Advanced Argillic Alteration Assemblages Research Articles

Advanced argillic alteration at Cave di Caolino, Lipari, Aeolian Islands (Italy): Implications for the mitigation of volcanic risks and the exploitation of geothermal resources

Four sites in the western sector of Lipari Island with still active hydrothermal activity are here considered. The petrography (mesoscopic observations and XRPD) and geochemistry (major, minor and trace elements chemistry) of ten representative and extremely altered volcanic rocks were characterized. Two types of parageneses of altered rocks are discriminable, one rich in silicate phases (opal/cristobalite, montmorillonite, kaolinite, alunite and hematite) and one in sulphates (gypsum, plus minor amounts of anhydrite or bassanite). The altered silicate-rich rocks are rich in SiO2, Al2O3, Fe2O3 and H2O, and depleted in CaO, MgO, K2O and Na2O, while the sulphate-rich ones are extremely enriched in CaO and SO4 in comparison with local unaltered volcanic rocks. The content of many incompatible elements is similar in altered silicate-rich rocks and lower in sulphate-rich ones with respect to the pristine volcanic rocks; conversely, almost all REEs are markedly enriched in silicate-rich rocks and heavy REEs are enriched in sulphate-rich altered rocks compared to unaltered volcanic rocks.Reaction path modelling of basaltic andesite dissolution in local steam condensate predicts the production of amorphous-silica, anhydrite, goethite, and kaolinite (or smectites and saponites) as stable secondary minerals and alunite, jarosite, and jurbanite as ephemeral minerals. Considering possible post-depositional reactions and admitting that the presence of two distinct parageneses is apparent, since gypsum is prone to form large crystals, it can be concluded that there is an excellent agreement between the alteration minerals occurring in nature and those predicted by geochemical modelling. Consequently, the modelled process is the main responsible for the production of the advanced argillic alteration assemblage of “Cave di Caolino” on Lipari Island. Since rock alteration is sustained by the H2SO4 solution produced by hydrothermal steam condensation, there is no need to invoke the involvement of SO2-HCl-HF-bearing magmatic fluids, in line with the absence of fluoride minerals.

Alunite supergroup minerals from advanced argillic alteration assemblage in the southern Atacama Desert as indicators of paleo-hydrothermal and supergene environments

Alunite supergroup minerals from advanced argillic alteration assemblage in the southern Atacama Desert as indicators of paleo-hydrothermal and supergene environments

Distribution of hypogene alteration and fluid evolution in the Los Humeros Geothermal Field (Puebla, Mexico): Multiple sourced fluids, interrelations, and processes in a superhot system

The Los Humeros Geothermal Field (LHGF) is one of the highest producing geothermal areas in Mexico. This paper aims essentially to determine high-resolution patterns of distribution of hypogene alteration assemblages and the distribution of temperatures in space and time of geothermal fluids in the LHGF. The zoning of hypogene alteration assemblages in the LHGF consists of (1) shallow advanced argillic alteration assemblages that grade into (2) argillic and (3) zeolitic assemblages with depth, followed by (4) low-temperature and (5) high-temperature propylitic, and (6) deep high-temperature high-silica advanced argillic alteration assemblages. The distribution of deep advanced argillic alteration assemblages is associated with the occurrence of acidic supercritical fluids prior to their deep boiling. Capping these, high-temperature propylitic assemblages are associated with the occurrence of subcritic brines (barely exceeding 13 wt% NaCl equiv.) that stemmed from magmatic supercritic fluids (upon degassing and/or conductive cooling) and that were diluted by deeply circulated and thermally equilibrated meteoric water. Conductively cooled subcritic fluids, which eventually kept being diluted on their way to the surface, would be likely associated with the generation of shallow low-temperature propylitic assemblages. Relatively fresh meteoric water might have been increasingly relevant in such dilution as upwelling fluids neared the surface. Shallow boiling occurred mostly between high- and medium-temperature propylitic zones, and generated acidic vapors that eventually condensated in the phreatic water table and, in part, in the argillic alteration zone. Shallow hypogene advanced argillic (and argillic) alteration consisting of kaolinite, platy alunite, native sulfur and the newly found cuprocopiapite occurred in steam-heated grounds. Fluid inclusion and present-day geothermal fluid data allow to introduce relative time as a variable in the characterization of hydrothermal fluids in the LHGF. These sets of temperature data allow to independently determine patterns of temperature distribution as isotherms. Concave shapes in isotherms are interpreted to spearhead preferential passageways for hydrothermal fluids. Some areas (for instance, in the southern part of the LHGF) record relatively stable passageways through time, but these may significantly shift their positions in other areas. Stable passageways, apparently undisturbed by geothermal exploitation, may identify robust portions of the system that ensure a long-lasting exploitation. They also correspond to sound inputs of high-salinity and hot magmatic fluids, which may be sandwiched between cooler low-salinity fluids, dominantly meteoric in origin, and collectively make a case for convection (and partial mixing). Also, we determined that “older” inclusion fluids are ∼50 °C higher than “younger” fluids from geothermal wells, but such decrease can be compatible with the cooling down of the system due to geothermal exploitation. Therefore, such differences in temperature do not necessarily reflect an actual cooling in the system.

