UNDERSTANDING SPHALERITE COLLOFORM GROWTH IN PRIMARY SULPHIDES USING EBSD AND S ISOTOPE ANALYSIS

Colloform textures are common in sulphide ore deposits and involve the sequential formation of texturally distinct microcrystalline layers. Here we present the first detailed study of the microstructure and S-isotope composition of colloform sphalerite from the Galmoy ore body in Ireland as part of an investigation into the utility of using colloform structures as proxies for changing environmental factors during mineral deposit formation. Detailed petrographic, crystallographic, chemical and in situ laser S isotope data reveal a series of layers which can be explained in terms of an 8 stage development history. In situ laser S-isotope analysis reveals dramatic variations in the δ³⁴S signature from -25‰ to +10‰, indicating the colloforms developed from two distinct sources of sulphur. Values lighter than around -5‰ suggest a bacteriogenic S source, whereas heavier values suggest a hydrothermal source. Stage 1 in the colloform development is characterised by a δ³⁴S of -25‰, a strongly bacteriogenic signature. Stages 2-4 show variable δ³⁴S between -20 and +5‰ resulting from mixing of bacteriogenic and hydrothermal S-sources. During stages 2-4 there has clearly been an excursion of a hydrothermal S source, which upon mixing with the bacteriogenic source has resulted in variable δ³⁴S. By stage 5 the principal S source is a hydrothermal one and characterised by a δ³⁴S of +9‰. Stage 6 has a variable δ³⁴S of between -4 and -8‰ suggesting that the hydrothermal source is waning and by stage 7 the S source is bacteriogenic with a δ³⁴S of -12 to -18‰. Crystallographic analysis using electron backscatter diffraction (EBSD) indicates that within the samples there is variation in sphalerite crystallographic preferred orientation (CPO) about either <100>, <110> or <111> crystallographic axes, and in some cases no preferred orientation developed. While there is some correlation between changes in CPO and the S source, it is remarkable that the dramatic environmental changes recorded only heralds that the CPO will vary, not what orientation will subsequently develop suggesting S source is not a primary control on sphalerite CPO. Thus, factors such as growth rate, temperature, degree of supersaturation and trace element availability may be important in controlling the crystallographic development of sphalerite colloforms.