Supergene Stage Research Articles

Characteristics, origin and distribution of dolomite reservoirs in Lower-Middle Cambrian, Tarim Basin, NW China

Abstract Based on cores, thin sections and drilling data of 18 wells and two outcrop profiles of the Lower-Middle Cambrian in the Tarim Basin, geochemical analysis of multi-parameters in micro-area (the micro-carbonate fabric which is formed under the same diagenetic event or has the same genesis) and reservoir dissolution modeling were carried out to find out the types, origin and distribution of Lower-Middle Cambrian dolomite reservoirs. There develop three types of dolomite reservoirs, margin reef-shoal reservoir, platform interior mound-shoal reservoir and platform interior gypsodolomite reservoir. The rock types include algae dolomite, grain dolomite, and gypsodolomite; the pore types in them include algae framework pores in algae dolomite; intergranular pores, intra-granular dissolved pores, inter-crystalline dissolved pores in grain dolomite and gypsum-dissolved pore gypsodolomite. The primary pores in sediments of reef-shoal facies and gypsodolomite flat are the key to the development of the dolomite reservoirs, some aragonite and calcite are the material basis of dissolution in early supergene stage, and the dolomite formed in penecontemporaneous dolomitization acts as strong rock skeleton to protect the primary pores from destructive effect in burial stage. The margin reef-shoal reservoirs, large in scale and good in physical properties, are the most practical exploration targets; furthermore, the platform interior reef-shoal reservoirs in Lower Cambrian Xiaoerbulake Formation and platform interior gypsodolomite reservoir in the Middle-Lower Cambrian have higher oil and gas potential.

Geological, geochemical, and geochronological characteristics of Caledonian W–Sn mineralization in the Baiganhu orefield, southeastern Xinjiang, China

The newly discovered large-scale Baiganhu W–Sn orefield, consisting of the Kekekaerde, Baiganhu, Bashierxi, and Awaer deposits, is located in Ruoqiang County, southeastern Xinjiang, China. These deposits comprise mainly three types of W–Sn mineralization: early-stage skarn-type, middle-stage greisen-type, and late-stage quartz-vein-type. In this study, we classified seven major vertical zones on the basis of petrographic characteristics, roughly from the bottom of the parental granitic intrusions upward, as (A) fresh syenogranite, (B) argillic alteration, (C) muscovite-dominated greisenization, (D) tourmaline-dominated greisenization, (E) marginal facies (including K-feldspar pegmatite and fine-grained granite), (F) aplitic apophysis, and (G1) skarn or (G2) infilled silification zones. According to the alteration–mineralization assemblages and cross-cutting relationships, five stages of mineralization are recognized in the orefield (I, skarn stage; II, greisen stage; III, quartz vein stage; IV, argillic alteration stage; and V, supergene stage), and reverse alteration zonation in the altered intrusion is also observed.The W–Sn deposits are spatially associated with syenogranite, which is part of the Caledonian Bashierxi magmatic series. Laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) zircon U–Pb dating of the syenogranite yields a weighted mean 206Pb/238U age of 413.6±2.4Ma (MSWD=0.36, n=30). Hydrothermal muscovite from three samples associated with W–Sn mineralization yields plateau 40Ar/39Ar ages of 411.7±2.6Ma (MSWD=0.21), 412.8±2.4Ma (MSWD=0.22), and 413.8±2.6Ma (MSWD=0.22), which is consistent with the zircon U–Pb age, thus indicating a temporal link between the emplacement of the syenogranite and the W–Sn mineralization. Our age data and previously published ages, along with geochemical data, for the granitoids in the Bashierxi magmatic series show a nearly contemporaneous evolution of A- and S-type granites, which were emplaced in a post-orogenic setting at ca. 432–413Ma. As compared with the A-type granites, the syenogranites with S-type affinities probably resulted from a lower degree of partial melting of metagreywackes, which was more likely to be enriched in the ore-forming elements W and Sn, as well as volatiles such as B and H2O. In addition, the syenogranites exhibit low oxidation states and underwent high degrees of factional crystallization, both of which favor post-magmatic W–Sn mineralization. We suggest that more attention should be given to buried syenogranites of S-type affinities during mineral exploration in this area, and that the proposed model of a vertical alteration zoning can act as a guide to the targeting of similar ore systems.

GEOLOGY AND ORE MINERALISATION OF NEOGENE SEDIMENTARY ROCK HOSTED LS EPITHERMAL GOLD DEPOSIT AT PANINGKABAN, BANYUMAS REGENCY, CENTRAL JAVA, INDONESIA

Low suphidation (LS) epithermal gold deposits were recently found in the Paningkaban area, Central Java province, Indonesia, with more than five hundred artisanal gold miners currently operating in the area. This study is aimed to understand the geological factors controlling the gold mineralization and to characterize the alteration and ore mineralogy of the deposit. Several epithermal veins/veinlets trending N–S, NW–SE, and NE–SW are hosted by Tertiary turbiditic volcanoclastic sedimentary rocks of the Halang formation. This formation is composed of looping gradation of sandstone and siltstone units. Pre- and syn-mineralization structures such as extension joints, normal sinitral fault and sinitral fault control the gold mineralization. Fault movements formed dilational jogs manifested by NW-SE-trending en-echelon tension gash veins. Four main alteration zones are identified: (a) phyllic, (b) argillic, (c) sub propylitic and (d) weak subpropylitic. Ore minerals consist of native gold, electrum, native silver, pyrite, chalcopyrite, sphalerite, galena, arsenopyrite, cubanite, marcasite, covellite and tennantite, which are commonly associated withargillic alteration. Vein structures such as massive, swarm and low angle veins, stockwork and veins dispersed in diatreme breccia are present. Normal banded, cockade, crustiform, bladed carbonates as well as, comb and saccharoidal features are the typical vein textures. It is noteworthy that the veins are basically composed of carbonate with minor quartz at gold grades of up to 83 g/t Au. Based on the vein structures and textures, four stages of ore mineralization were developed consisting of (a) early stage (fluidized breccia and quartz vein), (b) middle stage (carbonate base metal), (c) late stage (late carbonate), and supergene stage. Gold mineralization originated mainly during middle and late stages, particularly in association with cockade, crustiform, bladed carbonate base metal veins. Based on those various features, the LS epithermal deposit in the study area is categorized as carbonate-base metalgold mineralization type.

