Propylitic Zone Research Articles

Fluid inclusion noble gas and halogen evidence on the origin of Cu-Porphyry mineralising fluids

The naturally occurring noble gas isotopes (40Ar, 36Ar, 84Kr and 129Xe) and halogens (Cl, Br, I) have been determined simultaneously in irradiated quartz vein samples by noble gas mass spectrometry. Quartz vein samples were collected from the potassic and propylitic alteration zones of six porphyry copper deposits (PCD): Bingham Canyon, Utah; and Silverbell, Ray, Mission, Pinto Valley and Globe-Miami in Arizona. In addition, analyses of 3He/4He have been obtained from sulphide minerals hosted by the quartz veins at Silverbell, Ray, Pinto Valley and Globe-Miami.The majority of PCD fluids studied have Br/Cl and I/Cl ratios that overlap those of fluids included in mantle diamond, suggesting that the salinity had a juvenile origin. The high I/Cl (121 × 10−6 mole, M) in samples from the propylitic zone of Silverbell is attributed to the presence of sedimentary formation water.3He/4He ratios have R/Ra values in the range 0.3 to 1.72, and provide evidence for the involvement of a crustal component in addition to mantle volatiles. 40Ar/36Ar ratios vary from meteoric values of ∼317 in the propylitic zone of Bingham Canyon, and 323 in the skarn alteration of Mission up to 3225 in the potassic zone of Pinto Valley. Fluids in both the potassic and propylitic alteration zones of every deposit are a mixture of a low salinity end-member comprising meteoric water and air, and a high salinity end-member consisting of a mixed mantle and crustal fluid.The 40Ar/Cl ratio of fluid inclusions at Pinto Valley (∼10−4 M) is similar to values obtained previously for mantle fluids. The 40Ar/Cl value is two orders of magnitude lower at Bingham Canyon, where a depleted 36Ar concentration (0.2 × 10−6 cm3/g) below that of air saturated water (ASW), and a range of highly fractionated noble gas compositions (F84Kr = 13 and F129Xe = 160) indicate that boiling and pulsed fluid flow have occurred.

Mineral Paragenesis of the Lepanto Copper and Gold and the Victoria Gold Deposits, Mankayan Mineral District, Philippines

Abstract: Mineral paragenesis of the alteration, ore and gangue minerals of the Lepanto epithermal copper‐gold deposit and the Victoria gold deposit, Mankayan Mineral District, Northern Luzon, Philippines, is discussed.The principal ore minerals of the Lepanto copper‐gold deposit are enargite and luzonite, with significant presence of tennantite‐tetrahedrite, chalcopyrite, sphalerite, galena, native gold/electrum and gold‐silver tellurides. Pervasive alteration zonations are commonly observed from silicification outward to advanced argillic then to propylitic zone. The ore mineralogy of the Lepanto copper‐gold deposit suggests high fS2 in the early stages of mineralization corresponding to the deposition of the enargite‐luzonite‐pyrite assemblage. Subsequent decrease in the fS2 formed the chalcopyrite‐tennantite‐pyrite assemblage. An increase in the fS2 of the fluids with the formation of the covellite‐digenite‐telluride assemblage caused the deposition of native gold/electrum and gold‐silver tellurides.The principal ore minerals of the Victoria gold deposit are sphalerite, galena, chalcopyrite, tetrahedrite and native gold/electrum. The alteration halos are relatively narrow and in an outward sequence from the ore, silica alteration grades to illitic‐argillic alteration, which in turn grades to propylitic alteration. The Victoria gold mineralization has undergone early stages of silica supersaturation leading to quartz deposition. Vigorous boiling increased the pH of the fluids that led to the deposition of sulfides and carbonates. The consequent decrease in H2S precipitated the gold. Gypsum and anhydrite mainly occur as overprints that cut the carbonate‐silica stages.The crosscutting and overprinting relationships of the Victoria quartz‐gold‐base metal veins on the Lepanto copper‐gold veins manifest the late introduction of near neutral pH hydrothermal fluids.

Halogen and Ar–Ar age determinations of inclusions within quartz veins from porphyry copper deposits using complementary noble gas extraction techniques

Extension of Ar–Ar methodology has been used to determine mineralisation ages from mica inclusions and to simultaneously evaluate the noble gas and halogen composition of inclusion fluids within irradiated quartz vein samples from five porphyry copper deposits. Samples have been collected from the potassic and propylitic zones of Bingham Canyon, Utah, and four Arizonan deposits; Silverbell, Globe-Miami, Pinto Valley and Ray. Data obtained using three noble gas extraction techniques (laser ablation, in vacuo crushing and stepped heating) are compared with each other. Laser ablation provides a means for the analysis of individual fluid inclusions but is limited by blank levels and detection limits. Stepped heating and in vacuo crushing are bulk extraction techniques that preferentially release gases from solid and fluid inclusion phases, respectively, and can be used in combination to obtain accurate and meaningful ages of mineralisation. Ages obtained for porphyry copper deposits are as follows: Bingham Canyon 37.1±0.5, Ray 65.3±1.5, Globe-Miami 61.7±3.4, Pinto Valley 63.2±8.0 and Silverbell 55.8±1.8. The age of mineralisation at Ray is of particular interest as it enables a reported discrepancy in K–Ar ages to be understood. Halogen data for the Bingham Canyon samples gives insight into the partitioning of the heavy halogens (Br and I) between solid and liquid phases.

