Epithermal Veins Research Articles

Microtextural evidence for the recrystallization of opal-A to quartz in epithermal veins: A case study from the McLaughlin deposit, California

High-grade ores in low-sulfidation epithermal deposits commonly consist of banded veins containing quartz as the most abundant gangue mineral. Previous studies suggested that at least some of the quartz has formed as a product of recrystallization from a noncrystalline silica precursor. Detailed petrographic studies confirm that high-grade veins from the <2.2 Ma McLaughlin deposit in California were originally entirely composed of opal-AG. The noncrystalline silica is isotropic in crossed-polarized light and consists of compacted microspheres that are up to several micrometers in size. In many bands of the high-grade veins, the thermodynamically unstable opal-AG has matured to quartz. The recrystallization of the noncrystalline silica resulted in the development of quartz and ore textures that mask the original conditions of vein formation. Incipient recrystallization of the opal-AG caused the formation of lepispheres consisting of opal-CT as well as the development of concentrically banded silica spheres. Continued maturation led to the growth of elongated quartz crystals or complexly shaped quartz aggregates in the cores of the concentrically banded silica spheres. Amalgamation of quartz crystals caused the development of mosaic quartz or flamboyant quartz. Ripening produced large prismatic quartz crystals, which are characterized by zones of feathery appearance. Fluid inclusions within the quartz formed through recrystallization are typically highly irregular in shape and show inconsistent liquid-to-vapor volumetric proportions, but assemblages with consistent ratios are also present. Bands rich in fluid inclusions occurring along former recrystallization fronts in prismatic quartz crystals resemble growth zones in zonal quartz. Recrystallization of the silica matrix resulted in grain coarsening of the ore minerals and encapsulation of ore minerals by quartz. In areas where recrystallization of the noncrystalline silica proceeded to completeness, the quartz textures could be easily misinterpreted to indicate that quartz growth occurred in open space with the ore minerals infilling vug spaces. Correct recognition of recrystallization textures has significant implications for paragenetic investigations on vein material from epithermal deposits as well as the design of fluid inclusion studies.

Insights into fluid evolution and Re enrichment by mineral micro-analysis and fluid inclusion constraints: Evidence from the Maronia Cu-Mo ± Re ± Au porphyry system in NE Greece

Porphyry-epithermal veins hosting Re-rich molybdenite and rheniite (ReS2) from the Maronia Cu-Mo ± Re ± Au porphyry in Thrace, NE Greece, provide new insights into the hydrothermal processes causing extreme Re enrichment. Quartz trace element chemistry (Al/Ti, Ge/Ti), Ti-in-quartz thermometry, and cathodoluminescence imaging reveal multiple quartz generations in consecutive hydrothermal quartz-sulfide veins associated with potassic, sericitic, and argillic alteration. Fluid inclusions in different quartz generations indicate that phase separation and fluid cooling are the main ore-forming processes in the porphyry stage (~ 500 – 350 °C), whereas mixing of a vapor-rich fluid with metalliferous (e.g., Pb, Zn, Au) meteoric water forms the epithermal veins (~ 280 °C). These processes are recorded by trace element ratios in pyrite that are sensitive to changes in fluid temperature (Se/Te), fluid salinity (As/Sb, Co/As), and mixing between fluids of magmatic and meteoric origin (Se/Ge). Highly variable intra-grain δ34S values in pyrite record S isotope fractionation during SO2 disproportionation and phase separation, emphasizing the importance of in situ δ34S analysis to unravel ore-forming processes. High δ34S (~ 4.5‰) values of sulfides are indicative of low SO42−/H2S fluid ratios buffered by the local host rocks and mixing of the magma-derived fluid with meteoric water. The formation of Re-rich molybdenite (~ 6600 ppm) is favored by cooling and reduction of a magma-derived, high-temperature (~400 °C), oxidized, and Re-rich fluid triggering efficient Re precipitation in early veins in the potassic alteration zone. The systematic temporal fluid evolution therefore reveals that coeval cooling and reduction of oxidized Re-rich fluids cause extreme Re enrichment at the Maronia porphyry system.

Epithermal Gold Discoveries in the Emerging Khundii Metallogenic Province, Southwest Mongolia

