|Mournes - Lindsay's Leap, Thomas's Mountain||Down|
|Disused quarry at Lindsay's Leap, Thomas's Mountain, Mournes, Co. Down; old face obscured by extensive spoil heap of quarry scree; view from head of old mineral line leading to small harbour below.|
|Site Type: ||Quarry (disused)|
|Site Status: ||ASSI|
|District: ||Down District Council|
|Grid Reference: ||J373291|
|Rock Age: ||Tertiary (Eocene)|
|Rock Name: ||Granite|
|Rock Type: ||Granite, Greisen, Greisenized Granite|
|Minerals: ||Fluorite, Stilbite, Topaz|
|Other interest: ||Ornamental stone, mineral vein, Intrusion, Open moorland for sheep grazing and hill walking.|
Summary of site:
There are many veins of rock intruded into the Mourne Mountains granites but in this abandoned quarry there are unusual green veins of a rock called greisen that actually replaces the granite, in this case the second (G2) of the five in the Mournes.
The veins are up to 8 cm wide with rather diffuse walls and are near vertical or steeply inclined in parallel with the wall of country rock surrounding the granite. Examined with a hand lens they contain fine-grained green mica and grey quartz, often concentrated along the margins and in the very centre of the veins. The mica is often associated with topaz, colourless fluorite and dark green chlorite. Stilbite, a calcium aluminium silicate of a group of minerals called zeolites, has also been found. A 1930s chemical analysis of the mica suggested it was siderophyllite, a rare mineral.
In the late stages of cooling of granite magma, hot, chemically active fluids (excluding water) and gases are released and migrate along natural fractures such as joints. They are rich in volatile elements such as fluorine, chlorine, boron and hydrogen and as they migrate they alter the mineralogy of the walls of the veins. The process is called pneumatolysis and it creates rocks called greisens along the altered faces of the veins. At Lindsay's Leap, feldspars and biotite (a mica in the granite) have been replaced by the green mica and topaz. This change displaced calcium, potassium and sodium, some of which can be accounted for in the formation of the zeolite, stilbite, but not all. The alteration of the feldspar in the walls of the veins to kaolin, a clay mineral, was probably the first of a sequence of chemical changes. The biotite mica of the granite was chloritised to fluorite, also at an early stage and the other minerals following later. The quartz present is probably recrystallized original quartz.
Although greisens are recorded from many localities in the Mournes, Lindsay's Leap is by far the best example. Vein material is plentiful, particularly in the spoils, making this an ideal teaching locality and, despite its designation, a good site for judicious and sensitive collecting. The quarry is a designated Area of Special Scientific Interest (ASSI) conferring the highest level of protection, although there are no obvious threats.