The Pliocene Ixtacamaxtitlán low sulfidation epithermal deposit (Puebla, Mexico): A case of fossil fungi consortia in a steam-heated environment

The Ixtacamaxtitlán area in northern Puebla (central Mexico) contains middle Miocene Cu-Mo-Au porphyry/skarn and Pliocene low-sulfidation Au-Ag epithermal deposits that are geologically associated with the evolution of the Trans-Mexican Volcanic Belt (TMVB). In this paper, a new 40Ar/39Ar age (2.87 ± 0.41 Ma) is provided for rhombohedral alunite from a kaolinite + alunite ± opal ± cristobalite ± smectite advanced argillic alteration assemblage. This age contributes to the definition of a metallogenic province that is confined to the TMVB, a relevant feature for regional exploration. A ~12 My gap is established between the formation of the Cu-Mo-Au porphyry/skarn and low-sulfidation Au-Ag epithermal deposits, which rules out the possibility that their overlapping was the result of telescoping. Advanced argillic alteration is conspicuous throughout the mineralized area. This alteration assemblage consists of a widespread kaolinite-rich blanket that underlies silica sinters, polymictic hydrothermal breccias, and an alunite-rich spongy layer that consists of vertical tubular structures that are interpreted as the result of gas venting in a subaerial environment. The above indicate a shallow hypogene origin for the advanced argillic alteration assemblage—that is, formation by the partial condensation within a phreatic paleoaquifer of acidic vapors that were boiled-off along fractures that host epithermal veins at depth. The formation of the spongy alunite layer and silica sinters is interpreted to have been synchronous. Within the alunite-rich spongy layer, tubular structures hosted microbial consortia dominated by fungi and possible prokaryote (Bacteria or Archaea) biofilms. Such consortia were developed on previously formed alunite and kaolinite and were preserved due to their replacement by opal, kaolinite, or alunite. This means that the proliferation of fungi and prokaryotes occurred during a lull in acidic gas venting during which other organisms (i.e., algae) might have also prospered. Periodic acidic gas venting is compatible with a multi-stage hydrothermal system with several boiling episodes, a feature typical of active geothermal systems and of low-sulfidation epithermal deposits. The microstructures, typical for fungi, are mycelia, hyphae with septa, anastomoses between branches, and cord-like groupings of hyphae. Possible evidence for skeletal remains of prokaryote biofilms is constituted by cobweb-like microstructures composed of <1 µm thick interwoven filaments in close association with hyphae (about 2.5 µm thick). Bioweathering of previously precipitated minerals is shown by penetrative biobrecciation due to extensive dissolution of kaolinite by mycelia and by dissolution grooves from hyphae on alunite surfaces. Such bioweathering was possibly predated by inorganically driven partial dissolution of alunite, which suggests a lull in acidic gas venting that allowed living organisms to thrive. This interpretation is sustained by the occurrence of geometrical dissolution pits in alunite covered by hyphae. Fungal bioweathering is particularly aggressive on kaolinite due to its relatively poor nutrient potential. Such delicate microstructures are not commonly preserved in the geological record. In addition, numerous chalcopyrite microcrystals or microaggregates are found within the alunite layer, which could be related to sulfate reduction due to bacterial activity from the sulfate previously released by fungal bioweathering of alunite. Hydrogeochemical modeling constrains pH to between ~3.2 and ~3.6 and temperature to between 53 and 75 °C during the stage in which fungi and other organisms thrived. These waters were cooler and more alkaline than in earlier and later stages, which were characterized dominantly by steam-heated waters. The most likely process to account for this interlude would be mixing with meteoric water or with upwelling mature water that did not undergo boiling.

The Pliocene Xoconostle high sulfidation epithermal deposit in the Trans-Mexican Volcanic Belt: Preliminary study

The Xoconostle prospect in northeastern Michoacán state, south-central Mexico, is constituted by high sulfidation epithermal breccias and stockworks with Au and Hg prospective anomalies. The mineralization is hosted by latest Miocene to Pliocene rocks grouped into the El Terrero ignimbrite and the Siete Cruces dome complex and a stock of intermediate composition and undetermined (Pliocene?) age. Two alunite samples from deep hypogene advanced argillic alteration assemblages within the deposit yielded 40Ar/39Ar ages at 5.57 ± 0.44 (Messinian) and 3.67 ± 0.20 Ma (Zanclean). Such ages are in good agreement with those of volcanic rocks at a semi-regional scale, especially those associated with the nearby Amealco caldera. Assuming that the formation of Xoconostle deposit could be genetically related to any of the eruptive units in this caldera, it would be associated with dacitic-andesitic rocks at ~4.7 Ma or with bimodal andesite-basalt volcanism at ~3.7 Ma, with which rhyolites at the southwest rim of the caldera (nearer to the epithermal deposit) are contemporaneous. The obtained ages are also in good agreement with those determined for the youngest stages in the evolution of the Trans-Mexican Volcanic Belt (TMVB). In addition, such ages compare well with those established for the E-W striking Morelia-Acambay normal fault zone (or Acambay graben). The occurrence of E-W structural features in the study area support their correlation with those in the Acambay graben. Although the metallogenesis of the TMVB needs further endeavours that contribute to its understanding, the Xoconostle prospect adds up to other dated magmatic-hydrothermal deposits that may collectively constitute a Pliocene metallogenic province whose inception was geologically circumscribed to this volcanic arc. However, this and its companion papers in this issue confirm the metallogenic potential of the TMVB in most of its stages of evolution, particularly in the late Miocene-Pliocene stage of acid and bimodal volcanism.

