartz diorite porphyry are characterized by large ion lithophile element enrichment, depleted high field strength elements, low Yb/Lu and Y/Yb ratios, low εHf(t) (−8.29 to −12.02), εNd(t) (−6.93 to −7.42), and high 87Sr/86Sr(i) (0.705723 to 0.705802), suggesting a source comprised of both subduction-modified mantle- and crust-derived materials. The zircon U--Pb ages of the amphibole diorite porphyry (138.8 ± 3.0 Ma) and quartz diorite porphyry (137.6 ± 3.0 Ma) are consistent with their cross-cutting relationships and within error of the average molybdenite Re--Os age of 136.0 ± 1.3 Ma, suggesting that the Chating porphyry mineralization belongs to the early metallogenic event in the Middle-Lower Yangtze River Metallogenic Belt. Both the amphibole diorite porphyry and quartz diorite porphyry contain high and low Al amphiboles. The high Al amphiboles from the two intrusions show similar ranges of calculated temperatures (842 to 908 °C), pressures (143 to 229 MPa; corresponding to depths of 5.2 to 8.6 km), melt H2O contents (2 to 4 wt%), ΔNNO values (0.12 to 0.99), and Mg# (57 to 65), suggesting coeval crystallization in the same magma chamber. The low Al amphiboles with higher Mg# (68 to 75) formed at lower temperatures (711 to 812 °C) and pressures (49 to 89.3 MPa; corresponding depth of 1.9 to 3.4 km). The core-rim textures and abrupt compositional changes of amphibole and plagioclase from the amphibole diorite porphyry suggest that mafic magma was repeatedly injected into the magma chamber. In contrast, the patchy amphiboles and oscillatory zoned plagioclases in the quartz diorite porphyry may result from continuous fractional crystallization of biotite, amphibole, and plagioclase which would have increased the metal content and fO2 of the residual magma, and resulted in the exsolution of ore-forming fluids.

Magmatic sulfides in high-potassium calc-alkaline to shoshonitic and alkaline rocks

Abstract. We investigate the occurrence and chemistry of magmatic sulfides and their chalcophile metal cargo behaviour during the evolution of compositionally different magmas from diverse geodynamic settings both in mineralised and barren systems. The investigated areas are the following: (a) the Miocene Konya magmatic province (hosting the Doğanbey Cu–Mo porphyry and Inlice Au epithermal deposits, representing post-subduction) and (b) the Miocene Usak basin (Elmadag, Itecektepe, and Beydagi volcanoes, the latter associated with the Kişladağ Au porphyry in western Turkey, representing post-subduction). For comparison we also investigate (c) the barren intraplate Plio-Quaternary Kula volcanic field west of Usak. Finally, we discuss and compare all the above areas with the already studied (d) Quaternary Ecuadorian volcanic arc (host to the Miocene Llurimagua Cu–Mo and Cascabel Cu–Au porphyry deposits, representing subduction). The volcanism of the newly studied areas ranges from basalts to andesites–dacites and from high-K calc-alkaline to shoshonitic series. Multiphase magmatic sulfides occur in different amounts in rocks of all investigated areas, and, based on textural and compositional differences, they can be classified into different types according to their crystallisation at different stages of magma evolution (early versus late saturation). Our results suggest that independently of the magma composition, geodynamic setting, and association with an ore deposit, sulfide saturation occurred in all investigated magmatic systems. Those systems present similar initial metal contents of the magmas. However, not all studied areas present all sulfide types, and the sulfide composition depends on the nature of the host mineral. A decrease in the sulfide Ni∕Cu (a proxy for the monosulfide solid solution (mss) to intermediate solid solution (iss) ratio) is noted with magmatic evolution. At an early stage, Ni-richer, Cu-poorer sulfides are hosted by early crystallising minerals, e.g. olivine–pyroxene, whereas, at a later stage, Cu-rich sulfides are hosted by magnetite. The most common sulfide type in the early saturation stage is composed of a Cu-poor, Ni-rich (pyrrhotite mss) phase and one to two Cu-rich (cubanite, chalcopyrite iss) phases, making up ∼84 and ∼16 area % of the sulfide, respectively. Sulfides resulting from the late stage, consisting of Cu-rich phases (chalcopyrite, bornite, digenite iss), are hosted exclusively by magnetite and are found only in evolved rocks (andesites and dacites) of magmatic provinces associated with porphyry Cu (Konya and Ecuador) and porphyry Au (Beydagi) deposits.

