Propylitic Zone Research Articles

Geology, alteration geochemistry, and exploration geochemical mapping of the Ertsberg Cu-Au-Mo district in Papua, Indonesia

The Ertsberg-Grasberg district hosts the world's largest Cu-Au(Mo) skarn system. The mineralization is related to the Pliocene Ertsberg- and Grasberg intrusive complexes, which intruded the Jurassic-Cretaceous Kembelangan Group siliciclastics and Cenozoic New Guinea Limestone Group. Despite previous studies, elemental migration across the porphyry-skarn system and across the different ore stages is still unclear. Here, we attempt to tackle this issue by integrating ore deposit geology, alteration/mineralization paragenesis, whole-rock geochemistry of the Ertsberg granitoids and carbonate wallrocks, together with spatial mapping of hydrothermal minerals and target/pathfinder trace elements at the Deep Mill Level Zone of Ertsberg. Ores at Ertsberg are mainly skarn-hosted, whose formation comprises five stages: (I) prograde skarn; (II) retrograde alteration; (III) massive anhydrite replacement; (IV) late hydrothermal veining; (V) supergene alteration. Massive magnetite and main CuAu mineralization occurred in Stage II and III, respectively, although minor Cu mineralization persisted to Stage IV.The Ertsberg granitoids have medium SiO2 and high Al2O3 contents, and consist mainly of (quartz-)monzonite and minor (monzo)diorite. Least-altered samples have upper crust-normalized enrichments in certain large ion lithophile elements (LILEs, e.g., U and K) and depletions in all high field strength elements (HFSEs, e.g., Ti, Y, and Yb). The samples are fresh to moderately-altered, featured by garnet-diopside skarn and/or porphyry-style (potassic, propylitic, phyllic) alterations and anhydrite replacement. Elemental mass balance calculation indicates that the porphyry-style alteration zones are featured by feldspar destruction and the accompanying alkali depletions, although the loss of KRb (in potassic and phyllic zones) and Ca-Ba-Sr (in propylitic zone) are balanced by their respective capturing in secondary K-feldspar-mica and actinolite-epidote-anhydrite. Decarbonation of the dolomitic wallrocks likely released Mg-Ca-Sr, which facilitated the subsequent forsterite-diopside exoskarn and anhydrite alteration. The enrichments of various ore-related target/pathfinder elements in these altered samples are consistent with their mineralized nature, e.g., FeMn (magnetite), Cu-Fe-Bi-Se-Te (bornite), MoRe (molybdenite), Au-Ag-As-Bi-Te (auriferous pyrite/arsenopyrite), Zn-Fe-Mn-Sn-In (sphalerite), and Pb-Ag-Bi (galena).Alteration mineral and target/pathfinder elemental distribution patterns show that the ore fluids were likely originated from the porphyry hydrothermal (potassic) centers near the Ertsberg pluton margin. The fluids then spread out from the potassic centers, forming the surrounding propylitic zone, and migrated along the largely NW-SE-oriented intrusive contact and fault zones. This formed the endoskarn in the Ertsberg pluton, the forsterite-diopside exoskarn in the Waripi Formation dolomite, and the garnet-diopside exoskarn in the Ekmai Formation limestone/calcareous sandstone-siltstone. As the hydrothermal system started to wane, the prograde skarn formation is partially overprinted/enveloped by retrograde serpentine and actinolite/tremolite-epidote-chlorite alterations, and then by magnetite mineralization and massive anhydrite replacement in the carbonate wallrocks. The texturally-destructive skarn formation, and the dissolution of carbonate wallrocks and hydrothermal anhydrite were likely pivotal in creating the highly-fractured Broken Zone, which served as a structural trap for the high-grade Cu-Au-Mo ore deposition.

New Insights into the Pulang Porphyry Copper Deposit in Southwest China: Indication of Alteration Minerals Detected Using ASTER and WorldView-3 Data

The Pulang porphyry copper deposit (PCD), one of the main potential areas for copper resource exploration in China, exhibits typical porphyry alteration zoning. However, further investigation of the indicative significance of alteration minerals, additional insight into metallogenic characteristics, and prospecting guidelines continue to be challenging. In this study, ASTER and WorldView-3 data were used to map hydrothermal alteration minerals by employing band ratios, principal component analysis, and spectrum-area techniques; and subsequently, the indication significance of alteration minerals was studied in-depth. The following new insights into the metallogenic structure and spatial distribution of alteration zoning in Pulang PCD were obtained and verified. (1) A new NE trending normal fault, passing through the northeast of Pulang PCD, was discovered. (2) Two mineralization alteration centers, exhibiting alteration zoning characteristics of potassic-silicified, phyllic, and propylitic zones from the inside to the outside, were observed on both sides of the fault. (3) At the junction of the redivided potassic-silicification and phyllic zones, favorable prospecting potential areas were delineated. This study shows that the spectral/multi-sensor satellite data are valuable and cost-effective tools for the preliminary stages of porphyry copper exploration in inaccessible and remote areas around the world.