Mineralogy and Distribution of Critical Elements in the Sn–W–Pb–Ag–Zn Huanuni Deposit, Bolivia

The polymetallic Huanuni deposit, a world-class tin deposit, is part of the Bolivian tin belt. As a likely case for a “mesothermal” or transitional deposit between epithermal and porphyry Sn types (or shallow porphyry Sn), it represents a case that contributes significantly to the systematic study of the distribution of critical elements within the “family” of Bolivian tin deposits. In addition to Sn, Zn and Ag, further economic interest in the area resides in its potential in critical elements such as In, Ga and Ge. This paper provides the first systematic characterisation of the complex mineralogy and mineral chemistry of the Huanuni deposit with the twofold aim of identifying the mineral carriers of critical elements and endeavouring plausible metallogenic processes for the formation of this deposit, by means of a multi-methodological approach. With In concentrations consistently over 2000 ppm, the highest potential for relevant concentrations in this metal resides in widespread tin minerals (cassiterite and stannite) and sphalerite. Hypogene alteration assemblages are hardly developed due to the metasedimentary nature of host rocks, but the occurrence of potassium feldspar, schorl, pyrophyllite and dickite as vein material stand for potassic to phyllic or advanced argillic alteration assemblages and relatively high-temperature (and low pH) mineralising fluids. District-scale mineralogical zonation suggests a thermal zonation with decreasing temperatures from the central to the peripheral areas. A district-scale zonation has been also determined for δ34SVCDT values, which range −7.2‰ to 0.2‰ (mostly −7‰ to −5‰) in the central area and −4.2‰ to 1.0‰ (mainly constrained between −2‰ and 1‰) in peripheral areas. Such values stand for magmatic and metasedimentary sources for sulfur, and their spatial zoning may be related to differential reactivity between mineralising fluids and host rocks, outwardly decreasing from the central to the peripheral areas.

Hydrothermal Aluminum-Phosphate-Sulfates in Ash from the 2014 Hydrothermal Eruption at Ontake Volcano, Central Honshu, Japan

Aluminum-phosphate-sulfates (APS) of the alunite supergroup occur in igneous rocks within zones of advanced argillic and silicic alteration in porphyry and epithermal ore environments. In this study we report on the presence of woodhouseite-rich APS in ash from the 27 September 2014 hydrothermal eruption of Ontake volcano. Scanning electron microscope coupled with energy dispersive X-ray spectrometer (SEM-EDS) and field emission (FE)-SEM-EDS observations show two types of occurrence of woodhouseite: (a) as cores within chemically zoned alunite-APS crystals (Zoned-alunite-woodhouseite-APS), and (b) as a coherent single-phase mineral in micro-veinlets intergrown with similar micro-veinlets of silica minerals (Micro-wormy-vein woodhouseite-APS). The genetic environment of APS minerals at Ontake volcano is that of a highly acidic hydrothermal system existing beneath the volcano summit, formed by condensation in magmatic steam and/or ground waters of sulfur-rich magmatic volatiles exsolved from the magma chamber beneath Mt. Ontake. Under these conditions, an advanced argillic alteration assemblage forms, which is composed of silica, pyrophyllite, alunite and kaolinite/dickite, plus APS, among other minerals. The discovery of woodhouseite in the volcanic ash of the Ontake 2014 hydrothermal eruption represents the first reported presence of APS within an active volcano. Other volcanoes in Japan and elsewhere with similar phreatic eruptions ejecting altered ash fragments will likely contain APS minerals derived from magmatic-hydrothermal systems within the subvolcanic environment. The presence of APS minerals within the advanced argillic zone below the summit vent of Ontake volcano, together with the prior documentation of phyllic and potassically altered ash fragments, provides evidence for the existence within an active volcano in Japan of an alteration column comparable to that of porphyry copper systems globally.

Hydrothermal Alteration, Mineralization, and Structural Geology of the Zijinshan High-Sulfidation Au-Cu Deposit, Fujian Province, Southeast China

Zijinshan is a world-class Au-Cu district located in southwest Fujian Province, southeast China. It contains a diverse array of ore deposits hosted by the Zijinshan granite complex and surrounding volcano-sedimentary rocks. Associated deposits include high-sulfidation epithermal gold-copper, intermediate-sulfidation epithermal polymetallic silver-base metal, and porphyry molybdenum-copper deposits. The Zijinshan high-sulfidation Au-Cu deposit is located in the middle of the Zijinshan district, and ore zones are hosted in, below, and adjacent to the Zijinshan lithocap. The host rocks are part of a Jurassic granite complex emplaced between 165 and 157 Ma. 40Ar/39Ar age spectra of hydrothermal alunite indicate that hydrothermal activity at Zijinshan occurred at 102.86 ± 0.61 to 101.19 ± 0.60 Ma and was associated with dacite porphyry dikes that intruded at 104.8 ± 0.9 Ma. Hydrothermal activity produced pervasive silicic and advanced argillic alteration assemblages that are zoned from a central, massive quartz alteration domain, outward to massive quartz-dickite, disseminated quartz-alunite ± dickite, and disseminated quartz-muscovite-dickite zones. Copper sulfides occur in hydrothermally cemented breccias, veins, and minor disseminations in granite; copper minerals include covellite, digenite, and minor enargite. Oxide Au ores are associated with strongly weathered silicic alteration in the upper parts of the deposit. A set of subvertical NE-striking faults were active prior to mineralization at Zijinshan. During mineralization, a NW-striking fault system controlled the emplacement of veins, breccias, and dacite dikes. Kinematic indicators show that most of the NW-striking faults were active as normal oblique faults during mineralization. Postmineralization ENE-striking dextral strike-slip faults are associated with minor conjugate NNW-striking faults. The premineralization structures at Zijinshan likely formed during NW-directed compression. Synmineralization structures formed under subvertical contraction and subhorizontal NNE-plunging extension. Postmineralization structures developed in a strike-slip regime defined by a NW-oriented axis of compression and NE-trending axis of extension. Changes in the styles of pre- and synmineralization structures are related to fluctuations in the regional stress regime. The transition from compression to extension was fundamental to mineralization. Postmineralization faults are related to transient local stress regime variations at the deposit scale.

