Alunite Alteration Research Articles

Geology and fluid inclusion geochemistry of the Zijinshan high-sulfidation epithermal Cu-Au deposit, Fujian Province, SE China: Implication for deep exploration targeting

The giant Zijinshan Cu-Au deposit in the Zijinshan orefield, Fujian Province, southeastern China, is the first high-sulfidation epithermal deposit identified in mainland China. The Cu and Au orebodies occur as veins and pods in the NW-trending faults and breccias zones. Intensive and pervasive alteration is characterized by downward and outward zoning from intensively leached silicic alteration (or vuggy quartz, Q), through alunite-quartz-pyrite alteration (advanced aragillic alteration, Q-Alu) and the quartz-alunite alteration overprinting the sericite alteration (Q-Alu-Di-Srt), to sericite (or phyllic) alteration zone. Gold mineralization mainly occurs in the silicic alteration zone, while the copper mineralization is confined in the alunite-quartz alteration zone. On the basis of detailed petrographic study, an early porphyry type mineralization stage, characterized by chalcopyrite-bornite-molybdenite-pyrite-sericite assemblage is recognized at Zijinshan, which is subsequently and strongly overprinted by the sulfate alteration and high-sulfidation Cu-Au mineralization stages. A supergene stage is also identified at shallow depth of the deposit, with gold largely enriched but copper commonly leached. The ore-forming fluids related to the early sericite alteration and possibly porphyry type mineralization are high-temperature, high-salinity magmatic water, characterized by the presence of the halite-bearing inclusions. The CO2-H2O (C-type) inclusions are mostly vapor-rich and abundantly identified in the samples with advance argillic alteration (alunite alteration). They are regarded to be the buoyant vapor phase by fluid boiling of a single-phase, low- to moderate-salinity magmatic fluid at depth, where a separated saline phase and related porphyry mineralization might be expected. A group of secondary inclusions coexisting with enargite grains are recognized in the samples with alunite alteration and suggested to be trapped from the ore-forming fluids of the Cu-Au mineralization stage. The total homogenization temperatures and salinities of the secondary inclusions are below 300°C (peaking at 260–280°C) and under 10wt% NaCl eqv., respectively. Intensive fluid boiling is the major mechanism for the formation of the giant high-sulfidation Cu (covellite-, digenite-dominated) orebodies in the Zijinshan deposit. It is a deep-seated high-sulfidation epithermal deposit according to the estimated depth of 1.4–2.1km from the C-type inclusions in quartz grains from the alunite alteration zone. By fluid inclusion mapping of a nearly NW-trending cross section profile, the isotherms are extrapolated using the average total homogenization temperatures and the possible heat source is suggested. It is indicated that the heat source and possible concealed porphyry mineralization nearly coeval to the high-sulfidation Cu-Au mineralization at Zijinshan might be located at the southeastern Zijinshan deposit or the northern area between the Zijinshan and Wuziqilong deposits, where deep drilling is encouraged.

Pyrite Re-Os isotope systematics at the Zijinshan deposit of SW Fujian, China: Constraints on the timing and source of Cu-Au mineralization

Zijinshan is the largest high-sulfidation epithermal Cu–Au deposit on mainland China. The primary mineralization at the deposit is characterized by pyrite, digenite and covellite. Although some RbSr isochron ages of the fluid inclusions in quartz and the apparent K-Ar age of the Cu-bearing alunite alteration zone have been previously interpreted as the ore-forming age, the Rb-Sr and K-Ar dating systems are usually readily reset due to their closure temperature, and thus document the latest thermal event. In order to precisely determine the age of the Cu mineralization, eight-pyrite separates were Re-Os dated, and seven yielded an isochron age of 103±4Ma with an initial 187Os/188Os ratio of 0.45±0.14. This date is interpreted as the age of Cu mineralization, which is in agreement with geochronological data from molybdenite and the porphyritic granodiorite that hosts Cu-Mo mineralization at Luoboling (located 2km NE of Zijinshan), suggesting that these two deposits were formed during the same metallogenic event. The relatively low initial 187Os/188Os ratio indicates that the source of the Cu at Zijinshan is predominantly the crust with input from the mantle.

