Conditions of Mineral Deposition Related to Super-Eruptions

Abstract: The  Lake  City  Caldera  is  located  in  SW  Colorado  and  formed  during  an  erup:on  ca.  23  million  years  ago.  The  adjacent  region  hosts  precious  metals  (e.g.  gold  and  silver),  which  were  transported  near  the  surface  by  a  now  ex:nct  high-­‐temperature  hydrothermal  system  related  to  the  magma:c  ac:vity  following  caldera  forma:on.  The  excellent  exposure  allows  us  to  examine  composi:ons  of  hydrothermal  fluids  based  on  analyses  of  the  mineral  assemblage,  textures,  and  trace  element  composi:on  of  the  quartz  veins  present  to  determine  the  processes  involved  in  the  precious  metal  deposi:on.            We  used  laser  abla:on  induc:vely  coupled  plasma-­‐mass  spectrometry  (LA-­‐ICP-­‐MS)  to  measure  elemental  concentra:ons  in  quartz  veins  and  compared  it  with  textures  (i.e.  colloform,  zonal,  bladed,  etc.)  to  elucidate  the  fluid  evolu:on  history,  as  well  as  to  gain  a  beNer  understanding  of  the  condi:ons  required  for  metal  deposi:on.  In  par:cular,  the  elements  Al,  B,  Nb,  Rb,  Sn,  Ti,  and  Zn  provide  informa:on  regarding  chemical  mobility  in  these  systems  and  indicate  that  boiling  of  the  hydrothermal  fluid  occurred  and  likely  played  an  important  role  in  metal  precipita:on.  

Results and Data: Hypothesis: Precious metals were precipitated as a result of rapid boiling from decompression during caldera

collapse.

Methods: Fieldwork: Quartz vein samples were collected from many areas around the caldera from multiple lithologies to best represent the hydrothermal system. Petrographic Analysis: Samples were observed under microscope for textural identification. Textures of interest included colloform, jigsaw, zonal, and xenocrystic quartz. Cathodoluminescence (CL): CL imaging was used to provide a qualitative characterization of the quartz textures. The textures provide a context for geochemical analyses. Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS): LA-ICP-MS was used to determine the trace element composition of various quartz grains that form different types of textures.

Conclusions: Trace  elements  were  concentrated  in  veins  via  boiling  at  depth  based  on  the  concentra:ons  of  metals  found  in  the  colloform  texture  of  quartz  veins  at  the  Lake  City  Caldera.              Concentra:ons  of  B,  Zn,  Rb,  and  Sn  all  increase  across  colloform  texture  traverses,  and  based  upon  their  par::on  coefficient  (KD)  values,  we  interpret  that  the  elements  became  more  enriched  in  the  liquid  phase  rela:ve  to  the  vapor  one.  This  interpreta:on  is  consistent  with  previous  models  in  which  fluid  enrichment  and  ore  deposi:on  is  achieved  by  boiling  off  of  a  frac:on  of  the  hydrothermal  fluid  (Robb,  2004),  and  may  be  the  same  process  required  to  deposit  metals  in  economically  viable  systems.                Metals  are  in  lower  concentra:on  in  the  country  rock  rela:ve  to  the  jigsaw-­‐texture  vein.  This  could  poten:ally  indicate  that  trace  elements  originate  from  the  country  rock,  are  leached  via  hydrothermal  fluid,  and  subsequently  precipitated  in  the  vein.  More  research  is  needed  in  this  area,  however,  to  dis:nguish  between  country  rock  and  magma:c  trace  element  sources.            Our  approach  of  combining  elemental  trends,  textures  in  quartz  veins,  and  host  rock  chemistry  holds  great  promise  as  a  technique  to  evaluate  the  economic  poten:al  of  ancient  hydrothermal  systems.  

