Exploration geologists face the challenging task of having to target mineral deposits at increasingly greater distances to the primary mineralisation.
In order to see through the Earth’s cover and detect even subtle footprints of economic mineral systems, researchers are using a variety of techniques, including mineral geochemistry, to aid exploration efforts. Indicator minerals such as magnetite from stream sediments, glacigenic sediments, or regolith cover can provide vectors toward mineralised areas in environments where outcrops are scarce and access is difficult.
Recent years have seen an increased interest in the use of magnetite for provenance studies and as a pathfinder mineral for exploration.
A recent CSIRO study found that magnesium, aluminium, titanium, vanadium, chromium, manganese, cobalt, nickel, zinc, gallium and tin concentrations display systematic variations in magnetite from barren and mineralised rocks from different types of mineral deposits. In addition, the occurrence, abundance and composition of mineral inclusions in magnetite can also be a useful guide for exploration. For example, sulphide inclusions in magnetite are a characteristic feature for hydrothermal magnetite from sulphidic hydrothermal mineral deposits such as skarn or porphyry systems.
Figure 1 3-D block diagram illustrating the various settings in which indicator minerals such as magnetite can be employed for provenance studies and mineral exploration.
Magnetite is an important indicator mineral and commonly occurs in a variety of mineral deposits and their host rocks. It crystallises over a wide range of geological conditions and can incorporate a large number of minor- and trace-elements. Furthermore, magnetite is more resistant to weathering and transport than many other minerals, is easily identifiable, and can be easily magnetically separated due to its magnetic properties. These features make magnetite an ideal petrogenetic indicator. In combination with the development and improvement of analytical techniques such as laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and electron probe microanalysis (EPMA), that allow measurements with increasingly lower detection limits, hydrothermal and igneous magnetite can now be characterised in greater detail than previously possible. In-situ measurements on individual samples offer valuable information that complement small–scale petrographic observations, whereas bulk magnetic separates can be analysed rapidly with EPMA and LA-ICP-MS to provide indepth data that can be used to reveal large-scale trends in magnetite from stream sediments, regolith cover or drill core. “Such data can provide key insights into the likely locations of ore deposits and the type of mineralisation.”
Variations in the concentrations of minor- and trace-elements in magnetite reflect the formation conditions and the evolution of a specific geological setting and represent a unique compositional signature. The composition of magnetite is governed by a number of factors such as temperature, fluid composition, oxygen and sulphur fugacity, silicate activity, host-rock buffering, re-equilibration processes, and intrinsic crystallographic controls such as ionic radius and charge balance. These factors translate to a unique magnetite composition that can help explorers discriminate mineralised from barren rocks in greenfields exploration.
Figure 2 Median element concentrations in parts per million in hydrothermal and igneous magnetite from a variety of mineral deposits. Magnetite from the Inner Zone Batholith represents an example from an unmineralised porphyritic host rock.
A combination of multi-element statistics and element ratio plots can reliably identify magnetite from different types of mineral deposits and discriminate hydrothermal from igneous magnetite. Statistical data exploration is becoming an increasingly invaluable tool to reveal trends and patterns in large data sets. Explorers can use principle component or factor analysis and discrimination measures to determine underlying trends and multi-element inter-relationships that are often obscured in standard geochemical data processing and visualisation.
The composition and mineral inclusion inventory of magnetite is a cost-effective and reliable tool that can help explorers to target prospective areas in remote and deeply covered terranes.
* Patrick Nadoll is a geochemist at the CSIRO.
This article appears courtesy of the CSIRO.