Geochemistry of hot-springs at the SuSu Knolls hydrothermal field, Eastern Manus Basin: Advanced argillic alteration and vent fluid acidity

Abstract SuSu Knolls is an area of ongoing magmatic activity and recent volcanism located in the back-arc spreading environment of the Manus Basin in the Bismarck Sea, Papua New Guinea. In 2006, hydrothermal fluids were collected from three areas of submarine hot-spring venting and analyzed for the chemical and isotopic composition of major and trace species. Fluids were characterized by temperatures that varied from 226–325 °C, and formed grey to black smoke as they mixed with bottom seawater. The compositions of seawater derived vent fluids are regulated by the relative contributions of fluid-rock and fluid-sediment interaction, phase separation, and the addition of volatiles from magmatic degassing. In addition to phase separation, leaching of Cl from felsic rocks that compose the lithosphere in back-arc environments may produce Cl concentrations in excess of seawater values. The measured pH25°C of SuSu Knolls smoker fluids varied from 1.5–3.7, a range that includes values substantially more acidic than typically observed in fluids at mid-ocean ridge spreading centers. Late stage addition of magmatic volatiles in the shallow seafloor is directly responsible for the most acidic fluids (pH25°C values below 2). In contrast, the acidity of vent fluids characterized by pH25°C values between 2 and 3 is not the direct result of the direct addition of magmatically-derived acidic species. Instead, the pH of these fluids likely reflects reaction with rocks that were previously altered by highly acidic magmatic fluids to an advanced argillic alteration assemblage containing quartz-illite-pyrophyllite-anhydrite ± alunite in hydrothermal upflow zones. Fluids that do not react with advanced argillic alteration assemblages during upflow have measured pH25°C values between 3 and 4.

Mineralogical Study of the Advanced Argillic Alteration Zone at the Konos Hill Mo–Cu–Re–Au Porphyry Prospect, NE Greece

The Konos Hill prospect in NE Greece represents a telescoped Mo–Cu–Re–Au porphyry occurrence overprinted by deep-level high-sulfidation mineralization. Porphyry-style mineralization is exposed in the deeper parts of the system and comprises quartz stockwork veins hosted in subvolcanic intrusions of granodioritic composition. Ore minerals include pyrite, molybdenite, chalcopyrite, and rheniite. In the upper part of the system, intense hydrothermal alteration resulted in the formation of a silicified zone and the development of various advanced argillic alteration assemblages, which are spatially related to N–S, NNW–SSE, and E–W trending faults. More distal and downwards, advanced argillic alteration gradually evolves into phyllic assemblages dominated by quartz and sericite. Zunyite, along with various amounts of quartz, alunite, aluminum phosphate–sulfate minerals (APS), diaspore, kaolinite, and minor pyrophyllite, are the main minerals in the advanced argillic alteration. Mineral-chemical analyses reveal significant variance in the SiO2, F, and Cl content of zunyite. Alunite supergroup minerals display a wide compositional range corresponding to members of the alunite, beudantite, and plumbogummite subgroups. Diaspore displays an almost stoichiometric composition. Mineralization in the lithocap consists of pyrite, enargite, tetrahedrite/tennantite, and colusite. Bulk ore analyses of mineralized samples show a relative enrichment in elements such as Se, Mo, and Bi, which supports a genetic link between the studied lithocap and the underlying Konos Hill porphyry-style mineralization. The occurrence of advanced argillic alteration assemblages along the N–S, NNW–SSE, and E–W trending faults suggests that highly acidic hydrothermal fluids were ascending into the lithocap environment. Zunyite, along with diaspore, pyrophyllite, and Sr- and Rare Earth Elements-bearing APS minerals, mark the proximity of the hypogene advanced argillic alteration zone to the porphyry environment.

Physical and mechanical property relationships of a shallow intrusion and volcanic host rock, Pinnacle Ridge, Mt. Ruapehu, New Zealand

Shallow magmatic intrusions are prevalent in volcanic settings worldwide. Understanding how these intrusions interact and influence their volcanic host rocks is therefore relevant to many engineering geology, geothermal, and volcanological applications. In this study, we present the most comprehensive dataset for a shallow intrusion and its host rock in a volcanic setting to date, detailing the mechanical and physical properties of volcanic rocks from Pinnacle Ridge, Mt. Ruapehu, New Zealand. Based on the geomechanical properties of 194 measured samples, we identify seven geotechnical units: (1) unaltered dense coherent lava, (2) altered dense coherent lava, (3) unaltered brecciated lava margin, (4) altered brecciated lava margin, (5) unaltered intrusion, (6) altered intrusion, and (7) hydrothermal veining. We detail the mineralogy (andesite compositions ranging from primary to an advanced argillic alteration assemblage), porosity (0.7–31%), permeability (10−21–10−12 m2), elastic wave velocities (1994–5615 m/s), uniaxial compressive strength (1–332 MPa) of these geotechnical units. Our laboratory analyses indicate that primary lithology is the predominant control on the physical and mechanical properties of the geotechnical units. Additionally, the data suggest that there is a correlation between distance to the largest intrusion; this is particularly evident for the measurements on the brecciated lava margin samples. Towards the largest intrusion, this breccia shows decreasing porosity (30.92 to 5.49%) and permeability (10−12 to 10−17 m2) and increasing elastic wave velocities (1994 to 4157 m/s) and uniaxial compressive strength (3 to 61 MPa). Thin-section analysis suggests that these correlations are due to mineral precipitation within fractures and pores in the brecciated lava margins. These correlations with distance to the largest intrusion are not shared by the altered intrusions or dense coherent lavas. We suggest that the high primary permeability of the unaltered breccia facilitated efficient hydrothermal fluid circulation and mineral precipitation adjacent to the intrusion. The other geotechnical units are less affected because hydrothermal fluid flow, alteration, and mineral precipitation were limited due to low initial permeability (10−21–10−16 m2). Our study shows that the initial properties of the host rock (i.e. porosity and permeability) control the extent of hydrothermal alteration and the susceptibility to modifications of rock geomechanical properties. Modifications to porosity and permeability can influence edifice-scale behaviour; for example, a reduction in permeability can result in pore pressure augmentation, which exerts a primary control on volcanic slope stability, seismicity, and eruptive behaviour. This study provides the most comprehensive and complete geomechanical properties data suite on a shallow intrusion in volcanic host rock to date and will support monitoring and modelling of volcanic hazards associated with shallow igneous intrusions.