Spatial Relations of Gold-Ore Deposits and Cretaceous Granitoid Magmatism in the Chukotka Peninsula

Analysis of the spatial relations of gold ore occurrences and the enclosing geological environment in the Chukotka Autonomous Okrug (ChAO) has shown that Au–quartz ore occurrences in turbidites are located predominantly in areas of the Early Cretaceous granitoid magmatism with the thick collision-type continental crust. The Cu–Au porphyry deposits are located in areas with complete development of Early Cretaceous magmatism within the outer zone of the Okhotsk–Chukchi volcanoplutonic belt (OChVB). The Au–Ag epithermal deposits are located in the areas of the Late Cretaceous granitoid magmatism within the inner and outer zones of the OChVB. The prospects of island-arc complexes for searching for Kuroku-type Au-bearing ore objects are available.

Identification of mineralization pattern in high frequencies of geochemical data by using the new approach of DWT-PCA

summary The Dalli area has been introduced as copper–gold probably porphyry mineralized area in the central part of Iran. In this study, in order to determine the mineralization pattern, a new method based on coupling discrete wavelet transform (DWT) and principal component analysis (PCA) has been used. The surface geochemical data of 30 elements were transformed to position – scale domain using 2DWT and were decomposed to high and low frequencies in one level then PCA was performed on vertical and horizontal detail components separately. In the final, the elements of Au and Cu have been classified clearly using the combination of mineralization factors obtained of vertical and horizontal detail components. The results of this study demonstrate that the DWT – PCA combined approach is a modern method for geochemical data processing. Introduction In this paper, the position - scale domain of geochemical data, using 2dimensional discrete wavelet transform has been represented and analyzed on Cu–Au porphyry deposit in northern Dalli area and the results have been discussed. Wavelet analyses have led to very successful results in numerous scientific and engineering fields such as signal analysis and numerical applications. Methodology and Approaches WT is a tool for the analysis of signals. Wavelets are used as the basis functions for signal representation such as sines and cosines. In the DWT, detailed and approximation coefficients are obtained with the wavelet algorithm based on high-pass and low-pass filters. In this paper, a new method is proposed based on coupling Haar DWT and Principal component analysis (PCA) for mineralization elements forecasting applications. PCA is a multivariate statistical method for geo-information identification of geo-datasets. Results and Conclusions The wavelet coefficients of geochemical data in vertical and horizontal detail components have appropriate exploratory information. The results of this analysis on the Haar WT have desirably identified the mineralizing elements. Hence, the Haar wavelet is a suitable mother wavelet for interpretation of geochemical data. The surface information and the exploration drillings in the study area confirm the results of WT.

Zonation of Sulfate and Sulfide Minerals and Isotopic Composition in the Far Southeast Porphyry and Lepanto Epithermal Cu–Au Deposits, Philippines