Geology and structural controls of the Twin Peaks epithermal gold deposit, East Junggar, Xinjiang, China

The Twin Peaks epithermal gold deposit, located in East Junggar, China, is hosted by the late Paleozoic Kulankazigan island-arc complex. The deposit is structurally controlled by the extensional radial fractures associated with development of the Early Carboniferous Twin Peaks volcanic dome. It is stratigraphically restricted to the upper Batamayineishan Formation, which is subdivided into two units: the lower andesite unit that contains the ore bodies, and the upper rhyolite unit. Four hydrothermal alteration zones have been identified in both East and West Zones of the deposit. Each zone is characterized by a highly silicified core, and then there is an outward progression through adularia–sericite, argillic, and propylitic zones. The presence of adularia and sericite, in addition to the sulfide mineral association, suggests that the gold deposit is of the "adularia-sericite" or "low sulfidation" type. The East Zone ore body is offset by post-mineralization, high-angle reverse faults that strike parallel to the zone and small-scale, strike-slip faults that strike perpendicular to the zone.

Reaction Mechanisms of Smectite Illitization Associated with Hydrothermal Alteration from Ponza Island, Italy

AbstractA hydrothermally altered rhyolitic hyaloclastite from Ponza island, Italy, has four alteration zones with unique clay assemblages: (1) a non-pervasive argillic zone characterized by smectite; (2) a propylitic zone with interstratified illite-smectite (I-S) containing 10–85% illite (I); (3) a silicic zone composed of I-S with ≥90% I and pure illite; and (4) a sericitic zone with I-S ranging from 66% I to pure illite. Atomic force microscopy reveals abrupt changes in particle morphology with illitization, including initial changes from anhedral plates to laths and then to euhedral plates and hexagonal plates. I-S particles progressively thicken with illitization and mean particle area (basal plane) remains constant from pure smectite to I-S with 80% I. However, particle area increases from 90 to 100% illite. Computer modeling of I-S structural forms indicates octahedral cation ordering progressively changes from cis- vacant smectite to interstratified cis- and tnuis-vacant I-S, and then to trans-vacant illite. In addition, polytypes progressively change from 1 Md to 1M, and then to 2M, illite. Electron-microprobe and X-ray fluoresence analyses show that I-S chemistry progressively changes during illitization, evolving toward a phengitic composition with ∼0.89 fixed interlayer K+ per O10(OH)2. Octahedral Mg2+ shows little change with illitization, varying from 0.3 to 0.5 cations per O,10(OH)2. The layer charge of smectite is ~0.38 equivalents per O10(OH)2.On the basis of abrupt changes in morphology and progressive changes in polytype and chemistry, smectite illitization on Ponza involved a dissolution and recrystallization mechanism with multiple stages of nucléation and crystal growth. In this multi-step model, temperature of alteration provided the major control for the layer composition, polytype, and morphology of I-S crystallites. Other factors that may play a secondary role include: K+ availability, water-rock ratio, and permeability. Alternatively, the mechanism of I-S and illite formation at Ponza and other hydrothermal environments may occur by direct precipitation of I-S crystallites from rhyolite glass and may not involve progressive reactions of smectite precursors.

Palladium, platinum and gold concentration in porphyry copper systems of Greece and their genetic significance

Compilation of new and published data on precious metal and associated trace-element contents in mineralized porphyry copper intrusions of Greece indicates that the Pd content in the potassic–propylitic zone of the Skouries deposit is relatively high, ranging between 60 and 200 ppb (average 110 ppb), whereas in the porphyry copper intrusions of Gerakario and Pontokerasia, it is very low (3 ppb Pd). The oxidized zone of Fissoka (OP-65) exhibits low Pd and Cu contents, but higher Au, Te, As, Pb and Zn contents. The Pt content is much lower than Pd in the porphyry copper intrusions studied. The Pd/Pt ratio in the Skouries deposit ranges from 9 to 60, while it is much lower (<2) in the mineralized samples from Fissoka, Gerakario and Pontokerasia. A decrease of the Pd content and Pd/Pt ratio with increasing Te, As, Pb and Zn content may indicate that the Pd distribution in porphyry copper systems is effected by the evolution of the magma-hydrothermal system. Also, mineralogical and geochemical data on the Skouries deposit, coupled with textural relations between base metal sulphides, PGM and Au–Ag tellurides, indicate that the main Pd-bearing mineral, merenskyite, is associated with the vein-type chalcopyrite or bornite, and was deposited during the main stage of mineralization. The high magmatic oxidation state and the magmatic features of the hydrothermal system (limited participation of meteoric portion) in the Skouries intrusion may be a strong control of its Pd potential. Assuming that Pd is mainly associated with chalcopyrite in the porphyry copper deposits, the calculated Pd (from measured Pd) contents in chalcopyrite (average 3300 ppb Pd) are relatively high, and comparable to that in the chalcopyrite concentrate made after the processing of large composite drill-hole samples (2400 ppb Pd to 21 wt.% Cu). It is considered to be an encouraging economic factor for Pd as a by-product, probably along with Au, although the main product is Cu.

Mapping hydrothermal alteration in the Comstock mining district, Nevada, using simulated satellite‐borne hyperspectral data

Hydrothermal alteration mapping with spaceborne hyperspectral data was simulated in the Comstock mining district, Nevada in order to evaluate the mineral mapping capabilities of the proposed Australian Resource Information and Environment Satellite (ARIES‐1). As a result, a suite of hydrothermal alteration minerals, including kaolinite, dickite, illite, chlorite, alunite and carbonate was identified from the simulated data in the 0.4–2.5 μm wavelength region and their areal abundance variations mapped accordingly. The recognised alteration zoning shows a major change in alteration assemblages across the Comstock and Silver City Faults, and a gradual variation from north to south along the faults. In the bleached Miocene volcanic rocks, dickite, kaolinite, illite and alunite were recognised. Coexistence of dickite of relatively high temperature, high‐crystallinity kaolinite of medium temperature and low‐crystallinity kaolinite of low temperature suggests supergene processes overprinting earlier hypogene alteration. The bleached rocks probably represent hydrothermal alteration in the fluid up‐flow zones in the central and shallower parts of the hydrothermal system. Illite in the bleached zones is characterised by relatively short AI–OH band wavelengths (2190–2200 nm), indicating no or very low Fe and/or Mg contents. Fault‐controlled propylitic alteration is mapped in the central part of the district mainly in the footwall of the Comstock Fault. The associated illite is characterised mainly by medium AI–OH band wavelengths (2200–2208 nm). This propylitic alteration may be contemporaneous with Au–Ag mineralisation. Additional and more extensive propylitic zones, containing illite with long AI–OH band wavelengths (2204–2216 nm), were mapped in the southern part of the district. These zones resulted from either a pre‐mineralisation propylitic alteration, or the peripheral hydrothermal alteration in the fluid down‐flow zones of the Miocene hydrothermal system.

