SODIC ALTERATION AND FE-OXIDE-RICH HYDROTHERMAL SYSTEMS

Na-rich hydrothermal alteration has been long recognized as a feature of many Fe-oxide-rich hydrothermal systems (Lindgren, 1913, Mineral Deposits), yet its characteristics, origin and significance are still debated. A more complete understanding is central to evaluating the processes and sources of metals in Fe(-Cu-Au) deposits. New and published data on Na-rich alteration in such systems and in other kinds of igneous-related systems have systematic patterns in mineral associations, geochemistry, volumes and geological associations.

Sodic and sodic-calcic assemblages associated with Fe-oxide systems have Na plag ± Na scap, Mg- to Ca-rich mafics (chl, act, variably sodic amph & cpx), are voluminous (commonly 10s to 100s of km³), and show major mass transfer (at const. Al₂O₃) notably Na ± Ca addition and, typically, loss of large masses of metals (commonly millions of tonnes). These features occur in some porphyry Cu(-Au-Mo) systems, many unmineralized igneous centers, and, voluminously, in seafloor hydrothermal alteration. This class occurs in many geologic settings with diverse igneous rocks ranging from basaltic to rhyolitic, tholeiitic to alkaline. Fluid inclusion and isotopic data indicate formation from moderate- to high-salinity, low- to moderate-CO₂-bearing fluids with d¹⁸O typically resembling their host rocks but not requiring a magmatic fluid source.

Contrasting styles of Na alteration occur with compositionally restricted groups of igneous rocks. Their mineral assemblages, metasomatic changes, zoning patterns, associated metals, volumes all differ from the Na(Ca) group. Peralkaline-related systems generate Na fenites, typified by the presence of sodic mafic phases. Alb + musc (± qz ± K-feld ± fluor) assemblages are common, though volumetrically small in granitic systems, particularly where fluorine-rich. CO₂ is also common.

Magmatic and external fluids both create sodic alteration. Na(Ca) is most consistent with Na-rich external fluids and is not readily explained by any reported phase equilibria for a magmatic fluid source. Alkali-carbonate-bearing fluids are peralkaline; they can derive from peralkaline magmas and some external sources, but they can not form from metaluminous to peraluminous magmas nor can they lead to Na alteration by CO₂ unmixing.