For over 85 years, our understanding of how this deposit formed has been evolving. 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) form 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. HS!!