RE:Spatial Periodicity and Clusters
Article history:
Received 25 June 2015
Received in revised form
21 August 2015
Accepted 26 August 2015
Available online 14 September 2015
Keywords:
Tectonic setting
Porphyry copper-gold-molybdenum deposit
High sulphidation
Geology
Alteration
Structure
a b s t r a c t
The Oyu Tolgoi cluster of seven porphyry Cu-Au-Mo deposits in southern Mongolia, define a narrow,
linear, 12 km long, almost continuously mineralised trend, which contains in excess of 42 Mt of Cu and
1850 t of Au, and is among the largest high grade porphyry Cu-Au deposits in the world. These deposits
lie within the Gurvansayhan island-arc terrane, a fault bounded segment of the broader Silurian to
Carboniferous Kazakh-Mongol arc, located towards the southern margin of the Central Asian Orogenic
Belt, a collage of magmatic arcs that were periodically active from the late Neoproterozoic to Permo-
Triassic, extending from the Urals Mountains to the Pacific Ocean. Mineralisation at Oyu Tolgoi is
associated with multiple, overlapping, intrusions of late Devonian (w372 to 370 Ma) quartz-
monzodiorite intruding Devonian (or older) juvenile, probably intra-oceanic arc-related, basaltic lavas
and lesser volcaniclastic rocks, unconformably overlain by late Devonian (w370 Ma) basaltic to dacitic
pyroclastic and volcano sedimentary rocks. These quartz-monzodiorite intrusions range from early-
mineral porphyritic dykes, to larger, linear, syn-, late- and post-mineral dykes and stocks. Ore was
deposited within syn-mineral quartz-monzodiorites, but is dominantly hosted by augite basalts and to a
lesser degree by overlying dacitic pyroclastic rocks. Following ore deposition, an allochthonous plate of
older Devonian (or pre-Devonian) rocks was overthrust and a post-ore biotite granodiorite intruded at
w365 Ma.
Mineralisation is characterised by varying, telescoped stages of intrusion and alteration. Early A-type
quartz veined dykes were followed by Cu-Au mineralisation associated with potassic alteration, mainly
K-feldspar in quartz-monzodiorite and biotite-magnetite in basaltic hosts. Downward reflux of cooled,
late-magmatic hydrothermal fluid resulted in intense quartz-sericite retrograde alteration in the upper
parts of the main syn-mineral intrusions, and an equivalent chlorite-muscovite/illite-hematite assem-
blage in basaltic host rocks. Uplift, facilitated by syn-mineral longitudinal faulting, brought sections of
the porphyry deposit to shallower depths, to be overprinted and upgraded by late stage, shallower,
advanced argillic alteration and high sulphidation mineralisation. Key controls on the location, size and
grade of the deposit cluster include (i) a long-lived, narrow faulted corridor; (ii) multiple pulses of
overlapping intrusion within the same structure; and (iii) enclosing reactive, mafic dominated wall rocks,
focussing ore.
2015, China University of Geosciences (Beijing) and Peking University. Production and hosting by
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