Characterization Of Seabed Rocks For Mine Planning In The Eez
Authors: Cruickshank M.H. , Paul R.C. 
Published: October 2008

The preparation of Mining Development Scenarios for Environmental Impact Statements on proposed leasing actions in the Exclusive Economic Zone (EEZ) under the OCS Land Act has required many assumptions about operations for which there are no existing precedents. To obtain some data on the characteristics of the seabed on or in which the deposits occur, samples of seabed materials from the area of high cobalt manganese crust deposits in the Hawaiian EEZ were examined. The characteristics determined included, where appropriate, bathymetry, microtopography, and engineering properties of the deposit material, and substrate. These data were used to derive general ranges for thickness, roughness, and strength of the strata and from these ranges to determine feasible mining and processing scenarios. The results are still tentative, and insufficient for establishing a definite constitutive model. Nevertheless, there is an implication that these seabed materials are similar to cohesive soils or weak rocks. For the manganese crust tested in the laboratory, the derived cohesive strength was 2.9 MPa suggesting good ripability, and P wave velocity was 2.26 km/sec suggesting marginal ripability. Many more tests are required on additional samples to correlate the engineering properties measured in the laboratory with geophysical properties measured in situ by remote sensing, for which new tools are needed.

Introduction

Mining of hard-rock ore deposits beyond the shoreline has been generally limited to traditional underground mining systems developed from vertical or inclined shafts, collared on land or nearshore islands. In exceptional cases in shallow water, open pit mining methods have been used working from floating platforms with conventional tools (Fig. 1) or by isolating the deposits with coffer dams and working them dry. (Fig. 2) However, in the last 20 years, significant discoveries have been made of hard-rock deposits in the deep seabed for which no operating precedents are available. Those deposits which appear to have potential for exploration at this time are hydrothermal metalliferous sulfides and cobalt-rich manganese oxide crusts. The sulfide deposits of which over 30 have been discovered to date, occur at water depths between 2000 and 3500 meters in the form of surficial coalesced stacks, cemented breccia mounds or as stockwork veins which, if they are analogous to similar deposits found on land, may be expressions of deeper vein or massive sulfide ore bodies (Fig 3). Composition of the sulfides varies at each site and may include one or more of the metals Cu, Pb, Zn, Cd, Fe, Mo. Cr, Ba, Sr, Ag, or Au. Manganese crusts, which were first located on the Blake Plateau off the South Eastern United States, were more recently recognized in the Pacific Ocean where they appear to occur preferentially at depths between 800 and 2400 meters, coating the submerged slopes of islands and seamounts and the tops of guyots. The crusts form on rock substrate of basalt, hyaloclastite, carbonate or phosphoritic material as layers of predominantly iron-manganese oxides which vary in thickness from mere discolorations to as much as 30 centimeters, but usually between 3-5 cm (fig. 4).