Low-sulfidation Epithermal Au-Ag Research Articles

Specification and formation process of enriched portions in Au veins in an epithermal deposit via clustering and geostatistical approaches

Even though epithermal vein-type deposits are the main sources of Au and Ag worldwide, accurate three-dimensional modeling of vein structures and the extraction of high-grade zones are still difficult because of geologic structure heterogeneities and the scarcity of available data, mostly obtained via drilling. Accordingly, by selecting a low-sulfidation epithermal Au–Ag deposit in the Philippine archipelago, we develop an efficient and reliable workflow for determining the spatial distribution of Au-rich veins in detail and the transport of hydrothermal fluids. The studied deposit is mostly hosted in open fractures and breccias with a relatively large tonnage of low Au+Ag grades and/or a small tonnage of high Au+Ag grades. The whole-rock geochemical data (1877 core samples) and lithological data (56 drilling sites) are processed using hierarchical clustering analysis (HCA) and two geostatistical simulations: turning bands simulation for the Au grade distribution and interpretation of mineralization process and truncated Gaussian simulation for the spatial distribution of clusters with similar Au–Ag grade features as classified by HCA. The general correlation between the Au and Ag grades, with a linear correlation coefficient of ≈ 0.69, is based on the HCA application, and the clusters were likely formed by three-stage mineralization processes. A plausible three-dimensional geologic model is then constructed, delineating in detail hydrothermally mineralized and altered zones with Au enrichment and vein system structures in the study area.

A case of Te-rich low-sulfidation epithermal Au-Ag deposits in a calc-alkaline magmatic arc, NE China

Tellurium-bearing low-sulfidation epithermal Au-Ag deposits are significant producers of gold, silver, and potentially strategic elements if mineral processing methods are optimized for recovery. Although these deposits are generally related to alkaline magmatism, our study documents an unusual occurrence of Te-rich low-sulfidation epithermal systems in the North Heilongjiang Belt in northeast China that is spatially and temporally associated with calc-alkaline magmatism and tectonic extension in a continental arc‐setting.In the North Heilongjiang Belt, Te-bearing Au-Ag deposits are usually sited in dilatant zones, mostly along extensional NW structures or at their intersections with deep‐seated NE-striking faults such as the Nenjiang-Heihe and Jiayin-Mudanjiang lineaments. Of these deposits, Sandaowanzi is well‐known for its bonanza gold grades. These faults localized andesitic to rhyolitic volcano-plutonic centers that evolved from mantle‐derived mafic melts to intermediate compositions due to differentiation and crustal assimilation as documented by their mineralogy and Sr, Nd, and Pb isotope compositions. During assimilation of country rocks, fluids containing 3He and other volatiles derived from mantle magmas displaced or mixed with external groundwater containing radiogenic 4He derived from country rocks. In this belt, the bulk metal content of Au, Ag and Te was probably introduced by deep mantle‐derived mafic‐intermediate calc‐alkaline intrusions. However, our study does not exclude input of metals leached from underlying metasedimentary rocks and older Te‐bearing Au mineralization. The noble gas, hydrogen, oxygen and lead isotope compositions of fluid inclusions and ore minerals suggest that the deposits formed by mixing between magmatic fluids and convecting meteoric ground waters containing lead leached from surrounding country rocks. At deeper levels, isotopic evidence suggests that Au-Ag-Te (Bi) precipitated with pyrite, quartz, sericite, and carbonate minerals due to mixing with Te-rich fluids. At shallower levels, Au and Ag precipitated contemporaneously with base‐metal sulfides, hydrothermal quartz and sericite during episodic boiling as evidenced by silica morphology and mineral textures.The relatively uniform spacing of epithermal systems along NW- and NE-trending structures in the North Heilongjiang Belt was used to identify unexplored spaces within this mineral belt that are prospective for concealed low-sulfidation epithermal gold deposits. Although some of the Au-Ag deposits in this belt are enriched in Te, there is a lack of coeval alkaline igneous rocks that are commonly associated with Te-bearing gold deposits elsewhere in the world (e.g., Cripple Creek, Emperor, and Lihir Island).

