Vegrandisite (BaCl2) a new mineral from salt melt inclusions from the Biely Vrch porphyry gold deposit, Slovakia

Research Article| June 01, 2014 Magmatic salt melt and vapor: Extreme fluids forming porphyry gold deposits in shallow subvolcanic settings Peter Koděra; Peter Koděra 1Department of Geology of Mineral Deposits, Faculty of Natural Sciences, Comenius University, Mlynská dolina, 842 15 Bratislava, Slovakia Search for other works by this author on: GSW Google Scholar Christoph A. Heinrich; Christoph A. Heinrich 2Department of Earth Sciences, ETH Zürich, 8092 Zürich, Switzerland Search for other works by this author on: GSW Google Scholar Markus Wälle; Markus Wälle 2Department of Earth Sciences, ETH Zürich, 8092 Zürich, Switzerland Search for other works by this author on: GSW Google Scholar Jaroslav Lexa Jaroslav Lexa 3Geological Institute, Slovak Academy of Sciences, Dúbravská cesta 9, 840 05 Bratislava, Slovakia Search for other works by this author on: GSW Google Scholar Author and Article Information Peter Koděra 1Department of Geology of Mineral Deposits, Faculty of Natural Sciences, Comenius University, Mlynská dolina, 842 15 Bratislava, Slovakia Christoph A. Heinrich 2Department of Earth Sciences, ETH Zürich, 8092 Zürich, Switzerland Markus Wälle 2Department of Earth Sciences, ETH Zürich, 8092 Zürich, Switzerland Jaroslav Lexa 3Geological Institute, Slovak Academy of Sciences, Dúbravská cesta 9, 840 05 Bratislava, Slovakia Publisher: Geological Society of America Received: 06 Nov 2013 Revision Received: 05 Mar 2014 Accepted: 12 Mar 2014 First Online: 09 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 © 2014 Geological Society of America Geology (2014) 42 (6): 495–498. https://doi.org/10.1130/G35270.1 Article history Received: 06 Nov 2013 Revision Received: 05 Mar 2014 Accepted: 12 Mar 2014 First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Peter Koděra, Christoph A. Heinrich, Markus Wälle, Jaroslav Lexa; Magmatic salt melt and vapor: Extreme fluids forming porphyry gold deposits in shallow subvolcanic settings. Geology 2014;; 42 (6): 495–498. doi: https://doi.org/10.1130/G35270.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The recently discovered Biely Vrch deposit in the Western Carpathian magmatic arc is the most extreme example of a porphyry gold deposit, being practically free of copper, molybdenum or any other sulfide minerals. Microanalytical data on fluid inclusions in quartz veinlets, including a characteristic type of banded veinlets, show that this deposit formed from nearly anhydrous Fe-K-Na-Cl salt melts containing ∼10 ppm Au, coexisting with hydrous vapor of very low density. This exceptional fluid evolution required an Fe-rich dioritic source magma that was emplaced at shallow subvolcanic depth (<3.5 km), directly exsolving a hypersaline liquid and magmatic vapor at high temperature (∼850 °C). During ascent to the level of the porphyry intrusion (0.5–1 km), fluid expansion at high temperature but low pressure led to halite precipitation and further water loss to the vapor, generating an increasingly Fe-K-rich salt melt that transported high concentrations of Au but negligible Cu into the fractured porphyry stock. The low sulfur fugacity resulting from fluid expansion suppressed precipitation of sulfide, explaining the gold-only enrichment in this globally recurring but rare type of gold ore. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Fluid inclusion systematics in porphyry copper deposits: The super-giant Grasberg deposit, Indonesia, as a case study

This volume presents new data on a group of gold deposits that are hosted primarily within or in the immediate wall rocks to intrusions, and which have recently been suggested to comprise a distinct class of magmatic-hydrothermal system. These deposits have been called 'porphyry gold deposits' (Hollister 1992; Bakke 1995), 'intrusion-related stockwork-disseminated deposits' (Sillitoe 1991), 'plutonic-related gold deposits' (Newberry et al., 1988; McCoy et al., 1997) and 'intrusion-related gold deposits' (Thompson et al. 1999). Lang et al. (2000) preferred the term 'intrusion-related gold systems' because it reflects a tendency common to all magmatic-hydrothermal environments