AbstractThe world‐class Far Southeast (FSE) porphyry system, Philippines, includes the FSE Cu–Au porphyry deposit, the Lepanto Cu–Au high‐sulfidation deposit and the Victoria–Teresa Au–Ag intermediate‐sulfidation veins, centered on the intrusive complex of dioritic composition. The Lepanto and FSE deposits are genetically related and both share an evolution characterized by early stage 1 alteration (deep FSE potassic, shallow Lepanto advanced argillic‐silicic, both at ~1.4 Ma), followed by stage 2 phyllic alteration (at ~1.3 Ma); the dominant ore mineral deposition within the FSE porphyry and the Lepanto epithermal deposits occurred during stage 2. We determined the chemical and S isotopic composition of sulfate and sulfide minerals from Lepanto, including stage 1 alunite (12 to 28 permil), aluminum–phosphate–sulfate (APS) minerals (14 to 21 permil) and pyrite (−4 to 2 permil), stage 2 sulfides (mainly enargite–luzonite and some pyrite, −10 to −1 permil), and late stage 2 sulfates (barite and anhydrite, 21 to 27 permil). The minerals from FSE include stage 2 chalcopyrite (1.6 to 2.6 permil), pyrite (1.1 to 3.4 permil) and anhydrite (13 to 25 permil). The whole‐rock S isotopic composition of weakly altered syn‐mineral intrusions is 2.0 permil.Stage 1 quartz–alunite–pyrite of the Lepanto lithocap, above about 650 m elevation, formed from acidic condensates of magmatic vapor at the same time as hypersaline liquid formed potassic alteration (biotite) near sea level. The S isotopic composition of stage 1 alunite–pyrite record temperatures of approximately 300–400°C for the vapor condensate directly over the porphyry deposit; this cooled to &lt;250°C as the acidic condensate flowed to the NW along the Lepanto fault where it cut the unconformity at the top of the basement. Stage 1 alunite at the base of the advanced argillic lithocap over FSE contains cores of APS minerals with Sr, Ba and Ca; based on back‐scattered electron images and ion microprobe data, these APS minerals show a large degree of chemical and S‐isotopic heterogeneity within and between samples. The variation in S isotopic values in these finely banded stage 1 alunite and APS minerals (16 permil range), as well as that of pyrite (6 permil range) was due largely to changes in temperature, and perhaps variation in redox conditions (average ~ 2:1 H2S:SO4). Such fluctuations could have been related to fluid pulses caused by injection of mafic melt into the diorite magma chamber, supported by mafic xenoliths hosted in diorite of an earlier intrusion.The S isotopic values of stage 2 minerals indicate temperatures as high as 400°C near sea level in the porphyry deposit, associated with a relatively reduced fluid (~10:1 H2S:SO4) responsible for deposition of chalcopyrite. Stage 2 fluids were relatively oxidized in the Lepanto lithocap, with an H2S:SO4 ratio of about 4. The oxidation resulted from cooling, which was caused by boiling during ascent and then dilution with steam‐heated meteoric water in the lithocap. This cooling also resulted in the sulfidation state of minerals increasing from chalcopyrite stability in the porphyry deposit to that of enargite in the lithocap‐hosted high‐sulfidation deposit. The temperature at the base of the lithocap during stage 2 was ≥300°C, cooling to &lt;250°C within the main lithocap, and about 200°C towards the limit of the Lepanto orebody, approximately 2 km NW of the porphyry deposit. Approximate 300°C and 200°C isotherms, estimated from S isotopic and fluid inclusion temperatures during stage 1 and stage 2, shifted towards the core of the FSE porphyry deposit with time. This general retreat in isotherms was more than 500 m laterally within Lepanto and 500 m vertically within FSE as the magmatic–hydrothermal system evolved and collapsed over the magmatic center. During this evolution, there is also evidence recorded by large S isotopic variations in individual crystals for sharp pulses of higher temperature, relatively reduced fluid injected into the porphyry deposit.

The origin and distribution of critical metals (Pd, Pt, Te and Se) within the Skouries Cu–Au porphyry deposit, Greece

The origin and distribution of critical metals (Pd, Pt, Te and Se) within the Skouries Cu–Au porphyry deposit, Greece

Evolution of the Hazelton arc near Terrace, British Columbia: stratigraphic, geochronological, and geochemical constraints on a Late Triassic – Early Jurassic arc and Cu–Au porphyry belt

Understanding the development of island arcs that accreted to the North American craton is critical to deciphering the complex geological history of the Canadian Cordillera. In the case of the Hazelton arc (part of the Stikine terrane, or Stikinia) in northwestern British Columbia, understanding arc evolution also bears on the formation of spatially associated porphyry Cu–Au, epithermal, and volcanogenic massive sulfide deposits. The Hazelton Group is a regionally extensive, long-lived, and exceptionally thick Upper Triassic to Middle Jurassic volcano-sedimentary succession considered to record a successor arc that was built upon the Paleozoic and Triassic Stikine and Stuhini arcs. In central Stikinia, near Terrace, British Columbia, the lower Hazelton Group (Telkwa Formation) comprises three volcanic-intrusive complexes (Mt. Henderson, Mt. O’Brien, and Kitselas) that, at their thickest, constitute almost 16 km of volcanic stratigraphy. Basal Telkwa Formation conglomerates and volcanic rocks were deposited unconformably on Triassic and Paleozoic arc-related basement. New U–Pb zircon ages indicate that volcanism initiated by ca. 204 Ma (latest Triassic). Detrital zircon populations from the basal conglomerate contain abundant 205–233 Ma zircons, derived from regional unroofing of older Triassic intrusions. Eleven kilometres higher in the section, ca. 194 Ma, rhyolites show that arc construction continued for &gt;10 million years. Strata of the Nilkitkwa Formation (upper Hazelton Group) with a U–Pb zircon age of 178.90 ± 0.28 Ma represent waning island-arc volcanism. Telkwa Formation volcanic rocks have bimodal silica concentrations ranging from 48.1 to 62.8 wt.% and 72.3 to 79.0 wt.% and display characteristics of subduction-related magmatism (i.e., calc-alkaline differentiation with low Nb and Ti and high Th concentrations). Mafic to intermediate rocks form a differentiated suite that ranges from high-Al basalt to medium- to high-K andesite. They were derived from hydrous melting of isotopically juvenile spinel lherzolite in the mantle wedge and from subsequent fractional crystallization. Compared to basalts and andesites (εNd = +5 to +5.5), rhyolites have higher positive εNd values (+5.9 to +6.0) and overlapping incompatible element concentrations, indicating that they are not part of the same differentiation suite. Rather, the rhyolites formed from anatexis of arc crust, probably caused by magmatic underplating of the crust. This study documents a temporal and spatial co-occurrence of Hazelton Group volcanic rocks with a belt of economic Cu–Au porphyry deposits (ca. 205–195 Ma) throughout northwestern Stikinia. The coeval relationship is attributed to crustal underplating and intra-arc extension associated with slab rollback during renewed or reconfigured subduction beneath Stikinia, following the demise of the Stuhini arc in the Late Norian.

