Porphyry Systems Research Articles

The geology and mineralogy of the Stypsi porphyry Cu-Mo-Au-Re prospect, Lesvos Island, Aegean Sea, Greece

The Stypsi Cu-Mo-Au-Re prospect, Lesvos island, is a shallow porphyry-epithermal system hosted within a middle Miocene microgranite porphyry, which was emplaced along NNE-, NW- and NE-trending structures, within trachyandesites to trachydacites and felsic pyroclastics of the Stypsi caldera. The mineralization comprises three stages: Stage I is characterized by magnetite-actinolite ± quartz in transitional and banded quartz veins mostly developed within the microgranite and the surrounding lavas related to calcic-potassic and propylitic alteration of the host rocks. In the banded veins, quartz is botryoidal, suggesting crystallization from a gel. Abundant vapor-rich inclusions in the bands produce a dark grey to black color. Within the veins, magnetite, chalcopyrite, bornite and native gold were followed in time by pyrite, hematite, sphalerite and galena. Stage I mineralization was synchronous and also postdates quartz formation in the veins, since it crosscuts and/or fills vugs in the center of the veins. Sulfides are associated with various combinations of K-feldspar, actinolite, epidote, chlorite, and calcite gangue minerals. Pyrite-molybdenite-chalcopyrite (Stage II) and late intermediate sulfidation epithermal veins (Stage III) overprint earlier mineralization and alteration, and are associated with sericite-calcite and sericite-kaolinite alteration of the porphyry system and spatially associated lavas, respectively. Molybdenite is widespread in the first two mineralization stages, with Re content ranging from about 0.3 wt% in Stage I to 1.96 wt% in Stage II. A barren silicic and advanced argillic (alunite-kaolinite) lithocap, is exposed on top of the porphyry-style mineralization and is crosscut by non-mineralized high-sulfidation epithermal chalcedony-barite veins. Bulk ore analyses of surface samples from the Stypsi prospect yielded values of up to 276 ppm Mo, 978 ppm Cu, up to 0.5 g/t Au, up to 3 g/t Ag, and up to 70 ppb Pd and Re.Fluid inclusion data indicate that the Stage I transitional and banded quartz veins were deposited at 420 °C–530 °C and at pressures up to 450 bars, from boiling hydrothermal fluids. The fluid in the veins consists of a brine (40–61 wt% NaCl equiv) that coexists with a lower salinity (6–14 wt% NaCl equiv) liquid phase and a low-density vapor-rich fluid. Fluid inclusions in quartz of the Stage III intermediate-sulfidation epithermal veins are characterized by relatively low homogenization temperatures (231 °C–288 °C) and salinities (up to 1.9 wt% NaCl equiv), which was the result of subsequent dilution of the moderately saline fluids by circulating meteoric water. This study verifies earlier works suggesting that Au-enriched felsic magmas are able to crystallize Re-rich molybdenite, and that Re may also be redistributed and enriched in later stages during the deposition of porphyry-style mineralization. The Stypsi prospect ressembles in many respects (e.g., Au grades, Cu/Mo ratios, the Re content of molybdenite and the presence of ore-grade calcic-potassic alteration), other porphyry Cu-Mo-Re-Au systems hosted in calc-alkaline rocks in northeastern Aegean, Greece.

Using Rare Earth Elements (REE) to Decipher the Origin of Ore Fluids Associated with Granite Intrusions

A practical method is presented to estimate rare earth element (REE) concentrations in a magmatic fluid phase in equilibrium with water-saturated granitic melts based on empirical fluid–melt partition coefficients of REE ( k P R E E ). The values of k P R E E can be calculated from a set of new polynomial equations linking to the Cl molality ( m C l v ) of the magmatic fluid phase associated with granitic melts, which are established via a statistical analysis of the existing experimental dataset. These equations may be applied to the entire pressure range (0.1 to 10.0 kb) within the continental crust. Also, the results indicate that light REEs (LREE) behave differently in magmatic fluids, i.e., either being fluid compatible with higher m C l v or fluid incompatible with lower m C l v values. In contrast, heavy REEs (HREE) are exclusively fluid incompatible, and partition favorably into granitic melts. Consequently, magmatic fluids tend to be rich in LREE relative to HREE, leading to REE fractionation during the evolution of magmatic hydrothermal systems. The maximum k P R E E value for each element is predicted and presented in a REE distribution diagram constrained by the threshold value of m C l v . The REE contents of the granitic melt are approximated by whole-rock analysis, so that REE concentrations in the associated magmatic fluid phase would be estimated from the value of k P R E E given chemical equilibrium. Two examples are provided, which show the use of this method as a REE tracer to fingerprint the source of ore fluids responsible for the Lake George intrusion-related Au–Sb deposit in New Brunswick (Canada), and the Bakircay Cu–Au (–Mo) porphyry systems in northern Turkey.

