Adularia-sericite Type Research Articles

Ore deposits and epithermal evidences associated with intra-magmatic faults at Aïn El Araâr-Oued Belif ring structure (NW of Tunisia)

Hydrothermal ore deposits at Ain El Araâr-Oued Belif location are classified as epithermal deposits type. The ore bodies are hosted by upper Turonian (8–9 M.y) volcanic rhyodacitic complex. Polymetallic sulfide orebodies are mainly concentrated within intra-magmatic faults. Petrographic, XRD, and TEM–STEM investigations revealed that ore minerals are essentially, arsenopyrite, pyrite, chalcopyrite, pyrrhotite, hematite, goethite and magnetite with Au, Ag and Pt trace metals. Gangue minerals are mainly adularia, quartz, sericite, alunite, tridymite, chlorite, phlogopite and smectite. Epithermal alteration is well zoned with four successive characteristic zones: (1) zone of quartz–adularia–sericite and rare alunite; (2) zone of kaolinite and plagioclase albitization; (3) intermediate zone of illite–sericite; (4) sapropelic alteration type zone of chlorite–smectite and rare illite. This can be interpreted as a telescoping of two different acidity epithermal phases; low sulfidation (adularia–sericite) and high sulfidation (quartz–alunite), separated in time or due to a gradual increase of fluids acidity and oxicity within the same mineralization phase. Brecciated macroscopic facies with fragments hosting quartz–adularia–sericite minerals (low-sulfidation phase) without alunite, support the last hypothesis. Geodynamic context and mineral alteration patterns are closely similar to those of Maria Josefa gold mine at SE of Spain which exhibit a volcanic-hosted epithermal ore deposit in a similar vein system, within rhyolitic ignimbrites, altered to an argillic assemblage (illite–sericite abundant and subordinate kaolinite) that grades outwards into propylitic alteration (Sanger-von Oepen et al. (1990)). Mineralogical and lithologic study undertaken in the volcanic host rock at Ain El Araâr-Oued Belif reveals a typical epithermal low-sulfidation and high-sulfidation ore deposits with dominance of low-sulfidation. Host rocks in these systems range from silicic to intermediate for adularia–sericite type (low sulfidation) to rhyodacite for quartz–alunite type (high sulfidation).

Geology and stream sediment geochemistry of the Vilyuchinsky gold district, Southern Kamchatka, Russian Federation

The Kamchatka–Kuril island arc forms the northeastern margin of the Eurasian continent, including the Kuril Islands, Kamchatka Peninsula and parts of the Koryak Highlands. This composite island arc, over 2000 km long, contains abundant epithermal gold deposits with total resources currently assessed at approximately 30 Moz of gold. Epithermal gold deposits of the Kamchatka–Kuril arc cluster in several gold districts associated with Neogene volcanism, Koryak, Northern Kamchatka, Central Kamchatka, Kumroch and Southern Kamchatka (Vilyuchinsky), and there are also smaller deposits in the Kuril Islands.The Vilyuchinsky district comprises mainly intermediate and felsic volcanic rocks of Miocene and Pliocene age overlying a sedimentary flysch sequence, and locally overlain by Quaternary basalts. Shallow subvolcanic intrusions represent the roots of Neogene palaeovolcanic structures that were exposed by erosion. Epithermal gold deposits in the Vilyuchinsky district are of the adularia-sericite type and form north west trending linear arrays that are spatially associated with the Vilyuchinsky crustal fault that bounds the district to the North. Mineralisation is spatially associated with, and is commonly hosted by, subvolcanic intrusions of dacite and andesitic-dacite exposed in the deeply eroded palaeovolcanic centres.The Vilyuchinsky district was mapped and explored at a scale of 1 : 50 000. All creeks were systematically sampled at a density of approximately one sample per 500 m along the stream infilled to 1 : 25 000 in geologically favourable areas. Analysis of stream sediment assays indicates that gold has a multi-modal statistical distribution, suggesting the presence of different levels of geochemical anomalies. Gold assays ≥ 20 ppb are classified as anomalous, and values ≥ 100 ppb are classified as highly anomalous. In the Kamchatka region, deposits containing more than 1 Moz of gold coincide with stream sediments that contain ≥ 100 ppb of gold. In the Vilyuchinsky district, such samples are found only around the Porozhistoe deposit. Copper and lead values in stream sediments are classified as anomalous at ≥ 100 ppb and ≥ 30 ppb, respectively.Base metals and gold are commonly distributed in three separate zones surrounding diorite porphyry centres (e.g. at the Balaganchik prospect). Copper anomalies occur in the vicinity of diorite porphyry intrusions, lead is present 2–3 km from the centres, forming an intermediate geochemical halo, and gold veins and related stream sediment anomalies form an outer halo most distant, commonly >5 km from the diorite centres. The zoned nature of the stream sediment anomalies can be used as a geochemical vector to facilitate exploration.

