THE GEOCHEMICAL AND MINERALOGIC FOOTPRINT OF HYDROTHERMAL ALTERATION AT BUTTE, MONTANA

The giant Butte district hosts two porphyry Cu-Mo centers and the younger Main Stage system of polymetallic (Cu-Zn-Pb-Mn-Ag) veins that extend >6 km westward and track lateral fluid flow at ~300°C. Altered samples of the host Butte granite were analyzed using commercial lithogeochemistry (4 acid digestion and ICP & ICP-MS with detection limits near crustal abundance) to document chemical gains and losses. Hydrous minerals were identified with short-wave infrared (SWIR) spectroscopy, and selectively analyzed via EMPA and LA-ICP-MS.

Hydrothermal micas/clays characterize the sericitic selvages on Main Stage veins. In the central district, quartz-sulfide veins have wide selvages (m-scale) with inner pyrophyllite and outer muscovite, whereas distal quartz-carbonate veins have narrow selvages (dm-scale) containing relict igneous feldspar, local added K-feldspar, and mafic sites replaced by mixtures of chlorite and phengitic muscovite. The lateral gradient reflects decreasing acidity of fluids resulting from wall-rock alteration. In the central zones, late-formed phengitic illite, kaolinite, and smectites replace plagioclase and mafics to form intermediate argillic alteration at <300°C. “Sericite” compositions range from white muscovite in low-pH central zones lacking feldspar to pale green phengitic muscovite and illite in rock-buffered moderate-pH zones with feldspar. SWIR readily distinguishes the muscovite Al-OH absorption (2195 nm) from phengitic muscovite or illite (2204-2216 nm).

Main Stage ores are zoned from inner Cu(Ag-As) to Zn-Pb-Ag to Mn(Ag) to outer barren (quartz). Trace element anomalies in altered rocks also track this change via additions (inner vs outer zones) of chalcophile elements likely present in pyrite and Cu- and Zn-sulfides (As, Bi, Cd, Te, Se, Co, Ni), metals that form separate sulfide or oxide phases (Mo, Sn, W), and alkalis (K, Cs, Rb, Tl, Li, Ba) and metals (V, Cr, Sc) incorporated into micas and chlorite. Whereas some elements are likely supplied by the magma (Cu, Ag, As, Mo, Sn, W, K, Cs, Rb), other elements anomalous in distal Main Stage may be derived from wall-rock alteration in the central zones (Mn, Zn, Pb, V, Cr, Sc, Li). Regardless, anomalous metals with high valence form stable oxides so that they remain in weathered rocks and may provide effective vectors to central ores.