to form ores that manifest multiple styles, metal assemblages and spatial associations with their related intrusive centres. Although in its infancy, investigation and exploration of intrusion-related gold systems has accelerated markedly in the last five years, due in part to their global distribution and to the large number of deposits that contain a gold resource of >30 metric tons (Fig. 1). Major deposit examples include Fort Knox (~210 t Au), Donlin Creek (~315 t Au), Pogo (~160 t Au), and Dublin Gulch, True North and Brewery Creek (~40 t Au each) in Yukon and Alaska, as well as Mokrsko, Czech Republic (~120 t Au), Vasilkovskoe, Kazakstan (~300 t Au), Salave, Spain (~30 t Au), Korri Kollo, Bolivia (~160 t Au) and Kidston, Australia (~140 t Au). A paucity of detailed descriptions of individual intrusion-related gold systems, the plutonic provinces that host these systems, and the genetic processes critical to their formation currently limits our ability to either develop precise criteria for their definition or to formulate well-constrained geological and exploration models. The principal discussions (Sillitoe 1991; Hollister 1992; Newberry et al. 1988 and 1995; Lang et al. 1997; McCoy et al. 1997; Thompson et al. 1999; Lang et al. 2000; Goldfarb et al. 2000; Newberry 2000) suggest that there are several features common to most intrusion-related gold deposits and provinces, including: 1) metaluminous, subalkalic intrusions of intermediate to felsic composition that span the boundary between ilmenite- and magnetite-series, 2) carbonic hydrothermal fluids, 3) a metal assemblage which variably combines Au with elevated Bi, W, As, Mo, Te and/or Sb and low concentrations of base metals, 4) a low sulphide content (<5 volume %) with a reduced ore mineral assemblage that typically comprises arsenopyrite, pyrrhotite and pyrite, and which lacks magnetite or hematite, 5) areally restricted, commonly weak hydrothermal alteration, except in systems formed at the shallowest depths spanned by these deposits, 6) a tectonic setting of continental magmatism well-inboard of inferred or recognized convergent plate boundaries, and which commonly contains coeval intrusions of alkalic, metaluminous calc-alkalic and peraluminous compositions, and 7) a location in magmatic provinces best or formerly known for W and/or Sn deposits. Deposits that can be confidently included in the group formed during much of the Phanerozoic, but inclusion of some Proterozoic and even Archean deposits has also been proposed (e.g., Robert, this volume). The defining criteria as presently recognized show that intrusion-related gold systems and their associated plutonic provinces are globally widespread (Fig. 1), but adequate descriptions of the contained deposits are only now beginning to emerge. The Tintina Gold Belt of Alaska and the Yukon Territory in the northern part of the North American Cordillera (Fig. 1) is thus far the most extensively studied intrusion-related gold systems province. This belt is ~1000 km in length and contains gold deposits of Early Cretaceous to Eocene age (McCoy et al. 1997; Newberry et al. 1995; Lang et al. 2000; Goldfarb et al. 2000) that, as a group, span much of the globally recognized variation among these systems. As such, the Tintina Gold Belt is currently the primary standard against which deposits in other provinces can be compared, and is therefore emphasized in this introduction. This paper briefly considers the status of our knowledge of intrusion-related gold systems, and is intended only as an introduction to the major characteristics of these deposits rather than a comprehensive description. The discussion emphasizes the nature of the associated igneous rocks and their tectono-magmatic setting, the styles of deposits and their spatial distribution, the characteristics and evolution of hydrothermal fluids, and structural controls. It concurrently highlights some of the important gaps in our understanding of these systems that will be fruitful areas of investigation in ongoing and future research programs. The paper concludes with comments on deposit classification and the relationship of intrusion-related gold systems to other types of magmatic-hydrothermal systems.