Genesis of the Yanghuidongzi Cu deposit, NE China: Constraints from H-O-Pb isotopic compositions and geochronological study

• The Yanghuidongzi Cu deposit classified as a porphyry type Cu deposit. • H-O isotopes indicate a mixing of magmatic water and meteoric water for ore fluids. • Pb-Re isotopes indicated a deep seated magma mixed with crustal materials for ore metals. • The Yanghuidongzi Cu deposit formed during Early Jurassic . The newly-discovered Yanghuidongzi porphyry Cu deposit is located in the southeastern part of Heilongjiang Province in NE China and represents one of the rare Jurassic porphyry Cu deposits within the easternmost segment of the Central Asian Orogenic Belt (CAOB). The Cu mineralization occurs as veins and disseminations hosted by granodiorite porphyry, associated with typical porphyry copper deposit alteration zoning, which is characterized by from an inner K-silicate zone through to a quartz–sericite zone to an outer propylitic zone, the K-silicate alteration and quartz–sericite alteration are closely related to mineralization. LA-ICP-MS zircon U-Pb data, sericite Ar-Ar and Molybdenite Re-Os data, and H-O and Pb isotopic compositions are used to constrain the ore-forming age and genesis of the Yanghuidongzi Cu deposit. LA-ICP-MS U-Pb dating of zircons from three granodiorite porphyry samples yielded weighted mean 206 Pb/ 238 U ages of 194.3 ± 1.2 Ma, 192.0 ± 2.0 Ma, and 194.6 ± 2.1 Ma respectively, whereas zircon U-Pb age of granodiorite in the study area yielded weighted mean 206 Pb/ 238 U age of 199.5 ± 1.5 Ma that are interpreted to be the emplacement age of these intrusions. 40 Ar- 39 Ar dating of sericite from a quartz-pyrite vein and a quartz-polymetallic sulfide vein yielded plateau ages of 193.9 ± 2.1 Ma and 193.6 ± 1.9 Ma, respectively, whereas five molybdenite samples yielded a Re-Os isochron age of 194.8 ± 2.1 Ma, with a weighted mean model age of 193.6 ± 1.2 Ma. Mineralization ages are consistent with the emplaced ages of the granodiorite porphyry within the analytical uncertainties, indicating that the Cu mineralization is genetically related to the granodiorite porphyry. The fluid δ 18 O fluid and δ 18 D fluid values range from −4.22‰ to 6.5‰ and from −92.8‰ to −65.7‰, respectively, indicating that the ore fluids were mixtures of magmatic and meteoric water. The 206 Pb/ 204 Pb, 207 Pb/ 204 Pb, and 208 Pb/ 204 Pb values of the pyrite and chalcopyrite are 17.547–18.327, 15.446–15.505, and 37.408–38.113, respectively, suggesting a common, magmatic source for the sulfur. Taking Re (6.75–29.53 ppm) data into consideration, the sources of ore-forming materials may be from a mixture of crustal and mantle sources. Combining our new data with the Jurassic regional tectonic setting, the Yanghuidongzi Cu deposit and the related granitoids formed in a compressional tectonic setting are associated with the subduction of the Paleo-Pacific Plate beneath the Eurasian continent.

Geology Controlling Factors of the Top Reservoir; Muara Laboh Geothermal System Case Studies

Top of reservoir (TOR) determination is one of the resource keys features of the geothermal resource and significantly impacts the well’s productivity (Casing Point Determination). It is defined from the conceptual model process and refined by the well temperature data. This paper will describe the geologic controls on the top of reservoir geometries of the Muara Laboh (ML). ML has located in step over-pull apart basins of the Great Sumatra Fault Zone segment. The features then accommodate the emplacement of quaternary to tertiary volcanic and intrusion. The geoscientific data from the existing conceptual model, well cuttings, cores, MeB test, and image log data were combined with the drilling parameter data (mud temperature & mud loses) to characterize the geology of the TOR. Drilling results show that wells in the NE sector have a shallower TOR. The TOR is located near the quaternary Patah Sembilan volcanic unit’s contact with the undifferentiated silicic formation. The NE sector TOR corresponds with the change of alteration type from smectite rich argillic zone (<180°C) to the transitional zone (Chlorite, Chlorite-Illite, Chlorite-Smectite formed at 180°C to 240°C). The epidote – chlorite rich propylitic zone (>240°C) identified underlaying the NE reservoir sector’s transition zone. MeB Index below 10 seems to be corresponding with the base of conductive of 5 Ohm, and the NE TOR location. The SW sectors TOR sits a deeper elevation (∼300msl), lies within the intercalated dacite-andesite volcanic unit, higher temperature propylitic alteration zone, with epidote, epidote-adularia-quartz, and open space veins were observed below the TOR. Similar to the NE sector, the drop of the MeB index to less than ten, which is also related to the lithology contact, shows the possibility that the TOR has dropped in the SW sector. Below the low MeB zone, calcite infilled the fractures and sometimes encapsulated the pre-existing epidote vein; calcite then decreases with increasing epidote vein and epidote-adularia vein occurrence. The observation then suggests a rapid boiling process in the SW sectors due to the dropping of water levels, allowing the late-stage calcite vein to seal off the permeability within the shallow reservoir and not allowing the circulation of geothermal fluids within the interval.

