Black shales Research Papers - Academia.edu (original) (raw)

The world’s premier deposits of barite, copper,
gold, iron, lead, and zinc occur in sedimentary basins
(Laznicka, 1999). Many are associated with organicrich
shales and hydrocarbons (Fig. 1). For example,
the Pb and Zn ores of the American Mississippi Valley
including the Viburnum Trend, the world’s largest
known concentration of lead, occur on the peripheries
of basins containing hydrocarbon deposits and black
shales (Dozy, 1970; Coveney, 2000). Spatial correlations,
among other factors, have led to inferences
about critical roles for organic matter and organisms
in sourcing, transporting, and depositing ores. However,
in many cases, clear proof of a genetic relationship
between ores and organic matter remains
elusive—in some cases even dubious, despite spatial
coincidence. The Selwyn Basin of western Canada
and the Zechstein of northern Europe contain both
carbonate-hosted deposits and sedimentary exhalative
(sedex) deposits, which share many common characteristics,
including transport of ore constituents by
basinal brines (Sangster, 1990). Yet, certain major
shale basins, such as those of the Mississippi Valley,
USA, contain no known sedex ores, despite the
presence of major carbonate-hosted lead–zinc deposits,
black shales, and hydrocarbons. Moreover, some
black shale basins appear to contain neither type of
deposit, although this may be attributable to insufficient
exploration.
Besides the productive deposits alluded to above,
stratified rocks contain other deposits that may be
important in the future including non-placer resources
of Au and platinum group elements (PGE). For
example, Kucha (1982) reported remarkably rich
centimeter-scale Au and PGE laminae in the Kupferschiefer
of Poland, which are not economic because of
their thinness. Nevertheless, his results led to further
studies and the discovery of significant Au mineralization
in the accompanying Rote Fa¨ule facies in the
same mining district (Piestrzyn´ski and Wodzicki,
2000), which may become an important source of
precious metals. Similar possibilities exist for the
unusual decimeter-scale beds of Ni, Mo, As, and Zn
sulfides containing PGE and Au in China (Fan, 1983)
and the Yukon (Hulbert et al., 1992) (Fig. 1). Their
possible economic value is supported by reports of
thicknesses up to 1 m for the deposits in Guizhou,
China (Zeng, 1998; Mao et al., 2002). Moreover,
Goodfellow (2002) has documented three new occurrences
in the Yukon, separated by 400 km. These
workers suggest that the deposits may have been
deposited from normal sea water or in the wake of
an asteroid impact, respectively, but others favor
comparatively prosaic sedex origins (e.g., Hulbert et
al., 1992; Lott et al., 1999).