Genesis and Magmatic-Hydrothermal Evolution of the Yangla Skarn Cu Deposit, Southwest China

The Yangla skarn Cu deposit (150 Mt at 1.03% Cu) is located in the central segment of the Jinshajiang metallogenic belt within the Sanjiang (Three Rivers) region, southwest China. Skarn orebodies are mainly developed between different units of Devonian carbonate and quartz sandstone rocks with stratiform-like shapes, or within the contact zone between granitoids and marbles. Re-Os dating of molybdenite intergrowth with chalcopyrite yielded a well-constrained 187Re-187Os isochron age of 232.0 ± 1.5 Ma with a weighted average age of 231.8 ± 1.3 Ma, both coeval with the related intrusions (233.1 ± 1.4 and 231.0 ± 1.6 Ma at 2 σ by zircon U-Pb dating) from our previously published work. Field and textural relationships indicate three hydrothermal stages characterized by assemblages of prograde skarn (pre-ore stage), retrograde skarn and Cu-Fe-Mo-Bi sulfides (main ore stage), and Pb-Zn sulfides associated with calcite and quartz (late ore stage), as well as one supergene stage marked by secondary Cu mineralization (malachite and azurite). Skarns contain garnets with andraditic compositions (Ad96 Gr2~3Py0~1) and clinopyroxene (two series: Hd6Di94 and Hd86Di13Jo1) with low Mn/Fe ratios (<0.1), typical of Cu skarn deposits worldwide. Three stages of fluid evolution were observed by a detailed fluid inclusion study: (1) Early fluids were trapped under two-phase conditions, as evidenced by the coexistence of brine (homogenization temperatures = 560°–600°C, average salinity = 49.4 ± 1.7 wt % NaCl equiv, n = 33) and vapor-rich inclusions in pre-ore stage andradite and diopside (trapped at ~600 bars, or a depth of approximately 2 km assuming lithostatic pressure conditions). (2) Main ore stage fluid inclusions in quartz were also trapped under two-phase conditions (boiling), as identified by the coexistence of vapor- and liquid-rich fluid inclusions; liquid-rich inclusions homogenized between 312° and 389°C (average = 350° ± 24.7°C, n = 20), with salinities of 2.4 to 5.6 wt % NaCl equiv (average = 4.2 ± 0.9 wt % NaCl equiv, n = 13) and a depth of ~2 km (~200 bars, hydrostatic pressure conditions). (3) Late ore stage fluids are represented by inclusions in calcite, characterized by homogenization temperatures ranging from 220° to 290°C (average = 249° ± 27°C, n = 14) and salinities between 2.1 and 8.0 wt % NaCl equiv (average = 4.6 ± 1.5 wt % NaCl equiv, n = 11). Sulfur isotope compositions of sulfide minerals (pyrite, chalcopyrite, pyrrhotite, and molybdenite) from the main and late ore stage have a narrow range of δ 34S values from −1.9 to 2.6‰, consistent with a magmatic origin. Calcite in the late ore stage has δ 13C values ranging from −3.2 to −5.9‰ and δ 18O from 7.2 to 18.0‰, distinct from the host-rock marble compositions ( δ 13C = 1.2–4.3‰, δ 18O = 10.8–23.9‰). When corrected for temperature (250°C, estimated from fluid inclusion analysis in calcites), these calcite data correspond to ore fluid δ 13Cfluid values of −2.0 to −4.6‰ and δ 18Ofluid values of 0.5 to 11.2‰ (clustering at ~10‰), which are consistent with a magmatic origin. Lead isotope compositions of sulfides (206Pb/204Pb = 18.273–18.369, 207Pb/204Pb = 15.627–15.677, and 208Pb/204Pb = 38.445–39.611) are similar to those of the granitic intrusions and sedimentary wall rocks, but distinct from those of basalts (206Pb/204Pb = 18.282–19.133, 207Pb/204Pb = 15.564–15.665, and 208Pb/204Pb = 38.367–38.942) in the mining area. Taken together, these geologic, geochemical, and isotopic data confirm that Yangla is a typical Cu skarn deposit.