The phase relations in the Si-Al-Na-K-Li-F-H-O model granite system are studied experimentally at T = 800, 700 °C and P = 1 and 2 kbar, as well as at T = 600, 550, 500 and 400 °C and P = 1 kbar and different water content from 2 to 50 wt.%. The initial composition was set in such a way that the composition of the resulting silicate melt was close to the granite eutectic. It is shown that in the presence of Li, two immiscible melts are formed in the system—an aluminosilicate (L) and a salt alkali-aluminofluoride (LF). It is shown that at Т = 800 °С, Р = 1 kbar and 2 kbar and water content &gt; 10 wt. %, three phases are equilibrium in the system: L, LF, and fluid (Fl). Cryolite (Crl), which does not contain REE, begins to crystallize from the salt melt at 700 °C. Quartz (Qtz) crystallizes from the silicate melt at 600 °C and the equilibrium phases are L, LF, Crl, Qtz. At T = 500 °C Qtz, Na and K aluminofluorides and polylithionite crystallize from the aluminosilicate melt. The joint crystallization of Crl and Qtz is observed. Large crystals of cryolite and elpasolite are formed in both the salt and silicate melts. At the same time, the residual salt melt enriched in Li and REE is partially preserved. LF is completely crystallized at 400 °C, and L is in a metastable state. It is established that REE, Sc, Y and Li accumulate in the salt melt up to 500 °C with partition coefficients &gt;&gt; 1. REE and Sc enter into composition of the crystal phases at T = 500 °C and 400 °C. Sc partially isomorphically replaces Al. REE most often forms its own fluoride phases of the LnF3 type.

There are many examples of spatially associated porphyry and epithermal ore deposits; a genetic connection has been suggested for some and argued against for others. Nowhere is this spatial association better demonstrated than in the Mankayan district of northern Luzon, Philippines, where the Lepanto high-sulfidation epithermal Cu-Au deposit is superadjacent to the Far Southeast porphyry Cu-Au orebody; together they contain >3.8 million tons (Mt) Cu and >550 t Au.Quartz diorite porphyry dikes intruded Miocene basement rocks of metavolcanic and volcaniclastic rocks to a 300-m elevation. These intrusions postdate the Pliocene volcanic breccia and dacite porphyry that host much of the epithermal ore. K silicate alteration, consisting of biotite-magnetite and minor K feldspar, is centered on the quartz diorite porphyry. K-Ar ages of the biotite are 1.41 + or - 0.05 Ma (n = 6). Vitreous, anhedral quartz veins are associated with this early alteration and contain vapor-rich and hypersaline liquid inclusions with maximum homogenization temperatures of 450 degrees to 550 degrees C (and 50-55 wt % NaCl equiv salinities). Lithostatic pressure estimates indicate a paleosurface at a > or = 1,500-m elevation. Advanced argillic alteration formed over the top of the porphyry and consists of quartz-alunite, dated at 1.42 + or - 0.08 Ma (n = 5), synchronous with K silicate alteration. The lower limit of extensive quartz-alunite alteration is at a [asymp] 600-m elevation. Similar alteration and a core of leached, silicic alteration extend northwestward >4 km along the basement dacite contact, localized by the Lepanto fault. Chemical and S isotope zoning of alunite along strike indicates progressively lower temperatures away from the porphyry, from 350 degrees to 200 degrees C. K silicate alteration is overprinted by alteration consisting of chlorite plus hematite and/or sericite-illite, with a marginal zone containing pyrophyllite and an outer zone of propylitic alteration. The chlorite-sericite alteration is cut by veins of euhedral quartz that locally fill reopened anhedral quartz veins. The euhedral quartz veins contain anhydrite-white mica-pyrite + or - chalcopyrite + or - bornite and have halos of sericite; illite separated from these halos has ages of 1.30 + or - 0.07 Ma (n = 10). Fluid inclusions provide evidence for boiling on inception of this fracturing event (T h = 350 degrees C, 5 wt % NaCl equiv) and indicate a depth of 1,500 to 2,000 m below the paleowater table. This brittle-fracture event was followed by cooling and dilution of the hydrothermal fluid.The elevation of the enargite Au epithermal ore and its host of silicic alteration increases as the unconformity between the basement and dacite breccia rises from a 700- to 1,200-m elevation with increasing distance from the porphyry. Published data on enargite-hosted fluid inclusions (T h = 295 degrees -200 degrees C, 4-2 wt % NaCl equiv) indicate that the temperature and salinity both decrease with increasing distance from the porphyry. Epithermal ore consists of stage 1 euhedral pyrite-enargite-luzonite, and subsequent