Osmium isotopes fingerprint mantle controls on the genesis of an epithermal gold province

Abstract The formation of crustal blocks enriched with gold (Au) deposits above subduction zones is intimately bound to the genesis and evolution of magmatic-hydrothermal systems. A long-standing question, however, is whether the metal fertility of these systems stems from distinct sources that are anomalously enriched in Au or from subsequent processes occurring during crustal magma emplacement and hydrothermal activity. The Deseado Massif auriferous province in southern Patagonia (Argentina) is a unique place to test these contrasting hypotheses because Au-bearing mantle xenoliths indicate the presence of an underlying Au-rich lithospheric mantle reservoir. However, direct geochemical links between the Au-rich mantle source and the formation of the Deseado Massif auriferous province in the overlying crust remain to be established. To address this prominent gap in knowledge, we used sulfide Re-Os geochronology to identify the source of Au at Cerro Vanguardia, the largest low-sulfidation epithermal Au-Ag deposit in the Deseado Massif. Pyrite from high-grade Au quartz veins yielded an isochron age of 147.4 ± 2.9 Ma (mean square of weighted deviates = 1.04, n = 8) and an initial 187Os/188Os ratio of 0.26 ± 0.01, fingerprinting a dominant mantle control for the source of Os and, by inference, the source of Au. Our data provide a unique geochemical linkage between an Au-rich subcontinental lithospheric mantle source and the genesis of epithermal Au deposits, supporting the hypothesis that pre-enriched mantle domains may be a critical factor underpinning the global-scale localization of Au provinces.

Geometry and scale property of a gold-silver-bearing vein system associated with an oblique-slip fault zone at Gasado Island, Korea

Gasado Island, on the southwestern Korean Peninsula, comprises c. 50 Ma volcanic rocks that are cut by faults and fractures hosting low-sulfidation epithermal Au-Ag mineralization. In this study, we examined the relationship between the fault-fracture networks and auriferous vein systems to better understand the role of brittle structures in controlling hydrothermal fluid flow. Geometrical and fractal analyses were performed to express and quantitatively compare the vein geometries and thickness population. The results were as follows: (1) The geometrical analysis revealed a strong relationship between Au-Ag-bearing quartz veins and NE-SW-striking, oblique sinistral-normal faults, indicating that these structures are a key control on mineralization. (2) Box-counting fractal analysis applied to veins hosted by fault zone demonstrated that the D-values (fractal dimensions) are directly proportional to vein density – indicating that damage zones control vein density. (3) The linear traverse line analysis yielded cumulative frequency plots of vein thicknesses that showed obvious changes in the slope, which are reflected in the variable D-values. The cause of this slope variability is interpreted to be the presence of hybrid veins that fill secondary fault planes, which disturbs the original linear relationship shown by cumulative thickness distribution data. This study shows that vein density and physical properties are strongly controlled by the types, or existence, of fault damage zones. Understanding fault zone architecture may assist with the prediction of locations that concentrate mineralizing fluids and may benefit the initial stage of mineral exploration.

Characteristics and genesis of phyllosilicate hydrothermal assemblages from Neoproterozoic epithermal Au-Ag mineralization of the Avalon Zone of Newfoundland, Canada

