Table of Contents
The large massive sulphide deposits present a formidable challenge to the economic geologist. They represent such huge concentrations of iron, sulphur, base metals and precious metals that it is difficult to conceive how they were derived from the crust or mantle of the Earth. The problem of the origin of massive sulphide deposits has generated a geological controversy which has lasted two hundred years and shows no sign of abating.
Field Evidence of the Origin of Massive Sulphide Deposits
Although much experimental and theoretical work has been done towards elucidating the origin of the massive sulphide deposit, the most significant advances have come from field observations, particularly those made by geologists whose activities have permitted them to see many deposits.
It is beyond the scope of this paper to describe all of the characteristics of the massive sulphide deposits found in volcanic and sedimentary rocks. The four characteristics described in the following pages have been selected as having a particular bearing on the origin of the deposits.
Lithologic and Stratigraphic Associations
Massive sulphide deposits show certain marked regularities in their stratigraphic and lithologic associations. These relationships appear to be consistent, regardless of the age of the deposit. Geologists whose experience has been in volcanic terranes tend to stress the occurrence of massive sulphides in, or superjacent to, the uppermost portions of volcanic complexes. In such locales, two gross lithologic-stratigraphic associations are recognized:
The rhyolite-sulphide association
In certain volcanic sequences characteristic of orogenic regions, including the spilite-keratophyre and the basalt-andesite- dacite-rhyolite (calc-alkalic) kindreds, massive sulphides occur within, immediately above, or laterally peripheral to the youngest rhyolitic units. The host rocks are rhyolite flows, pyroclastics tuffaceous clastic sediments, or chemical sediments rich in silica, iron, and manganese. Graphite or other carbonaceous material commonly occurs in these pyroclastic and sedimentary rocks.
The basalt-sulphide association
A leas common environment for massive sulphides is at the tops of basalt complexes, or in the superjacent sedimentary rocks. Rhyolite is absent from this environment. The basalt complexes ore described by some writers as “ophiolitic” implying that they were formed by early eugeosynclinal mafic volcanism, whereas others interpret them as oceanic tholeiites formed at mid-oceanic ridges.
With several notable exceptions, the sedimentary hosts of the massive sulphide deposits have at least a minor volcanic component. This component can range from sparse mixtures of pyroclastic fragments with the normal sedimentary detritus, through thin tuff intercalations, to thick tongues of pyroclastic or flow rocks. The presence of a minor volcanic component may be extremely difficult to recognize for two reasons. The first is that small quantities of primary pyroclastic material (that is, volcanic clasts contributed directly to the sediments by explosive volcanic activity) cannot always be distinguished from detritus derived by normal processes of erosion and sedimentation from a volcanic terrane. The second problem arises when metamorphism has masked or obliterated the original characteristics of primary volcanic material.
Form and Conformability
The massive sulphide deposits under discussion are essentially tabular or lenticular, with many minor irregularities. They lie with their larger dimensions in the plane of stratification of the enclosing rocks. In structurally deformed areas the shapes of the massive sulphide bodies may reflect the changes produced by folding and faulting. In studying these deposits it is useful, where possible, to determine the top directions of the enclosing strata, and to mentally unfold and restore the strata (and the deposits) to their original upright, horizontal attitude. This technique is used in the following descriptions and such terms as upper, hangingwall, and thickness are used in the stratigraphic sense.
With minor exceptions, the upper contacts of the massive sulphide deposits are sharp and the overlying rock is not mineralized. The lower contacts may also be abrupt, but are commonly less well defined. In many deposits there is a downward transition from concordant massive sulphides to a root-like discordant zone of disseminated or veinlet sulphides.
Internal Structures and Textures
Some of the massive sulphide deposits have been described as “massive and structureless.” However, most of them are, at least in part, banded at some scale, and breccias of various types are seen in many deposits.
Banding – Compositional or textural layering involving ore and gangue minerals is a common feature of the massive sulphide deposits. “Banding” may be crude layering, with individual layers several cm. to a few metres in thickness, with vague boundaries and little lateral continuity, or it may consist of sharply defined, paper thin laminae with remarkable lateral persistence. The most striking form of banding, and one of the most common varieties, consists of thin layers of pyrite which alternate with layers of either sphalerite or galena, or both. The banding in massive sulphide deposits is parallel to the bedding of the enclosing strata, except where disharmonic folding has occurred. Bands a few mm. thick may maintain this parallelism faithfully through crenulations and larger folds.
Brecciation – Coarsely fragmental portions of massive sulphide deposits have been interpreted in the past as post-ore tectonic breccias or sulphide replacements of pre-ore breccias. More recently, some of the sulphide breccias have been restudied and hove been attributed to other mechanisms of formation.
Metamorphism of Massive Sulphide Deposits
Massive sulphide deposits occur in terranes which range from virtually unmetamorphosed to highly metamorphosed. The observation from field work or petrofabric studies that the fabric of many massive sulphide deposits is in harmony with the fabric of the enclosing metamorphic rocks has convinced many geologists that the sulphides were deposited prior to at least the most recent metamorphic event. On the other hand, the presence of apparently undeformed ores in deformed environments has been cited as evidence of post-metamorphism deposition.
In the vicinity of zinc-bearing deposits the occurrence of gahnite, the zinc spinel, is considered by the writer as evidence that moderate to high-grade metamorphism has involved both the ore and its host rocks.
An illustration of the inability of regional metamorphism to produce fundamental changes in massive sulphide orebodies is provided by the Broken Hill District of Australia. In this productive district, several lead-zinc orebodies occur in complexly folded Precambrian gneiscas which have attained the sillimanite-almandine facies of metamorphism. The orebodies are confined to two principal layers and a number of minor layers, which are conformable with each other and with the bedding plane foliation of the enclosing rocks.
Before a syngenetic origin can be accepted for massive sulphide deposits, several requirements must be satisfied. When the deposits have been deformed and metamorphosed, the task becomes more complex, and involves the following steps:
- Establishing that the deposit existed prior to metamorphism.
- Showing that it was formed before the structural deformation of the environment.
- Establishing that the deposit was formed at surface, that is, on the sea floor.
Careful field and microscope work by Vokes, Ramdohr, Kalliokoski and many others have furnished compelling evidence of a pre-metamorphic origin of many massive sulphide deposits. The persistent occurrence of orebodies at one or a few stratigraphic levels in such highly deformed districts as Broken Hill and Ducktown constitutes evidence that the ores were emplaced prior to deformation. The consistent stratigraphic relationship between lead-zinc and copper zones in certain massive sulphide orebodies, regardless of whether they are now horizontal, inclined, vertical or overturned, supports the interpretation fiat emplacement pre-dated folding.
Perhaps the strongest evidence of surface deposition is found in the “fragmental ores” of some Kuroko deposits and their analogs among older deposits. These ores ore breccia form mixtures of sulphide fragments, sedimentary detritus and volcanic rock fragments. Their mineralogical and textural heterogeneity and the presence of graded bedding precludes any possibility of a replacement or tectonic origin. These breccias were deposited on the sea floor, either by the explosive ejection of shattered, previously solidified sulphides, or by sea floor slumping of sulphides originally deposited near a vent.