Hydrothermal alteration of surficial rocks at Solfatara (Campi Flegrei): Petrophysical properties and implications for phreatic eruption processes

Solfatara crater is located within the Campi Flegrei caldera to the west of Naples (Italy). It is one of the largest fumarolic manifestations known, and the rocks hosting the hydrothermal system are affected by intense hydrothermal alteration. Alteration can result in changes of degassing behavior, and in the formation of a cap rock thereby increasing the probability of phreatic eruptions. Here, we investigate the effects of alunitic (solfataric) alteration on the mineralogy, the physical properties (porosity, density, permeability) and the mechanical properties (strength) of the rocks involved, as well as its influence on fragmentation and ejection behavior. Our results show that the pristine mineralogy of deposits from the vicinity of the Solfatara cryptodome and from Pisciarelli is almost completely replaced by amorphous silica and alunite. The differences in the degree of alteration among the samples series are reflected in the investigated properties and behavior as well as in the analysis of the experimentally generated particles. Alunitic alteration increases porosity and permeability, whereas it reduces density, elastic wave velocity and strength leading to higher fragmentation and ejection speeds for the sample series examined in this study. Our results also show that alteration results in the generation of a high fraction of fines (particle sizes <10μm) during fragmentation, mainly composed of alunite crystals. Due to their potential for inducing chronic disease, dispersion of such material should represent a serious health hazard on a local scale and the evaluation of precautions should be considered.

The Veladero High-Sulfidation Epithermal Gold Deposit, El Indio-Pascua Belt, Argentina: Geochronology of Alunite and Jarosite

The world-class Veladero high-sulfidation epithermal gold deposit is located near the northern end of the El Indio-Pascua metallogenic belt in Argentina. New 40Ar-39Ar alunite and jarosite ages were obtained at four mineralized centers in the Veladero district: the Amable and Filo Federico orebodies that compose the Veladero deposit, and the Argenta and Fabiana satellite centers. These ages indicate that magmatic-hydrothermal alteration and related gold mineralization were episodic, and activity in the Veladero district occurred over a period of approximately 3 m.y. The earliest magmatic-hydrothermal alteration in the Veladero district is recorded by alunite from Argenta that formed at 13.12 ± 0.18 Ma. At the Fabiana prospect, alunite is dated at 10.09 ± 0.08 Ma. At the Veladero deposit, the alteration and mineralization at the Filo Federico and Amable orebodies formed at different times despite their close proximity. Previously available data indicate that alunite at Amable formed at 11.93 ± 0.09 Ma, whereas Filo Federico alunite samples are younger and yield 40Ar-39Ar ages of 11.05 ± 0.12 and 10.34 ± 0.11 Ma. The oldest postmineralization jarosite alteration at Amable is dated at 11.8 ± 0.3 Ma and the youngest is dated at 8.58 ± 0.17 Ma. The age constraints suggest that the spatially overlapping Amable and Filo Federico orebodies formed approximately 1 m.y. apart, at about 11.9 Ma at Amable and after 11.1 Ma at Filo Federico. The results demonstrate that the Veladero district is host to the oldest centers of significantly mineralized magmatic-hydrothermal alteration within the El Indio-Pascua metallogenic belt. Magmatic-hydrothermal alteration and gold mineralization at the Veladero deposit broadly overlapped with isolated centers of middle to late Miocene volcanic activity in the region, dated at 12.7 ± 0.9 to 11.0 ± 0.2 Ma. In contrast to Veladero, the other high-sulfidation deposits in the belt are significantly younger with reported 40Ar-39Ar alunite alteration ages of other major deposits in the belt ranging from 9.4 (the Lama deposit at Pascua Lama) to 6.2 Ma (the El Indio deposit). These deposits formed during volcanic quiescence caused by shallowing of the Nazca plate below South America during the late Miocene, whereas Veladero formed prior to the cessation of volcanism. Fertile magmas of roughly 13 to 10 Ma were previously thought to be restricted to the Maricunga metallogenic belt approximately 200 km north of Veladero, but the present study demonstrates that mineralization in this age range occurred as far south as Veladero. These results are significant for regional exploration, in particular in the area between the Maricunga and El Indio-Pascua metallogenic belts.

Magmatic-vapor expansion and the formation of high-sulfidation gold deposits: Structural controls on hydrothermal alteration and ore mineralization