The Avalon Zone hosts well-preserved examples of high- and low-sulfidation epithermal Au-Ag mineralization and associated phyllosilicate alteration. Using examples from the Hope Brook and Hickey's Pond high-sulfidation systems (HSS) and the Heritage low-sulfidation system (LSS), this study presents new data on the mineralogical and chemical characterization of phyllosilicates associated with epithermal Au-Ag deposits. Electron probe microanalysis and X-ray diffraction studies revealed that the HSS examples contain Fe muscovite ((K0.62–0.79Na0.10–0.37Ca0.00–0.01)Al1.31–1.72(Mg0.01–0.08Fe0.05–0.19)(Al1.00–1.24Si2.76–3.00O10)(OH)2), paragonite ((K0.05–0.30Na0.78–1.02Ca0.00–0.01)Al1.66–1.84(Mg0.00–0.01Fe0.02–0.04)(Al1.01–1.11Si2.89–2.99O10)(OH)2), Al clinochlore ((Mg2.42–2.65Fe1.62–1.78)(Si2.57–2.69Al1.31–1.43O10)(OH)8), pyrophyllite, kaolin minerals, illite-smectite (I-Sme) and chlorite-smectite (C-Sme). Conversely, the LSS examples contain Fe-Mg muscovite ((K0.44–0.76Na0.00–0.02Ca0.00–0.01)Al1.55–1.82(Mg0.17–0.31Fe0.05–0.12)(Al0.55–0.71Si3.29–3.45O10)(OH)2), Al clinochlore ((Mg2.17–2.56Fe1.59–2.01)(Si2.71–2.92Al1.08–1.29O10)(OH)8), and I-Sme and C-Sme intermediates. Chlorite geothermometry indicated that HSS and LSS assemblages crystallized at average temperatures of 261 °C and 145 °C, respectively. Mica and chlorite from HSS and LSS are defined by Tschermak substitution and high-temperature polytypism (2M1 and IIb, respectively). In the HSS examples, one or more pulses of hot, acidic fluid reacted with host rocks generating an envelope of advanced argillic alteration (Fe-Ms, Kln, Pg, Prl) around a vuggy silica core. This is bounded by a zone of argillic alteration (Fe-Ms, Kln), with sericitic/chloritic alterations (Fe-Ms, Al-Clc) developed in distal parts of the systems. In the LSS, near-neutral hydrothermal fluids led to the formation of broad phyllic/chloritic zones (Fe-Mg Ms and Al-Clc). A low temperature overprint with 1M mica, Ib clinochlore ((Mg2.48–2.74Fe1.58–1.73)(Si3.05–3.15Al0.85–0.95O10)(OH)8), kaolinite and Sme-poor I-Sme and C-Sme is attributed to late waning stage hydrothermal activity and/or weathering. A close paragenetic link was established between Fe-Mg muscovite and Ag mineralization at Heritage, as well as between Fe-Mg muscovite and bladed calcite, suggesting boiling as the precipitation mechanism. The present study provides guidance on how phyllosilicate assemblages have the potential to discern HSS versus LSS mineralization during early prospection in terranes with limited exposure, and to contribute to the reconstruction of their hydrothermal record.

Induced polarization imaging applied to exploration for low-sulfidation epithermal Au–Ag deposits, Seongsan mineralized district, South Korea

The determination of mineralization boundaries during mineral exploration for undiscovered low-sulfidation epithermal Au–Ag deposits is a significant challenge because of the extensive survey areas required. Induced polarization (IP) imaging is an effective geophysical technique for the detection of sulfides or clay. Thus, this method is considered useful to determine the boundaries of subsurface mineralization and hydrothermal alteration associated with epithermal deposits. We used 2D and 3D IP imaging to define the silicification and mineralization boundaries of the Moisan deposit in the Seongsan mineralized district, which is geologically well-known. The boundaries of the silicification zone were defined by high resistivity values of 600 Ω-m, and those of the mineralization zone were defined by high global chargeability values of 3 mV V−1. The continuity of the high resistivity anomaly corresponded well to the silicification (quartz veins) exposed in outcrop. In addition, it is geologically reasonable that the chargeability anomaly, ≥3 mV V−1, associated with the mineralization/hydrothermal alteration zone was concentrated at near-surface depths, and extensively surrounding the resistivity anomaly, ≥600 Ω-m, associated with the silicification zone.

Low-sulfidation epithermal Au-Ag mineralization in the Sındırgı District, Balıkesir Province, Turkey

The Sındırgı District (Balıkesir, western Turkey) lies within the Western Anatolian volcanic and extensional province, adjacent to the WNW-trending Simav graben, approximately 130 km NE of İzmir. The Sındırgı mining district is underlain mainly by Miocene volcanic rocks and hosts several low-sulfidation epithermal Au-Ag deposits and prospects located near the towns of Sındırgı and Bigadiç. The Kızıltepe low-sulfidation epithermal gold-silver deposit is located southeast of Yusufçam village (Sındırgı, Balıkesir), and other prospects, including the Kepez, Kavaklıdüz, and Karadüz prospects, are located northeast of Kızıltepe. Potentially economic grades occur at Kızıltepe, which contains a measured and indicated resource of 1.754.790 Mt @3.0 g/t Au, 44 g/t Ag, hosted by quartz veins showing colloform/crustiform banding, quartz pseudomorphs after bladed calcite, and multiphase brecciations, all typical textures noted in low-sulfidation epithermal deposits. Alteration minerals include mixed-layer illite/smectite, high-crystallinity illite, and kandite group minerals (dickite and nacrite). Precious metal minerals include traces of electrum, acanthite, Au-rich acanthite, and Ag-Hg-Au-Tl-Pb series, occurring mainly within quartz. Pyrite is the most common opaque mineral at Kızıltepe. ^{40}Ar/^{39}Ar dating of adularia from the quartz veins indicates an age of mineralization of 18.3 ± 0.2 Ma. The ore mineralization is divided into three main phases. These comprise the deposition of: coarse-grained quartz, illite, pyrite, and minor precious metals (Phase I); major gold–silver-bearing medium-grained quartz, which commonly exhibits crustiform banding, carbonate replacement, and hydrothermal breccia textures (Phase II); and fine-grained chalcedonic quartz with colloform/crustiform banding (Phase III). Phase II is economically the most important in terms of precious metal content. Phase II quartz contains fluid inclusions, which range from predominantly vapor-rich to predominantly liquid-rich with homogenization temperatures (Th) varying from 157 to 330 °C, showing a cluster between 190 and 300 °C, and ice-melting temperatures (Tm) ranging from -0.2 to -2.9 °C (salinity from 0.5-4.8 wt.% NaCl equiv.). Moderate to strong positive correlations occur between Au-Ag (R = 0.8) and Au-Cu (R = 0.5), whereas there is no correlation between As and Au or Ag.