stage 2 Au is accompanied by tetrahedrite-chalcopyrite-sphalerite plus telluride and selenide minerals. Anhydrite and barite gangue minerals are followed by late vug-fulling quartz and minerals. The quartz-alunite alteration halo passes outward to kandite (kaolinite-nacrite-diclcite) alteration, then to chlorite or montmorillonite, depending on the host rock (basement or dacite, respectively).The dated minerals were also analyzed for their delta 18 O and delta D compositions, and their associated hydrothermal water values were calculated. Water in isotopic equilibrium with biotite averaged +6.3 and -45 per mil, respectively, typical of hypersaline liquid exsolved from felsic magma. The acidic water that deposited the alunite formed when magmatic vapor (+7ppm delta 18 O and -25ppm delta D) was absorbed by local meteoric water (-10ppm delta 18 O and -70ppm delta D) in a proportion of [asymp] 9:1 magmatic to meteoric. Lateral flow to the northwest and progressive mixing with ground water diluted the magmatic component to 1:1 at a distance of 4 km from the porphyry. At the depth of the porphyry, deposit, the later water isotopically stable with sericite was dominantly magmatic (+5.7ppm delta 18 O and -43ppm delta D) in the core. The marginal sericitie alteration (+1.5ppm delta 18 O and -51ppm delta D water values) indicates a maximum 20 to 30 percent component of local meteoric water. Pyrophyllite in both the porphyry and epithermal deposits formed from water with an isotopic composition similar to that which formed the sericitic alteration. The late euhedral quartz veining and sericitic alteration appear to have been associated with the majority of Cu and Au deposition. In addition, mineralogic, paragenetic, isotopic, and fluid inclusion evidence suggests that this water precipitated the enargite and Au within the epithermal deposit.Our results reinforce guidelines for exploration of such deposits. Advanced argillic (quartz-alunite) and K silicate alteration at Lepanto-Far Southeast are coupled in origin and result from vapor and hypersaline liquid separation. Thus, exploration programs for buried porphyry deposits should document carefully the geologic, morphologic, and temporal characteristics of exposed areas of advanced argillic alteration and its origin. Sericitic alteration at Far Southeast is associated with porphyry Cu and Au ore and appears to represent the roots of the main-stage Cu-Au mineralization in the epithermal deposit, hosted by silicic and quartz-alunite alteration that has a lower limit near the top of porphyry Cu-Au ore. In some cases, the sericitic overprint of a porphyry system, particularly where it is related to Cu and Au enrichment, may indicate a potential for nearby epithermal mineralization. Similarly, sericite and/or pyrophyllite underlying or overprinting a zone of hypogene advanced argillic (quartz-alunite) alteration indicates that mineralizing fluid may have ascended to epithermal depths. Epithermal ore at Lepanto-Far Southeast reflects a paleohydrologic regime dominated by lateral fluid flow, with a marked control by intersection of the Lepanto fault and a lithologic unconformity. Recognizing evidence for lateral flow is critical, as paleohydrology controlled the distribution of alteration and mineralization in many high-sulfidation epithermal deposits.

The porphyry gold deposits of the Refugio district and similar deposits in the Maricunga belt contain the lowest known copper to gold ratios (% Cu/ppm Au = ~0.03) of any porphyry-type deposit. The gold deposits are associated with subvolcanic andesitic to dacitic intrusions emplaced into coeval volcanic rocks. Both the Verde and Pancho deposits are zoned in space from a deeper zone of banded quartz veinlets associated with chlorite-magnetite-albite and/or pyrite-albite-clay alteration to a shallow zone of pyrite-albite-clay and local quartz-alunite ledges. Pancho contains an additional, deepest, porphyry copperlike zone, with quartz veinlets (A-veinlets) and potassic alteration. Relative to Verde, Pancho is telescoped, with all three zones present within a 400-m-vertical interval. The porphyry copperlike zone at Pancho is characterized by A-veinlets and pervasive potassic alteration, both restricted to intrusive rocks. A-veinlets range from hairline streaks of magnetite ± biotite with minor quartz and chalcopyrite, and K feldspar alteration envelopes to sugary quartz veinlets <1 cm in width with magnetite and chalcopyrite and no alteration envelopes. Hypersaline liquid inclusions coexisting with vapor-rich inclusions indicate temperatures above 600°C and salinities as high as 84 wt percent NaCl equiv. A pressure estimate of 250 bars indicates a depth of 