RECOGNITION OF THE HYDROTHERMAL FEEDER TO THE STRUCTURALLY INVERTED, GIANT BROKEN HILL DEPOSIT, NEW SOUTH WALES, AUSTRALIA

Abstract

Genetic models for the ca. 1690 Ma giant Broken Hill Zn-Pb-Ag deposit have been controversial because of the prolonged history of multiphase deformation and high-grade metamorphism and metasomatism that have affected the Broken Hill district. Most models propose a broadly synsedimentary exhalative to subsea-floor replacement origin, with minor to major structural-metamorphic modification, but various epigenetic, syn- to late-magmatic or syn- to late-tectonic models have also been proposed, with variable emphasis on structural relocation or even complete remobilization of ore lenses. Recent comprehensive reevaluation of the Broken Hill lodes in the southern operations (southern third of the orebody) strongly supports models that the orebody comprises a metamorphosed and tectonically inverted, yet broadly structurally intact, set of geochemically zoned ore lenses and lodes that resemble sedimentary-exhalative (SEDEX) systems in terms of their geometry. Importantly, the reevaluation clearly recognizes the previously elusive feeder-zone to this replacement-exhalative system as the siliceous, crosscutting Zn-rich C lode with associated blue quartz-gahnite-pyrrhotite alteration at the lithostratigraphic base of the lode system. The replacement-exhalative center is focused between Section 25, near the Zinc Corporation shaft, and Section 70, near the NBHC (New Broken Hill Consolidated) shaft, and is located around the C lode fault. It is characterized by the greatest volume of alteration, the greatest thickness and vertical extent of the C lode, the largest number of individual ore lenses, the presence of I lens, and the first appearance of the laterally extensive Lead lode orebody, among other anomalous features. Approximately 45 percent of the orebody mass is concentrated around this exhalative center along 13 percent of orebody strike. Although clearly exhalative in its geometry, and almost certainly controlled by a synsedimentary fault zone, the Broken Hill orebody differs from volcanic-hosted massive sulfide systems in that it is several orders of magnitude larger than other deposits in the district. It also differs from most SEDEX systems in terms of its very high Ag grades and anomalous concentrations of Mn, Ca, P, and F. Broken Hill, as is typical for all Broken Hill-type deposits, is metamorphosed to the granulite facies despite the lack of evidence for extensive crustal thickening, whereas most SEDEX deposits are preserved at much lower metamorphic grades. These features support an anomalous depositional environment for the exhalative system, probably related to lithospheric extension and continental rifting.