Type of Gold Deposit in Arinem Cisewu and Its Surrounding Garut Regency, West Java, Indonesia

Gold deposit based on the mineralization of gold and other ore minerals in Arinem area and its surrounding was found in the quartz vein. The research area is in Arinem area and its surrounding, Garut Regency, West Java. Arinem and the vicinity was the location for early stage of research program of mineralization in Papandayan area. This area has good mineralization, and according to previous studies, is a low sulfidation area with the presence of pyrite, chalcopyrite, galena and sphalerite (Antam, 2014). The presence of galena and sphalerite mineral has suggested that instead of low sulfidation, the area may be in intermediate sulfidation zone state supported by other data such as quartz breccia, banded quartz and shear as control structure. Lithology in the area consists of breccia Quaternary and lava unit of Jampang formation in Miocene and unit of andesite and basaltic in Quaternary and intrusion of dacite, andesite and diorite. The alteration and mineralization of research area was classified as propylitic alteration zone with the presence of chlorite, epidote and calcite mineral, argillic alteration zone with montmorillonite, kaolinite mineral, silicic alteration zone with the presence of quartz-sericite, a bit of calcite mineral. The observed ore minerals are pyrite, chalcopyrite, galena, sphalerite. The geological structure was controlled by horizontal fault in almost north-south direction of N 170O-180O E and northeast – southwest horizontal fault in N 40O – 50O E, while the mineralization zone was controlled or following fault pattern of N 170O - 175O E. The epithermal deposit of the study area can be classified as intermediate sulfidation epithermal deposit for zones with galena and sphalerite mineral particularly on several locations near fault zones and brecciated quartz. Meanwhile, the study area is generally low sulfidation epithermal type deposit.

Mapping hydrothermal alteration zone through aster data in Gadag Schist Belt of Western Dharwar Craton of Karnataka, India

Alteration zones are developed in a wide range of geological milieus such as volcano-plutonic rocks. Hydrothermal alteration zones are considered an important guide for mineral exploration and can act as a potential zone of mineralization. The minerals in alteration zones have very diagnostic absorption features in visible–near infrared (VNIR) and shortwave infrared (SWIR) regions of the electromagnetic spectrum. The Advanced Space-borne Thermal Emission and Reflection Radiometer (ASTER) data have the potential to identify mineral assemblages in different hydrothermal alteration zones because of its 14 spectral bands which cover a wide spectral range. In the present article, FCC, band ratio, and SPCA techniques were applied to map the hydrothermal alteration zones in Gadag Schist Belt (GSB) of Western Dharwar Craton (WDC) of Karnataka. FCC has highlighted lithology, alteration zones, and given a fair idea about the lithological control and alteration occurrences in the area. Band ratio technique for muscovite, kaolinite, chlorite, iron oxide gossan, and silica alteration have yielded promising results. SPCA analysis successfully mapped hydrothermal alteration and delineated phyllic, argillic, and propylitic zones in the area. These results were verified through the existing geological map and by geological fieldwork in the area and these results are satisfactory. The total area, altered by different alteration process, was also calculated. In the GSB, iron alteration zone, phyllic, argillic, propylitic, and silica alteration zones were effectively mapped with the help of ASTER data.

A fluid inclusion and stable isotope (O, H, S and C) study of the Archean IOCG Cristalino deposit, Carajás mineral Province, Brazil: Implications to ore genesis

The hydrothermal history of the Archean Cu-Au Cristalino deposit is recorded in moderately to intensely altered host rocks, which evolved from ~550 °C to ~150 °C. Distal sodic and proximal calcic-ferric, potassic and propylitic alterations developed successively with variable intensity. Two main mineralization types are also recognized: an earlier one characterized by a chalcopyrite + pyrite + magnetite + Au association that occurs as disseminations and breccias in calcic-ferric zones, and a later one represented by a chalcopyrite ± pyrite ± hematite ± Au association that occurs as breccias and veins mainly in potassic and propylitic zones. Estimated pressures (0.6 to 2.6 Kbar) are consonant with brittle and ductile-brittle deformational environments where both alteration and mineralization took place. The ore-bearing fluid was hot (T as high as ~550 °C), acidic and hypersaline (55.1% wt. NaCl equiv.) and chemically approached by the H2O-NaCl-CaCl2-CO2 ± MgCl2 ± FeCl2 system. Salinity might have exceeded 55 wt% NaCl equiv. in the early alteration stages but decreased progressively to 7.9 wt% NaCl equiv. from 250 °C onwards, due to incursion of surficial water into the deposit site. The fluid was initially 18O-enriched and D-depleted (δ18Ov-smow = +9.73 to +6.48‰; δDv-smow = −30.17 to −40.25‰) and most likely derived from magmatic sources. As result of dilution caused by mixing with meteoric water, the fluid became relatively 18O-depleted and D-enriched (δ18Ov-smow = +5.57 to −0.28‰; δDv-smow = −19.15 to −22.24‰). The δ13Cv-pdb values for vein and breccia calcite (−6.5 to −3.8‰) are consistent with a deep source for CO2, likely of mantellic origin. Most δ34Sv-cdt values for chalcopyrite show narrow variation (+1.6 to +3.5‰) and could indicate a homogeneous reservoir for sulfur. However, a few samples reveal significant influence of sedimentary rocks on their isotope composition (δ34Sv-cdt = −32.6‰), showing that sulfur may have been sourced from various reservoirs. It is inferred that most Cu and Au were transported as chloride complexes (>350 °C), and precipitated in response to decrease in temperature and Cl− activity, and increase in pH. The formation of the chalcopyrite ± pyrite ± hematite ± Au association was especially favored by an oxygen fugacity increase during the late stages of the hydrothermal alteration. Secondary fluid inclusions reveal that an aqueous, colder (200–150 °C) and relatively less saline (21–3.1 wt% NaCl equiv.) fluid of uncertain origin circulated in the Cristalino deposit area, which is likely related to a nearby Paleoproterozoic granitic intrusion. In comparison with other Archean Carajás IOCG deposits, particularly those that lie in the southern sector of the Carajás Domain, the Cristalino deposit shows similar structural styles, hydrothermal assemblages, and ore fluid composition and evolution. The data presented here support previous interpretations that consider Cristalino as of IOCG typology.

