FAQ-1. What is the nature of the "tuff marker beds"?
Distinctive layers within the mine sequence have been used for correlation of orebodies from the earliest mine mapping, where they were called "cross fracture beds-(XFB's)", referring to their tendency to form overprinting fractures and veins in their cherty basal portions, Figure 2, (Perkins, 1984, fig. 4). Two alternative origins have been proposed: a volcaniclastic origin, and evaporite brines altering a pre-existing sediment.
Figure 2. Marker beds of the “B” sequence.
Feldspathic marker bed (A) in drill core. Highly veined grey cherty layer. B is also a marker bed but is here obscured by sphalerite. No, 7 orebody B sequence. Locality unknown.
Croxford (1964) studied No 7 lead-zinc orebody at 6775mN on 8 Level. At this location the hangingwall marker beds (defined "A" sequence described in Perkins (1984) and "B" sequence of Perkins (1997), were obliterated by silica-dolomite, but the main (~60mm) footwall marker was well developed. Croxford studied this marker and many others throughout the mine, as well as similar layers in the district. The most important observation was of textures identified as glass shards indicating a volcanic origin. These textures were highlighted by dusty rutile along selected grain boundaries of microcrystalline potash feldspar. Other observations showed the shard outlnes in a groundmass of galena (Croxford, pers. comm. 1994). Further work on the composition of these layers showed them to be highly potassic, commonly containing up to 85% microcline. Croxford found this to be an unlikely original volcanic composition, but suggested that features such as embayed quartz crystal fragments and the absence of ferromagnesian minerals indicated possible rhyolitic affinities of the parent magma. He concluded that the volcanic glass "underwent post-depositional changes in order to adopt the most stable chemical and physical configuration,-in this case a low-temperature orthoclase as indicated by the low soda content". Since Croxford’s work the layers have been referred to as “tuff marker beds (TMB’s)”.
Although Croxford considered the possibility that a "hydrothermal concept of ore genesis for the lead-zinc ores might equate potash enrichment with hydrothemal mineralization", he preferred the interpretation that the volcanic glass of the cross-fracture beds derived their potassium from connate water. Noting that individual beds (other than the cross-fracture beds) also contained up to 8% potash, that potash feldspar was a dominant component of the mineralized sequence, and that "pellets" in some thin beds exhibited the same textures as the cross-fracture beds, Croxford concluded that the entire sequence was "to a large degree composed of potash-enriched volcanic material". The amount of potash in the gradational tops of the markers and the overlying sequences was regarded as a proxy for the original proportion of tuffaceous material in the sediments. It became routine in mine logging to stain suspected markers, with the intensity of yellow stain indicating the proportion of potash.
A volcanic origin for the Urquhart Shale was challenged by Neudert (1983) who recognized detrital K-feldspar and as overgrowths on grains, as well as the "pellets" described by Croxford and chert in the cross-fracture beds. Neudert found cherty K-feldspar to be most abundant in the Urquhart Shale, but to also occur in other parts of the Upper Mount Isa Group. He regarded the "pellet" forms as detrital in appearance and, in the mineralized zones of facies I, they were intergrown with other rock components and "often represent a major rock component" P. 142. Lath forms were replaced by K-feldspar ; calcite poikilotopes enclosed remnants of the dolomite-K-feldspar matrix and were surrounded by cherty K-feldspar. Neudert used these lines of evidence to indicate that evaporite brines were the source of the alkalis for the formation of authigenic feldspars. He disagreed (p167) with Croxford (1964) that K-feldspar had formed exclusively by volume-for-volume replacement of volcanic material, listing other forms which were not volcanic, and noting that not only the shards consisted of detrital K-feldspar but also the interstitial areas.
According to Paterson, D. J., (pers comm 2003) only about 10% of the marker beds contain recognizable shard textures. In this author's study of the lead-zinc orebodies (Perkins, 1997), the two markers bounding the 7 O/B "B" sequence (12 localities) and the footwall marker (5 localities), have been examined in thin section. In only the most distal locality ( 41000mN. Perkins, 1997, p.74) are shard textures recognized in these three markers. The footwall marker (30mm thick) contains the best-preserved shards (Figure 3) in the basal 3mm. The bed shows slight lamination suggesting very minor reworking. There is minimal dissolution indicating little shortening normal to bedding. It appears to be largely an air fall tephra. The central part of the marker consists of an interlocking mass of sericite. The same bed elsewhere is highly biotitic along strike from where it is a feldspathic chert (Perkins, 1997 p.74).
In the marker at the top of the "B " sequence only ~0.8 mm at the base contains well preserved shards in a carbonaceous groundmass. The remaining 5mm is a fine intergrowth of sericite (Figure 4). Boundary textures with the gradual disappearance of shard outlines into the sericitic area indicate sericitic alteration. These textures indicate that volcaniclastic textures are only locally preserved, within what are unequivocally correlatable layers, and that the feldspars and micas are alteration products.
Figure 3. TMB with shard textures.
Close-packed shard textures in a carbonaceous groundmass. Bedding is parallel to the short side of the photograph. Younging unknown. 7 O/B F/W TMB DDH QZ10 1039.38m. Field of view 1.7mm.
Figure 4. Micaceous alteration in a TMB.
Shard textures in carbonaceous groundmass. Younging is to left of the photograph. The lower grey area (on the right) is a dolomite vein system and the there is a gradational boundary (eg. at A) into fine sericite, indicating micaceous alteration. 7 O/B "B"sequence TMB QZ10 1034.85m. Field of view 1.7mm.
Neudert (1983) noted a direct textural association of authigenic feldspars (albite as well as K-feldspar) with metasomatic carbonate and canvassed the model of a hydrothermal origin for these minerals. He envisaged hydrothermal introduction from basin margin faults at stages from within the first few centimeters of sediment to late diagenesis but prior to final induration. Because of its zonal pattern consistent with other alteration features associated with lead-zinc mineralization, such as bleaching and "buff alteration", pyrrhotite deposition and iron-rich zones, Perkins (1997) argued that the cherty K-feldspar in the TMB's represented an early stage of hydrothermal alteration zoned around the deformed fault contact between the Urquhart shale and altered Eastern Creek Volcanics.
Neudert (1983) p.195 argued that his "feldspathic cherts" which did not contain recognizable shards could either be volcanic ash, deposits from turbulent suspension currents, or evaporites, and preferred that the generic term "tuffaceous marker beds" should not be used. From Perkins (1997), it is recommended that the distinctive and persistent cherty markers used by Mine geologists for correlation continue to be called TMB's. However, this author agrees with Neudert (1983) that the presence of authigenic K-feldspar does not signify a volcanic sequence, but is an alteration product, albeit much younger than that envisaged by Neudert.