Mapping Advanced Argillic Alteration at Cuprite, Nevada, Using Imaging Spectroscopy

Mineral maps based on Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) data were used to study late Miocene advanced argillic alteration at Cuprite, Nevada. Distributions of Fe-bearing minerals, clays, micas, sulfates, and carbonates were mapped using the Tetracorder spectral-shape matching system. The Al content of white micas increases toward altered areas and near intrusive rocks. Alunite composition varies from pure K to intimate mixtures of Na-K endmembers with subpixel occurrences of huangite, the Ca analogue of alunite. Intimately mixed Na-K alunite marks areas of relatively lower alteration temperature, whereas co-occurring Na-alunite and dickite may delineate relict hydrothermal conduits. The presence of dickite, halloysite, and well-ordered kaolinite, but absence of disordered kaolinite, is consistent with acidic conditions during hydrothermal alteration. Partial lichen cover on opal spectrally mimics chalcedony, limiting its detection to lichen-free areas. Pods of buddingtonite are remnants of initial quartz-adularia-smectite alteration. Thus, spectral maps provide a synoptic view of the surface mineralogy, and define a previously unrecognized early steam-heated hydrothermal event. Faulting and episodes of hydrothermal alteration at Cuprite were intimately linked to upper plate movements above the Silver Peak-Lone Mountain detachment and growth, collapse, and resurgence of the nearby Stonewall Mountain volcanic complex between 8 and 5 Ma. Isotopic dating indicates that hydrothermal activity started at least by 7.61 Ma and ended by about 6.2 Ma. Spectral and stable isotope data suggest that Cuprite is a late Miocene low-sulfidation adularia-sericite type hot spring deposit overprinted by late-stage, steam-heated advanced argillic alteration formed along the margin of the Stonewall Mountain caldera.

Epithermal Au and polymetallic mineralization in the Tulasu Basin, western Tianshan, NW China: Potential for the discovery of porphyry Cu[sbnd]Au deposits

The Tulasu basin is an important epithermal Au mineralization district in western Tianshan in northwestern China. Geochemical and geochronological studies of the volcanic host rocks (the Dahalajunshan Formation) and associated intrusive rocks, particularly the enrichment of large-ion lithophile elements and depletion of high field strength elements, suggest that the Tulasu basin was located in a magmatic-arc setting from Late-Devonian to Early-Carboniferous, associated with the southward subduction of the North Tianshan Ocean beneath the Kazakhstan–Yili plate. Geologic and geochemical characteristics of the AuCu–polymetallic deposits in the Tulasu basin suggest that the mineralization is related to the Late-Devonian to Early-Carboniferous arc magmatism, and is mainly of epithermal nature, including both the adularia–sericite type (Axi, Tawuerbieke and Tabei) and alunite–kaolinite or acid–sulfate type (Jingxi–Yelmend and Tieliekesayi). However, some evidence of porphyry-type mineralization has also been found, including the presence of mineralized porphyry enclaves in volcanic rocks hosting the Tawuerbieke Au prospect, and the development of porphyry-style Cu mineralization overprinted by vein-type mineralization at Kexiaxi. Considering the magmatic-arc setting that is favorable for both epithermal and porphyry types of mineralization, and the development of such mineralization systems in other parts of western Tianshan and the Altaid Belt (e.g., Uzbekistan, Kazakhstan and Kyrgyzstan), we suggest that major porphyry CuAu deposits may have been developed in the Tulasu basin, probably beneath or adjacent to known epithermal Au mineralization.

Genetic investigation and comparison of Kartaldağ and Madendağ epithermal gold deposits in Çanakkale, NW Turkey