Abstract Mineral exploration since 2005 in a previously underexplored region of southwestern Mongolia resulted in the definition of the Zuun Mod porphyry Mo-Cu deposit, followed by discovery of the Altan Nar and Bayan Khundii epithermal gold deposits along with several prospects and advanced exploration projects. These discoveries form the core of the emerging Khundii (“Valley”) metallogenic province, ~50 × 100 km in size, located within a single island-arc terrane of Middle Carboniferous to early Permian age and predominantly within an individual mapped subterrane. The province is situated ~700 km west-northwest of the late Devonian Oyu Tolgoi porphyry Cu-Au deposit in a belt of mid-Paleozoic island arcs that are part of the Central Asian orogenic belt, host to world-class porphyry Cu-Au and epithermal gold deposits that stretch from southern Mongolia to the west, into China, Kazakhstan, and beyond. The Zuun Mod porphyry Mo-Cu deposit (297 ± 4.8 Ma) is hosted by a granodiorite intrusion cut by B-type quartz-molybdenite-chalcopyrite veins with K-feldspar alteration selvages plus disseminated biotite and magnetite. After definition of this deposit, a regional exploration program was initiated in 2009 over 110,000 km2, based on the underexplored nature of the region. Exploration included compilation of existing geologic, geochemical, and geophysical data and interpretation of satellite imagery followed by ground exploration that included stream, soil, and rock-chip sampling and geologic and alteration mapping. The Nomin Tal Cu-Au prospect was discovered in early 2011, and based on the indications from initial soil sampling, a 400- × 400-m soil survey was conducted over the southern part of the exploration license, which identified a Pb-, Zn-, and Au-in-soil anomaly over an area of ~1.5 × ~5.5 km. The first drill hole within the soil anomaly in late 2011 resulted in the discovery of the Altan Nar Au-polymetallic epithermal deposit with veins of coarsely crystalline quartz-adularia (309.7 ± 0.5 Ma) and Ca-, Mg-, Mn-, and Fe-carbonate gangue that host the base metal sulfides. The Bayan Khundii gold deposit was discovered in 2015 as the result of prospecting, ~16 km southeast of Altan Nar. Subsequent discovery of the Khar Mori gold project was announced in early 2021, ~3 km north of Bayan Khundii along a structural trend, and later in 2021 drilling discovered wide zones of disseminated gold at Ulaan Southeast, ~800 m west of Bayan Khundii. The epithermal quartz-adularia-gold veins (336.8 ± 0.5 Ma) at Bayan Khundii have colloform bands with minor pyrite and are enveloped by proximal illite alteration. The epithermal veins and alteration overprint an earlier, unrelated alteration style of residual quartz and pyrophyllite ± dickite ± diaspore-kaolinite. Similarly, residual quartz and pyrophyllite-dickite at Khar Mori are overprinted by epithermal mineralization, including arsenopyrite. At the central Ulaan project, ~3 km northwest of Bayan Khundii, intense quartz-white mica-pyrite alteration is widespread at surface, including tourmaline bodies and local copper anomalies, associated with nearby residual quartz and related aluminosilicate alteration. These alteration styles indicate erosion of a lithocap to its base, exposing K-feldspar and magnetite plus quartz-white mica-pyrite related to the top of a porphyry deposit, as yet only tested by a few scout drill holes. The undated porphyry-related alteration was subsequently overprinted by the gold-bearing epithermal veins after significant erosion.

A Cretaceous siliceous sinter in NE China: Sedimentological and geochemical constraints on its genesis

Phanerozoic sinter deposits have been reported globally, with their identification mainly based on sedimentological, petrological, and mineralogical studies. In this study, a detailed geochemical investigation, combined with sedimentological characterization, was conducted on the Wugonglilu siliceous deposit, a Cretaceous (106 Ma) sinter in NE China, to examine its genesis. The deposit is inferred as a siliceous hot spring deposit (sinter) owing to its geological setting in an epithermal gold-mining district, its morphologically variable microbial textures typical of subaerial sinters, and nearly pure silica composition. Three lithofacies were recognized in this study, including laminated to thinly bedded sinter, silica-cemented breccia, and epithermal vein, which represent deposits from three contrasting hydrothermal environments. The 87Sr/86Sr ratios of the sinter are similar to the bedrock through which the geothermal fluids passed, namely the Upper Triassic Dajiahe Formation (T3dj), a marine siliceous rock unit, and the Lower Cretaceous Datashanlinchang Formation (K1d), a rhyolite unit. Results indicate that both underlying units are possible Sr sources of the silica in the Wugonglilu sinter. The REE + Y (rare-earth elements and yttrium) patterns of the sinter exhibit significant variability, primarily due to terrestrial detrital contamination. Differential REE + Y contamination by terrestrial detritus was striking in the white and dark laminae/beds of the laminated to thinly bedded sinter. The white laminae/beds, generally less contaminated, were probably formed during the dry season, whereas the dark laminae/beds exhibiting greater detrital contamination were likely formed during the wet season. When excluding the samples strongly contaminated by detritus, the sinter exhibits Y anomalies comparable to Post-Archean Australian Shale (PAAS), and to the adjacent underlying K1d rhyolite and T3dj siliceous rock samples, indicating that these anomalies were not inherited from rocks dominating the reservoirs. Furthermore, the sinter shows a (Ce/Ce*)N range close to that of the underlying marine siliceous rock and rhyolite samples, reflecting that the (Ce/Ce*)N of the sinter is largely inherited from the reservoir rock(s). The overall REE + Y patterns of the sinter range from nearly flat to LREE-depleted, similar to some samples of the K1d rhyolite and T3dj siliceous rock units. However, most samples of the K1d rhyolite and T3dj siliceous rock display (Pr/Tb)N and (Pr/Yb)N higher than many studied sinter samples. This is likely because of the formation of strong HREE–carbonate complexes during groundwater migration. Based on these results, a generalized formation model of the studied sinter system was constructed. This study suggests that integrated sedimentological and geochemical investigations can aid in interpreting the genesis of sinter deposits, in particular the link between sinter deposits and corresponding reservoir rocks.