Granite-hosted gold mineralization in the Midlands greenstone belt: a new type of low-grade gold deposit in Zimbabwe

In 1992, the Ford gold deposit was rediscovered during field work in the Kwekwe district near the Indarama mine, approximately 200 km southwest of Harare, Zimbabwe. Based on diamond drilling and open pit operations, estimated ore reserves are at least 3 Mt with an average gold content of 2.5 g/t. The gold deposit is located within a porphyritic granite dike with a thickness of 20–50 m, striking 800 m NNW-SSE. It dips 60–70° to the NE and intrudes a volcano-sedimentary sequence of tholeiitic basalts, acid volcanics, and banded iron formations of the Bulawayan Group (2900–2700 Ma). The intrusion of the dike occurred at 2541 ± 17 Ma (Pb/Pb step leaching technique) within a second order structure and is related to displacement along transcrustal deformation zones such as the Sherwood- and Taba-Mali deformation zones. Gold mineralization is confined to the s-shaped part of the dike intrusion. At the present stage of mining, the deposit is characterized by the absence of major veins, the occurrence of disseminated pyrite throughout the orebody, and a distinct alteration pattern comparable to that of porphyry copper deposits. The central zone of the dike shows a typical K-feldspar-albite-sericite-pyrite (±biotite?) alteration, followed by a narrow external propylitic zone. Native gold with an average Ag content of 5 wt.% and a grain size of 5–100 μm is rare and occurs within pyrite and secondary K-feldspar. Sulphide mineral separates of pyrite and minor arsenopyrite probably contain invisible gold (up to 120 ppm) amenable to cyanidation. Anomalously high gold values of ∼7 ppm have been found in the transition between the K-feldspar-albite-sericite-pyrite alteration and the propylitic zone, indicating that the mineralizing fluids have experienced major physico-chemical changes in the transition zone. The regional tectonic position of the orebody suggests that the emplacement of the granite and the gold mineralization are structurally controlled. The Pb isotope composition of several leachates of pyrite indicate isotope disequilibrium with magmatic minerals and point to a contamination of the mineralizing fluid by Pb from older (sedimentary?) sources. Stable isotope geochemistry of sulphides and carbonates as well as the metallogeny of the deposit compare to shear-zone hosted gold mineralization in the Kwekwe district, for which a deep crustal origin has been discussed. Although this study documents contrasting evidence for a porphyry-gold versus a shear-zone type of mineralization, it is suggested that gold-bearing fluids were syntectonically introduced into a ductile shear zone within the granite dike either during cooling of the intrusion or later in Archaean or early Proterozoic times.

Occurrences of Saplima Alunite Deposits and Properties in Sebinkarahisar (Giresun, NE Turkey) Volcanic Field

Alunite occurrences, related to Upper Cretaceous volcanic rocks of acid to intermediate composition from northern part of Sebinkarahisar, located in the south of eastern Black Sea region of Pontid provenance. Upper Cretaceous aged volcanic rocks of the Sebinkarahisar volcanic field intruded by same aged acidic plutonics and unconformably overlie Palaeozoic volcanic and ophiolitic rocks (Ayan and Dora, 1993). Eocene and Miocene-Palaeocene volcanic and sedimentary rocks covers the other volcanic and plutonic. Saphca volcanites remarkably altered by hydrothermal solutions and comprises of riyolite, dacite, and andesite lava and pyroclastics. Most of the replacement alunite deposits are located in the low hills of volcanic complexes and abundant in the centre of the advanced argillic alteration zone develops laterally outward from the alunite border. Most of the large replacement of alunite deposits in Sebinkarahisar volcanic field are located around the periphery of Saphca volcanites and contain alunite and kaolinite, quartz_ opal___ cristobalite. lllite and most rarely illite-smectite present whereas hematite, jarosite are common and more prevalent than pyrite and baryte or gypsum. Alunite replacement are generally observed in generally fault and contact zones and partly in pyroclastics by means of porosity controlled. Some spring waters are seen especially in these locations, pH of these waters are between 2.33 to 6.40. Vertically, the replacement of these deposits vary systematically downward from a quartzose cap though hematitic, jarositic, alunitic and finally propylitic zones. The complete sequence can be observed at only a few deposits but can be pieced together from several different localities (Celik and Karakaya, 1997). The replacement alunite forms massive, finegrained, white to light pink aggregates that are slightly denser than less altered rocks. Most of the texture of the pre-existing volcanic rock has been obliterated in the alunite zone, but relict clasts and phenocrysts can be discerned wither the transition zone to kaolinite. In thin section, the alunite appears as an interlocking and granular quartz with minor hematite dust. Small, spotty patches ofjarosite occur near the top of alunite zone. Most of the pre-existing volcanic rock here is replaced by an alunite-quartz mixture, but jarosite adds a golden-brown colour. Hematite forms a characteristic bright red stain on incompletely altered volcanic that contain some alunite and quartz. The hematite is distributed vertically as a band above the jarosite zone and beneath the flooded silica zone. Dense granular quartz of the flooded zone forms the top of the alteration sequence and commonly forms resistant caps on the highly coloured hills. Much of the quartz is massive, light grey has a concoidal fracture, and contains brecciated fragments of volcanic rocks. It consist of a microgranular intergrowth of quartz crystal locally containing disseminated aggregates of fine-grained hematite crystals. X-ray diffraction analysis of these samples showed that distribution of most trace elements correlates inversely with alunite content. K20 and A1203 in alunite deposits are found to be in the range 6.50% to 7.30% and 15.70% to 46.80%, respectively. Size of alunite crystals ranges between 2~tm and 15~tm. Alunite crystals sometimes observed in euhedral shape and especially amorphous crystals contain much silica (between 5% to 12%) in EDS analysis. The trace elements Y, Sc, Mo, Cr, Co, Ni, Zn to be mobile in acid aqueous systems and are nearly absent in the zone. In the surrounding kaolinitic envelope, on the other hand, these elements are present in background or slightly higher concentrations. In illitic zone, Rb and Sr content are high. Barium is highest close to the alunite zone because of the relative insolubility of baryte in acid solutions. The spring waters are meteoric in origin and they are rich in especially SO4, K, A1 and partly Fe, Si and Ca. The colour of waters varies colourless to brown and around the source of waters recent mineralization such as gypsum, alunite, goetite are determined.