Two epithermal gold deposits (Kartaldağ and Madendağ) located in NW Turkey have been characterized through the detailed examinations involving geologic, mineralogical, fluid inclusion, stable isotope, whole-rock geochemistry, and geochronology data.The Kartaldağ deposit (0.01–17.65ppm Au), hosted by Eocene dacite porphyry, is associated with four main alteration types with characteristic assemblage of: i) chlorite/smectite–illite±kaolinite, ii) quartz–kaolinite, iii) quartz–alunite–pyrophyllite, iv) quartz–pyrite, the last being characterized by three distinct quartz generations comprising massive/vuggy (early), fine–medium grained, vug-lining (early), and banded, colloform, comb (late) textures. Observed sulfide minerals are pyrite, covellite, and sphalerite. Oxygen and sulfur isotope analyses, performed on quartz (δ18O(quartz): 7.93 to 8.95‰ and calculated δ18O(H2O): −7.95 to 1.49‰) and pyrite (δ34S(pyrite): −4.8‰ and calculated δ34S(H2S): −6.08 to −7.20‰) separates, suggest a meteoric water source for water in the hydrothermal fluid, and an igneous source for the sulfur dissolved in ore-related fluids. Microthermometric analyses of primary fluid inclusion assemblages performed on quartz (late quartz generation) yield temperatures (Th) dominantly in the range of 245–285°C, and generally low salinity values at 0 to 1.7wt.% NaCl eq. Based on the quartz textures and the associated base metal concentrations, along with fluid inclusion petrography, the early vug-lining quartz is considered to have been associated with the mineralization possibly through a boiling and a late mixing process at >285°C.The Madendağ deposit (0.27–20.60ppm Au), hosted by Paleozoic mica schists, is associated with two main alteration types: sericite–illite–kaolinite, and quartz–pyrite dominated by two distinct quartz generations i) early colloform, comb and banded quartz and ii) late quartz, forming the cement in hydrothermal breccia. Whereas oxygen isotope analyses of quartz (δ18O(quartz): 9.55 to 18.19‰ and calculated δ18O(H2O): −2.97 to 5.54‰) suggest varying proportions of meteoric and magmatic sources for the ore bearing fluid, sulfur isotope ratios (δ34S(pyrite): −2.2‰ and calculated δ34S(H2S): (−3.63) to (−3.75) ‰) point to an essentially magmatic source for sulfur with or without contribution from sedimentary sources. Microthermometric analysis carried out on primary fluid inclusion populations of a brecciated sample (early quartz), give a temperature (Th) range of 235–255°C and 0.0 to 0.7wt.% NaCl eq. salinity. Based on the textural relationship, base metal and high gold contents, the ore precipitation stage is associated with late stage quartz formation via a possible boiling process.The presence of alunite, pyrophyllite and kaolinite, vuggy quartz and covellite suggest a high-sulfidation type of epithermal deposit for Kartaldağ. On the other hand, Madendağ is identified as an adularia-sericite type owing to the presence of significant sericite, neutral pH clays (mostly illite, chlorite/smectite, and kaolinite), low temperature quartz textures (e.g., colloform, comb, and banded quartz), and limited sulfide minerals.Given the geographical proximity of Kartaldağ and Madendağ deposits, the similar temperature and salinity ranges obtained from their fluid inclusions, and the similar ages of igneous rocks in both deposits (Kartaldağ: 40.80±0.36 to 42.19±0.45Ma, Madendağ: 43.34±0.85Ma) the mineralizing systems in both deposits are considered to be genetically related.

Endogene-supergene systems of epithermal deposits and occurrences of acid-sulfate and adularia-sericite type (cases from Bulgaria) - hypothesis and models

The epithermal acid-sulfate and adularia-sericite deposits аrе among the main sources of raw mаterials as well as among the most interesting subject of study and genetic modelling and conclusions оn the mineral deposits. Both types have а broad оссurеnсе in Bulgaria, which allows аn extensive research оn them. Hereafter, discussing the main features of the epithermal mineralization а new hypothesis fоr аn integrated endogene-supergene system is proposed. This idea is based оn published evidences from worldwide as well as Bulgarian examples. Its соrе аrе the detailed mineralogical and petrological characteristics of the epithermal systems. It is suggested, that under some circumstances both endogene and supergene parts of the systems mау bе observed as аn integrity. This integrity is observed as unanimity between the nature and human mind. Among the main prerequisites fоr the origin of such systems аrе spatial compatibility; dependence between endogene and supergene mineralizations; presence of metasomatic rеplacement and indications fоr а metasomatic zonation; соmmоn structural-tectonic agents; geomorphological and climatic agents; etc. The proposed models fоr the epithermal mineralisations аrе based оn Bulgarian occurrences, including some elements from the models of Hedenquist, Lowenstern (1994), Hedenquist (1994), Fournier (1999) and Chavez (2000). These models have аn endogene and а supergene part with а respective metasomatic zonation. The proposed hypothesis is obviously debatable and as аnу оnе hypothesis contains facts and suggestions. However, it is also supported bу evidences from porphyritic systems, where а transition between hypogene and supergene mineralization was already suggested bу Sillitoe (1973). Some debatable points of the proposed hypothesis could bе the cases where а significant gap in time bеtween the primary and supergene mineralizations is established. However, in such cases usually as supergene processes аrе accepted to bе only the recent ones, but those acting simultaneously with the endogene оrе forming so far have bееn neglected. The recent geothermal and hydrothermal volcanic systems аrе the best proof of the simultaneous occurrence of both endogene and supergene processes. The integrity of both processes is already recognized with the admission of the mixed character of the fluids, the convection and the participation of meteoric waters as well as atmospheric oxygen together with magamatic products into the forming of the endogene epithermal deposits.

