Jan van Eden|
Original publication in
Vol. 69, 1974, pp. 59‑79
Depositional and Diagenetic
Environment Related to Sulfide Mineralization, Mufulira, Zambia
J. G. VAN EDEN
The Mufulira copper deposits are in the
Lower Roan, lowest subdivision of the Katanga Sequence, overlying an early
Precambrian Basement Complex. The Lower Roan is described as a transgressive
marine sequence; at Mufulira three essentially strata bound ore bodies are
contained in identical phases of a cyclic succession. Large-scale
cross‑bedded units in the Footwall formation are interpreted as subaqueous
dunes in a high‑energy shallow marine environment, refuting their generally
accepted eolian origin. Analysis of cross‑bedding in the lower part of the
Ore formation and in the Footwall formation throughout the mine reveals a
bimodal distribution of paleocurrent directions with southerly offshore and
northeasterly longshore directions. In the west, southerly directions with
little variation around their mean vector indicate fluviatile channels.
The typical host rock of the
mineralization at Mufulira is a fine‑grained carbonaceous wacke. This wacke
was deposited in basin‑like depressions of the near shore seafloor,
sheltered from the open sea by shoals. Stagnating bottom waters, resulting
in anaerobic conditions, may have contributed to the exceptional conditions
under which synsedimentary sulfides could form. These original sulfide
deposits were probably of a low grade and were later enriched, possibly
during diagenesis as is indicated by diagenetic mottling in sandstones, by
mineral zoning, and by copper sulfide distribution in and around shale
fragments enclosed in coarse sandstone. Clean arenites and conglomeratie
sandstones in the lowest parts of the ore bodies and in their footwall beds
were ideal channelways for laterally migrating copper‑bearing fluids. The
economic concentration of copper was principally dependent upon the presence
of a reducing carbonaceous‑sulfureous environment that was associated with
the carbonaceous wacke and a mottled sandstone.
A unique relationship of copper
sulfides to a particular sedimentary facies is not observed, but
sedimentological trends are consistent with the distribution pattern of the
sulfides and can be used in exploration for this type of deposit.
Part 1 The Sedimentary Geology of
the Footwall and Ore Formations
THE Mufulira ore deposit is one of the
major copper sulfide occurrences in the world and is a typical example of
stratiform base metal deposit in arenaceous sedimentary rocks. The
vast lateral extent of the essentially stratabound copper sulfides and the
finely disseminated nature of ore minerals are principle features. The
geologic setting is described in detail by Brandt et al. (1961) and will not
be reviewed here.
All the major deposits of the
Copperbelt occur in metasedimentary rocks, and though the mode of ore
emplacement is uncertain, its relation to particular stratigraphic
horizons, as well as to sedimentary facies, is generally recognized.
Sedimentary geology has an important bearing on the understanding of base
metal deposits in the Copperbelt ; a number of authors have dwelt on the
subject and attempted
to interpret their depositional
environment. However, little rigorous sedimentologic analysis has been
carried out. The depositional environment of the host rocks to the ore
bodies is controversial; the consensus of opinion is that these sediments
were laid down in a nearshore marine environment, but in the past these same
rock types have been ascribed to such different environments as fluviatile
channel and marine turbidites.
An observed sedimentary control on
emplacement of the ore may be due to a direct relationship between copper
and the depositional environment assuming a synsedimentary origin‑or to a
later redistribution of ore minerals which concentrated the copper in
permeable or chemically receptive rocks associated with a particular
sedimentary environment. A similar relationship between economic
mineralization and sedimentary environment may exist in other areas of the
Copperbelt and in its regional extensions along the Katangan‑Damaran
orogenic belt, as pointed out by van Eden and Binda (1972).
Page 59 of the
FIG. 1. Generalized stratigraphic,
columns and graph showing transgressive‑regressive sedimentary cycles in
the Ore formation.
This paper is a study of the
depositional environment of the Footwall beds and Ore formation based on
field observations of sedimentary structures, paleocurrent analysis, and
microscopic study of the sedimentary lithology. Ore genesis is discussed in
the second part of the paper, using sedimentary environment as a starting
point. Particular attention is paid to synsedimentary and early diagenetic
oreforming processes related to the depositional environment.
and Sub‑Katanga Topography
The ore bodies at Mufulira occur in
metasedimentary rocks of the Lower Roan Subgroup, which is the lowest part
of the Katanga Sequence (nomenclature: Binda and van Eden, 1972). The
Katanga Sequence is of Late Precambrian age and lies unconformably on an
early Precambrian Basement Complex composed of schist and granite. The
unconformity is mostly visible as a sharp contact, with large boulders of
Basement rocks in the overlying metasediments. These basal conglomerates
form the start of a thick transgressive series totaling many thousands of
feet; the Lower Roan, Upper Roan, and Mwashia are composed predominantly of
coarse clastics and argillites, respectively, a series which reflects
increasingly deep‑water conditions (Fig. 1) .
The Lower Roan is divided into
and Hangingwall formations, a division
based on the presence or absence of copper minerals rather than on the
sediment type. The sequence is cyclic, a repeated slow marine transgression
being followed by a sudden minor regression. The first well developed cycle
starts at the base of the Upper Footwall, comprises the C Orebody and ends
in the Inter B/C (Fig. 1). The lithologic sequence shows a reduction in
grainsize from conglomeratic sandstone at the base, to argillitic sandstone
in the ore horizon, and to dolomite and argillite at the top, a sequence
which is twice repeated‑each cycle comprising an ore‑hearing horizon.
Since the sequence is repeated, it is
necessary to describe one cycle only and emphasis will be laid on the
Footwall formation and C Orebody, the largest of three ore bodies at
Mufulira, with a strike length of more than three miles. The Upper Footwall
formation is well exposed in the mine workings and provides data for the
interpretation of the depositional environment and paleogeography of the
mineralized C horizon.
Using the Mudseam, which is a thin
silty dolomite at the top of the C Orebody, as datum, a plan was prepared by
unrolling along dip sections from an arbitrary central Tine‑the 1,400 ft
(450 m) level. The unrolling procedure introduces little distortion, since
folding at Mufulira is simple with the main fold axes parallel and
horizontal. The unrolled plan presents a base on which to plot
paleogeographic data in close spatial relation to their original position
during time of deposition.
Page 60 of the
FIG. 2. Unrolled map with isopachs of
Footwall plus C Orebody in feet showing Basement topography; Basement highs
are reflected by low isopach values (shaded areas). Outcrop is near lower
edge of diagram.
Isopachs of the Footwall formation and
C Orebody plotted on the unrolled plan (Fig. 2) show that the Basement
topography consisted of rounded hills, ridges, and valleys, with a relief of
about 400 feet. Prominent features are ridges at approximately 59 and 34
mining blocks dividing the mining area into eastern, central, and western
parts. Troughs aligned in a north‑south direction at mining :blocks 72 and
43 suggest river valleys.
