Structural Controls on Hydrothermal Flow and Alteration at Sailor's Grave, Coromandel Peninsula, New Zealand

Abstract

The Sailor's Grave rocky shore platform, located in the eastern part of Coromandel Peninsula, New Zealand, is a well-exposed hydrothermally altered vein-style Au-Ag-epithermal system. The rock types are mainly basaltic andesite lavas and breccias. The deposit is well exposed, highly altered by hydrothermal fluids, and cross cut by faults and fractures. These attributes make it an ideal place to study the relationships between structural architecture and the associated hydrothermal system, thereby providing insight into possible localization of favorable sites for ore deposition. Based on petrographic and XRD analyses, four alteration types have been recognized. These are Phyllic/sericite alteration, Propylitic alteration, Argillic alteration and Potassic alteration. However, only one sample may belong to potassic alteration, which is located at the northern corner of the study area, and which may indicate a relatively shallow paleo-depth (<600m). Hydrothermal alteration intensity in this thesis is interpreted as: Minimally altered (Low), Moderately altered (Mid), Highly altered (High), Very highly altered (Very High), and Intensely altered (Max). There are three main vein types in the study area: calcite vein, quartz vein, and quartz + pyrite vein. Geochemical analysis provides a tool to identify the host rock type, geochemical haloes and mineralogical haloes. Isocon analysis can identify the element mobility. In this thesis, I have developed a new analysis method utilising mobile/immobile element ratios (MIM ratio) to understand the alteration halos, alteration intensity and to serve as an indicator for the vein zone and/or possible paleo-upflow zone. The main structures in the study area are NNW-striking normal faults and fractures, which are linked by NE/ENE-striking normal faults in the northern part of the study area. However, there also exist WNW-striking strike-slip faults and fractures with opposite senses of shear (right and left lateral). This study indicates that one of the highest rock alteration intensity and paleo-upflow zones is located at the center of Sailor's Grave platform, close to the cliff with the highest density of faults and fractures. The other is located at the linkage zone at South Otara Bay platform. The vein density and width also shows a strong correlation with the main faults. For example, the widest quartz veins are NNE-striking and dip steeply to the WNW and ESE, which has conformity with the main fractures and faults in the study area. According to the detailed structural field work, the paleo stresses in the study area are: σ1: 135/54(dip direction/dip), σ2: 332/35, σ3: 236/08. That may reflect the tilting process with an axis of 46°NE and 36°E tilt.