Epithermal Au–Ag and Ag Deposits of the Okhotsk Sector of the Okhotsk–Chukchi Volcanic Belt: Metallogeny, Mineral Paragenesis, and Fluid Regime

Magmatic sulphide/volatile interaction as a mechanism for producing chalcophile element enriched, Archean Au-quartz, epithermal AuAg and Au skarn hydrothermal ore fluids

The geotectonic units of Zhejiang Province include the Yangtze Plate in the northwest juxtaposed against the South China fold system in the southeast along the Jiangshan–Shaoxing fault. The South China fold system is further divided into the Chencai–Suichang uplift belt and the Wenzhou–Linhai geotectogene belt, whose boundary is the Yuyao–Lishui fault. The corresponding metallogenic belts are the Mo–Au(–Pb–Zn–Cu) metallogenic belt in northwest Zhejiang, the Chencai–Suichang Au–Ag–Pb–Zn–Mo metallogenic belt, and the coastal Ag–Pb–Zn–Mo–Au metallogenic belt. The main Mesozoic metal ore deposits include epithermal Au–Ag(Ag), hydrothermal vein-type Ag–Pb–Zn(Cu), and porphyry–skarn-type Mo and vein-type Mo deposits. These ore bodies are related to the Mesozoic volcanic-intrusive structure: the epithermal Au–Ag(Ag) deposits are represented by the Zhilingtou Au–Ag deposit and Houan Ag deposit and their veins are controlled by volcanic structure; the hydrothermal vein-type Ag–Pb–Zn deposits are represented by the Dalingkou Ag–Pb–Zn deposit and also controlled by volcanic structure; and the porphyry–skarn-type Mo deposits are represented by the Tongcun Mo deposit and the vein-type Mo deposits are represented by the Shipingchuan Mo deposit, all of which are related to granite porphyries. These metal ore deposits have close spatio-temporal relationships with each other; both the epithermal Au–Ag(Ag) deposits and the hydrothermal vein-type Ag–Pb–Zn deposits exhibit vertical zonations of the metallic elements and form a Mo–Pb–Zn–Au–Ag metallogenetic system. These Jurassic–Cretaceous deposits may be products of tectonic-volcanic-intrusive magmatic activities during the westward subduction of the Pacific Plate. Favourable metallogenetic conditions and breakthroughs in the recent prospecting show that there is great resource potential for porphyry-type deposits (Mo, Cu) in Zhejiang Province.

Numerous epithermal Au–Ag deposits and ore occurrences of the Chukchi Peninsula are localized in the Cretaceous Okhotsk–Chukotka (OCVB) continent-marginal and Late Jurassic–Early Cretaceous Oloi (OVB) island arc volcanic belts and in Early Cretaceous postcollisional volcanic troughs. Volcanotectonic depressions, calderas, and volcanic domes control the location of the deposits. The orebodies of the deposits are quartz–adularia veins, sometimes en-echelon ones forming extending vein zones, as well as isometric and linear stockworks. The auriferous veins of most deposits display complex breccia–crustification structures. The vein ores have rhythmically and colloform–banded structures, with a predominantly fine distribution of ore mineral grains, often with banded clusters of ore minerals (ginguro). Native gold is of low fineness; the dispersion of this index varies from low to high. Acanthite is widespread in the ores. Its highest contents are specific to deposits with the repeated redistribution of substance (Kupol, Corrida, and Valunistoe). Based on the results of mineralogical studies, most of the epithermal Au–Ag deposits of the Chukchi Peninsula can be assigned to the Se type. The ores of some deposits (Valunistoe, Dvoinoe, etc.) contain both Se and Te minerals. The telluride-richest sites of the Sentyabr’skoe and Televeem deposits are far from the main orebodies. Most of the Chukchi epithermal Au–Ag deposits have many common characteristics (low and moderate temperatures of fluids, low fluid salinity, domination of carbon dioxide over methane, etc.) typical of low-sulfidation deposits. The maximum temperatures and salinity are specific to fluids in the Central Chukchi sector of the OCVB and in the Baimka zone of the OVB, and the minimum ones are typical of fluids in the East Chukchi flank zone and inner zone of the OCVB. The average salinity of mineral-forming fluids in the inner zone of the OCVB is half as high as the salinity of fluids in the East Chukchi flank zone of this belt, although the sulfate content is higher. At the same time, the fluids in the inner zone of the OCVB are richer in carbon dioxide and bicarbonate ion than the fluids in the East Chukchi flank zone of this belt. The fluid inclusion data permit the Vesennee deposit (Baimka zone) to be regarded as an intermediate-sulfidation one and suggest the presence of epithermal high-sulfidation deposits in the inner zone of the OCVB.