Insights from Pb Isotopes for Native Gold Formation During Hypogene and Supergene Processes at Rich Hill, Arizona

The origin of gold nuggets in placers is enigmatic as they can be mechanically separated from the primary vein deposit or can be of secondary origin, formed in the placer environment. Pb isotopes can be a valuable tracer as a match in the isotopic ratios between vein and placer gold will indicate that the placer nuggets are simply mechanically separated from the hydrothermal vein and unmodified by weathering. Reconnaissance elemental analyses revealed that vein and placer gold from the Rich Hill district, Arizona, contain up to thousands of ppm Pb and therefore both are suitable for Pb isotope analyses. We measured Pb isotopes in placer gold nuggets and compare the results to primary vein gold and sulfides and also gangue minerals and host rocks in the Rich Hill area. Primary vein gold and galena show similar Pb isotope ratios with 206Pb/204Pb ranging from 18.364 to 18.486, 207Pb/204Pb from 15.574 to 15.593, and 208Pb/204Pb from 37.150 to 37.359. This indicates that the ore metals were derived from a similar source during a hypogene hydrothermal event in the geologic past. Present-day Pb isotope compositions of local rocks in the area show distinct values compared to the primary vein mineralization. Age-corrected and modeled Pb isotope compositions for local magmatic rocks do not provide a clear picture for a possible hydrothermal ore metal source due to possible alteration processes and the long time elapsed since the mineralization event. The distinct present-day Pb isotopes in local lithological units, however, provide an excellent tracer for deciphering the origin of the placer gold. All the placer gold nuggets show very distinct Pb isotope compositions when compared to their respective hydrothermal vein source represented by the vein gold and galena. The placer gold Pb isotopes show very wide variations, with 206Pb/204Pb ranging from 17.985 to 20.057, 207Pb/204Pb from 15.494 to 15.749, and 208Pb/204Pb from 36.863 to 38.606. The placer nuggets follow the present-day Pb isotope trend observed in rocks and sediments in the area. In addition, the Pb isotopes in majority of the placer gold nuggets are far more radiogenic compared to the vein gold, sulfides, and even gangue minerals. This is indicating that a significant part, if not all, of the Pb in the placer gold was relatively recently acquired from the placer sediments. The observed distinct Pb isotope compositions between primary and placer samples suggest that gold undergoes significant chemical changes during the supergene stage. Considering a number of possible scenarios, the most plausible explanation for the observed present-day Pb isotope similarity between placer gold and local rocks is formation of a significant part of the total mass of these nuggets within the placer environment.

Au–Ag–Te Mineralization of the Low‐Sulfidation Epithermal Aginskoe Deposit, Central Kamchatka, Russia

AbstractMineralogic studies of major ore minerals and fluid inclusion analysis in gangue quartz were carried out for the for the two largest veins, the Aginskoe and Surprise, in the Late Miocene Aginskoe Au–Ag–Te deposit in central Kamchatka, Russia. The veins consist of quartz–adularia–calcite gangue, which are hosted by Late Miocene andesitic and basaltic rocks of the Alnei Formation. The major ore minerals in these veins are native gold, altaite, petzite, hessite, calaverite, sphalerite, and chalcopyrite. Minor and trace minerals are pyrite, galena, and acanthine. Primary gold occurs as free grains, inclusions in sulfides, and constituent in tellurides. Secondary gold is present in form of native mustard gold that usually occur in Fe‐hydroxides and accumulates on the decomposed primary Au‐bearing tellurides such as calaverite, krennerite, and sylvanite. K–Ar dating on vein adularia yielded age of mineralization 7.1–6.9 Ma. Mineralization of the deposit is divided into barren massive quartz (stage I), Au–Ag–Te mineralization occurring in quartz‐adularia‐clays banded ore (Stage II), intensive brecciation (Stage III), post‐ore coarse amethyst (Stage IV), carbonate (Stage V), and supergene stages (Stage VI). In the supergene stage various secondary minerals, including rare bilibinskite, bogdanovite, bessmertnovite metallic alloys, secondary gold, and various oxides, formed under intensely oxidized conditions. Despite heavy oxidation of the ores in the deposit, Te and S fugacities are estimated as Stage II tellurides precipitated at the log f Te2 values −9 and at log fS2 −13 based on the chemical compositions of hypogene tellurides and sphalerite. Homogenization temperature of fluid inclusions in quartz broadly ranges from 200 to 300°C. Ore texture, fluid inclusions, gangue, and vein mineral assemblages indicate that the Aginskoe deposit is a low‐sulfidation (quartz–adularia–sericite) vein system.

Characteristic Analysis of Texture and Structure of Ores and Genesis of Luziyuan Pb-Zn Polimetallic Deposit in Zhenkang, Yunnan, China

The main ore structure of the deposit include: disseminated structure, emulsion droplet-like structure, banded structure, vein-like structure, the massive structure, brecciated structure, etc.; The main ore texture include: crystalline granular texture, the replacement texture, solid solution separation texture, interstitial texture, etc. According to the ore fabric characteristics and metal minerals and gangue mineral combination, type and shape since the degree and mineral in space analysis of characteristics of the relationship, the mining area can be roughly divided into three periods: sedimentary diagenesis stage, the initial formation of the sources, hydrothermal reformation metallogenic stage and supergene stage. It was considered that Luziyuan Pb-Zn polymetallic deposit is the hydrothermal deposits for early deposition, the late hydrothermal transformation.

Zoning and Exploration Significance of the Maoping Pb-Zn Deposit, Northeastern Yunnan Province, China

Based on the lithofacies mapping and petrographic studies, the mineralization-alteration zoning and mineralization process of the Maoping Pb-Zn deposit are discussed. Three ore body groups of the Maoping Pb-Zn deposit have similar transversal zoning, which is characterized by different mineral assemblages from the ore body center to outside. The mineralization-alteration can be divided into diagenetic stage, massive sulfide-carbonate stage, veined sulfide-carbonate stage, and supergene stage. The massive sulfide-carbonate stage is main Pb-Zn mineralization stage. The established zoning model and indicators can be used for ore prospecting.

