Rock Types of Guyana: A Geological Overview

Guyana's geology is dominated by the Guiana Shield—one of Earth's oldest stable continental cores (Archean to Proterozoic, 3.5–1.6 Ga). This ancient basement shapes the country's mineral wealth and landscape.

1. Igneous Rocks (Plutonic & Volcanic)

• Granite / Granodiorite: Widespread in basement complex. Coarse-grained, felsic. Associated with gold, rare earths.
• Diorite / Gabbro: Mafic intrusions. Greenstone belt hosts. Nickel, chromium, platinum potential.
• Rhyolite / Dacite: Volcanic felsic units in greenstone belts (Barama-Mazaruni Supergroup)
• Basalt / Andesite: Volcanic mafic units. Pillow lavas indicate submarine eruption.

2. Metamorphic Rocks (Greenstone Belts & High-Grade Terrains)

• Schist / Gneiss: High-grade metamorphism of sedimentary/volcanic protoliths. Garnet, kyanite, sillimanite zones.
• Phyllite / Slate: Low-grade metamorphism. Cleavage well-developed.
• Quartzite: Metamorphosed sandstone. Resistant ridges (Pakaraima Mountains).
• Iron Formation (BIF): Banded iron — potential iron ore (not yet economic)

3. Sedimentary Rocks (Cover Sequences)

• Roraima Group (Proterozoic): Sandstone, conglomerate, shale — forms iconic tepuis (table mountains: Roraima, Kukenán, Ayanganna). Quartz arenites, cross-bedding.
• Takutu Basin (Mesozoic): Red beds, sandstone, shale — rift-related. Oil/gas potential (exploratory).
• Coastal Plain (Cenozoic): Unconsolidated sands, clays, shell beds, peat — recent alluvial/marine.

Economic Geology Links

• Gold: Orogenic, in greenstone belts (Omai, Aurora, Karouni) — hosted in shear zones in metavolcanics/metasediments
• Bauxite: Lateritic weathering of Roraima sandstone/granite (Linden, Kwakwani, Aroaima)
• Diamonds: Alluvial in Mazaruni, Puruni rivers — kimberlite source not confirmed
• Manganese: Matthews Ridge (supergene enrichment)
• Oil & Gas: Offshore Stabroek Block (ExxonMobil) — Cretaceous turbidites — >11B barrels discovered (transformational)

Guyana's rocks tell a story from Archean craton to Cretaceous rifting to modern petroleum province.