The Basement topography influences the
paleocurrent pattern, the lithofacies of the Katanga sediments, and also
Lithology of Footwall and C Orebody Formations
Footwall sandstones are composed of
quartz and feldspar, with occasional rock fragments and accessory minerals
in a variable amount of fine detrital matrix. Coarse detrital feldspar is
invariably microcline, while the matrix may contain appreciable amounts of
albite or oligoclase, which is probably authigenic. Rock fragments are
quartz‑sericite aggregaten with a fine granular mosaic structure. Most of
the matrix is of primary origin and is composed of a fine granular mixture
of quartz, feldspar, muscovite, and sericite, though in part it is the
product of secondary alteration such as sericitization of feldspar and the
solution of finer quartz particles followed by chemical precipitation of
silica. Because of the amount of primary matrix, most Footwall sandstones
are wackes and, because of their high feldspar content, the rocks are either
feldspathic wackes or arkoses. Some of the cleaner sandstones have a
carbonate or anhydrite cement. Unlike the
bedded anhydrite occurrences in the
Upper Roan dolomites, the Lower Roan anhydrite is interstitial or forms
concretionary concentrations and veins. The anhydrite may constitute a
chemically precipitated cement that formed in the rock shortly after
deposition, hut there is no indication of an evaporite environment.
Footwall metasediments of the western
area are coarse, gray, and pink arkoses, feldspathic wackes, conglomerates,
pink siltstones, and subordinate dark colored siltstones and argillites. The
coarseness of the clastics and their feldspathic character are typical of
continental clastic wedges. The more uniform feldspathic arenites and wackes
of the eastern area are characteristic of peripheral marine deposits.
The C Orebody sandstones have a higher
quartz content and slightly more abundant dolomitic or anhydritic cement
than the Footwall sandstones. The lowest part of the C Orebody consists of
feldspathic arenites showing a more extensive recrystallization of quartz
and feldspar; the lesser amount of primary matrix may have favored
The characteristic rock type of the
middle and upper C Orebody is a sericitic quartzitic sandstone that is
composed of detrital quartz (up to 70 percent) and feldspar in a groundmass
of sericite and some carbonate cement. Referring to its primary
composition, this rock will be called argillitic sandstone or wacke. The
normai argillitic sandstone is laterally transitional into a wacke that may
contain more than one percent of organic carbon and that han a
characteristic dark color. This carbonaceous wacke has wrongly been named
The C Orebody sandstones have a
somewhat different composition from the Footwall sandstones, which may be
due to a different provenance. The higher quartz and lower feldspar
contents, however, point rather to selective weathering or attrition of the
detritus, perhaps in high‑energy littoral environments, before it was laid
Page 61 of the
FIG. 3 A. Photomicrograph of
well‑preserved detrital grains in feldspatic sandstone (Lower Footwall) ;
abundant interstitial sericite suggests an originally high clay content;
large patches of sericite are altered feldspar clasts; sample 16; crossed
B. Photomicrograph of carbonaceous
wacke; quartz clasts in matrix of copper sulfides and sericitecarbonaceous
material; sample 24; plane light.
Lateral variation in composition,
in particular the proportion of fine matrix, reflects energy conditions in
the environment. It is not clear whether the more abundant dolomitic cement
as compared to the Footwall beds reflects conditions of the depositional
environment or is merely a result of its stratigraphic position beneath
The sedimentary rocks at Mufulira have
been subjected to deep burial and folding and show a variable degree of
recrystallization. In thin section, however, most rocks show the original
boundaries of detrital quartz grains, which are distinguishable by .their
"dust" rings, visible in ordinary transmitted light. Remarkably
well‑preserved sedimentary textures can be observed in some specimens (Fig.
3A), hut textural analysis of the whole rock remains elusive because of the
sericitization of the matrix and of clastic feldspar. Locally, cleaner
sandstones have lost their original sedimentary texture almost completely
by recrystallization of quartz and feldspar to a mosaic texture.
Quantitative measurements of textural parameters for these rocks are
inevitably sub j ective.
Surface texture or even rounding of
quartz grains is difficult to observe, because of pitting of the grain
boundary by pressure solution and the development of overgrowths. The
observed roundness varies from subangular to subrounded, without appreciable
differences between the Footwall beds and C Orebody ; rounded or
well‑rounded grains seem to be absent.
Page 62 of the original publication
FIG. 4. Size frequency distribution of
long axes of quartz grains, measured in thin section; Lower Footwall (16),
Upper Footwall (17, 18, 19), C Orebody lowest (23), C Orebody carbonaceous
wacke (24) ; note the similarity of all Footwall sandstones, the relatively
good sorting of the lowermost C Orebody and the poor sorting of C Orebody
TABLE 1. Grainsize Parameters of
Sediments from the Mudseam, C Orebody, and Footwall Formations (Long axis of
quartz grains, measured in thin section)
Grainsize measurements were made on a
small number of selected samples; the data presented in Table1 are not
statistically reliable, but illustrate general characteristics of typical
Grainsize parameters: Grainsize was
measured in thin section using techniques described by Friedman (1958) .
Only detrital quartz grains larger than 10 microns were measured; the
exclusive use of quartz grains avoids bias from the variable amount of
feldspar that is destroyed by sericitization. Data are expressed in phi
units, providing a convenient logarithmic scale that is commonly used in
sedimentology. (Phi to micron conversions can be found along the horizontal
axis in Fig. 4.) Grainsize parameters presented in Table I are: phi
arithmetic mean (Xф), phi arithmetic
standard deviation (Gф), phi skewness
(SKф), and phi kurtosis (Kф)
(Griffiths, 1967). All parameters are derived for the population of quartz
grains computed to 100 percent, excluding the fraction smaller than 10
Mean grainsize is in agreement with the
field observation ,that sandstones in the Upper Footwall beds are slightly
coarser than those of the Lower Footwall beds and the middle part of the C
Orebody. Though the average standard deviation is higher in the Lower
Footwall beds than in the Upper Footwall beds, ‑suggesting a lesser degree
of sorting for the former, differences are insignificant. The C Orebody
sediments, particularly the carbonaceous wacke, have a relatively high
standard deviation, indicating poor sorting. An exception to this is the
base of the C Orebody ~which like the uppermost Footwall beds is
characterized by a low standard deviation ; these sandstones are rather
coarse, well‑sorted arenites.
Though the significance of skewness and
kurtosis questionable, these parameters clearly differentiate the
sandstones. Most obvious is the low skewness and kurtosis of the uppermost
Footwall and lowest C samples and the particularly high kurtosis for the C
Cumulative size frequency graphs for
part of the data discussed above are shown in Figure 4. The graph of the C
carbonaceous wacke (24) is distinctly different from those of the Footwall
sandstones (16, 17, 18, 19), reflecting its poor sorting. The lowest part
of the C Orebody (23) has the graph with the steepest slope, indicating
relatively good sorting.
Metasediments of the Footwall beds are
composed largely of sandstones with a uniform composition and texture, but
subordinate conglomerates and argillite beds are important constituents.
Grainsize of the metasediments has considerable lateral variation, with the
conglomerates abundant in the west, gradually decreasing toward the east
(Fig. 5) , and thin argillite intercalations in the top of the Footwall
formation more abundant in the western area. Relevant lithologic criteria
may be summarized in the following idealized sequence from basal
unconformity to Mudseam in the east : (1) several hundred feet of uniform
Footwall sandstones, wackes, or not‑soclean arenites, incorporating some
conglomeratic lenses, (2) a few tens of feet of clean arenites in the
uppermost Footwall or lowest C, (3) argillaceous sandstones or carbonaceous
wackes approximately 40 feet thick, and (4) thin dolomitic siltstone,
sandstone, and argillite layers on top.
Page 63 of the original publication
Fig. 5. Unrolled map showing abundance
of conglomerates and conglomeratic sandstones in the Upper Footwall ; 150
feet isopach, traced from Figure 2, shows Basement highs; dashed lines show
position of major mining levels and surface outcrop of Mudseam.