High-sulfidation copper–gold lode deposits such as Chinkuashih, Taiwan, Lepanto, Philippines, and Goldfield, Nevada, formed within 1500 m of the paleosurface in volcanic terranes. All underwent an early stage of extensive advanced argillic silica–alunite alteration followed by an abrupt change to spatially much more restricted stages of fracture-controlled sulfide–sulfosalt mineral assemblages and gold–silver mineralization. The alteration as well as ore mineralization stages of these deposits were controlled by the dynamics and history of syn-hydrothermal faulting. At the Sulfate Stage, aggressive advanced argillic alteration and silicification were consequent on the in situ formation of acidic condensate from magmatic vapor as it expanded through secondary fracture networks alongside active faults. The reduction of permeability at this stage due to alteration decreased fluid flow to the surface, and progressively developed a barrier between magmatic-vapor expansion constrained by the active faults and peripheral hydrothermal activity dominated by hot-water flow. In conjunction with the increased rock strength resulting from alteration, subsequent fault-slip inversion in response to an increase in compressional stress generated new, highly permeable fractures localized by the embrittled, altered rock. The new fractures focused magmatic-vapor expansion with much lower heat loss so that condensation occurred. Sulfide Stage sulfosalt, sulfide, and gold–silver deposition then resulted from destabilization of vapor phase metal species due to vapor decompression through the new fracture array. The switch from sulfate to sulfide assemblages is, therefore, a logical consequence of changes in structural permeability due to the coupling of alteration and fracture dynamics rather than to changes in the chemistry of the fluid phase at its magmatic source.

REE and HFS element behaviour in the alteration facies of the Erenler Dağı Volcanics (Konya, Turkey) and kaolinite occurrence

Mainly high-K, calc-alkaline, Late Miocene to Pliocene volcanic rocks cropped out of the Konya area in Central Anatolia, Turkey. The volcanic rocks are predominantly andesitic to dacitic in composition and rarely basalt, basaltic andesite, basaltic trachyandesite and pyroclastics. Kaolinite, illite, Ca-montmorillonite, alunite, jarosite, minamiite and silica polymorphs were formed by widespread and intense hydrothermal alteration in or around the volcanic products. To investigate the effects of hydrothermal alteration on the chemistry of volcanic rocks, the whole rock chemical composition (major and trace elements, including rare-earth elements (REE) was analysed. The results of the study demonstrate notable differences in the REE behaviour in the different sample groups. REE trends of fresh parent rocks to weakly-, moderately-altered, kaolinitic and alunitic rocks are characterised by strong LREE enrichment ((La/Lu) cn = 14.57, 11,8 to 15.20, 4.54 to 13.30, 12.5 to 24.2 and 34.6 to 47.26, respectively). Most of the samples have pronounced negative and/or weakly-negative Eu anomalies ranging from 0.75 to 0.98 while three samples have weakly-positive Eu anomalies. LRE element contents are higher than those of HREE in the samples. The LRE elements were strongly enriched in the kaolinitic and alunitic alteration; in weakly- and moderately-altered rocks. LREE are nearly immobile whereas HRE elements show different behaviour in different rock groups. The HFS and TRT elements are slightly mobilised in weakly-altered rocks, but enriched in other alteration types. Elements commonly assumed to be immobile (e.g. Y, Zr, Nb, Hf, TiO 2, Al 2O 3, REE) show variation in mass calculation. LIL elements showed enrichment over LREE and MREE, and similar behaviour, in contrast with HFSE. A clear increment of trans-transition elements (TRTE) was found mainly in alunitic and partly in kaolinitic samples.

Fluids in early stage hydrothermal alteration of high-sulfidation epithermal systems: A view from the Vulcano active hydrothermal system (Aeolian Island, Italy)

High-sulfidation (HS) epithermal systems have elements in common with passively degassing volcanoes associated with high T, acid fumarole fields or acid crater lakes. They are considered to form in two stages, the first of which involves advanced argillic alteration resulting from intense, strongly acidic fluid–rock interaction. The La Fossa hydrothermal system (Vulcano Island) represents a classic example of such an active HS system and can be considered as a modern analogue of this early stage of alteration, resulting in a core of intense silicic (90–95% pure SiO 2) alteration surrounded by alunitic alteration zones. This paper focuses on a geochemical and stable isotope study of the surficial alteration facies of Vulcano – particularly the horizon characterized by strong silicic alteration – and on deep seated xenoliths ejected during the last eruption of La Fossa volcano (1888–90) that can be considered as representative of fragments of the deep conduit system of La Fossa volcano. Using directly measured temperatures at the sites of sampling, we have calculated fluid composition in isotopic equilibrium with the alteration products. The large range of measured silica δ 18O (12.3 to 29‰) reflects the wide range of formation temperatures (80–240 °C). The fluid compositions calculated for intense silicic alteration vary from − 0.9 to + 6.5‰. These are significantly heavier than local meteoric water (− 6‰), and are consistent with derivation from the condensation of high-temperature fumarolic gases, dominated by magmatic fluids and rich in acid gases (SO 2, H 2S, HCl, HF), into shallow groundwaters of meteoric origin, with dynamically variable ratios of fumarolic steam/meteoric water. The calculated δ 18O and δD of water in equilibrium with alunite also suggest the mixing of magmatic and meteoric waters for the fluids involved in the genesis of advanced argillic alteration facies. The calculated δ 18O of water in equilibrium with hedenbergitic clinopyroxene, found in a veinlet in a metasomatized xenolith is + 8.9‰. This value cannot reasonably result from water–rock interaction between the host volcanic rocks and surface water. Instead, it most likely represents a fluid (brine) exsolved from magma, which was responsible for high temperature metasomatism in the deep conduit system.