Hydrothermal alteration of a rhyolitic hyaloclastite from Ponza Island, Italy

A rhyolitic hyaloclastite from Ponza island, Italy, has been hydrothermally altered producing four distinct alteration zones based on XRD and field textures: (1) non-pervasive argillic zone; (2) propylitic zone; (3) silicic zone; and (4) sericitic zone. The unaltered hyaloclastite is a volcanic breccia with clasts of vesiculated obsidian in a matrix of predominantly pumice lapilli. Incomplete alteration of the hyaloclastite resulted in the non pervasive argillic zone, characterized by smectite and disordered opal-CT. Obsidian clasts, some pumice lapilli, and pyrogenic plagioclase and biotite are unaltered. Smectite has an irregular flakey morphology, although euhedral particles are occasionally observed. The propylitic zone is characterized by mixed-layer illite/smectite (I/S) with 10 to 85% illite (I), mordenite, opal-C and authigenic K-feldspar (akspar). The matrix of the hyaloclastite is completely altered and obsidian clasts are silicified; however, plagioclase and biotite phenocrysts remain unaltered. Flakey I/S replaces pumice, and mordenite, akspar and silica line and fill pores. I/S particles are composed predominantly of subequant plates and euhedral laths. The silicic zone is characterized by highly illitic I/S with ≥ 90% I, quartz, akspar and occasional albite. In this zone the matrix and clasts are completely altered, and pyrogenic plagioclase shows significant alteration. Illitic I/S has a euhedral lath-like morphology. In the sericitic zone the hyaloclastite altered primarily to illitic I/S with ≥ 66% I, quartz, and minor akspar and pyrite. Clay minerals completely replace pyrogenic feldspars and little evidence remains of the original hyaloclastite texture. Unlike other zones, illitic I/S is fibrous and pure illite samples are composed of euhedral laths and hexagonal plates. The temperatures of hydrothermal alteration likely ranged from 30 to 90 °C for the argillic zone, from 110 to 160 °C for the propylitic zone, from 160 to 270 °C for the silicic zone, and were possibly as high as 300 °C for the sericitic zone. The four zones occur as linear bands that increase in intensity north of the bentonite mine at Cala dell'Acqua. The alteration zones have two orientations and may be structurally controlled by E-W- and NE-SW-trending faulting which is consistent with the dominant structural trends of the Pontine archipelago. Finally, hydrothermal alteration most likely involved seawater based on the geologic evolution of Ponza.

Evolution of a mineralized geothermal system, Valles Caldera, New Mexico

The 20-km-diam Valles caldera formed at 1.13 Ma and had continuous postcaldera rhyolitic eruptions until 0.13 Ma. Hot springs and fumaroles are surface manifestations of a hydrothermal reservoir (210 degrees -300 degrees C; 2-10 X 10 3 mg/kg Cl) that is most extensive in fractured, caldera fill tuffs and associated sedimentary rocks, located in specific structural zones. Fluids are composed of deeply circulating water of (primarily) meteoric origin that have a mean residence time in the reservoir of 3 to 10 k.y. The only component of clearly magmatic origin is anomalous 3 He although it is possible that some magmatic water, carbon, and sulfur is contributed to present hydrothermal fluids. Host rocks show intense isotopic exchange with hydrothermal fluids. Alteration assemblages are controlled by temperature, permeability, fluid composition, host-rock type, and depth. A generalized distribution from top to bottom of the system consists of argillic, phyllic, propylitic, and calc-silicate assemblages. Typical alteration minerals in phyllic and propylitic zones are quartz, calcite, illite, chlorite, epidote, and pyrite, whereas common vein minerals consist of the above minerals plus fluorite, adularia, and wairakite. Argentiferous pyrite, pyrargyrite, molybdenite, sphalerite, galena, chalcopyrite, arsenopyrite, stibnite, barite, and tetradymite (?) have been found at various depths in the Valles system. Fluid inclusion studies show that these mineral assemblages formed from liquid water (0-5.5 wt % NaCl equiv) at temperatures ranging from 175 degrees to 310 degrees C, depending on location in the system. Fluid inclusion studies also show that the top of the liquid-dominated zone has descended, leaving behind a low-pressure vapor cap. Dating of spring deposits, core samples of vein minerals, and altered host rocks by K-Ar, U-Th, U-U, and paleomagnetic methods indicates that the hydrothermal system was created at about 1.0 Ma and has been continuously active to the present. However, these dates in combination with geologic and fluid inclusion evidence suggest that the vapor zone formed at about 0.5 Ma, after a breach of the southwest caldera wall drained widespread intracaldera lakes and lowered the hydraulic head on the hydrothermal reservoir. Although petrologic and geophysical evidence indicates that residual pockets of melt still reside in the pluton beneath the caldera, the size of the hydrothermal system has shrunk since initial formation. Thus, the Valles caldera contains a mature hydrothermal system that remains hot, that contains a classic geothermal configuration, and that displays many analogies to epithermal mineral deposits exposed in older volcanic environments.