Natural growth of gold dendrites within silica gels

Abstract High-grade ores in low-sulfidation epithermal precious metal deposits include banded quartz veins that contain gold dendrites. The processes by which dendrite growth takes place have been subject to debate for decades, especially given that these deposits are known to form from dilute thermal liquids that contain only trace amounts of gold. It is shown here that growth of gold dendrites in epithermal veins at the McLaughlin deposit in California (western USA) originally took place within bands of gel-like noncrystalline silica. The gel provided a framework for the delicate dendrites to form. The high permeability of the gel allowed the diffusion and advection of gold from the thermal liquids flowing across the top of the silica layers to the sites of crystal growth within the gel. Over time, the gel hardened to form opal-AG. This silica phase is thermodynamically unstable and recrystallized to quartz that has a distinct mosaic texture.

Fluid inclusion, zircon U-Pb geochronology, and O-S isotopic constraints on the origin and evolution of ore-forming fluids of the tashvir and varmazyar epithermal base metal deposits, NW Iran

Tashvir and Varmazyar deposits are part of the epithermal ore system in the Tarom–Hashtjin Metallogenic Belt (THMB), NW Iran. In both deposits, epithermal veins are hosted by Eocene volcanic-volcaniclastic rocks of the Karaj Formation and are spatially associated with late Eocene granitoid intrusions. The ore assemblages consist of pyrite, chalcopyrite, chalcocite, galena, and sphalerite (Fe-poor), with lesser amounts of bornite and minor psilomelane and pyrolusite. Fluid inclusion measurements from the Tashvir and Varmazyar revealed 182–287 and 194–285°C formation temperatures and 2.7–7.9 and 2.6–6.4 wt.% NaCl equivalent salinities, respectively. The oxygen isotope data suggested that the mineralizing fluids originated dominantly from a magmatic fluid that mixed with meteoric waters. The sulfur isotope data indicated that the metal and sulfur sources were largely a mixture of magma and surrounding sedimentary rocks. LA-ICP–MS zircon U–Pb dating of the granitoid intrusion at Tashvir and Varmazyar, yielded a weighted mean age of 38.34–38.31 and 40.85 Ma, respectively, indicating that epithermal mineralization developed between 40.85 and 38.31 Ma. Our data indicated that fluid mixing along with some fluid boiling were the main drives for hydrothermal alteration and mineralization at Tashvir and Varmazyar. All these characteristics suggested an intermediate-sulfidation epithermal style of mineralization. The THMB is proposed to be prospective for precious and base metal epithermal mineralization. Considering the extensional tectonic setting, and lack of advanced argillic lithocaps and hypersaline fluid inclusions, the THMB possibly has less potential for economically important porphyry mineralization.

Mонгол орны зарим алтны ордуудын хүдэржилт, гарал үүслийн харьцуулалт

The gold mineralization of Mongolia has been described by many geologists. Mongolian gold deposits and occurrences can be divided into 23 metallogenic belts. It shows that Mongolia has a very high potential in gold production. Epigenetic gold deposits and occurrences in Mongolia are genetically classified into several types: Au-Ag epithermal vein, orogenic, intrusion related, skarn, and porphyry, iron oxide-copper-gold deposit types. Mongolian orogenic deposits are structurally controlled and related to shear zones or folds, and are mostly hosted in granitoids or metasedimentary rocks, and metamorphic rocks. These Au deposits commonly occur as massive auriferous quartz veins that contain sulfides and less commonly occur as disseminated and stockwork. In this paper, the geological characteristics, sulfur isotopic geochemistry, and geochemical data of ore-forming fluids of five gold lode deposits in Mongolia such as Tavt deposit located in the Eg river-Dhzida metallogenic belt of the Eg-Dzhida metallogenic province, Gatsuurt, Harganat, Ereen, Biluut, Sujigtei, Ulaanbulag deposits of the Dzuunmod ore district located in the North Khentii metallogenic belt of the Khangai- Khentii metallogenic province, and Olon Ovoot group gold deposits located in the Ulziit metallogenic belt and Undurnaran deposit located n the Kharmagtai-Khunguut-Oyut Ulaan metallogenic belt of the South Mongolian metallogenic province, and Chandman Uul deposit in the of South Kherlen-Buyant metallogenic belt of the Central-Eastern Mongolia metallogenic province are comprehensively compared. The features of gold ore mineralization, fluid inclusion, and S isotope of gold deposits are summarized and analyzed.

Hydrothermal evolution from porphyry- to epithermal-style mineralization in the Naozhi deposit, NE China