The Pierina epithermal Au–Ag deposit, Ancash, Peru: paragenetic relationships, alunite textures, and stable-isotope geochemistry

The Pierina high-sulfidation epithermal Au–Ag deposit (9°26.5′S; 77°35′W) was emplaced in the Middle Miocene into a hypabyssal-to-extrusive “pumice-tuff” and an underlying, older, dacitic flow-dome complex, both of which are cut by hydrothermal breccias and small dacitic domes. Stage I advanced argillic alteration generated a core of vuggy silica, focused in the tuff, and surrounded successively by zones of quartz–alunite, dickite±kaolinite±pyrophyllite, and illite–montmorillonite±kaolinite. Laser-ablation ICP-MS analysis of the sulfide minerals of the succeeding, Stage II, Cu (–Pb, Bi, Sb, Zn, As)–barite mineralization, largely confined to the vuggy-silica zone, reveals that both Au and Ag were introduced at that time. This assemblage was almost entirely obliterated during Stage III, when oxidation by low-temperature meteoric waters generated botryoidal hematite–goethite assemblages, which are now the main precious-metal hosts. Stage IV barite–acanthite mineralization shows limited temporal overlap with Stage III, but dominantly overprints the hematite–goethite assemblage. The deposit incorporates alunite exhibiting a wide range of modes of occurrence, grain size, and morphology. Disseminated alunite dominates Stage I alteration, which replaced phenocrysts and fragments, and locally hosts corroded, ≤40 μm alumino-phosphate-sulfate (APS) inclusions. The alunite shows limited Na substitution [molar Na/(Na+K)=<0.2], but alunite–natroalunite assemblages occur sporadically throughout the quartz–alunite alteration zone. Isotopic analysis yields δ 34S values of 16.6–31.0‰, consistent with a magmatic–hydrothermal origin. Rare occurrences of disseminated alunite and pyrite in textural equilibrium occur in unoxidized areas of the deposit, and yieldΔ 34S alu-py precipitation temperatures of 179 to 250 °C, with the majority below 200 °C. Disseminated alunite in shallower, oxidized portions of the deposit, where pyrite has been destroyed, yield δ 18O SO 4 values of 6.3‰ to 14.4‰. The lighter compositions occur along the axis of the quartz–alunite alteration zone, and imply precipitation at higher temperatures, albeit with some meteoric water involvement. Porcelaneous alunite (10–100 μm) forms the matrix of breccias that cut Stage I alteration. Sodium contents are low [Na/(Na+K molar≃0.1) and APS inclusions have not been identified. The δ 34S values of 21.8–27.1‰ are consistent with a magmatic–hydrothermal origin, but the δ 18O SO 4 values of 11.4‰ to 14.6‰ indicate deposition under cooler temperatures and the involvement of meteoric water that interacted substantially with igneous country-rocks. Coarse, open-space-filling alunite, which occurs as veins or coatings on breccia fragments, exhibits limited Na substitution, is not associated with natroalunite, and does not host APS inclusions. Its δ 34S and δ 18O SO 4 values of 12.9‰ to 26.2‰ and 6.5‰ to 8.6‰, respectively, imply a magmatic–hydrothermal origin and deposition at higher temperatures. Dike-like bodies and mantos of friable alunite have distinctive δ 34S (16.2–19.3‰) and δ 18O SO 4 (7.6–9.2‰) values, and may record a steam-heated environment. However, these compositions may also indicate loss of H 2S gas during oxidation, or overprinting of steam-heated alunite by magmatic–hydrothermal alunite (or vice versa). All forms of alunite exhibit higher δ 18O SO 4 values in the central part of the deposit, and the coherent isotopic chemistry casts doubt on the reliability of the widely accepted textural criteria for the origin of this mineral in hydrothermal systems.

