Vugs are small to medium irregular cavities, pores, or voids in rocks often lined with mineral crystals whose composition differs from the surrounding rock.
These cavities form via tectonic activities like folding and faulting, fluid exsolution from magma melt, or rock dissolution. They can form in various rocks but are common in igneous rocks.
Usually, vugs are larger than interparticle pores, i.e., the spaces between crystals or gains. Their typical sizes are on a centimeter scale. However, some are much larger.
Also, they can be separate, i.e., not directly connected, except via interparticle pores, i.e., separate vugs or touching. Touching vugs are directly interconnected to each other, forming a network.
The term vuggy is an adjective that describes the quality of having vugs or the formed fabric.
Lastly, these cavities are also known as bugholes, vugular, or vughs. Other less common names are vogle, vugg, vough, or voog. Some authors use the term vughs to mean larger vugs.
How do vugs form?
Vugs form when fluids exsolve from magma or a part of a rock dissolved. Also, they can form from tectonic activities.
1. Tectonic activities
Most vugs form from tectonic activities like folding and faulting. These activities will cause cracks and fissures. Then, minerals like calcite, quartz, zeolites, etc., will later line these voids.
2. Exsolved fluids
Exsolved fluids in highly crystalline magmas can collect and create angular cavities or a vuggy fabric.
This often happens during the final emplacement of plutonic rocks like granite when most of the melt has crystallized. The resultant high viscosity and surrounding crystals will confine exsolving gas bubbles, forming highly irregular shapes.
These cavities will differ from the nearly spheroidal vesicles formed as solidifying magma traps some evolved gases.
Similarly, as pegmatites crystallize quickly inward from outside, they can trap fluids, forming large vugs towards their cores.
Pegmatites are igneous rocks with abnormally large interlocking crystals, some several meters in size.
Lastly, it is possible for volatile fluids to exsolve from magma melt due to decompressing and forming irregular cavities or vugs. Decompression happens as magma rises towards the surface.
3. Dissolution
Solutions, especially acidic, can dissolve rocks, forming small irregular cavities or vugs to extremely large cavities. They are common in rocks high in carbonate and limestone rocks.
For instance, during the development of karts, dissolution will form small pores and cavities, which can enlarge to form caves and caverns.
This process starts at intergranular or interparticle pores, expanding to vugs that cut textural elements.
In some cases, dissolution will create vuggy porosity with irregular voids, sometimes as high as 50% of the volume.
Long-term dissolution can create large vuggy pore networks, allowing people and water to pass through. Examples are Mammoth Cave, Kentucky, and Big Room at Carlsbad Caverns, New Mexico, in the USA.
Associated textures
Some of the textures or fabrics associated with vuggy fabrics are:
1. Miarolitic cavities or miarolitic texture
Miarolitic cavities or miarolitic texture are vugs lined with euhedral vapor phase crystals. These irregular cavities are also rich in incompatible elements.
They are common in granitic rocks, especially the central portion of granitic pegmatites. However, they can occur in phaneritic rocks and other igneous rocks.
These miarolitic cavities are widely spaced, several grains diameter apart.
2. Diktytaxitic texture
Diktytaxitic texture is common in medium-grained basalts or diabase lava flow. It has small angular vugs interspaced among larger grains or laths of pyroxene and plagioclase.
Which minerals line vugs?
Vugs are lined by various minerals, most forming large, euhedral crystals, i.e., well-formed crystals with sharp faces that are easily recognizable. It happens due to unrestricted growth in the cavities.
These cavities have both magmatic or primary and secondary. Primary minerals crystalize from magma, while hydrothermal fluids and groundwater deposit secondary minerals.
Common primary minerals in vugs include quartz, mica, and feldspars like albite or microcline.
Some secondary minerals include exotic minerals like topaz, fluorite, aquamarine, phenakite, tourmaline, and beryl, including milarite.
Others are epidote, scapolite (marialite), dioptase, amazonite, zeolites, carbonates, okenite and apophyllite, zeolites, and sulfate minerals.
Usually, the best zeolite crystals, like those of chabazite, analcime, modernite, heulandite, and stilbite, occur in vugs.
Calcite is a common carbonate mineral. Others are aragonite, dolomite, fibrous malachite, smithsonite, and azurite.
Besides quartz and cristobalite, these irregular cavities will have other silica polymorphs and quartz varieties like agate, chalcedony, smoky quartz, and opal.
Minerals like fayalite can be primary minerals in rhyolites and obsidian (silica-rich volcanic glass) in small amounts, vugs, and lithophysae.
Lastly, nepheline, a feldspathoid mineral, can occur in vugs in volcanic alkali-rich rocks.
Which rocks form vuggy fabrics
Vugs are common in crystalline plutonic rocks like granites and rocks that commonly form pegmatites like granite, diorite, gabbro, and syenite. Plutonic rocks are those that form deep inside the Earth’s crust.
Also, vuggy fabrics are common in rhyolites. Such will have irregularly shaped pockets or cavities with exsolved volatiles in a crystalline matrix. These pockets have vapor-phase crystals that grow freely in the trapped volatile fluid.
Some of the vapor-phase precipitates include quartz and hematite. Some peraluminous rhyolite can have topaz and iron-manganese garnets.
Peraluminous means that the rhyolite rock has more aluminum oxide than combined sodium, potassium, and calcium oxides.
Lastly, basalt vuggy fabrics occur. They are often filled with okenite, apophyllite, and other minerals. Okenite forms cotton balls and is associated with zeolites.
Why are vugs important?
Some of the large vugs host the most beautiful, well-formed, and much-sought crystals of minerals like topaz, tourmaline, beryl, aquamarine, and large quartz crystals.
For instance, Maine and Southern California have pegmatites with gem-quality tourmaline.
Secondly, some host native gold. Fluids like sulfur dioxide, hydrogen sulfide, or hydrogen chloride from magma can result in high sulfidation deposits.
When these deposits interact with groundwater, they will form strong acids. These strong acids can eat wall rocks, leaving a porous, spongy residual silica known as vuggy silica or quartz.
Vuggy silica can host native gold as clustered or discrete grains along fractures and lining vugs.
Frequently Asked Questions (FAQs)
Druses are crystals with projecting faces that line vugs, amygdales, and other cavities. They usually have the same minerals as enclosing rocks, while these minerals differ in vug.
Yes. They also occur on the moon. The 15595 and 15596 lunar mare samples from the Apollo mission are fine-grained, porphyritic basalts in which vugs account for 15-30 % of their volume.
Vugs differ from amygdales since they are cavities, with some only partially filled, while amygdales are partially or fully filled vesicles, i.e., gas bubbles trapped by solidifying lava during a volcanic eruption.