The Influence of Geological Structures on Gold Deposits: Faults, Shear Zones, and Folds

1. Introduction

Gold mineralization is a complex process influenced by a multitude of geological factors, among which structural controls play a pivotal role. This article, based on experiences of Dr. Vinay Sahay in gold exploration, presents the various structural settings that influence gold deposit formation, including fault zones, shear zones, and fold systems. By examining these structures, the mechanisms of gold transport and deposition can be better understood, providing valuable insights for exploration and mining. This article highlights key examples from Archean greenstone belts, mesothermal/orogenic gold deposits, and epithermal gold systems, illustrating the diverse structural environments that host significant gold mineralization.

2. Geological Setting

Gold deposits occur in various geological settings, each characterized by specific structural features. These include orogenic gold deposits, epithermal systems, and Carlin-type deposits (sediment-hosted disseminated gold deposits), among others. The general geological setting of gold deposits involves the interaction of hydrothermal fluids with pre-existing rock and their structures, which create pathways and traps for gold mineralization. Structural component plays a crucial role in this context by influencing fluid flow, pressure-temperature conditions, and the creation of depositional environments.

3. Structural Controls on Gold Mineralization

3.1 Fault Zones

Fault zones are significant structural features that control hydrothermal fluid flow and gold mineralization. Faults can act as conduits for mineralizing fluids, providing pathways for their migration and deposition. The types of faults associated with gold mineralization include normal, reverse, and strike-slip faults.

• Normal Faults: These faults, characterized by extensional tectonics, can create open spaces that facilitate fluid flow and gold deposition. An example is the Carlin-type gold deposits in Nevada, where normal faulting has played a crucial role in localizing mineralization.
• Reverse Faults: Associated with compressional tectonics, reverse faults can create over pressured environments conducive to gold deposition. The Kalgoorlie gold deposit in Western Australia is a prime example of reverse fault-controlled mineralization.
• Strike-Slip Faults: These faults, which involve lateral displacement, can create complex fracture networks that enhance fluid flow and gold deposition. The Mother Lode gold belt in California is an example where strike-slip faulting has significantly influenced gold mineralization.

3.2 Shear Zones

Shear zones are regions of intense deformation that can act as major conduits for hydrothermal fluids. The characteristics of shear zones, such as their high permeability and the presence of extensive fracture networks, make them favorable sites for gold concentration.

In India, significant gold deposits are found in the Archean greenstone belts, particularly in Karnataka's Dharwar Craton. The Kolar Gold Fields, now defunct, were located within a tightly folded and sheared sequence of amphibolites and schists. Gold was primarily concentrated in quartz veins within the sheared zones, illustrating the role of both folding and faulting in controlling mineralization.

Similarly, in the Hutti-Maski Greenstone Belt, gold mineralization is structurally controlled by a series of shear zones and associated quartz veins. The Hutti Gold Mines are a prime example where gold-bearing quartz veins are hosted within sheared amphibolites and metasediments, emphasizing the interplay between shear zones and host rock lithology.

3.3 Fold Systems

Folding can influence gold deposition by creating structural traps and influencing fluid flow patterns. Folds can act as barriers or channels for hydrothermal fluids, depending on their orientation and geometry.

• Structural Traps: Anticlines and synclines can trap mineralizing fluids, creating zones of high gold concentration. The Bendigo goldfield in Australia is a classic example where fold systems have played a crucial role in localizing gold deposits.
• Influence on Fluid Flow: Fold systems can enhance or impede fluid movement, affecting the distribution and concentration of gold within a deposit. Understanding the relationship between folding and fluid flow is essential for predicting gold mineralization patterns.

4. Mechanisms of Gold Transport and Deposition

Gold transport and deposition are controlled by the dynamics of hydrothermal fluids and the geochemical environment. Key mechanisms include:

• Hydrothermal Fluid Dynamics: The movement of hydrothermal fluids is influenced by pressure gradients, temperature, and the presence of structural conduits. These fluids, often rich in gold, migrate through fractures and faults, depositing gold as conditions change.
• Precipitation Mechanisms: Gold is typically transported in solution as complexes with ligands such as chloride or bisulfide. Precipitation occurs due to changes in temperature, pressure, pH, or fluid composition. Common mechanisms include boiling, fluid mixing, and chemical reactions with wall rocks.
• Structural Traps: The interaction between hydrothermal fluids and structural features such as faults, shear zones, and folds creates traps where gold can accumulate. These traps are crucial for the formation of economically viable gold deposits.

5. Conclusions

Structural aspects play a critical role in the formation and localization of gold deposits. Faults, shear zones, and fold systems provide pathways and traps for mineralizing fluids, creating favorable conditions for gold deposition. By understanding these structural controls, geologists can improve exploration strategies and increase the likelihood of discovering economically viable gold deposits.