The Naozhi deposit in the Yanbian district of northeastern China contains polymetallic epithermal veins and subeconomic porphyry-style Cu mineralization within 2 km of each other. These two mineralization styles are genetically associated with 130–126 Ma andesites and dioritic–granitic intrusions. Isotopic ages determined on sericite (128.2 ± 2.9 Ma; 40Ar–39Ar plateau age) and sulfide (124.6 ± 2.7 Ma; Rb–Sr isochron) associated with the epithermal mineralization constrain the hydrothermal activity to at approximately 128 Ma. Three types of fluid inclusions, including halite-bearing, vapor-rich, and liquid-rich inclusions were trapped in quartz from porphyry-style veinlets associated with an assemblage of K-feldspar–biotite–magnetite. Based on microthermometry and the homogenization behavior of halite-bearing inclusions, they were trapped at lithostatic pressures of > 500 bars and at temperatures of ≥ 350 °C, whereas the vapor- and liquid-rich inclusions were trapped at pressures and temperatures of 110 ± 35 bars and ≤ 350 °C.Epithermal mineralization comprises four vein stages, which, from oldest to youngest, are quartz–pyrite, quartz–base metal sulfide, quartz–precious metal, and quartz–carbonate. The ore assemblage precipitated from low-salinity fluids (<10 wt% NaCl equiv.) at temperatures and pressures of ≤ 350 °C and ≤ 100 bars. The δ18OH2O (2.5 to −2.4 ‰) and δDH2O (−85 to −114 ‰) compositions of fluids related to porphyry- and epithermal-style mineralization indicate a mixture of magmatic and meteoric waters. The δ34SH2S (−3.1 to 4.5 ‰) values of fluids become isotopically heavier from the quartz–pyrite stage to the quartz–base metal sulfide stage, reflecting continuous reduction during epithermal mineralization. The mineral assemblage and chemical composition of sulfide minerals indicate a transition from oxidizing, high sulfidation porphyry-style fluids to reducing, intermediate–low sulfidation epithermal fluids, with log ƒO2 values ranging from −31 to > −33, and log ƒS2 values ranging from −7.0 to ≤ −13.4 during the epithermal veining stage. Metal deposition in the Naozhi magmatic–hydrothermal system resulted from cooling, sulfidation, neutralization, and boiling of predominantly magmatically sourced hydrothermal fluids that mixed with meteoric water.

Application of complex resistivity survey in an epithermal Au-Ag deposit, South Korea

Electrochemical reactions at the interface between groundwater and sulphides are remarkable. Sulphides in mineralised zones are relatively abundant compared to hydrothermal alteration zones and host rocks. Complex resistivity is a geophysical tool for visualising difference between various subsurface electrochemical reactions. The epithermal Au–Ag mineralisation at Moisan hill (South Korea) occurs in the extensively disseminated pyrite zone, a typical feature of advanced argillic and argillic alteration zones. The epithermal vein at Moisan had a strike length of >500 m horizontally and approximately 300 m vertically and was controlled by the WNW fault zone. In this context, the deposit was subjected to a test bed to demonstrate the applicability of the complex resistivity survey for mineral exploration. To compare complex resistivity results with geological characteristics of epithermal mineralisation, we visualised the complex resistivity survey results and Au–Ag mineralised zones confirmed by drilling cores in three dimensions. The quartz veins of the targets showed high resistivity and a strong phase response; however, both the alteration zones and host rocks showed lower resistivity and a weaker phase response than the target zones. Through a step-by-step clustering analysis, a simple map integrating both kinds of the geophysical models was generated, to identify the boundary between the target and background. Geologic survey and drilling investigations indicate that the target is well-localised in a mineralised zone. The complex resistivity survey is a useful tool for exploring epithermal Au–Ag deposits.

TRACE ELEMENTS IN QUARTZ: INSIGHTS INTO SOURCE AND FLUID EVOLUTION IN MAGMATIC-HYDROTHERMAL SYSTEMS

AbstractQuartz trace elements record information about fluid evolution as well as metal migration and precipitation. Here, we summarize most of the reported (including this study) quartz trace element data (N = ~4,600) generated by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) on various textural types and paragenetic stages of quartz in I-type porphyry-epithermal (Cu-Mo-Au-Ag-Te) and S- and A-type granitegreisen (Sn-W and rare metal) systems in the world. The results show that Li versus Al diagrams, combined with Ti-Ge-As-Sb contents, can be used to decipher the source and evolution of fluids in magmatic-hydrothermal systems. In I-type porphyry-epithermal systems, magmatic quartz has low Li/Al ratios from 0.001 to 0.173 (N = 483) with a mean of 0.039 ± 0.032. Hydrothermal quartz has progressively higher Li and Al concentrations that are dominated by cooling along fluid pathways. Quartz evolves from Ti rich to Ge rich from early to late stages in porphyry hydrothermal veins and is As and Sb rich in epithermal veins. In S- and A-type granite-greisen systems, magmatic quartz has high Li/Al ratios from 0.007 to 0.502 (N = 604) with a mean of 0.130 ± 0.063 and from 0.009 to 0.327 (N = 325) with a mean of 0.126 ± 0.065, respectively. Hydrothermal quartz has progressively lower Li and Al concentrations that are dominated by fluid-rock reactions and cooling along fluid pathways. Quartz evolves with decreasing Ti concentrations from magmatic to hydrothermal stages. Ge is abundant in pegmatite quartz in S-type systems. Variations in pH or precipitation rate along fluid pathways have a small influence on Li/Al ratios. The variation of quartz trace elements with elevation in individual systems suggests that they can be used as a vector to guide exploration in magmatic-hydrothermal systems.