Geometallurgy of the Pebble Porphyry Copper-Gold-Molybdenum Deposit, Alaska: Implications for Gold Distribution and Paragenesis

The Pebble Cu-Au-Mo deposit in southwest Alaska is one of the world's largest porphyry deposits. The deposit contains over 80 billion pounds (Blbs) Cu, 107 million ounces (Moz) Au, and 5.6 Blbs Mo in all resource categories. Copper and gold grades vary across diverse hydrothermal alteration assemblages with higher grades associated with sericite- and pyrophyllite-rich assemblages in an advanced argillic alteration zone. Moderate grades are associated with the potassic and sodic-potassic alteration assemblages that dominate the deposit and low grades are found in quartz-illite-pyrite assemblages. These variations reflect fluctuations in both the temperature and composition of the magmatic-hydrothermal fluids in time and space. QEMSCAN scanning electron microscope-based mineral mapping and in situ laser ablation inductively coupled plasmamass spectrometry methods designed to document the spatial and temporal variations in metal deportment across alteration zones show that gold occurs as inclusions of electrum and high-fineness gold in chalcopyrite and pyrite and as free grains hosted by silicate minerals, with the proportion of each related to the principal host alteration assemblage. Gold associated with the earliest high temperature sodic-potassic and potassic assemblages occurs as electrum inclusions in chalcopyrite and to a lesser extent pyrite, and pyrite trace element signatures have high gold, silver, and copper concentrations (pyrite-1). During cooling of the hydrothermal system lower temperature illite and illite-kaolinite alteration overprinted the earlier assemblages with varying degrees of recrystallization and precipitation of more pyrite-rich assemblages with pyrite having a gold-poor trace element composition (pyrite-2). Gold released during recrystallization formed high-fineness gold inclusions in newly formed pyrite and narrow gold-rich pyrite rims (pyrite-3). A second magmatic-hydrothermal event resulted in a sericite- and pyrophyllite-rich advanced argillic overprint, which introduced more copper and gold to the system and recrystallized preexisting sulfides into both high and low sulfidation assemblages with high-fineness gold inclusions in pyrite and chalcopyrite and solid solution gold in bornite. A second generation of pyrite-1 with elevated palladium concentrations is associated with this hydrothermal fluid pulse and also possibly high arsenic pyrite (pyrite-4). Understanding the controls on gold deportment provides genetic constraints on ore deposit genesis. Alteration and sulfide assemblages and gold compositions provide information on the hydrothermal fluid compositions, pH, and temperature evolution of the magmatic-hydrothermal system. The presence of electrum inclusions in chalcopyrite and pyrite in potassic and sodic-potassic alteration is consistent with these zones forming from early high-temperature magmatic fluids because both gold and silver are transported together in such fluids. High-fineness gold inclusions in younger pyrite related to low-temperature clay alteration are evidence that the gold in early electrum was partially remobilized and reprecipitated. The low-temperature clay alteration most likely formed from a mixture of H2S-rich vapor with meteoric waters, conditions where gold is transported as a bisulfide complex and silver is not, resulting in the separation of the two metals. A second, structurally controlled pulse of magmatic fluids formed a well-mineralized advanced argillic alteration assemblage containing high-fineness gold inclusions in chalcopyrite and pyrite, the only part of the deposit where high-fineness gold is hosted by chalcopyrite. In the pyrophyllite stability field acidic fluid compositions transport gold and silver as different complexes and gold solubility is significantly lower than silver leading to the precipitation of highfineness gold inclusions. Palladium is transported by the same complex as gold under these conditions, consistent with the elevated palladium content of pyrite in the advanced argillic alteration assemblage. The temporal and spatial studies of variations in gold deportment across the Pebble deposit provide critical inputs to optimization of mineral processing design. The greatest influence on metallurgical gold recovery at Pebble is the proportion of gold that is hosted by pyrite. Pyrite-hosted gold may require different mineral processing methodologies compared with gold hosted by chalcopyrite. Therefore, defining domains of consistent hydrothermal alteration, and sulfide mineralogy and gold deportment is the key to the geometallurgical characterization of the deposit.

The role of granites in volcanic-hosted massive sulphide ore-forming systems: an assessment of magmatic–hydrothermal contributions