The Angouran Zn (Pb) deposit, NW Iran: Evidence for a two stage, hypogene zinc sulfide–zinc carbonate mineralization

The unusually high grade hypogene zinc ore at Angouran in northwestern Iran (40.4% Zn, 1.9% Pb in the sulfide ore, 28.1% Zn, 4.4% Pb in the carbonate ore, and 110g/t Ag) formed from an initially highly saline, reduced, relatively acid hydrothermal brine at two successive sulfide and carbonate ore stages. The early ore stage consists of multiple phases of sphalerite dominated sulfide ore breccia with subordinate amounts of galena (±Pb sulfosalts), minor pyrite, and abundant barite. Sphalerite precipitated at moderate temperatures (≥155°C) because of pH increase in the presence of hangingwall marble. Smithsonite precipitated at a higher pH value (≥7) and at lower temperatures (≤120°C) from dilute solutions (salinities close to zero) by mixing of the Zn bearing brines with cool, HCO3− bearing waters. The first melting points of the primary (LV) fluid inclusions in sphalerite and in hydrothermal quartz are unusually low (≤−60°C), close to the eutectic point of the ZnCl2–H2O system (−62°C). Total salinities taken from the ZnCl2–H2O system as a best approximation correspond to 26–41eqmass % ZnCl2. The initial brine evolved to a CaCl2–NaCl rich solution with 27eqmass % salinity. Gas densities (≤0.1gcm−3 for water vapor and ≤0.18gcm−3 for CO2) in the fluid inclusions indicate low pressure (≤5bar for water vapor, and ≤100bar for CO2) at the entrapment temperatures.At the first carbonate ore stage sulfides continued to precipitate characteristically as arsenopyrite with minor amounts of galena and pyrite. The abrupt change of the fluid composition at the sulfide–carbonate boundary was accompanied by a change of the fluid temperature and pressure that produced brecciation of the sulfide ore matrix and an almost total dissolution of barite and replacement by the hypogene smithsonite. Alteration is restricted to ore deposition and consists of weak sericitization and silicification with local dolomitization at the sulfide ore stage, and pervasive dissolution of the hangingwall marble, in particular at the carbonate ore stage.The breccia orebody at Angouran is sited at the crestal portion of a domed antiform at the lithological and thrust boundary of Neoproterozoic–Lower Cambrian footwall schists and the hangingwall marble in rapidly uplifted and exhumated Angouran Block east of the Geynardjeh Thrust Fault. The footwall schists occupy a detachment fault zone above imbricated nappe sheets of the basement metamorphic complex of the Sanandaj–Sirjan zone. During the Pliocene, the nappe sheets were thrust toward the southwest onto the Miocene felsic volcanic rocks of the Urumieh Dokhtar Volcanic Belt that are intruded by 10Ma late Miocene basalts. The 11.9Ma and 18.4Ma zircon ages of the felsic volcanic rocks indicate the lower age limit of the ore body emplacement.The associations with large scale, mid-late Miocene, felsic volcanism along the active Tethyan belt, as well as the ubiquitous presence of the volcanic rock clasts in the sphalerite ore matrix, provide strong evidence of the involvement of hydrothermal processes at Angouran. Ore fluids were successively and pulsatorily generated within the seismically active region. A following geothermal activity appears to have had a significant input in the formation of the carbonate ore of the hypogene, as well as the supergene stage. Stable isotope data suggest complex interaction of element sources and processes. Allowing a broad interpretation, the sulfur isotopic composition of the sulfides δ34S (3.9 to 7.4‰) suggests that the sulfur could be sourced from evolving, mixed magmatic–basinal brine. The isotopic composition of the hypogene smithsonites (δ13C: 2.72 to 5.51‰, δ18O: 18.4 to 22.8‰) broadly supports the local geology and field relationships, which comply with a marble wallrock source for the carbonate ores. They lend support to the assumption that smithsonite was deposited from solutions with isotopic composition similar to those involved in the hydrothermal dolomitization of the marbles. The excess of dissolved marble precipitated as large volumes of travertine and as late calcite veins (δ13C: 18.8 to 20.3‰, δ18O: 3.1 to 6.4‰) at the mineralization site. Isotope values of the travertine (δ13C: 4.5 to 6.6‰, δ18O: 20.1 to 21.1‰ V-SMOW) are consistent with the involvement of CO2 derived from thermogenic decarbonization of the host marble by waters of dominantly meteoric origin, most likely concomitantly with ore forming processes.The Angouran deposit is the only reported case of a two stage, hypogene zinc sulfide–zinc carbonate mineralization, and represents a new style of nonsulfide zinc mineralization.