Geochemistry of metabasalts and hydrothermal alteration zones associated with c. 3.45 Ga chert and barite deposits: implications for the geological setting of the Warrawoona Group, Pilbara Craton, Australia

Relatively unaltered metabasalts of the Archaean Coonterunah and Warrawoona Groups, Pilbara Craton are compared with altered metabasalts from immediately beneath bedded cherts of these groups to provide evidence for the depositional environment and hydrothermal alteration processes of crust formation. The geochemistry of relatively unaltered basalt, stratigraphy, and inherited zircon data indicate that the lower Warrawoona Group (3.53–3.43 Ga) formed as an oceanic plateau complex built on a sialic basement to 3.724 Ga, following an analogue with the Phanerozoic Kerguelen oceanic plateau, and not as a mid-ocean ridge or convergent volcanic-arc complex as previously proposed. Advanced argillic, argillic, phyllic, and propylitic alteration zones in footwall basalts of this succession are products of repeated episodes of seafloor hydrothermal circulation, syngenetic with bedded chert deposition, in the distal parts of high-sulphidation epithermal systems. The upper part of the Warrawoona Group (3.350–3.315 Ga Euro Basalt) represents a continental flood basalt event, up to 8 km thick, that erupted onto the older succession across a regional unconformity on which the Strelley Pool Chert was previously deposited. Widespread silica–alunite alteration of dolomitic chert protoliths and phyllic and propylitic alteration of footwall basalts are interpreted as products of fluid circulation driven by heat from the overlying, newly erupted lavas.

Alunite alteration of tuffaceous layers and zircon dating, Upper Permian Kennedy Group, Carnarvon Basin, Western Australia

Alunite layers were found in two locations in the Binthalya Formation, Upper Kennedy Group, of Late Permian age, in the Merlinleigh Sub‐basin, onshore Carnarvon Basin. Alunite has not previously been reported from the Kennedy Group, or from a similar setting in Western Australia. The alunite occurs in discrete layers that have different chemical signatures to surrounding, unaltered sedimentary rocks. The lack of alteration in surrounding sedimentary rocks suggests that alteration was localised by initial differences in lithology, rather than by fortuitous localisation of alteration in a single layer. Alunite is thought to be the product of alteration of K‐feldspars, clay minerals and pyrite under acidic, specifically sulfuric, conditions. It is found in hydrothermal systems and weathering profiles. Because the precursor minerals of alunite are believed to be feldspars, clays and sulfur‐rich minerals, tuffaceous layers are the probable precursors of the alunite layers. Sulfur for the reaction was derived from within the tuffaceous deposits or from the oxidation of pyrite, which was probably present as cementation of trace fossils in other beds. The alteration is thought to have occurred during extensive oxidation, weathering and laterite formation during the Cenozoic, because the low density and powdery nature of the alunite indicates that it developed at a shallow depth. U–Pb dates obtained from zircons extracted from the alunite layers returned ages ranging from ca 2690 Ma to ca 270 Ma. The majority of zircons are inherited from the same source as the Kennedy Group sediments, indicating mixing of the initial tuffaceous layers with background sedimentation, either during the sedimentation process or afterwards by reworking or biological disturbance.

Thermodynamic basis for distinguishing the acid-sulphate and adularia-sericite alteration types of rock in epithermal deposits and genetic analogy with the metasomatites of the formation classification

The study of mineral equilibria in the K2O-Al2O3-SiO2-H2O-SO3 system permits to propose thermodynamic basis for distinguishing the acid-sulphate and adularia-sericite alteration types of volcanic rokcs and frequently associated subvolcanic intrusive rocks in the epithermal deposits. The fields of the acid-sulphate and adularia-sericite types of alteration are divided by the fields of the advanced argillic alteration without sulphatebearing minerals and the sericitiztion in the prognostic paragenetic diagram of the mineral equilibria in the system studied. This study presents theoritical arguments for the existence of the two end paragenetic incompatible types of wall rock alteration in epithermal deposits. The advanced argillc alteration of the rocks is subdivided of the acid-sulphate and acid-chlorine types of alteration. The mineralogical synonyms of the acide-sulphate type are kaolinite-alunite and alunite alteration types and of the acid-chlorine type are kaolinite, pyrophyllite and dickite-pyrophyllite types wallrock alteration. The study of the system is basis for genetic interpretations of the advanced argillic, sericitic, greisen, K-silicate and adularia-sericite alteration.