ASTER 분광분석을 통한 해남지역 강고령토변질 암석의 식별

해남 천열수 광화대는 우리나라 남서부에 위치하고 있으며, 저유황계 천열수 금-은 광상(은산-모이산)과 점토광상(옥매산, 성산, 춘산)을 배태하고 있다. 백악기 화산활동과 관련된 천열수 광상과 이와 함께 수반되는 열수변질작용은 열수변질광물들로 구성된 대규모의 대상분포를 야기 시켰다. 명반석-석영, 명반석-딕카이트-석영, 딕카이트-카올리나이트-석영의 광물조합을 갖는 강고령토변질작용으로 변질된 암석들은 옥매산, 성산, 춘산지역에 노출되어 있다. 세 개의 가시근적외선 밴드, 여섯 개의 단파장적외선 밴드, 다섯 개의 열적외선 밴드로 구성된 Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER)는 해남 천열수 광화대 내 강고령토변질작용을 받은 암석들을 식별하는데 사용되었다. 열수변질광물들의 특정한 분광특성들은 연구지역내 변질 받지 않은 암석들로부터 강고령 토변질작용을 받은 암석들의 식별을 가능하게 한다. 연구지역 내 강고령토변질작용을 받은 암석들을 주로 구성하는 명반석, 딕카이트, 카올리나이트는 대표적인 함 Al-O-H 광물로 <TEX>$2.20{\mu}m$</TEX>에서 강한 흡수특성을 갖는다. ASTER 밴드조합 및 밴드비 변환은 강고령토변질작용을 받은 암석과 비변질 암석으로부터 획득된 DN 값의 차이를 증가시킨다. 옥매산, 성산, 춘산지역으로부터 강고령토변질작용을 받은 암석들 내 명반석과 딕카이트-카올리나이트는 각각 1.523과 1.737의 평균 DN 값을 갖는다. 이러한 DN 값은 비변질 지역으로부터 획득된 DN (각각 평균 1.211과 1.308)값에 비해 훨씬 높다. ASTER 영상은 지표환경에서 열수변질광물의 분포특성을 원격적으로 제공할 수 있다. 이러한 측면에서 ASTER 분광분석과 야외지질조사 결과의 좋은 상관성은 ASTER 분광분석이 광물자원탐사의 초기 단계에서 유용한 탐사방법이 될 수 있음을 제시한다. The Haenam epithermal mineralized zone is located in the southwestern part of South Korea, and hosts low sulfidation epithermal Au-Ag deposit (Eunsan-Moisan) and clay quarries (Okmaesan, Seongsan, and Chunsan). Epithermal deposits and accompanying hydrothermal alteration related to Cretaceous volcanism caused large zoned assemblages of hydrothermal alteration minerals. Advanced argillic-altered rocks with mineral assemblages of alunite-quartz, alunite-dickite-quartz, and dickite-kaolinite-quartz exposed on the Okmaesan, Seongsan, and Chunsan area. Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER), with three visible and near infrared bands, six shortwave infrared bands, and five thermal infrared bands, was used to identify advanced argillic-altered rocks within the Haenam epithermal mineralized zone. The distinct spectral features of hydrothermal minerals allow discrimination of advanced argillic-altered rocks from non-altered rocks within the study area. Because alunite, dickite, and kaolinite, consisting of advanced argillic-altered rocks within the study area are characterized by Al-O-H-bearing minerals, these acid hydrothermal minerals have a strong absorption feature at <TEX>$2.20{\mu}m$</TEX>. The band combination and band ratio transformation cause increasing differences of DN values between advanced argillic-altered rock and non-altered rock. The alunite and dickite-kaolinite of advanced argillic-altered rocks from the Okmaesan, Seongsan, and Chunsan have average DN values of 1.523 and 1.737, respectively. These values are much higher than those (1.211 and 1.308, respectively) of non-altered area. ASTER images can remotely provide the distribution of hydrothermal minerals on the surface. In this way good relation between ASTER spectra analysis and field data suggests that ASTER spectral analysis can be useful tool in the initial steps of mineral exploration.

