Shallow Low-temperature Hydrothermal Gold Deposits

Hypothermal gold deposits are mainly associated with near-surface volcanic and subvolcanic activity on the continental surface, occurring in continental volcanic rock series or adjacent rocks ranging from basaltic andesite andesite to dacite and rhyolite. These deposits form below the paleowater table, with mineralization depths generally less than 1.5 km, occasionally reaching 2.0 km (see Figure 1), and mineralization temperatures mostly below 300°C.

Figure 1. Key metallogenic geological elements and metallogenic depth of major types of gold deposits.

1. Mineralization Characteristics

Based on the main mineral composition, alteration characteristics, pH, and sulfur redox state, hypothermal gold deposits have been classified by different researchers into various types: low-sulfidation, high-sulfidation, and alkaline rock types; feldspar-sericite type and kaolinite-alumite type; low-sulfidation and high-sulfidation types; low-sulfidation, high-sulfidation, and medium-sulfidation types. Among these, the intermediate sulfidation type, as a transitional type, has limited practical significance in ore deposit research and exploration. Currently, the widely accepted ore deposit classifications are low-sulfidation and high-sulfidation types. Low-sulfidation refers to ore-forming fluids dominated by reduced sulfur species (HS-, H2S), corresponding to the feldspar-sericite type, but the latter emphasizes the near-neutral nature of the ore-forming fluids. High-sulfidation refers to ore-forming fluids containing at least one important oxidizing sulfur species (HSO4-, SO42-, SO2), corresponding to the kaolinite-alumite type, but the latter emphasizes the acidic nature of the ore-forming fluids.

Epithermal gold deposits occur in volcanic-subvolcanic geothermal systems composed of volcanic structures and surrounding tectonic expansion spaces. The ore-forming elements mainly originate from magma exhaust. Ore-controlling structures include annular, arcuate, and radial faults around the crater, as well as secondary fault and fracture systems; fault and fracture systems in volcanic domes or tectonic depressions; cryptovolcanic breccia pipes; and high-angle fault and fracture systems in the basement rocks. Ore bodies are mainly hosted in volcanic rocks and contemporaneous volcanic sedimentary rocks, with some also occurring in various underlying basement rocks. Ore bodies are predominantly vein-like, veinlet-like, and disseminated (see Figure 1).

1.1 Low-sulfidation Gold Deposits

Low-sulfidation gold deposits occur at moderate to considerable distances from the crater, with volcanic rocks as host rocks, and some also occurring in basement rocks. Ore bodies mainly occur as veins and veinlets, with smaller amounts occurring as breccia pipes and disseminated altered rocks. The ore body patterns exhibit vertical zonation, with travertine deposits (silica or travertine) at the surface, veinlets below, and veins further downwards. The ore types are mainly quartz vein type and veinlet type, with minor amounts of breccia type and siliceous rock type. The ore textures mainly include vein-like, veinlet-like, banded, laminated, crust-like, cluster-like, and brecciated structures; the ore structures are mainly colloidal, lattice, euhedral, and subhedral granular structures. The main ore minerals are pyrite, argentite, native gold, pyrrhotite, sphalerite, galena, and arsenopyrite, with relatively low sulfide content (generally 1%~5%); the gangue minerals are mainly quartz, chalcedony, opal, sericite, feldspar, calcite, rhodochrosite, illite, chlorite, and epidote. The main ore-forming elements are Au, Ag, Zn, and Pb, with minor amounts of Cu, Sb, As, and Hg, etc. The w(Au)/w(Ag) value varies considerably, generally ranging from 0.01 to 10.00. The surrounding rocks exhibit strong alteration and distinct zoning. Generally, from the ore body outwards, silicification, argillaceous alteration, and propylitic alteration zones appear sequentially. The main altered minerals and assemblages in the silicification zone are quartz, feldspar, illite, and pyrite; the argillaceous alteration zone mainly consists of illite, sericite, montmorillonite, and pyrite; and the propylitic alteration zone mainly consists of illite, chlorite, calcite, epidote, and pyrite . The ore-forming fluids are mostly atmospheric precipitation and may contain some magmatic volatiles . The temperature of the ore-forming fluids ranges from 100°C to 300°C, mainly concentrated between 150°C and 250°C; the salinity of the ore-forming fluids is 1% to 8%, usually less than 5%.

1.2 High-sulfidation Gold Deposits

High-sulfidation gold deposits occur near volcanic structures, with the host rocks primarily being volcanic rocks. The ore bodies are mainly of the disseminated altered rock type, with minor amounts of volcanic breccia pipe type and quartz vein type. Ore types include siliceous rock type, breccia type, and quartz type. Ore textures mainly include disseminated texture, vein texture, porous texture, brecciated texture, and massive texture; ore structures mainly include metasomatic texture, euhedral texture, and subhedral granular texture. Ore minerals include pyrite, chalcopyrite, arsenopyrite, covellite, argentite, native gold, and tellurides, with relatively high sulfide content (10%~80%); gangue minerals include quartz, alunite, barite, kaolinite, and pyrophyllite. Ore-forming elements are mainly Cu, Au, Ag, and As, with minor amounts of Pb, Hg, Sb, Te, etc., and the w(Au)/w(Ag) value is generally less than 1. The wall rock alteration is well-developed, exhibiting obvious zoning. Generally, from the ore body outwards, there are sequentially occurring silicification zones, advanced argillaceous zones, argillaceous zones, and propylitic zones. The main alteration minerals and assemblages in the silicification zones are quartz and alunite; those in the advanced argillaceous zones are kaolinite, pyrophyllite, sericite, and illite; those in the argillaceous zones are montmorillonite, illite, and chlorite; and those in the propylitic zones are chlorite, epidote, and calcite. Vertically, the top consists of steam-heated lacustrine sediments, commonly containing native sulfur or pyrite, alunite, and kaolinite. Opal and kaolinite develop near ancient erosion surfaces, and chalcedony veins can form near ancient water table. Downwards, there are sequentially strongly leached porous alteration mineral assemblages such as quartz-alunite-kaolinite, dickite-sericite-pyrophyllite-anhydrite, and quartz-pyrophyllite-sericite. The ore-forming fluids are primarily magmatic water, with some mixing from atmospheric precipitation; the temperature of the ore-forming fluids ranges from 140°C to 300°C, mainly concentrated between 200°C and 300°C; the salinity of the ore-forming fluids ranges from 1% to 25%, mostly concentrated between 4% and 8%.

