| Popis: |
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. |
