Lode Au Deposits Research Articles

Gold endowment of the metasomatized lithospheric mantle for giant gold deposits: Insights from lamprophyre dykes

The metasomatized sub-continental lithospheric mantle (SCLM) is increasingly proposed to play a key control in the formation of giant gold (Au) deposits. However, the extent of Au enrichment in metasomatized SCLM and the Au contents of its derivative partial melts remain poorly constrained. Lamprophyres are derived from low-degree melting of metasomatic and fusible components in the SCLM, and could provide fundamental information about the metasomatized source and metal release compared to mantle peridotites and/or pyroxenites. The giant Jiaodong Au province (> 5000 tons, ∼ 120 Ma) in the eastern part of the North China Craton (NCC) is an ideal locality to assess the Au endowment of metasomatized SCLM which may serve as the source of Au, as indicated by the presence of mantle-dominated volatiles in auriferous fluids associated with the deposits. Based on detailed petrological studies, we analyzed Au, S, Cu and platinum-group element (PGE) contents of the SCLM-derived lamprophyres that are temporally (130–121 Ma) and spatially associated with the Jiaodong lode Au deposits. The lamprophyre dykes were emplaced in the deep crust and did not experience significant degassing and sulfide segregation, and the sulfide saturation occurred at a late stage during magma cooling, as demonstrated by sulfide petrography, S contents, Cu/S and Cu/Pd ratios of the lamprophyres. The relatively oxidized (ΔFMQ +1 – +2), hydrous (2–4 wt.% H2O) lamprophyres display variable and generally low Au contents (0.04–3.59 ng/g, mainly 1–3 ng/g, n = 31), as well as low PGE and Cu contents. Their high Au/Cu(N) and Au/Pd(N) ratios together with high Ba/Nb indicate a discernible Au addition to their source, resulting from multi-stage mantle metasomatism related to subduction. However, the average Au contents of the metasomatized SCLM before large-scale Au mineralization are heterogeneous and overall in a range of 0.5–1.5 ng/g. The values are higher than that of depleted SCLM (< 0.5 ng/g) but are still lower than or similar to the fertile upper mantle (1.2–1.7 ng/g). These results indicate that mantle metasomatism related to subduction replenished some amount of Au into previously depleted SCLM, leading to local or regional Au enrichment. However, such processes evidently did not result in remarkable Au enrichment of the mantle source beneath the eastern part of the NCC. We thus propose that the metasomatized SCLM and magmas derived from it, though without anomalously high Au contents, could still provide large amounts of Au for large-scale Au mineralization. Importantly, metasomatic volatiles from the metasomatized SCLM exert a key role, and they enable efficient release of Au from mantle sources, promote Au transportation along trans-lithospheric fault channels and subsequent enrichment and precipitation via magmatic-hydrothermal processes for giant Au deposits.

Tectono-magmatic controls on decratonic gold deposits

Magmatic-hydrothermal gold–copper deposits in post-subduction settings represent essential targets for mineral exploration, but controls on their formation remain controversial. The early Cretaceous lode Au districts that formed during lithosphere destruction of the North China Craton provide an ideal opportunity to better understand the key tectono-magmatic factors responsible for the genesis of Au-rich deposits in post-subduction settings. Here, we present a LA-ICP-MS study of silicate melt inclusions and sulfide inclusions from ore-related mafic to intermediate rocks in the central Taihangshan Au district in the interior of the North China Craton to constrain the content and evolution of magmatic ore metals ± volatiles. The results, combined with numerical modeling, suggest that the ore-related magmas contained only a few ng/g Au, which is similar to the Au content of non-mineralization-related mafic to intermediate magmas worldwide. The low Au content of the lode Au-related magmas suggest that large volumes of magmas had to accumulate in the middle to lower crust through trans-lithospheric fault systems to produce the lode Au deposits. It is further suggested that the lode Au-related magmas were alkali-rich, hydrous, oxidized and relatively rich in sulfur and chlorine (mafic melt inclusions contain 0.14‒0.24 wt% S and 0.1‒0.2 wt% Cl). These properties are considered critical for the generation of auriferous ore fluids. By comparing the tectono-magmatic setting of the giant Jiaodong Au province (~ 4000 t Au) with the central Taihangshan district (~ 150 t Au), we propose that the much larger total Au tonnage of the Jiaodong district results from the accumulation of a much larger volume of ore-forming magmas at deep crustal levels, induced by a stronger degree of lithosphere modification. In addition, given that the composition of lode Au-related magmas is similar to that of porphyry Cu–Au-related magmas, the lack of giant, early Cretaceous porphyry Cu–Au deposits in the North China Craton suggests that strong extensional settings favor the formation of lode Au deposits instead of porphyry Cu–Au deposits. The present study, therefore, has general implications for the genesis of Au-rich deposits in strongly extensional settings.