Sodic-Calcic Family of Alteration in Porphyry Systems of Arizona and Adjacent New Mexico

Sodic-Calcic Family of Alteration in Porphyry Systems of Arizona and Adjacent New Mexico

Partial sub-pixel and pixel-based alteration mapping of porphyry system using ASTER data: regional case study in western Yazd, Iran

ABSTRACT This study uses remote sensing image-processing to enhance recognition of areas with mineral abundance halo, which can be associated with porphyry mineralisation of the Urumieh-Dokhtar Magmatic Belt (UDMB) in western Yazd. Granitoid intrusions of Oligocene-Miocene age in UDMB hosts the most important Porphyry Copper Deposits (PCDs) of Iran such as Darezereshk and Aliabad PCDs. Diagnostic spectral absorption features of six shortwave infrared (SWIR) and three visible and near-infrared (VNIR) bands of the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) data were applied to determine the alteration minerals and their abundance. In this research reflectance data from various representative alteration types throughout the study area were measured by the FieldSpec-3 spectroradiometer and applied for mapping minerals. Different remote sensing analytical techniques, including the Principal Component Analysis (PCA), band ratio, relative absorption band depth, and MTMF, were applied to enhance mapping the alteration minerals. Field works including rock sampling, petrography and field spectroradiometer followed up the alteration mapping. Field observation implies that, the vein-type mineralisation above the cupola of this porphyry system is preserved in the western part, although it was eroded in the eastern area. Consequently, PCDs potential is expected to be higher in the western area.

Incursion of meteoric water triggers molybdenite precipitation in porphyry Mo deposits: A case study of the Chalukou giant Mo deposit

There has been a long-term debate about the timing of the incursion of meteoric water into porphyry systems and whether it plays an important role in ore precipitation. To evaluate the role of meteoric water in the mineralization in porphyry Mo deposits, typical hydrothermal veins of different stages from the Chalukou giant porphyry Mo deposit are selected for combined scanning electron microscope cathodoluminescence (SEM-CL), fluid inclusion microthermometry and high-resolution secondary ion mass spectroscopy (SIMS) oxygen isotope analyses. Multiple generations of quartz are identified in almost all of the veins. The barren quartz veins and quartz-magnetite veins of the early Mo mineralization stage are dominated by granular CL-bright quartz (EMQ1), which usually shows dissolution and re-deposition textures and are overprinted by CL-dark quartz (EMQ2)-filled healed fractures. The molybdenite precipitation in the quartz-molybdenite veins of the main Mo mineralization stage is synchronous with small volumes (<15%) of CL-dark quartz (MMQ2) and postdates the large volume (>85%) of CL-bright quartz (MMQ1). Similar to the quartz-molybdenite veins, the precipitation of pyrite in the quartz-pyrite veins during the transitional stage and sphalerite in the quartz-sphalerite-pyrite veins during the Zn-Pb mineralization stage are also directly associated with CL-dark quartz (TSQ2, ZPQ2). The SIMS oxygen isotopic values of EMQ1 and MMQ1 fall into the range of 5.42–9.79‰ with calculated equilibrium fluid compositions of 2.07–6.45‰, indicating a magma-dominated fluid composition, whereas MMQ2, TSQ2 and ZPQ2 have very low oxygen isotopic values from −4.68 to 4.29‰ with corresponding fluid compositions of −13.64 to −5.94‰, implying that the fluid formed by the mixing of magmatic water and a large proportion of meteoric water. The contrasting SEM-CL features and oxygen isotopic compositions of the two generations of quartz in the quartz-molybdenite veins demonstrate that the incursion of meteoric water into the hydrothermal system occurred earlier than the molybdenite precipitation. Based on these results and the crosscutting relationships and spatial distribution of the veins, we reconstructed the quartz evolutionary sequence during the vein development process. After the early precipitation of barren quartz, the reduction in fluid pressure from lithostatic to hydrostatic conditions allowed the incursion of convecting cool meteoric water into the relatively hot hydrothermal system, and the temperature of the evolved Mo-rich fluid decreased rapidly from >500 °C to <400 °C, leading to concentrated molybdenite precipitation with small volumes of quartz. Fluid cooling resulting from the incursion of meteoric water may have played a greater role in the metal precipitation in the Chalukou Mo deposit than wallrock interactions and fluid boiling. Moreover, this process may have also occurred in other porphyry Mo deposits.

Exsolution depth and migration pathways of mineralising fluids in porphyry systems – examples from the Yerington District, Nevada

Porphyry copper deposits provide around 75% of the world’s copper and are an important source of gold and other metals (Sillitoe 2010). One aspect of porphyry deposit models that is poorly understo...