Assessment of geological, geochemical and isotopic data indicates that a significant subgroup of volcanic-hosted massive sulphide (VHMS) deposits has a major or dominant magmatic–hydrothermal source of ore fluids and metals. This group, which is typically characterised by high Cu and Au grades, includes deposits such as those in the Neoarchean Doyon-Bousquet-LaRonde and Cambrian Mount Lyell districts. These deposits are distinguished by aluminous advanced argillic alteration assemblages or metamorphosed equivalents intimately associated with ore zones. In many of these deposits, δ34Ssulphide is low, with a major population below −3‰; δ34Ssulphate differs from coexisting seawater and Δ34Ssulphate–sulphide ∼ 20–30‰. These characteristics are interpreted as the consequence of disproportionation of magmatic SO2 as magmatic–hydrothermal fluids ascended and cooled and as a definitive evidence for a significant magmatic–hydrothermal contribution. Other characteristics that we consider diagnostic of significant magmatic–hydrothermal input into VHMS ore fluids include uniformly high (>3 times modern seawater values) salinities or very 18O-enriched (δ18O > 5‰) ore fluids. We do not consider other criteria [e.g. variable salinity, moderately high δ18Ofluid (2–5‰), δ34Ssulphide near 0‰, metal assemblages or a spatial association with porphyry Cu or other clearly magmatic-hydrothermal deposits] that have been used previously to advocate significant magmatic–hydrothermal contributions to be diagnostic as they can be produced by non-magmatic processes known to occur in VHMS mineral systems. However, in general, a small magmatic–hydrothermal contribution cannot be excluded in most VHMS systems considered. Conclusive data that imply minimal magmatic–hydrothermal contributions are only available in the Paleoarchean Panorama district where coeval seawater-dominated and magmatic–hydrothermal systems appear to have been physically separated. This district, which is characterised by chloritic and sericitic alteration assemblages and lacks aluminous advanced argillic alteration assemblages, is typical of many VHMS deposits around the world, suggesting that for “garden variety” VHMS deposits, a significant magmatic–hydrothermal contribution is not required. Other than deposits associated with advanced argillic alteration assemblages, the only deposit for which we ascribe a major magmatic–hydrothermal contribution is the Devonian Neves Corvo deposit. This deposit differs from other deposits in the Iberian Pyrite Belt and around the world in being extremely Sn-rich, with the Sn closely associated with Cu and in having formed from high 18O-rich fluids (δ18Ofluid ∼8.5‰). We consider these characteristics, particularly the last, as diagnostic of a significant magmatic hydrothermal contribution. Our analysis indicates that two subgroups of VHMS deposits have a major magmatic–hydrothermal contribution: Cu/Au-rich deposits with aluminous alteration assemblages and reduced, very Sn-rich deposits in which Sn was introduced in a high-temperature ore assemblage. Comparison with “normal” VHMS deposits suggests that these subgroups of VHMS deposits may form in specialised tectonic environments. The Cu/Au-rich deposits appear to form adjacent to magmatic arcs, an environment conducive to the generation of hydrous, oxidised melts by melting metasomatised mantle in the wedge above the subducting slab. This contrasts with the back-arc setting of “normal” VHMS deposits in which relatively dry granites (In this contribution, we use the term granite sensu latto) formed by decompression melting drive seawater-dominated hydrothermal circulation. The tectonic setting of highly Sn-rich VHMS deposits such as Neves Corvo is less clear; however, thick continental crust below the ore-hosting basin may be critical, as it is in other Sn deposits.

History and Geologic Overview of the Yanacocha Mining District, Cajamarca, Peru

Discovery and recognition of the Yanacocha district as a belt of northeast-trending, near-surface, high sulfidation-type epithermal gold deposits with strong supergene oxidation was made over a four-year period (1984–1988) through a joint-venture alliance operated by Newmont Mining Corporation with partners Compania de Minas Buenaventura and CEDIMIN (BRGM). Open-pit mining began in the district in late 1993 with initial full-year gold production in 1994 of 300,000 oz and a reported end-of-year reserve of 4 Moz. The production growth curve at Yanacocha over 15 years (1994–2008) has been about 10 times the first full year of production in 1994. In February 2007, Minera Yanacocha (Newmont 51%; Buenaventura 44%; International Finance Corp 5%) poured its 20-millionth gold ounce life-of-mine. Reported 2008 end-of-year oxide reserves within the operating district are >13 Moz gold. Since exploration began in the district, over 20 hard-rock deposits and two gravel fan deposits (La Quinua, La Quinua Sur) have been developed. Cumulative historical discovery-to-reserve conversion costs average <$10/oz Au. The estimated oxide plus sulfide drill-indicated gold endowment within the district exceeds 70 Moz (2,170 tonnes). The high-sulfidation epithermal gold deposits are hosted by volcanic rocks that occur at the southern terminus of the northern Peruvian volcanic belt, a continuous sequence of a north-northwest–trending Miocene-Pliocene suite of bimodal andesite to rhyolite volcanic rocks that extend into southern Ecuador. In the Yanacocha district, the volcanic pile has been subdivided into three groups: (1) the lower andesite sequence, consisting of an intercalated sequence of block and ash flow tuffs, flow sequences with rare, associated flow domes, and an upper zone dominated by ignimbrites and fine-grained, laminated epiclastic sequences that show a transition into the overlying Yanacocha pyroclastic sequence; (2) the Yanacocha pyroclastic sequence, a variable sequence of lithic to lithic crystal tuffs, extensively altered in the central portion of the district and primary host to the majority of gold deposits within the district; (3) the upper andesite-dacite sequence, consisting of intercalated units of andesite to dacite flows, dominated by multiple flow dome complexes in its upper portion. Ar-Ar age dating within the district has yielded ages ranging from 19 Ma (basal lower andesite) to >12 Ma (upper andesite sequence). The entire volcanic pile has been crosscut by multiple phases of phreatic (vapor phase dominant), phreatomagmatic (intrusive component) and hydrothermal breccias, and intruded by multiple late-stage phases of andesite dikes and dacite to quartz dacite plugs, dikes and stocks (10–8 Ma), the latter of which are associated with shallow Au-Cu porphyry-style mineralization that underlies the high-sulfidation epithermal deposits. Alteration in the district consists of multiple stages of advanced argillic assemblages, typical in nature, that are zoned outward from a central core of massive to vuggy quartz to broader envelopes of advanced argillic alteration (alunite-pyrophyllite) to still broader zones of intermediate argillic and propylitic alteration halos. On a district scale, a generally southwest to northeast progression of multiple alteration centers with various timing has been documented through Ar-Ar dating of hydrothermal alunite, with ages ranging from 11.5 to 8.5 Ma. In places the advanced argillic alteration assemblage is overprinted by a later event consisting of creamy colored cryptocrystalline chalcedony plus barite associated with intermediate sulfidation type feeders. Gold mineralization is a late-stage event that was superimposed over all alteration types, most commonly occurring in late-stage fractures with massive, vuggy, and granular quartz, and especially where higher grade gold mineralization (>3.0 g/t Au) is associated with indurated replacement zones of creamy chalcedony. Mineral deposits are commonly localized within pyroclastic host rocks and phreatic breccias that envelop the margins of less permeable phreatomagmatic and within hydrothermal breccia pipes and andesite-dacite dome margins. Geophysical surveys have been employed as part of an integrated approach from the earliest exploration stages in the district. Resistivity and magnetic and potassium radiometric response anomalies are spatially associated with gold on a district-wide and deposit scale. Gravity surveys have added to understanding of basement geometry and have outlined volcanic subbasins. The east-northeast alignment of the Yanacocha gold deposits is interpreted to have been caused by the intersection of a northeast-trending trans-arc crustal break (Chicama-Yanacocha structural corridor). This is superimposed over older, north-northwest to northwest Andean-parallel folded and thrusted Cretaceous-age sedimentary basement terrane (Paleocene-Eocene). Both structural orientations dominate in the Yanacocha district, controlling emplacement of breccias and intrusions as well as formation of gold mineralization. Discontinuous east-west–oriented faults and fracture zones are interpreted to be extensional and control gold mineralization on a deposit scale. The transition from near-surface, gold-dominant mineralization to underlying copper-dominant sulfide mineralization associated with shallow quartz eye-bearing monzonite to tonalite intrusions has been documented by deep (500–1,200 m) diamond drilling beneath the Yanacocha complex (Sur-Norte-Oeste), the largest single gold deposit in the district with an endowment exceeding 18 Moz.