거풍구리광상: 산출공물, 유체포유물 및 안정동위원소 연구

거풍구리광상은 트라이아스기 청산화강암 내에 발달된 NE 계열의 열극대를 충진한 2개조의 석영맥으로 구성된 열수맥상광상이다. 본 광상의 석영맥은 주로 괴상으로 산출되며 일부 정동 및 각력상 조직이 관찰되고 연장성은 500 m, 맥폭은 0.2에서 2.2 m 정도이다. 이들 석영맥의 광화작용은 hypogene 시기와 supergene 시기로 구분된다. Hypogene 시기의 광물은 견운모, 황철석, 석영, 녹니석 및 점토광물로 구성된 열수변질광물과 황철석, 유비철석, 자류철석, 백철석, 섬아연석, 황석석, 황동석 및 방연석으로 구성된 황화광물이 관찰된다. Supergene 시기에는 침철석이 생성되었다. 유체포유물 자료에 의하면, 광화시기 광석광물의 침전과 관련된 균일화온도와 염농도는 각각 <TEX>$163{\sim}356^{\circ}C$</TEX>, 0.2~7.2 wt.% NaCl 로서 광화유체가 천수의 혼입에 의한 냉각과 희석작용이 있었음을 지시한다. 황(<TEX>${\delta}^{34}S$</TEX>: 4.3~9.3‰)의 기원은 주로 화성기원과 일부 모암내의 황에서 유래된 것으로 해석된다. 산소 (<TEX>${\delta}^{18}O$</TEX>: 0.9~4.0‰)와 수소(<TEX>${\delta}D$</TEX>: -86~-69‰) 동위원소값의 자료로 볼 때, 이 광상의 광화유체는 마그마 기원 또는 천수 기원의 유체로 생각되며 광화작용이 진행됨에 따라 기원이 다른 천수의 혼입이 작용한 것으로 해석할 수 있다. The Geopung Cu deposit consists of two subparallel quartz veins that till the NE-trending fissures in Triassic Cheongsan granite. The quartz veins occur mainly massive with partially cavity and breccia. They can be followed along strike for about 500 m and varies in thickness from 0.2 to 2.2 m. Based on the mineralogy and paragenesis of veins, mineralization of quartz veins can be divided into hypogene and supergene stages. Hypogene stage is associated with hydrothermal alteration minerals such as sericite, pyrite, quartz, chlorite, clay minerals and sulfides such as pyrite, arsenopyrite, pyrrhotite, marcasite, sphalerite, stannite, chalcopyrite and galena. Supergene stage is composed of geothite. Fluid inclusion data from quartz indicate that homogenization temperatures and salinity of hypogene stage range from 163 to <TEX>$356^{\circ}C$</TEX> and from 0.2 to 7.2 wt.% eq. NaCl, respectively. They suggest that ore forming fluids were progressively cooled and diluted from mixing with meteoric water. Sulfur (<TEX>${\delta}^{34}S$</TEX>: 4.3~9.2‰) isotope composition indicates that ore sulfur was derived from mainly magmatic source although there is a partial derivation from the host rocks. The calculated oxygen (<TEX>${\delta}^{18}O$</TEX>: 0.9~4.0‰) and hydrogen (<TEX>${\delta}D$</TEX>: -86~-69‰) isotope compositions suggest that magmatic and meteoric ore fluids were equally important for the formation of the Geopung Cu deposit and then overlapped to some degree with another type of meteoric water during mineralization.

Accessory minerals of fluorite and their implication regarding the environment of formation (Nabburg–Wölsendorf fluorite district, SE Germany), with special reference to fetid fluorite (“Stinkspat”)

The Nabburg–Wölsendorf vein-type fluorites in SE Germany which are characterized by a pronounced timebound variation in mineral color (blue → black → green → white/colorless → yellow → brown) were investigated for their accessory minerals included in or associated with these different fluorite types. Nabburg–Wölsendorf is not a unidirectional fluorite mineralization but a polyphase mineralizing process with several repetitions of the various color types during the vein mineralization event. Accessory minerals of the fluorite mineralization include silicates, sulfates, oxide/hydroxides, phosphates, sulfides and carbonates. These minerals are intimately intergrown with fluorite in the vein-type mineralization and are efficient tools to broaden knowledge on the depositional environment of fluorite that is otherwise poor in marker minerals for redox conditions, temperature, alkalinity/acidity and fluid viscosity. Pyrite, galena and cinnabar, are indicative of hypogene processes, while uranyl phosphates and APS minerals furnish evidence for supergene mineralizing processes. Some minor constituents may be used as marker minerals of redox conditions (hematite, galena, and goethite) or held to be characteristic for the changing pH regime (kaolinite and smectite). Oxidizing conditions prevailed during fluorite mineralization, excluding some phases when fetid and white fluorite precipitation ran through a maximum. Yellow-brown honey spar marks the oxidizing physical–chemical conditions by its brown mineral color at the end of fluorite mineralization. Oxidizing conditions prevailed also during supergene mineralization. The pH of the mineralizing fluids of the hypogene stages was greater than pH 7. During the supergene stage, Eh fluctuated, but was mainly > 0; the pH of fluids ranged between 7 and 4. The black color of fetid fluorite and its pungent odor resulted from the U present primarily in its tetravalent state. Incomplete haloes recognized in some fetid fluorites reflect “frozen” uranium disequilibria and an early phase of supergene alteration. In this respect, the fetid fluorite (“Stinkspat”) plays an important part and held to be a marker for the radioactive “hot spot” as well as “ghost” uranium mineralization. Fetid fluorite is defined as to its mineral color, morphology and characteristic spectral features so as to allow for a precise distinction from other dark blue or green fluorite varieties by its outward appearance and easy-to-handle methods. The accessory minerals under study are indicative of the depocentre of the fluids which lies close to an unconformity but does not, however, deliver information on the source of the mineralizing fluids. The nearness to the potential source rock of the fluids can only concluded from the presence of cerite-(Ce) which formed prior to fluorite. This REE silicate is cast as a marker of the root zone of the fluorite veins which fade out into late Variscan granites. Cerium is assumed to have been derived from monazite an accessory mineral largely found outside the fluorite veins, proper, in the late Variscan granites. In the waning stages of fluorite mineralization different types of REE-bearing phosphates, belonging to the rhabdophane solid solution series, and some APS minerals (aluminum–phosphate–sulfate solid solution series) evolved.

