Siliceous sinters as dipsticks for epithermal mineralisation

Abstract

Siliceous sinters, or silica-rich hot spring deposits, are primarily composed of hydrothermally derived silica precipitates occurring in bedded, erosion-resistant mounds and sheet deposits, metres to tens of metres thick, that contain a variety of sedimentary textures and other features of environmental significance (i.e. lithofacies). Sinters form as surface manifestations of geothermal systems, where thermal fluids of varied compositions are discharged at the intersection of the water table and the Earth's surface. They potentially may be spatially associated with Au-Ag ore producing adularia sericite epithermal deposits at depth, particularly those derived from fluids of nearneutral pH alkali chloride composition. This thesis by publication utilises lithofacies associations, especially textural identification, together with trace elemental analysis within a paragenetic framework to demonstrate how siliceous sinter deposits in the geologic record can be used as a tool in exploration for epithermal ore deposits, via three main themes of identification, characterisation and mineralisation, outlined further below. This thesis is mainly focused on case studies from the Hauraki Goldfield, Coromandel Volcanic Zone, New Zealand, a Miocene age, 200 × 40 km region of 50 gold-silver bearing, adularia-sericite type epithermal deposits and several porphyry copper prospects. Moreover, this study also examines other sinters and superficially similar deposits of various ages (presently forming to Pliocene) throughout North Island, New Zealand, as well as global examples within known epithermal regions, including the Jurassic Deseado Massif, Argentina, and the Miocene deposits of the Great Basin, California and Nevada, USA, in order to compare features to improve identification, characterisation and mineralisation understanding of siliceous sinters. Identification of siliceous sinters in the geologic record is primarily achieved through textural differentiation of the varied hot spring related lithofacies as compared to other silicified sedimentary and volcanic deposits often associated with sinters, the latter of which may appear similar with respect to geomorphology and/or outcrop features and/or be interbedded with or cross cut sinters. These similar surface features are known as pseudosinters, many of which will have no known direct association with epithermal deposits at depth. Herein, the lithofacies of siliceous sinters and pseudosinters, from numerous examples ranging in age from Jurassic to modern, are described and their formation environments inferred, thereby systematically distinguishing the former from the latter. Furthermore, trace elemental analysis is utilised in conjunction with textural analysis of silicified features - including sinters as well as silicified travertines, fluvial and lacustrine sediments, volcanics, chalcedonic veins and inferred mud pool deposits - for various Miocene age sites in the Coromandel Volcanic Zone, to distinguish between different formation environments of sinters and pseudosinters. Characterisation of preserved (i.e. ancient) siliceous sinters through comparison of lithofacies to relevant modern analogues allows reconstruction of the paleoenvironmental formation conditions. Sinter lithofacies transitions from vent and proximal slope, through middle and distal apron areas, to the commonly widespread, geothermally influenced marshes along a thermal continuum (~100 °C - ambient), may be used to infer thermal fluid type, direction and relative fluid volume discharged. Paleoenvironmental reconstructions of 18 Miocene sinter deposits of the Coromandel Volcanic Zone Identified in this thesis, through the analysis of lithofacies and geological associations, aid in delineating and vectoring towards potential zones of epithermal fluid upflow. Additionally such reconstructions in a paragenetic framework can characterise various changes - fluid composition (e.g. acidic overprinting with a drop in water table), hydrological, seasonal and post depositional - within a given hydrothermal system over time. The mineralisation potential of several occurrences of sinters, ranging in age from modern to Miocene, revealed that the concentrations of 47 elements in 30 siliceous sinter deposits from three calk-alkaline volcanic arcs of North Island, New Zealand, were found to be highly variable in pathfinder elements, alkali metals and immobile elements. The evaluation of this variability suggests an association with several factors including geothermal reservoir conditions, protolithic material type, as well as weathering and diagenetic history. Moreover, gold was observed to be primarily affiliated with the arsenic that is hosted within sulphides contained within some sinter deposits. The sulphides are inferred to have formed at depth and were transported to the surface and incorporated into siliceous sinters during or post sinter deposition, providing important information on reservoir conditions and allowing assessment of possible epithermal ore deposit associations.