At the Van Deemen Mine a 7° west-dipping detachment fault has placed Miocene andesitic lavas, volcanic breccias, and conglomerate over Precambrian gneiss, schist, and phyllite (Figure 7). The footwall Precambrian rocks locally exhibit a mylonitic and/or ultramylonitic fabric, probably imparted on the rock during late Mesozoic or early Tertiary time. The volcanic and sedimentary rocks of the upper plate are rotated as much as 75 0 to the east and are truncated abruptly down dip by the underlying detachment fault. Rotation of the upper-plate Tertiary rocks is accommodated by a series of north-striking, west-dipping high-angle normal faults which merge with the underlying detachment fault.
In the area of the Van Deemen, the detachment fault is marked by a complex zone of intense low-angle shearing and brecciation that varies from 15' to 100' thick. This highly structurally prepared zone present along the detachment fault at the base of the Tertiary rocks is referred to as the ''detachment breccia zone". The top and bottom of this zone is in many cases a sharp contact between broken, but not brecciated, volcanic and sedimentary rocks above, intensely chloritized and moderately to poorly broken Precambrian gneiss and schist below. The detachment breccia zone at the Van Deemen is exactly analogous to the breccia zone separating Tertiary volcanic rocks from Precambrian granite and gneiss at the Picacho Mine near Yuma, Arizona. Ore at the Picacho Mine is hosted exclusively in this detachment breccia zone of sheared and brecciated Precambrian granite and gneiss.
The rocks beneath the detachment breccia zone at the Van Deemen are poorly broken and are cut by widely-spaced, narrow low-angle shear zones. These footwall shear zones are weakly mineralized and not of sufficient thickness to be of economic interest. Also developed in these chloritized footwall rocks are high-angle normal faults and fractures generally oriented north-south. These faults ordinarily do not offset the overlying detachment and are perpendicular to the direction of extension as indicated by the east dips of upper-plate Tertiary rocks. For this reason, these high-angle footwall structures are interpreted as tensional fault and fracture features developed in response to movement along the detachment fault above. These faults may act as important feeders for mineralization into the detachment breccia zone.
Post detachment fault structures are generally oriented east-west and are high-angle, dip-slip slip faults with minor displacement.