A new look at the Afton copper mine in the light of mineral distributions, host rock geochemistry and irreversible mineral-solution interactions
Creators: name: Kwong, Yan-Tat John
Publisher: name: University of British Columbia
Year: 2010

description: Embodied in three main parts, this study incorporates field observations, theoretical modelling and geochemical data to interpret the various aspects of the copper mineralization at Afton Mines. Part I describes and interprets mineral distributions within and about the Afton orebody. The copper mineralization occurred in a roof pendant environment characterized by a diversified array of rock types with intricate mutual relationships. Excluding the large scale zoning of pyrite and magnetite established by Carr and Reed (1976) and the apparent association of hydrothermal biotite with ore, distribution of most primary and secondary minerals is rather sporadic. High temperature alterations at Afton probably proceeded with a low ratio of fluid to rock such that primary mineralogy was able to control the appearance of secondary minerals. The absence of intense phyllic and argillic alterations is interpreted to be caused by the lack of abundant meteoric water participating in the hypogene mineralization event. Intense carbonate alteration, however, is likely to be indicative of concentrated juvenile hydrothermal activity. The mode of occurrence of ore minerals, on the other hand, suggests that supergene alteration at Afton was dominated by irreversible mineral-solution interactions. The extent of the alteration at depth appears to be largely fracture-controlled. Part II presents a thermodynamic reconstruction of the supergene alteration at Afton. It is demonstrated that the dominance of native copper and the lack of copper enrichment in the supergene zone are related to the relatively mafic composition of the wall rocks and the absence of abundant sulfides in the hypogene zone. Supergene alteration of porphyry copper deposits in general can be interpreted in the framework of a log f-log f plot with respect to two arbitrary "limiting" curves characterized by extreme values of acidic and basic pH respectively. The closer a reacting fluid evolves parallel to the upper or acidic "limiting" curve, the more likely is the formation of a well-developed leached cap with an enriched supergene ore blanket underneath. In contrast, a reacting fluid evolving parallel and close to the lower or basic "limiting" curve will lead to a spectrum of copper oxides and native copper with little enrichment. Part III summarizes and relates major element geochemical data, petrographic observations and field evidence to the evolution and copper mineralization of the various intrusive phases of the Iron Mask batholith. The four major phases of the batholith, namely, the Iron Mask Hybrid, Pothook, Sugarloaf and Cherry Creek units, appear to be related by the process of fractional crystallization. The Afton protore is likely a by-product of magmatic differentiation generated at the early Cherry Creek stage when portions of the fractionating magma intruded into a subvolcanic environment. Mineralogical characteristics of the orebody, including the scarcity of molybdenite, reflect significant influence of host rock geochemistry on late stage copper mineralization and the associated hypogene alteration. Taking into account all the information acquired, the evolution of the Afton orebody appears to have involved four major events: i) fractional crystallization of a parent Iron Mask magma at depth with continual magnetite separation; ii) transfer of portions of the early Cherry Creek magma to a subvolcanic environment where rapid cooling enhanced supersaturation of copper sulfides and where some loss of sulfur occurred through volcanic emanations; iii) local trapping of late magmatic fluid to remobilize and concentrate the originally disseminated sulfides to form the hypogene ore; and iv) subsequent supergene alteration constrained to take place in a basic pH domain to stabilize abundant native copper without much enrichment.