Copper inclusions in sheet silicates from porphyry Cu deposits
Economic geologists have studied the Cu contents of biotite for at least a quarter of a century as potential guides for exploration and as clues to the genesis of porphyry copper deposits1–12. These earlier studies used bulk analytical techniques, and a common, although cautious, assumption was that Cu substituted for Fe in the octahedral sites of the biotite. Here we report data from high-resolution and X-ray analytical transmission electron microscopy which demonstrate that anomalous Cu in biotite is present primarily as submicrometre inclusions of pure native Cu in the interlayer regions of the mica. The inclusions occur with greatest density and size in mixed-layer sheet silicate domains which form by alteration of biotite and chlorite. Less abundant and smaller Cu inclusions are associated with biotites that show subtle alteration only on the high-resolution transmission electron microscopy scale. The association of Cu inclusions with alteration domains, coupled with mass-balance considerations, suggests that these sheet silicates act as Cu sinks, and not as sources for primary mineralization.
Geochemistry of biotites from mineralized and barren intrusive systems
Ninety-two biotite separates from 35 different igneous systems in western North America and the Caribbean region have been analyzed for Cl, F, H 2 O, Cu, Zn, Fe, and Mg. Biotites in fresh magmatic samples from igneous systems genetically related to significant mineralization (dominantly porphyry copper type) form a population with average chlorine and fluorine contents greater than and average copper content less than the average of the remaining barren intrusive systems (at the 90 percent confidence level). However, the magnitude of this halogen enrichment is too small to permit its use as a routine exploration tool. There are no systematic differences in Cl:F, F:H 2 O or other multicomponent ratios of biotite compositions that can be used to distinguish between mineralized and barren host intrusions with a high degree of confidence. Although the average halogen content of biotites from both mineralized and barren potassium-rich intrusions is higher than that for potassium-poor intrusions, biotites from mineralized potassium-rich intrusions are slightly enriched in halogens with respect to their barren counterparts. Magmatic biotites from the Boulder batholith do not exhibit systematic spatial variations in halogen abundance related to known areas of mineralization, although biotites from altered rocks around Butte exhibit chlorine depletion. The copper content of biotites from the same samples indicates the location of mineralization at Butte more reliably, however.
Time and Heritage
As a historian studying what can be considered a form of intellectual history, I have slowly come to adopt Michel de Certeau’s observation made at the end of the 1980s that ‘objectifying the past, for the last three centuries, has undoubtedly left unconsidered time within a discipline that has continued to use it as a taxonomic instrument’. To a certain degree, time has become commonplace for the historian who has preserved or instrumentalized it. It is not considered because it is inconceivable, but because we do not think of it or, more simply, we do not think about it. As a historian attempting to pay attention to the time I’m living in, I have thus, like many others, noticed the swift development of the category of the present until it has become obvious that the present is omnipresent. This is what I refer to here as ‘presentism’.