Izu Peninsula in central Japan, the northern tip of the Izu‐Bonin arc, hosts numerous epithermal Au–Ag vein deposits of low‐sulfidation style. All have similar vein textures, mineralogy, and alteration. Geochemical data from fluid inclusions in vein quartz, the mineralogy and mineral chemistry of alteration, and stable isotope data indicate that auriferous hydrothermal activity occurred under subaerial conditions. The K–Ar ages of auriferous vein minerals are <1.5 Ma, indicating that the mineralization took place after extensive submarine volcanism for the host rocks. These observations suggest that Au–Ag mineralization was synchronous with the development of an extensional regime of the Izu block after its collision with the Honshu arc after 1.5 Ma. This collision resulted in the shifting of the Izu block far from the trench to the rear position, and the subduction of the Izu block along the Suruga trough to the west and along the Sagami trough to the east. The reararc position of the Izu block and double subduction resulted in crustal extension, upwelling of asthenospheric mantle, and tholeiitic magmatism reflected by mafic dyke swarms and subsequent monogenetic volcanic activity in the Izu peninsula. The timing of the Au mineralization in the Izu Peninsula during the beginning of lithospheric extension is similar to that of the Sado Au–Ag deposit on Sado island in the Japan Sea. Two mineralization events coincide with extensive tholeiitic mafic volcanism and injections of dyke swarms related to the back‐arc opening of the Japan Sea. The geological setting of the Au–Ag mineralization in Izu and Sado is also similar to that of the epithermal Au–Ag deposits in northern Nevada, where mineralization was contemporaneous with crustal extension and tholeiitic mafic magmatism derived from the asthenospheric mantle. This study suggests that epithermal Au mineralization at shallow crustal depths is a product of large‐scale lithospheric evolution.

Evolution of hydrothermal fluids of HS and LS type epithermal Au–Ag deposits in the Seongsan hydrothermal system of the Cretaceous Haenam volcanic field, South Korea

The Birkachan deposit in the Middle Paleozoic Kedonsky Volcanic Belt (KVB) is unique in Northeast Russia for its stockwork epithermal Au–Ag ores. The deposit is located in the northeastern part of the Gurnik volcanotectonic depression, in the Kedonsky segment of the KVB. The deposit is localized in the lying side of a large sublatitudinal thrust fault, due to which it was overlain in the Jurassic–Early Cretaceous by allochthon rocks and, therefore, was slightly eroded, as evidenced by the preserved fragments of the argillisite cap. The ore-bearing Gurnik sequence is composed of felsic tuffs and lavas with intercalations of ignimbrites. The main ore-bearing zone of the Birkachan deposit is 4.5 km long and 200–300 m wide consists of several echelon-like vein zones of northeast strike, falling to the southeast at angles of 55°–70°. Most of the ore bodies are linear stockworks. Single, lenticular rich ore bodies represented by mineralized breccias have been found at deep horizons. In the section, the system of ore bodies of the Birkachan deposit forms a fan-shaped structure. The main textures of ores are vein-disseminated, breccia, and rhythmically banded. The ores are enriched in comparison with the upper continental crust in a rather narrow range of elements (Au, Ag, Sb, As, Mo, W, and Li) and are poor in rare-earth elements, among which light lanthanides predominate. The sulfide grade of ores is 0.1–0.5%. The vein minerals in ores are dominated by quartz, sericite, and siderite. Among the ore minerals, pyrite dominates, with fahlore, native gold and chalcopyrite, and minerals of the series acanthite → Se–acantite → naumannite, pearceite, and Se–pearceite being less common. The average fineness value of native gold is 643‰. The parameters of mineral-forming solutions correspond to typical medium-temperature fluids of epithermal low-sulfidized deposits: homogenization temperature of 93–291°С, salt concentration of 0.2–7.0 wt % eq. NaCl, and fluid density of 0.71–0.99 g/cm3. The main indicators of fluid composition are CO2/CH4 = 15.8–23.6, Na/K = 2.3–4.3, and K/Rb = 2007. The prospects for increasing the reserves of Au and Ag are associated with further study and exploration of the flanks and deep horizons of the deposit.

This paper is concerned with the geochemical features and conditions of generation for Paleozoic Au–Ag mineralization in the pre-accretionary Kedon (D2-3) volcano-plutonic belt (KVB), which is situated within the Omolon cratonic terrane (Northeast Russia). We present new data on the compositions and concentrations of trace elements and rare-earth elements (REEs) in the host rocks and volcanogenic ores of epithermal Au–Ag fields. The ores are found to be enriched in a wide range of trace elements. The general features of the ores under study include a low level of REE concentrations, an obvious enrichment in light REEs, and considerable variations in europium anomalies, ranging from low negative to low- and strongly positive. Fluid inclusions were studied to show that the ore-forming solutions were hydrocarbonate potassium in composition. The mineral content of the fluid increases toward later, low-temperature phases of the mineralization. We found a tendency of potassium increasing from the earlier oreless quartz to productive quartz going to great depth, as well as a slow decrease in the concentrations of Na+, Ca++, and Cl–. The productive quartz shows direct correlation in Ag–K and a reverse correlation in Ag–Na. The results indicate andesite magmas and meteoric water as the most likely sources of the fluids that have formed epithermal Au–Ag ores in the KVB deposits.

