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Geology of Endako Mine, British Columbia Dawson, Kenneth Murray

Abstract

Endako molybdenite deposit and surrounding area were examined in detail, with particular attention to petrology and genesis of Topley Intrusions, regional structural setting and structural history of Endako orebody, sulphide and alteration mineralogy, fluid inclusion geothermometry of vein quartz, and minor element content of orebody pyrite. The host rock, Endako Quartz Monzonite, is a relatively early phase of Topley Intrusions that range in composition from diorite to granite, and in age from Middle Jurassic to Lower Cretaceous. Endako stockwork was localized by wrench faulting and doming generated by cooling of the batholith and intrusion of pre-mineral dykes. Hydrothermal fluids that altered and mineralized the stockwork were generated contemporaneously with the cooling of Endako pluton. Abundant early potassic alteration and relatively high fluid inclusion paleotemperatures attest to the paramagmatic affiliation of vein and alteration mineral assemblages. Cross-cutting relations indicate a relative age sequence among the potassic, sericitic and argillic alteration stages that is in agreement with a chemical control based primarily on the activity ratio of K+/H+. Alteration zonation suggests decreasing temperature from the ore zone towards the south, implying similar temperature variations for sulphide deposition. Fluid inclusion studies support this trend. Average minimum filling temperatures for fluid inclusions in quartz veins from potassic, sericitic and argillic assemblages are 500°C+, 480°C and 400°C, respectively. Minor element content of pyrite shows a similar temperature-dependent trend. In Q-mode factor analysis of 12-element spectrochemical data from 67 orebody pyrites, Mn, Ni and Sn define Factor I which coincides with the ore zone, whereas Factor II (Co and Cu) coincides with the pyrite zone. Correlation of Factors I and II to ore and pyrite zone respectively implies that the factors are related to mineralizing processes that give rise to the two mineralogically distinct zones.

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