Isotopic constraints on the processes favouring endoskarn- over porphyry-style mineralisation in Cu porphyry systems: an example from the Daye district, China

Isotopic constraints on the processes favouring endoskarn- over porphyry-style mineralisation in Cu porphyry systems: an example from the Daye district, China

Platinum transport in chloride-bearing fluids and melts: Insights from in situ X-ray absorption spectroscopy and thermodynamic modeling

Abstract Hydrothermal chloride-rich fluids are identified at the late stages of the formation of platinum group element (PGE) deposits in giant layered intrusions, and are considered as the PGEs transport media in Cu(-Mo,Au) porphyry systems. In order to quantify the hydrothermal mobility of Pt we performed an investigation of the speciation of Pt in hydrothermal chloride-bearing fluids and dry melt by means of X-ray absorption spectroscopy (XAS). The experiments consisted in recording the Pt L3-edge X-ray absorption near edge structure/extended X-ray absorption fine structure (XANES/EXAFS) spectra of Pt-bearing fluids obtained by dissolution of Pt metal in KCl/HCl and CsCl/HCl fluids in the temperature range from 450 to 575 °C at pressures from 0.5 to 5 kbar. A spectrum of Pt dissolved in dry CsCl/NaCl/KCl + K2S2O8 melt was recorded at 650 °C. The capillary method, when the experimental solution together with Pt(cr) is sealed inside a silica glass capillary, was used. As was determined from the XANES spectra, in all the experimental systems Pt existed in the +2 oxidation state. Analysis of EXAFS spectra showed that Pt is coordinated by four Cl atoms with RPt-Cl = 2.31 ± 0.01 A independently of the T-P-compositional parameters. No evidence of the formation of complex with alkali metal cations in the second coordination sphere of Pt was found by the analysis of the EXAFS spectra of concentrated CsCl brines and melt. Our results imply that PtCl42− is the main Pt-Cl complex which predominates in hydrothermal fluids at t > 400 °C and fluid density d > 0.3 g·cm−3. Experimental data obtained for dry melt of alkali metal chlorides suggest that Pt-Cl complexes can dominate Pt speciation in chloride-bearing aluminosilicate melts where Cl exhibits a salt-like atomic arrangement and ionic bonding. The literature data on the Pt solubility constant, P t ( c r ) + 2 H C l ( a q ) o + 2 C l - = P t C l 4 2 - + H 2 ( a q ) , are compiled and fitted to the simple density model equation log K s o ( P t C l 4 2 - ) = 0.973 - 8202 · T ( K ) - 1 - 5.505 · log d w + 2223 · log d ( w ) · T ( K ) - 1 , where d(w) is the pure water density in g·cm−3. The equation, combined with the extended Debye-Huckel equation for activity coefficients, can be used to calculate the solubility of Pt up to 1000 °C/5 kbar. It accurately predicts the solubility of Pt in concentrated chloride brine (up to 50 wt% NaCl) at parameters of magmatic-hydrothermal transition (800 °C/1.4 kbar). At fluid/vapor density below 0.3 g·cm−3 a neutral complex PtCl2°(aq) is suggested as the dominant Pt species. Our data demonstrate that Pt is highly mobile in high-temperature oxidized chloride-rich hydrothermal fluids. For example, at 800 °C/2 kbar the concentration of Pt can reach a few wt% in the 1 wt% HCl/50 wt% NaCl fluid which is in equilibrium with magnetite-hematite buffer. Once a Cl-reach fluid exsolves from alumuinosilicate melt, Pt follows Cl and enriches the fluid phase where it exists mostly in the form of PtCl42−. Decrease of temperature, acidity, and fluid chlorinity results in precipitation of Pt from the fluid phase.

Mineral Mapping and Vein Detection in Hyperspectral Drill-Core Scans: Application to Porphyry-Type Mineralization

The rapid mapping and characterization of specific porphyry vein types in geological samples represent a challenge for the mineral exploration and mining industry. In this paper, a methodology to integrate mineralogical and structural data extracted from hyperspectral drill-core scans is proposed. The workflow allows for the identification of vein types based on minerals having significant absorption features in the short-wave infrared. The method not only targets alteration halos of known compositions but also allows for the identification of any vein-like structure. The results consist of vein distribution maps, quantified vein abundances, and their azimuths. Three drill-cores from the Bolcana porphyry system hosting veins of variable density, composition, orientation, and thickness are analysed for this purpose. The results are validated using high-resolution scanning electron microscopy-based mineral mapping techniques. We demonstrate that the use of hyperspectral scanning allows for faster, non-invasive and more efficient drill-core mapping, providing a useful tool for complementing core-logging performed by on-site geologists.