Geology and geochemistry of Summitville, Colorado; an epithermal acid sulfate deposit in a volcanic dome

Geology and geochemistry of Summitville, Colorado; an epithermal acid sulfate deposit in a volcanic dome

Hydrothermal alteration and stable isotope systematics of the Babine porphyry Cu deposits, British Columbia; implications for fluid evolution of porphyry systems

The Eocene Babine Intrusive Suite of west-central British Columbia hosts a number of porphyry copper deposits, most significantly Bell, Granisle, and Morrison. All deposits feature central potassic zones containing the ore zones and peripheral propylitic zones. In addition, Granisle and Bell feature superimposed sericite-carbonate zones between the potassic and propylitic zones, and the Bell deposit also possesses a superimposed phyllic stockwork zone, which hosts most of the ore.Calculated fluid compositions from potassic zone biotites of all deposits yield a range from delta 18 O (sub H 2 O) = 6 to 8 per mil, whereas plagioclases yield delta 18 O (sub H 2 O) = 1 to 7 per mil. Since plagioclase samples yield more 18 O-depleted fluids than coexisting biotites and delta (sub plagioclase-biotite) values are very small, it appears that isotopically lighter fluids were present in later stages of potassic alteration. Oxygen isotope values from the sericite-carbonate and phyllic alteration zones yield fluid compositions more 18 O-depleted than magmatic fluids, requiring the incorporation of isotopically light ground waters.Calculated hydrogen isotope values of potassic, propylitic, and sericite-carbonate fluids fall in the range of -40 to -110 per mil. These results, in combination with the delta 18 O data, suggest that the alteration events were caused by mixtures of magmatic and meteoric waters. Phyllic and argillic alteration fluids (delta D (sub H 2 O) = -100 to -130 and -140ppm, respectively) indicate fluids dominantly of meteoric origin.The stable isotope data, in combination with the petrologic and field relations, suggest that early alteration fluids were derived from the magma, and in peripheral areas fluids evolved at very low water/rock ratios. Incorporation of external fluids in later alteration was responsible for the texturally destructive, cation-leaching alteration and remobilization of the ores.Differences in the stable isotope systematics and salinities of porphyry copper deposits compared to meteoric hydrothermal systems and epithermal ore deposits indicate that the systems are fundamentally distinct. The strong density contrast between the high-salinity magmatic fluids and low-salinity meteoric waters most likely inhibits mixing, resulting in separation of the hydrothermal systems.

Bulagidun prospect: a copper, gold and tourmaline bearing porphyry and breccia system in northern Sulawesi, Indonesia

The Bulagidun CuAu prospect, in a remote area of north Sulawesi, was identified during follow-up of stream sediment and panned concentrate geochemical anomalies. Soil geochemistry and drilling outlined areas of disseminated and fracture controlled CuAu mineralization. Soil sampling shows a close association of Cu and Au anomalies with three mineralized breccia zones. Numerous weak metal anomalies also correlate with peripheral quartz-sulfide veining. The three separate bodies contain a geological resource of more than 14.4 Mt at 0.68 ppm Au and 0.61 wt.% Cu. A series of intrusions into regionally-widespread andesitic volcanic rocks (ca. 9.4 Ma) vary in composition with time, from early diorite to quartz diorite to late tonalite and post-mineral andesitic dykes. Early actinolite alteration is overprinted by hydrothermal biotite + magnetite. Coarse biotite + magnetite ± quartz ± K-feldspar ± pyrite ± chalcopyrite open space filling occur within hydrothermal breccia, and postdates the pervasive biotite + magnetite alteration. This later alteration, and retrograde chlorite, co-exist with up to ten percent chalcopyrite and pyrite. A propylitic (chlorite ± epidote ± carbonate) zone surrounds the actinolite and biotite zones, and is accompanied by weak vein-controlled AuPbZn and local As mineralization. A fracture-and permeability-controlled overprinting of sericite + clay + chlorite ± quartz ± carbonate alteration (8.75 Ma) partially destroys earlier silicates. Disseminated blebs of radiating tourmaline are widespread in the breccia bodies, and only partially co-eval with visible sulfide mineralization. Bulagidun is characteristic of island-arc porphyry CuAu mineralization. However, the abundant tourmaline is unusual in the southwest Pacific; this characteristic is similar to tourmaline-bearing breccia pipes in Chile, Arizona and elsewhere that only rarely contain economic-scale disseminated Cu ores.

