The structure and petrology of rocks close to the Broken Hill Lode.

Abstract

The thesis consists of a review of Broken Hill ‘mine area’ structure and the Archaean stratigraphy on which it is based. A fourfold division was made.

Part 1: This contains a review and discussion of the structural geology close to the main lode. The concept of ‘en echelon’ folding as opposed to ‘parallel’ folding (advocated by past geologists) was introduced in the latest structural interpretation.

Part 2: The second section is a comparative petrographic study of several ‘marker horizons’ at Broken Hill viz. Potosi Gneiss, Hanging Wall Gneiss, Aplite. The recognition of several distinct horizons of each of these rock types was stressed in Part 1. Potosi-like gneisses occur at three separate stratigraphic levels, and the Hanging Wall gneisses at two. A microscopic examination led to the conclusion that these rock types are petrologically different. The aplites and ‘quartzites’ collected from widely separated sources were studied with a view towards possible petrological correlation. Such was not possible.

Part 3: The petrogenesis of several structurally important rock types was reviewed. Namely, Banded Iron Formation, Amphibolite Sillimanite Gneiss, Sericite Schist, Plagioclase ‘quartzite’, Potosi Gneiss, Hanging Wall Gneiss. A microscopic study provided information for a reappraisal of petrogenetic ideas. The role of metasomatism in the genesis of sericite schist, Potosi Gneiss, ‘quartzite’ and Hanging Wall Gneiss was stressed, as opposed to a ‘dry transformist’ approach. On the other hand, the amphibolites were regarded as originally igneous bodies that suffered intense metamorphism with a lesser degree of metasomatism.

Banded Iron Formations and sillimanite gneisses were undoubtedly sedimentary rocks prior to metamorphism.

Part 4: Part 4 consists of thin section descriptions which provide data applied in Parts 1 and 3. A division was made into Part 2 thin section descriptions and, secondly, those specimens and rock suites used in the study of rock origins.

The correlation of the granitic-type gneisses (Part 2) was satisfying enough, but conflicting evidence as to their origins still leaves room for much speculation. The answer must lie in shrewd observation of field relationships that may be exposed by future exploration and development. The application of sound chemical principles to such processes as metasomatism and granitisation is also desired. Controversy concerning the origin of the rocks will not cease until theories of ‘wet’ and ‘dry’ diffusion and suchlike have been clarified by indisputable evidence.