Landsat-7 and ASTER remote sensing satellite imagery for identification of iron skarn mineralization in metamorphic regions

The use of satellite remote sensing imagery for mineral exploration is a fast and low-cost approach for indicating high potential zones. Exploration of iron skarn mineralization in metamorphic regions is challenging during the fieldwork campaign. In this study, Landsat-7 and Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) satellite imagery were used for mapping alteration zones and geological structures associated with iron skarn mineralization in the Galali region, NW, Iran. Band ratio and Directed Principal Component Analysis (DPCA) and Automatic Lineament Extraction methods were applied to Landsat-7 data. Relative Absorption Band Depth (RBD) and Constrained Energy Minimization (CEM) algorithms were implemented to ASTER data. Fieldwork and laboratory analysis were conducted to verify the remote sensing results. Results indicate that propylitic alteration zone and intersection of N–S and NW–SE and curvilinear features are typically associated with iron skarn mineralization and some high potential zones are identified for future exploration projects.

Shallow structure of Los Humeros (LH) caldera and geothermal reservoir from magnetotellurics and potential field data

SUMMARYThis study focuses in the analysis of the internal structure of the upper 3 km of Los Humeros (LH) caldera and the relation of electrical and hydrothermal anomalies. For this purpose, we measured, processed and interpreted 78 broad-band magnetotelluric (MT) soundings. We performed a 3-D inversion of the data set (ModEM) using all MT soundings, although only half of the available frequencies per sounding due to limited computed power. We also carried out the 2-D inversions (NLCG) of the invariant determinant along two orthogonal profiles (EW and NS) crossing the caldera structure; their comparison yields similar resistivity and structural models results. The resistivity modelling is complemented with the results of a joint 3-D inversion of an accurate gravity database of 720 stations, and total field aeromagnetic data (SGM) from the caldera crater. The combined results provide novel details about the structure of the shallow geothermal reservoir of the resurgence caldera complex hosting the active hydrothermal system. Density and resistivity models show the existence of a composed crater basin structure separated by an EW high-density structure; the northern basin is associated to the LH crater, whereas the southern basin associates to the emergent Los Potreros (LP) caldera basin. The magnetization model indicates that there is a common source for the magnetic volcanic products observed at the caldera surface, and that the LP fault is the more magnetized fault of the geothermal system. The propylic zoning under the geothermal field, which according to the MT model results has resistivities above ∼100 Ω-m, was extrapolated using this and additional criteria to obtain the distribution of other hypothetical propylitic zones of hydrothermal potential.

Spaceborne mapping of hydrothermal alteration zones associated with the Mundiyawas-Khera copper deposit, Rajasthan, India, using SWIR bands of ASTER: Implications for exploration targeting

This paper discusses the effectiveness of spectroscopic techniques for identifying hydrothermal alteration zones associated with the Mundiyawas-Khera copper deposit, Alwar Basin, western India, and assesses implications for mineral exploration targeting in the greater Alwar basin. Mineral specific SWIR bands of ASTER were spectrally enhanced to highlight areas of anomalous Al-OH and Mg-OH responses, representing key mappable expressions of the Mundiyawas-Khera copper deposit. The hydroxyl-bearing mineral zones were identified at 98% and 95% threshold pixels, respectively. Kaolinite ± scapolite, sericite (muscovite), tremolite and dolomite, contained in the felsic volcanic and dolomitic copper host rocks, were identified from spectroscopic data and corroborated with petrographic and XRD data. On this basis, distinct argillic, phyllic and propylitic hydrothermal alteration zones could be identified at Mundiyawas-Khera. The integrated approach presented here (i) identified a well defined phyllic alteration pattern in the felsic volcanic rocks, (ii) resulted in a comprehensive remotely sensed hydrothermal alteration map illustrating the spatial distribution of the copper-related hydrothermal alteration zones (i.e., an eastern argillic, a central phyllic, and a western propylitic zone), (iii) helped to delineate potential copper-rich zones coincident with a known lithostructural trend, and (iv) has implications for copper exploration in the greater Alwar basin.

Mineralogy and mineral geochemistry of the Tuwu porphyry Cu deposit, Eastern Tianshan, NW China: implication for the ore-forming condition and Cu mineralization

The Tuwu deposit is one of the largest porphyry copper deposits in Eastern Tianshan, NW China. The widespread hydrothermal alteration is associated with porphyries and their wall-rocks in the porphyry deposit can be divided into potassic, chlorite–sericite, phyllic, and propylitic zones. In this study, we conducted a detailed study on the mineralogy of hydrothermal magnetite, mica, and epidote. Magnetite is closely related to the potassic alteration, and evolves from earlier disseminated magnetite (Mt-I) to the later magnetite–quartz ± biotite ± pyrite ± bornite ± chalcopyrite veins (Mt-II). The significant decrease in the Cr2O3 and V2O3 contents of magnetite from early to late period indicates the gradual increase of oxygen fugacity. The crystallization of magnetite supplies enough reduced sulfur that promotes the sulfide precipitation. The dominance of muscovite (Mc-I) in the early phyllic stage and the increasing of phengite muscovite (Mc-II) in the late phyllic stage suggest the evolution of hydrothermal fluid from a high-temperature and acidic environment to a low-temperature and less acidic environment. This process also results in the decrease of sulfide mineral deposition in the late phyllic stage. Epidote from propylitic zones at Tuwu has similar characteristic with the overprinting epidote in the Yandong porphyry Cu systems. Propylitic Ep-I have higher FeO content than Ep-II. The interaction of Fe-rich propylitic epidote with hydrothermal fluid enhanced both Ca2+ activity and pH value, and favored the precipitation of sulfides (e.g., chalcopyrite). In addition, the Fe-rich and less acidic fluids, which derived from the propylitic zones and flow though the sericitic rocks in the phyllic zones, also promote the deposition of Cu mineralization. Our results, combined with previous research on fluid inclusion, isotopes, and mass transfer, suggest that the changes in temperature, oxygen fugacity, and pH caused by the fluid–rock interaction played a critical role for the Cu mineralization at Tuwu.