Hydrothermal Alteration of the Saplica Volcanic Rocks, Sebinkarahisar, Turkey

A widespread, intense hydrothermal alteration zone has developed in the Cretaceous Saplica volcanics as a result of the intrusion of Late Cretaceous-Paleocene granitoids. The propylitic, phyllitic (sericitic), and argillic alteration along with hematite, silica polymorphs, and two types of tourmaline mineralization developed under a wide range of Eh and pH conditions. Alunite, kaolinite, and silica are abundant in the argillic alteration, whereas sericite dominates in the phyllic alteration. Most of the major alunite deposits are located along the periphery of the Saplica volcanic rocks and in addition contain alunite, kaolinite + quartz ± opal ± cristobalite. Illite and pyrite, barite, and gypsum also occur in small amounts. Major and trace elements are concentrated in, or were leached from, the volcanic rocks, depending upon the alteration types. In general, Al + K and Mg + Ca + Fe were enriched in the alunitic + sericitic and propylitic alteration types, respectively. On the other hand, Ca, Mg, and Fe were leached during argillic alteration, and Fe was concentrated in hematite formation. Strong leaching of Na was determined for alteration types. Silica generally decreased in argillitic (kaolinitic and alunitic) alteration zones. Most trace elements were mobile during hydrothermal alteration. Y, Sc, Mo, Cr, Co, Ni, and Zn tend to be mobile in acid aqueous systems, and thus are nearly absent in these alunitic alteration zones. In the surrounding kaolinitic envelope, these elements are present at background (average) or slightly higher concentrations. Rb and Sr contents are high in the alunitic and kaolinitic zones. Barium is highest near the alunite zone because of the relative insolubility of barite in acidic solutions. Pb and Cu contents increase in the propylitic zone. Such hydrothermal alteration zones can be used effectively in the exploration and evaluation of mineral resources of the eastern Black Sea region.

Alteration-mineralization zoning and fluid inclusions of the high sulfidation epithermal Cu-Au mineralization at Zijinshan, Fujian Province, China

The Zijinshan mine in Fujian Province in the southeast of China is the first recognized example of high sulfidation epithermal Cu-Au mineralization of Cretaceous age in mainland China. The Cu-Au mineralization occurs around a dacitic volcanic pipe in the central part of a calc-alkaline, trachydacitic to dacitic volcanic dome of Cretaceous age that has intruded an S-type Jurassic granite. The Cu-Au orebodies are subparallel, northwest-trending breccias, veins, and massively silicified rocks which have developed along extensional fractures in wide hydrothermal alteration zones. At depth the dacite porphyry pipe changes downward into a potassic-altered granodiorite porphyry associated with porphyry copper-style mineralization, indicating a close spatial and temporal association between high sulfidation Cu-Au and porphyry copper mineralization.Zoning of hydrothermal alteration and ore mineralization assemblages at Zijinshan is typically recognized from the deep and central zones toward shallower and outer portions within the epithermal system. A deep sericite + quartz + pyrite assemblage in the phyllic alteration zone gives way upward to a dickite + quartz + pyrite + or - zunyite + or - sericite assemblage in the dickite alteration zone and successively to the alunite + quartz + pyrite assemblage in the funnel-shaped alunite alteration zone at depths between 350 and 1,200 m from the land surface. High sulfidation copper mineralization is typically developed within the alunite alteration zone and shows a gradual change of ore mineralogy from digenite and enargite preferentially at deeper levels to covellite and gold in shallow levels. The silicic alteration and associated gold mineralization are restricted at shallow depths (less than 350 m). Vertical metal zoning in the sequence Au-Ag-As --> Cu --> Cu-Pb-Zn is also recognized within the silicic and alunite alteration zones.Paleogeographic reconstruction indicates depths of 2 was probably formed by phase separation of a deep magmatic fluid and subsequently ascended northwestward. The upwelling magmatic vapors condensed into meteoric water at shallow levels (higher than 1,700 m below the paleosurface), forming the high sulfidation epithermal fluids (

The timing of adularia-sericite-type mineralization and alunite-kaolinite-type alteration, Mount Skukum epithermal gold deposit, Yukon Territory, Canada; 40 Ar- 39 Ar and U-Pb geochronology