Copper–arsenic decoupling in an active geothermal system: A link between pyrite and fluid composition

Over the past few decades several studies have reported that pyrite hosts appreciable amounts of trace elements which commonly occur forming complex zoning patterns within a single mineral grain. These chemical zonations in pyrite have been recognized in a variety of hydrothermal ore deposit types (e.g., porphyry Cu–Mo–Au, epithermal Au deposits, iron oxide–copper–gold, Carlin-type and Archean lode Au deposits, among others), showing, in some cases, marked oscillatory alternation of metals and metalloids in pyrite growth zones (e.g., of Cu-rich, As–(Au, Ag)-depleted zones and As–(Au, Ag)-rich, Cu-depleted zones). This decoupled geochemical behavior of Cu and As has been interpreted as a result of chemical changes in ore-forming fluids, although direct evidence connecting fluctuations in hydrothermal fluid composition with metal partitioning into pyrite growth zones is still lacking. In this study, we report a comprehensive trace element database of pyrite from the Tolhuaca Geothermal System (TGS) in southern Chile, a young and active hydrothermal system where fewer pyrite growth rims and mineralization events are present and the reservoir fluid (i.e. ore-forming fluid) is accessible. We combined the high-spatial resolution and X-ray mapping capabilities of electron microprobe analysis (EMPA) with low detection limits and depth-profiling capacity of secondary-ion mass spectrometry (SIMS) in a suite of pyrite samples retrieved from a ∼1km drill hole that crosses the argillic (20–450m) and propylitic (650–1000m) alteration zones of the geothermal system. We show that the concentrations of precious metals (e.g., Au, Ag), metalloids (e.g., As, Sb, Se, Te), and base and heavy metals (e.g., Cu, Co, Ni, Pb) in pyrite at the TGS are significant. Among the elements analyzed, As and Cu are the most abundant with concentrations that vary from sub-ppm levels to a few wt.% (i.e., up to ∼5wt.% As, ∼1.5wt.% Cu). Detailed wavelength-dispersive spectrometry (WDS) X-ray maps and SIMS depth vs. isotope concentration profiles reveal that pyrites from the TGS are characterized by chemical zoning where the studied elements occur in different mineralogical forms. Arsenic and Co occur as structurally bound elements in pyrite, Cu and Au in pyrite can occur as both solid solution and submicron-sized particles of chalcopyrite and native Au (or Au tellurides), respectively. Pyrites from the deeper propylitic zone do not show significant zonation and high Cu–(Co)–As concentrations correlate with each other. In contrast, well-developed zonations were detected in pyrite from the shallow argillic alteration zone, where Cu(Co)-rich, As-depleted cores alternate with Cu(Co)-depleted, As-rich rims. These microanalytical data were contrasted with chemical data of fluid inclusions in quartz and calcite veins (high Cu/As ratios) and borehole fluid (low Cu/As ratios) reported at the TGS, showing a clear correspondence between Cu and As concentrations in pyrite-forming fluids and chemical zonation in pyrite. These observations provide direct evidence supporting the selective partitioning of metals into pyrite as a result of changes in ore-forming fluid composition, most likely due to separation of a single-phase fluid into a low-density vapor and a denser brine, capable of fractionating Cu and As.

