Nickeliferous laterite deposits comprise in situ lateritic weathering products developed from peridotites. Nickel enrichment in the laterite profile is largely derived from olivine or serpentine. Four zones are noted in normal, uneroded profiles: (1) the upper zone—transported limonite and ferricrete, (2) in situ limonite, (3) intermediate zone—either nontronite or silica boxwork, and (4) lower zone—saprolite. Profiles are classed into four major types on the basis of the serpentine content of the host and the presence or absence of the intermediate zone.
Profiles without an intermediate zone are characteristic of the humid equatorial zone or other locales with very high rainfall and minimal dry season. They may result from efficient leaching without reaching supersaturation conditions for smectite clays in the saprolite zone. They show marked mineralogical and structural variation from profiles developed over unser-pentinized peridotite (type Ia) through partly serpentinized hosts (type Ib) to serpentinite hosts (type Ic). Type Ia is characterized by a heterogeneous structure of partially leached joint blocks of saprolite with fresh rock cores and interblock veins of quartz and garnierite. Type Ic consists of a relatively homogeneous saprolite in which the principal mineral is parent rock serpentine which has undergone partial substitution of Mg by Ni and Fe+3. Type Ib, the commonest, has characteristics intermediate between Ia and Ic.
Profiles with silica boxwork zones or nontronite zones typically occur in less humid tropical climates with a marked dry season. In general they result from inefficient or relatively slow leaching attaining supersaturation conditions for smectite clays within the saprolite zone.
Elements may be strongly leached, supergene enriched, or residually concentrated as follows: leached: Mg, Si, Ca; supergene: Ni, Mn, Co, Zn, Y; and residual: Fe, Cr, Al, Ti, S, Sc, Cu. The leached elements show a loss which decreases approximately exponentially with depth. The supergene elements are concentrated in Mn, Co oxides; only nickel is enriched in supergene silicate phases.
Nickel enrichment preferentially occurs in the saprolite zone, commonly on hills or moderate slopes where relatively strong fracturing or closely spaced jointing favors downward leaching. Differences in topographic control of laterite formation and nickel enrichment suggest three types of morphogenic development: (1) downwasting of an initially mountainous terrain to low foothills hosting nontronitic profiles possibly below a thick limonite; (2) downwasting of an initially mountainous terrain with cycles of renewed erosion, multiple terrace development, and deep Ni-enrichment root zones; and (3) erosion of a continental peneplain with development of a silica boxwork mesa and peripheral Ni-enrichment zones.
Laterite ores develop from the decomposition of 20 to 100 m of peridotite rock. Many laterites are currently evolving at a rate such that a million years is adequate to develop most ore profiles; however, indirect age and geomorphic evidence suggest that development has, in most cases, taken place in stages over a time span of about mid-Tertiary to the present.
Figures & Tables
Seventy-Fifth Anniversary Volume
The first notions of a new journal came to J. E. Spurr during the closing days of 1904. When he shared his thoughts with friends in Washington, D. C., they were so enthusiastic about the suggestion that they formed themselves into an ad-hoc committee to seek ways to implement the idea. The ad-hoc group met informally for several months and by May of the following year was ready to announce the birth of an unusual new publishing company and the journal the company would produce. The first formal meeting of the Economic Geology Publishing Company took place on May 16, 1905. The first issue of the new journal appeared in October of the same year, and the first volume was completed in December 1906. The birthing was not easy, but it was successful because the founders provided much of the financing as well as the first papers. The story of those earliest days and the many struggles of the fledgling journal is engagingly recounted by Alan M. Bateman in an article published in the Fiftieth Anniversary volume.
From inception, management of the journal has differed from the management of most scientific journals. There was no sponsoring society, so the founders raised capital by incorporating and selling shares in the venture. The journal has been owned and published by the Economic Geology Publishing Company ever since. There is no record that the founders experienced difficulties in selling shares in the Company, but they must have had some because the Publishing Company had a goal that other corporations(and presumably many of the investors) would have found difficulty in understanding: the new corporation was committed to keeping the books balanced but not to making a profit.
Initially incorporated in the District of Columbia, the Publishing Company was reincorporated in 1970 as a nonprofit membership corporation in Delaware. The modification in corporate status came in response to a suggestion made by the Internal Revenue Service.
The affairs of the Publishing Company are controlled by a Board of Directors, and the journal is sold to the public by direct subscription. Day-to-day operations of paper selection, review, and printing are in the hands of the Editor, while business matters, such as subscriptions and advertising, are in the hands of the Business Editor.
The one tie the Publishing Company has with a society was instituted many years after the journal. was founded—with the Society of Economic Geologists. When the Society was founded in 1920 it first considered publishing its own bulletin. Because the venture seemed financially questionable, and the coffers of the new society were bare, an arrangement was reached whereby members of the Society first received offPrints of papers written by its members and eventually Economic Geology as