Secular Variation of Magmatic Sulfide Deposits and Their Source Magmas

摘要

Magmatic sulfide deposits are divisible into two major groups, those that are valued primarily for their Ni and Cu and that are mostly sulfide rich (>10% sulfide), and those that are valued primarily for their PGE and tend to be sulfide poor (<5% sulfide). Most members of the Ni-Cu group formed as a result of an interaction of mantle-derived magma with the crust that gave rise to the early onset of sulfide immiscibility. Of the different classes of deposit in this group, the komatiite-related class ranges from 2.7 to 1.9 Ga in age, the Flood basaltrelated class from 1.1 to 0.25 Ga, and the Mg basalt- and basalt-related group from the Archean to the present. There is only one example each of anorthosite complex- and impact-related deposits, so that one cannot generalize about their secular distribution, except to say that anorthosite complexes are Proterozoic. Ural-Alaskan intrusions are dominantly Phanerozoic (some Archean deposits have been included with this group), but as yet no examples have been found with economic sulfide bodies. Seventy-five percent of known PGE resources occur in three intrusions-the Bushveld, Great Dyke, and Stillwater, the rocks all of which have crystallized from two magma types, an unusual, high SiO_2, MgO, and Cr and low Al_2O_3 type (U-type) that was emplaced at an early stage and a later, normal tholeiitic-type magma (T-type); the PGE are concentrated in layers close to the level at which the predominant crystallization switches from one magma type to the other. The U-type magma is interpreted as a PGE-rich, komatiitic magma (possibly the product of two-stage mantle melting) that has interacted to varying degrees with the crust, becoming SiO_2 enriched in this way. These three intrusions are Neoarchean to Paleoproterozoic in age. All known examples of komatiites, with one exception, are Paleoproterozoic or older and their secular distribution is thought to be due to cooling of the Earth. Known deposits do not occur in the oldest (>3.0 Ga) komatiites but appear at around 2.7Ga in continental (Kambalda, Western Australia) pr island-arc (Perseverance- Mount Keith, Western Australia) environments, possibly because it was these environments that offered the opportunity for interaction with felsic rocks. It is suggested that the development of these environments in the Archean was an additional control on the age distribution of these deposits. It is postulated that the restricted secular distribution of PGE-enhanced intrusions is also due to the need for a hot mantle to give rise to U-type magmas.