Types of cross‑bedding
Many sedimentary structures have been
reported from Mufulira; Garlick (1967) describes crossbedding, mud cracks,
shale slabs, ripple marks, load casts, slumps, wash‑outs, and other (but
less easily explained) structures. Cross‑bedding is the most common
sedimentary structure and is easily observed throughout the Footwall beds
and in .parts of the C Orebody. The foresets are often enhanced by a
distinctive color banding due to calcification of the coarser layers and
silicification of the finer grained layers (Fig. 6). Two principal types of
crossbedding can be distinguished by their shape and scale.
Small‑scale cross‑bedding, typical of
the Upper Footwall formation in the west is trough‑shaped with an average
set‑thickness of 6 inches (15 cm), and numerous lenticular sets form large
cosets. This "festoon" type cross‑bedding is generally considered to have
formed in a fluviatile environment.
The large‑scale cross‑bedding is
planar, the sets being essentially tabular. Foresets are tangential, with a
long toe gently inflecting toward the base of the set (Fig. 6) . Truncation
planes are remarkably straight and some units persist over several hundred
feet. The set thickness of the majority of these cross‑bedded units is two
to three feet, but thicknesses up to six feet have been observed. Some of
the units are grouped in cosets of two or three sets, but more than half
occur as isolated units, lying between horizontally bedded sandstone. This
type of cross‑bedding may be attributed to dunes in a marine shelf, tidal
channel, or other high‑energy marine environment. This interpretation of
environment contrasts with the accepted eolian origin of the largescale
cross‑bedded units at Mufulira (Brandt et al., 1961; Brandt, 1962; Garlick,
1965, 1967; Fleischer, 1967) ; only Dott (1967) has expressed doubt about
their eolian deposition.
The so‑called eolian sandstones form a
large part of the Footwall and have greatly influenced the interpretation
of the paleogeography and the depositional environment during Footwall and C
Orebody times. Brandt (1962) assumes deposition of copper sulfides in
isolated basins in a desert. Garlick (1967) shows that on the 1,150 level in
the east, the C Orebody interdigitates with so‑called eolian sandstones,
which are generally barren. In the light of the controversy about the origin
of "eolian" sandstones and their importance to the paleogeography, it seems
appropriate to discuss the large‑scale cross‑bedded units in more detail.
Origin of large‑scale cross‑bedding
In the past, the large scale of
particular crossbedded units has been taken as a criterion for eolian
origin. Large‑scale cross‑bedding, with a set thickness between one and six
feet is indeed most common in eolian dunes (MCBride and Hayes, 1962; McKee,
1966) ; however, large‑scale cross‑bedding is also described from modern
subaqueous environments. Sand waves of up to 30 feet in amplitude have been
recorded for the lower course of the Mississippi River (Potter and
Page 64 of the original publication
FIG. 6. Large‑scale cross‑bedding with
distinct color alterations along foresets ; Upper Footwall, eastern area.
Large‑scale cross‑bédding, in
units up to 40 meters thick, is allo found in deposits on shallow
continental shelves (Houboult, 1968) . It is bevond doubt that cross‑bedding
of equal or larger scale than that found in the Footwall beds at Mufulira
can be formed in a subaqueous environment.
Steeply dipping foresets that have
occasionally been observed and interpreted as eolian are also considered no
reliable criterion. It is difficult to differentiate on the basis of this
criterion even in modern deposits (McKee, 1957). In folded rocks one may
correct for the effects of tectonic tilt and axial plunge (Ramsay, 1961) and
obtain accurate current directions, hut the internal deformation of beds and
its influence on the inclination of foresets is difficult to assess. At
Mufulira, very high angles of 40° and more in apparently undeformed rocks
show the considerable steepening of primary angles. Primary angles of
foresets, measured in 152 beds, average around 20°, while maximum angles can
be as high as 32°. Angles in the so-called eolian beds, however, were not
found to be significantly different from those measured in accepted aqueous
Truncation planes of the large‑scale
cross‑bedded units at Mufulira are remarkably straight and mostly parallel
to the normal bedding. Examples from the literature show that truncation
planes in recent eolian dunes are almost always inclined (McKee, 1940).
McKee (1966) finds many bounding surfaces of sets dipping from 20° to 28°
in a downwind direction. Straight, near‑horizontal truncation planes in
eolian deposits point to exceptional conditions. Stokes (1968) has
explained such planes in the Navajo and Coconino sandstones as a result of
sediment removal by strong winds down to the water table. His
interpretation, however, refers to erosional planes that truncate composite
sets of intricate eolian cross‑bedding quite different from the individual
cross‑bedded units at Mufulira. Straight, near‑horizontal erosional and
depositional surfaces are more easily explained in an aqueous environment
(McKee, 1940) .
Exposed faces of Recent eolian deposits
typically show numerous sets of medium‑ to large‑scale, often interlocking
units of wedge or polyhedral shape (MeKee, 1940, 1966; Shrock, 1948). Many
of the large‑scale cross‑bedded units at Mufulira, however, are single units
sandwiched between horizontally bedded sandstones, indicating subaqueous
rather than eolian deposition. In the aqueous environment the horizontally
bedded sandstones originate either from sheet flow under conditions of
strong currents or in a lower energy flow regime from very low amplitude
sand waves (Smith, 1971) .
Page 65 of the original publication
The alternation of large‑scale
cross‑bedded sandstone with thin beds of conglomeratic sandstone or
argillite in the Upper Footwall has been thought to originate from
alternating aqueous and eolian environment (Garlick, 1967) . However,
without criteria for an eolian origin of the large‑scale crossbedding, the
definitive aqueous characteristics of some of the beds would rather point to
a subaqueous origin for the whole sequence.
Lithologic characteristics may
sometimes also provide reliable criteria for the recognition of eolian
deposits. Eolian sand of Recent deposits is well sorted and the grafins
usually show good rounding (Shepard and Young, 1961 ; Bigarella, 1969).
Desert sandstones characteristically have well‑sorted laminae a few mm thick
and commonly sharp differences in maximum grainsize between laminae
(Glennie, 1970). Silt and clay fractions in modern dune sand of desert as
well as coastal environments are generally less than ene percent (McKee,
1966; Shepard and Young, 1961; Friedman, 1961) . The Footwall formation at
Mufulira consists of poorly sorted feldspathic sandstones with a silt and
clay fraction of up to 15 percent. Well‑rounded grains and laminae of
contrasting grainsize are seldom observed. Only some sandstones at the very
top of the Footwall beds are better sorted and contain less matrix, hut here
.the environment is generally accepted as aqueous. Though the sedimentary
texture is obscured in the partially altered and recrystallized rocks,
recognizable textural parameters exclude an eolian origin for most Footwall
rocks at Mufulira.
Reinterpretation of the large‑scale
cross‑bedding as subaqueous considerably simplifies the interpretation of
the sedimentary environment and the paleogeography. Marine Footwall
sandstones in the cast are overlain by similar C Orebody sandstones and no
major change in enviroment from continental to marine has to be assumed
within this uniform sequence. The amount of conglomerate in the Upper
Footwall beds in the western alluvial‑littoral environment decreases towards
the cast and is primarily a function of distance from the coastline in a
nearshore marine environment, and absence of conglomerates in some parts
of the eastern area does not indicate an eolian origin for those sandstones.
Large‑scale cross‑bedding reported by Garlick (1967) to flank the C Orebody
in the south and east marks a transition to a more open, higher energy,
marine environment. The implications of
this interpretation with regard to ore
genesis will be discussed later.