Geology and Alteration‐Mineralization Characteristics of the Cibaliung Epithermal Gold Deposit, Banten, Indonesia

Abstract. The Cibaliung gold project is located at the central portion of the Neogene Sunda‐Banda magmatic arc. Gold‐silver mineralization in the area is hosted in a thick sequence of sub‐aqueous basaltic andesite volcanics with intercalated sediments intruded by sub‐volcanic andesite to diorite plugs and dykes, and subsequently cut by a cluster of diatreme breccias. These host rocks are unconformably overlain by dacitic tuffs, younger sediments and basalt flows.The gold prospects in Cibaliung occur within a NW‐trending structural corridor that is 3.5 km wide by at least 6 km long. It is fault‐bounded and is considered to be a graben. Two aligned NNW‐trending sub‐vertical shoots, Cikoneng and Cibitung, host the currently defined resource within the steeply dipping vein system with a minimum strike length of 1,300 m. As of July 2001, exploration has defined an inferred + indicated mineral resource of approximately 1.3 million tonnes at 10.42 g/t gold and 60.7 g/t silver at a 3 g/t Au cut‐off. This equates to approximately 435,000 ounces of gold and 2.54 million ounces of silver.Gold‐silver mineralization occurs as quartz veins characteristic of the low‐sulphidation epithermal adularia‐sericite type. Progressive dilation with a general increase in gold grade has produced multi‐stage veining and brecciation that grades from early to late stages as: pre‐mineral fluidized breccia, quartz vein stockwork, massive vein, crustiform vein, colloform‐crustiform vein with progressive increase in chloritic clay bands, clay‐quartz milled matrix breccias with a progressive increase in clay content, and synto post‐mineral fault gouge with vein clasts. Wall rock alteration is characterized by pro‐grade chlorite+adularia flooding that is locally overprinted by a low temperature argillic alteration (smectite, illite and mixed layered clays). Generally, the argillic alteration becomes weak with depth. The major mineral constituents of the veins are quartz, adularia and clay. In the early gold‐poor hydrothermal stages, quartz and adularia dominate with minor calcite and clay (smectite, poorly crystalline chlorite, interlayered chlorite‐smectite and illite‐smectite). In the later gold‐rich hydrothermal stages, clay with variable amounts of carbonate increases whereas the abundance of quartz and adularia decreases. Gold occurs mainly as electrum while silver occurs as argentite‐aguilarite‐naumannite and electrum, and rarely as native silver, sulphosalts and tellurides. Sulphides generally comprise <1 vol % of the vein, with pyrite as the most common species. Together with pyrite, traces of very fine‐grained base metal sulphides dominated by chalcopyrite, sphalerite and galena are in most cases intimately associated with electrum and silver minerals. Partial supergene oxidation generally extends down to about 200 m below the surface at Cikoneng and further down to more than 300 m at Cibitung.The hydrothermal system responsible for the gold‐silver mineralization in the area may be related to rhyolitic magmatism focused on a volcanic intrusive center during back arc rifting that formed a graben or pull‐apart basin. The dominant mechanism for the higher grade gold deposition is fluid mixing of up welling metal‐bearing hydrothermal solutions with relatively near surface cool, oxygenated condensate and/or steam‐heated meteoric fluids, as opposed to retrograde boiling. The strongly focused dilational structural environment is thought to have been the mechanism for focusing fluid flows, both up welling and descending, forming pipe‐like mineralized bodies in the rhomboidal dilation zones. It is interpreted that mineralization took place under low temperature conditions (<150–220d̀C) at a minimum depth of around 200–250 m below the palaeo‐water table.

Shallow to near-surface, vein-type epithermal gold mineralization at Lalab in the Sibutad gold deposit, Zamboanga del Norte, Mindanao, Philippines

The Sibutad vein-type epithermal gold deposit is the most promising economically feasible gold mineralization found in recent years in Zamboanga del Norte province in Zamboanga Peninsula, Mindanao island. The Sibutad gold deposit occurs in Pliocene to Pleistocene volcanic rocks, resting on a deformed island arc block to the east of the tectonically active Sindangan–Cotabato–Daguma Lineament. The host rocks are the Malindang Volcanics, composed of lower and upper members, which are both intruded by andesite porphyry. The lower member is made up of andesite flows, dacite tuff and tuffite, whereas volcanic breccia and tuff breccia characterize the upper member. The Sibutad gold deposit is subdivided into the Larayan and Lalab prospects. In Lalab, which is the prospect studied, hydrothermal activity and gold mineralization occur in the andesite flows. The geology, alteration and mineralization of the Lalab orebody are of adularia-sericite type that suggests the gold was precipitated from reduced, near-neutral pH solutions within a shallow to near-surface environment. The following events produced the Lalab orebody: (1) pre-breccia wallrock alteration, (2) hydrothermal brecciation, (3) post-breccia wallrock alteration, and (4) gold mineralization. Gold precipitated in quartz veins was a response to boiling, followed by fluid mixing. Prolific gold zones occur between 30 m below sea level and 200 m above sea level.