Mineral Zoning and Geochemistry of Epithermal Polymetallic Zn-Pb-Ag-Cu-Bi Mineralization at Cerro de Pasco, Peru

Mineral Zoning and Geochemistry of Epithermal Polymetallic Zn-Pb-Ag-Cu-Bi Mineralization at Cerro de Pasco, Peru

Alteration Mineralogy at the Cerro La Mina Epithermal Prospect, Patagonia, Argentina: Field Mapping, Short-Wave Infrared Spectroscopy, and ASTER Images

The Cerro La Mina prospect, located in the Los Menucos area, Argentina, is currently being explored for epithermal gold mineralization. Triassic-Jurassic hydrothermal activity produced intense alteration of rhyolites, andesites, ignimbrites, and tuffs of the Late Triassic Los Menucos Group, in the Somun Cura Massif. A detailed surface map of the principal alteration assemblages at Cerro La Mina was produced employing field mapping, supported by short-wave infrared (SWIR) field spectroscopy, petrography, X-ray diffraction (XRD), and scanning electron microscopy (SEM). Advanced argillic, argillic, and silicic alteration zones and their internal mineral assemblage variations were recognized at Cerro La Mina. The silicification consists of massive bodies of fine-grained quartz and minor vuggy quartz, locally containing disseminated aggregates of pyrite, and rutile. Advanced argillic alteration of magmatic-hydrothermal origin occurs adjacent to the silicic alteration zones and can be subdivided into three mineral assemblages: (1) stage 1 alunite + quartz ± dickite ± kaolinite ± pyrophyllite ± diaspore ± rutile ± pyrite ± barite; (2) dickite + kaolinite ± pyrophyllite ± diaspore ± pyrite ± barite; and (3) dickite + quartz ± diaspore. The advanced argillic and silicic alteration grades to argillic alteration, comprising three mineral assemblages: (1) kaolinite ± dickite ± quartz ± illite ± smectite; (2) kaolinite ± illite ± smectite ± ferroan clinochlore; and (3) illite ± quartz ± muscovite ± kaolinite. Supergene alteration, which locally overprints the prior alteration zones, consists of jarosite + hematite + goethite ± stage 2 alunite ± aluminum phosphate sulfate (APS) ± poorly crystalline kaolinite. Consistent discrimination of the main alteration minerals of this prospect was also achieved by means of satellite multispectral remote sensing. The application of image processing techniques, specifically designed for mineral mapping, to selected spectral bands of the Terra/ASTER multispectral sensor resulted in a detailed alteration map for the entire prospect. Comparison between the results achieved through remote sensing with field data on the Cerro La Mina confirmed the accuracy of the former. The silicification and minor vuggy quartz, the abundant dickite, and the coarse-grained hypogene alunite, among other advanced argillic alteration assemblages, suggest that Cerro La Mina corresponds to a volcanic-hydrothermal leached environment, possibly controlled by permeable lithological units. These characteristics led us to interpret this prospect as a possible lithocap similar to those that typically host subsequent high-sulfidation mineralization.

Evolução paragenética e dos fluidos no depósito Epitermal Cupro-Aurífero Golden Hill, Camaguey, Cuba

