ODDS’N’SODS — Nova Scotia’s Meguma terrains

Many geologists who visit Nova Scotia scratch their heads over the great number of gold occurrences in the fold belts of Ordovician-age metasediments, known locally as Meguma terrains.

They do so because hundreds of gold showings, as well as old reported occurrences, prospects and past-producing mines, occur in fold belts stretching from one end of the mainland to the other.

One well-known geochemist suggested, years ago, that the amount of gold occurring in such a large terrain is one of the world’s most startling geochemical anomalies. And, in gold exploration circles today, there is a growing school of thought according to which the anomalous amounts of gold in the Meguma belts are much more than a unique curiosity.

A simplified theory, which is gaining more and more acceptance as a working hypothesis, suggests that the yellow metal in these types of deposits was leached (or remobilized) from underlying and/or surrounding rocks containing elevated trace amounts of gold. It was then transported in hydrothermal solutions to favorable structures, or chemically receptive host rocks, where it precipitated with other metals and minerals in large structural traps and carbonaceous, sedimentary rocks in higher structural levels of reduced pressures and temperatures.

One of the most promising regions in the world, where gold explorers can use this basic concept when exploring for large deposits, exists in numerous Carboniferous-age sedimentary basins in Nova Scotia. Unfortunately, few geologists realize the Meguma terrains in the province contain so much gold, with several basins containing the favorable plumbing systems (deep rifts and allied faults), heat generators (syntectonic granite intrusives), traps (large dilation zones at the junction of intersecting deep-seated faults) and favorable hosts (thick beds of carbon-bearing shales, limestone and sandstones) — all of which are necessary for the largest types of gold deposits.

Of all the world’s major-size gold deposit types, those of Nova Scotia have the greatest potential for being similar to those that occur in the Carlin region of Nevada. By definition, these are deposits in carbonaceous shales and limestones and in which gold solutions have been pumped back into beds of rock containing organic matter which precipitated gold and other metals above mineralization conduits.

In several of the province’s Carboniferous-age basins, and elsewhere in the Maritimes, large blocks of strata were deeply rifted (or faulted). Faulted sediments of the older, gold-rich, Meguma-age rocks occur below many fault blocks.

In some areas, the Meguma was faulted against older rock near a major suture which became part of a rift system in late geological-structural events and which is now part of a vast plumbing system through which great waves of hydrothermal solutions moved, at times, during the Carboniferous period. These hydrothermal systems were generated by great heat centres which, in turn, were developed by underlying granite plutons that penetrated the rift zones and allied structures at various intervals during the Carboniferous period. These thermal units are manifest in huge deposits of iron carbonates (ankerite, siderite, etc.) which occur in various rift terrains. The deposits consist of large replacement lodes, fissure veins, replaced beds, stockworks and breccia zones.

Large iron carbonate zones are the surface manifestation for great volumes of carbon dioxide which was transported, in the presence of iron-bearing solutions, through large conduits upwards into thick beds of carbon-bearing, sedimentary rocks.

Recent work in Nova Scotia has identified gold in thick beds of black shale near large iron carbonate lodes adjacent to a rift. Ongoing geochemical work by the private sector and the Nova Scotia Department of Natural Resources has found geochemical anomalies in streams and tills near promising structural targets. Large gold-bearing conduits have been recognized.

— Avard Hudgins, a frequent contributor, resides in Truro, N.S.