Other sedimentary structures
Many of the smaller sedimentary
structures are more difficult to recognize, but are remarkably well
preserved, considering that these Precambrian rocks are folded and partially
recrystallized. Ripple marks, mud cracks, load casts, etc., point to a
particular depositional environment. Mud cracks, occurring in places near
the base of the C Orebody in the western and central areas, show that
mudflats were occasionally exposed to air, dessicated, and cracked. Shale
slabs, several inches long, that are found within conglomeratic sandstone at
approximately the same horizon, must have been ripped from partially
indurated shale beds and laid down after only short transport.
These and other structures are
characteristic of tidal mudflats in a littoral environment, they are
localized near the base of the C Orebody and mark the transition from
continental to marine deposits in the western area. The transition is
characteristic of paralic conditions, but their subordinate occurrence
between thick alluvial fan conglomerates and marine sandstones suggests that
the coast must have been topographically steep and the transition of the
continent to the sea rapid, not allowing the development of an extensive
Paleocurrent directions and direction
of sediment transport are vital factors for the analysis of the
paleogeography, and directional characteristics facilitate extrapolation of
geologie data to areas as yet unexplored. Though directional structures like
current ripples, scour‑and‑fill, and preconsolidation slumping occur,
directional data have been interpreted from cross‑bedding only. The
correction of the direetional data for tectonic tilt has been made by a
single rotation of the bedding plane about fits strike, a simple method that
is acceptable since fold axes in the mining area are horizontal or plunge at
less than 15°.
Paleocurrent directions in the Footwall
beds fall into two distinctive groups, coinciding with the Upper and Lower
Footwall stratigraphic units. In the Lower Footwall beds paleocurrents have
a dominant northerly direction, but on the deeper mine levels, particularly
in the west, southerly directions have been found (Hodgson, 1969). The
small number of observations (approximately 100) on only two mining levels
does not allow a detailed analysis, but northerly current directions
indicate a net transport of sand toward the coast. A shallow marine
environment is envisaged in which the cross‑bedding was formed in submarine
sandbars moving landward similar to those described from recent shelf
environments (Tanner, 1955 ; Werner, 1963) .
Page 66 of the original publication
FIG. 7. Unrolled map of Upper Footwall
beds showing major sedimentary environments and current directions.; dashed
line indicates trend of shore line; 150 feet isopach, traced from Figure 2,
shows Basement highs.
The Upper Footwall beds and C Orebody
have consistent paleocurrent directions to the south and, less prominently,
to the northeast and are therefore considered as one group. It should be
stressed, however, that cross‑bedding is scarce in the C Orebody and
analysis of the paleocurrent pattern relies heavily on data of the Upper
Footwall beds. The development of the paleogeography of the C Orebody is
thought to reflect gradual transition from conditions similar to those under
which the Footwall beds had been deposited.
A relationship has been observed
between current directions and the topography of the Basement. In the west
currents are generally toward the south, following the downslope direction
of a valley. Current directions over the eastern flank of the ridge on
mining block 59 (Fig. 2) reflect the local topography of this hill ; current
directions north of the ridge are more often to the north, while south of
the ridge a dominant southerly direction is found, corresponding in both
instances to a downslope direction. The ridge on block 35 and the area
farther east show similar relations. The strong influence of the underlying
Basement topography on current directions illustrates its controlling
influence on the depositional environment of Upper Footwall beds and C
Orebody. Grouped data of current directions plotted in Figure 7 show two
types of distribution : (1) unimodal distributions with low variability
around a mean direction toward the south, and (2) bimodal distributions with
more variable directions toward
the south and a second mode to the
east. The unimodal paleocurrent distributions occur mainly in the west
where most cross‑bedding is small scale and forms large cosets; its
fluviatile origin is unmistakable. The great vector strength of the
southerly current suggests a steep‑gradient coastal area with a sediment
supply from the north. The bimodal paleocurrent directions are in the
south‑central and throughout the eastern area where cross‑bedding is
generally of the large‑scale planar type.
Bimodality and high variability of the
paleocurrent directions are indicative of marine nearshore areas, though
larger inland lakes may develop similar bimodal characteristics. The
southerly mode, which is most consistent, is parallel to the unimodal
directions in the western fluviatile deposits and is interpreted as an
offshore direction. The northeasterly mode is more variable. Directions of
the two modes are nearly opposite at some locations in the east, indicating
tidal currents. In the west‑central area current directions are found at
right angles to each other, which suggests a stronger shoreline influence ;
the easterly mode reflects longshore currents, while the southerly mode is
the normai offshore direction. The trend of the shoreline, being at right
angles to the offshore current directions, may be delineated accurately
(Fig. 7), and its location was to the north of the Mufulira ore deposit.
Depositional Environment and Paleogeography
The sediments of the Katanga Sequence
were deposited during a major marine transgression over the early
Precambrian Basement, which was the source of the Katanga sediments and must
have formed an extensive land mass in the north.
Page 67 of the original publication
FIG. 8. Elements of the paleogeography
during C Orebody sedimentation: (1) carbonaceous wacke and (2) mottled
argillitic sandstone, both (1) and (2) represent a low‑energy depositional
environment in which synsedimentary or diagenetic copper sulfides could
have formed; (3) and (4) coarse often cross‑bedded, sandstones of a higher
energy depositional environment, littoral and off shore, respectively; 150
feet isopach, traced from Figure 2, shows underlymg Basement hills; line A‑B
is location of section in Figure 9.
Immature conglomeratic arkoses found
locally at the base of the Katanga Sequence represent continental materials
in situ on the ancient Basement and indicate a swift transgression. The
Basement landscape was characterized by low rounded hills of schist or
granite, which early in .the Katanga transgression formed small offshore
islands. The Lower Footwall formation varies in thickness from 0 to 300
feet, as it fills valleys in the irregular Basement surface.
The uniform sandstone facies of the
Lower Footwall beds reflects a marine environment, in which the sand was
derived from older coastal or residual continental deposits. Net transport
of sand, as indicated by the northerly directions of large‑scale crossbeds
was locally toward the coast.
The Lower Footwall beds are truncated
by an unconformity and overlain by conglomeratic sandstones derived from the
north. The unconformity marks a minor regression, probably caused by a
gentle tectonic uplift of the northern hinterland. Coarse Upper Footwall
arkoses in the west were deposited in a fluviatile‑littoral environment as
alluvial fans by small steep‑gradient rivers. Toward the east alluvial
conglomerates become less abundant and marine sandstone prevails. A sandy
coastline exposed to strong wave action is envisaged, on which offshore and
longshore currents were active. The topography was now flat with most of the
Basement covered by sediments, though locally it was still exposed on the
seafloor. The abundance of conglomerates in valleys
in the west and near‑absence over
Basement hills suggests deposition of conglomerates on channel floors in
either a fluviatile or nearshore marine environment. In the east
conglomerates are more abundant over the low Basement hills that probably
formed relatively shallow areas on the sea floor (Fig. 5) ; such shoals
would have been exposed to currents and waves that winnowed out the fine
sand, leaving only the coarser components. Relatively coarse sediment is not
uncommon over topographic elevations in recent marine deposits. Kofoed and
Gorsline (1963) show that topography is the most important single factor
controlling the distribution of sedimentary material in a nearshore marine
C Orebody formation
Toward the end of Footwall time the
western area was transgressed by the sea and the alluvial deposits were
overlain by marine sands of the C formation. In the east, where both
Footwall beds and the C formation consist of marine sandstones, the start of
the C is difficult to establish. The lowest beds of the C formation are
clean, rather well‑sorted sandstones that often exhibit cross‑bedding; they
reflect the high‑energy environment possibly of a beach or an offshore
barrier. Except for these lowest beds, the C Orebody is composed of
argillitic sandstones laid down under relatively quiet conditions. As with
the Upper Footwall beds the shallower parts of the depositional area were
above Basement hills, which were now buried ; the topography, however,
tended to be rejuvenated by compaction of the thick unconsolidated sand
sequence in the valleys.
