Page 136 - Handbook of Gold Exploration and Evaluation

Geology of gold ore deposits 115

2.21 Typical cross-section of seafloor hydrothermal system (derived from
Hertzig and Hannington, 1995). (From Ore Geology Reviews, Volume 10, P
Hertzig and M Hannington, Polymetallic massive sulphides at modern seafloor,
pp 95±115, 1995, with permission from Elsevier).

2.4.2 Mesothermal gold ores
Mesothermal gold ores comprise mainly high fineness (low silver) quartz vein
systems and disseminated replacement deposits. The orebodies are emplaced
either in or near supracrustal belts dominated by volcanic rocks, or in supra-
crustal belts dominated by elastic sedimentary rocks. Table 2.9 offers a broad
classification of mesothermal gold deposits for exploration purposes based upon
host rock type and form of mineralisation. Whilst acknowledging the
shortcomings of this grouping, Hodgson stresses that factors such as the
geometrical and mineralogical-chemical characteristics of mineralisation
commonly relate to host-rock type and point to the significance of the sulphide
content in iron formation deposits. The main associated minerals in mesothermal
ore zones are pyrite, pyrrhotite, base metal sulphides, arsenopyrite, tourmaline
and molybdenite. In some areas, deposit size is significant. For example, in the
Timmins-Kirkland Lake area, Toronto, Canada, Hodgson and Troop (1988)
found that scheelite, tourmaline, arsenopyrite, tellurides, sphalerite and galena