Distribution, Geometry, and Origin of Heavy Mineral Placer Deposits on Oregon Beaches

Abstract

ABSTRACT Beach segments (0.5-3 km in length) enriched in heavy minerals (20-100% by weight abundance) are located south of six prominent headlands that interrupt the 175-km coastline between Cape Lookout and Cape Perpetua, Oregon. Beach-face retreat in the winter of 1982-83 exposed underlying placers containing the economic minerals garnet, zircon, ilmenite and chromite. In each case, the highest concentration of placer minerals occurs in an area of maximum shoreline curvature at a distance south of the north bounding headland that is proportional to seaward extent of the headland. Cross-sections and magnetic surveys of one 8-km coastal section, Otter Rock to Yaquina Head, define a small placer body (heavy minerals > 50%) some 0.75 km in length and 80 103 m3 in volume that comprises 10% of the total beach sand between the bounding headlands. Mineral grain-size analyses of winter beach samples along this coastal section confirm earlier models of mineral segregation by entrainment processes; the light minerals are more susceptible to fluid shear stress than the heavy minerals. Maximum segregation and heavy mineral enrichment occur in the backshore and at shoreline inflection points where decreasing flow velocities produce gradients of fl id shear stress, allowing light minerals to be winnowed from more stable, heavy minerals. Development of placer deposits is controlled by shoreline configuration as the first-order factor and seasonal variations of wave climate, and longshore currents as second-order factors that delineate shape and concentration of heavy minerals in placer bodies.