Page 68 of the original publication
Fig. 9. Idealized section of C Orebody
and Footwall beds during a late phase of C Orebody sedimentation; Basement
forms a threshold, isolating part of the coastal sea floor from the open
sea; line A‑B in Figure 8 shows location of section.
The eastern, central, and western
areas were shallow depressions on the coastal sea floor separated from each
other, and from the open sea, by shoals. The sediments deposited on these
shoals were relatively coarse and often cross‑bedded, while fine‑grained
sand with a high percentage of silt and clay, sometimes laminated, was laid
down in the tentral, deeper parts. This fine sediment reflects a low‑energy
environment, below wave base, sheltered from the open sea and is the
characteristic host for a large part of the ore at Mufulira. The two main
topper‑bearing rock types of the C Orebody are a carbonaceous wacke and a
fine mottled sandstone, to be discussed in more detail below.
Carbonaceous wacke: The carbonaceous
wacke ("Graywacke") is differentiated from the normai argillitic sandstone
by its Bark color, which is caused mainly by finely disseminated carbon and
the high percentage of sulfides. The fine‑grained carbonaceous wacke is
typically massive, but is thinly bedded or laminated in some of its lighter
colored variants near its fringes. The abundante of sericitic matrix in the
carbonaceous wacke and its low feldspar content as compared to the Footwall
sandstones may be partly due to sericitization of feldspar clasts, as
pointed out by Darnley (1960) . Several features indicate, however, that
the original sediment contained an appreciable amount of clayey material.
The homogeneous composition or occasional laminated structure and the
absence of current indicators such as crossbedding, imply a low‑energy
depositional environment where clay would be expected. The extremely poor
sorting of quartz grains, as compared to other rocks at Mufulira, suggests a
mixed sediment with a large clay and silt fraction. Furthermore, the
abundance of finely disseminated organic carbon could probably only have
been preserved in a clayrich sediment. Present day deposits show a
systematic increase of organic carbon with a decrease in grainsize, and
disseminated organic remains are clearly associated with clayey sediments
Though a carbonaceous wacke occurs
locally in the west, it forms a major sedimentary body only in the east
(Fig. 8). It was formed in a slightly deeper coastal basin whose shape was
inherited from the Basement (Fig. 9). The carbonaceous wacke is flanked by
coarser and cleaner sandstones located over protruding Basement hills, which
acted as a kind of threshold, sheltering the toast from the open sea. Thus
the carbonaceous wacke was laid down in a partially isolated basin in which
bottom waters may have become stagnant, providing anaerobic conditions in
which organic matter was only partly decomposed. In comparable modern
basins carbon content increases towards the center, reaching a maximum in
zones of less mobile water (Strakhov, 1969). Minor amounts of detritals
would be derived from the nearby littoral zone and brought in by occasional
slow‑moving currents or gentle wave action.
The carbonaceous wacke has been
interpreted as a channel deposit by Darnley (1960) ; Brandt (1962) supported
this interpretation and suggested more specifically a deltaic environment
with a river streaming toward the north and west. This assumed direction of
flow is contrary to paleocurrent directions measured in underlying and
laterally adjacent sandstones.
Page 69 of the original
Fig. 10. Mottled sandstone of lower C
Orebody; western area.
The abundante of sericitic matrix, the
absence of conglomerates and cross‑bedding, as well as the geometry of the
"Graywacke" body are strongly opposed to a fluviatile channel deposit, as
already pointed out by Garlick (1967), who, on the other hand, favors
deposition by turbidity currents. Typical turbidite successions, however,
are made up of a marked alternation of argillites and sandstones, the Jatter
having sharply defined bottom surfaces and usually indistinct top surfaces
(Dzulynsky and taalton, 1965) . Sole markings such as flute casts, groove
casts, or bounce casts should occur frequently. None of these
characteristics is found in the carbonaceous wacke. Graded bedding is
reported by Garlick (1967), but this by itself is not sufficient proof of a
Mottled sandstone: This is the lateral
equivalent of the carbonaceous wacke, and apart from the mottling, it
differs from the latter mainly in its lower content of preserved organic
carbon. A lateral transition occurs from even‑colored to well‑mottled rock
consisting of light and dark gray diffuse patches subparallel to bedding or
randomly distributed (Fig. 10). Copper minerals and organic carbon are more
abundant in the dark gray parts, the ligtiter parts contain more carbonate
and are sometimes more porous, probably due to solution of the carbonate and
The mottling is a result of a chemical
reorganization, which probably started in the unconsolidated sediment. From
recent sediments it is known that physiochemical conditions may be patchy
and that mineral transformations during diagenesis may eventually result in
a mottled rock (Strakhov, 1969). The white spots in the C Orebody sandstones
are carbonate concretions; the rather small percentage of carbonates in
these spots is normal for concretions in sandy rocks, where the coarse
clastics cannot be removed easily and carbonate is limited to the pore
spaces (Strakhov, 1969).
Malan (1964) and Paltridge (1968)
observed the association of mottled sandstones with a stromatolite reef that
occurs stratigraphically above the C horizon and they suggest that part of
the mottling may be due to algal activity. Biological activity is indeed
possible and algae may be a likely source for organic carbon in these
The mottling and apparent absence of
regular bedding were interpreted by Paverd (1965) as a result of submarine
slumping. Garlick (1965 ; 1967) described the mottled sandstone as a "slump
breccia," associated with slump and turbidity flow mechanisms. Though slump
structures are observed, these are only local phenomena, and the mottling in
general is not caused by slumping. The uniform texture, but variable mineral
composition, shows that the mottling is a result of chemical redistribution
in an otherwise undisturbed sedimentary rock.
Part 2 The Mineralization
The association of mineralization with
particular stratigraphic horizons and the intricate relations between
sulfides and sedimentary structures are commonly attributed to a syngenetic
origin for the ore (Garlick, 1961, 1965, 1967, 1972; Maree, 1960; Brandt,
1962; Fleischer, 1967). During the years of discovery of the Copperbelt
deposits, however, the ore bodies were considered to originate from
hydrothermal solutions (Gray, 1932; and others) and some authors still favor
a metasomatic or other epigenetic origin on the basis of mineralogy and
chemistry of the rocks and the numerous replacement and mobilization
features (Darnley, 1960). As with many such controversial issues, the truth
may be somewhere in between ; emplacement of the ore by a combination of
synsedimentary and postsedimentary processes should not be excluded.
The generally accepted synsedimentary
process leading to ore genesis is chemical precipitation of the copper as
sulfides under anaerobic conditions, possibly assisted by biological
processes involving bacteria or algae.
Page 70 of the original publication
Such a synsedimentary origin is
quite possible for shale‑type deposits, hut is it also for sulfides in
arenites? At Mufulira ore minerals, mainly chalcocite, bornite, and
chalcopyrite, occur finely disseminated or in coarser blebs of replacable
habit in rocks ranging from argillite to coarse conglomeratic sandstone, but
the typical host rock is sandstone, and the high‑energy depositional
environment of some of the host rocks cannot be considered favorable for
chemical precipitation of sulfides. The mere variety of mineralized rock
types suggests that not one, but a number of ore‑forming processes were
Synsedimentary Ore‑Forming Processes
deposition of sulfides
In Recent deposits copper‑bearing
minerals are sometimes found along lamination planes in fine clastic
sediments, and malachite or azurite, forming incrustations on organic
remains, can be transported and deposited as clastic particles (Botvinkina
and Yablokov, 1964). The concentration of copper transported in this way
would, however, remain extremely low, as pointed out by Davidson (1962).
