Copper in the North Mountain Basalts, Nova Scotia

Abstract

The North Mountain basalts are quartz-normative tholeiites stretching from Cape Split to Whipple, Nova Scotia. The lavas have been subjected to zeolite facies metamorphism. Native copper occurs in trace quantities associated with zeolites and in some fault zones such as the one at Cape d'Or from which copper was mined between 1900 and 1907.

A total of 56 rocks were analysed for Cu, Zn, Mn, Co, Ti, Cr and Ni, using atomic absorption spectroscopy. Variation diagrams of Ti, Cr and Ni show a rather well-defined differentiation trend indicating that these metals probably remained relatively immobile during burial metamorphism. The Cu does not correlate with Zn, while poor co-variances exist between Zn and Mn, and Cu and Mn. On a Cu - Zn - Mn diagram, zinc enrichment occurs only at Ross Creek and in a fault zone at Cape Split.

When Cu, Zn, and Mn were plotted against the differentiation index, Ti/Cr + Ti, generally poor correlations were obtained, showing that the distribution of Cu, Zn and Mn was controlled by different processes from those affecting Ti and Cr. Prior to alteration, Cu, Zn and Mn are found in interstitial glass or replace Fe2+ in magnetite and pyroxene. During alteration by hydrothermal brines Cu, Zn and Mn were liberated, first from the glass and then from pyroxene and magnetite. Copper was precipitated by reduction to Cuo indicating a low fS2. Oxidation of magnetite to hematite supplied electrons for copper reduction. Precipitation of Zn is made difficult by the lack of sulphide. Some Zn, Mn and Cu may be absorbed by clays, chlorite or zeolite.

The North Mountain basalts have not bee metamorphosed to the same degree as the Keweenawan lavas (prehnite-pumpel-lyite facies). The behaviour of Cu, Zn and Ni in the North Mountain basalts is similar to the Keweenawan lavas, where Cuo is deposited, Zn is leached away and Ni remains unchanged except for zeolite dilution. However, at North Mountain, copper concentration does not reach economic levels as in the Keweenaw Peninsula.

Keywords: Pages: 95 Supervisors: D. B. Clarke