The golden transformation of the Cretaceous plate subduction in the west Pacific

Long-term couplings between the subducting and overlying plates are very important to understanding plate tectonics, in particular intraplate evolutions. Geological records of this coupling however, are usually not well preserved. Here we show a good example in eastern China where Cretaceous tectonic evolution matches remarkably well with the drifting history of the Pacific plate. The most pronounced phenomenon is that the eastern China large-scale orogenic lode gold (Au) mineralization occurred contemporaneously with an abrupt change of ~ 80° in the drifting direction of the subducting Pacific plate, concurrent with the formation of the Ontong Java Plateau. Given that lode Au deposits usually form at the onset of compressional or transpressional deformations, the Au deposits dated the major tectonic change from extension to transpression in eastern China, coherent with the subduction regime. The Cretaceous drifting history of the Pacific plate also tallies with other major geological events in eastern China, e.g., the evolution of the Tan-Lu fault and magmatic activities, suggesting that the major geological events in eastern China in the Cretaceous were mainly controlled by the subduction of the Pacific plate, and that plate interactions during subduction are important driving forces for geological evolution in eastern China and intraplate tectonics in general.

Gamma-Ray Data Processing and Integration for Lode-Au Deposits Exploration

A large number of mineral deposits are associated with hydrothermal processes, especially auriferous deposits. In such processes, studies on percolating fluids may indicate the presence of potash (K), among other elements. In this study, aerogammaspectrometric data-processing methodologies are evaluated, especially those methods based on the suppression of the primary contribution of potassium, the result of lithological and soil variations, and to environmental conditions. Resulting maps point out the contribution of hydrothermal K. This processing procedure was used because of the association of hydrothermal K and auriferous mineralizations according to the deposit model defined for the studied region. Intensity maps locate the areas with great influence of hydrothermal K. Data integration required to improve a change in the gammaspectrometric data processing in order to positively correlate hydrothermalised areas. Data integration could distinguish high and medium favorable targets for mineral exploration of lode-Au deposits in the studied region.

Geodynamics and metallogeny of the Lachlan Orogen

The Palaeozoic Lachlan Orogen of eastern Australia is an accretionary orogen made up of structurally thickened oceanic successions, including turbidites from deep‐sea fans, andesitic volcanics from remnant island arcs, forearc sediments and slices of oceanic crust. Accretion of these units has been inferred to occur by collapse of a marginal basin during double divergent subduction within a complex southwest Pacific‐style oceanic setting. The small basin closed behind, and in the backarc position to, a major, long‐lived subduction system off Gondwanaland in the Palaeozoic. Stepwise deformation and metamorphism from Late Ordovician through Early Carboniferous times formed three distinct geological subprovinces that have interacted with each other through time and space. Metallogeny relates directly to these three provinces and their position within this tectonic framework. In the Western Subprovince, Ordovician turbidites host major lode Au deposits (e.g. Bendigo and Ballarat, central Victoria) because of structural thickening and metamorphism within a coupled thick turbidite wedge ‐ oceanic crustal layer above an inferred flat, west‐dipping subduction zone. In the Central Subprovince a Late Ordovician to Early Silurian high‐T metamorphic belt hosts Sn–W ± Mo skarn/greisen deposits as part of a more evolved, differentiated magmatic arc associated with northwest‐trending, east‐dipping subduction beneath the Tabberabbera Zone. In the Eastern Subprovince, porphyry Cu–Au deposits (e.g. Goonumbla and Lake Cowal, central‐northern New South Wales) formed within an Ordovician oceanic island arc, and volcanic‐hosted massive‐sulfide deposits (e.g. Captains Flat and Woodlawn, eastern New South Wales) formed in younger Silurian forearc sediments. Both deposit types are related to the outboard, long‐lived, west‐dipping subduction zone off Gondwanaland.