Some of the rich mineralized conglomerates and sandstones at Mufulira are
reminiscent of placer deposits, but is mechanical concentration of copper
sulfides possible? Minerals like diamond, zircon, gold, etc., which are
mechanically and chemically more resistant than many rock‑forming minerals,
are concentrated by selective weathering, but a similar process seems
impossible for the relatively soluble and soft copper sulfides.
Nevertheless, detrital pyrite has been reported in ancient conglomerates of
the South African Witwatersrand in concentrations of several percent (Hoefs
et al., 1968), showing that detrital sulfides can survive.
At Mufulira, thin laminar
concentrations of copper sulfides frequently occur on the foresets of
crossbedding in sandstones, and visual observation may suggest a detrital
origin for the sulfides. However, chemical rock constituents, such as
carbonates, also occur concentrated along foreset planes; foresets have a
variable grainsize at right angles to their laminae, therefore permeability
will vary considerably more across the foreset planes than along them and
pore fluids flowing through the rock would tend to follow coarse‑grained
foreset planes. Thus, laminar concentrations of copper sulfides could have
formed from rnigrating pore fluids. Permeability control on copper
mineralization was suggested by Hamilton (1967) for a similar relationship
of copper distribution and cross‑bedding at White Pine, Michigan.
Chemical precipitation of sulfides
Chemical processes may, under certain
conditions, explain the precipitation of copper from solution in sea water.
Biochemical processes, whereby hydrogen sulfide is liberated by anaerobic
bacteria, are capable of precipitating copper as sulfides in a sedimentary
environment, a process suggested by Garlick (1961, 1965) for Copperbelt
deposits. Many fèatures of the mineralization are, however, more readily
explained by diagenetic processes. Early diagenetic processes are inherent
to any newly formed sediment and have a decisive influence on the mineral
composition of a rock. Physico‑ or biochemical conditions are not strictly
bounded by the depositional interface; a newly formed sediment is not a
closed .system, it is in contact with interstitial, as well as overlying
water, and diffusion óccurs by liquid flow or ionically (Love, 1967).
Chemical processes that can precipitate and concentrate copper sulfides from
sea water can also precipitate copper sulfides from pore waters, while
conditions of Eh and pH favorable for precipitation of sulfides (Krumbein
and Garrels, 1952) are more commonly found in the sediment than in the
overlying water. A pronounced exchange of elements occurs between the bottom
waters and interstitial waters, so that sulfur can be found in higher
concentration in pore waters than in the overlying sea water. Eventual
saturation of interstitial waters may then lead to precipitation of sulfides
(Larsen and Chilingar, 1967) .
The formation of iron sulfides
(predominantly pyrite) by the activity of sulfate‑reducing bacteria is well
known (Love, 1964). Other heavy‑metal sulfides (such as chalcopyrite) are
also shown to be formed during early diagenesis (Muller, 1955; Haussiihl and
Muller, 1963). Modern bottom deposits may contain colloidal metal compounds
and organic carbonaceous‑sulfureous matter, from which sulfides could be
formed, possibly by bacterial action. The early stages of diagenesis, in
which new minlerals are formed, occur in recent sediments to a depth of up
to 10 m (Larsen and Chilingar, 1967).
The diagenetic environment below the
depositional interface of a newly formed sediment is conditioned by the
overlying bottom water and the type of sediment. Coarse‑grained marine
sandstones with open water circulation are oxygenated to well below the
depositional interface, whereas in finer grained sediments reducing
conditions normally prevail immediately below the sediment surface. Though
fineness facilitates deoxygenation of a sediment, it is not an absolute
requirement for the formation of sulfides during early diagenesis. The
critical factor in the formation of sulfides is impermeability of the
sediment to oxygen migration from the overlying water, and sorting is
therefore of main importance. Sulfides may develop in sandstones if they
are poorly sorted and contain enough organic material (Evans, 1965 ; Love,
Page 71 of the original publication
FIG. 11. Fragment of shale slab,
embedded in conglomeratic sandstone; base of C Orebody, western area.
Thus the poorly sorted and carbon‑rich
argillitic sandstones and carbonaceous wackes of the Mufulira ore bodies are
suitable sediments for diagenetic formation, and the sedimentary
environment, in which these sandy host rocks were laid down, is similar to
that of the ore‑shale of other Copperbelt deposits.
Features of the Mineralization and Their Genetic
In the mottled sandstones the light
gray carbonaterich parts are almost devoid of copper, while the dark parts
are rich in copper minerals. Strakhov (1969) shows that during the growth of
carbonate concretions, an impoverishment of elements like Cu must be
expected in the concretionary bodies. Thus, it is not impossible that copper
sulfides were present
and homogeneously distributed in the
rock at the time that diagenetic carbonate concretions formed. However, the
concretionary development affected the distribution of organic matter as
well. Dark parts of the mottled sandstone contain abundant organic matter,
and these parts of the host rock may have been particularly receptive to
later introduced copper.
Though mottling occurs occasionally in
the Footwall rocks, it is largely characteristic of ore body sandstones ;
this may be explained by the high carbon content of the Jatter sandstones.
Strakhov (1969) states that the abundance of organic matter in tbe initial
sediment is a critical factor in the redistribution of carbonates.
Decomposition of buried organic material generates CO2 which facilitates
solution and migration of carbonates and which eventually leads to the
formation of concretions. The mottled sandstones, laterally equivalent to
the carbonaceous wackes, probably originally contained an appreciable
amount or organic carbon.
Argillite fragments occur in the
conglomeratic sandstone near the base of the C Orebody in the west. The
angular argillite slabs must have been ripped from partially consolidated
shale beds and redeposited after transport over a short distance. The cores
of some of these argillite slabs contain microscopic disseminated bornite,
amounting to about 0.5 percent in the specimen illustrated (Fig. 11) . The
lighter colored borders contain less copper (Table 2), which occurs as blebs
that are visible to the naked eye.
TABLE 2. Chemical Composition of Argillite Slab,
Dark colored core
Light colored fringe
These data are obtained by wet‑chemical
quantitative analysis; additional semi‑quantitative
spectrographic analyses on 26 other elements did not show a
significant difference between the dark colored core and the
light colored fringe of the specimen.
Page 72 of the original publication
FIG. 12. Unrolled map showing C Orebody
mineralization in percent copper; numbers 1 to 5 are locations of lithologic
columns in Figure 14.
The fine‑grained copper sulfides of the
core coincide with a higher organic carbon and a lower carbonate content
than in the rim. Assuming an original uniform copper and carbon distribution
before partial consolidation, the pancity of carbon round the rim would be
due to oxidation by exposure to oxidizing conditions in the porous
conglomeratic sandstone in which it was deposited. These oxidizing
conditions could also remove copper and precipitate carbonate.