Reducing subjectivity in multi-commodity mineral prospectivity analyses: Modelling the west Kimberley, Australia

Predicting realistic targets in underexplored regions proves a challenge for mineral explorers. Knowledge-driven prospectivity techniques assist in target prediction, and can significantly reduce the geographic search space to a few locations. The mineral prospectivity of the underexplored west Kimberley region was investigated following interpretation of regional gravity and magnetic data. Emphasis was placed on identifying geological structures that may have importance for the mineral prospectivity of the region. Subsurface structure was constrained through combined gravity and magnetic modelling along three transects. Crustal-scale structures were interpreted and investigated to determine their depth extent. These interpretations and models were linked to tectonic events and mineralization episodes in order to map the distribution of minerally prospective regions using a knowledge-driven mineral systems approach. A suite of evidence layers was created to represent geological components that led to mineralization, and then applied to each mineral system where appropriate. This approach was taken to provide a more objective basis for prospectivity modelling. The mineral systems considered were 1) magmatic Ni-sulphide, 2) carbonate-hosted base metals, 3) orogenic Au, 4) stratiform-hosted base metals and 5) intrusion-related base metals (including Sn–W, Fe-oxide–Cu–Au and Cu–Au porphyry deposits). These analyses suggest that a geologically complex belt in the Kimberley Basin at the boundary to the King Leopold Orogen is prospective for magmatic-related hydrothermal mineral systems (including Ni, Au and Cu). The Lennard Shelf is prospective for carbonate-hosted base metals around a feature known as the 67-mile high, and parts of the King Leopold Orogen are prospective for stratiform-hosted base metals. These results show that knowledge-driven mineral system modelling is effective in identifying prospectivity in regional-scale studies of underexplored areas, as well as drastically reducing the search space for explorers working in the west Kimberley.

The chlorite proximitor: A new tool for detecting porphyry ore deposits

The major, minor and trace element chemistry of chlorite were evaluated as a tool for mineral exploration in the propylitic environment of porphyry ore deposits. Chlorite from eighty propylitically altered samples, located up to 5km from the Batu Hijau Cu–Au porphyry deposit in Indonesia, was analyzed using electron microprobe and laser ablation inductively-coupled plasma mass spectrometry. The results show that a variety of elements, including K, Li, Mg, Ca, Sr, Ba, Ti, V, Mn, Co, Ni, Zn and Pb, are probably incorporated in the chlorite lattice and display systematic spatial variations relative to the porphyry center. Ti, V and Mg decrease exponentially in concentration with increasing distance, whereas the others increase. Ratioing the former to the latter provides a variety of ratios that vary up to four orders of magnitude, providing sensitive vectoring parameters. Chlorite geothermometry suggests that Ti is substituted into chlorite as a function of crystallization temperature and thus maps out the thermal anomaly associated with the mineralized center. By contrast, Mn and Zn display a maximum in chlorite at a distance of ~1.3km that mirrors the whole rock anomaly for these metals, reflecting their lateral advection into the wall rocks by magmatic-hydrothermal fluids. The recognizable footprint defined by chlorite compositions extends to at least 4.5km, significantly beyond the whole rock anomalism (≤1.5km) and thus represents a powerful new exploration tool for detecting porphyry systems. Variations in chlorite chemistry are very systematic in the inner propylitic zone (to distances of ~2.5km), thereby providing a precise vectoring tool in a domain where other tools are typically ineffective. In this zone, equations of the form:x=lnRabcan be formulated, where the distance to center, x, is predicted based on a variety of element ratios in chlorite R, and where a and b are exponential fit parameters. Importantly, distal chlorite compositions in porphyry-related propylitic alteration systems are also shown to be distinct from metamorphic chlorite, allowing the external fringes of porphyry-related hydrothermal systems to be distinguished from “background” regional metamorphism or geothermal alteration.

