Outlines of the main methods used in our work on volcanic stratigraphy,
VMS deposits, and hydrothermal alteration are given in the following
Barrett, T.J. and MacLean, W.H., 1999. Volcanic sequences, lithogeochemistry,
and hydrothermal alteration in some bimodal volcanic-associated massive
sulfide systems. In: Volcanic-Associated Massive Sulfide Systems: Processes
and Examples in Modern and Ancient Settings (C.T. Barrie and M.D. Hannington,
editors). Reviews in Economic Geology, Volume 8, p. 101-131.
Barrett, T.J. and MacLean, W.H., 1994. Chemostratigraphy and hydrothermal
alteration in exploration for VHMS deposits in greenstone and younger
volcanic rocks. In: Alteration and Alteration Processes Associated with
Ore-Forming Systems. D. R. Lentz, editor.
GAC-MAC Annual Meeting, 1994, Waterloo, Canada. Geological Association
of Canada, Short Course Notes, v. 11, p. 433-467.
Barrett, T.J. and MacLean, W.H., 1994. Mass
changes in hydrothermal alteration zones associated with VMS deposits
in the Noranda area.
Exploration and Mining Geology, v. 3, p. 131-160.
MacLean, W.H., 1990. Mass change calculations in altered rock
Mineralium Deposita, v. 25, p. 44-49.
MacLean, W.H., 1988. Rare earth element mobility at constant inter-element
ratios in the alteration zone of the Phelps Dodge massive sulphide deposit,
Mineralium Deposita, v. 23, p. 231-238.
MacLean, W.H. and Barrett, T.J., 1993. Lithogeochemical methods
using immobile elements.
Journal of Exploration Geochemistry, v. 48, p.109-133.
MacLean, W.H. and Kranidiotis, P., 1987. Immobile elements as
monitors of mass transport in hydrothermal alteration: Phelps Dodge massive
sulfide deposit, Matagami.
Economic Geology, v. 82, p. 951-962.
Volcanic stratigraphy in the area of the
Norbec and East Waite VMS deposits of the Noranda camp, showing their
position above the Waite Rhyolite and below the Amulet Andesite. Alteration
pipes extend below the Waite Rhyolite into the Waite Andesite.
Problems in VMS exploration: primary stratigraphic relations are obscured by hydrothermal alteration, or distorted by tectonic effects such as folding and fault offsets, making it difficult to locate target horizons.
Using lithogeochemical data and immobile element relationships, specific volcanic units can be identified and traced. The upper plot shows highly altered volcanic rocks at the Phelps Dodge deposit, in the Matagami area, which were all derived from one homogeneous unit. The magmatic affinity of a series of volcanic rocks can also be determined, as in the lower plot, where the Ansil mafic to felsic volcanic rocks, in the Noranda area, are of transitional affinity.
Once the individual volcanic units are identified using their lithogeochemical fingerprints, their 3-D distribution can be plotted.At the Mobrun deposit, sulfide lenses are generally located at the contacts between chemically different rhyolite units.
Once a fractionation trend is established using a suite of least altered samples from a particular volcanic sequence, the altered samples can be superimposed on a binary immobile-element plot to examine the effects of alteration. In the diagram, the effects of net mass gains and losses of mobile elements are shown for a dacite to rhyolite series. Samples plotting in the mass loss field have generally experienced leaching due to hot fluids, leaving sericite-chlorite-rich residues. Samples plotting in the mass gain field have experienced precipitation of components such as silica, carbonates and sulfides, commonly at cooler temperatures.
From the immobile element relations, mass changes can be calculated for the mobile elements for each sample, as schematically shown. This allows quantitative assessment of hydrothermal effects. The results often give a picture of alteration which differs considerably from taking the untreated lithogeochemical data at face value.
Commonly, VMS deposits are hosted by only two or three main volcanic units, and thus single-precursor treatments can be applied. However, more complex volcanic systems also exist which are related by continuous fractionation. These are treated using the multiple precursor system, which is similar in principle, but requires a few extra steps to obtain proper mass change results.
Using a spreadsheet calculation procedure which relates a series of altered rocks back to their appropriate precursor compositions, mass changes are calculated for each element. Mass changes which are due to hot fluids can be recognized, as can those due to cooler fluids -- the former are of greater interest to the explorationist as they are associated with mineralization. The mass changes results, which provide hydrothermal alteration vectors, can be plotted down individual drillholes, and can be contoured on sections to show large-scale alteration trends.
In the world of VMS stratigraphy and deposits, chemical effects near mineralization produce major magnifications and diminutions in the abundances of various elements. However, some of these effects are 'illusory' in the sense that immobile elements which appear to increase or decrease, such as Al and Ti, in fact go nowhere. In the example shown, which is for altered rhyolites at a VMS prospect, silica additions and depletions are mainly responsible for the large apparent changes in Al and Ti values from precursor composition (shown by the whited dot).
To make matters more potentially confusing, mobile elements which really have been significantly removed or added by alteration can end up with apparently normal concentrations. These various effects make rock identification in altered terrains a clouded or even hopeless task, as normal relations get turned inside out. Even Dr. Who might be perplexed.