Copper sulfides in the conglomeratie
sandstone have a pronounced preference for the immediate surroundings of
argillite fragments; the argillite slab of Figure 11 is also enveloped in
bornite and some chalcopyrite. These high concentrations of copper sulfides
along the borders of argillite fragments may be explained as a result of a
permeability barrier and the relative low Eh and pH associated with
carbonaceous shale, under which conditions copper could be arrested as
sulfides from alkaline and oxygenated ground waters. This process may become
active immediately after deposition of the shale slab in the sand and
continue until the present. The appreciable content of organic carbon and
sulfide in the core of the argillite specimen indicates its continued
reducing capacity. Ontward from the argillite fragment there is first, a
thin rim of recrystallized quartz associated with chalcopyrite, then
coarsely recrystallized carbonate and massive particles of bornite are
found. This mineral segregation across the argillite fragment and sandstone
contact shows that a physicochemical gradient of outwardincreasing Eh and
pH may have existed at some stage, e.g., during recrystallization.
The observations suggest that (1)
copper and sulfur formed synsedimentarily in euxinic clayey sediments,
though in a low concentration, and (2) copper mineral constituents were
moved around at more than one stage and reemplaced in or close to sediments
containing organic carbon, i.e., in rocks with a relatively high reducing
over Basement hills
Copper mineralization in the C Orebody
is continuos over approximately 15,000 feet along strike except for two
low‑grade gaps around mining blocks 57 and 38 (Fig. 12). These major gaps
and some smaller low‑grade areas are located over Basement hills. The C
Orebody thins slightly .in these areas and a change from copper sulfides to
pyrite makes them uneconomic.
The pyrite zones over the Basement
hills are not easily explained. The host rock reflects a depositional
environment of high current and wave energy, in which the sandstones were
well oxygenated. Conditions were unfavorable for the development of
diagenetic sulfides, and the sulfides which now fill the pore spaces must
have been introduced at a later stage.
An introduction of these sulfides from
above is suggested by the inverted coneshape of the pyrite lenses that hang
from the pyrite‑rich top of the C Orebody, and the occurrence in some
locations of a thin layer of bornite between the pyrite lens and the
Basement hill. Toward the end of C Orebody sedimentation more quiescent and
slightly deeper water conditions prevailed, pyrite was formed throughout the
area, and its constituents may have migrated downward into the hitherto
unmineralized sediments that overly the Basement hills.
Page 73 of the original publication
FIG. 13. Unrolled map of C Orebody
showing generalized zones of dominant sulfide minerals.
Ore minerals at Mufulira are pyrite,
chalcopyrite, bornite, and chalcocite, and though all of these are
widespread, there are distinct zones in which particular minerals
predominate. Mineral zoning occurs stratigraphically as well as laterally.
In general the stratigraphic sequence of dominant minerals is, from bottom
to top: chalcocite, bornite, chalcopyrite, and pyrite. Laterally, in the C
Orebody, the most obvious features are the pyritic zones that overlie
Basement hills, a chalcopyrite zone that coincides roughly with the
carbonaceous wacke environment, and chalcocite zones associated with fringes
and nearsurface portions of the ore body (Fig. 13). In the B Orebody a
large central part of the carbonaceous wacke has dominant chalcopyrite
mineralization. The A carbonaceous wacke mineralization is also dominantly
chalcopyrite with rich chalcocite fringes. A more detailed description of
the mineral zoning and its relation to lithofacies is given by Garlick
The genetic process responsible for the
mineral zoning is not clear, but some comments can be made. First, the
chalcopyrite of the carbonaceous wackes in all three ore bodies and the
pyrite of some small occurrences of carbonaceous wacke in the western part
of the C Orebody point to a relation between these minerals and lithology.
Within the series of minerals here discussed, pyrite and then chalcopyrite
have the strongest affiliation with a reducing environment and their
synsedimentary formation in the euxinic sediments of these areas .is not
impossible. Similarly, iron‑rich sulfides in the top of the C Orebody are
coincident with a host rock of a quiescent sedimentary environment.
The chalcocite is associated with
fringe areas and occurs as a dominant mineral in many conglomeratic
sandstones underlying the ore bodies. Figure 13
clearly demonstrates its occurrence
close to the surface in the C Orebody and for these parts a supergene
origin is obvious. Where it occurs deeper down, supergene enrichment was
facilitated by highly porous conglomeratie sandstones.
A broad zonal sequence from copper‑rich
to ironrich sulfides representing depth zones parallel to the shoreline, as
described by Garlick (1961), cannot be confirmed by the present author. This
study of the sedimentary environment disproves such a relationship between
mineral zoning and distance to the shore.
Stratigraphically the fringe between
the cupriferous zone and the overlying barren pyritic rocks is of the
greatest interest. The narrow Cu‑Fe transition at the top of the C Orebody
generally occurs just below the stratigraphic marker of the Mudseam, hut
rises above it in the western area, as shown in Figure 14 (2). The
configuration of this mineral zoning and its relation to the lithologic
facies is remarkably similar to that described by White and Wright (1966)
and Brown (1971) for the White Pine copper deposit of Michigan. They
conclude that copper was deposited in a sulfur‑rich reducing environment,
largely by replacement of original pyrite along a laterally and upward
advancing mineralization front. Application of such a mechanism to Mufulira
would explain better the pyrite over the Basement hills as a result of
insulation from the mineralizing solutions moving primarily upward from
supply channels in the Footwall sandstones, since these sandstones exist
only between the Basement hills (W. S. White, pers. commun., 1973). Thus,
the assumption of a White Pine model for the Mufulira deposits is tempting,
but for the moment a more detailed study of the mineral zoning is required
to arrive at a well‑defined answer.
Page 74 of the original
FIG. 14. Lithologic columns with copper
assay graphs and type of mineralization; chalcocite (Cc), bornite (Bn),
chalcopyrite (Cp), and pyrite (Py) ; location of columns indicated in Figure
It is evident that ore‑forming
processes were not limited to one phase in the history of these rocks and
explaining the rich concentrations of copper sulfides requires a combination
of sedimentary and diagenetic mechanisms. Ore principally occurs in two
lithotypes: (1) carbonaceous wacke and argillitic sandstone, and (2)
conglomeratie, often cross‑bedded arenites, locally intercalated with
argillite beds. These types of ore that occur in distinct parts of the ore
bodies (see identifying numerals in Fig. 16) point to diverse genetic
processes and will be discussed separately.
(1) Ore in a host rock of fine‑ to
medium‑grained argillitic wacke occurs as a stratiform body within the area
of economie mineralization. This type of mineralization is related to a
particular paleogeographic realm as well as to a particular stratigraphic
sequence. Stratigraphically the mineralization occurs in fine‑grained
argillitic wackes that form part of a transgressive sequence, underlain by
coarse alluvial or littoral sediments and overlain by deeper water marine
argillites. Occurrence of the highest grade copper mineralization in the
three wacke horizons of a repetitive cyclic succession illustrates
convincingly the stratigraphic‑sedimentary control on the mineralization
(Fig. 15) .
With respect to host rock
paleogeography, this ore is bound to sediments of the nearshore marine
environment; it forms a relatively narrow belt stretching along the coast,
fringed on one side by the highenergy shore environment and on the other
side by submarine sand bars or deeper marine environment (Fig. 8) . In some
places the ore bodies are cut off by large‑scale cross‑bedded, barren
sandstones. The absence of copper sulfides in these rocks need not be due to
an eolian origin as proposed by Garlick (1967) ; a high‑energy marine
environment flanking an area of quiescent conditions would result in a
similar cutoff of mineralization, due to a rapid lateral change in
lithofacies from a favorable to an unfavorable host rock. Mineralization
occurred in the rocks of marine nearshore basins, sheltered from the open
sea by shoals that were located over protruding Basement hills, thus
associating mineralization with a pronounced Basement topography (Fig. 9),
An original low‑grade copper content in
the carbonaceous wacke may partly be explained by synsedimentary processes.