The Kupol Epithermal Au-Ag Vein District, Chukotka, Far Eastern Russia

Abstract The Kupol epithermal Au-Ag vein district is located in the northern part of the Okhotsk-Chukotka volcanic belt, a Late Cretaceous subduction-related continental volcanic arc exposed for &amp;gt;3,000 km along the eastern coast of Russia. High-grade veins are hosted in the Kupol andesite sequence, a 300- to 1,000-m-thick, subhorizontal, layered sequence of andesite flows, sills, and ash tuffs, dated at 97 to 96 Ma (Cenomanian). The Kupol andesite sequence is underlain by mixed mafic-felsic volcanic units plus sedimentary rocks (“older volcanics”) and overlain by a &amp;gt;1-km-thick “upper felsic” sequence of dacitic-rhyolitic tuffs and associated dikes and flow domes, dated at 95 to 85 Ma, with local sequences of fluvio-lacustrine sedimentary rocks. The epithermal veins occupy N-striking, steeply dipping normal faults that cut thick coherent andesite flows and sills in the central-upper part of the Kupol andesite sequence. The district is dominated by the large Kupol vein (180.7 tonnes (t) Au and 1,986 t Ag produced to 2020), hosted by the 5.5-km-long Kupol fault, which accommodates normal, east-side-down displacement of up to 190 m. The Moroshka and Providence veins, 5 km east-southeast of Kupol, occupy shorter faults (1- to 2-km strike) with smaller vertical displacements (to 70 m). The Moroshka vein is dated at 93.5 ± 1.5 Ma (Turonian; 40Ar/39Ar method on adularia), and the timing of vein mineralization here and at Kupol overlaps with the early stage of upper felsic sequence magmatism. Veins contain subhorizontal ore shoots, controlled by the intersection of the steep faults with flat-lying Kupol andesite sequence stratigraphy and by steepening of the faults to a more dilational orientation as the inferred paleosurface is approached. Local structural controls are also evident, reflecting a component of oblique slip on the Kupol fault, with the thickest vein segments at steeply pitching jogs and relays. Main-stage veins grew via repeated encrustation by quartz-chalcedony ± amethyst ± lattice bladed calcite (replaced by quartz), with Au-Ag–bearing crustiform adularia ± clays ± sulfides/sulfosalts/electrum ± chlorite ± hematite bands. The main controls on Au grade are inferred to have been boiling, resulting in sharp vertical limits to high metal grades typical of epithermal veins, coupled with optimal dilation of the vein system where the hosting normal fault steepens near surface with decreasing differential stress. Although much of the displacement on the controlling faults is pre-mineralization in timing, lithified cataclastic breccia, coeval with some vein stages, and vein geometry patterns indicate that some vein development occurred contemporaneously during late normal displacement along the fault system. Waning of the hydrothermal system is marked by late carbonate fill, initially Fe dolomite, then coarse calcite as veins, matrix to vein breccia, and central vein cavity fill. The Kupol district veins have proximal adularia-quartz alteration (haloes meters wide), within an extensive (hundreds of meters in scale) clay alteration halo. Clays are zoned both vertically and laterally with respect to veins, with inner illite-chlorite that was magnetite-destructive (at highest paleotemperature; &amp;gt;220°C), grading outward and upward to illite/interlayered illite-smectite with kaolinite, then to an outer zone (or upper blanket) of smectite, at lowest paleo-temperature (&amp;lt;150°C). The boundary between the illite and smectite zones is interpreted to mark the interaction limit of paleo-hydrothermal systems with cooler groundwater. District-scale pathfinder element zonation correlates with clays, with S-Te-Bi-As in the illite-chlorite core and Sb-Cs-Tl(-As-Li) in the smectite blanket. Pathfinder zonation patterns at Kupol point to a magmatic source at depth or, more likely given the scale of the anomalies, multiple magmatic sources, with the surface clay zonation indicating the extent of coalesced paleo-hydrothermal systems associated with upflow plumes. This is the best-defined alteration record with geochemical signature for a complete district hosting a large, high-grade vein deposit. Early definition of clay and pathfinder element patterns across an entire epithermal district can be carried out at low cost to provide useful constraints on vein targeting.

VEINS SYSTEM AND THEIR MINERALOGICAL AND MICROTHERMOMETRIC CHARACTERISTICS WITHIN THE HUMPA LEU EAST PORPHYRY COPPER-GOLD MINERALIZATION AT HU'U DISTRICT, SUMBAWA ISLAND, INDONESIA

The study area, i.e Humpa Leu East is a porphyry prospect located in Hu'u district, Sumbawa Island, Indonesia. This study is aimed to understand the characteristics of veins, the distribution of veins and mineralogy, microthermometric conditions of ore fluids, distribution of elements, and their implications for exploration and deposit model. The Hu'u district is a paleo-volcano member of the Miocene to Plio-pleistocene volcanic rocks. Hydrothermal alteration evolved to get out from the tonalitic body, consists of potassic, propyllitic and overprinted by phyllic and advanced argillic. The mineralization is dominated by chalcopyrite, associated with quartz±anhydrite veins, and hydrothermal breccia. Hydrothermal fluids temperature measured at the value of 109.9 °C - 525.3 °C and &gt; 550 ° C with an averaging range of Th is 296.5 -329.7 °C, and salinity of 10.9 wt% NaCl eq. Quartz veins occur as a package or series of porphyry type veins designated as EDM-M1-A1, A2-A3-Apsb-anh, A3-A2, M2-Apsb-A2-A3, M1-B-C, C-D-anhydrite, and epithermal veins. Hydrothermal fluids possibly have mixed by high-temperature hyper-saline fluids to medium temperature low saline fluids. The Humpa Leu East can be identified as a 'push-up porphyry system' that still remains more extensive system underneath or in the side area. These results can be used for understanding the texture of veins as a vectoring to ore, metal distribution in veins and rocks. Early type veins such as type A have more intensive metal content and are followed by type B, C and weaken until the latest stage.

