Greenlaws Mine, Daddry Shield, Weardale

Geological Context

Greenlaws Mine is situated in the Northern Pennine Orefield, a region shaped by complex geological processes over millions of years. The area is characterised by Carboniferous sedimentary rocks, primarily limestones, sandstones, and shales, deposited approximately 359-299 million years ago. These rocks were subsequently affected by tectonic activity and mineralisation events.

The tectonic setting of Greenlaws Mine is influenced by the presence of the Weardale Granite, a Devonian intrusion that underlies much of the region. This granite played a crucial role in the area's mineralisation, creating a thermal gradient that drove hydrothermal fluid circulation.

Significant geological events that shaped the area include:

1. Deposition of Carboniferous sediments in a shallow marine environment
2. Intrusion of the Weardale Granite during the Devonian period
3. Uplift and faulting during the Variscan Orogeny
4. Hydrothermal mineralisation events during the Carboniferous and Permian periods

The primary host rock at Greenlaws Mine is the Great Limestone, a Carboniferous bioclastic packstone rich in crinoid debris, brachiopods, and corals. This limestone is thickly bedded with interbeds of shaly mudstone partings.

Faulting and Flats

The Northern Pennine Orefield, including Greenlaws Mine, is characterised by extensive faulting and the presence of mineralised "flats". These structural elements played a crucial role in the distribution and concentration of minerals in the area.

Faulting: The region is crisscrossed by a network of faults, primarily trending east-northeast to west-southwest. These faults were instrumental in creating pathways for mineralising fluids to circulate through the Carboniferous limestone sequence. The most significant faults in the area are often associated with the richest ore deposits.

Flats: "Flats" are horizontal or gently inclined replacement orebodies that formed along bedding planes in the limestone. These flats are particularly common in the Great Limestone, which hosts much of the mineralisation at Greenlaws Mine. The flats often extend laterally from vertical or steeply inclined veins associated with faults.

Economic and Historical Significance

Greenlaws Mine has been a significant part of the Weardale mining industry, which has played a crucial role in the region's economy for centuries. Mining activity in the area dates back to at least the 14th century, with leases granted by the Bishop of Durham. The mine itself has been active since 1725.

During its economic heyday, Greenlaws Mine was owned by the Blackett-Beaumont Lead Mining Company and later passed to the Weardale Lead Company towards the end of the 19th century. The mine primarily produced lead ore, but also yielded fluorite (fluorspar) and other minerals.

The decline of lead mining in the late 19th century led to a shift towards fluorspar production, which extended the life of the mine. Today, the site is protected as a Scheduled Ancient Monument and is on the Buildings at Risk Register. While commercial mining has ceased, there are ongoing specimen recovery operations at Greenlaws Mine, contributing to its geological and mineralogical significance.

Mineral Paragenesis and Sequencing

The mineral assemblage at Greenlaws Mine, as indicated by the Mindat data, consists of calcite (CaCO₃), cerussite (PbCO₃), fluorite (CaF₂), galena (PbS), quartz (SiO₂), siderite (FeCO₃), smithsonite (ZnCO₃), and sphalerite (ZnS). This assemblage is typical of Mississippi Valley-Type (MVT) deposits, which are common in the Northern Pennine Orefield.

The paragenetic sequence likely follows this general order:

1. Early-stage minerals: Quartz (SiO₂), siderite (FeCO₃)
2. Main-stage minerals: Fluorite (CaF₂), galena (PbS), sphalerite (ZnS)
3. Late-stage minerals: Calcite (CaCO₃), cerussite (PbCO₃), smithsonite (ZnCO₃)

The formation of these minerals was influenced by the circulation of metal-rich brines through the fractured Carboniferous limestones. The Weardale Granite provided the heat source for this hydrothermal activity.

The presence of faults and flats significantly influenced the mineral paragenesis and sequencing:

1. Fault-related mineralisation: The main stage minerals (fluorite, galena, and sphalerite) often precipitated along fault planes and in associated breccias. These fault-filled veins served as the primary conduits for mineralising fluids.
2. Flat formation: As the fluids moved laterally from the faults, they reacted with the limestone host rock, creating the characteristic flat orebodies. This process involved both dissolution of the limestone and precipitation of ore minerals.
3. Zoning: The interaction between faults and flats often resulted in a zoned distribution of minerals, with different assemblages occurring in the vertical veins compared to the horizontal flats.

Alteration Processes:

Galena (PbS) often alters to cerussite (PbCO₃) through oxidation and carbonation processes. Similarly, sphalerite (ZnS) can alter to smithsonite (ZnCO₃) under similar conditions. These alterations typically occur in the upper, oxidised zones of the deposit.

Factors Influencing Mineral Formation:

1. Temperature: Decreasing temperatures as fluids moved upwards through fractures
2. Pressure: Changes in fluid pressure due to tectonic activity and depth
3. Fluid Composition: Metal-rich brines interacting with carbonate host rocks
4. Time: Multiple mineralisation events over millions of years
5. Structural controls: Faults and flats guiding fluid flow and mineral precipitation

Associated Minerals

While not listed in Mindat data, other minerals commonly associated with this type of deposit in the Northern Pennine Orefield include:

1. Dolomite (Ca,Mg(CO₃)₂)
2. Ankerite (Ca(Fe,Mg,Mn)(CO₃)₂)
3. Aragonite  (CaCO₃)
4. Pyrite (FeS₂)
5. Late stage Quartz (SiO₂)

These minerals often play important roles in the paragenetic sequence. For example, aragonite is typical late-stage mineral while pyrite can form throughout the sequence. Ankerite/dolomite may form as an alteration product of earlier carbonate minerals.

The interplay between faulting and flat formation created a complex network of mineralised structures, contributing to the rich and varied mineral assemblage found at Greenlaws Mine. This structural control on mineralisation is a key feature of Mississippi Valley-Type deposits in the Northern Pennine Orefield and significantly influenced the economic viability of mining operations in the area.

Greenlaws Mine, with its rich mineralogical diversity, continues to be of interest to geologists and mineral collectors, contributing to our understanding of the complex geological processes that shaped the Northern Pennine Orefield.