Volcanic Framework of the Pliocene El Dorado Low-Sulfidation Epithermal Gold District, El Salvador

The Pliocene El Dorado low-sulfidation epithermal Au-Ag vein system, located within an 84-km2 district in north-central El Salvador, is hosted by volcanic rocks formed along the Caribbean plate margin in response to the subduction of the Cocos plate during the Tertiary. The principal volcanic host rocks to epithermal veins, and effective basement in the district, consist of a >400-m-thick sequence of basaltic to andesitic lava flows and volcanogenic sedimentary rocks. Locally, porphyritic basaltic to andesitic domes and dikes intruded this sequence during the late Miocene (10.7 ± 1.9 Ma). The mineralized volcanic basement rocks, outcropping mainly in north and central El Dorado, represent different parts of a complex volcanic sequence composed of multiple superimposed volcanic centers. Vein formation in the district occurred in the Pliocene between 4.7 ± 0.2 Ma and 4.06 ± 0.29 Ma, based on K-Ar and 40Ar-39Ar plateau ages on vein adularia. Vein formation overlapped with Pliocene felsic volcanism (3.94 ± 0.29 Ma to 3.36 ± 0.49 Ma), based on field relationships and 40Ar-39Ar geochronology. Pliocene felsic volcanic rock sequences crop out widely in southern El Dorado at lower elevations relative to central and northern El Dorado. The felsic rocks have sinters intercalated with pyroclastic and sedimentary rocks at the base of the sequence that define the paleosurface of the epithermal vein system, at least in this part of the district. The depth of erosion in central and northern El Dorado, the primary Au-Ag resource, is below the paleosurface, as only the low sulfidation–style epithermal veins are present. Field relationships and geochronology suggest that the onset of extensive felsic volcanism marks the waning stage of hydrothermal activity associated with vein formation and mineralization. From north to south, the El Dorado district represents an oblique cross section through the volcanic and epithermal vein system formed during east-west extension. Northern and central El Dorado, dominated by mineralized mafic to intermediate volcanic basement rocks, represent the deeper parts of the volcanic and hydrothermal system. Southern El Dorado is the shallow level of the volcanic field and epithermal vein system, preserving the surface and shallow subsurface environments, including the sinters and contemporaneous to postmineral felsic volcanic rocks. Although we cannot preclude that the distribution of volcanic rocks represents lateral facies changes in a Pliocene felsic dome field or that the variations in depth within the low-sulfidation epithermal system represent simply lateral flow driven by topography in the geothermal system, the preponderance of evidence suggests that the oblique section is principally due to postmineral deformation associated with formation of large Pliocene and Pleistocene Rio Titihuappa basin lying to the immediate south.

Identification of quartz and carbonate minerals across northern Nevada using ASTER thermal infrared emissivity data—Implications for geologic mapping and mineral resource investigations in well-studied and frontier areas

ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) thermal infrared imagery over a 389 km × 387 km area in northern Nevada (38.5°–42°N, 114°–118.5°W) was analyzed to evaluate its capability for accurate and cost-effective identification and mapping of quartz and carbonate minerals at regional to local scales. The geology of this area has been mapped at a wide range of scales and includes a diversity of rock types and unconsolidated surficial materials, many of which are composed primarily of quartz and carbonate minerals. This area is also endowed with a wide variety of economically and scientifically important ore deposit types that contain an array of commodities (Au, Ag, Pb, Zn, Cu, Mo, W, Sn, Be, F, Mn, Fe, Sb, Hg, and barite). The hydrothermal systems that generated these deposits frequently deposited large amounts of quartz where fluids cool, and generally smaller amounts of calcite or dolomite by other mechanisms. To identify and map quartz and carbonate minerals, band ratioing techniques were developed based on the shapes of laboratory reference spectra and applied to ASTER Level 2 surface emissivity products of 108 overlapping scenes. These mineral maps were mosaicked into a single coverage that was overlain with published, vector-format geologic maps of various scales to determine which geologic terranes, formations, and geomorphic features correspond to identified quartz or carbonate. Where quartz or carbonate minerals were mapped in rocks composed primarily of other minerals, they were inferred to be hydrothermal in origin and compared to known occurrences of hydrothermal alteration and mineralization. The ASTER-based quartz mapping identified thick sequences of quartzite, bedded radiolarian chert, quartz sandstone, conglomerates with clasts of quartzite and chert, silicic and/or altered rhyolites, and silicic welded tuffs. Alluvial fan surfaces, sand dunes, and beach deposits composed of quartz and/or carbonate are prominent, well-mapped features. Quartz was also identified in smaller bodies of jasperoid, quartz-alunite, and quartz-sericite-pyrite alteration, hot spring silica sinter terraces, and several diatomite and perlite mines and prospects. The ASTER-based carbonate mapping identified thick sequences of dolomite, limestone, and marble, as well as small hot spring travertine deposits. Eolian carbonate was identified in several playas. Dolomite exhibited a stronger carbonate response than calcite, as predicted based on their thermal spectral characteristics. Quartz was detected at lower concentrations than carbonates because of the greater strength of the quartz reststrahlen features in the thermal infrared compared to the bending-related spectral features of carbonates. The 90 m ground pixel size of the ASTER thermal imagery prevents the identification of small-scale features. Despite this limitation, numerous bodies of hydrothermal quartz were detected in or near known Carlin-type gold deposits, distal disseminated Au-Ag deposits, high- and low-sulfidation epithermal Au-Ag deposits, and geothermal areas. Detection of hydrothermal carbonate was rare and mainly in geothermal areas. The ASTER-based thermal quartz and carbonate mapping demonstrated here can be used in well-studied or frontier areas to verify the accuracy of existing geologic maps, guide future detailed stratigraphic and structural mapping in lithologically complex terranes and allochthons, and identify hydrothermal features for exploration and resource assessment purposes.

Numerical Simulation of Multiphase Fluid Flow in Hydrothermal Systems

Ore-forming hydrothermal systems often show signs of boiling. In certain classes of deposits—such as low-sulfidation epithermal Au-Ag veins—the chemical consequences of boiling are often considered the cause of ore mineral precipitation, while in others the role of boiling may rather lie in enforcing a favorable pressure-temperature regime. In yet other cases it may, however, be just a passive record of the conditions during ore formation, with little or no direct causal relation to the actual metal enrichment. A quantitative understanding of the occurrence, the spatial and temporal extent, and the dynamic evolution of boiling zones in hydrothermal systems on a generic level, as well as for specific systems, can help in determining the role of boiling and is thus a potentially valuable tool in economic geology. However, while numerical simulation of the simultaneous flow of coexisting fluid phases (water, oil and gas) is a routinely used key tool in hydrocarbon exploration and reservoir management (Gerritsen and Durlofsky 2005), it has not yet found a similar position in the study and exploration of mineral resources. This is at least partly due to the fact that the geological context of hydrothermal mineral deposits is more difficult to resolve with geophysical methods and, hence, the overall geometry for which simulation of fluid flow should be carried out, is often less clear. Nevertheless, proper simulation methods have emerged rather recently and have found some application in the hydrothermal regime. Simulations are now a tool to understand and manage individual geothermal fields. O’Sullivan et al. (2001) provide a recent review and Pruess (1990) gives a summary of the basic simulation methods. So-called high-enthalpy geothermal systems often operate under conditions of boiling and are considered to be at least hydrologically equivalent to epithermal hydrothermal systems (Hedenquist et al. 1992). However, studies that have specifically investigated the …

Sm–Nd fluorite dating of Proterozoic low-sulfidation epithermal Au–Ag deposits and U–Pb zircon dating of host rocks at Mallery Lake, Nunavut, Canada

