The Petrochemistry of Altered Volcanic Rocks Surrounding the Louvem Copper Deposit, Val d'Or, Quebec

Abstract

The Louvem copper deposit, a carrot-shaped body of mineralized silicic pyroclastic rock, appears generally conformable with surrounding, steeply dipping volcanic rocks, but otherwise closely resembles the cross-cutting feeder pipes that underlie many Archean stratiform volcanogenic massive sulfide deposits. It is, like many such deposits, associated with peraluminous and calc-alkaline rocks in the felsic upper portion of a volcanic sequence.Naming of the Louvem volcanic host rocks by means of their chemical composition is rendered difficult by intense local alteration which has changed their original compositions. Of the four classification schemes tried, that based on sample SiO2 content appears to provide results that are least affected by this alteration and which therefore reflect most clearly the original compositions of the rocks surrounding the ore deposit.The calc-alkaline nature of Louvem volcanic rocks is apparent even for very altered near-ore samples. This is revealed by Ol–Ne–Qz and AFM diagrams, which appear to be suitable for the genetic classification of such altered rocks.The chemical nature of the wallrock alteration in and around the deposit is revealed by certain petrologic diagrams. All rocks in the study area show magnesium enrichment, but no petrologic diagram illustrates this very clearly. Outside the orebody, the alteration consists mainly of Na and Ca depletion, and those diagrams which show such depletion are the most useful. Of these, the AKF, AFM, and ACF plots appear to be most practical.