Fluid boiling during high-grade gold deposition in an epithermal gold-telluride deposit, Guilaizhuang, China

Boiling is a well-established mechanism for precious metal mineralization in low-sulfidation epithermal veins, and is considered a key process in the formation of high-grade bonanza zones in those systems. In contrast, the potential role of fluid boiling in the formation of epithermal gold-telluride veins associated with alkalic porphyries has rarely been discussed. The Guilaizhuang gold-telluride deposit, Shandong Province, China, is an ideal location to investigate the possible role of fluid boiling because the deposit comprises several distinct styles of orebodies, ranging from finely-disseminated gold mineralization hosted in carbonate rocks at depth, through to very high-grade mineralized breccia zones at shallower levels. Here, we examine the relationships between ore-forming fluids and mineralization styles in this deposit, based on paragenesis, vein textures and fluid inclusions. In the deeper carbonate-replacement style orebodies, we found no evidence of fluid boiling and, as such, alternative mechanisms such as fluid mixing or fluid-rock reactions were likely occurring. In contrast, the shallower, high-grade breccias show clear evidence of fluid boiling, which we infer contributed to the formation of exceptional bonanza zones. Our results thus indicate that fluid boiling can play a key role in precious metal mineralization in gold-telluride deposits, analogous to the low-sulfidation epithermal deposits. Furthermore, our results illuminate how contrasting mechanisms of gold mineralization in these deposits (i.e., boiling versus non-boiling) can give rise to the diversity and distribution of gold grades and reserves within an epithermal gold-telluride system.

Colloids and Nanoparticles: Implications for Hydrothermal Precious Metal Ore Formation

Abstract Research on nanoscale processes is expanding in many scientific and technical disciplines, and economic geology is no exception. The idea that colloidal gold particles (or nanoparticles) may have played a significant role in ore formation was initially based on textural evidence from high-grade epithermal veins. More recently, the discovery of gold nanoparticles in some present-day geothermal systems and several orogenic gold deposits suggests that their formation could be a common process in gold-supersaturated hydrothermal solutions. Rare laboratory studies indicate that gold nanoparticles typically aggregate to form dendritic clusters. Over time, these dendrites perhaps can evolve to form coarser-grained crystals as subsequent annealing and recrystallization occurs. Due to the ephemeral nature and later recrystallization of dendrites, evidence of their former existence is commonly obscured. However, newer nanoscale imaging technologies have resulted in an increased recognition of their presence in hydrothermal gold ores, and thus their role in ore-forming processes merits further research. In particular, does their nucleation and deposition lead to forming higher-grade ores?

Breccia and vein mineralization of the Balatoc Diatreme, Acupan gold deposit, Baguio Mineral District: An example of a diatreme-hosted epithermal deposit in the Philippines

The Acupan epithermal gold deposit is one of the Philippines’ largest gold camps, having produced over 200 t Au in the last century with average grades of ∼ 6 g/t Au mostly from well-studied vein orebodies hosted by the Virac Granodiorite. However, vein and breccia mineralization (90/99 veins and GW orebodies, respectively) currently being mined in the northeastern portion of the Acupan deposit is hosted by the less-studied Balatoc Diatreme. This study demonstrates the role of the diatreme as a precursor to epithermal mineralization at Acupan, providing structural control in focusing the fluid flow along the diatreme margin. Mineralization in the Balatoc Diatreme formed across five mineralization stages. Stage I, the main gold mineralization stage, is characterized by gray quartz with pyrite + marcasite + arsenopyrite + electrum + sphalerite ± chalcopyrite. Stage II is typified by white quartz associated with pyrite + electrum + chalcopyrite. Stage III, a newly recognized stage for the Acupan gold deposit, is composed of clear quartz hosting pyrite + stibnite + chalcopyrite + galena + sphalerite + electrum (±pyrite + marcasite + arsenopyrite). Stage IV and Stage V are associated with calcite and gypsum, respectively, hosting trace amounts of pyrite and sphalerite.Fluid inclusions hosted by Stage I and Stage II vein quartz revealed homogenization temperatures (Th) ranging from 220 to 230 °C and 280 to 290 °C, respectively. The fluid inclusions in Stage III quartz breccia cement recorded bimodal homogenization temperatures, 230 to 240 °C and 280 to 290 °C. Fluid inclusion and textural evidence from Stage I quartz and Stage II quartz suggest boiling conditions during ore formation of veins transecting the Balatoc Diatreme. Fluid mixing, on the other hand, is proposed for the formation of the base-metal rich Stage III mineralization of the GW orebodies quartz cement. Pyrite and sphalerite δ34S values (0.8 to 1.5 ‰) from Stage I to Stage IV veins and breccias indicate reduced ore-forming conditions in an H2S-dominated system. Meanwhile, the negative δ34S values (−1.6 to −1.5‰) measured from the pyrite of Stage V breccia, first reported in this study, imply partitioning of the heavier isotopes to the gypsum sulfate in an oxidizing environment and suggest possible spatial variations of sulfur isotope signatures across the Acupan epithermal vein system. This study emphasizes the significance of fluid mixing in diatreme-hosted epithermal deposits such as Acupan. The diatreme possibly provided pathways between the contrasting magmatic-hydrothermal and meteoric environments, resulting in ore precipitation.

