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DRILL-HOLE DESIGN FOR DILATIONAL ORE SHOOT TARGETS IN FAULT-FILL VEINS

Eric P. Nelson

Department of Geology and Geological Engineering, Colorado School of Mines, 1500 Illinois Street, Golden, Colorado 80401

E-mail, enelson{at}mines.edu

Ore along fault-fill veins typically has an uneven distribution, with semilinear high-grade ore shoots separated by lower-grade zones or barren fault segments. Therefore, a successful drill campaign must be optimally designed for intersection of ore shoots and not the intervening zones. However, the typical drill-hole design for fault-fill veins, which targets the vein directly downdip from the highest-grade outcrops, will miss most ore shoots that rake less than ~80° to 90° in the structure. Dilational ore shoots form along pre- or synmineralization faults in localized, elongated structural openings perpendicular to the slip vector. The rake of such ore shoots in fault-fill veins can be predicted using models of stress-controlled fault-fracture kinematics. Equations are presented that use fault-kinematic data to design drill holes for optimal success in targeting such ore shoots. The equations calculate the universal transverse mercator (UTM) coordinates of the drill target given fault-fill vein orientation, slip-vector rake (usually determined from slickenlines), vertical depth to intersect the ore shoot, and the UTM coordinates of geochemically anomalous outcrops along the fault vein. Although the method assumes that fault kinematics control ore-shoot orientation, and is not applicable to all fault-vein ore shoots (such as those formed at intersections of faults or shear zones with permeable beds, other faults, or dikes), the method increases the probability of success for drill-testing ore shoots through optimal drill-hole design.