1,000 m, assuming lithostatic pressure. Potassic alteration consists of a central zone of magnetite-K feldspar-oligoclase that changes outward to a biotite-rich zone. Total sulfide content, predominately as chalcopyrite, is generally <1 vol percent, whereas magnetite content is 2 to 5 percent. Where A-veinlets and potassic alteration predominate, grades are typically 0.1 wt percent hypogene copper and 0.5 to 1 ppm gold. Banded quartz veinlets are present at both Verde and Pancho, where they occur mostly above A-veinlets and cut A-veinlets where they overlap. They are less than 2 cm in thickness and lack alteration envelopes. Dark gray bands, whose color is due to abundant vapor-rich fluid inclusions and micrometer-sized grains of magnetite, commonly occur as symmetric pairs near the vein walls. The bands are commonly botryoidal and are continuous through quartz grains, suggesting that the quartz recrystallized from a silica gel. Rare liquid-rich fluid inclusions in quartz indicate temperatures <350°C and salinities <35 wt percent NaCl equiv. Estimated pressures are <200 bars, suggesting depths of 190 to 1,500 m under hydrostatic pressure. Gold occurs both in the dark bands with magnetite and outside the dark bands with pyrite, chlorite, illite, and K feldspar. Banded veinlets occupy steeply dipping radial and shallowly dipping concentric fractures. Zones of abundant banded veinlets without early A-veinlets generally contain 0.5 to 2 ppm gold and <0.05 wt percent hypogene copper. Most of the differences between porphyry gold deposits at Refugio and porphyry copper deposits can be attributed to shallower depths of formation—less than 1 km compared to 1.5 to 4 km that is typical for porphyry copper deposits. Shallower depths resulted in lower sulfide concentrations, local garnet veinlets, widespread albite-bearing alteration, and most importantly banded quartz veinlets, which are unique to porphyry gold deposits. Banded quartz veinlets are a direct result of episodic intrusion of magma to within 1 km of the surface and exposure of high-temperature magmatic fluids to hydrostatic pressures. Episodic rupturing of a brittle-ductile boundary surrounding the intrusive centers at Verde and Pancho led to flashing of magmatic fluids, loss of sulfur to vapor, and low sulfide/gold ratios in ore.

Volatiles, magmatic degassing and eruptions of Mt. Somma-Vesuvius: Constraints from silicate melt inclusions, Cl and H2O solubility experiments and modeling

The Refugio, Aldebaran, and La Pepa districts in the Maricunga belt of northern Chile contain advanced argillic alteration zones that locally host high-sulfidation epithermal gold deposits in proximity to porphyry gold (± copper) deposits. The spatial association suggests a genetic link. Mineralized zones are characterized by four main vein types that formed at different times and have specific zonal relationships. A-veinlets are the earliest and deepest vein type. They are restricted to potassic alteration zones in intrusive rocks. A-veinlets contain variable amounts of quartz, magnetite, biotite, and chalcopyrite and locally have K feldspar halos. They have nonmatching, irregular vein walls and lack internal symmetry. Hypersaline liquid-rich inclusions coexisting with vapor-rich inclusions in A-veinlets indicate temperatures as high as nearly 700°C and pressures between 200 and 400 bars. Assuming a lithostatic load, depths of 0.8 to 1.6 km are inferred. Zones of abundant A-veinlets contain mostly <1 ppm gold and 0.1 to 0.4 percent hypogene copper. Banded quartz veinlets occur mostly above A-veinlets and cut A-veinlets where they overlap. Dark gray bands, the color resulting from a high density of vapor-rich fluid inclusions and micron-sized grains of magnetite, commonly occur as symmetric pairs near the vein walls. Vein walls are parallel and slightly wavy, vuggy vein centers are common, and alteration envelopes are absent. Data from rare liquid-rich inclusions in banded quartz veinlets indicate temperatures <350°C at pressures between 20 and 150 bars. Assuming a hydrostatic load, depths of 0.2 to 1.5 km are inferred. Zones of abundant banded quartz veinlets generally contain 0.5 to 2 ppm gold and <0.1 percent hypogene copper. D-veins are pyrite veins with quartz-sericite-pyrite halos. They are widespread and crosscut A-veinlets and banded quartz veinlets. The brittle nature of D-veins and limited fluid inclusion data suggest temperatures <400°C. D-veins serve as important time lines. They are nowhere truncated or crosscut by intrusions, A-veinlets, or banded quartz veinlets. Quartz-alunite replacement veins, referred to as ledges in this paper, are typical of the high-sulfidation epithermal environment. They