Andacollo copper–gold district, La Serena, Chile: preliminary data from the porphyry copper and possible relationships between Cu and Au mineralization

The Andacollo mining district is located in the Coquimbo region of Chile at 30°14’ south, 71°06’ west, some 55 km southeast of La Serena, at a mean elevation of 1030 m within a semi-arid hilly landscape (Fig. 1). Current mining activity in the district is concentrated on copper and gold. These metals are mined, respectively, from a porphyry copper deposit and epithermal, manto and vein gold deposits of adularia–sericite type.11,13 Other types of mineralization include mercury veins hosted by carbonate rocks. The gold veins are controlled by a northwest-trending set of normal faults, whereas the manto-type mineralization is strata-bound and largely confined to andesite breccias, dacites and sites of strong fracturing. The lateral and vertical continuity of the mantos is strongly controlled by rock type, faulting and intensity of fracturing. The gold deposits have been the focus of a recent study,11 but comparable information on the Andacollo porphyry has not become available. A brief, updated geological perception of the porphyry is now presented and possible relationships between the copper and gold deposits are analysed.

Geological Structure and Ore Mineralogy of the Juliette Gold-Silver Deposit, Northeast Russia

Julietta is a rich epithermal gold-silver deposit of the low-sulfidation, adularia-sericite type, located in the Cretaceous Okhotsk-Chukchi volcanic-plutonic belt 250 km northeast of Magadan. The deposit was discovered in 1989 by a regional soil geochemical survey in an area previously considered barren on the basis of a regional stream-sediment survey. The deposit has not been completely explored, but presently is in the feasibility stage; proven reserves are 26 metric tons of Au (grades averaging 23 g/t) and the Au/Ag ratio is about 1:10. The deposit occurs on the periphery of a large volcanic-tectonic depression. Host andesite, andesite-basalt lava, corresponding subvolcanic bodies, and tuff are cut by Early Cretaceous quartz diorite stocks. Six vein zones occur in tensional and compression fissures. Ore shoots and smaller bonanzas comprising most of the gold reserves are located in flexures of the ore-host fissures. Ore mineralization was preceded by intense voluminous propylitization and linear seric...

The Rosita Hills epithermal Ag-Base metal deposits, Colorado, USA A stable isotope and fluid inclusion study

The Rosita Hills volcanic centre is an alkalicalcic, mid-Tertiary complex overlying orthoand paragneissic basement, on the eastern margin of the Rio Grande Rift in south central Colorado, USA. The centre contains vein-hosted, adularia-sericite type, epithermal Ag and base-metal mineralisation with minor Au. Stable isotope studies (O and H) of whole rock and mineral separate (quartz and sericite) samples from veins and hydrothermal eruption breccias show that the hydrothermal fluid had both magmatic and meteoric components. The δD and δ18O values of the hydrothermal fluid, calculated from mineral values, range from -22‰ to -103‰ and 0.5‰ to 5.9‰ respectively. Fluid inclusion data from vein minerals (quartz, baryte and sphalerite) and from an advanced argillic lithocap overlying the veins again show that the hydrothermal system had more than one component fluid. Fluid inclusions have salinities which range from 1.7 to 25.1 wt% NaCl equivalent and show evidence of boiling in the advanced argillic lithocap. Homogenisation temperatures range from 135°C to 298°C. Liquid CO2 is present in some inclusions. These data indicate that a saline, isotopically heavy fluid mixed with a dilute, isotopically light fluid to precipitate the ore. We argue that the saline, isotopically heavy fluid is magmatic and derived from a resurgent rhyolitic magma below the mineralisation.