POST-MIOCENE AND BRONZE-AGE SUPERGENE Cu-Pb ARSENATE - HUMATE- OXALATE - CARBONATE MINERALIZATION AT MEGA LIVADI, SERIFOS, GREECE

As a result of the Late Miocene skarn-type Fe mineralization at Mega Livadi on the Isle of Serifos, Greece, arsenical copper mineralization of hydrothermal origin formed in calcareous roof-rocks of a granodioritic intrusion. The hypogene mineralization, with pyrite, As-rich tennantite-group minerals and arsenopyrite, underwent strong alteration during the Pliocene and Quaternary by descending meteoric waters. We subdivide the geogenic and anthropogenic processes responsible for this alteration into five mineralizing stages, differing from each other by their element budget, Eh and pH. The supergene stages I to III comprise a variegated assemblage of oxides, hydroxides and arsenates: stage I (goethite, mixture of fine-grained Fe–As–Sb–Cu minerals, pharmacosiderite), stage II (Sb-bearing beudantite, REE-bearing beudantite), stage III (arseniosiderite). The anthropogenic stages IV and V are characterized by a set of hitherto unknown K–Cu humates and oxalates (stage IV) and the common Cu carbonates malachite and azurite (stage V). Chemical weathering induced by the (sub)tropical climatic conditions during the Late Miocene and Pliocene was responsible for the supergene associations of stages I through III at Mega Livadi. In the course of this alteration, the content of Fe decreased and As increased. Anomalously high contents of LREE in beudantite are used as a “minerostratigraphic” tracer to correlate the mineralization of stage II with a phase of pervasive formation of laterite in the Aegean region during the Pliocene. After a hiatus, an anthropogenic episode of mineralization was triggered by ancient mining activities during the early Bronze Age, around 3325 to 2890 BC, when miners began exploiting the soft arsenical Fe and Cu ore at shallow depth. This mineralization evolved under more temperate climatic conditions as a result of ventilation during mining. Bronze-age K–Cu humate–oxalate aggregates and Cu carbonates precipitated from pedogenetic fluids as a function of variable redox conditions and a pH value fluctuating around neutral. This sequence of per descensum mineralization with arsenates, humates and carbonates at Serifos, Greece, is of importance in three ways. It offers an insight into the most recent weathering processes of base-metal mineralization in the Aegean Sea region. It gives an overview of mining activities across Europe from Cyprus, the “Cradle of Cu mining”, to Great Britain during the Bronze Age. Also, the physicochemical results obtained from the study of this multistage alteration may be employed to explain the compositional variation under (sub)tropical through temperate climatic conditions in tailings derived from beneficiation of As–Cu ores elsewhere.

HERMIONE, EVOLUTION OF ATe-BEARING EPITHERMAL MINERALIZATION, ARGOLIS, HELLAS

The Cu-Te-bearing pyrite deposits of Hermione, Argolis are hosted in Miocenic ophiolites. The ophiolites are overlain by a shale-sandstone formation with intercalations of limestones and manganiferous sedimentary rocks. The ore deposits form irregular lenticular or stratiform ore bodies, and veins. These ore bodies are related to volcanic activity in an arc-related rift at the margins of a palaeocontinent. Late N- to NNE-trending, sinistral, milky quartz-pyrite-calcite veins cut the host ophiolites. Alteration haloes of quartz-calcite, albite-sericitechlorite, and chalcedony-epidote-clay minerals are developed in the lavas as concentric shells, or as envelops that parallel the quartz veins. The telluriumbearing mineralization is developed in two successive stages, characterized by the assemblages: pyrite-(pyrrhotite)-magnetite-chalcopyrite-sphalerite (Stage I) and galena-sphalerite-freibergite-marcasite-chalcocite (Stage II), followed by a supergene stage. The cobaltiferous pyrite-chalcopyrite geothermometer defined two ranges of last-equilibration temperatures: 220° to 250°Cfor Stage I, and 120° to 195°Cfor Stage II. The calculated δ18 Ο and SD compositions of the mineralizing fluids, at 200° and 250°C, reflect the dominance of a magmatic component. The calculated δ SH2S fluid values reveal a magmatic source for the sulphur, with minor contribution from submarine sediments, whereas tellurium is proposed to be derived from a mafic-ultramafic source.

Evolution of the magmatic-hydrothermal acid-sulfate system at Summitville, Colorado: integration of geological, stable-isotope, and fluid-inclusion evidence