2. Ore-forming Conditions

Hypnotic hydrothermal gold deposits primarily form in near-surface environments of calc-alkaline to alkaline magmatic arcs, including oceanic island arcs and continental arcs, with the main geodynamic background being the subduction of convergent plate boundaries. Furthermore, this type of gold deposit also occurs in intraarctic, back-arc extensional, and post-collisional rift environments. The intrusive rocks associated with hypothermal mineralization are oxidizing, consistent with porphyry copper-gold deposits, while gold deposits associated with intrusive rocks and Carlin-type gold deposits are associated with reducing intrusive rocks. Globally, hypothermal gold deposits are mainly concentrated in three giant metallogenic domains: the Circum-Pacific metallogenic domain, the Tethys metallogenic domain, and the Laurasia metallogenic domain. Mineralization occurs during volcanic and subvolcanic activity. Generally, the mineralization temperature is less than 300°C, the mineralization depth is less than 2 km, and the mineralization pressure is less than 50 MPa. The activity of ore-forming fluids requires abundant fracture and fissure systems and high-porosity surrounding rocks to provide sufficient expansion space.

3. Significance of mineral exploration

Shallow-level hydrothermal gold deposits are shallow in depth and easily exposed or eroded during later geological processes. Furthermore, some high-sulfidation gold deposits form in the shallow parts of active volcanic structures; subsequent reactivation of volcanoes may destroy these deposits or early geothermal systems. Currently, the surviving shallow-level hydrothermal gold deposits are mainly formed in the Mesozoic to Cenozoic eras, with a small number formed in the Late Paleozoic. Therefore, terrestrial acidic and alkaline volcanic-subvolcanic activity zones (belts) since the Cretaceous are the main target areas for finding shallow-level hydrothermal gold deposits. Shallow-level hydrothermal gold deposits, together with their underlying porphyry gold deposits and transitional sulfide-quartz vein gold-silver deposits, constitute a complete metallogenic system. The presence of epithermal gold deposits may indicate the development of porphyry gold deposits at their deeper levels, and vice versa. Under certain geological conditions, the presence of porphyry gold deposits may indicate the development of epithermal gold deposits at their upper or peripheral levels; that is, under certain geological conditions, the two can serve as mutual prospecting indicators. Generally, high-sulfidation gold deposits are located approximately 1 km from deep, concealed porphyry bodies, while low-sulfidation gold deposits are located at greater distances. Volcanic structures and their surrounding fault and fracture systems are favorable locations for ore bodies. Ore bodies often occur in clusters and belts, sometimes covering an area of ​​up to 200 km². Characteristic alteration minerals and assemblages, alteration zoning, and related fault and fracture structures are the most important and effective prospecting indicators. At the same time, the degree of subsequent erosion and the modification of the distribution of ore bodies and altered rocks by tectonic structures must be fully considered. Therefore, large-scale tectonic-alteration facies mapping is an effective geological prospecting method.

Airborne and ground gravity measurements provide excellent resolution for volcanic and subvolcanic areas (zones). Low resistivity anomalies caused by mineralization or argillaceous alteration, high resistivity anomalies caused by mineralization or silicification, and ore-controlling faults can all be revealed using high-density electrical resistivity tomography (EDT). Airborne magnetic surveys can reflect low magnetic anomalies formed by hydrothermal alteration. Radiometric measurements in volcanic rock areas can obtain anomalies of enhanced radioactivity mainly caused by potassic alteration.

Geochemical surveys of stream sediments, soils, and rocks are highly effective in obtaining information on epithermal gold mineralization and alteration. Geochemical anomalies of ore-forming elements such as Au, As, Sb, and Hg, and rock-forming elements such as K, Na, Si, and Mg, indicate potential epithermal mineralization and alteration zones. Portable shortwave infrared spectroscopy is used for alteration mapping of epithermal gold deposits, enabling rapid and effective identification of alteration traces, mineral assemblages, and zoning, especially clay minerals. It can also reveal subtle variations in the chemical composition of muscovite (illite-sericite) and chlorite, enhancing the directional significance of alteration zoning and mineralization location, thereby guiding sample collection during the prospecting phase and reducing drilling workload. In arid and semi-arid regions, remote sensing hyperspectral information can effectively identify alteration minerals and mineral assemblages for regional and mining-scale remote sensing alteration mapping. By acquiring continuous spectral image data in many very narrow intervals across multiple bands including visible, near-infrared, shortwave infrared, and thermal infrared, it is easy to distinguish advanced argillaceous, argillaceous, and silicified zones associated with mineralization in high-sulfidation gold deposits. Combining hyperspectral information with field geological information allows for mapping of illite and illite-montmorillonite alteration in low-sulfidation gold deposits.