Geological Context

The Gwynfynydd Gold Mine is situated on the eastern flank of the Harlech Dome, approximately 8 kilometres north of Dolgellau. The area is characterised by Cambrian rocks, including the Vigra (Maentworg), Clogau, and Gamlan Formations.

Tectonic Setting

The mine lies within the Dolgellau Gold Belt, a geological formation spanning approximately 26 km long by 6 km wide around the south-eastern and eastern margins of the Harlech Dome. The area has undergone multiple stages of uplift and folding, with the gold-bearing veins cutting through the main fold structures and north-south trending cleavage of the Dome.

Rock Types

The primary host rocks at Gwynfynydd include:

• Clogau Formation: A pyritic and graphitic black mudstone, which is particularly auriferous where intersected by quartz veins.
• Greenstone: Intrusive sills associated with the sedimentary rocks.
• Bryn-Teg Volcanic Formation: Not exposed on the surface but proven in boreholes, this formation is considered a potential source of metals.

Significant Geological Events

1. Deposition of Cambrian sediments and volcanic activity (Bryn-Teg Volcanics).
2. Intrusion of greenstone sills.
3. Formation of gold-bearing quartz veins, dated at 405±6 million years.
4. Post-metamorphic uplift and dewatering at the close of the Caledonian Orogeny.
5. Development of the Trawsfynydd Fault Zone, showing movement from the Ordovician to the Mesozoic.

Economic/Historical Significance

Gwynfynydd Gold Mine holds a significant place in the economic and historical landscape of Welsh gold mining:

Historical Development

• Gold was first discovered at Gwynfynydd in 1863, but commercial development didn't begin until 1887.
• The mine became one of the most important in the Dolgellau Gold Belt, alongside Clogau-St David's Gold Mine.
• Over its history, Gwynfynydd has produced around 45,000 ounces (approximately 1.3 tonnes) of gold.

Economic Impact

• Gwynfynydd and Clogau-St David's mines together contributed about 95% of the total gold output from the Dolgellau Gold Belt.
• The historical mining grade at Gwynfynydd was an impressive 15 grams of gold per tonne.
• Gold from Gwynfynydd has been used in modern awards, including the Glyndŵr Award for excellence in the arts in Wales.

Significant Periods

• The late 19th century saw the mine's development under William Pritchard Morgan, known as the "Welsh gold king".
• The most recent production period was between 1991 and 1999, when 10,000 tonnes of ore were extracted annually.
• In 1986, Queen Elizabeth II was presented with a kilogram of Welsh gold from the mine to celebrate her 60th birthday.

Current Status and Future Prospects

• Commercial operations at Gwynfynydd ceased in 1999 when the gold price was only around US$300 per ounce.
• In 2020, Alba Mineral Resources acquired the exploration rights to Gwynfynydd, aiming to revive gold mining in the area.
• With modern exploration techniques and the current higher gold prices, there's potential for renewed economic activity at Gwynfynydd.

Mineral Paragenesis and Sequencing

Based on the Mindat data provided, the typical mineral assemblage at Gwynfynydd includes:

Acanthite, Calcite, Chalcopyrite, Galena, Goethite, Gold (var. Electrum), Halotrichite, Marcasite, Pickeringite, Pyrite, Pyrrhotite, Quartz, Sphalerite, Sylvanite, Tellurobismuthite

Sequence of Mineral Deposition

1. Primary Sulphides: Pyrite, Pyrrhotite, Chalcopyrite, Galena, Sphalerite
2. Gold Mineralisation: Gold (var. Electrum), possibly associated with Sylvanite and Tellurobismuthite
3. Gangue Minerals: Quartz, Calcite
4. Secondary Minerals: Goethite, Halotrichite, Pickeringite, Acanthite (from alteration of primary sulphides)

The geological context of Gwynfynydd Mine influences the mineral formation through:

• Host Rock Interaction: The Clogau Formation's carbonaceous nature provides a reducing environment favourable for gold deposition.
• Hydrothermal Activity: Post-metamorphic fluids, likely sourced from the Bryn-Teg Volcanics, transported metals and formed the quartz veins.
• Structural Controls: The normal faults and oblique-slip components facilitated fluid flow and mineral deposition.

Associated Minerals

Additional minerals commonly associated with this assemblage in similar settings might include:

• Arsenopyrite: Often found with gold in mesothermal deposits
• Chlorite: As an alteration product in greenstone wallrocks
• Sericite: Observed in alteration zones around veins
• Dolomite: Noted in carbonate alteration of host rocks

These minerals play crucial roles in the paragenetic sequence, with chlorite and sericite indicating hydrothermal alteration of host rocks, and arsenopyrite often closely associated with gold mineralisation.