Cirotan, West Java, Indonesia; a 1.7 Ma hybrid epithermal Au-Ag-Sn-W deposit

The Cirotan gold deposit, dated at 1.7 + or - 0.1 Ma, is a mineralized fault with ore shoots contained within a system of fractures cutting a Miocene volcano-sedimentary series (9.5 Ma) intruded by Pliocene microdiorite (4.5 Ma). The mineralization, hosted by a right-lateral strike-slip fault that evolved into a normal fault, has been partly eroded.The deposit is of the adularia-sericite type with strong vertical ore zoning (Ag/Au = 64 at the top and only 7 in depth) and resulted from a single mineralizing event with five stages of deposition: silicification (stage a), siliceous breccias (stage b), cockade breccias with polymetallic sulfide ores and local rhodochrosite-rhodonite (stage c), precious brecciated ore including bonanzas (stage d), and late vuggy quartz (stage e). This evolution was accompanied by an increase in W, Sn, Au, and Bi reflected in the late appearance of abundant ferberite, scheelite, gold, cassiterite, and rare minerals such as Te-bearing canfieldite and lillianite. Fluid inclusion studies indicate a slight increase in temperature (235 degrees -255 degrees C) and salinity (2.89-7.15 wt % NaCl equiv) from the initial silicification to the bonanza ore, and an absence of volatiles other than H 2 O; that this could reflect an increased participation of magmatic fluids is supported by the sulfur isotope compositions of pyrite and galena (avg delta 34 S = 2.2 and 1.0ppm, respectively) close to the 0 per mil isotopic value of magmatic sulfur. Lead isotope results establish a common origin for both the Cirotan gold deposit ( 206 Pb/ 204 Pb = 18.77 and 207 Pb/ 204 Pb = 15.67) and the Pliocene andesitic-dacitic magmas to which the gold is related, and indicate a remobilization of Precambrian continental basement.Although Cirotan shows strong affinities with classic epithermal gold deposits, the late geochemical and mineralogical evolution with W, Sn, and Bi is extremely unusual, if not unique. In the context of the Malaysia-Sumatra tin belt, this is considered to reflect a genetic link with porphyry tin deposition and is explained by a progressive upward migration of Pliocene magma favored by strike-slip faulting. The occurrence of the two types of mineralization within a single structure thus makes Cirotan a hybrid deposit.

Mineralization, antiforms and crustal extension in andesitic arcs

AbstractThe distribution and stratigraphic position of porphyry copper and epithermal gold deposits in andesitic arcs of the western Pacific and eastern Europe suggest that porphyry copper and epithermal vein deposits of adularia–sericite type develop successively under different stress regimes in an evolving arc, rather than being genetically related as commonly supposed. Absence of coeval high-level stocks in the root zones of many adularia-sericite deposits suggests that circulation of the dominantly meteoric hydrothermal fluids is not driven by shallow intrusions. The location of several world-class deposits on basement geanticlines, and on more localized antiforms of which at least one has been interpreted as a metamorphic core complex, implies that elevation of the arc, emplacement of magmatic sills at depth and adularia–sericite type gold mineralization are genetically related to subduction-induced crustal extension. Ascent of deep hydrothermal fluids, predominantly meteoric but with a metamorphic or magmatic component, may be controlled by regional low-angle structures at depth, analogous to those inferred for some mesothermal gold deposits. Mineralization at shallow (epithermal) depths in high-angle structures largely reflects the high geothermal gradient and mixing of deep fluid with cool meteoric water in or at the base of the permeable volcanic cover. Andesitic magmatism may resume following porphyry copper mineralization, adularia–sericite epithermal gold mineralization, or continued extension to form a ‘back arc’ spreading system, depending on the relative plate motion.

Epithermal gold deposits in West Java, Indonesia: geology, age and crustal source

Epithermal gold mineralization of the adularia-sericite type in West Java is hosted by strike-slip faults cutting Miocene to Pliocene volcanic and plutonic formations. Mineralization consists of two styles of discordant gold-bearing quartz veins: crustiform banding veins of the “Pongkor” type, with electrum, some minor sulphides and manganese oxides, and breccia veins of the “Cirotan” type with rhodochrosite, rhodonite, electrum, abundant polymetallic sulphides and some cassiterite and wolframite. Except for the Pongkor ore deposit, whose age is Miocene (8.5 Ma), K/Ar dating of adularia gave a Pliocene-Pleistocene (2.1-1.5 Ma) age for both styles of epithermal gold deposits. Most are hosted by volcanic and intrusive rocks related to extensive Pliocene magmatism that we dated in the centre of the Bayah Dome as forming between 5.7 and 2.0 Ma. Lead-isotopic compositions of Miocene volcanic rocks show mantle affinities, whereas the lead of Pliocene volcanic rocks and Miocene-Pliocene gold deposits is highly radiogenic and clearly crustal ( 207 Pb/ 204Pb = 15.66 to 15.72). This indicates that this lead and possibly associated metals have their source in an underlying Precambian crust, which must extend at depth from West Java to the Tin Islands. Mineralogical data, especially the presence of magmatic-related minerals, sulphur isotopes with δ 34S clustered around 0‰, and lead isotopic results, all suggest that the metal content and probably most of these ore-forming fluids of epithermal gold deposits originated from a crustal magmatism through a reworking of the underlying Precambrian crust.