Highly differentiated juvenile crust-derived magmas linked with the Xilekuduke porphyry Mo (Cu) deposit in East Junggar, NW China

The East Junggar region links the Kazakhstan-Junggar plate with the Siberian Craton along the western part of the Central Asian Orogenic Belt. Among the major ore deposits in this region, the Xilekuduke molybdenite deposit is a unique example of Mo-dominated porphyry. Molybdenum and copper mineralization here occur as disseminated sulfides or veinlets mainly in granite porphyry and monzogranite, with a prominent alteration from an inner potassic, through sericitic to an outer propylitic zone. Zircon LA-ICP-MS U–Pb ages indicate that the granite porphyry and monzogranite were emplaced at 326.4 ± 2 Ma and 328.2 ± 1.8 Ma respectively. Both rocks show enrichment in light rare earth elements (LREE), and Rb, K, Sr, Hf, Zr, but depletion in Nb, Ta, Ti and P, suggesting magma generation in a post-collisional setting. A relatively high oxidation state is inferred from zircon δCe values and oxygen fugacity of parental magma. The ore hosted intrusions also show high Differentiation Index (DI), indicating a highly oxidized parental magma and prolonged differentiation process. The range of zircon εHf (t) values (+11.5 to +17), young zircon Hf model ages, and consistent but slightly low zircon δ18O values (+4.84 to +5.20), together with the range of Mg# (35–53), suggest derivation of the parental magma through partial melting of juvenile crust with minor contribution from mantle components and sediments. We propose that melting of juvenile oceanic components at depth with high degree of differentiation played a significant role in generating the Mo mineralization in Xilekuduke deposit within a post-collisional setting in the East Junggar domain.

An overview of the Muara Laboh geothermal system, Sumatra

The Muara Laboh geothermal system is a liquid-dominated, fracture-controlled reservoir showing some characteristics of both intrusion-related and fault circulation systems. The developed reservoir has moderate to high-temperature (230–310 °C) with a long NNW outflow (160–230 °C) extending to the Sapan Malulong boiling chloride springs. Reservoir fluids generally have low salinity (∼400-1600 ppm Cl) with benign chemistry and low non-condensable gas (NCG) content (∼0.5 to 2 wt% in steam). The proven reservoir is divided into distinct SW and NE sectors. The principal deep upflow zone (270 to 310 °C) is located in the SW and is associated with Patah Sembilan volcano and satellite vents (Anak Patah Sembilan). The SW upflow is characterized by relatively low salinity (∼400-600 ppm Cl), whereas fluids in the NE emerge from a second upflow along faults and fractures at 240–250 °C (1200–1600 ppm Cl) and ascend to an initial-state steam cap near the Idung Mancung fumarole. The shallow NE part of the geothermal system is hosted mainly by Quaternary to Miocene Age andesitic to rhyolitic rocks, whereas the deeper SW part of the system occurs in a Mesozoic to Cenozoic Age plutonic complex and its host rocks that are cut by younger dikes. Rock porosity and permeability trends can be related to reservoir age, volcanic deposit type, and alteration history. Fracture permeability is high, whereas matrix porosity (<1-7%) and permeability are moderate to low and decrease with depth. The volcanic sequence is capped by tuffs dated at ∼34,000 to 41,000 years BP, incapsulated in debris flows. This sequence may have formed during and shortly after likely sector collapse episodes of the Patah Sembilan volcano. The shallow clay cap of the system wraps around the crater, suggesting that it was excavated during crater formation.Permeable fracture patterns and their origins differ somewhat between the deep SW and the shallow NE reservoir. In the deep SW reservoir, fluids ascend along fractured margins of stock and dike intrusions. Steeply dipping intrusions and open fractures strike from WNW to NNW, subparallel to the main Great Sumatra Fault trend, with a secondary set of N to NE fractures and intrusions parallel with the inferred maximum horizontal stress direction (SHMax). In the shallower NE steam cap and outflow, N to NE steeply dipping fractures dominate, with bounding N to NNW faults. Bulk rock alteration includes shallow argillic and transitional clay zones overlying phyllic and propylitic zones. High-T propylitic alteration (secondary amphibole) and rare potassic alteration (secondary biotite) are associated with intrusion. Vein paragenesis in the SW reservoir indicates that portions of the initially permeable propylitic zone have been sealed by calcite and quartz ± prehnite. Late calcite veins suggest ingress of steam-heated bicarbonate waters contributed to resealing of the system. These relationships suggest that the SW system underwent extensive boiling, fluid loss, and inflow of dilute bicarbonate waters, possibly related to sector collapse and partial excavation of the topseal of the system (now Patah Sembilan crater).

