Page 141 - Handbook of Gold Exploration and Evaluation

120 Handbook of gold exploration and evaluation

2.24 Schematic representation of Au, Cu-Au and Cu-Mo porphyries, such as at
Fort Knox, Alaska.

are thought to be initially hot (500±600 ëC) but zoned alteration assemblages are
formed at lower temperatures as the metal-bearing fluid cools and reacts with
the country rock as it moves away from the intrusion. Deeply eroded volcanic-
plutonic arcs characterised by stocks and widespread volcanic rocks of inter-
mediate composition may provide attractive targets for alluvial gold exploration.
Sillitoe (1993) reconstructs a typical intrusion-centred gold district charac-
terised by carbonate wall rocks in Fig. 2.25. Geological evidence and fluid-
inclusive studies in Sillitoe's text support depths ranging from 1 to 3 km beneath
the palaeo-surface.

2.4.4 Carlin trend type gold deposits

The Carlin trend forms the largest and most prolific accumulation of gold
deposits in North America (Teal and Jackson, 1997). Discovered in 1961, more
than 40 separate deposits of disseminated gold mineralisation in carbonate rocks
have contributed 25 million ounces of gold from 26 working mines. The Carlin
trend is a 60 km long north-northwest trending alignment of gold deposits
located in northeastern Nevada. Gold mineralisation is hosted in a variable
package of Ordovician through lower Mississippian. The current north-
northwest alignment of the Carlin trend reflects an apparent pre-existing zone
of crustal weakness that transects present north-south trending basin and range
topography. Inception of basin and range extension is interpreted to have begun
with the onset of regional intrusive activity during the late Eocene ( 37 Ma)
(Christiansen and Yeats, 1992). Tectonism may have begun during early
Miocene ( 20 Ma), but remains a subject of debate (Teal and Jackson, 1997).
Carlin-trend geologists suggest a range of interactions involving system,
structure and host rock as essential components for the formation of a gold
deposit. Within this context, the major geologic parameters that have
contributed to the genesis of gold deposition on the Carlin trend are: