The clay mineral assemblages and geology of the Mahakirau hydrothermal alteration zone, Coromandel Peninsula (NZ)

Wall clays and vein silicates from the fossil hydrothermal system at Mahakirau, Coromandel Peninsula are shown to consist of discrete mineral assemblages. Those in quartz veins are: - K-feldspar - K-mica - kaolinite K-feldspar - K-mica kaolinite - hematite K-mica K-mica – kaolinite K-mica - Mg-Chlorite The last three assemblages also occur as wall clay and may be used to subdivide the quartz sericite or phyllic alteration types previously used for Coromandel-style wall rock alteration. The mafic phyllosilicates are significant in distinguishing wall clay assemblages, which include: - K-mica K-mica - kaolinite K-mica - Mg-Chlorite K-mica - Fe/Mg-Chlorite Fe/Mg-Chlorite 29Å chlorite-vermiculite (± ~low K, K-mica) Detailed XRD and EPMA analysis of the above assemblages is presented, and the effectiveness of the techniques compared. The distinctive textures and colouring of wall clays correlate well with mineral assemblages, making possible the mapping of clay mineral assemblages in the field. The 3km by 6km portion of the elongated, NNE trending Mahakirau alteration zone (MAZ) investigated here contains quartz veins having fluid inclusion filling temperatures ranging from 290° C at an inferred upflow zone in the south to 250° in the north. A relatively dilute fluid (0.1 - 0.5 eq. wt.% NaCl) has been responsible for predominantly late stage precipitation of Ag, Te, Sb sulphosalts and Cu,Pb,Zn sulphides. Metallic mineral assemblages are :- pyrite pyrite - chalcopyrite (± Sb sulfosalts) pyrite - chalcopyrite - sphalerite - galena - (± Sb sulfosalts) pyrite - chalcopyrite - sphalerite - galena - hessite - (± Sb sulfosalts) The early to late sulphides in any particular vein are usually precipitated in the above sequence of assemblages. There are indications of similar sequences of early to late vein silicate assemblages but they are not investigated in detail. It is suggested that at Mahakirau these sequences are the result of an ordered set of preferential mineral precipitations and relatively steady state (non chaotic) deposition mechanisms. This contrasts with the situation where the rock-water ratio is high as in wall rock alteration zones, where primary igneous minerals having a range of susceptibilities to dissolution buffer changes to fluid composition. The absence of an unstable primary mineral limits alteration. With a late saturated fissure fluid the absence of a precipitating phase will limit modifications to fluid composition. Relatively complex vein silicate assemblages correlate in general with similarly complex metallic mineral assemblages indicating that whatever the mechanism of deposition, the effect on both silicate and sulphide minerals has been similar. Do vein silicate and sulphide mineral assemblages precipitated from the same fluid have predictable inter-relationships? Detailed investigation of the identity and relative amounts of all silicate and sulphide phases in sequences of individual vein zones or selvages should be carried out. Multilevel underground mines elsewhere on Coromandel Peninsula will be better locations for further investigation, also, more 'barren' quartz veins need to be studied. Observations of lateral and vertical variations of vein temperature and of silicate and sulphide assemblage sequences may allow a more dynamic appreciation of epithermal systems