The Anatomy of an Alkalic Porphyry Cu-Au System: Geology and Alteration at Northparkes Mines, New South Wales, Australia

The Late Ordovician-early Silurian (∼455–435 Ma) Northparkes system is a group of silica-saturated, alkalic porphyry deposits and prospects that developed within the Macquarie island arc. The system is host to a spectacular and diverse range of rocks and alteration-mineralization textures that facilitate a detailed understanding of its evolution, in particular the nature and controls of porphyry-related propylitic alteration. The first intrusive phase at Northparkes is a pre- to early-mineralization pluton that underlies all the deposits and varies in composition from a biotite quartz monzonite to alkali feldspar granite. Prior to total crystallization, this pluton was intruded by a more primitive quartz monzonite that marks the onset of a fertile fractionation series. Toward its upper levels, the quartz monzonite is porphyritic and locally rich in Cu sulfides. Subsequently, a complex series of synmineralization quartz monzonite porphyries was emplaced. The quartz monzonite porphyry intrusions have a distinct pipe-like morphology and are ubiquitously K-feldspar–altered with a crystal-crowded porphyritic texture. The textures of the quartz monzonite porphyries and common occurrence of porphyry-cemented contact breccias indicate they were forcibly emplaced and of relatively low viscosity. The quartz monzonite porphyries are therefore interpreted as crystal-bearing, silicate melt-aqueous fluid slurries that represent the conduits through which deep-seated magmatic-derived ore fluid was discharged into the shallow crust (1–2 km depth). Each deposit is centered on a multiphase cluster of quartz monzonite porphyry intrusions that drove discrete hydrothermal systems. Initial fluid evolution was similar in all the deposits, with three major alteration facies developed as largely concentric zones around the quartz monzonite porphyry complexes. The innermost zone is host to Cu sulfide ore and dominated by K-feldspar alteration. This transitions outward through a shell of magnetite ± biotite alteration, with pyrite and minor chalcopyrite, to an outer halo of propylitic alteration. Generally, epidote, chlorite, and pyrite are abundant in the most deposit-proximal propylitic zone, with a decrease in the abundance of pyrite, and then epidote, with increasing distance away from deposit centers. Propylitic alteration, particularly within relatively low permeability rocks, is fracture-controlled and a hierarchy of veins is observed. Veins of chlorite-quartz-pyrite ± calcite ± hematite ± epidote ± chalcopyrite (P1) appear to represent the principal fluid conduits. They are surrounded by pervasive and intense alteration halos with a distinct mineralogical zonation from vein-proximal chlorite-sericite (phengite) ± epidote ± pyrite, through hematite-sericite-chlorite ± epidote, ultimately to a vein-distal hematite-albite ± chlorite ± epidote assemblage. These P1 veins are surrounded by regions in which smaller epidote-chlorite ± calcite ± quartz ± pyrite veins (P2) are abundant, again with zoned alteration envelopes: vein-proximal chlorite-sericite (phengite) ± epidote ± pyrite grades out into an epidote-rich zone, which in turn transitions into vein-distal albite-hematite ± chlorite ± epidote. Areas of weakest propylitic alteration, distant from both P1 and P2 veins, are characterized by small epidote-only veinlets (P3) with albite-hematite halos. Mineralogical transitions across the propylitic zone are therefore repeated in the evolution from P1 to P3 veins, as well as in the halos around these veins. It is the overall vein abundance and overlap of associated alteration halos that controls the intensity and appearance of propylitic alteration in most rocks. Such scale invariance and spatial relationships strongly suggest the transition from P1 to P3 veins reflects a broadly decreasing outward flux of (magmatic-derived?) fluid that passed through the fracture network and progressively reacted with country rocks. Further support for this hypothesis comes from crosscutting relationships and Rb-Sr dating of epidote (returning an age of 450 ± 11 Ma), which demonstrate the bulk of propylitic alteration was coeval with mineralization and potassic alteration. Late-stage fluid evolution at each deposit was unique. Much of the E48 orebody, and locally the GRP314 deposit, was overprinted by texturally destructive, white sericite-albite-quartz-alunite ± chlorite alteration. In the E26 deposit and in regions of the GRP314 deposit a series of quartz-anhydrite ± pyrite ± Cu sulfide veins with distinctive, vein-proximal, sericite-dominant alteration halos cuts the primary, deposit-concentric alteration facies. The vein-distal mineralogy of these alteration halos is controlled by their distance from deposit centers, changing from K-feldspar ± biotite in deposit-proximal veins to chlorite ± epidote-albite in depositdistal veins. Late-mineralization quartz monzonite porphyries at E26 and GRP314 also appear to be related to the generation of anhydrite-quartz ± sphalerite veins and a set of quartz-calcite-pyrite-sphalerite ± chalcopyrite ± galena veins. Postmineralization magmatic activity produced relatively primitive and barren monzonite porphyries and younger alkali basalt dikes.