Episodic porphyry Cu (-Mo-Au) formation and associated magmatic evolution in Turkish Tethyan collage

Continental collision between Eurasia and Afro-Arabia in Turkey along the Pontide subduction zone, İzmir-Erzincan-Ankara suture zone, Bitlis-Zagros suture zone, and Aegean subduction zone generated a fertile metallogenic environment with calc-alkalic and alkalic porphyry Cu (-Mo-Au) and magmatic hydrothermal systems emplaced in narrow arc segments over limited time durations within major magmatic events in the late Cretaceous to late Miocene. The major magmatic events represent multiple subduction zones that gradually stepped from north to south as small oceans closed during collisional events and the trench retreated or shifted southward as a result. Based on geological, new geochemical, and 40Ar/39Ar, U-Pb LA-ICP-MS geochronological data presented herein along with published data for igneous and hydrothermally altered rocks, four metallogenic episodes are defined; late Cretacecous, early-late Eocene, late Oligocene- early Miocene, and middle-late Miocene, in four geographically distinct regions, the Pontides in the north, the western Anatolian province in the west, central Anatolian Crystalline Complex, and the Southeastern Anatolian Orogenic Belt in the east central part of Turkey. Within those regions, porphyry Cu and intrusion-related hydrothermal systems begin in the late Cretaceous to the north, and generally young southward. They begin as porphyry Cu-Mo type, and tend to be more Au-rich from late Cretaceous to late Miocene. Published and new ages indicate early-late Cretaceous magmatic rocks in the Pontides were emplaced between ca. 131 and 65 Ma, peaking between ca. 88–76 Ma. Apart from the Re-Os molybdenite ages, published and new ages for porphyry Cu-related rocks and hydrothermal minerals from the Ayder, Konak, Esendal, Börekli, and Dereköy prospects indicate porphyry Cu formation near the end of that magmatic event (ca. 83–76 Ma). Published data indicate a somewhat shorter late Cretaceous magmatic event (86.5 to 70.3 Ma) peaking at ca. 86 to 80 Ma at the southeastern Anatolian orogenic belt with porphyry Cu systems reported to have formed contemporaneously. The latest Paleocene to early-late Eocene magmatism and porphyry-style hydrothermal systems are common across Turkey, with porphyry Cu systems formed in the eastern Pontides (58–37.9 Ma) based on the ages of their host rocks and syn-mineral stocks; in the western Anatolian province in the Tavşanlı porphyry belt (ca. 49.1–46.7 Ma) and the Biga porphyry belt (40.1–39.5 Ma), and in the Southeastern Anatolian orogenic belt (ca. 47 to 43 Ma). Oligocene-Miocene magmatism (30–18 Ma) and associated hydrothermal systems dominate during ca. 27.4 to 24.8 Ma the western Anatolian province in the Biga porphyry belt, but appear less common in other parts of Turkey except for the Cevizlidere porphyry deposit in the Southeastern Anatolian orogenic belt. Middle-late Miocene magmatism (ca. 16–8 Ma) and associated porphyry Cu systems predominate within the Afyon-Konya porphyry belt in western Anatolia, and contains the youngest porphyry related magmatic and hydrothermal systems (14.4–8.9 Ma).Calc-alkalic porphyry Cu ± Mo-Au systems dominate all belts except the Afyon-Konya porphyry belt where alkalic porphyry Au-Cu systems are present. The porphyry systems are dominantly metaluminous and medium to high-K rocks with a range in composition from diorite to granite, with monzonite and syenite associated with the alkalic porphyry systems. Late Cretaceous rocks have typical arc magma compositions except the Central Anatolian Crystalline complex, whereas Eocene to Miocene collisional to post-collisional rocks have compositional characteristic of arc magmas that over time interacted and assimilated by ancient crust and evolved toward slightly higher Sr/Y and La/Yb trace element characteristics. The trace element compositions along with rare earth element patterns share characterristics, typical of rocks associated with porphyry Cu-related magmas reflecting the hydrous and oxidized nature of the magmas.The late Cretaceous porphyry systems common in the Pontides form a portion of a belt of porphyry Cu deposits that included world-class deposits in Serbia and Bulgaria, and extend eastward into the Caucasus of Georgia, Armenia and Iran. Early-late Eocene porphyry systems in Pontides, western Anatolian province and southeastern Anatolian orogenic belt form clusters not extending the in the Balkans. Oligocene and Miocene porphyry systems are common in the adjoining countries in the Balkans and Iran, but are only known in the western Anatolia region of Turkey. Overall, the distribution of porphyry Cu systems reflects complex plate interactions involving subduction, collision of continental blocks, closure of small oceanic basins, and rapidly shifting axes of magmatic activity.