Discovery, Development, and Production of a Blind Ag-Au Bonanza in the Calera Vein, Orcopampa District, Peru

Epithermal precious metal veins of the adularia-sericite type in the Orcopampa district were worked during Spanish times and since 1965 by the Buenaventura Group. Ore is hosted by generally ENE- to NE-striking normal faults that cut Miocene volcanic rocks of silicic and intermediate composition. Early-produced ores contained silver as freibergite and had very high Ag/Au ratios. The potential importance of the largely buried Calera structure was recognized during district-scale geologic mapping in 1972. Exploration began in 1975 and ore was discovered by diamond drilling in 1978. A large tonnage of rich ore containing gold as well as silver was discovered in 1983. Reserves of 2,383,240 short tons with an average of 16.2 ounces Ag per short ton and 0.15 ounces Au per ton had been defined by 1986, mostly in one exceptionally large ore shoot. Ore tends to occur in strongly fractured rock at the junctions of footwall splits with the Calera vein. The presence of several ore-grade paragenetic stages contributed ...

Contrasting styles of epithermal precious-metal mineralization in the southwestern Nevada volcanic field, USA

The southwestern Nevada volcanic field contains epithermal precious-metal deposits hosted by Miocene volcanic rocks and pre-Tertiary sedimentary rocks with production+reserves greater than 60 t of gold and 150 t of silver. The volcanic rocks consist predominantly of ash-flow tuffs erupted between 15 and 7 Ma during three major magmatic stages: the main stage (ca. 15-13 Ma); the Timber Mountain stage (ca. 13-9 Ma); and the late stage (ca. 9-7 Ma). Hydrothermal activity and precious-metal mineralization in the southern part of the field took place between ca. 13 and 8.5 Ma, coinciding with portions of all three magmatic stages. Regional extension during this period produced imbricate normal and detachment faulting that provided structural control for some of the mineralization. Contrasts in the style and geochemistry of mineralization, together with stratigraphic and radiometric age data and differences in geologic setting reflect the variable nature of hydrothermal activity during development of the southwestern Nevada volcanic field. During the main magmatic stage, silver-rich vein mineralization of the adularia-sericite type occurred in an intermediate volcanic center at Wahmonie. Secondary high-salinity fluid inclusions in felsic subvolcanic intrusions, a trace element suite that includes bismuth and tellurium, and geophysical data support the presence of a buried porphyry-type magmatic system at Wahmonie. Hydrothermal activity at Bare Mountain took place during the main magmatic stage, and may have continued into the Timber Mountain magmatic stage. Bare Mountain contains gold-rich, disseminated Carlin-type deposits with high arsenic, antimony, mercury and fluorine in sedimentary and igneous rocks. In northern and eastern Bare Mountain, mineralization is associated with felsic porphyry dikes that contain secondary high-salinity fluid inclusions. A genetic relationship between porphyry magmatism and shallow Carlin-type gold deposits seems likely at Bare Mountain. Sedimentary-rock-hosted mineralization at Mine Mountain is spatially associated with a thrust and was apparently deposited, in part, by a hydrothermal system active during the Timber Mountain magmatic stage. The silver:gold ratio is high and base-metal, arsenic, antimony, mercury and selenium contents are very high. Mine Mountain mineralization shares features with vein and disseminated silver deposits at Candelaria, Nevada. Gold-silver deposits in the areally extensive Bullfrog district comprise the largest known precious-metal resource in the volcanic field. They are mainly quartz-carbonate±adularia veins with alteration and mineralization styles similar to other adularia-sericite-type deposits in the Great Basin. Deposits in the Rhyolite area and at the Gold Bar mine have very low contents of arsenic and mercury compared to other epithermal deposits in the Great Basin, although copper and antimony are locally elevated. Similarities in mineralization style and assemblages, which include two occurrences of the rare gold-silver sulfide uytenbogaardtite, indicate deposition under similar conditions in different parts of the district. Hydrothermal activity in the Bullfrog district was coeval with extensional tectonism and may have continued from the Timber Mountain stage into the late magmatic stage. Mineralization at some deposits in the Bullfrog and Bare Mountain districts is spatially associated with, and, in part, structurally controlled by a regional detachment fault system. However, significant differences in age, mineralization style and geochemistry indicate that mineralization in the two districts is unrelated.

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.