are mostly limited to overlying volcanic rocks. They contain local core zones of vuggy residual quartz that can contain enargite or, at higher elevations, barite. Of the three districts studied only La Pepa has mineable quartz-alunite ledges, which contain an average gold grade of about 20 ppm. A spectrum of porphyry-style deposits exists. Cerro Casale at Aldebaran shares many characteristics of porphyry copper deposits worldwide, whereas Verde at Refugio is a true porphyry gold deposit. Potassic alteration zones and A-veinlets are strongly developed at Cerro Casale, whereas they are absent at Verde. Banded quartz veinlets predominate at Verde, whereas they occur only at the upper levels of Cerro Casale. The Pancho deposit at Refugio and the Cavancha deposit at La Pepa are telescoped systems in which banded quartz veinlets overprint potassic alteration zones and A-veinlets. A-veinlets and banded quartz veinlets cut and are cut by intrusions, indicating multiple cycles of intrusion→potassic alteration→A-veinlets→banded quartz veinlets during formation of porphyry-style mineralization. Banded quartz veinlets are thought to have formed by flashing of magmatic fluids during episodic transitions from lithostatic to hydrostatic pressure. Loss of sulfur to the vapor phase during flashing inhibited formation of copper-sulfides in banded quartz veinlets and, therefore, resulted in high gold/copper ratios. Where rising magmatic vapors condensed into overlying meteoric water along faults, barren quartz-alunite ledges formed. This conclusion is supported by equivalent 40Ar/39Ar dates on hydrothermal biotite associated with porphyry-style ore and alunite from barren ledges at Aldebaran. 40Ar/39Ar dates at La Pepa indicate alunite formed at least 140,000 years to as long as 900,000 years after hydrothermal biotite. Within the high-sulfidation epithermal environment, the development of ore depends on the ability of late, moderate-salinity magmatic fluids to reach the surface without condensing a brine upon ascent. Cooling and boiling of the moderate-salinity fluid below its critical temperature results in the formation of sericite at depth and alunite near the surface that is essentially synchronous with high-sulfidation ore formation. The timing of the switch from lithostatic pressures to brittle hydrostatic conditions, relative to the life of the hydrothermal system, might determine how much porphyry-style ore forms relative to high-sulfidation epithermal ore.

Based on the method of investigation of melt inclusions using electron and ion microprobe, the magma composition and formation conditions of comendites of the Early Mesozoic Adaatsag volcanic association (Mongolia) were studied. The mechanisms leading to the accumulation of rare and rare earth elements in them were determined. Melt and fluid inclusions are found in quartz from the comendites, collected from different parts of the volcanic sequence. The melt inclusions consist of glass, a gas bubble and daughter minerals represented by fluorite, polylithionite and potassium feldspar. The use of the Raman spectroscopy method made it possible to study the composition of the gas phase in melt inclusions. It is determined that the predominant components of the gas phase are water and hydrogen. Fluid inclusions are represented by aqueous solutions that correspond to KF in composition. The concentration of KF in the solution reach up to 4.0–4.1 wt. %. Thermometric experiments with melt inclusions and analysis of the composition of glasses of homogenized melt inclusions in quartz of the comendites showed that the crystallization of magmas of these rocks occurred from water-saturated rare-metal melts with high contents of Li, Zr, F, Rb, Nb, Y and Th at temperatures of 880–930°C and at a pressure of 1000 bar at a depth of ~3.5 km and was accompanied by degassing processes. Comparison of the obtained data on the study of melt inclusions in the phenocrysts of the alkali-salic rocks of the same age volcanic associations Adaatsag, Dzarta-Кhuduk and Sant, developed within the Kharkhorin rift zone, revealed general patterns of their magma evolution. This allowed us to propose a similar mechanism of their formation, involving the accumulation of many rare and rare-earth elements, as well as volatile components (F, H2O) in the process of crystallization differentiation. Subsequently, a salt melt rich in Li, F and water could be separated from such comendite magmas. The detection of fluoride aqueous inclusions in quartz allows us to suppose the further evolution of the salt melt leading to the appearance of a concentrated aqueous fluid and the possible participation of the latter in metasomatic processes.