The lower Eocene Mount Skukum epithermal Au deposit, southwestern Yukon Territory (60 degrees 13' N, 135 degrees 28' W), exhibits both hypogene alteration and mineralization facies characteristic of the adularia-sericite deposit type, and barren advanced argillic alteration of the alunite-kaolinite type, the two being separated by only 750 m on the present erosional surface.The advanced argillic alteration forms a crudely cylindrical, possibly downward-tapering body, with an alunite-quartz core and surrounding pyrophyllite + or - kaolinitc zones, and is centered on a small (< or = 150-m diam)rhyolitic stock. The delta D and delta 34 S values of the alunite, which average -175 + or - 0.5 (1Sigma ) per mil and +2.93 + or - 0.43 (1Sigma ) per mil, respectively, suggest that this alteration conforms to the hypogene type formed by magmatic SO 2 mixing with meteoric water, an inference that is consistent with mineral associations and textures that imply fairly high temperatures (pyrophyllite stable). The 40 Ar- 39 Ar age spectra for alunite, generated by furnace heating, exhibit good multi-step plateaus and show that during slow heating with small-increment steps alunite releases virtually all of its argon through volume diffusion at temperatures below 540 degrees C. The preferred age for the alunitic alteration is 55.69 + or - 0.24 (2Sigma ) Ma, the error-weighted average of four plateau dates that range from 55.00 + or - 0.55 to 56.12 + or - 0.45 Ma. Calculated closure temperatures for alunite average 280 degrees + or - 20 degrees C (2Sigma ) at a cooling rate of 50 degrees C/m.y.Gold-bearing quartz-calcite(-adularia) veins are hosted by andesitic volcanic strata of the Eocene Skukum Group and by rhyolitic dikes. The mineralized veins conform entirely to the adularia-sericite class and were controlled by strike-slip faults and fracture zones. A wide rhyolite dike cut by the most important ore zone was emplaced at 56.3 + or - 0.4 (2Sigma ) Ma (earliest Eocene), as defined by U-Pb zircon dating. Step-heating 40 Ar- 39 Ar dating of vein adularia, with orthoclase structure, yields plateaus, which suggest that no significant Ar loss occurred. The error-weighted average of two plateau dates for the Lake zone and the lowest age steps in two slightly saddle-shaped plateaus for the Cirque zone, 54.05 + or - 0.31 Ma, is taken as the age of adularia-sericite-type mineralization, which is slightly but significantly younger than the alunite-kaolinite-type alteration. A swarm of intermediate dikes, some of which cut ore veins, was intruded over an interval that embraced the time of mineralization. One of these dikes, which cuts the wide rhyolite dike, yielded a U-Pb zircon age of 55.7 + or - 0.3 Ma, but this does not further constrain the age of the mineralization.The ages of alunite and adularia from Mount Skulmm indicate a succession from the barren alunite-kaolinite-type hydrothermal system to the gold-bearing adularia-sericite-type system, both of which formed during ongoing magmatism in the Mount Skukmn Volcanic Complex.

The application of imaging spectrometry data to mapping alteration zones associated with gold mineralization in southern Spain

Abstract The gold mineralization at the Rodalquilar mine in southern Spain is of the acid-sulphate type. We have assessed the ability of AVIRIS imaging spectrometer data to detect the hydrothermal alteration mineralogy mapped using traditional ground-based techniques at Rodalquilar. Four methods of retrieving apparent surface reflectance have been evaluated, the empirical line method and three methods of radiative transfer modelling. This study indicates that radiative transfer modelling, using only atmospheric information derived from the imaging spectrometry data, can satisfactorily correct the atmospheric effects involved in retrieving apparent surface reflectance. The alteration at Rodalquilar is characterized by propylitic (vermiculite), illitic, kaolinitic (pyrophyllitic), alunitic and silicic zones with an increasing degree of alteration towards the formative hydrothermal cells in whose fossil cores the gold mineralization is found. However, several factors act against the usefulness of the AVIRIS data for mapping these mineral zones: the mine site is heavily disturbed with exposed workings, often of high albedo altered rocks, whilst unworked areas have partial iron-rich soil and dry vegetation cover; the alteration itself is highly discontinuous at the scale of 10 m and the shortwave infrared (SWIR) data where much of the diagnostic information for these hydrothermal minerals lies is very noisy. Nevertheless it has proved possible to map successfully, at the individual pixel level, the zones of gold-bearing alunitic alteration using the joint presence of alunite absorption features at 1480 and 1760 nm. The data from the 2000-2400 nm spectral range allow the areas with general absorption features centred around 2200 nm to be mapped but zonation based on individual minerals identification is not possible.