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.

Au–Ag mineralization of the Olcha and Teploe epithermal deposits underwent thermal metamorphism due to porphyritic intrusions. The presence of Bi-bearing galena and matildite in the ores (Teploe), Cu–Te-bearing naumannite (Olcha), the occurrence of middle- and high-temperature facies of metasomatic rocks (epidote and actinolite), and temperature formation conditions are related, firstly, to the influence of granitoids on the ore process, which supplied not only Cu and Mo, but also Bi, Te, and, secondly, to the heating of host rocks containing pre-porphyritic epithermal Au–Ag mineralization. The abundance of Cu–Ag sulfides and Cu-acanthite resulted from the enrichment of later mineral phases in Cu and Ag under the substance redistribution with the formation of Ag-acanthite ores. The data considered in the paper are of practical importance for regional forecasting of metallogenic constructions, exploration, and evaluation of the epithermal Au–Ag deposits.

Cretaceous epithermal gold–silver mineralization and geodynamic environment, Korea

This paper considers the geochemistry and conditions of generation for the Mesozoic Au–Ag epithermal deposits in the Okhotsk–Chukchi volcanic–plutonic belt (OChVB) in Northeast Russia. We provide new data on the composition and concentrations of trace elements, including REEs, in the ores of epithermal Au–Ag deposits. The ores were found to be enriched in a wide range of trace elements. The REE distributions of these ores are dominated by light “hydrophile” lanthanoids of the “cerium” group. The Eu anomalies were found to vary between high negative to low and high positive levels. Comparative analysis over the classes of gold concentration showed a synchronous enrichment of the ores in similar sets of trace elements. A study of fluid inclusions revealed that the ore-forming solutions had hydrocarbonate potassium or hydrocarbonate sodium compositions. The fluids had high concentrations of sulfate ions for most deposits. The salinity of the fluids was frequently found to increase toward later low-temperature mineralization phases. We identified the tendency of increasing K+ percentage in the fluid from the earlier oreless quartz to productive quartz with increasing depth, as well as some decrease in the percentages of Na+, Ca++, and Cl–. The results indicate magma chambers of andesite magmas and meteoric waters as the most likely sources of the fluids that generated the epithermal Au–Ag ores in the OChVB deposits.

We present 40Ar/39Ar data acquired by infra-red (CO2) laser step-heating of alunite crystals from the large Miocene Colquijirca district in central Peru. Combined with previously published data, our results show that a long (at least 1.3 My) and complex period of magmatic-hydrothermal activity associated with epithermal Au–(Ag) mineralization and base metal, Cordilleran ores took place at Colquijirca. The new data indicate that incursion of magmatic SO2-bearing vapor into the Colquijirca epithermal system began at least as early as ∼11.9 Ma and lasted until ∼10.6 Ma. Four alunite samples associated with high-sulfidation epithermal Au–(Ag) ore gave 40Ar/39Ar plateau ages between ∼11.9 and ∼11.1 Ma (compared to the previously documented ∼11.6 to ∼11.3 Ma). By combining individually these new ages with crosscutting relationships, the duration of the Au–(Ag) deposition period can be estimated to at least 0.4 My. Three new 40Ar/39Ar plateau ages on alunite associated with the base-metal Cordilleran ores are consistent with previously obtained ages, all of them between 10.83 ± 0.06 and 10.56 ± 0.06 Ma, suggesting that most of the sulfide-rich polymetallic deposits of Smelter and Colquijirca formed during this short period. The recognition of consecutive alunite-bearing and alunite-free mineral assemblages within both the Au–(Ag) and the base-metal Cordilleran ores may suggest that SO2-bearing magmatic vapor entered the epithermal environment as multiple discontinuous pulses, a number of which was not necessarily associated in time with ore fluids. It is likely that a period of SO2-bearing vapor degassing longer than 11.9 to 10.6 Ma may be recognized with further more detailed work.

Metallogenic epochs in the Inner Zone of southwest Japan

Meso-Cenozoic tectonics of the southern Patagonian foreland: Structural evolution and implications for Au–Ag veins in the eastern Deseado Region (Santa Cruz, Argentina)

A reassessment of the geochemistry of stream sediments collected by the British-led North Sumatra Project from 1975 to 1980 in central Aceh with spectrum–area and local singularity fractal techniques refines previously determined areas with anomalous Cu and delineates As anomalies (proxy for Au) that correspond to porphyry Cu and epithermal Au–Ag deposits and prospects discovered in the 1980s and 1990s. Additionally, the application of these methods to stream sediment samples collected by an Australian company in the mid-1990s from part of central Aceh identifies Au, As, and Cu anomalies that are aligned with regional geochemical trends. Integrating singularity metal-enriched areas and the distribution of mineral occurrences and structures reveals northerly and easterly tectono-magmatic corridors controlling epithermal Au–Ag and porphyry Cu mineralisation. The above results attest to the usefulness of legacy data for first-pass mineral assessments and for providing evidence layers for a future mineral prospectivity map.