Mineralisation at Mount Greenland and Mount Rangitoto, Westland

• Main Author: Jury, A.P.
• Format: Other/Unknown Material
• Language: English
• Published: University of Canterbury. Geological Sciences 1981
• Subjects: Cedar Creek; Greenland
• Online Access: http://hdl.handle.net/10092/5433; https://doi.org/10.26021/5783

Gold- and silver-bearing veins located in Mine Creek, Mount Rangitoto and Cedar Creek, Mount Greenland have been worked sporadically since 1875, for small returns. The Mine Creek workings (1875 - 1904) yielded unknown but presumably small quantities of silver, and possibly gold, before operations ceased due primarily to poor recovery of the lower than expected silver grades encountered. The Cedar Creek workings yielded some 112kg (3,969oz) of gold from mineralised quartz veins, worked intermittently between 1885 and 1941, before operations finally ceased due to the poor financial position of the last company to work the ground, Mount Greenland Gold Limited, and their purchasing of an over-estimated ore reserve. The Mine Creek veins consist of a quartz-pyrite-sphaleritegalena- minor chalcopyrite - electrum ± tourmaline ± calcite assemblage hosted in narrow shear zones in hornfelsed Greenland Group country rock. The mineralised shears which strike northeast and dip at 20 - 40° NW, are located near the crest of the Mine Creek Anticline, a horizontal or gently plunging upright close fold, but cross-cut both bedding and the axial surface of the fold at high angles. Mineralisation of the shears probably occurred during the latest stages of emplacement of the strongly differentiated Triassic (c.214 Ma) aged Kakapotahi Granite. Magmatic mineralising fluids rich in Fe, Pb, Zn, Sb, As, Ag and Au, but poor in Cu, derived from the Kakapotahi Granite, passed upwards along the Mine Creek Fault propylitically altering the Rangitoto Granite adjacent to the fault, and into the mineralised veins. Sub-economic disseminated gold-silver mineralisation accompanied this alteration. The mineralising fluids were probably Co2-rich with a salinity of approximately 18% NaCl equivalent, as indicated by fluid inclusion study. Deposition of the ore minerals in the veins occurred as a fissure infilling process contemporaneous with fault movement, at estimated temperatures of 350° - 500°C and PH20 pressures of 1-2 kbars. The observed ...