Abstract:Red clay type gold deposits, located in the south of China, are situated not only in orogenic belts, but also in inner cratons, where climate is tropical‐subtropical with clear arid and humid. The lateritic weathering crust often can be divided into five zones, including topsoil, siliceous duricrust zone, multi‐color zone (or red clay zone in some deposits), pallid zone and saprolite zone from surface to the base rock, several of which are absent in some deposits. The base rocks are composed mainly of carbonate rocks with minor clastic rocks, intermediate‐basic volcanic rocks and intermediate‐acid and alkalic intrusions. The orebodies are mainly located in the multi‐color zone with part of them in the pallid and saprolite zones. The ore sources include orebodies of Carlin‐type gold deposits and porphyry gold deposits, as well as gold‐rich base rocks. The red clay type gold deposits experienced early‐stage endogenic gold mineralization and laterization during the Tertiary and Quaternary. The areas with endogenic gold deposits, especially Carlin‐type gold deposits and porphyry gold deposits in karst depressions on the plateau, structual erosional platforms in the middle‐lower mountains, and intermountain basins in southern China are well worth studying to trace red clay type gold deposits.

The composition and formation conditions of magmas of comendites of the Early Mesozoic Adaatsag volcanic association are determined using electron and ion microprobe analysis of melt inclusions, and mechanisms favorable for the accumulation of trace and rare earth elements are identified. The melt and fluid inclusions are found in quartz of comendites sampled in various parts of the section of a volcanic sequence. The melt inclusions consist of glass, a gas bubble, and daughter minerals (fluorite, polylithionite, and K-feldspar). The composition of a gas phase in melt inclusions was studied using Raman spectroscopy. The dominant gas phase components include water and hydrogen. The fluid inclusions are aqueous fluids probably corresponding to KF. The thermometric studies of melt inclusions and the analysis of glasses of homogenized melt inclusions in quartz phenocrysts of comendites showed that magmas crystallized from water-saturated rare metal melts with high Li, Zr, F, Rb, Nb, Y, and Th contents at a temperature of 880–930°C and a pressure of 1000 bar at a depth of ~3.5 km accompanied by degassing. The comparison of our data of the study of melt inclusions in phenocrysts of alkaline salic rocks of the coeval Adaatsag, Dzarta-Khuduk, and Sant volcanic associations, which occur within the Kharkhorin Rift Zone, revealed common principles of magma evolution. This allowed us to propose a similar mechanism of their formation, involving the accumulation of many rare and rare-earth elements, as well as volatile components (F, H2O) in the process of crystallization differentiation. Subsequently, a salt melt rich in Li, F and water could be separated from such comendite magmas. The detection of fluoride aqueous inclusions in quartz allows us to suppose the further evolution of the salt melt leading to the appearance of a concentrated aqueous fluid and the possible participation of the latter in metasomatic processes.

Decratonic gold deposits constitute the most important type of gold endowment in the North China Craton (NCC), although the nature and evolution of ore-related fluids in these deposits remain controversial. Here we investigate geochemistry of hydrothermal zircon grains from the Dahu gold deposit in the Xiaoqinling region in the southern margin of the NCC. We also compile the published information on trace elements in hydrothermal zircon from deposits in the other major gold districts of the NCC. The zircon U-Pb data show ages in the range of 101–148 Ma reflecting prolonged hydrothermal alteration that occurred within an extensional tectonic setting. Based on the trace elements, three distinct groups of hydrothermal zircons are recognized. Group 1 shows low La contents (<1 ppm), high Ce/Ce* (>10), and steep positive LREE patterns ((Sm/La)N > 20); Group 2 possesses high La contents (<10 ppm), low Ce/Ce* (<10), and weak positive LREE patterns ((Sm/La)N < 20); Group 3 displays ultra-high La contents (>10 ppm), low Ce/Ce* (<10), and flat LREE patterns ((Sm/La)N < 20). This classification is also applicable to orogenic and porphyry gold deposits. Group 1 grains initially crystallized from magma and underwent subsequent modification by post-magmatic fluids, whereas Group 2 and Group 3 directly crystallized and regrew from zirconium-saturated aqueous fluids. The La-enriched signature in hydrothermal zircons may mainly originate from undetected REE-bearing phases within zircon that formed during the hydrothermal alteration events. The fluid flow has a significant effect on the redox state of gold metallogenic system. The hydrothermal zircons from decratonic gold deposits display higher Ce/Ce* ratios (1–258) and Ce/Nd ratios (0.7–32.2) than those from orogenic and porphyry gold deposits, indicating that the zircon grains formed from a relatively oxidized fluid.

