Geology Reference
In-Depth Information
•
Reefs
(Monseur 1973; Etminan et al. 1984; Lagny
1984) are common host rocks, because of their high
primary porosity and the possibility that ongoing ce-
mentation forms a seal.
•
Hardgrounds
associated with condensation horizons
in basinal settings (e.g. Mn mineralizations; Krainer et
al. 1994).
•
Slopes
of shelf-basin transitions provide settings with
reduced sedimentation favoring e.g. manganese min-
eralizations (Maynard 1990).
may have been accelerated by the availability of abun-
dant organic matter and microbes as postulated for the
Middle Devonian Pine Point deposits. The lithofacies
of many Pb-Zn host rocks is characterized by the pre-
dominance of carbonates formed in very shallow
depths, tidal and lagoonal sediments, the abundance of
organic matter in limestones and dolomites indicating
reducing diagenetic conditions, abundant microbial tex-
tures, and evaporitic minerals. The multi-phase diagen-
esis of the Pb-Zn ores often starts within an open sys-
tem where precipitation can take place due to sulfate-
reducing bacteria or triggered by seepage and evapo-
rative pumping mechanisms. The second phase char-
acterized by diagenesis of both carbonate grains and
ore minerals occurs within a closed system and is con-
nected with the formation of diagenetic crystallization
rhythmites and syngenetic breccias.
Pb-Zn deposits
in carbonate rocks.
Carbonate-
hosted Pb-Zn ores known from Cambrian to Cretaceous
limestones and dolomites are excellent examples of syn-
and epigenetic carbonate-hosted mineral deposits. Fa-
cies controls are especially evident for Mississippi-
Valley Type and Bleiberg Type deposits (Fontboté and
Amstutz 1983; Buelter and Guillemette 1988; Mazzullo
and Gregg 1989; Cerny 1989; Bechstädt and Boni 1994;
Zeeh et al. 1995; Dejonghe 1998; Sass-Gustkiewicz
and Dzulynski.1999; Zeeh et al. 1999; Liaghat et al.
2000). The epigenetic (diagenetic) Mississippi-Valley
Type ores occur near platform margins or in epeiric
carbonates. (e.g. Missouri, North West Canada, Poland,
Sardinia, North Africa). The input of metal solutions
took place via saline connate waters from adjacent ba-
sins. The precipitation of metals occurred in solution
breccias and paleokarst structures and in reef zones.
The distribution of the syngenetic Bleiberg Type ores
is distinctly controlled by facies and regional tecton-
ics.
17.2.2 Microfacies and Ore Deposits
Microfacies studies of host rocks and mineral deposits
are relevant in evaluating the depositional environment
of the sediments affected by mineralizations, under-
standing the spatial and temporal relationships between
host rocks and ore bodies, and answering metallo-
genetic questions. Ore minerals must be studied in po-
larized reflected light of polished sections.
Reconstruction of sedimentary environments.
Microfacies data described in Chap. 4 and Chap. 5 al-
low paleoenvironmental controls (Chap. 12) to be iden-
tified and syngenetic mineralizations to specific facies
zones and facies models (Chap. 14) attributed. These
data assist in evaluating the overall depositional frame-
work affected by mineralizations.
Strata-bound and stratiform Pb-Zn deposits occur
in different paleogeographic positions: (a) at the base
of transgressive surfaces above the basement near ba-
sin margins (e.g. Mississippi-Valley Type deposits and
deposits in Europe), and (b) within thick carbonate se-
quences far from transgressive horizons (e.g. Bleiberg
or Spain). Both types correspond to shallow-marine
sediments, the former with partly high clastic input.
The joint criteria of both types are mineralizations of
tidal and lagoonal sediments, evaporitic environments,
and abundance of dolomitization (Cerny 1989).
Many mineralizations are associated with bedded
platform interior limestones exhibiting cyclic sedimen-
tation patterns (Bechstädt 1975) and indications of sub-
aerial exposure, and with reef limestones. Most reefs
are shallow-water platform reef mounds and platform-
margin reefs, both of which are prone to sea-level fluc-
tuations causing karstification and solution and cavern
porosities. In contrast, Pb-Zn deposits within mud
mounds formed in deeper outer ramp positions are of-
ten fault-controlled (e.g. Early Carboniferous mounds
in Ireland). The mineralization postdates mud mound
formation. The precipitation of Pb-Zn sulfides in reefs
Host rock/mineralization relationships.
Answering
this question requires a thorough thin-section analysis
of carbonate beds below and above the mineralized
horizon (e.g. Hagenguth 1984) and the differentiation
of microfacies types (e.g. Cisternas 1986).
Metallogenetic problems.
Sedimentary fabrics of
carbonate rocks (Chap. 5) and fabrics observed in syn-
genetic ores exhibit many coincidences. These coinci-
dences are used to prove the syngenetic origin of strati-
form deposits.
Relevant sedimentary ore fabrics have been sum-
marized by Schulz (1976):
•
Stratified conformable arrangement of ore bodies
within bedded carbonate sediments.
•
Parallel bedding of mineralized strata and carbon-
ate strata.