The Shumake volcanic dome-hosted epithermal, precious metal deposit, western Mojave Desert, California

Several Tertiary volcanic-hosted precious metal deposits occur in the Mojave block in southern California which is bounded on the north by the Garlock fault and on the southwest by the San Andreas fault. The Middle Buttes volcanic dome complex hosts several low-grade gold and silver orebodies and is comprised of Miocene-age coalescing lava domes, flows, vent breccias, and pyroclastic deposits lying unconformably on Cretaceous basement of the Sierra Nevada batholith. The volcanic rocks range in composition from andesite through rhyolite and are cut by numerous faults.The Shumake deposit, over 5 million tons of ore with an average grade of 0.043 oz per ton (1.5 ppm) Au and 0.4 oz per ton (14 ppm) Ag, is centered around two large quartz veins and the intervening altered rhyolites. The large veins are characterized by pyrite, arsenopyrite, and silver sulfosalts, with minor base metal sulfides and gold. Gold in the altered rhyolite occurs with stockwork quartz veinlets and an oxidation assemblage of iron oxides, scorodite, and red kaolinite. Alteration of the rhyolites is best characterized by sericite, composed of fine-grained illite and muscovite, and stockwork quartz veinlets with associated adularia. Fluid inclusions indicate that the stockwork quartz formed around 210 degrees C. The quartz, sericite, and minor adularia assemblage is underlain by propylitic alteration and overlain in one part of the deposit by alunite and white kaolinite alteration. These main-stage alterations were followed by an oxidation assemblage which is interpreted to be related to collapse of the hydrothermal system. The deposit is dominated by anomalous abundances of Au, Ag, Hg, As, and Sb. Spatial geochemical variations suggest that the two massive quartz veins were dominant components of the hydrothermal plumbing system.The paleohydrologic system at the Shumake deposit is analogous to the adularia-sericite type of volcanic-hosted epithermal precious metal deposits. Faults and fractures provided the pathways for hydrothermal fluid movement. Vein textures suggest episodic mineralization that can be explained by seismic pumping of hydrothermal fluids. Since a close temporal relationship exists between volcanism and ore deposition, magmas probably provided heat to the hydrothermal system. Thus, for the Shumake deposit, faulting provided both the pathways and driving mechanism for the hydrothermal system while heat was provided by magmas.

Thermal infrared spectra and images of altered volcanic rocks in the Virginia Range, Nevada

Abstract Airborne Thermal Infrared Multispectral Scanner (TIMS) data and laboratory thermal infrared reflectance spectra were collected from sites of hydrothermally altered andesitic volcanic rocks near Virginia City, Nevada. Alunitic, kaolinitic, illitic, and propylitic alteration types have distinct laboratory spectral curves, although the individual spectra are difficult to interpret mineralogically. TIMS emittance spectra show a general shift in the wavelength of the silicate emittance minima to shorter wavelengths with increasing intensity of alteration, owing to the increasing abundance of secondary framework and sheet silicate minerals. Strongly altered volcanic rocksare identifiable on TIMS radiance and emittance imagery.

Geology and K-Ar geochronology of the Paradise Peak Mine and the relationship of pre-Basin and Range extension to early Miocene precious metal mineralization in west-central Nevada

The Paradise Peak mine is a major gold-silver-mercury deposit located in the southwestern part of the Paradise near the eastern edge of the Walker Lane in the western Great Basin, Nevada. The Tertiary volcanic rocks of the area can be divided into a sequence of 26- to 24-Ma intermediate lavas, a sequence of 24- to 22-Ma silicic ash-flow tuffs, and a sequence of 20- to 15-Ma intermediate lavas. We classify these rocks as the older, middle, and younger sequences. Sedimentary rocks, and latites and basalts 12 Ma old or younger, locally overlie the younger intermediate lavas south of the mine. Silicified rhyolite tuff in the lower part of the middle tuff sequence is the principal host of the precious metal ore. Most ore occurs under an acid-leached zone of alunitic alteration and is contained in hydrothermal breccias that crosscut early quartz-pyrite and alunite alteration.Field relations and K-Ar ages of hypogene alunite indicate that precious metal mineralization and alunitic and silicic alteration formed at about 19 to 18 Ma, several million years after eruption of the main host rock. Mineralization formed during high-angle faulting related to crustal extension. Low-angle normal faults are present 5 km northeast of the Paradise Peak mine, but they formed more than 2 Ma after formation of the Paradise Peak deposit. Silicic and alunitic alteration and precious metal mineralization are generally absent in areas of low-angle faulting.Regional stratigraphic relations and K-Ar ages indicate that volcanism changed from silicic ash-flow tuffs to intermediate lavas at about 20 to 19 Ma. Regionally extensive angular unconformities indicate that a period of and Range crustal extension occurred between about 22 to 19 Ma. This extension was penecontemporaneous with the shift in the style of volcanism and with gold-silver mineralization in the Paradise Peak mine and in the Goldfield and Tonopah districts of western Nevada. The close temporal and spatial relationships of precious metal mineralization with pre-Basin and extension suggest that extension was a major factor in the genesis of early Miocene precious metal deposits in the western Great Basin.