Automated Identification of Magnetic Responses from Porphyry Systems

Porphyry-style mineralisation often appears as near circular anomalies within magnetic data. This article presents an automatic grid analysis system to detect such responses. Our approach follows three steps: (1) Find circular features using the radial symmetry transform; (2) Validate the detected features by the presence of high magnetic contrast at the feature location; and (3) Highlight the alteration zone boundary using deformable splines. The outcome of this system is two-fold. First, provide estimates of the location of potential porphyry systems, and second, accentuate the appearance of potential exploration targets to aid manual data inspection. Experiments were conducted on survey data from Reko Diq, Pakistan, a region known to contain numerous occurrences of porphyry-style mineralisation. The prediction results of our system closely matched the location of the known deposits in this region thus rendering confidence in the effectiveness of our approach. It is suggested that this system be used as an initial screening tool for large datasets, therefore reducing the time and cost imposed by manual data inspection in the exploration targeting process.

Gold behavior in intermediate sulfidation epithermal systems: A case study from the Zhengguang gold deposit, Heilongjiang Province, NE-China

The Zhengguang gold deposit, a typical intermediate-sulfidation epithermal deposit, is located in the southeastern part of the Duobaoshan orefield, west of the Hegenshan-Heihe suture zone, in the eastern part of the Central Asian Orogenic Belt. The deposit comprises five ore zones with total Au reserves exceeding 35 tonnes, with potential additional resources at depth. All vein-type orebodies are hosted by Paleozoic volcanic rocks and comprise multiple vein sets 1–100 cm in thickness. Although gold generally occurs in native form, or as electrum in epithermal deposits like Zhengguang, both pyrite and sphalerite are known to accommodate modest concentrations of invisible gold. This study employs a combination of petrography and sulfide chemistry to determine the role of invisible gold in the Zhengguang ores and the mechanisms of gold incorporation into epithermal sulfides. Three sulfide stages are identified: an early quartz + pyrite (Py1a, Py1b) ± chalcopyrite (Ccp1) stage; a subsequent quartz + sphalerite (Sph2a, Sph2b) + pyrite (Py2a, Py2b, Py2c, Py2d) + chalcopyrite (Ccp2a, Ccp2b) ± galena ± calcite stage; and a late stage containing deformed quartz + pyrite (Py3a, Py3b) ± sphalerite. Petrography and sulfide chemistry allow three groups of pyrite (Au-poor, Au-rich, and a distinct Sb-rich group) to be distinguished, alongside three groups of chalcopyrite (Bi-rich, intermediate-Bi, and Bi-poor), and two groups of sphalerite (Au-poor, Au-rich). A potential porphyry system is indicated beneath the epithermal system by the appearance of Au-poor pyrite and Bi-poor chalcopyrite. After precipitation of early Au-poor sulfides, inflow of relatively low temperature epithermal fluids led to alteration and replacement of early porphyry-related sulfides, and to precipitation of Au-rich pyrite, Bi-rich and intermediate-Bi chalcopyrite, and sphalerite. Gold-rich pyrite contains up to 140 ppm Au, interpreted as both as lattice-scale substitution (Au1+) and as included particles of native gold (Au0). Epithermal chalcopyrite is an important silver carrier but, although Au is measurable, it is a not a good carrier for gold. A strong positive correlation between Au and Cu in pyrite from the first two stages indicate that gold and other metals were likely sourced from magma-derived hydrothermal fluids. The deposit was formed in the Early Paleozoic but some gold ores appear deformed and partially destroyed by a later metamorphic event during which a distinct Sb-rich pyrite crystallized. This study should catalyze exploration in the orefield as it provides further support for an as-yet undiscovered porphyry system close to the Zhengguang deposit.

Magnetite Chemistry in the Porphyry Copper Systems of Kerman Cenozoic Magmatic Arc, Kerman, Iran

The present work attempts to characterize magnetite chemistry in some collisional (Chahfiruzeh and Keder; Miocene) and pre-collisional (Reagan and Daralou; Eocene–Oligocene) porphyry copper systems hosted in the Kerman Cenozoic Magmatic Arc (KCMA). EPMA results for magnetite samples show collisional deposits have higher average contents of Fe and Ca compared to those of pre-collisional systems. Nevertheless, magnetite samples of pre-collisional porphyry Cu systems contain elevated Si values. Highlightly, the frequency of V and Ca in magnetite have a direct relation with mineralization potential of studied deposits decreasing from Chahfiruzeh to Keder, Daralou and Reagan. V/Cr and V/Al in the analyzed magnetite samples show that Reagan porphyry system is quite different than other studied deposits. Moreover, average concentrations of Mo and Cu reach the highest in collisional and pre-collisional deposits, respectively. According to Fe/(V + Ti) diagram, most of the studied magnetite samples have re-equilibrated magmatic nature. Al + Mn versus V + Ti and Ni (Cr + Mn) versus Ti + V diagrams also confirmed that all magnetite samples from collisional and pre-collisional deposits are analogous to those reported previously for porphyry Cu systems. Regarding the condition of oxygen fugacity, based on Ni versus V diagram, the Reagan deposit (pre-collisional) has the highest oxygen fugacity, while the Daralou deposit (pre-collisional) and collisional deposits (Chahfiruzeh and Keder) exhibit moderate to high fugacity. Ti + V versus Al + Mn show that selected deposits were approximately formed in the same temperature conditions.

