Geologic research in the fields of thermodynamics, geochemistry and plate tectonics during the past 20 years has greatly improved our understanding of crustal evolution and the distribution of metals in the earth's crust. Throughout the world in Canada, Africa and Australia, major gold deposits are found within a suite of rocks referred to as greenstones. The name greenstone refers to the color of the rock acquired during metamorphism: greenschist facies metamorphism. The Mother Lode and Alleghany mining districts also lie within a greenstone belt that contains world-class gold deposits, one of which is the Sixteen to One vein system with production of more than 31 metric tons of gold.
Our understanding of how this deposit formed has been evolving for over 85 years. A process called carbonate metasomatism formed ribbon quartz veins along en echelon faults that are secondary to the crustal suture known as the Melones fault. These secondary veins (the Sixteen to One), formed by replacement as aqueous fluids ascending through cracks in the earth's crust, removed accessory cations from wall rock minerals.
The Melones fault is the remnant of a compressive or transpressional environment common to convergent plate margins. This fault marks the boundary along which oceanic crustal rock is emplaced against the continent. The fracture system along which the quartz veins formed is an expression of the forces and motion along the crustal suture. The Alleghany fracture system originates at the Melones fault which is the conduit for the large volume of fluid that formed the vein system. The volume of fluid can be calculated with reasonable accuracy because the fluid composition is known from fluid inclusion chemistry. The volume of the end product, which is quartz, is known from mining. Fluid chemistry assures us that there is vein continuity to the depth of the Melones fault.
To evaluate the model extending the Sixteen to One ore chute at its recorded rake to the junction with the Melones fault, we compare pressure/temperature boundary conditions to a model developed during 20 years of thermodynamic research. The model states that gold precipitation occurs at 300-500 degrees C. and 1-3 kb. Fluid inclusion studies conducted on vein quartz from the Alleghany district indicate formation temperatures that lie within the above boundaries. Sensitivity of the system to the temperature gradient along the fluid path is simpler and more predictable than its sensitivity to the pressure gradient which is responsive to block motion in addition to depth. Considering Ferguson and Gannett's estimate of a two mile thickness of pre-Cretaceous overburden, the 2600 foot level is at 1 kb (PL). Sinking another 3,200 feet vertically to the Melones fault would only add another 0.25 kb to the boundary conditions. Another 23,000 feet of overburden could be added before reaching the 3 kb limit.
Projecting the rake of the Sixteen to One pay chute to the Melones fault indicates a distance as great as 12,600 feet. Presently the Sixteen to One gold deposit has been mined to a depth of 3,000 feet with production of more than a million ounces of gold. At a production rate of one million ounces per 3,000 feet, the potential exists for the production of several million ounces before the system dives steeply into the Melones fault.