The Cu-Au mineralization at the Golden Hill deposit in the Las Tunas Province, Eastern Cuba, is characterized by brecciahosted massive and semi-massive sulfide bodies confined to the Crucero Contramaestre Formation in the Cretaceous Volcanic Arc. The deposit is enveloped by a silicic, argillic and advanced argillic alteration assemblage and the Cu-Au orebodies are particularly associated with the latter, typical of high sulfidation epithermal deposits. Within the advanced argillic alteration zone, two stages of alteration were defined on the basis of the mineralogical and textural associations: (i) Stage I with quartz–kaolinite–alunite–zunyite–diaspore, disseminated pyrite, enargite and, probably, sub-microscopic Au; (ii) Stage II with quartz – natroalunite – pyrophyllite, closely related to pyrite – enargite – luzonite – famatinite and Au. Barite commonly occurs filling fractures and cavities and may have been introduced late during Stage II. These assemblages indicate that, in both stages, the advanced argillic alteration and the Cu-Au mineralization took place under conditions of low pH ( 10 -32.8 bar) and high SO 4 - /H 2 S ratios, with f S 2 > 10 -9.7 bar and f S 2 > 10 -8.7 bar for stages I and II, respectively. Additionally, temperatures for the stage I were constrained between 250°C and 275°C, based on the coexistence of alunite-woodhouseite and quartz-kaolinite-diaspore, whereas for stage II they varied from 188°C and 330°C, according to the total homogenization temperatures of fluid inclusions in quartz and the stability of pyrophyllite-bearing assemblages. Fluid inclusion investigations, restricted to stage II quartz and barite, revealed that part of the advanced argillic alteration and the Cu-Au mineralization took place in the presence of CO 2 - poor (< 0.85 molal), low, but variable, salinity (0,5 – 9,3 wt% eq. NaCl) aqueous solutions. The presence of CO 2 and the ample interval of salinity and total homogenization values of the inclusion fluids, together with Wand Bi-Mo-bearing minerals in Stage II, are evidence to envisage the hydrothermal system of the Golden Hill deposit as a result of a progressive interaction of more diluted and lower temperature surface fluids, with more saline and higher temperature magmatic solutions. This would imply in possibilities of the occurrence of shallow epizonal intrusions, which are generally considered important sites for porphyry Cu-Au mineralization.

Geochemical dispersion about the Western Tharsis Cu–Au deposit, Mt Lyell, Tasmania

The Western Tharsis disseminated Cu–Au orebody, which occurs within the Cambrian Mt Read Volcanics of Western Tasmania, is surrounded by a pyritic halo that extends 100–200 m stratigraphically above and below the ore zone. Although this halo extends laterally along the same stratigraphic position to the south, it probably closes off to the north based on limited surface and drill hole data. The ore zone is characterized by extreme enrichment (the enrichments and depletions referred to herein are relative to background; these have not been established using mass balance techniques) in As, Bi, Ce, Cu, Mo, Ni, S and Se; with the exception of Mo, these elements are also enriched, but at a much lower level, in the pyrite halo. Pronounced depletion in K, Cs and Mg occurs in 20–30 m wide stratiform zones that flank the orebody on both sides within the pyritic halo. These anomalies and depletions in Be, Ga, Rb, Y, MREE and HREE are associated with a pyrophyllite-bearing alteration zone that wraps around the main pyrite–chalcopyrite-bearing ore zone. This zone is also characterized by positive Eu anomalies which persist up to 150 m both into the hanging wall and footwall of the orebody. The depletion of these elements is consistent with the advanced argillic alteration assemblage developed about acid-sulfate Cu–Au deposits. The pyrite halo is surrounded by a peripheral carbonate halo which is highly enriched in C, CaO and MnO, and weakly enriched in Zn and Tl. Zinc and Tl are most enriched in the upper 100–150 m of the stratigraphically lower halo. In the stratigraphically upper halo, Zn and Tl values are anomalously high but erratic. Barium and Sr enrichment, although mainly restricted to the pyrite halo, extends into the stratigraphically lower carbonate halo by up to 100 m. A Na depletion anomaly extends from 150 m below the orebody and to at least the Owen contact (i.e. ≥400 m)in the hanging wall. The dispersion patterns observed at Western Tharsis are quite unlike those of Zn–Pb-rich volcanic-hosted massive sulfide (VHMS) deposits in western Tasmania. Rather, the dispersion patterns observed at Western Tharsis are more akin to those surrounding porphyry Cu deposits and related acid-sulfate Cu–Au deposits.

Deep hypogene oxidation of porphyry copper potassium-silicate protore at Butte, Montana; a theoretical evaluation of the copper remobilization hypothesis

The oxidation effects of potassium-silicate protores from heated meteoric water have been thermodynamically computed and confirm previously described copper vein-forming mechanisms involving hypogene leaching and enrichment of chalcopyrite-bearing protore (Brimhall, 1979). Intense copper leaching of protore chalcopyrite is shown to be related to a state of peroxidation in which the only stable minerals are quartz, pyrite, and muscovite following the complete destruction of the wall-rock oxygen fugacity buffer assemblage including hematite dissolution. Because modification of the protore continues to higher fluid/rock ratios, copper is extracted from the fluid phase and again is fixed in the rock as sulfides, first as bornite and ultimately as chalcocite-covellite associated with the advanced argillic alteration assemblage. This end-stage assemblage should occur only where extremely high fluid/rock ratios are reached in zones of unusually high permeability near the source of the heat responsible for the fluid motion.The results of the calculations are interpreted to mean that certain intervals of fluid/rock ratio are characterized by net base metal leaching and other intervals by metal fixation. Attainment of only intermediate fluid/rock ratios may result in the major amount of the copper in a protore being remobilized into solution and leaving the system through ground-water circulation. In contrast, in highly evolved systems where the fluid/rock ratio has increased to extremely high values, the presence of an advanced argillic alteration assemblage with chalcocite and covellite strongly suggests the presence within a district of a partially oxidized, potassium-silicate, disseminated sulfide protore, including a strongly leached and sericitized quartz-pyrite protore relict.Near-surface vein mineralization may conceivably be related to primary or unoxidized protore at depth in two distinct ways, depending upon the origin of the intrusives and the fracture systems responsible for circulation of oxidizing meteoric water. First, late intrusives which are cogenetic with the magmatic heat source of the protore may remobilize ore-forming elements upon deep hypogene oxidation of pyritic disseminated sulfides. Second, much later, totally unrelated intrusive activity and deep crustal faulting, perhaps related to entirely different petrotectonic regimes, may provide access for meteoric water to penetrate much older protores, resulting in the formation of near-surface ore deposits by element recycling.It is therefore suggested that near-surface hydrothermal base and precious metal deposits be explored to considerable depths for possible low-grade protore potential.