The Andacollo strata-bound gold deposit, Chile, and its position in a porphyry copper-gold system

The Andacollo mining district is in the Coastal Cordillera of central Chile, 400 km north-northeast of Santiago. The Andacollo deposits are the products of a complex hydrothermal system and consist of a porphyry copper-gold deposit and peripheral strata-bound manto gold deposits and veins with minor associated base metals. The hydrothermal system was part of the Pacific porphyry copper belt which was generated during development of an Early Cretaceous magmatic arc displaying shoshonitic petrochemical affiliations. Rocks that crop out in the area include a volcanic sequence, the Arqueros and Quebrada Marquesa Formations, consisting of andesitc and dacite flows, volcanic breccias, and pyroclastic rocks of Early Cretaceous age. Intrusive rocks range from diorite to granodiorite in composition and date between 87 and 130 Ma. The porphyry copper-gold deposit is zoned vertically downward from a leached capping through a supergene enrichment blanket to a hypogene sulfide zone. Alteration is characterized by central potassic (K feldspar-biotite), phyllic, and peripheral propylitic zones. Abundant northwest-trending tensional fractures were superimposed on the porphyry copper-gold deposit and surrounding areas during the later stages of the evolving mineralized system. The fractures channeled mineralizing fluids from the central parts of the porphyry copper deposit outward for up to 5 km. Replacement by adularia and sericite took place together with deposition of gold-bearing pyrite and minor amounts of zinc and copper where these fluids encountered permeable dacite flows and andesite flow breccias. The alteration process caused remobilization of aluminum and alkalies and addition of K 2 O, which attains values of 12 to 13 wt percent. The Andacollo system is interpreted to be a porphyry copper-gold deposit that is transitional outward to distal epithermal, adularia-sericite-type contact metasomatic gold orebodies.

Petrology and geochemistry of propylitic alteration at Southwest Tintic, Utah

Systematic variations in mineral assemblages, fluid inclusion characteristics, mineral chemistry, and isotopic ratios document the chemical and thermal evolution of hydrothermal fluids responsible for late alteration in a small porphyry copper system near Tintic, Utah. Propylitic alteration of volcanic rock can be subdivided into actinolite, epidote, and chlorite subzones. An actinolite subzone containing actinolite, epidote, chlorite, and calcite extends 250 m away from the secondary biotite zone. An epidote subzone containing epidote, chlorite, and calcite is present up to 500 m away from the secondary biotite zone. A chlorite subzone of chlorite and calcite is present up to 2 km from the secondary biotite zone. Chlorite which occurs in each subzone varies systematically in Fe, Mg, and Mn content. The ratio Fe/(Fe + Mg) increases from 0.27 at the secondary biotite zone to 0.48 in the outer propylitic zone accompanied by a decrease in activity of clinochlore from 0.15 to 0.02. MnO increases from 0.23 to 0.44 wt percent. The Fe (super +3) content of epidote also systematically decreases from the actinolite subzone outward. The activity of clinozoisite in the epidote ranges from 0.038 at the secondary biotite zone to 0.21 700 m away from the secondary biotite zone in the outer epidote subzone. Systematic changes in chlorite and epidote compositions reflect increasing log a (sub Mg (super +2) ) /a 2 (sub H (super +) ) and log a (sub Fe (super +2) ) /a 2 (sub H (super +) ) with increasing distance from the hydrothermal biotite zone and decreasing temperature and salinity.Fluid inclusion homogenization temperatures decrease away from the secondary biotite zone with a horizontal thermal gradient of 78 degrees C/km. Temperatures range from 351 degrees to 160 degrees C in propylitic alteration. Salinities also decrease outward from 7.3 to 0.0 equiv wt percent NaCl.Hydrothermally altered rocks are systematically depleted in 18 O outward from the phyllic alteration zone into the chlorite subzone, as recorded by whole-rock samples (10.9 to -1.7ppm), calcite (10.4-1.6ppm), and quartz veins (2.8 to -0.8ppm). Calculated delta 18 O and delta D values for the actinolite subzone are +5 and -55 per mil, respectively; values as low as -15.1 and -120 per mil are recorded at the outer limits of sampling in the chlorite subzone.These latter values indicate that outer propylitic alteration is dominated by isotopically unevolved local meteoric water. However, the large variations in hydrogen and oxygen isotope compositions recorded across the zoned propylitic alteration are too large to have been produced from an isotopically homogeneous fluid, given the measured range of alteration temperatures (350 degrees -200 degrees C). Rather, the variations in isotopic compositions and salinity of the propylitic alteration fluids are consistent with the progressive mixture of unevolved ( 18 O- and D-depleted), low-salinity local meteoric water with a much more 18 O- and D-enriched, higher salinity fluid. The later phyllic alteration also formed from a similar, 18 O- and D-enriched fluid. We suggest that these isotopically enriched fluids more likely evolved from local meteoric water through extensive exchange at very low water/rock ratios with the igneous rocks at depth in the hydrothermal system at Tintic. An alternative explanation is that 18 O- and D-enriched fluids represent a late incursion of magmatic water unlike many other porphyry copper systems.Participation of such 18 O- and D-enriched fluids in the waning stages of hydrothermal alteration at Tintic contrasts with the usual domination of isotopically unevolved meteoric water at these stages (e.g., propylitic, phyllic) in many porphyry copper systems (Sheppard et al., 1971; Taylor, 1974; Gustafson, 1978). At Tintic, isotopically enriched fluids coexist with and even displace meteoric water during propylitic and phyllic alteration, respectively.

Epithermal and subjacent porphyry mineralization, Acupan, Baguio District, Philippines: a fluid-inclusion and paragenetic study