Lode Gold and Epithermal Deposits of the Mallina Basin, North Pilbara Terrain, Western Australia

The Mallina basin, in the central west part of the North Pilbara terrain, is one of the more mineralized parts of the Pilbara craton. Three styles of Au mineralization are present in the region: (1) lode Au deposits associated with sericite-carbonate-pyrite alteration assemblages, (2) lode Au deposits associated with pyrophyllite-bearing alteration assemblages, and (3) lode Sb-Au deposits. Gold resources have been identified at the Withnell and Camel deposits just to the north of the east-trending Mallina shear zone. East-trending mineralized zones at the Withnell deposit are typical of turbidite-hosted lode Au deposits in that they are late tectonic (syn- to late- local D3) and associated with extensive carbonate-sericite alteration. Gold mineralization occurs in planar quartz carbonate veins and with pyrite and arsenopyrite in the host rocks to the veins. Fluid inclusion analyses indicate mainly low-salinity, carbonic inclusions with moderate homogenization temperatures. The lode Sb-Au deposits have a similar structural setting and fluid characteristics. At the Camel 2 deposit Au is associated with syn- to late-D3 pyrophyllitic shear zones that have overprinted an earlier carbonate alteration assemblage. Structural relationships and limited Pb isotope data suggest that the lode Au mineralization in this district formed at about 2900 Ma. In addition to the lode Au and Sb-Au deposits, north-northwest–trending quartz veins have textures typical of low-sulfidation epithermal deposits. The lithogeochemistry of samples collected from these veins indicates local enrichment of Au, As, Sb, Ag, Te, and Hg, which is consistent with an epithermal origin. Based on geologic relationships and Pb isotope data, these deposits are interpreted to have formed in the Mallina basin at ca. 2750 Ma. If they are true epithermal deposits, then they are among the oldest known. Their presence suggests that preservation, not Archean age, is the key control on the temporal distribution of epithermal deposits.

The Timing of Mineralization in the Archean North Pilbara Terrain, WesternAustralia

Mineralizing events in the North Pilbara terrain of Western Australia occurred between 3490 and 2700 Ma and include the oldest examples in the world of many ore deposit types. These events were pulsed and associated with major volcano-plutonic episodes (volcanic-hosted massive sulfide, porphyry Cu, Sn-Ta pegmatite, mafic-ultramafic-hosted Ni-Cu-PGE, and epithermal deposits) and deformation events (lode Au deposits). In many cases, the mineralizing events are associated with extension, either in rifts, pull-apart or back-arc basins, or as the consequence of doming associated with granitoid diapirism. Although mineralizing events occurred throughout the evolution of the North Pilbara terrain, the largest deposits are related to the development of the Mallina basin, which is located between the older East and West Pilbara granite-greenstone terranes. Four volcano-plutonic and three deformation events occurred in and around the Mallina basin between 2950 and 2840 Ma. Mineralization in the East and West Pilbara granite-greenstone terranes is less intense and occurred over a much longer period. Compared to other Archean granite-greenstone terrains, the North Pilbara terrain is poorly endowed; the only known world-class deposit in this region is the Wodgina Ta-Sn pegmatite deposit, but large iron ore deposits are being mined in the northeastern North Pilbara terrain. The long history of crustal development and slow rate of crustal growth may be responsible for the diversity of mineral deposits and could account for the apparent poor endowment of the North Pilbara terrain.

Regional and local-scale gold grain and till geochemical signatures of lode Au deposits in the western Abitibi Greenstone Belt, central Canada

Abstract This paper is an overview of drift exploration methods for lode Au deposits in areas of thin and thick cover of glacial sediments within the Abitibi Greenstone Belt of central Canada. It summarizes a large volume of data produced by government regional surveys and case studies as well as that from industry-led gold exploration programs. Regional till surveys can be used as targeting mechanisms for further Au exploration. Anomalies are defined by a series of samples with elevated Au concentrations that lie along significant bedrock structures, occurring in clusters or as isolated samples in areas of low sample density. Thresholds between background and anomalous Au grain abundances or Au concentrations are variable and depend on location within the Abitibi Greenstone Belt. Case studies around known deposits provide examples of geochemical and mineralogical signatures of Au deposits that can be expected in till down-ice. These serve as sources of information on appropriate sampling methods and size fractions to analyse, and on ice flow patterns, local glacial stratigraphy and suitable till units for sampling. Two methods for measuring the Au content of till are commonly used: (1) a count of visible Au grains and (2) geochemical elemental analysis. Close to source, till contains thousands to hundreds of thousands ppb Au and several hundred Au grains. The Au grains vary from coarse sand to silt sizes and have pristine shapes. The presence of high Au concentrations in till indicates that the ore zones subcrop and that glacial processes have produced Au dispersal trains down-ice.

