Mineralogy and morphology of speleothems from Bulmer Cavern, North West Nelson, New Zealand

Cross, Travis.

Six types of speleothem were investigated from Bulmer Cavern, Northwest Nelson, New Zealand, plus selected sediments and marble bedrock. Mineralogical and morphological investigation wasundertaken using XRD, XRF, electron microprobe and SEM to determine speleothem mineralogy, structure and speleogenesis. It was hoped the source of elements found in the speleothems could be determined by bedrock analyses. Mineralogical analyses determined five of the speleothem types are sulphates with one carbonate. Two of the investigated speleothems, which are known as -· and • are fibrous, growing from cave walls and/or ceiling. Both are composed of gypsum (CaS04.2H20). Angel hair grows up to 1050 mm long and between ~53-500 IJ.m wide and is seen as two morphologies. One morphology is a single prismatic crystal fibre, sometimes with a minor surface powder coating and the other with extensive secondary crystal and powder growths on the fibre surface. Cave wool is made up of numerous randomly oriented inter-fingered acicular gypsum crystals, typically between ~2-12 IJ.m thick. A floor deposit called - was investigated, which was determined to have a hydromagnesite (Mg4(C03)4(0H)2AH20) composition. The moonmilk blankets a ~50 m length of passage from a thin covering, up to ~40 cm thick. Another floor deposit investigated were - which cover a ~ 10 m2 area of passage with composed of white, broken-up chunks ofmacrocrystalline aggregates of gypsum between ~5-30 mm thick. are composed of swallowtail twinned gypsum, up to ~60 cm in length, growing at random orientations from relatively dry, sandy sediments on the cave floor. rock fractures, and are composed ofmirabilite (Na2S04.10H20). grow from Analyses suggest marble host rock is a relatively pure carbonate. Most of the elements found in the speleothems can be derived from the marble and water i.e.Ca, Mg, H, 0, Sr, Ba and C, with the exception of Na and S. Sediment and cave crusts contained higher element concentrations than in the host rock.

v, 96 leaves : ill., maps ; 30 cm. + 3 folded sheets.

2003Cross