Orogenic gold deposits, constituting over 75% of historically recovered gold, are hydrothermal mineral deposits influenced by rock structure. This article delves into their formation, emphasizing the primary role of rock structure in controlling both the transportation and deposition of mineralized fluids. Hosted by shear zones in orogenic belts, particularly in metamorphosed fore-arc and back-arc regions, these deposits are associated with the syn- to late metamorphic stages of orogeny. The structural evolution and geometry of the lithospheric crust play a crucial role, as hydrothermal fluids navigate through pre-existing and active discontinuities generated by tectonic processes. This journey through faults, shear zones, and lithological boundaries facilitates the deposition of gold-bearing fluids and other metallic elements, resulting in the formation of vertically extensive quartz veins at upper-crustal levels.
Orogenic Gold Deposits Simply Explained.
Temporal Patterns and Fluid Sources in Orogenic Gold Deposits.
This article explores the temporal distribution and fluid sources associated with orogenic gold deposits. The formation of these deposits is intricately linked to the cycles of amalgamation of continental masses, known as Wilson cycles, leading to significant changes in the geochemical, mineralogical, and structural nature of the lithosphere. Orogenic gold deposits exhibit a temporal pattern, primarily concentrated in three epochs of Earth’s history: Neoarchean (2.8–2.5 Ga), Paleoproterozoic (2.1–1.8 Ga), and Phanerozoic (0.500–0.05 Ga), with a notable absence in the period 1.80–0.8 Ga, considered a phase of general minimum ore-forming activity.
The temporal occurrence of orogenic gold deposits is aligned with the breakup or formation of supercontinents, emphasizing the dynamic geological processes at play. The article also discusses the fluid source in orogenic gold deposits, comparing magmatic and hydrothermal systems. Unlike magmatic systems where ores and host rocks are derived from the same fluid, hydrothermal fluids carrying metals are predominantly aqueous and younger than the host rocks. Various rock types have been proposed as the source of orogenic gold, but the complexity of host rocks throughout Earth’s history adds uncertainty to their relation to gold formation processes.
Furthermore, age dating reveals significant time gaps between mineralization and the formation of host rocks, suggesting a genetic independence of fluid formation from local lithologies. These insights contribute to a better understanding of the temporal dynamics and geological complexities involved in the formation of orogenic gold deposits.
Mineralogy and Geochemistry of Orogenic Gold Deposits: Insights into Formation Conditions
This article delves into the mineralogy and geochemistry of orogenic gold deposits, with a focus on gold-bearing quartz-carbonate veins. Understanding the geochemical characteristics of these veins is crucial for determining the temperature, pressure, and chemical composition of the fluids involved in their formation. Quartz typically dominates these veins, but there are variations, including gold-bearing carbonate-dominant veins in orogenic deposits.
Ore bodies in orogenic gold deposits are primarily characterized by sulfide minerals (≤ 3–5%), commonly arsenopyrite in metasedimentary host rocks and pyrite/pyrrhotite in meta-igneous rocks. Additionally, carbonate minerals (≤ 5–15%), such as ankerite, dolomite, and calcite, are present. An intriguing feature of orogenic gold lodes is the widespread presence of carbonate alteration zones, including ankerite, ferroan dolomite, siderite, and calcite. This alteration is significant in understanding the mineralization processes.
Orogenic gold deposits exhibit a tendency for gold to be transported preferentially as a sulfide complex, contributing to the near absence of base metals (Cu, Pb, Zn) in the same mineral systems. This phenomenon is explained by the formation of complexes with sulfur, rather than chlor, by base metals.
The geochemical composition of hydrothermal fluids associated with orogenic gold deposits is characterized by low salinities (up to 12 wt% NaCl equivalent), high H2O and CO2 contents (> 4 mol%), with lesser amounts of CH4 and N2, and near-neutral pH. High salinity fluids may result from the dehydration of evaporite sequences, while the wide variety in CO2 content indicates diverse fluid production temperatures, with CO2-rich fluids suggesting temperatures exceeding 500 °C. These findings contribute valuable insights into the complex conditions governing the formation of orogenic gold deposits.
Gold Deposit Formation: Genetic Models and Fluid Sources
This article explores the intricate processes involved in the formation of orogenic gold deposits, focusing on various genetic models and fluid sources. Orogenic gold deposits, found in metamorphosed terranes, exhibit a diverse range of characteristics with little in common except for complexity and low mean stress.
The discussion begins by examining the magmatic-hydrothermal fluid source model, where felsic-intermediate magmas release fluids during crystallization. While this model has its merits, challenges arise, such as the lack of age relationships between gold mineralization and granitic intrusions in many gold provinces. The hybrid model, combining magmatic and metamorphic sources, is considered a more common scenario.
Another model involves a mid-crustal fluid source, where metamorphic fluids release gold and other elements during subduction-related scenarios. The low salinity of these hydrothermal fluids is attributed to devolatilization of minerals associated with metamorphic phase reactions. Fluid-rock interactions along the pathway and coupling between fluid flow and structural deformation play key roles in mineralization.
The sub-crustal fluid source model, similar to the mid-crustal model, involves fluid ascent from devolatilization of a subducting slab and overlying sediment wedge. Serpentinite, formed through slab mantle hydration, may play a crucial role due to its lubricating effect and enhanced permeability for hydrothermal fluids. The model suggests that slab dewatering may trigger fluid migration along faults, leading to gold deposits, with the end of subduction or stalling of the slab acting as potential triggers.
While the sub-crustal fluid source model provides a more robust description, acknowledging both source and mechanism, limitations exist, particularly in Precambrian gold deposits without thick sedimentary successions. Overall, the article contributes to a deeper understanding of the geological dynamics and processes shaping orogenic gold deposits.
Tectonic Controls on Orogenic Gold Formation: Structural Geometries and Geodynamic Settings
This article investigates the intricate relationship between tectonics and the formation of orogenic gold deposits, shedding light on the structural geometries and geodynamic settings that influence ore-fluid processes. While specific deformation structures associated with orogenic gold deposits are challenging to define, various fault types have been identified as hosts for gold deposits. Despite this diversity, orogenic gold deposits exhibit repetitive structural geometries that play crucial roles in controlling the formation, transport, and precipitation of ore-fluids.
The geodynamic setting and architecture play a pivotal role, with large-scale lithospheric deformation structures correlating with gold endowment. Active structural permeability in the crust is influenced by the prevailing tectonic stress field. The article emphasizes the connection between orogenic gold deposits and specific geodynamic settings, particularly in primary orogenic belts.
Accretionary orogens, including metamorphosed fore-arc and back-arc regions, are significant locations for orogenic gold deposits. These deposits are closely associated with lamprophyres, felsic porphyry dikes, and sills, indicating deep lithospheric connections for fluid conduits. Structural discontinuities, such as faults, fractures, dilatation zones, and shear zones, exhibit a spatial relationship with orogenic gold deposits. The hosting structures, often subsidiary faults or shear zones, are related to major regional-scale deformation structures like lithospheric boundaries and suture zones.
The article highlights the discordance of deformation structures hosting gold deposits with the stratigraphic layering of host rocks. These structures, including slickensides formed under hydrothermal conditions, indicate syn- to post-mineralization displacements. The geometry of vein systems is primarily influenced by dynamic stress changes and variations in fluid pressure. Overall, the comprehensive exploration of tectonic controls on orogenic gold formation contributes to a deeper understanding of the geological processes involved.
Leave a Reply