Structure and alteration in the Broken Hills District, Coromandel Peninsula, New Zealand

Abstract

Au-Ag deposits result from mineral deposition during circulation and upwelling of hydrothermal fluids in the shallow crust. The Broken Hills District, located on the Coromandel Peninsula, northern New Zealand, comprises the fossilised remnants of a hydrothermal system, containing a number of historical mines, and the active Broken Hills Goldmine. Epithermal veins in the district are hosted in flow-banded Whitianga Group rhyolite and ignimbrite, which is presumed to overlie Coromandel group andesite. Mineral deposition at the Broken Hills mine is believed to have occurred approximately 7.1 Ma, in extensional settings which probably prevailed throughout the southern Coromandel Volcanic Zone. This extension facilitated hydrothermal fluid flow, causing widespread clay alteration, strong localised silicification, and quartz veining. Research on the smaller historical mining sites in the district has taken place and has been documented in a number of exploration reports by mining companies, investigating the potential of these sites as future exploration prospects; mapping has also been undertaken at various scales, notably by Moore (1979) and Rabone (2006). In this study, geological mapping of lithology, structure and alteration in the district was carried out, followed by geochemical analysis using short-wave infrared spectroscopy, and inductively coupled plasma mass spectrometry and atomic emission spectrometry (ICPMS/ AES). An improved geologic map was produced using both field and geochemical data, and from this a conceptual model was developed of the structural paleohydrology controlling mineralisation in the district. A satellite system mapped to the east of the Broken Hills goldmine exhibits structural and geochemical properties which may have been conducive to ore mineralisation, possibly in a separate fluid regime from the structures at Broken Hills and Golden Hills. Geomorphic analysis and structural mapping, combined with previous regional geophysical studies, suggest that reactivated basement structures and regional tectonic regimes exhibit a strong influence on the structural characteristics controlling local mineralisation. The greater relevance of these results extends beyond the potential for future mining in the Broken Hills District. The results highlight the district's unique location on a regional structural intersection, archiving major turning points in the tectonic and volcanic history of the Coromandel Volcanic Zone.