Ordovician marginal basin evolution near the palaeo-Pacific east Gondwana margin, Australia

Although the Ordovician Macquarie Arc, part of the eastern Lachlan Orogen of southeastern Australia, has long been considered to be an intra-oceanic arc within an accretionary orogen, key characteristics contrast with more typical examples of accreted arcs. Significantly, multiple stacked phases of mafic to intermediate volcanic rocks, with suprasubduction-zone chemistry, are flanked to the east and west by extensive, coeval continental margin turbidite, chert and black shale sequences. By analysing stratigraphic and contact relationships within and between the volcanic rocks, ophiolitic components ( sensu lato ) and turbidite sequences, constrained by precise biostratigraphy, we document a repeated cycle of uplift, upper-plate extension and collapse common to all sequences. This cycle is interpreted as resulting from localized extension (rifting or hyper-extension and lower-crustal delamination) within a continent-margin sequence developed upon an already established marginal or back-arc basin of probable middle to late Cambrian age. Our interpretation provides a counter-example to prevailing arc accretion models by inferring extensional tectonics at the palaeo-Pacific east Gondwana margin during the Ordovician with development of alkalic and calc-alkalic Cu–Au porphyry deposits away from an active arc system.

Hydrothermal fluids in epithermal and porphyry Au deposits in the Central Slovakia Volcanic Field

Abstract The Neogene Central Slovakia Volcanic Field in the Carpathian arc contains various Au deposits, hosted by central zones of large andesite stratovolcanoes. Fluids involved in mineralization have been studied at three different types of deposit, mostly by fluid inclusion and stable isotope techniques. The Rozália mine in the Štiavnica stratovolcano hosts intermediate sulphidation-style Au–Ag epithermal mineralization in subhorizontal veins related to hydrothermal activity during an early stage of caldera collapse. Associated fluids of low salinity underwent extensive boiling at 280–330 °C on transition from suprahydrostatic towards hydrodynamic conditions at shallow depths ( c. 550 m) from fluids of mixed magmatic and meteoric origin. The Kremnica ore field hosts a large system of low sulphidation-style Au–Ag veins contemporaneous with rhyolite magmatism and situated on resurgent horst faults. Fluids were of low salinity, predominantly of meteoric origin, and showed gradual decrease in temperature along the system ( c. 270–140 °C) related to a decrease in erosion level from c. 500 to c. 50 m. The Biely Vrch Au-porphyry deposit in the Javorie stratovolcano is associated with quartz stockwork in diorite porphyry intrusion. The major type of ore-bearing fluid was high temperature magmatic vapour (720–&lt;380 °C) accompanied by Fe-rich salt melt. Gold precipitated in a high-temperature but low-pressure subvolcanic environment. Supplementary material: Stable isotope data and fluid inclusion microthermometric data are available at http://www.geolsoc.org.uk/SUP18752 .

Tethyan mantle metasomatism creates subduction geochemical signatures in non-arc Cu–Au–Te mineralizing magmas, Apuseni Mountains (Romania)

The history of processes that have affected the lithospheric mantle can be reflected in the chemical characteristics of magmas that are emplaced millions of years later during crustal extension. As a result, ore deposits normally associated with subduction can form in environments where they are not conventionally expected. Calc-alkaline magmatism in the Apuseni Mountains, Romania, occurred in response to Miocene extension and formed rich Au–Ag–Te epithermal and Cu–Au porphyry deposits. We present major, trace element and Sr–Nd–Pb isotopic data from a comprehensive suite of unaltered Apuseni samples. This dataset indicates that the Apuseni magmas were derived from partial melting of a lithospheric mantle source enriched in volatiles and incompatible elements. Mantle-like Sr and Nd isotopic signatures, combined with large ion lithophile element enrichments, Nb–Ta depletions, and Pb enrichments, are best explained by mantle refertilization during subduction. Subduction metasomatism of the Apuseni source region occurred at least 50Ma prior to extensional magma generation in the Miocene, most likely during NE-dipping subduction associated with closure of the Neotethys Ocean in the Mesozoic.We suggest that the different types of mineralization in the region are generally related to the degree of Miocene extension and consequent partial melting of the mantle source. Initial extension is correlated with the largest Au deposits in the Apuseni Mountains. The magmatic rocks associated with these deposits contain the most isotopically enriched Pb–Sr–Nd ratios, higher SiO2 and lower Mg-number, consistent with significant lower crustal influence during the initiation of extension and associated crustal anatexis. As extension progressed, increased mantle input resulted in magmas with more isotopically depleted Pb–Sr–Nd ratios, lower SiO2 and higher Mg-numbers. Uniquely rich Te-rich Au–Ag epithermal deposits are associated with these magmas, and are followed by porphyry Cu–Au deposits. Exceptional endowment in Te-rich epithermal deposits is common to other provinces associated with extensional magmatism preceded by much older mantle metasomatism, and we suggest that these deposits are ultimately linked to this metasomatism.

