Environmental Assessment
Volume E-3.2
Tailings Facility
Geology and Geochemistry
Ambatovy Project
25
January 2006
3.2
GEOLOGY AND GEOCHEMISTRY
3.2.1
Introduction
This section discusses impacts on geology and geochemistry for the tailings
facility of the Ambatovy Project (the project). Changes in geochemistry are not
considered as direct residual impacts in themselves, but are rather used in the
water quality Environmental Assessment (EA).
The study area for geology and geochemistry is the same as the soils study area
shown in Volume A, Section 7, Figure 7.2-3.
3.2.2
Baseline Summary
3.2.2.1 Geology
A series of valleys west of Toamasina make up the area of the planned tailings
facility. The valleys are located largely within the Vohibory formation consisting
of weathered gneissic mica schist. The geology of the area is extensively
weathered. Highly weathered, sandy silt to silty sand saprolite material is present
to depths of between 10 and 30 m. The rock formation seems less weathered and
fractured below 40 m.
The main part of the proposed tailings facility is underlain by gneiss, containing
biotite and migmatite lenses. Dolerite dykes and sills have intruded the older
gneiss rocks. Dykes have a predominantly northeast-southwest to north-south
strike direction. The western part of the tailings dam area has been intruded by a
large dolerite sill, which is mined for aggregate stone. Alluvium is found along
streams and consists of clay, sand, silt and gravel, formed by weathered gneiss,
washed from the higher areas. The thickness of this material is thought to be less
than 5 m.
3.2.2.2 Geochemistry
A geochemical characterization program was performed on several samples of
neutralized tailings from Dynatec’s pilot plant processing of Ambatovy ore.
Details are provided in Volume I, Appendix 2.1. The purpose of the geochemical
testing was to determine the chemical composition and assess the environmental
stability of these materials. In particular, the goals were to assess the following:
•
the chemical behavior of the tailings when exposed to the environment;
and