Metallogeny of the southern Sierras Pampeanas, Argentina: geological, 40Ar– 39Ar dating and stable isotope evidence for Devonian Au, Ag–Pb–Zn and W ore formation

Early to mid-Paleozoic metamorphic and igneous rocks of the southern Sierras Pampeanas tectono-stratigraphic terrane, northwestern Argentina, host diverse metallic mineral deposits of Au, Ni–Cu, Ag–Pb–Zn, W, Li, Be, Nb, LREE and Th. Integration of deposit geology, 40Ar– 39Ar geochronology and stable isotope studies with regional geological mapping and U–Pb zircon dating, has led to the resolution of three principal metallogenic epochs. Ordovician intrusion-hosted Ni–Cu sulfide deposits and rare-metal-enriched pegmatites were succeeded by a province-wide metallogenic epoch and concurrent tectonic processes during the Devonian. This epoch was characterized by mesothermal lode Au deposits, high-level Ag–Pb–Zn veins, W±Cu-bearing veins and replacements, and various pegmatite- and granite-related lithophile element deposits. Finally, Miocene–Pliocene epithermal Au–Ag mineralization developed during Andean tectonic events. Sericitic alteration associated with mesothermal lode Au (±Cu±Ag) in the Sierra de Las Minas and Sierras de Córdoba yields Devonian 40Ar– 39Ar step heating ages in the ranges 382–393 and 351–378 Ma, respectively. These ages span the timing of Ag–Pb–Zn sulfide–quartz veining in the El Guaico district (386±4 Ma, sericite), and W-bearing quartz vein mineralization in the Aguas de Ramón and El Morro districts which yielded muscovite ages of 366±1 and 363±1 Ma, respectively. Oxygen isotope δ 18O SMOW compositions of the Au, Ag–Pb–Zn and W ore-forming fluids, calculated from quartz, white mica and chlorite isotope ratios, range from 5.2‰ to 10.8‰ ( n=32), whereas calculated δD SMOW compositions range from −61‰ in the La Rioja Au systems to −141‰ in the W veins ( n=9). Sulfide δ 34S values range from 1.5‰ to 10.0‰, averaging 6.5‰ ( n=15). The oxygen and hydrogen stable isotope data suggest contributions to the ore fluids of either D-depleted meteoric waters that have reacted extensively with metasedimentary rocks, or fluids equilibrated with degassed felsic magmas, or mixtures of both. This mineralization formed during and up to 20–30 Ma after widespread emplacement of high-level peraluminous to metaluminous granites, which yielded U–Pb zircon ages of 404±6–382±6 Ma. The Devonian Au deposits are localised in transcurrent and reverse fault and shear zones that may be related to collision of the Chilenia terrane with the western margin of Gondwana during the newly defined Achalian orogeny. These deposits are considered to be members of the mesothermal lode Au family of systems found worldwide.

Gold-telluride mineralisation in the Birimian at Diabatou, Burkina Faso: the role of CO2[sbnd]N2 fluids

Gold-telluride bearing quartz veins at Diabatou occur in rocks of the Palaeoproterozoic Birimian Supergroup of Burkina Faso. The main Au-telluride mineralisation and wallrock alteration occurred at temperatures between 300 and 380°C and pressures of about 3–5 kbar. The Au-telluride- bearing quartz veins predominantly contain (>90%) CO2N2 fluid inclusions, some of which display densities of up to 0.99 g cm−3, much in contrast to other lode-Au deposits of greenstone affiliation. The highest density isochores match the P-T conditions of the mineralisation. CO2N2 inclusions and Au-telluride mineralisation occur in all investigated samples, while aqueous inclusions are almost absent. Therefore, the CO2N2 inclusions at least represent residues of mineralising fluid, although the fluid composition calculated from thermodynamic modelling of wallrock mineral assemblages disagrees with the fluid inclusion composition. Post-entrapment modifications due to grain-boundary migration recrystallisation during deformation are suggested to be responsible for the formation of most CO2N2 inclusions.

