ABSTRACT
Testing Established Models of Hydrothermal Fluid Distribution Around Porphyry Deposits: The
Application of Fluid Inclusion Research to Porphyry Deposit Exploration
By
Wyatt M. Bain
Dr. Jean S. Cline, Examination Committee Chair
Professor
University of Nevada, Las Vegas
The Kabba porphyry copper prospect in northwestern Arizona contains two zones of
porphyry style mineralization and alteration, which are spatially separated by a north-south
trending normal fault. These two zones are referred to as the Kabba footwall and hanging wall
and are hypothesized to represent the root and intermediate depth levels of the same porphyrylike hydrothermal system, respectively. If this hypothesis is correct, then fluid inclusions found
in the hanging wall and footwall should have the physical and chemical characteristics of fluid
inclusions in the root and intermediate depth levels of other known porphyry deposits.
Vein and alteration petrography, fluid inclusion petrography and microthermometry, and Raman
and LA-ICP-MS analyses of individual fluid inclusions from Kabba footwall and hanging wall
samples lead to the following observations: 1) the textures and paragenetic sequence of alteration
and mineralization in the Kabba hanging wall is almost identical to the textures and paragenetic
sequence of alteration and mineralization in the footwall, 2) the mineralization and alteration in
iii the Kabba prospect is consistent with the footwall and hanging wall having formed in the deep
roots and intermediate zones to a porphyry-like hydrothermal system, respectively, 3) the
distribution of fluid inclusion types in the footwall and hanging wall broadly matches the
distribution of fluid inclusion types in the deep roots and intermediate zones of other porphyry
systems, 4) the chemical and physical conditions of fluid inclusions in the Kabba system are very
similar to the chemical and physical conditions of fluid inclusions in the Butte porphyry system
in Montana, 5) Cu concentrations in fluid inclusions from Kabba are highest in inclusions with
the lowest Sr, Rb, and Ba, and 6) Cu concentrations were consistently high in vapor-rich fluid
inclusions. These observations strongly suggest that the mineralization and alteration in the
Kabba system formed at great depth in a porphyry-like hydrothermal system, and that the
hanging wall and footwall represent the root and intermediate depth levels of that system. Based
on the microthermometic behavior of fluid inclusions from the hanging wall and footwall it is
interpreted that, 1) the mineralization and alteration in the Kabba system formed via 5 episodes
of hydrothermal fluid flux through the footwall and hanging wall, 2) immiscibility may have
played a role in forming the mineralization in the hanging wall, 3) fluids went from being H2Oto CO2-rich over time, 4) the Kabba footwall and hanging wall formed at pressures between
2500-3000 bars (10-12 km) and 1500-2000 bars (6-8 km), respectively, and 6) fluids in the
footwall and hanging wall were trapped between <300-545°C and <300-525°C, respectively.
iv CHAPTER 8
CONCLUSION
The objective of this thesis project was to test the hypothesis that the structurally
separated areas of mineralization and alteration in the Kabba footwall and hanging wall formed
together within the same hydrothermal system, and represent the deep root and intermediate
zones of a single porphyry-like hydrothermal system. Based on the results of this study that
includes comparison between data from the Kabba hanging wall and footwall and to established
fluid inclusion distribution models, the following conclusions have been identified.
1. The alteration types observed in the hanging wall and footwall are consistent
with what has been observed in intermediate zones and root zones of other
porphyry systems and suggest that the Kabba system is similar to some other
productive porphyry systems found around in the world.
2. The mineralization, alteration, and distribution of fluid inclusion types from
the footwall and the hanging strongly suggest that the hanging wall and
footwall formed as part of the same hydrothermal system, and that the
hanging wall formed at a shallower depth than the footwall.
3. Based on the microthermometric behavior of the fluid inclusions found in the
hanging wall and footwall it is possible for fluids found in the footwall to have
evolved into the fluids found in the hanging wall, suggesting a genetic link
between the two areas.
4. LA-ICP-MS data from the individual fluid inclusion groups found in the
hanging wall and footwall indicate that the fluids trapped in the fluid
inclusions associated with the mineralization and alteration in the hanging
wall and footwall were capable of forming the mineralization found
throughout the Kabba system.
5. Patterns in the concentrations of Sr, Ba, and Rb relative to Cu in all inclusions
groups in the hanging wall and foot wall are consistent with those found in
other porphyry systems and this further suggests that the Kabba system is
similar to other productive porphyry systems found around in the world.
6. Five distinct generations of hydrothermal fluid ascended from depth, traversed
the footwall and the hanging wall, cooled, and precipitated the mineralization
and alteration found in and adjacent to the veins in each area.
7. The fluids involved in forming the mineralization and alteration in the
hanging wall and footwall changed from being H2O to CO2-bearing over time.
8. Chalcopyrite mineralization in the hanging wall was associated with a lowdensity vapor and the high Cu content of the vapor suggests that vapor
transport may have been important in the Kabba system.
9. The lithostatic pressure recorded by fluid inclusions in the footwall was
consistently at or between ~2500-3000 bars. The pressure conditions in the
hanging wall fluctuated between lithostatic pressures of ~1500-2000 bars, and
hydrostatic pressures of ~550-750 bars. These pressures correspond to depths
between 6-8 Km for the hanging wall and 10-12 Km for the footwall, and
indicate that the Kabba system formed at depths greater than those determined
for the Butte porphyry deposit.
10. Temperatures in the Kabba system fluctuated from high to low over time with
mineralization in the footwall forming between <300-545°C, and
mineralization in the hanging wall forming between <300-525
11. The exploration model used to guide drilling in the hanging wall, which is
predicated on the assumption that the hanging wall and the footwall formed as
part of the same hydrothermal system and have since been separated along a
north-south trending normal fault, is broadly supported by the comparison
between the vein minerals, alteration types, and fluid inclusion data from the
hanging wall and footwall.
12. Additional drilling is needed to determine the economic potential of this
system and to further delineate economic mineralization and alteration in the
hanging wall that may have formed as part of the fossil hydrothermal system
present at Kabba.