Mineralisation at Mount Greenland and Mount Rangitoto, Westland
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
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 paragenetic sequence pyrite - sphalerite - galena - chalcopyrite - electrum,in order of decreasing age, reflects deposition of the ore minerals under conditions of decreasing activities of sulphur and FeS, probably accompanied by decreasing temperature. The major gold- and silver-bearing minerals, electrum and galena respectively, occur as replacements of earlier formed pyrite, a chemically favourable host for these minerals. The Cedar Creek veins consist of up to a few per cent in total, of pyrite - chalcopyrite - arsenopyrite - sphalerite - galena - boulangerite - (jamesonite) - bournonite - tetrahedrite - gold hosted in a quartz ± calcite vein system up to 4m wide. Although the vein system located in the normally faulted crest of the William Tell Anticline is traceable for over a kilometre, known economic gold mineralisation is restricted to the ore shoot worked in the William Tell levels. The ore shoot which plunges steeply to the northwest along the strike of the vein and reputedly thins with depth, is probably located in a former zone of high permeability within the vein system, possibly due to increased fracturing. Vein formation and mineralisation occurred penecontemporaneously with emplacement of the Rangitoto Granite into the previously folded overlying Greenland Group sediments, approximately 295 - 314 Ma ago. Magmatically derived mineralising fluids, metals and sulphur migrated upwards, probably along bedding planes, into the crest of the William Tell Anticline and thence upwards along the William Tell Fault, parallel to the axial plane of the anticline. Deposition of the ore minerals from the fluids occurred as fissure infillings associated with fault movement, at estimated temperatures of 230 - 400°C and maximum PH20 pressures of less than 2 kbars. The observed paragenesis pyrite - arsenopyrite – chalcopyrite - sphalerite - galena - boulangerite - bournonite - tetrahedrite reflects ore deposition at successively lower temperatures, the late stage sulphosalts being deposited at estimated temperatures of 230° - 300° C. Changes in the composition of the mineralising fluids accompanied the decreasing temperature. The activity of sulphur decreased from approximately 10-12atm – 10-4atm during deposition, and was accompanied by the decreasing activity of PbS. Increases in the activity of Sb2S3 and Cu concentrations accompanied the above decreases resulting in the late stage paragenesis (galena - bou1angerite - (jamesonite) - bournonite - tetrahedrite), observed. Gold, the youngest of the ore minerals present, was probably deposited as a response to cooling of the mineralising fluids resulting in the decreased solubility of gold in the aqueous solutions.