The Mallery Lake area contains precious metal-bearing quartz–chalcedony stockworks that are pristine examples of ancient low-sulfidation epithermal deposits. Fluorite extracted from these epithermal deposits define a Sm–Nd errorchron age of 1434 ± 23 Ma mean square of weighted deviates (MSWD) = 4.8. This date is interpreted to have age significance because (1) a simple linear trend does not exist between the 143Nd/144Nd ratios of the fluorite with respect to their 1/Nd concentrations as would be expected for mixing of two geochemical end members; (2) microthermometric studies indicate that the fluorite analysed in this study has an intimate association with a single high-salinity, calcic brinal fluid; and (3) the age determined from seven fluorite samples extracted from a single outcrop location yielded an identical age result (1434 ± 60 Ma; MSWD = 5.5) compared to the fluorite composite. Rhyodacites of the Pitz Formation and syenites from the Nueltin suite (intrusive equivalent to the rhyodacites) are the youngest volcanic–plutonic rocks that are observed in outcrop in the Mallery Lake area, and they were dated by U–Pb zircon analysis at 1760 ± 43 Ma and 1755.4 ± 1.8 Ma, respectively. The ~320 million year age difference between the epithermal deposits and the hosting rhyodacitic flows suggests that the epithermal stockworks may have formed by a regional hydrothermal event unrelated to this earlier Paleoproterozoic volcanic activity. Uranium deposits in the Thelon and Athabasca basins, to the northwest and south of the Baker Lake Basin, were determined to have similar ore emplacement ages with no evident heat source.

Genetic implications of fluid inclusions in skarn chimney ore, Las Animas Zn–Pb–Ag(–F) deposit, Zimapán, Mexico

Three tectonic and physiographic provinces are present in the study area: (1) the Sierra Madre Oriental (SMOR; Suter, 1987; Carrillo-Martinez and Suter, 1982) formed by the succession of anticlines and synclines with a consistent NNW– SSE trend; (2) the Basin and Range (BR), formed by horst and grabens oriented NE–SW and NW– SE; and (3) the Trans-Mexican Volcanic Belt (TMBV; Aranda-Gomez et al., 2000), a continental volcanic arc on the southwest margin of the NorthAmerican plate resulting from the subduction of the Rivera and Cocos plates along the Acapulco trench (Fig. 1). The SMOR hosts a huge variety of ore deposits: Skarn Pb +Zn +Ag+(Hg–Sb) type ore deposits of Paleocene–Eocene age are found mostly at the El Pinon anticline; low sulfidation epithermal Au–Ag

Proterozoic low-sulfidation epithermal Au-Ag mineralization in the Mallery Lake area, Nunavut, Canada

The Mallery Lake area contains pristine examples of ancient precious metal-bearing low-sulfidation epithermal deposits. The deposits are hosted by rhyolitic flows of the Early Proterozoic Pitz Formation, but are themselves apparently of Middle Proterozoic age. Gold mineralization occurs in stockwork quartz veins that cut the rhyolites, and highest gold grades (up to 24 g/t over 30 cm) occur in the Chalcedonic Stockwork Zone. Quartz veining occurs in two main types: barren A veins, characterized by fine- to coarse-grained comb quartz, with fluorite, calcite, and/or adularia; and mineralized B veins, characterized by banded chalcedonic silica and fine-grained quartz, locally intergrown with fine-grained gold or electrum. A third type of quartz vein (C), which crosscuts B veins at one locality, is characterized by microcrystalline quartz intergrown with fine-grained hematite and rare electrum. Fluid inclusions in the veins occur in two distinct assemblages. Assemblage 1 inclusions represent a moderate temperature (Th=150 to 220 °C), low salinity (~1 eq. wt% NaCl, with trace CO2), locally boiling fluid; this fluid type is found in both A and B veins and is thought to have been responsible for Au-Ag transport and deposition. Assemblage 2 inclusions represent a lower temperature (Th=90 to 150 °C), high salinity calcic brine (23 to 31 wt% CaCl2-NaCl), which occurs as primary inclusions only in the barren A veins. Assemblage 1 and 2 inclusions occur in alternating quartz growth bands in the A-type veins, where they appear to represent alternating fluxes of dilute fluid and local saline groundwater. No workable primary fluid inclusions were observed in the C veins. The A-vein quartz yields δ18O values from 8.3 to 14.5‰ (average=10.9±1.7‰ [1σ], n=30), whereas δ18O values for B-vein quartz range from 11.2 to 14.0‰ (average=13.0±0.9‰, n=12). Calculated δ18OH2O values for the dilute mineralizing fluid from B veins range from –2.6 to 0.2‰ (average=–0.8±0.9‰, n=12) and are consistent with a dominantly meteoric origin. No values could be calculated for the brine, however, because all A-vein quartz samples contain mixed fluid inclusion populations. However, the fact that A-vein quartz samples extend to lower δ18O values than the B veins suggests that the brine had a lighter isotopic signature relative to the dilute fluid. Hydrogen isotopic ratios of fluid inclusion waters extracted from eleven quartz samples of both vein types range from δDFI=–56 to –134‰, but show no particular correlation with vein type. In most respects, the mineralogical and fluid characteristics of the Mallery Lake system are comparable to those of Phanerozoic low-sulfidation deposits, and although the presence of high salinity brines is unusual in such deposits, it is not unknown (e.g., Creede, Colorado). In addition, one of the few other examples of well-preserved, Precambrian, low-sulfidation epithermal deposits, from the Central Pilbara tectonic zone, Australia, contains a similarly bimodal fluid assemblage. The significance of these saline brines is not clear, but from this study we infer that they were not directly involved with Au-Ag transport or deposition.

