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Heat and fluid flow simulations in submarine volcanic terrains and implications for the formation of massive sulfide deposits

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Schardt, C (2003) Heat and fluid flow simulations in submarine volcanic terrains and implications for the formation of massive sulfide deposits. PhD thesis, University of Tasmania.

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Abstract

Finite difference numerical modeling has been used to study buoyancy-driven heat
and fluid transport in submarine volcanic environments and its consequences for the
formation of volcanic-hosted massive sulfide deposits in ancient and modern
submarine seafloor environments. Two different geological settings were chosen to
enable comparison of modeling results with field observations and determine the
extent and significance of key modeling parameters (rock properties, fault structure,
model geometry) on fluid migration, rock alteration and base metal accumulation
under a variety of conditions.
The first model combines field data with theoretical assumptions to examine
heat and fluid flow in the Lau basin, a modern back -arc seafloor setting. An
extensive sensitivity analysis of relevant rock and model parameters confirmed
several key observations in the Lau basin, such as the location of major fluid
discharge zones and measured discharge temperatures. Other results such as fluid
velocities and heat flux are comparable with theoretical predictions and general
seafloor measurements. New insights were obtained with regard to the controls on
recharge-discharge patterns, deep-seated fluid migration and the life span of
hydrothermal convection as well as the timing and minimum requirements for
massive sulfide ore body formation on the seafloor.
The second model researched the evolution of an ancient seafloor
hydrothermal system in the Panorama district, Western Australia. The Archean
Panorama district provides unprecedented access to a complete section of a
hydrothermal system underlying massive sulfide deposits, which allowed for the first
time the construction of a locally accurate model involving realistic stratigraphy,
fault geometry and fault distribution. The unique preservation of the Panorama
district enables a direct comparison of modeling results with field observations and
helps to simulate the main features of the hydrothermal system, such as alteration
zonation, temperature distribution and discharge temperatures. Results also allow the
establishment of minimum fluid and rock property requirements for the formation of
confirmed, and potential, ore bodies and assess the potential of the hydrothermal
system to host significant (> 5 Mt) massive sulfide deposit. Results from both models suggest that the most important factors and processes
controlling heat and fluid transport in submarine volcanic settings are fault and rock
permeability, basement topography, and the nature of the heat source. Discharge
temperatures are primarily controlled by rock permeability and range between 150QC
and about 400QC. Discharge fluid velocities depend mostly on fault permeability
variations and range between - 1x10-8 m/s and 4x10-6 m/s for individual fault
structures, which equates to values of about 3 m/s for a typical chimney orifice. Both,
predicted discharge temperatures and fluid discharge velocities, compare well with
seafloor observations and theoretical calculations and do not require a thermal
cracking front and associated permeability constraints.
The control on recharge-discharge patterns depends on the nature of the heat
source and model geometry, but is generally determined by fault spacing (Panorama
model) and their placement relative to the heat source (Lau basin model), and not by
the size of developing convection cells. Significant deep-seated fluid migration is
predicted to occur at the sheeted dike - gabbro interface due to inferred permeability
differences and basement topography. The life span of ancient and modern seafloor
hydrothermal systems producing fluid discharge temperatures greater than 150QC for
individual faults is predicted to range between about 2,000 years at high
permeabilities and greater than 200,000 years as permeabilities decrease.
Heuristic mass calculations based on modeling results indicate that
hydrothermal fluids with a 10 ppm base metal content and low deposition efficiency
(10 percent) can form significant seafloor massive sulfide deposits(> 0.5 Mt of 10%
Cu+Zn) within a time frame of about 6,000 years. World-class or giant massive
sulfide deposits (> 4 Mt of Cu+Zn) require a convection system, which runs for
about 40,000 years or hydrothermal fluids with a much higher deposition efficiency
and/or higher base metal content. The spacing, distribution, and potential size of base
metal deposits is predicted to depend primarily on the shape/extent of the heat source
as well as the spacing and position of fault structures relative to the heat source.

Item Type: Thesis (PhD)
Additional Information:

Copyright 2003 the Author - The University is continuing to endeavour to trace the copyright owner(s) and in the meantime this item has been reproduced here in good faith. We would be pleased to hear from the copyright owner(s).

Date Deposited: 30 Aug 2011 04:56
Last Modified: 11 Mar 2016 05:54
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