Mass changes in hydrothermal alteration zones associated with VHMS deposits in the Noranda area

Barrett, T.J. and MacLean, W.H., 1994. Exploration and Mining Geology, v. 3, p. 131-160.
Mass changes in hydrothermal alteration zones associated with volcanogenic massive sulfide (VMS) deposits yield quantitative information on hydrothermal alteration and fluid-rock interaction, and provide another criterion for selecting exploration targets for locating sulfide mineralization. Immobile element methods allow determination of primary fractionation trends in volcanic rocks, subsequent mass changes, and recognition of volcanic chemostratigraphic units in altered sequences. Mass changes associated with VMS deposits in the Noranda area fall into two main groups. 1) Large silica and alkali mass losses and Fe+Mg mass gains in discordant feeder zones under small tonnage (<5 Mt) deposits of the bimodal Central Mine Sequence (CMS), e.g. the Ansil, Corbet and Norbec deposits. Volumetrically limited silica-sericite zones of mass gain occur lateral to these feeders. 2) Large volumes of silica-sericite alteration with net mass gain associated with the large tonnage Horne (60 Mt) and Mobrun (>15 Mt) deposits, which lie in mainly felsic volcanic terrains outside the CMS.

In the recharge zone of a seafloor hydrothermal convection system, cool seawater moving through the volcanic rocks introduces Mg and K mass, and also some silica dissolved from volcanic glass in higher lavas. In the discharge zone, water-rock ratios and alteration temperatures are much higher, and the main mass changes involve significant Fe mass gain, near-total Na and Ca losses, major silica loss, and local K loss.

As hot Fe-rich fluids rise and penetrate into the wallrocks, gradients in FeO mass and Na-Ca-K also will occur laterally away from highly discordant pipes. By contrast, the silica gradient should show a lateral crossover, as silica removed by leaching within the pipe is transported laterally and precipitated. Rare-earth elements may be leached from the hottest parts of pipes, and precipitated in cooler peripheral zones. In other alteration zones, the REE are immobile. Around Central Mine Sequence deposits, oxygen isotope ratios in the footwall show generally lower d18O values towards the orebody, partly due to temperature increase, but partly also due to silica mass loss. Host rocks of the felsic-hosted VMS deposits outside the CMS have strongly positive oxygen isotope anomalies related to generally lower temperature alteration and silica mass addition. Calculated mass changes provide a useful means of distinguishing chloritization associated with synvolcanic hydrothermal systems from regional chloritization at low water/rock ratios. In areas distal to VMS deposits, mass changes in Fe and Mg are near-zero, as chlorite forms due to hydration of Fe-Mg-Al-Si components already in the rock. Close to footwall pipes, however, chloritized zones show addition of Fe+Mg mass and silica loss, as a result of hot fluids and high water/rock ratios. Alteration around late fault zones may show Na depletion but not most other mass changes typical of seafloor mineralizing fluids; these areas are therefore low-priority targets. Within large Na depletion zones in general, smaller zones can be selected as high-priority targets if they additionally show Fe-Mg-Si mass changes of the type associated with VMS deposits.

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