The Acupan gold-silver deposit is a major precious-metal producer in the Baguio gold district, Northern Luzon, Philippines. Paragenetic and fluid-inclusion studies have confirmed the presence of two contrasting hydrothermal fluid types, each characterized by distinctive fluid-inclusion populations, ore and gangue associations and alteration assemblages. Mineralization related to both porphyry and epithermal ore-forming events are recognized. The spatial proximity of magmatic-hydrothermal to epithermal mineralization has not previously been described from an adularia-sericite deposit such as Acupan. Two vein stages of porphyry origin (stages I and II) are related to emplacement of a subvolcanic porphyry dacite. Stage I (quartz-magnetite-chalcopyrite) veinlets are associated with K-silicate alteration. Biotite, magnetite and anhydrite are the dominant alteration minerals. ‘White sericite’ and propylitic alteration halos envelop stage II (anhydrite-quartz-pyrite-chlorite) veinlets, which cross-cut the earlier stage I mineralization. The association K-mica-quartz-pyrite-anhydrite-chlorite typifies the white sericite assemblage. Propylitic alteration is dominated by chlorite-pyrite-anhydrite; epidote is notably absent. Paragenetic stages III to V of epithermal character occur together as veins up to one meter wide and are superimposed on porphyry-style mineralization. The highest precious metal concentrations are present in fine-grained ‘grey quartz’ (stage III), recognized predominantly along vein margins in the upper levels of the deposit and as clasts enclosed by later paragenetic stages. Fragments of altered wall rock are hosted by stage III gangue. Stage IVa mineralization is coarse-grained and typified by ‘white quartz’ together with K-mica, adularia, rhodochrosite and mangano-calcite. Opaque minerals include pyrite, sphalerite, chalcopyrite, galena, free gold and gold-silver tellurides. Vuggy, colloform and brecciation textures are all recognized in stage IVa gangue. Later-stage IVb gangue infills central portions of the veins and is dominated by coarse-grained calcite and quartz. Stage V (anhydrite-pyrite-quartz) veins reopen and crosscut stages III and IV. Stage IVb and V veinlets develop locally intense propylitic envelops where they are isolated from the larger veins. Sericitic alteration (K-mica-quartz-pyrite-illite) is in general symmetrically disposed about the epithermal veins in envelopes up to a meter wide, and is surrounded by a large, weakly altered propylitic zone characterized by the mineral association epidote-chlorite-pyrite-carbonate-magnetite. Fluid-inclusion studies reveal that stage I veinlets were deposited from hypersaline (34 to over 74 eq. wt. % NaCl) magmatic fluids. Homogenization temperatures ( T H) ranged from 350°C to in excess of 600°C. Stage II was deposited from saline fluids (31–63 eq. wt. % NaCl), and T H measurements from 300°C to in excess of 600°C were observed. Entrapment temperatures for stage III fluids were between 240°C and 320°C, with salinities from 0 to 2.6 eq. wt. % NaCl. Stage IVa formed from low-salinity fluids (0–5.1 eq. wt. % NaCl) at temperatures between 220°C and 310°C. Deposition of stage IVb occurred between 180°C and 280°C from low-salinity fluids (0–1.7 eq. wt. % NaCl). Temperatures declined approximately 60°C from the beginning of stage III deposition to the end of stage IVb. A late resurgence in temperatures is present during anhydrite deposition (stage V), with T H between 214°C and 299°C and salinities from 0.2 to 3.4 eq. wt. % NaCl. Magmatic-hydrothermal fluids exsolved during crystallization of the subvolcanic porphyry dacite produced stage I and II mineralization. A co-existing magmatic vapor generated by phase separation was ubiquitous throughout these stages. Epithermal mineralization commenced at least 1 Ma after stage II, with near-surface rupturing of a hydrothermal seal producing hydrothermal breccias, widespread boiling and gold mineralization (stage III). Later self-seal/rupture cycles and minor hydrothermal brecciation produced apparent paragenetic reversals of stage IVa gangue sealed by stage III, and stage IVb gangue sealed by IVa. Boiling, recognized at greater depths in stage IVa gangue, was more restricted in extent than for stage III. Gold deposition below the boiling zone in stage IVa was localized at structural intersections and controlled by pressure fluctuations. Stage IVb may have been deposited in response to boiling and/or fluid mixing. Temperature increase and/or fluid mixing are the likely mechanisms for stage V deposition.

The Climax-Alma granite batholith of oligocene age and the prophyry molybdenum deposits of Climax, Colorado, U.S.A.

The porphyry molybdenum deposits of Climax, Colorado, are stockworks of quartz-molybdenite veinlets. They are apical to intrusive cupolas of leucocratic rhyolite-granite porphyry of Oligocene age (33 to 24 Ma). The upper parts of the orebodies are overprinted by overlying zones of phyllically altered rocks, and by stockworks of greisen-like veinlets, containing quartz, pyrite, topaz and huebnerite. The phyllic and/or greisen-like zones are surrounded by a propylitic zone, characterized by quartz-chlorite-hematite veinlets and chlorite after biotite. Late veins, radial and peripheral to the intrusive center, are common but widely scattered and discontinuous. Such veins typically contain quartz, fluorite, rhodochrosite, pyrite, dark sphalerite, galena, tetrahedrite, and traces of huebnerite, molybdenite, argentite, native gold and/or electrum. The Alma district, 4 to 10 km southeast of Climax, contains widely scattered clusters of veins and replacement deposits. The veins cut chloritized to sericitized silicate host rocks, and the replacement deposits are in carbonate host rocks. The ore- and gangue-mineral assemblages of the veins of the Alma district resemble those of veins peripheral to the Climax molybdenum deposits. Veins of the Alma district cut dikes of Late white rhyolite (35 Ma), which resemble the rhyolite porphyries of Climax. A swarm of such dikes is parallel to the long axis of an elliptical drainage pattern, which bounds a structural dome, here called the outer Alma dome. The outer Alma dome (8 × 12 km) surrounds an inner Alma dome (6 × 8 km). Veins of the Alma district generally are axial and/or marginal to the outer Alma dome, and/or radial to the inner Alma dome. Veins and alteration minerals associated with the outer Alma dome formed between 35 and 27 Ma ago. Those associated with the inner Alma dome formed about 27 Ma ago, as indicated by fission-track age determinations on thermally annealed zircon grains from hydrothermally altered rocks, within and around the inner Alma dome. Negative Bouguer gravity anomaly patterns indicate: (1) a regional, northeast-trending gravity low, associated with the composite monzonite-granodiorite-granite B batholith of the Colorado mineral belt (average density about 2.62 g/cm 3); (2) a semi-regional gravity trough, within the gravity low of the Colorado mineral belt, extending northeastward across the Mosquito Range; (3) an oval pattern of gravity lows, coincident with the Alma domes; and (4) a closed gravity low associated with the Climax stock (density about 2.56 g/cm 3) (Tweto and Case, 1972; Behrent and Bajwa, 1974; Corry, 1981). The semi-regional gravity trough is interpreted as the gravimetric expression of a granite batholith, here called the Climax-Alma batholith. The gravity lows of the Climax and Alma districts are interpreted as expressions of cupolas on this granite batholith. The highly differentiated rhyolite-granite intrusions of Climax plunge toward the Alma domes. The Climax intrusions are interpreted as products of apical differentiation of the inferred Climax-Alma granite batholith. Magmatic-hydrothermal fluids, associated with highly differentiated apical magmas, produced the Climax molybdenum orebodies. Most of the veins and replacement deposits of the Alma district are associated with lower, broader cupolas of the inferred granite batholith. The shape of the composite Climax-Alma magmatic-hydrothermal system resembles that of a teapot, with Climax above the spout and the Alma district above the lower, broadly domed lid.