Lithogeochemistry of Intrusive Rocks in the Halo Porphyry Copper-Molybdenum Prospect, Northeast Cambodia

The Halo copper-molybdenum prospect is a porphyry system in Ratanakiri province, northeastern part of Cambodia. There is only one research was carried out on this prospect about geological mapping and short wave infrared (SWIR) spectroscopy on alteration mineral identification. The purpose of this research is to confirm the deposit type from previous Angkor Gold’s report and find the centre of porphyry deposit based on characteristic of intrusive rocks at surface and subsurface, characteristics of the intrusive rocks and alteration lithogeochemistry of intrusive and volcanic rocks by using Pearce Element Ratio (PER) analysis. PER analysis was used to examine the nature and extend of the alteration halos in the porphyry Halo copper-molybdenum prospect. The intrusive rocks and volcanic rocks in Halo, range from diorite to granite (quartz feldspar porphyry) in composition as well as dacite to trachyandesite (andesite porphyry) in composition, respectively. They were formed in a subduction-related tectonic setting, likely a volcanic arc. Trace elements spider diagrams were normalized to primitive mantle display strong enrichment in large-ion lithophile elements such as Rb, Ba and K and depletion in some high-field strength elements such as Nb and Ti, suggesting magmas generated in a subduction-related tectonic setting. PER analysis indicates a moderate to high degree of sericite alteration of dacites, quartz feldspar porphyries, andesite porphyries and granodiorite porphyries. Moreover, three alterations in the Halo prospect such as potassic (secondary K-feldspar and biotite), phyllic (sericite) and propylitic (epidote) alteration were identified, which are consistent with petrography analysis. The potassic alteration zone is vectoring center of the hydrothermal system which may represent the locus of mineralization. Therefore the geochemical signature of potassic alteration within the quartz feldspar porphyry, andesite porphyry and granodiorite porphyry with high grad of copper ranges up to 2670 ppm and molybdenum ranges up to 5297 ppm represents a character for further exploration in the Halo porphyry copper-molybdenum prospect.

Hydrothermal fluid evolution in collisional Miocene porphyry copper deposits in Iran: Insights into factors controlling metal fertility

The Cenozoic Urumieh-Dokhtar Magmatic Belt (UDMB) in Iran is considered as one of the major Cu-bearing regions in the world with high potential for the occurrence of giant/large porphyry Cu ± Mo ± Au systems. In the UDMB, porphyry Cu mineralization is strictly associated with Miocene intrusions originating mostly from collisional-related partial melting of subduction-modified juvenile lower crust. In this study fluid inclusion coupled with laser Raman analysis is used to characterize and compare the fluid evolution of the large Meiduk deposit and smaller low grade porphyry copper systems (e.g., Keder, Sarkuh, and Iju) in the UDMB. It can be shown that enhancement of metal fertility of a porphyry system is controlled by several factors: (1) The salinity of early hydrothermal ore fluids; maximum ore-efficiency is seen in porphyry deposits showing supersaturation of NaCl in the pristine high temperature fluids (e.g., Meiduk), (2) Presence of CO2 and fast exhausting of CO2 vapor components during early stages of mineralization, (3) Temperature decrease especially during the main ore stage, and (4) High oxygen fugacity (near to the magnetite – hematite oxygen buffer) of the most primitive fluids in the early stages of hydrothermal system.

Geochemical characteristics of collisional and pre-collisional porphyry copper systems in Kerman Cenozoic Magmatic Arc, Iran: Using plagioclase, biotite and amphibole chemistry

