Research Paper GEOSPHERE Controls on hydrothermal fluid flow in caldera-hosted settings: Evidence from Lake City caldera, USA 1 1 1 2 3 GEOSPHERE; v. 13, no. 6 Thomas O. Garden , Darren M. Gravley , Ben M. Kennedy , Chad Deering , and Isabelle Chambefort 1Department of Geological Sciences, University of Canterbury, Private Bag 4800, Christchurch, New Zealand 2 doi:10.1130/GES01506.1 Geological and Mining Engineering and Sciences, Michigan Technological University, Houghton, Michigan 49931, USA 3GNS Science, Wairakei Research Centre, Taupo 3377, New Zealand 10 figures; 3 tables ABSTRACT tion thereof (Table 1). It is not well understood why some calderas host hydro- CORRESPONDENCE: thermal systems while others do not or what factors promote fluid localization thomas .o .garden@gmail .com Silicic caldera volcanoes are often associated with hydrothermal systems in certain parts of a caldera. In particular, caldera “ring faults” are commonly economically important for electricity generation and localization of ore de- suggested to be important structures for localizing fluid flow (e.g., Duex and CITATION: Garden, T.O., Gravley, D.M., Kennedy, B.M., Deering, C., and Chambefort, I., 2017, Controls posits. Despite their potential importance, the poor exposure that is typical in Henry, 1981; Wood, 1994; Guillou-Frottier et al., 2000; Stix et al., 2003; Kissling on hydrothermal fluid flow in caldera-hosted settings: caldera settings has limited the number of detailed studies of the relationship and Weir, 2005); yet no studies to date have focused on a thorough examina- Evidence from Lake City caldera, USA: Geosphere, between caldera structures and fluid flow. We use field mapping, outcrop scale tion of their permeability structure. v. 13, no. 6, p. 1993–2016, doi:10.1130/GES01506.1. scanline transects, and petrographic analyses to characterize fault rocks, alter- Hydrothermal systems occur in a range of crustal settings. This paper is ation, and veins in the well-exposed 22.9 Ma Lake City caldera fossil hydro- primarily focused on the upper ~2 km of a silicic caldera-related setting, where Received 30 January 2017 Revision received 18 July 2017 thermal system. The caldera margin consists of relatively straight segments topography has less effect on reservoir fluid flow than in stratovolcano set- Accepted 12 September 2017 linked by more structurally complex intersections; these structural intricacies tings (Henley and Ellis, 1983). This corresponds with the mineralization and Published online 19 October 2017 produce a zone of deformation that can reach >300 m wide. Structural analy- discharge zones of Rowland and Simmons (2012), which are above the feed ses show that the wide (up to ~60 m) fault core of the ring fault contains abun- zone that extends down to the brittle-ductile transition and the base of convec- dant subparallel veins, with orientations similar to that of the caldera margin. tion. It is in this upper portion of hydrothermal systems that discrete high-flux Smaller displacement faults inside the caldera generally have narrow (<1 m), fluid conduits are important; the formation of these conduits strongly depends hydrothermally cemented fault cores with more variably oriented veins in the on the interplay of structure and lithology (Rowland and Simmons, 2012; surrounding damage zone. These findings at Lake City illustrate that fluid flow Vignaroli et al., 2015). The location of fluid conduits is, thus, expected to be is controlled by lithology and the location and displacement of faults, e.g., influenced by caldera-related structures and lithology. ring fault versus intracaldera fault. Fault connectivity is another key control. Ring faults accommodate the bulk of caldera collapse and comprise part We propose a conceptual model where fluid flow in caldera-hosted settings is of the structural margin of a caldera. In this study, we use the term “structural influenced by: (1) the presence of favorable lithologies (proximity to magmatic margin” for the zone of deformation at the caldera margin and the term “ring intrusions and/or the presence of permeable lithologies), (2) a high density of fault” for the portion(s) of this with the highest strain, if present. Fault rocks faults and fractures, and (3) favorable orientations of faults and fractures that (e.g., breccia, gouge, cataclasite, and pseudotachylyte) will generally form in, promote the formation of discontinuity intersections. and be indicative of, the highest strain (i.e., ring fault) portions of the caldera margin. The topographic margin of a caldera is where the intracaldera fill is juxtaposed against pre-caldera rocks or a significant scarp slope exists, but INTRODUCTION there is no evidence of faulting along this contact (Smith and Bailey, 1968; Spray, 1997; Lipman, 2000; Cole et al., 2005; Branney and Acocella, 2015). In Calderas have long been recognized as hosts for hydrothermal systems many calderas, the contact between intracaldera fill and basement can be lo- that can be economically important for geothermal power and the localization cated, but exposure is insufficient to ascertain whether this defines the topo- of ore deposits (e.g., Smith and Bailey, 1968; Rytuba, 1976; Duex and Henry, graphic margin or the structural margin. In this study, we call this potentially 1981; Stelling et al., 2016). In addition, characterizing hydrothermal activity ambiguous contact between intracaldera and extracaldera rocks the “caldera in large, modern calderas worldwide (e.g., Yellowstone, USA, Campi Flegrei, margin discontinuity.” Ring faults differ from other types of faults by: (1) a high Italy, Taupo, New Zealand) has been important in understanding their restless strain rate; large displacements (>1 km for some calderas) are accommodated behavior and associated hazards (Peltier et al., 2009; Rinaldi et al., 2010; Hur- in a short period of time (i.e., during and/or immediately posteruption) (Spray, For permission to copy, contact Copyright witz and Lowenstern, 2014). These systems may be associated with the margin 1997) and (2) frequent modification by magmatism (e.g., dike intrusion into the Permissions, GSA, or [email protected]. of a caldera, its interior, adjacent regional-scale structures, or some combina- fault) and/or landsliding soon after their creation (Branney and Acocella, 2015). © 2017 Geological Society of America GEOSPHERE | Volume 13 | Number 6 Garden et al. | Controls on hydrothermal fluid flow at Lake City caldera Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/13/6/1993/3990768/1993.pdf 1993 by guest on 27 September 2021 Research Paper TABLE 1. THE SPATIAL RELATIONSHIP BETWEEN COMPONENTS OF A SILICIC CALDERA-FORMING VOLCANO- TECTONIC SYSTEM AND THE LOCATION OF HYDROTHERMAL FLUID FLOW Location of hydrothermal flow On regional Quality of CalderaReference(s) Marginal Intracaldera structures constraints Bachelor/San Luis, USA Bethke et al., 1976 XGood Bonanza, USA Pride and Hasenohr, 1983; Lipman et al., 2015XXGood Campi Flegrei, ItalyAllard et al., 1991 XX Poor Cerro Aguas Calientes, ArgentinaPetrinovic et al., 2010 XModerate Citorek, Indonesia Marcoux and Milési, 1994 XPoor Chacana, Ecuador Beate et al., 2010 XXXModerate Chegem, Russia Lipman et al., 1993 XX Good Chianti Mountains, USA Duex and Henry, 1981 XX Moderate Colli Albani, ItalyGiordano et al., 2014XXModerate Creede, USABethke, 2001 XExcellent Hamada, Japan Matsuhisa et al., 1980 XGood Indio Muerto District, Chile Gustafson et al., 2001XPoor Infiernito, USADuex and Henry, 1981 XModerate Ischia, ItalySbrana et al., 2010 XGood Karymshina, Russia Leonov and Rogozin, 2010 XModerate Lake City, USAHon, 1987; Sanford et al., 1987XXGood La Pacana, Chile Gardeweg and Ramirez, 1987XPoor Long Valley, USASuemnicht and Varga, 1988; Sorey et al., 1991; Hildreth, 2017 XXGood Los Azufres, Mexico Ferrari et al., 1991 XPoor Luingo, ArgentinaGuzmán and Petrinovic, 2010 XPoor Mangakino, New ZealandKissling and Weir, 2005 XGood McDermitt, USA Rytuba, 1976; Castor and Henry, 2000 X Good Okataina, New ZealandWood, 1994; Rowland and Simmons, 2012; Caratori Tontini et al., 2016X XGood Onikobe, Japan Klein et al., 1990 XPoor Pantelleria, ItalyFulignati et al., 1997 XPoor Pongkor, Indonesia Milési et al., 1999 XXModerate Porco, Bolivia Cunningham et al., 1994 XXX Good Questa, USA Lipman, 1992; Klemm et al., 2008 XXX Good Reporoa, New ZealandWood, 1994; Rowland and Simmons, 2012XXModerate Rodalquilar/Lomilla, Spain Arribas et al., 1995 XX Good Rotorua, New ZealandWood, 1992; Milner et al., 2002XGood Round Mountain, USAHenry et al., 1997 X Good Silverton, USA Lipman et al., 1976 XXX Good Snowdon, UK Reedman et al., 1985 XX Good Soledad, Bolivia Redwood, 1987 XXGood Summitville/Platoro, USABethke et al., 2005 XXGood Taupo, New ZealandWhiteford, 1992 XXModerate Toba, Indonesia Hochstein and Sudarman, 1993 XXPoor Valles, USAHulen and Nielson, 1986; Goff and Gardener, 1994 XX Excellent Waihi, New Zealand Smith et al., 2006 XPoor Whakamaru, New ZealandWood, 1995; Wallis et al., 2013 XXX Poor Xela, Guatemala Bennati et al., 2011 XXGood Yellowstone, USAFournier, 1989 XXX Good Zacatecas, Mexico Ponce and Clark, 1988 XXX Good Note: The hydrothermal system in many of these systems is only briefly mentioned in volcanologically focused studies; therefore, interpretation was necessary in order to classify some calderas. GEOSPHERE | Volume 13 | Number 6 Garden et al. | Controls on hydrothermal fluid flow at Lake City caldera Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/13/6/1993/3990768/1993.pdf