Geology, mineralization, stable isotope geochemistry, and fluid inclusion characteristics of the Novogodnee–Monto oxidized Au–(Cu) skarn and porphyry deposit, Polar Ural, Russia

The Novogodnee–Monto oxidized Au–(Cu) skarn and porphyry deposit is situated in the large metallogenic belt of magnetite skarn and Cu–Au porphyry deposits formed along the Devonian–Carboniferous Urals orogen. The deposit area incorporates nearly contemporaneous Middle–Late Devonian to Late Devonian–Early Carboniferous calc-alkaline (gabbro to diorite) and potassic (monzogabbro, monzodiorite- to monzonite-porphyry, also lamprophyres) intrusive suites. The deposit is represented by magnetite skarn overprinted by amphibole–chlorite–epidote–quartz–albite and then sericite–quartz–carbonate assemblages bearing Au-sulfide mineralization. This mineralization includes early high-fineness (900–990 ‰) native Au associated mostly with cobaltite as well as with chalcopyrite and Co-pyrite, intermediate-stage native Au (fineness 830–860 ‰) associated mostly with galena, and late native Au (760–830 ‰) associated with Te minerals. Fluid inclusion and stable isotope data indicate an involvement of magmatic–hydrothermal high-salinity (>20 wt.% NaCl-equiv.) chloride fluids. The potassic igneous suite may have directly sourced fluids, metals, and/or sulfur. The abundance of Au mineralization is consistent with the oxidized character of the system, and its association with Co-sulfides suggests elevated sulfur fugacity.

Alteration, zoning model, and mineralogical structure considering lithogeochemical investigation in Northern Dalli Cu–Au porphyry

Dalli Cu–Au porphyry deposit was occurred in the igneous diorite, quartz diorite porphyry (QDP), and volcanic rocks such as porphyritic amphibole andesite, andesite (AND), dacite, and pyroclastics during the late Miocene to Pliocene. Regolith investigations and Advanced Spaceborne Thermal Emission and Reflection Radiometer images were used to identify the anomalous areas. According to lithogeochemical survey (from boreholes and trenches) in Northern Dalli Cu–Au porphyry, the potassic, chlorite, sericite, propylitic, and argillic alterations have been found and mineralization was basically associated with potassic and quartz–sericite alterations. The alteration is dominantly moderate quartz chlorite ± sericite magnetite with 1–10 mm wide quartz ± magnetite veinlets. The elevated copper–gold values are correlated with density of stockworking and mineralization. The intensity of the mineralization is high in the contact of QDP and AND with increases in pyrite and chalcopyrite values. Malachite, native Cu, and bornite were used to identify supergene, transition, and hypogene zone. In addition, molybdenum increased near to the center of granodiorite intrusion. And besides, from depth to surface in DDH03 and wall rock to mineralization zones, a sequence of Mo→Cu (Au)→Au (Cu) was recorded and the mineralization temperature cooled down (from high to low). The alteration is characterized by specific pattern and structure in Dalli Cu–Au porphyry deposit. The alteration model was followed from the modified Lowell and Gilbert model. The porphyry is stockworked by quartz veins and by quartz magnetite veins. Vein distribution and ore mineralogy vary between the different alteration zones. Due to the formation of an iron cap in the supergene, especially in the southern hills, supergene grade was higher than hypogene zone. Also, hematite, as a dominant Fe oxide in DDH03 borehole with minor limonite, jarosite, and goethite created thickness about 150–270 m in supergene zone; finally, this finding show a possibility of an extensive mineralization.