Gold deposit continuum: A genetic model for lode Au mineralization in the continental crust

A model is presented dealing with the relations between convection in the continental crust and the formation of lode Au deposits. Sufficient permeability exists in the brittle rheologic regime in large sections of the continental crust to permit convection of meteoric water on both local and regional scales. The solubility of Au in this fluid is high enough to permit the mobilization of quantities of Au which are sufficient to form ore-grade deposits. This general process is shown to be responsible for a continuous series of deposit types ranging rom 1-2-km-deep epithermal deposits, to intermediate-depth Carlin-type deposits, to 10-12-km-deep mesothermal deposits. Major geologic and geochemical differences among the deposit types are products of fluid convection at variable depths and the resulting effects on water/rock ratios and gas solubilities.

Fluids and thrusting

Recent studies of the structure of the continental crust by COCORP and BIRPS have shown that major thrust structures in the lower crust are ubiquitous. Plate-tectonic processes of continental crust destruction or construction at sites where subduction or collision occurs (the Himalayas or the Andes) involve thrusting of continental materials or ocean crust beneath continents, and local overthrusting may accompany transcurrent motion. In all such cases massive fluid flow must occur. As first stressed by M.K. Hubbert and W.W. Rubey, thrusting mechanisms require that fluid pressures are close to, or greater than, lithostatic pressures. When thrusting occurs, compaction of the underplate must occur rapidly, expelling pore water. Crustal thickening leads to gradual heating of the underplate with prograde metamorphism near the top and even melting at the base. If the overthrust plate is thick and hot at the base, retrograde metamorphism will occur at the base of the overthrust plate and rising fluids will encounter inverted thermal gradients. In such a region veins need not occur but leaching phenomena may dominate. If an underthrust plate is hydrated, and/or contains sedimentary aquifers, large fluid volumes are expelled through shear zones into the overthrust plate. The scale of fluid-release processes can be large. Thus, in thin-skinned tectonics where the overthrust plate is 10–15 km thick, induced fluid release can easily reach 4 · 10 9 g m −2 of thrust surface. The flow of such fluids will be controlled by the thrust surface and lithology, both of which will influence hydraulic fracture mechanisms and spacing of fractures. The chemistry of thrust-derived fluids may be highly variable depending on lithology and the time constants of thrusting. Vertical thermal and redox environments will be similarly dependent. In the subduction process, impressive quantities of fluids must pass back up the thrust surface, and such fluids have recently been directly observed during drilling. Dewatering of spilites, serpentinites and sediment layers of the underthrust oceanic lithosphere must produce massive fluid flow back to the surface but some of these fluids may form hydrated minerals in the overlying mantle and, at great depth, flux mantle melting. Extreme metasomatism in blueschist belts must result from such fluids. A case of particular interest involves the cessation of subduction when a high-level slab reaches thermal equilibrium. In general, flow regimes in the thrust and fault zones associated with collisions follow a sequence from conditions of high T- P with locally derived fluids at low water/rock ratios during initiation of the structures, to high fluxes of reduced metamorphic fluids along conduits as the structures propagate and intersect hydrothermal reservoirs. Later in the tectonic evolution, and at shallower crustal levels, there may be incursion of oxidizing near-surface fluid reservoirs into the faults. These fluids may have extremely low δ 18O-values, where mountain ranges form on rebound faults, and high-altitude depleted fluids penetrate down the rebound structure. Extensive mineralization may be associated with such thrust-derived fluids. Examples which will be discussed include the Mother Lode Au deposits of California, U.S.A., and the large U deposits of Lagoa Real, Brazil.