Identification of alteration zones using a Landsat 8 image of densely vegetated areas of the Wayang Windu Geothermal field, West Java, Indonesia

Remote sensing is a method usually used for conducting geothermal reconnaissance surveys by mapping surface alteration anomalies, which are captured by a satellite sensor. This study analyzes the characteristics of the surface alteration zones in the Wayang Windu geothermal field (WWGF). It employs a remote sensing method using multispectral Landsat 8 imagery and is validated by ground truth data from field surveys. The WWGF is located in Pangalengan, West Java with an elevation of 1500-2600 m.a.s.l., and lies in a quaternary volcanic arc. The rock types in Wayang Windu consist of andesite, basalt, tuff, breccia, and pumice. Fractures and faults are identified as lineaments in this area and based on their structures, were directed to be oriented northwest-southeast and northeast-southwest. This research combined a field survey and remote sensing methods to enhance the spatial data. Field surveys yield 18 spots for obtaining soil samples and laboratory analyses were performed. Spectral reflectance analysis was performed to determine the reflectance and mineral composition of the samples, X-ray diffraction was performed to determine the mineral composition, and X-ray fluorescence was performed to determine the abundance of elements. A scene from Landsat 8 image acquired on September 10, 2013 was evaluated using a principal component analysis-based method. The surface alteration zones in the WWGF correlated with joints and faults. Based on the mineral composition, the surface alteration zones in WWGF were identified as advanced argillic zone with the occurrence of secondary minerals such as cristobalite and halloysite, and a propylitic zone with the occurrence of secondary minerals such as epidote and chlorite.

The layered model of the Copahue geothermal reservoir, Argentina

An exhaustive review of the available data related to the reservoir of the volcano-hosted Copahue geothermal system is presented. Reservoir interpretation to date suggests a vapor-dominated nature. However, the review provided reveals the limitations of this model, regarding the transition from shallow argillic to deep propylitic alteration zones, gas geothermometers calibrated to near-neutral pH environment, production tests, and low recharge of the feed zone, among the other features. We propose that a layered model with a shallow vapor zone above a fluid pressurized reservoir better accounts for the known features of the geothermal reservoir. Temperature profiles define the 800-m-thick upper thermal boundary that includes the impermeable clay cap. An argillic alteration layer (smectite + chlorite + illite + kaolinite) with low electric resistivity response, consistent with the presence of smectite, composes the clay cap. A shallow steam cap develops immediately below the impermeable layer. This vapor zone has 200–215 °C, low seismic activity, and high electrical resistivity, and is highlighted by the isothermal segment of the well logs. The bottom part of the reservoir is subjected to controversy because of the lack of direct measurements. The deep liquid-dominated reservoir has a temperature of ~ 280 °C, develops below 1500 m depth under near-neutral pH condition, and probably has a pervasive propylitic alteration, and wairakite + laumontite + chlorite + epidote + prehnite + actinolite is the likely mineral assemblage. Pressure collapse might trigger the development of the shallow steam cap during deflation–inflation volcanic cycles or Pleistocene-to-recent tectonic deformation. This work points out the main weak points of the previous conceptual model, providing an alternative one based on the first comprehensive compilation of data of the Copahue reservoir. Further research, including a deep exploration drilling stage, is required to establish the physicochemical reservoir state precisely and to validate the proposed layered model.

Geochemistry and mineral chemical behavior of hydrothermal alteration of the Tuwu porphyry copper deposit, Eastern Tianshan, Northwest China

The Tuwu deposit is one of the largest porphyry copper deposits in the Eastern Tianshan, Northwest China. Ore bodies of the deposit are associated with the tonalite porphyry, which intruded the Early Carboniferous Qi´eshan Group. The hydrothermal ore‐forming processes include the porphyry mineralization episode with potassic, chlorite‐sericite/albite, phyllic, and propylitic alteration and the overprinting mineralization episode. These hydrothermal alteration episodes were studied on the basis of mass transfer and element mobility. The mass balance data and isocon diagrams reveal that Cu is highly associated with the chlorite‐sericite (albite) stage and followed by phyllic and potassic stages. On the basis of electron probe analysis, the SO3 contents of the igneous apatite from the tonalite porphyries range from 0.016 to 0.288 wt.%, which are higher than those from the diorite porphyries (SO3 = 0.088–0.179 wt.%). Similarly, the apatite saturation temperature (861–926°C) and magmatic S content (mean = 52.16 ppm) from the tonalite porphyries are also higher that from the diorite porphyries (T = 813–877°C; S = 28.25 ppm). These results suggest that the S content of the apatite is related to the temperature and S content of the melt. In the hydrothermal alteration zones, the F favours partitioning into apatite under lower temperature, the F contents of hydrothermal apatite increase from the potassic zones to the propylitic zones accompanying with the decrease of temperature. Cl element is much more sensitive to the pH, pressure, and fluid composition than F element. The Cl contents of the apatite increase from the potassic zones (0.18–0.46 wt.%) to the chorite‐sericite (albite) zones (0.28–0.58 wt.%), resulting from the decrease of fluid pH and pressure. The lower Cl contents of apatite (0.07–0.48 wt.%) in the phyllic zones were due to the lower Cl content of hydrothermal fluids. The highest Cl contents (0.09–0.69 wt.%) of apatite in the propylitic zones were caused by the decrease of pressure. The δ18Owater values of quartz in chlorite‐sericite (albite) zones and phyllic zones range from −0.8‰ to −0.4‰ and from −5.3‰ to 4.8‰, and the δD values vary from −57‰ to −47‰ and from −52‰ to −44‰, respectively. Gaseous compositions of the fluid inclusions in quartz are dominated by H2O with minor CO2, and the liquid compositions mainly consist of Na+ and Cl−. These data imply that the ore‐forming fluids belong to the H2O‐NaCl system and evolved from magmatic water to meteoric water in origin.

