SHEETED DYKE SWARMS - LAYERED GABBROIC ROCKS - FAULT ZONES - STOCKWORKS THAT DON'T WORK AND MASSIVE SULPHIDE DEPOSITS

The most common sulfide mineral in VMS deposits is pyrite, which is often associated with other sulfides such as pyrrhotite, chalcopyrite, sphalerite, and galena (Galley and others, 2007). Other possible nonsulfide minerals associated in VMS deposits include magnetite, hematite, and cassiterite; barite can be present as a gangue mineral. All these minerals have relatively high values of specific gravity (4.0–7.5 g/cm3; table 7–2), which is in strong contrast to the significantly lower densities measured in their sedimentary or volcanic host rocks. Thomas (2003) measured densities of 2.70–2.84 g/ cm3 for the host rock at the Bathurst mining camp.
Electrical Signature
Electrical methods are highly effective in identifying VMS targets because they respond to the electrical conductivity of the rocks and minerals, which can vary by 20 orders of magnitude (Grant and West, 1965). Electrical methods are unique in being able to detect such a large range of magnitudes; no other physical property has such a wide range. Because of this large potential range in values, a variety of electrical techniques have been developed that capitalize on these differences, such as measurement of conductivity, resistivity (the inverse of conductivity), induced polarization, electromagnetism, and gamma ray spectra (table 7–1). Electrical methods are currently the most used technique in surveying for VMS deposits; a variety of survey types (for example, MegaTEM, Titan24, and borehole techniques) are pushing the limits of detectable depth ranges.
Volcanogenic massive sulfide deposits have high conductivities (fig. 7–2B) exceeding 500 mS/m (millisiemens per meter) and are similar in magnitude to graphite and saltwater (Ford and others, 2007). Compared to igneous and metamorphic rocks with typical conductivities of <1 1="" 500="" a="" also="" and="" anoxic="" are="" as="" associated="" be="" between="" body.="" body="" br="" by="" can="" complicating="" conductive="" conductivities="" conductivity="" contain="" content="" contrast="" could="" definitive="" deposits.="" deposits="" difficult="" distinguish="" distinguishable="" economic="" effectively="" electromagnetic="" exploration="" factor="" from="" fully="" graphite="" highly="" horizons="" host="" however="" in="" increase="" introduced="" its="" m="" mask="" massive="" may="" ms="" noneconomic="" not="" of="" or="" ore="" other="" overlying="" physical="" potentially="" property.="" pyrite-rich="" pyrrhotite-rich="" reducing="" rock="" rocks="" sedimentary="" sediments.="" signal="" significant="" so="" some="" substantially="" such="" sulfide="" techniques="" that="" the="" themselves.="" thus="" to="" tools="" types="" typically="" unit="" useful="" vms="" water-rich="" water="" with="">Electrical resistivity surveys are useful in calculating the apparent resistivity of the subsurface at different depths resulting in the generation of cross sections of true resistivity (Ford and others, 2007). These can be used to produce three-dimensional geometries of ore bodies at depth. Resistivity surveys also are used to estimate the thickness of overburden, which
7. Geophysical Characteristics of Volcanogenic Massive Sulfide Deposits
By Lisa A. Morgan
118 7. Geophysical Characteristics of Volcanogenic Massive Sulfide Deposits
Figure 7–1. Schematic diagram of the modern Trans-Atlantic Geothermal (TAG) sulfide deposit on the Mid-Atlantic Ridge, depicting a cross section of a volcanogenic massive sulfide deposit with concordant semi-massive to massive sulfide lens underlain by a discordant stockwork vein system and associated alteration halo. From Hannington and others (1998) and Galley and others (2007). Modified from Hannington and others (2005).
then can be used to improve interpretation of ground gravity surveys (Ford and others, 2007). Conductivity, the inverse of resistivity, also can be used to map overburden.
Induced polarization (IP) surveys measure the chargeability of the ground and the time variance in the response of the electromagnetic field, which is related to ability of the material to retain electrical charges. Induced polarization surveys are very effective in detecting disseminated sulfide bodies. Typically, these sulfides occur in the altered halo surrounding the massive sulfide ore body and may be associated with clays, which also produce significant IP responses (Ford and others, 2007).
The techniques associated with electromagnetic (EM) surveys, collected both on ground and in air, are the most common electrical methods employed in mineral exploration. Electromagnetic techniques can directly detect conductive features such as base metal deposits where significant contrasts in conductivity values occur between the ore bodies and their resistive host rocks (Thomas and others, 2000). Values for the conductivity of soils, rocks, and ore bodies, measured in milliSiemans per meter, span several orders of magnitude ranging from 3.57×109 mS/m for graphite to 5×108 mS/m for pyrrhotite to 0.01 mS/m for gravel and sand (Thomas and others, 2000). Both airborne and ground electromagnetic techniques are effective in detecting