Features of Large Magmatic–Hydrothermal Systems in Japan: Characteristics Similar to the Tops of Porphyry Copper Deposits

Abstract

Volcanic complexes in Japan such as Kusatsu Shirane, Honshu, and Kuju‐Hatchobaru and Kirishima, Kyushu, host active magmatic–hydrothermal systems that are several kilometers in diameter and typically asymmetric to the intrusive center. Their heat flow is driven by multiple intrusions at <5–10 km depth over a period on the order of ~105 years. These hydrothermal systems consist of advecting fluids of magmatic origin (either vapor with acidic components condensed into meteoric water, or a subsequent liquid of varying reactivity) that interact with convecting meteoric water. The amount of the magmatic component depends on location relative to the intrusive center, local controls on permeability, and the temporal evolution of the intrusions. The proportion of meteoric water diluent typically increases from proximal to distal locations (over several kilometer distance), and with time after intrusion (> several thousands of years). Some of the neutral pH solutions in these large hydrothermal systems, up to ~5 km from volcanic vents, support geothermal energy developments, with strong structural control on fluid flow. Near active volcanic vents, the magmatic vapor component of hydrothermal systems can form acidic condensates (pH ~ 1.5) that are responsible for strong alteration of the host rock. The resulting residual quartz and advanced argillic minerals are typical of shallow‐formed lithocap alteration that is associated with deeper porphyry copper deposits in similar arc‐hosted volcanic settings. Around the world, this near‐surface lithocap alteration, influenced by structures as well as lithology, may be barren in metals, even where associated with mineralized porphyry intrusions at depth. Such intrusion‐proximal alteration is widespread in both active and extinct settings in Japan. In addition, distal geothermal hot springs typically have a magmatic water component, albeit minor, with a pH that varies from ~3 to near‐neutral; deep alteration intersected in drill holes is characterized by alunite–dickite–pyrophyllite or chlorite–wairakite, respectively. These alteration assemblages are typical of their extinct equivalents, intermediate sulfidation epithermal veins, with mineralogical complexities caused by temporal evolution as well as spatial variation (e.g. along different structures). Such active and extinct magmatic–hydrothermal systems signify the presence of shallow degassing or degassed intrusions, respectively. Where similar intrusions in volcanic arcs around the world are eroded to ~1 to 2 km depth, the tops of porphyry copper deposits are commonly exposed, marked by a transition of white mica upward to pyrophyllite‐dominant alteration. Active magmatic–hydrothermal systems on the flanks of arc volcanoes in Japan are dynamic – both cycling as well as evolving – and share characteristics with extinct systems that host epithermal and porphyry ore deposits.

Pleistocene hydrothermal activity on Brokeoff volcano and in the Maidu volcanic center, Lassen Peak area, northeast California: Evolution of magmatic-hydrothermal systems on stratovolcanoes

“…Hydrothermally altered rocks at Brokeoff and Maidu locally are enriched in many "pathfinder" elements present in epithermal gold-silver deposits and in the upper parts (lithocaps) of porphyry copper systems (Table 4; Saunders et al, 2014;Halley et al, 2015). Alteration types and hydrothermal features, such as hydrothermal breccias, residual quartz ledges, and banded quartz ± carbonate veins, are similar to characteristics of these deposits (e.g., Hedenquist et al, 2000;Simmons et al, 2005;Sillitoe, 2010;Hedenquist et al, 2018).…”

“…Based largely on geothermal drill holes, Reyes et al (1993) and Rae et al (2003) describe alteration on the flanks of active volcanoes in the Philippines, and Moore et al (2008) present similar data for Galunggung volcano, Indonesia. Hedenquist et al (2018) summarize alteration and present a composite hydrothermal model for several active stratovolcanoes in Japan. All these studies show the complexity and evolving nature of Research Paper magmatic-hydrothermal systems on the flanks of stratovolcanoes and indicate that these systems are characteristic features of arc-related stratovolcanoes.…”

“…Volcanic hydrothermal activity forms when hot gases rise and condense in shallow ground water and flow through and react with volcanic and underlying basement rocks below volcanic edifices. The resulting hydrothermal alteration resembles shallow-level alteration associated with porphyry copper and epithermal gold-silver mineralization (e.g., Sillitoe, 1973Sillitoe, , 1975Sillitoe, , 2010Sillitoe and Bonham, 1984;Hedenquist et al, 1993Hedenquist et al, , 2018Hedenquist and Lowenstern, 1994;Proffett, 2003;Simmons et al, 2005). This hydrothermal alteration also can decrease rock strength and edifice stability, which leads to potentially catastrophic debris flows (Lopez and Williams, 1993), as observed at Mount Rainier, Washington (USA) (Vallance and Scott, 1997;Finn et al, 2001;Reid et al, 2001;Zimbelman et al, 2005;John et al, 2008).…”

“…Most studies are examinations of surficial features, such as recent ash deposits, fumaroles, and crater lakes, on active volcanoes (e.g., Hedenquist et al, 1993;Ohba and Kitade, 2005;Minami et al, 2016;Caudron et al, 2017;Lowenstern et al, 2018;Takahashi and Yahata, 2018) or drill-hole studies of geothermal systems on the margins of active stratovolcanoes (e.g., Reyes, 1990;Reyes et al, 1993;Rae et al, 2003;Moore et al, 2008). Other studies examine hydrothermal alteration associated with epithermal and porphyry mineral deposits in more deeply eroded stratovolcanoes and subvolcanic intrusions where it is difficult to link volcanic and hydrothermal activity (e.g., Gustafson and Hunt, 1975;Hedenquist et al, 1998Hedenquist et al, , 2018Longo et al, 2010). Few studies directly link mineralization temporally and stratigraphically to stratovolcano evolution (Sillitoe, 1975).…”

Rapid alteration of fractured volcanic conduits beneath Mt Unzen

“…At the upper end of this range, the high temperatures for kaolinite would require very low alkali metal cation to proton ratios (Hemley and Jones 1964) and thus acidic conditions for early alteration. This, together with the first appearance of sulfides, probably indicates a magmatic component in the clay alteration fluid (Hedenquist et al 2018). Early acidic fluids must also be responsible for leaching of Ca-rich bands in plagioclase and hornblende and thus for formation of voids for fluid circulation and clay precipitation.…”