ALTERATION GEOCHEMISTRY OF THE BOURBON IRON OXIDE-APATITE AND BOSS IRON OXIDE-COPPER-GOLD DEPOSITS, SOUTHEAST MISSOURI, USA

To recognize metasomatism associated with primary and secondary alteration in iron oxide-apatite (IOA) and iron oxide-copper-gold (IOCG) deposits, whole rock and pXRF analyses were conducted on variably altered host rocks, ore breccias, and veins from the Bourbon IOA and Boss IOCG deposits. Major element data and mineral compositions were plotted on molar K/Al vs. Na/Al plots and Al-K-Fe, Al-K-Mg, and Al-K-Ca ternary diagrams.

At Bourbon, most altered rocks have high K/Al ratios that extend towards the K-feldspar-biotite end member, but a minority have high Na/Al ratios that extend towards albite, indicative of major K and minor Na metasomatism. Some samples have low K/Al and Na/Al ratios that are shifted towards the clay end member, indicative of hydrolytic metasomatism. On the Al-K-Fe ternary diagram altered rocks plot along mixing lines between K-feldspar (or albite) and magnetite end members, indicative of strong Fe metasomatism. Samples with argillic alteration extend between hematite, chlorite, and muscovite (or clay) end members. Ore breccias and veins cluster at the magnetite, actinolite, phlogopite, and apatite end members.

At Boss, altered rocks primarily have high K/Al ratios that extend towards the K-feldspar-biotite end member, indicative of major K with little to no Na metasomatism; some samples also have low K/Al and Na/Al ratios indicative of hydrolytic metasomatism. On the Al-K-Fe ternary diagram, most altered rocks plot along mixing lines between K-feldspar and biotite end members with a minority that extend towards magnetite end member, indicative of moderate Fe metasomatism. On the Al-K-Ca ternary diagram altered rocks extend towards epidote, indicative of minor Ca metasomatism. Ore breccias and veins cluster between magnetite and biotite end members with a minority that extend towards K-feldspar and muscovite; others show extension towards garnet, epidote, and carbonate (or fluorite) end members.

Whole rock and pXRF analyses of altered rocks in the studied IOA and IOCG deposits agree with primary and secondary mineral assemblages identified by logging and support major element classification schemes.