Application of mineral equilibria modeling to constrain T and X _CO2 conditions during the evolution of the Magdala gold deposit, Stawell, Victoria, Australia

Abstract

Mineral equilibria modeling involving solid solution calculations has been combined with mineral assemblage information from the alteration zones associated with gold mineralization to determine the T and X _CO2 conditions for the formation of the Magdala gold deposit at Stawell, Victoria, Australia. Economic gold mineralization is primarily hosted within the stilpnomelane alteration zone of the Stawell Facies that is adjacent to the Magdala Basalt. Evolution of the Magdala gold deposit involved at least three fluid infiltration events: (1) a CO₂-bearing fluid during the D₂ deformation event produced carbonate spots throughout the chlorite zone; (2) a CO₂–S–K-bearing fluid, accompanied the D_3–4ab deformation and produced a muscovite zone and siderite rims on ankerite; and (3) a CO₂–K–S–Au-bearing fluid during the D_4c deformation event produced the stilpnomelane zone of the Stawell Facies, the proximal and distal alteration zones within the Magdala Basalt, and the main economic gold mineralization. Mineral equilibria modeling constrains the temperature of formation of the Magdala deposit to T = 345–390°C at 3kbar, substantially lower than indicated by other previous classical thermobarometry methods. Furthermore, this method has allowed the characterization of the mineralizing fluid and constrained its composition to X _CO2 < 0.08 at 3kbar. The timing and composition of the mineralizing fluids are similar to that of metamorphic fluid generated from devolatilization of a greenstone pile with peak of metamorphism occurring earlier and at deeper levels in the crust.

Appendix

Fe–Mg stilpnomelane

Generation of a thermodynamic model for stilpnomelane is difficult due to the lack of thermodynamic or experimental data and uncertainties in its formula. The assumed formula for stilpnomelane is ${\text{K}}_{1.26} {\text{Mg}}_{6.02}^{2 + } {\text{Fe}}_{6.02}^{2 + } {\text{Al}}_{2.26} {\text{Si}}_{15.8} {\text{O}}_{38.64} \left( {{\text{OH}}} \right)_{18.04}$. Fe–Mg mixing occurs on one octahedral site. With uncertainties on the thermodynamic properties of both end-members involved in the binary Fe–Mg stilpnomelane model, the thermodynamic properties of other end-members are combined in the correct proportions, as shown below. The enthalpies of formation of both end-members were then adjusted so that the (1) Fe–Mg partition between stilpnomelane and pumpellyite is appropriate (Potel et al. 2002) and so chlorite + K-feldspar = biotite + stilpnomelane reactions occur at temperatures a little lower that the disappearance of actinolite + biotite, in agreement with field observations (Brown 1967).

For binary Fe–Mg stilpnomelane, there is one compositional variable:

$$x_{{\text{sti}}} = \frac{{{\text{Fe}}}}{{{\text{Fe}} + {\text{Mg}}}}.$$

The resulting individual site fractions are:

$$\begin{aligned} x_{{\text{Mg}}} = 1 - x_{{\text{sti}}} , \\ x_{{\text{Fe}}} = x_{{\text{sti}}} . \\ \end{aligned} $$

For binary, single-site mixing stilpnomelane, end-member proportions are equivalent to the corresponding site fractions:

$$\begin{aligned} p_{{\text{msti}}} = 1 - x_{{\text{sti}}} , \\ p_{{\text{sti}}} = x_{{\text{sti}}} . \\ \end{aligned} $$

With a multiplicity of 6, the ideal activity expressions are:

$$\begin{aligned} a_{{\text{ideal,msti}}} = \left( {x_{{\text{Mg}}} } \right)^6 , \\ a_{{\text{ideal,sti}}} = \left( {x_{{\text{Fe}}} } \right)^6 . \\ \end{aligned} $$

The resulting data file looks like:

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sti 2

$$\begin{array}{*{20}l} {x\left( {sti} \right)} \hfill {0.85} \hfill \\ \end{array} $$

%———————————————————————————————

$$\begin{array}{*{20}l} {p\left( {msti} \right)11} \hfill {11 - 1x} \hfill \\ {p\left( {sti} \right)11} \hfill {0\,1\,1\,x} \hfill \\ \end{array} $$

%———————————————————————————————

ideal

%———————————————————————————————

$$\begin{array}{*{20}l} 2 \hfill {x\left( {Mg} \right)1\,1} \hfill {1\,1\, - 1\,x} \hfill \\ {} \hfill {x\left( {Fe} \right)1\,1} \hfill {0\,1\,1\,x} \hfill \\ \end{array} $$

%———————————————————————————————

$$\begin{aligned} \begin{array}{*{20}l} {} \hfill {} \hfill {} \hfill {} \hfill {} \hfill {} \hfill {} \hfill \\ {} \hfill {} \hfill {} \hfill {} \hfill {} \hfill {} \hfill {} \hfill \\ \end{array} \\ \begin{array}{*{20}l} {msti} \hfill {1\,1} \hfill {x\left( {Mg} \right)6} \hfill {} \hfill {} \hfill {} \hfill {} \hfill \\ {} \hfill {make} \hfill {5\;mu\; - 0.72} \hfill {cel\;1.44} \hfill {clin\;0.97} \hfill {phl\; - 0.09} \hfill {q\;1.66} \hfill \\ {DQF} \hfill {10\;0\;0} \hfill {} \hfill {} \hfill {} \hfill {} \hfill {} \hfill \\ {fsti} \hfill {1\;1} \hfill {x\left( {Fe} \right)\;6} \hfill {} \hfill {} \hfill {} \hfill {} \hfill \\ {} \hfill {make} \hfill {5\;mu\; - 0.72} \hfill {fcel\;1.44} \hfill {daph\;0.97} \hfill {ann\; - 0.09} \hfill {q\;1.66} \hfill \\ {DQF} \hfill { - 30\;0\;0} \hfill {} \hfill {} \hfill {} \hfill {} \hfill {} \hfill \\ \end{array} \\ \end{aligned} $$

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