Figure 9 shows surface waters in connection with the open sea, while bottom
waters were restricted in their circulation by the shape of the sea floor
hollows. Copper ions in the sea water could have precipitated as sulfides on
encountering deoxygenated bottom waters, as proposed by Garlick (1961) and
others, and conditions in these bottom sediments would certainly be
favorable for the formation of early diagenetic sulfides.
If the copper content of some
argillites, up to 0.5 percent, may be taken as a guide to possible
syngenetic copper grades, enrichment must have .taken place. During the
process of enrichment these same carbonaceous wackes formed a particularly
favorable host rock, due mainly to their reducing capacity.
page 75 of the original
FIG. 15. Typical lithologic section of
Ore formation and an assay graph for copper; eastern area.
(2) Part of the economic mineralization
is in clean, often cross‑bedded arenites or conglomeratic sandstones. The
high‑energy environment in which these rocks are laid down contrasts with
the quiescent conditions necessary for the synsedimentary mineralization of
type (1) and it is therefore considered as a genetically different type of
ore. Type (2) mineralization is always spatially associated with type (1),
and it constitutes only a minor part of the volume of the ore bodies.
As mentioned above, the type (1)
mineralization in argillitic‑carbonaceous sandstones must have been enriched
postsedimentary, and the underlying permeable arenites could have formed an
ideal channelway for mobilized pore fluids. The greater thickness of the
mineralization in the west, where abundant conglomerates underlie the C
horizon (Fig. 5), and the concentration of ore minerals along small unconformities such as washouts suggest a
controlling influence of permeability on emplacement of the ore. The lower
boundary of the C economic mineralization in the east is in well‑sorted
arenites that form a unit a few tens of feet thick in the top of the
Footwall formation. This lower ore boundary often follows a particular bed
over some distance, but no apparent lithofacies change is observed between
the mineralized and underlying barren rocks. That only the top part of the
Footwall arenite unit is mineralized shows that, though permeability must
have controlled the flow of copper‑bearing fluids, other factors were of
importante in precipitating the copper. Favorable physicochemical
conditions, such as a reducing environment associated primarily with the
carbonaceous wackes, may have extended somewhat below the actual lithologic
boundary within the clean arenites : water squeezed out of the overlying
wackes and argillites during compaction may have influenced the diagenetic
environment, a process similar to that suggested by Hamilton (1967) for
Copper sulfides in conglomeratie
sandstones of the western area are mostly conspicuously concentrated around
argillite fragments (Fig. 11) or near intercalated argillite layers. The
argillite intercalations interpose permeability barriers and they may have
physically retarded the flow of migrating copperbearing pore fluids. In
addition, the argillite, which contained a relatively high amount of organic
carbon, must have induced a reducing environment in which topper sulfides
could be arrested.
The sedimentary succession of the Lower
Roan has commonly, in the past, been described as consisting of windblown
continental sands overlain by marine sediments reflecting increasingly deep
water (Brandt et al., 1961). In this paper, the eolian origin of the
Footwall sandstones at Mufulira is rejected and a transgressive marine
succession starting at the Basement unconformity, or locally above the basal
conglomerates, is proposed. The same reasoning may well apply to similar
stratigraphic horizons in other areas of the Copperbelt. For instance, at
Luanshya large‑scale crossbedded sandstones, previously considered to be of
eolian origin, have also been reinterpreted as subaqueous by Binda and van
Eden (1968) .
A reinterpretation of large‑scale
cross‑bedded sandstones is not without precedent. The coppershale deposits
at Mansfeld, Germany, in many ways analogous to the Copperbelt deposits of
Zambia (Garlick, 1961, p. 147), are underlain by sandstones of the
Weissliegendes which for more than 50 years had been interpreted as eolian ;
these have now been shown to be of marine origin (Pryor, 1971).
Page 76 of the original publication
FIG. 16. Idealized longitudinal section
through Footwall and C Orebody, showing spatial relationship of (1)
stratiform ore in carbonaceous‑argillitic sandstone and (2) ore in
conglomeratic and cross‑bedded arenite, so‑called Footwall mineralization.
Though the Mufulira deposits contain
elements that point to synsedimentary sulfide deposition, processes of
enrichment during diagenesis are also evident. The economic concentration
of copper is, though partly dependent upon permeability characteristics,
principally controlled by the reducing carbonaceous‑sulfureous environment
that was associated with the carbonaceous wacke and mottled sandstone. The
Mufulira ore bodies may be described as containing a nucleus of
carbonaceous wacke, enveloped by mottled sandstone that was, at least
originally, also rich in organic carbon, and fringed by cleaner,
non‑carbonaceous arenites or conglomerates. Mineralization varies from
pyrite and chalcopyrite in the carbonaceous core to chalcopyrite and
bornite in the enveloping rocks, and chalcocite in the (near‑surface)
fringes. The deposition and preservation of carbonaceous material is
limited to strictly defined sedimentary environments, which may be outlined
by study of the paléogeography. At Mufulira the carbonaceous wackes are
spatially related to a shore line and to a paleotopography that was
inherited from the Basement.
Uniform, large‑scale cross‑bedded
sequences, underlying and laterally flanking the C Orebody in the eastern
area, formed excellent permeability systems, but lacked precipitants and
are therefore barren. The eastern fringe of the ore body is largely
determined by the sedimentary geology; mineralization in argillitic and
carbonaceous sandstones of nearshore quiescent environment ends where the
whole sequence below the Mudseam consists of crossbedded arenites of a high‑energy open
marine environment (previously considered eolian) that occurs to the
southeast. In the western area mineralization is not related to sedimentary
environment; the orebody fringe occurs arbitrarily within a laterally
uniform lithofacies and was probably governed by a physicochemical gradient
toward the favorable carbonaceous host rocks that occur to the east. The poor correlation of the mineralization with the sedimentary environment in
this area may be partly due to the effects of supergene enrichment, a
process that has affected the western part of the C Orebody to a greater
depth because of the locally abundant porous conglomerates.
Techniques of sedimentary petrology
have shown their value in this study of the cupriferous beds at Mufulira.
Major trends in the paleogeography are consistent with the distribution of
the copper sulfides and the sedimentary facies is seen to have influenced
both synsedimentary deposition of sulfides and the distribution of later
I am indebted to F. Mendelsohn and W.
G. Garlick who stimulated my initial interest in the geology of the
Copperbelt and who outlined the direction of this study in its early stages.
F. Mendelsohn also critically read the manuscript. Thanks are due to P. L.
Binda, M. E. Green, and J. L. W. Dolly for helpful criticism on drafts of
this paper and valuable suggestions that came forward in our many
discussions about concepts of ore genesis. W. S. White of the U. S.
Geological Survey reviewed the paper and is gratefully acknowledged for his
Page 77 of the original
The author wishes to acknowledge the
Management of Roan Consolidated Mines Ltd. for permission to publish this
GEOLOGIC RESEARCH DEPARTMENT
ROAN CONSOLIDATED MINES LTD.
Present Address [ this is the address
at the time of publishing the paper in Economic Geology]:
J.G. VAN EDEN
GEOLOGIC RESEARCH DEPARTMENT
JOHANNESBURG CONSOLIDATED INVESTMENT
P.O. BOX 2, RANDFONTEIN REPUBLIC OF SOUTH AFRICA
February 5; August 15, 1973
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Page 79 of the original publication
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