Cosmic-Ray Muography as a New Tool for the Density Characterization of Plumbing Systems of Hydrothermal Fluids: Structurally Controlled Carbonate Alteration as an Example

Carbonate alteration is a common feature in proximity to many hydrothermal ore deposits, including some orogenic gold, porphyry, Carlin, skarn, epithermal and VMS deposits. It is also present in Mars. Amongst the common carbonate alteration minerals are calcite (2.71 g/cm3), dolomite (2.842.86 g/cm3), ankerite (2.933.10 g/cm3), and siderite (3.96 g/cm3), with the measured densities given in parentheses. Their respective portions in altered rocks are governed by, e.g., the volume of fluid, extent of the hydrothermal system, and fluid-rock reactions driven by chemical imbalance. In many cases carbonate alteration is particularly widespread and vertically most continuous along major shear zones that were once tapped into the CO2-rich fluid source. In typical situations, hydrothermally introduced carbonates replace pre-existing silicates. Mapping of the extent of carbonate alteration assists in recognition of the past fluid pathways. These, in turn, are useful as vectors towards structures and mineralization. We propose that the extent of intense carbonate alteration can be followed by applying a novel density variation imaging method called muography. It is based on the ambient secondary cosmic-ray induced muon particles that constantly radiate from the upper atmosphere with relativistic energies (speeds). The muons are highly penetrative in rocks and as they interact with material, they lose energy and can penetrate shorter distance inside rocks the higher is their average density. The resulted attenuation in the muon flux in different directions is exploited for reconstructing density variations models of the imaged volume of geological materials. Applying muography requires that the density changes associated with the added carbonate assemblages are not compensated by removal or addition of other hydrothermal minerals (i.e., the protoliths have a different density than the carbonated rocks). Hence, if carbonate alteration has generated notable density contrasts in rocks and those are large enough for detection (1% increase in density increases attenuation by 3%), muography can be applied for carbonate alteration mapping. Figure 1 proposes some examples of detector arrays that can be used for this type of surveys. The method is applicable to other alteration styles if the same premises are met.