The Summitville Au–Ag–Cu deposit is a classic volcanic dome-hosted high-sulfidation deposit. It occurs in the Quartz Latite of South Mountain, a composite volcanic dome that was emplaced along the coincident margins of the Platoro and Summitville calderas at 22.5±0.5 Ma, penecontemporaneous with alteration and mineralization. A penecontemporaneous quartz monzonite porphyry intrusion underlies the district and is cut and overlain by pyrite–quartz stockwork veins with traces of chalcopyrite and molybdenite. Alteration and mineralization proceeded through three hypogene stages and a supergene stage, punctuated by at least three periods of hydrothermal brecciation. Intense acid leaching along fractures in the quartz latite produced irregular pipes and lenticular pods of vuggy silica enclosed sequentially by alteration zones of quartz–alunite, quartz–kaolinite, and clay. The acid-sulfate-altered rocks host subsequent covellite+enargite/luzonite+chalcopyrite mineralization accompanied by kaolinite, and later barite–base-metal veins, some containing high Au values and kaolinite. The presence of both liquid- and vapor-rich fluid inclusions indicates the episodic presence of a low-density fluid at all levels of the system. In the mineralized zone, liquid-rich fluid inclusions in healed fractures in quartz phenocrysts and in quartz associated with mineralization homogenize to temperatures between 160 and 390 °C (90% between 190 and 310 °C), consistent with the range (200–250 °C) estimated from the fractionation of sulfur isotopes between coexisting alunite and pyrite. A deep alunite–pyrite pair yielded a sulfur-isotope temperature of 390 °C, marking a transition from hydrostatic to lithostatic pressure at a depth of about 1.5 km. Two salinity populations dominate the liquid-rich fluid inclusions. One has salinities between 0 and 5 wt.% NaCl equivalent; the other has salinities of up to 43 wt.% NaCl equivalent. The occurrence of high-salinity fluid inclusions in vein quartz associated with mineralization, as well as in the deep stockwork veins, suggests that brines originating deep in the system transported the metals. The δ 34S values of sulfides in magnetite (−2.3‰) and of sulfate in apatite (5.4‰) in unaltered quartz latite indicate that δ 34S ΣS was near 0‰. The δ 34S values of coexisting alteration alunite and pyrite are 18.2‰ to 24.5‰ and −8.1‰ to −2.2‰, respectively. Deep in the system, most of the change in δ 34S values occurs in the sulfates, indicating that the fluids were initially H 2S-dominant, their redox state buffered at depth by equilibration with igneous rocks. However, in the main alteration zone, most of the change in δ 34S values occurs in pyrite, indicating that the fluids moved off the rock buffer and became SO 4 2−-dominant as pyrite precipitated and SO 2 disproportionation produced the sulfuric acid requisite for acid leaching. The δ 34S values of the late-stage barite and sulfides indicate that the system returned to high H 2S/SO 4 2− ratios typical of the original rock-buffered fluid. The δD H2O of alunite parent fluids was near −45‰ and their δ 18O ranged from 7‰ to −1‰, depending on the degree of exchange in the alteration zone at low water–rock ratio, or mixing with unexchanged meteoric water. The low δD values of some alunite samples are interpreted to result from postdepositional exchange with later ore fluids. Fluid exsolved from the magma at depth had δD H2O and δ 18O H2O values near −70‰ and 10‰, respectively. During and following migration to the top of the magma chamber, the fluid underwent isotopic exchange with the partially crystallized magma and its solid and cooler, but still plastic, carapace just below the transition from a lithostatic to hydrostatic pressure regime. These evolved magmatic fluids had δD H2O and δ 18O H2O values close to −40‰ and 5‰, respectively, prior to release into the superjacent hydrostatically pressured fracture zone, wherein the fluids separated into a low-density phase and hypersaline brine. The low-density phase rose to the level of acid-sulfate alteration, where disproportionation of SO 2 commenced upon condensation of the vapor plume causing the acid-sulfate alteration. Hydrogen and oxygen isotopic compositions estimated from deep kaolinitic wallrock alteration and fluid inclusions in quartz stockwork veins indicate the brine mixed with highly exchanged meteoric water at depth and rose to the ore zone, producing the Cu–Au–Ag mineralization. The composition of inclusion fluids in a single sample of enargite also indicates mixing between ore fluids and unexchanged meteoric water at shallow depths. The sulfate in fluids responsible for precipitation of most barite samples was of magmatic origin, but the δ 34S of a single sample suggests mixing with sulfate derived from the atmospheric oxidation of H 2S in an overlying steam-heated zone.

THE KOMAN DAWSONITE AND REALGAR ORPIMENT DEPOSIT, NORTHERN ALBANIA: INFERENCES ON PROCESSES OF FORMATION

The deposit of dawsonite and realgar–orpiment in the Koman area, northern Albania, is aligned along the NE–SW-trending tectonic line joining the Krasta–Cukal and Mirdita structural-tectonic zones. The deposit contains the following main paragenet ic assemblages: i) marcasite – greigite (Fe-sulfide stage), ii) stibnite – realgar – orpiment (As–Sb-sulfide stage), iii) dolomite – calcite – dawsonite – aragonite – barite – gypsum (carbonate–sulfate stage), iv) native As – gibbsite (supergene stage). There was lithostratigraphic control of mineralization; carbonate-rich wallrocks reacted with the mineralizing fluids emanating from a bu ried magmatic body and migrating along Albanian transversal faults, rather than argillaceous lithotypes. Values of � 18 O and � 13 C indicate that dawsonite and hydrothermal dolomite are derived at the expense of carbonate rocks, which occur extensively in the stratigraphic sequence of the host rocks. The water:rock ratio during the carbonate–sulfate stage of deposition was probably small. Moreover, oxygen and carbon isotopic exchange during metasomatic transformation of the rocks, recrystallization and late involvement of groundwater, probably all occurred. The sulfur involved in realgar and orpiment deposition may be of magmatic origin, and there was enrichment of heavier S during late hydrothermal processes. Both marcasite and greigite have low Co/Ni values reflecting low-temperature conditions during their formation; the amount of As in the fluids was relatively high during both Fe- and As–Sb-sulfide stages of deposition. The wallrocks were the source of Al and Na involved in dawsonite deposition. Al, Na, K and Si were the elements most effectively leached from rocks during hydrothermal alteration.

