Page 137 - Handbook of Gold Exploration and Evaluation

116 Handbook of gold exploration and evaluation

Table 2.9 Broad classification of mesothermal gold deposits based upon rock type
and form of mineralisation (after Hodgson, 1993)

Host rocks Dominant form of mineralisation Examples

In or near supracrustal belts dominated by volcanic rocks
a
Volcanic rocks, classic 1. Auriferous quartz veins and Sigma, Kirkland Lake, a
sedimentary rocks veinlet systems Hollinger-McIntyre, a
b
and intrusions Kalgoorlie, Mother
Lode, c
a
2. Disseminated pyritic quartz- Hemlo, Blanket d
albite and (or) potassium
feldspar- carbonate replacement
zones
Oxide-facies iron- Breccia, stockwork and bedding- Mount Magnet e
formation replacement zones
In supracrustal belts dominated by clastic sedimentary rocks
Silicate-facies and (or) Quartz stockwork and Morro Velho f
carbonate-facies iron- replacement zones Homestake g
formation
f
Oxide-facies iron- Breccia, staockwork and Sao Bento, Cuiaba, f
¬
formation bedding-replacement zones Raposos f
h
Clastic sedimentary Stawell, Bendigo h
rocks
a Abitibi-Wawa Belt, Candian Shield. Norseman-Wiluna Belt,Yilgarn Block, Australia. Foothills
b
c
d
e
metamorphic belt, California. Gwanda Belt, Zimbabwe Craton. Murchison Belt,Yilgarn Block,
f
g
h
Australia. Quadrilatero Ferrifero, Brazil. Black Hills, South Dakota. Lachlan fold belt,Victoria,
Australia.
were more common in the larger deposits of the area than in the smaller. They
noted also, that the mineral distribution varied between deposits associated with
quartz-bearing and quartz-free porphyries.
Mesothermal deposits occur mostly in low to medium grade metamorphosed
supracrustal belts and mineralisation appears to depend upon intrusion by felsic
to intermediate plutons. Important Mesozoic and Palaeozoic mesothermal gold
deposits in northern China and Korea include metamorphic granitic to inter-
mediate plutons and intermediate to high-grade Precambrian gneiss (Sang and
Ho, 1987). Age dating has shown that the timing of mesothermal ore formation
in both volcanic and clastic sedimentary belts is late in the orogenic history of a
region and is relatively universal between principal mineralised districts. Host
structures are mainly faults and folds and there is a regular association of the
deposits with fluid passages that are fault-controlled. The faults are nearly
always second-order structures, subsidiary to large barren strike-slip faults
(Sibson, 1987). Strike-slip shear zones parallel to the fault slip vector are more
conducive to dilation than the large and straight major faults.