The Faride epithermal silver-gold deposit, Antofagasta region, Chile

The Faride epithermal silver-gold deposit belongs to the adularia-sericite type and is associated with an eroded volcanic center. This volcanic center is located within a geotectonic environment dominated by the development of a magmatic arc from the beginning of the Upper Cretaceous to the early Tertiary.The Faride deposit consists of a series of epithermal veins, containing precious metals and associated base metals that lie within a structural system. The veins strike roughly east-west to N 45 degrees W and extend for approximately 2 km. Within this system a total of eight ore shoots presently have been recognized and explored. These deposits are deeply oxidized down to 200 m, so that the present mineralogy consists of a suite of complex chloride, carbonate, sulfate, and oxide minerals. Below 200 m the primary mineralization can be recognized, which consists of specular hematite, pyrite, chalcopyrite, galena, sphalerite, and copper-silver sulfosalts. Gangue minerals are quartz, barite, rhodochrosite, and minor siderite. Quartz textures are typical of open-space filling, showing banding or crustification.Two main paragenetic stages of veins are described: the earlier stage (I) is gold rich and consists of quartz-hematite-pyrite-chalcopyrite, and the later stage (II) is silver rich and consists primarily of barite-galena-sphalerite-chalcopyrite and Ag sulfosalts.Hydrothermal alteration is weakly developed within the intrusive host rock, forming quartz-sericite envelopes around the veins. At depth abundant quartz-adularia veinlets predominate.Fluid inclusions in quartz associated with the gold-rich stage have homogenization temperatures that range from 200 degrees to 320 degrees C; temperature increases with depth. Salinities in this stage range between 0.4 and 8.2 wt percent NaCl equiv. Inclusions associated with the late, silver-rich stage range in homogenization temperature from 170 degrees to 245 degrees C and do not vary with depth. Salinities of the second stage are similar to those of the first, however, ranging from 1.3 to 8.0 wt percent NaCl equiv.Based on mineralogic data and fluid inclusion studies, it is suggested that a combination of boiling, mixing, and conductive cooling decreased the temperatures of ascending ore-bearing fluids, causing precipitation of the primary base metal sulfides and precious metals. For metal migration and deposition within the oxidation zone, two possible mechanisms are considered to have acted at the Faride deposit: reduction by ferrous iron, and breakdown of Au, Ag, and base metal complexes by evaporation in the vadose zone.

Mineralogic alteration patterns in volcanic rocks of the La Libertad gold mining district and its surroundings, Nicaragua

Several alteration zones characterized by secondary mineral assemblages of successively higher formation temperatures can be recognized in the Tertiary volcanic country rocks surrounding a swarm of auriferous quartz veins at La Libertad, Nicaragua. The outermost zone (I) is affected by regional burial diagenesis at low zeolite facies (mordenite subfacies) and a thermal gradient < 50 degrees C/km. The secondary phases are smectite and cristobalite-tridymite (+ or - mordenite and heulandite). Passing to zone II cristobalite-tridymite are replaced by quartz, and heulandite becomes more abundant (heulandite subfacies). Next zone (III; laumontite subfacies) is marked by the appearance of laumontite instead of heulandite and mordenite, swelling chlorite instead of smectite (in drill core samples), and calcite. Chlorite then replaces swelling chlorite, and wairakite and epidote become local constituents at depth (zone IV; transition to wairakite subfacies).A steep thermal gradient at La Libertad, exceeding 150 degrees C/km, reveals that the mining district is a fossil geothermal field. The influence of the geothermal system (zones II-IV) can be noticed more than 5 km from the district. Strongly altered rocks forming aureoles around quartz veins define a local alteration pattern characterized by quartz, chlorite, adularia, illite, and pyrite, suggesting that the deposit is of adularia-sericite type. An illite-kaolinite-bearing supergene cap (zone V) covers most of the mining district. The geologic history can be inferred from the study of the regional to local alteration patterns.

The geology of hydrothermal gold deposits in Chile

A detailed geologic study has been undertaken of the characteristics of 20 of the best known hydrothermal gold deposits in Chile. Characteristics include production and reserve data, metal grades, geotectonic setting, morphology, mineralogy, zonation, alteration assemblages and fluid-inclusion studies. Based on this geologic data base, two broad categories of gold deposits have been recognized: volcanic-hosted epithermal deposits of the adularia-sericite and acid-sulphate types and porphyry-related deposits. Volcanic-hosted epithermal deposits of the adularia-sericite type appear to be more abundant than the acid-sulphate type. Very few examples of porphyry-related deposits have been recognized. The majority of the acid-sulphate type deposits were formed during the Oligocene-Quaternary time-span and no adularia-sericite type examples are yet known from this period. This latter epithermal type was formed exclusively during the Mesozoic and Early Tertiary. Presently accepted ore-deposit models have been applied to the Chilean epithermal gold deposits, and an empirical model for adularia-sericite epithermal system is presented.