Mineralisation and Hydrothermal Alteration

Hydrothermal alteration facies

Above: Diagrammatic cross-section of the basin margin fault zone and the area containing the Rhynie cherts, North of Rhynie village, showing the variation in hydrothermal alteration facies. The K-feldspar-quartz assemblage represents the zone of highest alteration, with the highest gold and arsenic contents, and mainly affects the slices of sandstone and andesite within the basin margin fault zone.  

 

The Early Devonian hydrothermal complex at Rhynie is now dismembered, principally by vertical faulting, and so different levels are now exposed within and adjacent to the basin margin fault zone. Coring and trenching at various locations in the different fault blocks along the basin margin fault zone has enabled us to sample the rocks and examine the different assemblages of minerals associated with the hydrothermal mineralisation and alteration.  

The highest level of the hydrothermal complex is found in the Dryden Flags Formation on the south-eastern side of the fault zone and contains sinters, the Rhynie and Windyfield cherts, which are the classic surface expression of hydrothermal activity (see inset above). This sequence is weakly mineralised and contains thin calcite and quartz veins, sparse pyrite and patchy gold, arsenic, tungsten and molybdenum anomalies. The alteration assemblage of the sediments is chlorite and calcite plus silica alteration in the sinter-rich intervals.  

Deeper levels are exposed within the basin margin fault zone. Here intense silification, brecciation and veining can be traced continuously along strike for 2km and follow the basement- Devonian contact (inset right) (see also the section on Geology and Setting). This is presumed to be the main conduit for rising hydrothermal fluids at the current erosion level.

 

 

 

 

Silicified fault breccia

Right: Cored section through intense brecciation and silicification in the basin margin fault zone. Many of the brecciated fragments are coated by numerous overlays of chert.

 

Between this contact and the sinter-bearing sequence lay fault slices of sandstone and andesite that are intensely altered and veined (see insets above, right and below). These slices are interpreted to represent the faulted lateral equivalents of the Quarry Hill and Tillybrachty Sandstone Formations in the Rhynie area. Evidence from textures such as bladed quartz (see inset right) and the mineralogy, K-feldspar in veins (inset below left) and altered rock (inset below right), indicate this was a zone of boiling.

 

Bladed quartz crystals in brecciated andesite

Right: Bladed quartz crystals (arrowed) in brecciated andesite from one of the many fault slices in the basin margin fault zone at Rhynie.

 

Quartz and K-feldspar vein Altered rock
Above: Quartz and K-feldspar vein. The K-feldspar is flesh pink in colour (for a close-up click on the image ). Above: Highly altered brecciated andesite from the basin margin fault zone. The andesite has been altered to quartz and K-feldspar (adularia) (for a close-up click on the image).

 

The veins and altered rocks in the fault slices, especially within the andesites, contain the highest levels of gold, arsenic, and tungsten yet found at Rhynie (see cross-sections below).

 

Arsenic in boreholes

Above: Diagrammatic cross-section of the basin margin fault zone and the area containing the Rhynie cherts, North of Rhynie village, showing the variations in arsenic content of rocks recovered from selected cored boreholes (MRD/1, MRD/2, MRD/3, 97/2, 97/8 and 97/9).

 

Gold in boreholes

Above: Diagrammatic cross-section of the basin margin fault zone and the area containing the Rhynie cherts, showing the variations in gold content of rocks recovered from selected cored boreholes (MRD/1, MRD/2, MRD/3, 97/2, 97/8 and 97/9).

 

Geochemical analysis of the rocks at Rhynie suggest there is a correlation between the abundance of pyrite and gold. Ion probe analysis of pyrite crystals in altered andesites from the basin margin fault zone indicates most of the gold is probably located in zones of arsenian pyrite (see inset right). 

Arsenian pyrite

Right: Polished thin section of altered andesite showing a cluster of pyrite crystals viewed as a back-scattered image on the  SEM. The bright zones relate to arsenic-rich areas (arsenian pyrite) in which gold has been detected by ion probe analysis (Rice et al. 1995).
 

The hydrothermal fluids were dominated by meteoric water (Rice et al. 1995). The features described above are typical of a low- sulphidation epithermal system.

 

The information and images on this page have kindly been supplied by Dr. Clive Rice, Dept. of Geology & Petroleum Geology, University of Aberdeen.