Low sulfidation epithermal deposits are important sources of gold and silver,
and are considered the extinct analogues of modern geothermal systems. The
aerial extents of alteration halos that surround twenty-two epithermal deposits
range from 4 to 315 km2, with many less than 60 km2. Deposits with alteration
halos <30 km2 typically have well defined limits, whereas deposits with>
alteration exceeding 30 km2 are commonly complicated by overlapping alteration
centres related to adjacent epithermal deposits or other deposit types (i.e.
high sulfidation, porphyry). Irrespective of the physical coverage of vein or
alteration halos, both large (>60 km2) and small (<20 km2) deposits can>
contain large amounts of gold and silver. The size ranges for twenty-one
analogous liquid-dominated geothermal fields from around the world have a
restricted size range of 5 to 60 km2 and irrespective of physical size, commonly
have multiple zones of focused fluid upflow.
From an exploration perspective the comparison between epithermal deposits
and geothermal fields provide the following four important points. 1) Alteration
halos surrounding epithermal deposits as well as those for analogous geothermal
systems show a range of sizes, although most are less than 60 km2. Therefore,
where alteration exceeds 60 km2 multiple vein systems related to several
paleogeothermal systems are predicted. 2) Multiple upflow zones are common in
geothermal fields of all sizes (5 to 60 km2) and although difficult to
demonstrate in epithermal deposits there is a strong possibility of discovering
several discrete adjacent mineralised centres. 3) The orientation of veins
parallels the principle orientation of the surrounding alteration and where
alteration is wider than the vein strike length, this could suggest the
occurrence of adjacent veins. 4) The physical size of both veins and alteration
zones is not important in relation to amount of contained gold, as both large
and small can contain large amounts of gold. These conclusions provide insight
into the physical size limits and hydrologic structure of these paleogeothermal
systems and highlight the potential for multiple mineralisation centres within a
single alteration halo.