A porphyry gold deposit has been documented for the first time at Marte in the Maricunga belt of the Andean Cordillera, northern Chile. Exploration conducted from 1981 through 1987 resulted in definition of 66 metric tons of contained gold. The deposit entered production in late 1989 as an open-pit, heap leach operation.The gold deposit is part of a linear, calc-alkaline volcanoplutonic arc constructed during the mid- to late Miocene as eastward-directed subduction was shallowing. The consequent compression of continental basement underlying the deposit and its environs created a series of high-angle, reverse faults. Stock emplacement and gold mineralization took place within a coeval andesitic stratovolcano at 13 to 14 Ma. The hornblende-biotite diorite stock is subdivided into three phases: two mineralized porphyries and a weakly porphyritic, late mineralization microdiorite. Much of the microdiorite constitutes the matrix of an intrusion breccia, which is transected locally by a closely related hydrothermal breccia.Gold mineralization is coincident with a stockwork of quartz veinlets surrounded by chloritesericite-clay alteration of intermediate argillic type. Pyrite and iron oxides are both present in dominantly disseminated form, with each making up as much as 10 vol percent of the rock. Hematite exceeds magnetite in abundance and was generated in large part by hypogene martitization. Minor chalcopyrite and even lesser quantities of molybdenite, bornite, tennantite, and enargite also occur. Gypsum after anhydrite commonly exceeds 5 vol percent and calcite and tourmaline exist as traces. Isolated remnants of hydrothermal biotite and alkali feldspar attest to the former presence of K silicate alteration, which is believed responsible for introduction of much of the veinlet quartz, magnetite, and gold. Chlorite-dominated alteration affected the late-stage microdiorite and its associated breccias. An erosional remnant of the andesitic volcanic roof to the stock is largely replaced by a chalcedony- and alunite-rich advanced argillic assemblage, which is part of a formerly more extensive tabular alteration zone. Abundant pyrite, gypsum, native sulfur, and barite and trace amounts of enargite and subeconomic gold are present in this advanced argillic cap.The relative impermeability of the cap caused development of an inverted supergene profile in which sulfide-bearing ore is underlain by jarositic leached ore. The steep, elongate orebody is made up of both ore types, which do not differ in gold content. However, the hypogene copper content, about 550 ppm, was more than halved by supergene oxidation. The gold ore is poor in silver (Ag/Au = 0.44), but highly anomalous in molybdenum as well as copper and weakly anomalous with respect to lead, arsenic, and mercury. The advanced argillic cap is marked by pronounced Au, Bi, Hg, Tl, Pb, Mo, and As anomalies, which contrast with a broad zinc halo to the orebody.Preliminary fluid inclusion studies suggest that the auriferous stockwork is a product of boiling fluids which ranged in temperature from 155 degrees to 375 degrees C and in salinity from 2 to 20 wt percent NaCl equiv. A reconstruction of the volcanic edifice combined with pressure estimates based on the fluid inclusion data implies that mineralization took place about 600 to 700 m below the paleosurface. The geologic, alteration, and mineralization characteristics of the Marte gold deposit are closely similar to those of gold-rich porphyry copper deposits, especially those in the Philippines. The only significant mineralogic difference is a deficiency of copper at Marte. The fluid inclusion population at Marte is also more reminiscent of porphyry rather than epithermal environments, although high-salinity inclusions common in porphyry copper deposits were not encountered in the few samples studied.

Geology, geochemistry and geochronology of Lindero porphyry gold deposit in the Southern Puna plateau, Argentina

The sources and physical–chemical conditions of gold mineralization in the Qi189 porphyry deposit, Qiyugou gold orefield, Eastern Qinling