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.
Copper, manganese, and zinc in coexisting mafic minerals from Laramide intrusive rocks in Arizona
The partition of Cu, Mn, and Zn between coexisting hornblende and biotite was investigated in two Laramide intrusions in the Arizona porphyry copper province. Roozeboom diagrams suggest that an equilibrium distribution was attained and that changes in the partition coefficient are closely related to textural variations within a single rock type and to differences in rock types. Equilibrium partitioning of Cu was well developed, even though Cu concentrations in biotite varied from 15 to 3,658 ppm. Analyses of chlorite show that its Cu content is more variable than its Zn content and suggest differences between the magmatic and hydrothermal behavior of Cu and Zn. It is concluded that the Cu, Mn, and Zn concentration of hornblende and biotite were magmatically controlled and that high concentrations did not result from later introduction of material during subsolidus hydrothermal activity.The Cu content of biotite progressively increases during the early stages of magmatic crystallization. The decrease of Cu in biotite during the later stages is correlated with the appearance of a hydrothermal phase into which Cu was fractionated from the melt. Escape of this phase removed Cu from the system causing biotites growing in the residual molten portions of the intrusions to be impoverished in Cu. Metal balance calculations indicate that Cu lost from biotite can account for all the Cu in the genetically associated sulfide deposits.A practical evaluation of the data indicates that the variation of Cu in biotite can be a useful exploration tool. Copper analyses of biotite yield larger anomalous haloes and greater contrast between background and anomaly than other geochemical techniques.
A combined ion probe/spark source analysis system☆
Abstract Spark source mass spectrometry and secondary ion mass spectrometry have complementary advantages in the elemental analysis of solids. The AEI IM20 ion probe and MS702R mass spectrometer form a composite instrument incorporating both techniques; the design and different operating modes are described. A primary ion beam diameter as low as 2 μm can be obtained under favourable circumstances; the minimum detectable concentration of a typical element is derived as a function of this diameter. The mass resolution is greater than 5000 (10% valley definition) with electrical detection and 10,000 (50% density definition) with photographic detection; this resolution is much higher than that obtained with other secondary ion mass spectrometers. Two typical applications illustrate that such high mass resolution is a necessary prerequisite to quantitative interpretation in secondary ion mass spectrometry: precise isotopic analysis of titanium is demonstrated and the concentrations of impurity transition elements derived from ion probe and spark source analyses of a geological olivine are compared.