Jordan Lubbers1, Chad Deering2, Marcel Guillong3, Olivier Bachmann3, Eric Hiatt1, University of Wisconsin-Oshkosh1, MichiganTechnological University2, Eidgenössische Technische Hochschule Zürich3

Introduction: Lake  City  Caldera  in  Lake  City,  Colorado  is  the  youngest  of  15  Ter:ary  calderas  in  the  San  Juan  Volcanic  Field  of  Southwestern  Colorado,  and  formed  22.93±0.02  Ma  with  the  erup:on  of  the  Sunshine  Peak  Tuff  (Steven  &  Lipman,  1976).  Extreme-­‐topographic  relief  and  post-­‐caldera  resurgence  (uplie)  dominate  the  area,  which  resulted  in  exposure  and  uplie  of  over  2km  of  intra-­‐caldera  Sunshine  Peak  Tuff  and  several  magma:c  intrusions.  Within  the  Lake  City  Caldera  large  pyroclas:c  and  intrusive  rocks  are  well  exposed,  providing  evidence  for  a  large,  evolving,  magma:c  system  (Kennedy  et  al.,  2012).  Shortly  aeer  caldera  collapse,  a  large  hydrothermal  system  formed  and  resulted  in  some  of  the  intra-­‐caldera  rock  being  altered  (Larson  &  Taylor,  1986).    

Elemental Source: Trace metals (e.g. Au, Ag) found in quartz veins can originate from fluid derived from the magma body at depth and/or surrounding country rock by remobilization via hot, hydrothermal fluid leaching processes. These elements are then precipitated in quartz veins as temperature drops. This relationship makes understanding country rock chemistry crucial. Previous researchers suggested that precious metals found in the Lake City Caldera were derived from leaching of the surrounding rock, however, the processes involved are poorly understood.

Boiling: Boiling of the hydrothermal fluid is a potentially important mechanism for metal deposition. Most metals preferably stay in the hydrothermal liquid phase rather than the vapor phase, so as boiling of the hydrothermal fluid occurs, the concentration of metals in the remaining fluid increases to the point where the fluid becomes oversaturated with respect to metals. They can be subsequently deposited in the form of gangue minerals and ores (Robb, 2004). In order for this boiling to occur, however, the system must not only reach a high temperature, it must have a pressure release via fracturing or some other mechanism, as the lithostatic pressure that is normally imposed on a fluid prohibits boiling of the system.

Jigsaw Quartz Colloform Quartz

References: Kennedy, B., Wilcock, J., Stix, J., 2012, Caldera resurgence during magma replenishment and rejunenation at Valles and Lake City calderas: Springer-Verlag 2012. Larson, P.B., Taylor, H.P. Jr., 1986, An Oxygen Isotope Study of Hydrothermal Alteration in the Lake City Caldera, San Juan Mountains, Colorado: Journal of Volcanology and Geothermal Research, v. 30, p. 47-82 Lipman, P.W., 2006, Geologic map of the central San Juan caldera cluster, southwestern Colorado: U.S. Geological Survey Map I-2799, scale 1:50,000. Robb, L, 2004, Introduction to Ore Forming Processes, Oxford, Wiley- Blackwell, p. 155-156 Steven, T.A., Lipman, P.W., 1976, Calderas of the San Juan volcanic field, southwestern Colorado: USGS Professional Paper, v. 958, p. 1-35.

LA-ICP-MS at ETH-Zürich:

Geochemistry:

Lipman, 2006

Maps:

Lake City Caldera

Jigsaw Colloform

#1

#17

#1

#23

Legend:

Nb (ppm)

Zn (ppm)

Ti (ppm)

Al (ppm) Zn (ppm)

Sn (ppm) B (ppm)

Rb (ppm)

Monzonite and dacite

Syenite

Upper Sunshine Peak Tuff Middle Sunshine Peak Tuff Breccia

Lower Sunshine Peak Tuff

CL image of Jigsaw quartz vein CL image of Colloform quartz vein

San Juan Volcanic Field

Lipman, 2012

Acknowledgements: We thank Dr. Olivier Bachmann and Dr. Marcel Guillong of ETH-Zürich for their generosity in helping us obtain the geochemical data and photomicrographs presented here. This project was supported by funds by the UW-Oshkosh Collaborative Grants program through the UW Oshkosh Office of Grants and Faculty Development.