Hydrothermal Deposits - Metals and Society: An Introduction to Economic Geology - page 107

In the unconformity-related deposits deposits, pods, veins, and semimassive
replacements of uraninite are located close to unconformities between early Prote-
rozoic conglomeratic sandstones in the lower portions of intracratonic basins and
metamorphosed basement rocks (Fig. 4.12a ). Fluids in the sandstones are oxidized
and as they circulate they dissolve uranium from detrital minerals such as monazite,
alanite and apatite, which were derived initially from granites of the basement.
When these fluids come into contact with pockets of pelitic, organic-rich schist in
the basement, the uranium precipitates to form an ore body. The exact location of
the ore body is strongly influenced by variations in the permeability of the
sandstones and the presence of faults that control the fluid circulation.
Sandstone deposits are the principal source of ore in the USA, mainly from
deposits of the Wyoming Basin and Colorado Plateau. These deposits are contained
within medium to coarse-grained sandstones deposited in a continental fluvial or
shallow marine sedimentary environment. There are two main types, referred to
as “tabular” and “roll-front” deposits. In the former, impermeable shale or mudstone
units are interbedded in the sedimentary sequence and occur immediately above and
below the mineralized horizon. The fluid within the sandstone is low-temperature,
low-salinity, oxidized meteoric water that flows readily through the permeable
sandstone, transporting with it dissolved uranium; the fluid in the shales and
mudstones is a relatively stagnant, salt-rich basinal brine. Mixing between the two
fluids at the interface between the sedimentary units precipitates uranium minerals.
“Roll-front” deposits form in a similar geological setting but via a different
mechanism. The host rock is permeable sandstone which at depth contains
a reduced assemblage of pyrite, calcite and organic matter. Oxidized fluid flowing
from nearer the surface down the permeable horizon reacts with the reduced
material creating at redox front - a cusp-shaped zone where the two types of fluid
mix and react (Fig. 4.12b ). The uranium minerals initially precipitate at the front,
which continues to migrate down and along the sandstone layer. As it moves it
sweeps up the dispersed uranium in the reduced material concentrating it at the
redox front, thus creating a richer and richer deposit.

4.4.3 Iron-Oxide Copper Gold (IOCG) Deposits

An IOGC deposit is defined as a polymetallic, breccia-hosted deposit in which ore
is spatially and temporally associated with granite and with iron oxide alteration.
The Olympic Dam deposit is located within a funnel-shaped, hematite-rich hydro-
thermal breccia that formed close to the surface through progressive, polyphase
fracturing and alteration of the upper part of a granitic intrusion. The ore minerals
are diverse and complicated, comprising some 30 varieties of Cu, U, Au, Ag, Ni, Co
sulfides, sulfosalts, oxides, carbonates, and native metals. They occur as veins,
disseminations, irregular patches and breccia fillings that occur together with zones
of intense calcic-sodic, iron and potassic alteration.