Fluid inclusion and stable isotope constraints on the genesis of epithermal base-metal veins in the Armaqan Khaneh mining district, Tarom-Hashtjin metallogenic belt, NW Iran

Rashtabad, Aqkand and Jalilabad are neighbouring epithermal base-metal deposits in the Armaqan Khaneh mining district of the Tarom-Hashtjin metallogenic belt, northwestern Iran. Mineralised veins in these deposits consist of pyrite, chalcopyrite, galena, sphalerite with lesser bornite and minor amounts of specular hematite. The gangue mineralogy includes quartz, chlorite, and calcite in ore veins and sericite/illite, and sericite–epidote–chlorite–calcite in vein haloes. At Rashtabad and Jalilabad, primary LV fluid inclusion assemblages homogenise into liquid at 150–285 °C and 120–342 °C, respectively. The corresponding salinities, respectively, vary between 7.4–12.9 and 7.8–13.5 wt% NaCl equiv. At Aqkand, LV inclusions show a T h(total) of 229–353 °C and salinities of 7.4–9.8 wt% NaCl equiv. Calculated δ18Owater values of quartz samples at Rashtabad, Aqkand, and Jalilabad are +1.0‰ to +9.6‰ Vienna-Standard Mean Ocean Water, signifying that the ore-forming fluid progressed from magmatic to meteoric water. Sulfur isotopic values of sulfides range from −6.0 to −2.2‰ Vienna Cañon Diablo Troilite (averaging −3.7‰), indicate sulfur was derived from a rather homogeneous magmatic source that was oxidised by meteoric water through fluid mixing. Our data suggest that Rashtabad, Aqkand and Jalilabad are an intermediate-sulfidation type of epithermal mineralisation formed from a well-mixed source that experienced extensive fluid mixing and boiling. KEY POINTS The Rashtabad, Aqkand and Jalilabad deposits of the Armaqan Khaneh mining district are intermediate-sulfidation style of epithermal mineralisation. Fluid boiling and mixing facilitated hydrothermal alteration and mineralisation. Oxygen isotope values suggest that the ore–fluid system evolved from magmatic to meteoric. Sulfur derived from a magmatic source was oxidised by meteoric water through fluid mixing.

Characteristics and evolution of the Etili silica sinter epithermal deposits, Çanakkale – Turkey: Relation to alkali chloride vs acid-sulfate fluids

The Oligo-Miocene Etili epithermal deposits are a well-preserved fossil geothermal system in the Çanakkale Region and is one of the largest fossil siliceous hot spring deposits of Turkey. Main E-W and NE-SW trending faults systems occur with minor NW-SE fracture systems perpendicular to the main faults. Silica-rich deposits are commonly observed in or on the calc-alkaline tuffs, ashes and pyroclastic rocks that were the products of the magmatism controlled by the extensional tectonic regime.The Etili epithermal system was examined at two representative locations including the Hamamtepe and Muratlar sites. Morphologies of the proximal apron were defined by lithofacies that included silica infiltrate, spring conduits, nodular and finely laminated geyserite, sinter clast breccia, silicified volcanic rocks, and epithermal veins. Microfossils were detected in mat structures developed in the proximal apron. The silica deposits have low abundances of altered mineral assemblages (e.g., kaolinite, halloysite, and alunite), which are otherwise commonly observed in the region. The origin of the Etili Fossil Silica Sinter Region (EFSSR) was constrained by using geochemical and isotopic data. δ18O and δD isotopic values of kaolinites ranged +9.4/+9.6 ‰; to 84/−74 ‰ respectively. δ18O isotopic values from siliceous and silicified samples ranged from 8.3 to 18.4 ‰, which shows two different formation types for the Etili Fossil Silica Sinter Region. We hypothesized hypogene origins for the kaolins and a mixed hypo/supergene origin for siliceous and silicified samples. The calculated model formation temperature from the δ18O values of the silica and siliceous samples is in the range of 63° to 140 °C. δ34S isotopic values of alunite ranged from −19.6 to +16.6 ‰, which also indicated two different sources that include magmatic hydrothermal and meteoric waters affected by bacterial activity. The timing of acid-sulphate alteration can be grouped into three periods by 40Ar/39Ar dating of alunites. These ages are: (A) 32.4 ± 1.2 to 22.6 ± 0.22 Ma in the Muratlar silica site, (B) 12.3 ± 0.3 to 15.2 ± 0.3 Ma in the northern part of the Hamamtepe site and (C) 5 ± 0.18 to 7 ± 0.3 Ma in the southern part of the Hamamtepe silica site. These consistent findings show that the emplacement periods of the Evciler pluton and the ages of alteration deposits are similar to each other. Silica sinters and geyser mounds represent very late stage near-neutral pH alkali chloride geothermal activities. The epithermal deposits in the EFSSR comprises a complex history of three different hydrothermal processes, each of which have different compositional and temporal emplacement periods related to the Evciler pluton.

The Chah-Mesi epithermal Cu-Pb-Zn-(Ag-Au) deposit and its link to the Meiduk porphyry copper deposit, SE Iran: Evidence from sulfosalt chemistry and fluid inclusions

The polymetallic Chah-Mesi epithermal vein deposit is located about 2.5 km south of the Meiduk porphyry Cu-(Mo-Au) deposit in the Kerman Cenozoic Magmatic Arc (Kerman Belt), representing the southeastern sector of the Urumieh-Dokhtar Magmatic Belt in Iran. The veins are N-S to NNE-SSW oriented and hosted in volcanic rocks of basaltic, andesitic to dacitic composition and pyroclastics, which were affected by intense local silicification and sericitization in proximity to mineralized veins. Propylitic, argillic, as well as potassic alteration show a more regional distribution; the latter only observed closer to the Meiduk deposit. Mineralization occurs in various types of veins (massive, banded, crustiform) and to minor extent in replacement and breccia bodies. It encompasses several stages: An early higher-sulfidation stage, characterized by pyrite, chalcopyrite, enargite/luzonite and bornite, is followed by the main intermediate-sulfidation state assemblage with pyrite, chalcopyrite, tetrahedrite group minerals and sphalerite. The late Pb-Zn rich stage with sphalerite, galena, chalcopyrite and pyrite overprints the earlier associations. Gold and silver of up to 7 g/t and 150 g/t, respectively, are associated with the main and late stages of mineralization. The main carriers of these precious metals are Ag-rich gold (electrum) and tetrahedrite; other Ag-bearing sulfosalts like pearceite are rare. The chemical composition of tetrahedrite group minerals ranges from tennantite-(Fe) to tetrahedrite-(Fe) and tetrahedrite-(Zn) exhibiting a positive correlation between Sb and Ag contents. The tetrahedrite group minerals show complex zoning and display an increase of Sb away from the Meiduk deposit. Primary fluid inclusions in sphalerite and quartz from the banded and crustiform veins are low saline aqueous H2O-salt inclusions with only traces of CO2. Homogenization temperatures of two-phase LV inclusions (Th LV→L) have an average of 210 °C in sphalerite and 260 °C in quartz. Salinity values range from 1.2 to 9.9 and 2.1 to 9.2 mass% NaCl equivalent in sphalerite and quartz, respectively with an average of c. 6 mass% NaCl equiv. Low CO2 concentrations in the vapor phase detected by Raman spectroscopy together with previously published stable isotope data indicate fluids of magmatic origin as the main fluid source that were mixed with meteoric water. Mineralization is linked to ascending hydrothermal fluids which evolved from high- to intermediate-sulfidation state due to cooling, dilution with meteoric water and progressing fluid-wallrock interaction. Similar low salinity, but higher CO2-bearing fluids were previously reported from the nearby Meiduk porphyry deposit. Conclusively, Chah-Mesi is classified as an intermediate-sulfidation epithermal deposit that developed in the peripheral parts of the Meiduk porphyry system.

Lower to Middle Jurassic volcanism and Au–Ag mineralization at Cerro Moro District, Deseado Massif, Argentine Patagonia

In the Cerro Moro Mine area, a horst block exposes a volcanic sequence with ages ranging from 191 Ma to 167 Ma, extending the Jurassic volcanic history of the Deseado Massif back to the Lower Jurassic (Sinemurian-Pleinsbachian boundary). Three volcanic sequences are recognized: a lower volcanic complex comprising welded tuffs, rhyolitic volcanic breccias, andesite lavas and intercalated continental sediments from 191 to 186 Ma, an intermediate sequence of welded ignimbrites, block-and-ash flow deposits and epiclastic sediments with ages ranging from 182 to 181 Ma, and an upper sequence of rhyolitic domes and welded ignimbrites from 180 to 167 Ma. Epithermal vein mineralization was formed in a series of pulses beginning at 180 Ma and culminating in a bonanza event at 171 Ma, coincident with the eruption of the upper volcanic sequence. Increasing grades of precious and base metals with time leading up to the bonanza event are interpreted as the result of repeated fertilization of the hydrothermal system by successive fluid pulses. Detrital, entrained and inherited zircons in the Jurassic rocks have ages ranging from the Triassic to Paleoproterozoic.

Preliminary Study of Geology, Alteration and Ore Mineralisation at East Motoling Area, South Minahasa District, North Sulawesi, Indonesia

The East Motoling area is one of the prospect areas in the Minahasa region of North Sulawesi, which has indications of low sulfidation epithermal-type mineralization. The research was conducted as a preliminary study to determine the characteristics of geological conditions, alteration, and ore mineralization in the epithermal system. The research method is divided into two main parts, such as fieldwork including surface geological mapping (lithology, stratigraphy, geomorphology, structural geology, alteration and mineralization) and laboratory analysis methods including petrographic analysis. The stratigraphy of the study area consists of altered volcaniclastic breccia, altered lapilli tuff, altered tuff, limestone, welded lapilli tuff, and andesitic breccia. Volcaniclastic breccia, altered lapilli tuff, and altered tuff, member of the Volcanic Rock Formation which is Late – Middle Miocene age, are the host rock for ore mineralization and hydrotermal alteration process. There are 3 types of alterations that have developed, namely argillic (illite + quartz ± kaolinite), sericitic (sericite + illite ± chlorite), and propylitic (chlorite + epidote ± illite). The dextral slip fault with NW – SE trend present as a main control structure to formation of extention fracture/vein. The epithermal veins are relatively north-northeast – south-southwest, north-northwest – south-southeast, and northwest – southeast. The textures of the veins divided into 7 main groups, namely that is bladed-quartz, breccia, calcedony, colloform, comb, mold, and massive quartz. Ore mineralization is forms in the veins as pyrite and banded sulfide. Apart from that, the disseminated pyrite also limitedly found around the veins.