Geology, porphyry CuAu, and epithermal CuAuAg mineralization of the Tombulilato district, North Sulawesi, Indonesia

The geology of the Tombulilato district, North Sulawesi, is characterized by an island arc-type volcano-sedimentary pile, > 3400 m thick and of late Miocene(?)-Pleistocene age, which is made up of submarine to subaerial basic to acid volcanic rocks interbedded with marine and continental sedimentary rocks. The sequence is intruded by high-level stocks and dikes, and cut by diatreme breccias of late Pliocene and Pleistocene age, some of which are associated intimately with porphyry CuAu and epithermal CuAuAg mineralization. A main compressive deformation event took place in the Pliocene. Three main mineralization types are present in the district: porphyry CuAu; high-sulfidation epithermal CuAuAg; and low-sulfidation epithermal AuAg. Porphyry CuAu mineralization is present at Cabang Kiri, Sungai Mak, Kayubulan Ridge, and Cabang Kanan. Hypogene CuAu mineralization is associated typically with magnetite-bearing K-silicate assemblages, which are partially to completely obliterated by sericite, illite and chlorite. Copper as chalcopyrite and bornite, and Au show a positive correlation. Multiphase intrusions and alteration-mineralization events are commonplace. Biotite-bearing K-silicate alteration at Cabang Kiri and Kayubulan Ridge yields KAr ages of 2.93±0.06 and 2.36±0.05 Ma, respectively. Cabang Kiri possesses a gold-rich zone (> 1.5 ppm Au) open at depth, and the bulk of the mineralization at Sungai Mak is contained in a supergene chalcocite blanket. High-sulfidation epithermal CuAuAg mineralization is present at Motomboto, which displays an alteration pattern consisting of a core of residual (vuggy) silica, bordered outward by zones of quartz-alunite, quartz-kaolinite, and chlorite. This alteration zoning is interpreted to have formed by magmatic-volatiles charging a meteoric convection cell, with the resulting fluids becoming progressively neutralized outward by fluid-rock interaction. CuAuAg mineralization is contained in the silica core associated with multiphase hydrothermal breccias, which are typically cemented by pyrite, enargite, luzonite, marcasite, and minor sphalerite and galena. Late-stage barite contains trails of secondary liquid + vapor-rich inclusions with T h between 154 and 178°C. Available K-Ar dating on alunite suggests that advanced argillic alteration at Motomboto was active between 1.89 and 0.93 Ma. Low-sulfidation epithermal AuAg mineralization at Kaidundu, and elsewhere in the district, is confined to narrow (to 1 m wide) silica veins. They consist of chalcedony and quartz displaying comb and cockade textures, and abundant open vugs. The veins contain <5 vol.% sulfides (dominantly pyrite), and illitic alteration is restricted to narrow selvages and occluded wallrock fragments. Gold is free in quartz, and also associated with pyrite and tellurides in millimeter-scale bands and disseminations. Crystalline quartz contains primary liquid + vapor-rich fluid inclusions with T h between 166 and 324°C and salinities of up to 0.8 equiv. wt.% NaCl. Preliminary geologic reconstructions suggest that these mineralization types in the Tombulilato area were generated over about 2 M yr (between 2.9 and 0.9 Ma) as part of a district-scale hydrothermal system. Continuous syn-mineralization uplift and erosion, which are interpreted to have removed some 2 km of rock in the last 3 M yr, were responsible for the progressive unroofing of the hydrothermal system and the superposition of epithermal environments over relics of higher temperature, deeper-seated mineralization. In the near-surface environment, intense uplift accompanied formation of a chalcocite blanket at Sungai Mak.