Element transport and enrichment during propylitic alteration in Paleozoic porphyry Cu mineralization systems: Insights from chlorite chemistry

Chlorite is one of the most widely developed minerals in propylitic alteration around porphyry deposits and can be used to investigate element transport and enrichment processes during hydrothermal alteration. The Paleozoic Tuwu porphyry Cu deposit (metal reserve: 0.7 Mt Cu @ 0.67%), located in the southern margin of the Central Asian Orogenic Belt, is one of the most economical and largest porphyry Cu deposits in NW China. The Tuwu porphyry Cu deposit comprises an early porphyry Cu mineralization (including potassic, propylitic and phyllic alteration stages) and later overprinting Cu mineralization periods with chlorite found in both.Chlorite from the propylitic zone at Tuwu is geochemically similar to propylitic chlorite in younger Cenozoic porphyry systems, but enriched in Mg and K and depleted in Fe and Al relative to the overprinting chlorite. Scandium, V, Ti and Ga are enriched, and Li, Sr, Mn and Zn are depleted in the Tuwu propylitic chlorite proximal to the orebody, consistent with Cenozoic deposits. Compared with metamorphic chlorite from Proterozoic metamorphic terranes in Australia, Tuwu propylitic chlorite has higher Mg and K, lower Fe, Al and As. Our results suggest propylitic alteration in the Tuwu district is probably the product of cooling of fluids derived from the magmatic-hydrothermal system, but not from peripheral waters, and trace element variations in chlorite are mainly controlled by temperature although other factors such as fault development could affect variation trends. The common elemental features of chlorite in both Paleozoic and Cenozoic porphyry Cu deposits indicate it could act as a potential tool for mineral exploration.

Geology and geochronology of Naruo large porphyry-breccia Cu deposit in the Duolong district, Tibet

The Duolong district is located in the south Qiangtang terrane of Tibet and is the most significant ore cluster within the Bangongco-Nujiang metallogenic belt. Duolong contains one giant, three large and two medium to small-sized porphyry (±epithermal ± breccia) copper deposits and several other mineralized porphyry bodies. All deposits are closely associated with early Cretaceous (123–115 Ma) intermediate-felsic intrusions. Naruo is a poorly studied porphyry-breccia copper deposit in the north of the Duolong district. Hydrothermal alteration surrounding the ore-bearing granodiorite at Naruo is characterized by an inner potassic zone and an outer propylitic zone, overlapped locally by minor phyllic and argillic alteration assemblages. A detailed paragenetic study has identified five distinct hydrothermal veins (M, A, B, C, D) within the porphyry system. Hydrothermal B veins are strongly related to copper mineralization. Strong propylitic alteration is also observed throughout the hydrothermal breccias identified at Naruo. Sandstone breccia, diorite-bearing breccia and granodiorite-bearing breccia were identified according to the distribution and composition of clasts. U-Pb zircon dating has determined the ages of the ore-bearing granodiorite (121.6 ± 1.3 Ma) and a barren intrusion (115.5 ± 1.1 Ma) within the porphyry system, diorite clasts (122.3 ± 0.9 Ma) and later diorite matrix (120.5 ± 1.0 Ma) in the hydrothermal breccia system, suggesting that with the exception of the late barren intrusion, they all belong to the same mineralizing event at Duolong. The geological and geochemical evidence presented in this study suggest that the porphyry and breccia systems may have originated from the same magma source, but are now spatially independent.

Paragonite-chloritoid alteration in the Trafalgar Fault and Fimiston- and Oroya-style gold lodes in the Paringa South mine, Golden Mile, Kalgoorlie: 2. Muscovite-pyrite and silica-chlorite-telluride ore deposited by two superimposed hydrothermal systems

The Golden Mile deposit (1751 t Au to 2017) in the Archean Eastern Goldfields Province, Yilgarn Craton, Western Australia, is located in a km-scale envelope of chlorite-calcite-albite alteration overprinting metamorphosed tholeiitic rocks. The propylitic zone (310–340 °C) is centered on D2 strike-slip faults segmenting D1 folds. At the southeast margin of the deposit, 200 m wide hydrolytic paragonite-ankerite-chloritoid alteration ( 1.5 km northwest to the Paringa South mine. In the mine, D2 faults control Fimiston ore locally overprinted by Oroya ore, whereas D3 reverse faults offsetting the D2-system control only Oroya ore. Representative ore bodies (lodes) were studied by petrographic, geochemical, and thermometric methods. Fimiston ore (4–20 g/t Au) is refractory, characterized by 10% pyrite ± chalcopyrite ± tennantite, and varies in oxidation state (magnetite stable to hematite stable). Gold and tellurides form inclusions in pyrite. In propylitic GMD, lode alteration is zoned from albite stable in veins and selvages (400 °C) to outer hydrolytic phengite-quartz-ankerite replacement (370 °C). The younger Oroya ore in D3 faults contains free gold and is oxidized (5 g/t Au; pyrite-hematite) or reduced (50–120 g/t Au; pyrite-pyrrhotite) close to carbonaceous black schist. Oroya Stage 1 silica-chlorite-siderite replacement (400–425 °C) and As-rich pyrite-arsenopyrite-sphalerite mineralization is crosscut by Stage 2 silica-ankerite fill (385 °C) and Sb-rich pyrite-tetrahedrite-telluride mineralization associated with vanadian muscovite. Mass-balance calculations indicate that the Oroya fluid carried higher concentrations of silica, Fe, V, Ni, and Te than the older Fimiston fluid.