Calculation of the physicochemical parameters ΣS, ΣC, aH2S, I and of the metal ions content of the hydrothermall fluid that has precipitated Au-Ag-Te ores in Panormos - Liardades Bay area, Tinos Island, Cyclades

The Guldberg - Waage chemical equilibration method was applied to calculate the ionic content of the hydrothermal fluid that has precipitated Ag-Au-Te ores with milky and clear quartz in a stockwork of veins at Panormos - Liardades Bay. The Au-Ag-Te mineralization is developed in nine (I to IX) successive stages with Base Metals Tellurides, Native precious metals and a supergene stage at the end. The metallic and non-metallic ions content of the hydrothermal fluid was calculated for temperatures between 300°C and 190°C. Reconstruction of the evolution of pHysicochemical conditions in the fluid within the same temperature interval yielded calculated values of ΣΟ= IO"1 to 10°, a=10'29 to 109 2 and 1= 0.4

Genesis and development of porosity in carbonate reservoirs—Fluid inclusion evidence

Fluid inclusions in minerals filled in pores of reservoir rocks can be used as a good indicator of pore genesis and development so as to shed light on oil generation, migration and accumulation. The relationship between pore evolution and oil generation has been established based on fluid inclusion studies on the Lower Ordovician carbonate reservoir strata in the Ordos Basin, Northwest China. Seven stages of porosity development can be recognized, i. e., the penecontemporaneous, the early and middle-late diagenetic, the supergene, the early, middle and late re-burying stages. The dissolution pores and fissures formed in the supergene and middle-late re-burying stages and the structural fractures formed in the late re-burying stage constitute the major traps of oil and gas. The major phase of oil migration and accumulation took place between Late Jurassic and Cretaceous, corresponding to the middle and late re-burying stages. The generation and accumulation of oil can be closely related to Yenshanian tectonics.

The Ag-Mn-Pb-Zn vein, replacement, and skarn deposits of Uchucchacua, Peru; studies of structure, mineralogy, metal zoning, Sr isotopes, and fluid inclusions

Uchucchacua is an Ag-Mn-Pb-Zn vein, replacement, and skarn mineral district in the central Andes of Peru. Host rocks are massive Jumasha Formation shelf limestones of Turonian age that have been folded into an asymmetric northeast-verging anticline of Andean trend. Strata near the fold crest are cut by minor dacitic intrusions and have been displaced by a conjugate set of steep wrench faults that strike northwest-southeast and northeast-southwest. Most ore occurrences are restricted to host rocks that lie below marly limestone at the top of the middle Jumasha Formation. Vein ores located along the fracture system have formed by fissure infill and by replacement of limestone wall rocks. Larger sheetlike replacement orebodies are parallel and adjacent to a large fault. Replacement is in zones of brecciation adjacent to fault bends which were dilatent during sinistral slip of the master fracture. Such fracture belts may have been subject to paleokarst solution processes before mineralization.Four paragenetic stages have been identified. Fe, Mn, and Si were introduced at the exoskarn stage (I) as the anhydrous silicates ferroan tephroite, johannsenite, rhodonite, and bustamite. During the early main stage (II) ferroan tephroite was replaced by friedelite and magnetite under oxidizing conditions at a relatively low pH. Pb, Zn, Fe, Cu, and B were introduced; principal sulfides are pyrrhotite, Fe-rich sphalerite, Mn-rich wurtzite, alabandite, galena, chalcopyrite, and tetrahedrite. Pyrrhotite was replaced by other sulfides during later stage II. Main gangue minerals were calcite, kutnohorite, rhodochrosite, and quartz. Ag, As, and Sb were introduced during the late stage (III) in the form of sulfosalts, principally pyrargyrite. Redistribution of metals introduced at stage II resulted in the growth of Fe-poor sphalerite and alabandite accompanied by calcite gangue. Decreasing Fe contents of alabandite and sphalerite during late stage II and stage III, together with the appearance of pyrite, indicate an increase in sulfur fugacity and/or decrease in temperature over this period. The supergene stage (IV) affects the upper 30 to 150 m of most veins and involves the growth of Mn hydroxides, goethite, orpiment, marcasite, cerussite, and siderite.Distribution patterns of metal ratios and high metal values define ore bands, with a succession of antiforms and synforms. Ore-band locations are determined by vein width, itself a function of wall-rock reactivity, and fracture permeability. Metal ratio distributions in the Luz vein show symmetric dispersion of metals at right angles to the ore-band axis. Ag shows the least dispersion with progressively greater dispersion of Pb and Zn.Geologic considerations indicate mineralization at a minimum depth of 1,600 m. Most primary fluid inclusions in calcite of probable stage II origin were trapped over a temperature range of about 200 degrees to 337 degrees C (hydrostatic pressure) or about 225 degrees to 362 degrees C (lithostatic). Fluid inclusions from late stage II quartz were trapped at about 165 degrees to 205 degrees C (hydrostatic) or 190 degrees to 230 degrees C (lithostatic). Salinity ranged from 0.5 to 29.7 equiv wt percent NaCl-CaCl 2 with little relation to temperature. Calcite 87 Sr/ 86 Sr values of 0.70743 to 0.71122 extend well outside the range of values both determined for magmatic rocks of the region and estimated for the host limestones but are probably compatible with older sedimentary rocks of the mine district. Salinity levels and NaCl/CaCl 2 ratios are similar to basinal brines associated with Mississippi Valley-type deposits. It is concluded that basinal brine, expelled from the deforming Mesozoic sequence, was a significant component of the ore fluid. This fluid was probably heated by dacitic magmas or cooling dacitic intrusions; a polygenetic high-salinity magmatic-basinal fluid then rose along existing strike-slip fractures toward the mineralization site where it mixed with heated low-salinity ground water.