Geological Methods in Mineral Exploration and Mining / Roger Marjoribanks

Serpantinit, Sadece Değişime Uğramış Bir Kayaç Olmayıp, Aynı Zamanda Mineralleşmenin Lokomotifidir

Çamurtaşları, serpantinitler ve piritli kireçtaşlarından oluşan karmaşık stratigrafik dizilerde sondaj yaparken, cevhere rastlamak sadece şans olmayıp, apısal kontrollerin ve jeokimyasal değişimlerin çok iyi değerlendirilmesi ve yorumlanması ile ilgilidir.

Ekte sunulan yazıda ve makalede, serpantinit içeren sistemlerde mineralleşmeyi yönlendiren spesifik mekanizmalar olabildiğince derinlemesine incelenmiştir. İster bir fay zonunu haritalandırıyor olun, ister bir sondajı 3 boyutlu olarak modelliyor olun, bu ortamları anlamak, yeni keşifler için en güçlü ve en güvenilir yoldur.

Makalede ele alınan başlıca konular şöyle özetlenebilir;

🔹 Hidrotermal ve Metasomatik Kapanlar: Serpantinizasyon ve ani redoks değişimlerinin Altın, Arsenik ve Baz Metal Sülfürleri için mükemmel ortamlar yaratması.

 🔹 Besleyici Fayların Gücü: Yapısal geçirgenliğin, mineralleştirici sıvıların tam olarak nerede biriktiğini ve çökeldiğini belirlemesi.

 🔹 Ortomagmatik Kalıntılar: Parçalanmış Podiform Kromit ve PGE'lerin magma odasına kadar izlenmesi.

 🔹 İkincil Zenginleşme: Yüksek indirgeyici ortamlarda Nikel'in serbest bırakılmasıyla Awaruit ve Heazlewoodit oluşumu.

 🔹 Süperjen Süreçler: Büyük Nikel Laterit yataklarının ardındaki yoğun ayrışma mekaniği.

Akışkanın yollarının ve keskin litolojik sınırların ve geometrisini anlamak, bu sistemlerin kilidini açmanın anahtarıdır.

Mineralization in serpentinites is directly linked to the serpentinization process itself—the hydrothermal alteration of ultramafic rocks such as peridotite and dunite—as well as the distinct geochemical and structural environments this process creates. The mechanisms of mineralization can generally be grouped into magmatic remnants, hydrothermal/metasomatic alterations, and supergene processes.

Particularly in deep drilling projects and complex stratigraphic sequences (such as alternating mudstones, serpentinites, and pyritic limestones), understanding the nature of the mineralization requires well-defined structural controls and accurate identification of alteration zones.

Here are the primary mineralization mechanisms and deposit types associated with serpentinites:

1. Hydrothermal and Metasomatic Processes (Listvenitization and Sulfides)

In deep-seated systems and fault zones, hydrothermal mineralization is the most common type encountered. During the serpentinization reaction, a significant amount of hydrogen gas H₂ is released, creating a highly reducing environment (low oxygen fugacity).

• Listvenitization: When CO₂ and sulfur-rich hydrothermal fluids interact with serpentinized masses, they convert the serpentinite into listvenite—a rock dominated by carbonates (magnesite, ankerite) and quartz. This process creates an excellent structural and chemical trap for gold (Au), arsenic (As), and antimony (Sb) mineralization.
• Redox Precipitation (Sulfide Traps): Reducing hydrothermal fluids experience abrupt redox shifts when they come into contact with sedimentary units possessing different geochemical properties, such as pyritic limestones or mudstones. These contact zones and feeder faults serve as ideal depositional environments for base metal sulfides (copper, nickel, zinc).

2. Structural Controls and the Role of Feeder Faults

Serpentinite masses typically behave in a ductile manner and often act as slip surfaces within shear zones.

• Permeability and Fluid Pathways: Brittle-deformed wall rocks (e.g., limestones) or fault breccias within the serpentinite itself serve as the primary fluid pathways for hydrothermal systems.
• Ore Geometry: In 3D geological modeling, the ore body is frequently concentrated along the hanging wall or footwall of these feeder faults, forming veins that run parallel or at an angle to the lithological contacts. The dip and displacement of the fault ultimately dictate where the mineralizing fluids pool and precipitate.

3. Orthomagmatic Remnants (Chromite and PGEs)

These mineralizations are not a byproduct of serpentinization; rather, they are primary deposits formed within the magma chamber during the initial fractional crystallization of the ultramafic rock.

• Podiform Chromite: Typically found as lenses or pods within Alpine-type peridotites. During serpentinization and subsequent tectonic movements, these rigid ore bodies are dismembered, faulted, and subjected to boudinage, often losing their original continuous geometry.
• Platinum Group Elements (PGEs): These elements can be enriched within the chromitite lenses or associated with base-metal sulfide segregations (such as pentlandite and pyrrhotite).

4. Secondary Magmatic/Sulfide Enrichment (Awaruite and Heazlewoodite)

The strongly reducing environment generated during serpentinization allows primary nickel, originally bound within the crystal lattice of silicates (e.g., in olivine), to be released.

• In environments with low sulfur activity, this liberated nickel combines with iron to form a native metal alloy known as awaruite (Ni₃Fe).
• If sufficient sulfur is present, secondary nickel sulfides such as heazlewoodite (Ni₃S₂) and pentlandite precipitate within fault fractures and surrounding alteration halos.

5. Supergene Processes (Nickel Laterites)

These deposits form through the intense surface weathering of serpentinite masses, typically in tropical or subtropical climates.

• Highly mobile elements like silica and magnesium are leached out of the profile, leaving behind a residual enrichment of iron, nickel, and cobalt.
• Minerals like garnierite (nickeliferous smectite) concentrate deeper in the profile within the saprolite zone, while iron-rich limonitic zones form massive, low-grade deposits closer to the surface.

In Summary:

If economic mineralization is the target within a serpentinite sequence intersected by deep drilling, the primary focus must be detailed structural mapping. The spatial geometry of feeder faults, the extent of listvenite alteration halos, and sharp lithological boundaries (such as pyritic limestones) that trigger abrupt shifts in the pH and redox balance of hydrothermal fluids are the strongest vectors for locating vein systems and massive sulfide accumulations.

KAYNAK

https://allaboutgeology.com/mineralization-in-serpentinites/