Kerman Cenozoic Magmatic Arc (KCMA) located in the southeastern part of Urumieh-Dokhtar Magmatic Bet (UDMB) hosts a number of world class and moderate to small porphyry copper deposits. The present work compares the silicate chemistry (plagioclase, biotite and amphibole) of Miocene collisional porphyries (e.g., Chahfiruzeh and Keder) with Eocene-Oligocene pre-collisional porphyry systems (e.g., Reagan and Daralou) in the KCMA. P (<200Mpa) and H2Omelt, in the all studied porphyry systems is analogous to those reported previously for other porphyry copper systems. All analyzed biotites are high-Mg with subduction-related calc-alkaline and medium to high oxygen fugacity character. However, the Al2O3 and TiO2 contents of amphibole indicate that studied porphyries had a crust-mantle mixed source. Pre-collisional porphyry systems with lowest degree of Cu mineralization simultaneity show lowest An% and Al* compared with that of collisional porphyries. The amphiboles represent calcic – calc alkaline moderately oxidant magmatic character (NNO). The plagioclase chemistry indicates that magmatic system in the collisional deposit is relatively closed exhibiting the low rate of mixing compared with pre-collisional deposits. The biotite chemistry indicates temperature decreasing from Keder to Reagan, Daralou and Chahfiruzeh, respectively. The lack of regular temperature die line in the pre-collisional porphyry systems could be effective in low grade mineralization. High biotite halogen fugacity ratios (fH2O/fHF and fH2O/fHCl > 1) imply water-rich nature of studied porphyry systems. The relative enrichment F in the collisional porphyry deposits (e.g., Keder) could be linked with enrichment of Mg in these systems.

The alteration characteristic of Cu-Au skarn and porphyry-style alteration in The Deep Mlz, Ertsberg District, Papua, Indonesia

The Deep Mill Level Zone (DMLZ) is a mining zone within the lower part of the East Ertsberg Skarn System (EESS), a world class copper-gold skarn deposits that linked to porphyry system. Macroscopic observation along with microscopic analysis such as Petrography, SEM, and Mineragraphic analysis were made on each core samples to determine the zonation of Skarn alteration that superimposed with porphyry style alteration system within the mineralization zone. The DMLZ hosted in the Paleocene Waripi Formation that was intruded by Diorite to Quartz Monzodiorite. The Diorite characterized by its equigranular texture with the appereance of Hornblenda-Biotite phenocrysts, meanwhile later Quartz Monzodiorite shows porphyritic texture with abundant Pyroxene-Biotite phenocrysts embedded in allotriomorphic granular quartz groundmass. The DMLZ is hosted by mixed assemblages of silisiclastic and dolomitized carbonate that associate with the multiphase intrusive complex. The Skarn alteration zonation can be identified based on dominant lithology, such as: 1) Forsterite-diopside Skarn 2)Anhydrite 3)Magnetite Skarn 4)Hornfels 5)Marble and 6)Endoskarn. Meanwhile Porphyry system that identified in the diorite to quartz monzodiorite intrusion consist of: Secondary biotite-K- Feldspar±actinolite±tremolite ±anhydrite-magnetite; Sericite-pyrophyllite-illite-secondary quartz; Chlorite-clay minerals-calcite-epidote-secondary quartz; Calcite ± dolomite ± siderite;. Some alteration zones occur together in several depth indicating overprinting of different alteration episodes, but there is evidence that the carbonate alteration is the latest event. Cu-Au mineralization consists of bornite and chalcopyrite predominantly, both within skarn system and also porphyry copper system.

Petrophysics and Exploration Targeting: The Value Proposition

There is still much that needs to be understood about the physical properties of rocks in mineralised geological environments. This knowledge gap becomes more important as the transition to deeper exploration targets under cover occurs, with an associated greater reliance on geophysical exploration methods. The major challenge associated with understanding petrophysical data is not making the measurement, but rather understanding the results. The interpretation of the data is a cross disciplinary problem. Fundamentally it is necessary to understand the rock mineralogy and geochemistry to put the petrophysics in context with the geophysical results. Several case studies are presented where the petrophysics have determined not only which geophysical techniques to apply but whether a geophysical target has indeed been tested. Drill testing EM plate approximations for nickel sulphide and volcanogenic massive suphide (VHMS) ore deposits can benefit from inductive conductivity measurements on core as it can determine whether an EM conductor has been intersected. Chargeability highs associated with porphyry copper mineralisation is indicative of disseminated pyrite in the propylitic and pyrite +/- chalcopyrite +/- bornite in the potassic alteration zones and higher chargeability does not necessarily mean more copper. In most porphyry systems magnetite is coarse-grained, therefore a world class porphyry deposit should not have dominant remanent effects and the only likely source of remanence features in younger terrains are oxidised mafic intrusions and skarns. Furthermore, porosity and specific types of alteration (argillization) display the strongest correlations with resistivity and can be tied to gold distribution in Carlin Type Deposits.

