Abstract

The recently proposed syndeformational model for copper mineralization at Mount Isa has tremendous implications for metallogenesis and exploration, and hence, a regional assessment of its validity was made. Detailed studies on the structural setting, timing, and microstructural development of four brecciated sediment-hosted copper deposits situated northwest of Mount Isa, plus the assessment of many small prospects in the same area, reveal that all primary copper mineralization is strikingly similar and involves syndeformational (syn-D 3 ) veining and alteration. Variation in and between deposits can be attributed to their host-rock lithologies and local structural relationships. "Silica dolomite" replacement as recognized at Mount Isa mine is associated with all copper mineralization in dolomitic host rock. Sulfide (+ chlorite) replacement in arenaceous host rock, for example at the Mammoth deposit, occurs concomitantly with silica dolomite alteration in other deposits that comprise both host-rock types. The matching alteration history of the deposits indicates a common origin for the mineralization, and together with a large number of similar small-sized copper occurrences, suggests that a major regional copper mineralizing event occurred during the third deformation phase throughout the western Mount Isa block. Local structural relationships, common to several deposits, such as the presence and degree of bedding discordance across low-angle (D 1 ) faults together with later (D 2 ) folding, may constitute suitable structural traps for subsequent (D 3 ) mineralization. Therefore, the structural geometry, together with the alteration features, is essential in exploration and assessment of prospects.

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