Amphibolites from the Broken Hill district

Abstract

Abstract The amphibolites of the Broken Hill district fall into two distinctive groups. One group, characteristically developed in the Broken Hill Basin, contains common green hornblende and lesser brown hornblende, and in places alman‐dine. These amphibolites are intimately associated with pyroxene‐brown horn‐blende‐bytownite granulites, in which hypersthene is the chief pyroxene. Every stage of transition can be recognized within the distances of a few feet. These rocks occur as sills or interbedded formations, in places prominently banded. The banding is partly inherent from the granulites, but was intensified by conversion of dark felspars and hornblende to white zoisite and clinozoisite along the foliation during alteration of the granulites to amphibolites. The rocks have the chemical composition of basalts, and the conversion of granulite to amphibolite results merely from a reaction between pyroxene and plagioclase. The granulites are interpreted as regionally metamorphosed basic igneous rocks; and the amphibolites as products of their retrograde metamorphism. The second group of amphibolites appears restricted to the western sector, in the Peak Hill‐Silver Rock‐Apollyon Valley areas. It consists of strongly banded rocks, in which darker bands characterized by magnesian hornblende are interlayered with lighter bands rich in diopside. The banding corresponds to bedding. Locally these rocks contain scapolite and wollastonite. In places they carry seams of dark green actinolite, and of coarse‐grained, iron‐rich hornblende, accompanying small bodies of coarse‐grained magnetite and weak sulphide mineralization. They lack the general retrograde metamorphic changes shown by the first group of amphibolites and differ from them in texture. Chemical analyses, and their mineral compositions, indicate that they are regionally metamorphosed impure limestones and magnesian limestones.