DETERMINATION OF THERMODYNAMIC PROPERTIES OF GOLD FROM ITAGUNMODI, SOUTHWEST NIGERIA

Abstract

The Thermodynamic Properties (TP) of gold usually gives an insight into the nature of gold occurrences. However, only geophysical characterization of gold deposits are often undertaken to confirm mineralisation with little attention paid to the TP of gold. Therefore, this work was designed to determine the TP of gold around Itagunmodi and their consequences on and or correlation with temperature profile in the lithosphere.

Thirty samples of gold were purposively collected from selected locations in Itagunmodi Southwestern Nigeria. One gramme of each unrefined gold sample was analysed for variations in volume when subjected to high pressure (600 MPa) and temperature (3000 ℃) using highly improved-pressure vessel. The TP (bulk modulus, thermal pressure, compressed volume and volume strain) were determined using standard methods. The parameters served as input to the Mie-Grüneisen equation of state for lithological classification. The temperature profile from the crust based on average values of volume and temperature to the lithosphere was determined using: Q ̇=Q ̇_MC+q ̇_0 z_0-∫_0^z▒〖q ̇dz〗, where q ̇,q ̇_0,z_0,z, are mean continental, mantle crustal heat flux, exponential variation of radiogenic heat with depth, mean radiogenic heat flux, fixed reference depth for crustal part of the lithosphere and varying reference depths for crustal part of the lithosphere, respectively. Lithospheric stress was determined using standard procedure, while the variation of melting point with pressure was determined using the plot of temperature against pressure. Data were analysed using descriptive statistics.

The bulk modulus of gold was obtained within 0.68-9.85 GPaand it decreased with temperature rise. The compression in volume of the unrefined gold samples increased with increase in both temperature and pressure because the increase in temperature will not cause any increase in volume as a result of the application of pressure. The initial average values of pressure, thermal pressure, temperature and compressed volume ranged from 436.40-549.00 MPa,0.150-0,831 GPa, 2327.30-2871.70 ℃and0.041-0.69×〖10〗^(-3) m^3, respectively. The strain rate was 〖10〗^(-5) s^(-1), which conformed to literature laboratory values for materials (〖10〗^(-5)-〖10〗^(-9) s^(-1)) such as copper, aluminium and polycrystalline tantalum. The calculated temperature profile ranged from 27-1483 ℃and the depth from 0-150 Km. The calculated base value of 1483 ℃, agreed with literature values (1280 ℃and 1600 ℃). The stress in the lithosphere increased at 33 MPa/Km, while bulk modulus increased at 1.136 GPa/Km. The temperature of the melting curve value from 1064 ℃and 1394.3 ℃in the pressure ranged between 0 and 6500 MPa. This corresponds to temperature range of 1045 and 1483 ℃and pressure range of 0 and 4900 MPain the lithosphere.

The determined thermodynamic properties to the depth of the lithosphere of gold deposit at Itagunmodi showed that the nature of the gold existed as solid mass at the source.