Arkose-hosted, aquifer-controlled, epithermal Au-Ag mineralization, Wenatchee, Washington

The Wenatchee district, site of the Cannon mine, is located on the east flank of the Cascade Range in central Washington. Epithermal gold mineralization of Eocene age (44 Ma) is hosted by the Eocene Chumstick Formation, a sequence of interbedded arkosic sandstone, conglomerate, and mudstone. This study focuses on the structure and hydrothermal alteration and mineralization south of the Cannon mine.Hydrothermal alteration and mineralization is hosted by Chumstick arkose, minor Eocene dacite, and andesite dikes, and locally, Eocene felsic volcanic rocks (flows, ash flows, eruption breccia) and extends for at least 4 km southeast of the Cannon mine. The district-scale structure is dominated by a postmineralization, northeast-verging fold and thrust belt which consists of three major southwest-dipping reverse faults and associated folds produced by the propagation of the faults.Arkose-hosted alteration consists of zones of silicification which grade symmetrically outward to argillization and a widely dispersed, district-scale propylitic alteration. Alteration is strata bound within units of arkosic sandstone and conglomerate bound or capped by mudstone-rich sections. Features of the propylitic zone include the growth of pyrite within detrital biotite and the replacement of plagioclase by carbonate and epidote. The argillic assemblage includes hydrothermal kaolinite and sericite. Silicification consists of silicic hydrothermal breccias and pervasive quartz flooding closest to the argillic zone; hydrothermal K feldspar is common. Typically, a transitional subzone containing abundant quartz veins (quartz stockwork) occurs between the argillic and silicic zones.In decreasing order of abundance, metallic phases are pyrite, arsenopyrite, marcasite, stibnite (latest in paragenesis), chalcopyrite, hessite, electrum, native silver, sphalerite, galena, pyrargyrite, and boulangerite. Gold contents are highest in silicic alteration and the quartz-stock-work subzone. Where arsenic contents are high, gold is concentrated in the argillic zone. In the latter case, strong correlations between gold and arsenic suggest that gold may have been transported as a gold thioarsenide complex; precipitation of the gold and arsenic may have been promoted by the progressive sulfidation of biotite.Two laterally extensive strata-bound alteration zones (> or =2 km along strike) have been delineated, one largely bound by mudstones in the hanging wall of a postmineralization reverse fault, and one in the footwall of the fault and bound above by a mudstone and ash-flow tuff. The absence of alteration stratigraphically above the footwall zone indicates that it is the stratigraphically highest mineralized aquifer in the district. Hydrothermal fluids migrated laterally along at least two laterally extensive aquifers which were stratigraphically stacked. The shallower footwall zone is enriched in As, Sb, and Au relative to the deeper hanging wall zone, which is richer in Ag, Cu, Te, and Se. This variation in depth is similar to patterns observed in active geothermal systems and epithermal precious metal deposits and implies some degree of exchange of hydrothermal fluids between the aquifers, possibly via leaks in aquitards.

Fluid evolution, wallrock alteration, and ore mineralization associated with the Rodalquilar epithermal gold-deposit in southeast Spain

At Rodalquilar gold mineralization is found in Late Tertiary volcanic rocks of the Sierra del Cabo de Gata and is related to a caldera collapse. Radial and concentric faults were preferred sites for gold deposition. Hydrothermal activity produced a specific alteration zoning around gold-bearing vein structures, grading from an innermost advanced argillic via an argillic into a more regionally developed propylitic zone. Advanced argillic alteration with silica, pyrophyllite, alunite, and kaolinite extends down to several hundred m indicating a hypogene origin. High-grade gold mineralization in vein structures is confined to the near-surface part of the advanced argillic alteration. Fine-grained gold is associated with hematite, jarosite, limonite, or silica. At a depth of about 120 m, the oxidic ore assemblage grades into sulfide mineralization with pyrite and minor chalcopyrite, covellite, bornite, enargite, and tennantite. Two types of fluids from different sources were involved in the hydrothermal system. Overpressured and hypersaline fluids of presumably magmatic origin initiated the hydrothermal system. Subsequent hydrothermal processes were characterized by the influx of low-salinity solutions of probable marine origin and by interactions between both fluids. Deep-reaching, advanced argillic alteration formed from high-salinity fluids with 20–30 equiv. wt% NaCl at about 225°C. Near-surface gold precipitation and silification are related to fluids with temperatures of about 175°C and 3–4 equiv. wt% NaCl. Gold was transported as Au(HS) 2 − , and precipitation resulted from boiling with a concomitant decrease in temperature, pressure, and pH and an increase in fO2. All features of the Rodalquilar gold deposit reveal a close relationship to acid-sulfate-type epithermal gold mineralization.