Assessing Thallium Elemental Systematics and Isotope Ratio Variations in Porphyry Ore Systems: A Case Study of the Bingham Canyon District

The Bingham Canyon porphyry deposit is one of the world’s largest Cu-Mo-Au resources. Elevated concentrations of thallium (Tl) compared to average continental crust have been found in some brecciated and igneous samples in this area, which likely result from mobilization of Tl by relatively low temperature hydrothermal fluids. The Tl-enrichment at Bingham Canyon therefore provides an opportunity to investigate if Tl isotope ratios reflect hydrothermal enrichment and whether there are systematic Tl isotope fractionations that could provide an exploration tool. We present a reconnaissance study of nineteen samples spanning a range of lithologies from the Bingham district which were analysed for their Tl content and Tl isotope ratios, reported as parts per ten thousand (ε205Tl) relative to the NIST SRM997 international standard. The range of ε205Tl reported in this study (−16.4 to +7.2) is the largest observed in a hydrothermal ore deposit to date. Unbrecciated samples collected relatively proximal to the Bingham Canyon porphyry system have ε205Tl of −4.2 to +0.9, similar to observations in a previous study of porphyry deposits. This relatively narrow range suggests that high-temperature (&gt;300 °C) hydrothermal alteration does not result in significant Tl isotope fractionation. However, two samples ~3–4 km away from Bingham Canyon have higher ε205Tl values (+1.3 and +7.2), and samples from more distal (~7 km) disseminated gold deposits at Melco and Barneys Canyon display an even wider range in ε205Tl (−16.4 to +6.0). The observation of large positive and negative excursions in ε205Tl relative to the mantle value (ε205Tl = −2.0 ± 1.0) contrasts with previous investigations of hydrothermal systems. Samples displaying the most extreme positive and negative ε205Tl values also contain elevated concentrations of Tl-Sb-As. Furthermore, with the exception of one sample, all of the Tl isotopic anomalies occur in hydrothermal breccia samples. This suggests that ε205Tl excursions are most extreme during the migration of low-temperature hydrothermal fluids potentially related to sediment-hosted gold mineralization. Future investigation to determine the host phase(s) for Tl in breccias displaying both chalcophile element enrichment and ε205Tl excursions can potentially provide new information about hydrothermal fluid composition and could be used to locate sites for future porphyry exploration.

Feldspar-brine alteration at high-temperatures

A magmatic volatile phase (MVP) may be exsolved from a silicate melt through decompression during ascent and/or the crystallization of anhydrous mineral phases. MVPs are commonly chloride-rich fluids and are hypothesized to induce hydrothermal alteration in and around igneous intrusions. Feldspars are common alteration products in these systems with K-feldspar (potassic alteration zone) often found in areas directly above cupolas of the igneous intrusions. Albite-bearing alteration assemblages are found generally on the flanks of the intrusions, below the cupolas, or more distally. Mineral alteration reactions near the intrusion are likely influenced wholly or partially by a subcritical, vapor undersaturated high-salinity liquid phase (brine). Whereas the K-feldspar-MVP system is well studied, there is a dearth of data on albite-brine systems at elevated temperatures and pressures. This study addressed mineral phase stability and equilibrium brine compositions in the albite-andalusite-quartz-brine system (65 wt% NaCl equivalent; NaCl-HCl-H2O) at 600, 650, and 700 °C and 80 MPa. The stable mineral phases were ascertained through an examination of crystal morphology, optical microscopy, and by chemical compositions. The albite + andalusite + quartz + brine phase boundary was found to be at CNaClbrine/CHClbrine molality values of 87.5 ± 12.5, 67.5 ± 7.5, and 40 ± 10 at 600, 650, and 700 °C, respectively. Apparent equilibrium constants, not accounting for activity coefficients, for the reaction: albite + HCl = 0.5 andalusite + NaCl +2.5 quartz +0.5 H2O were calculated as 70 ± 10, 54 ± 6, and 32 ± 8 at 600, 650, and 700 °C, respectively. The alteration of albite to andalusite occurs at much lower HCl concentrations than is required to alter K-feldspar to andalusite for equivalent concentrations of chloride salts. Thus, HCl concentrations of brines within the potassic and/or sericitic alteration zones can be much higher than in albite-bearing alteration zones. The data presented in this study indicate that albite-rich mineral zones on the flanks of igneous intrusions are most likely produced by a low-acidity fluid with an elevated CNaClfluid/CKClfluid. Fluids that meet these specifications, and may be significant sources of sodic alteration, include non-magmatic fluids flowing towards the intrusion or a descending magmatic brine depleted in cations and metals by mineral alteration reactions near the cupola. Although the sodic alteration zone is unlikely to host significant concentrations of metals in porphyry ore systems, it can provide significant constraints on metasomatic reactions and fluid flow pathways. Sodic and sodic-calcic alteration sequences in iron-oxide‑copper‑gold deposits are likely produced by a low-acidity, saline fluid and, unlike porphyry systems, correlate with high metal contents which indicate that the precipitation mechanisms controlling ore distribution in these systems may not be as dependent on neutralization reactions as in potassic and sericitic alteration sequences.