Stratabound Massive Sulphides

    There is a morphological term for base metal sulphide deposits that happen as part of a sequence of sedimentary or volcanic rocks and conform to their host rock’s bedding. That they happen as part of the sequence is very strong evidence that they formed along with their host rocks, instead of being implaced later. There is the chance that they would start with the shapes of lens or tabulas, but the possibility of later deformation can fold them into shapes which are complex or they can also break them into pieces. When referring to the expression of massive sulphide it does not mean by any means that it has something to do with the size, instead, the mineral bodies are nearly homogenous and are made up almost completely of sulphides.

    Volcanogenic massive sulphides are the stratabound deposits which are found in volcanic rocks. Volcanic vent areas and the dykes, stocks and sills that feed them are the sources of heat and, as a consequence, they are the centres of exhalative or hydrothermal activity. Circulating waters carrying dissolved metals travel through fractures in the volcanic rocks, at times they deposit sulphide minerals in the fractures themselves. The heat forces the fluids in an upward direction to the top of the volcanic sequence, where they are vented or exhaled.

    The immediate change in temperature as the fluids leave the hot rock makes it seemingly impossible for the dissolved species to remain in solution, due to this they precipitate at the surface of the volcanic pile. It is very common for this process to be happening at many different places at the same time and many deposits have the chance of forming at a single mineralized horizon or stratigraphic level.

    If the volcanic pile is found not on land but under water, the exhaled fluids intertwine with the seawater and the sulphides are deposited around the hydrothermal vent, which form layers of massive sulphide material. They may be moved and carried for some distances by ocean-floor currents or they may slump into ocean-floor depressions.

    If the volcanic pile is found above sea level, the fluids that were at one time under immense pressure inside the fractures in the rock are immediately released into the lower pressure of the atmosphere. The fluids respond by immediately turning into steam. Dissolved minerals precipitate as a sinter around the vent area, very similar to the scale around the mouth of a kettle.

    Massive sulphide deposits are apt to contain base metal sulphides such as galena, sphalerite and chalcopyrite. The iron sulphides pyrite and pyrrhotite very frequently happen along with them. The main minerals of ore provide lead, zinc and copper, and metals such as gold, silver, tin and cadmium are very common byproducts in the deposits mentioned. There are also barren massive sulphide bodies which contain only massive pyrrhotite or pyrite.

    It is very common for the rocks which are stratigraphically under the mineralized horizon to have stockwork or stringer mineralization, that is, sulphide minerals which are in veinlets or are disseminated through the rock. This mineralization can contain sufficient metal to be considered ore, even though the grade is a lot lower than that of the massive ore in the true stratabound body above it. The host rocks surrounding it show very strong hydrothermal alteration and at times are converted completely to chlorite, a green colored alteration mineral.

    It is of much use to take notice that the stockwork mineralization is younger than its host rocks, or epigenetic, while the stratabound body is conidered synergetic, even though the two were formed at about the same time.

    Sedimentary exhalative deposits are very similar to the volcanogenic massive sulphides; they are massive sulphide bodies that happen in sedimentary rocks, most often in thick sequences of shale. These deposits also have stringer or disseminated mineralization in the rocks which are under the mineralized layer and, similar to their volcanic cousins, they can be deformed or faulted into shapes that are very complex.

    Some typical volcanogenic massive sulphides are the Bathurst and Kidd Creek deposits of Canada. A typical deposit of the sedimentary exhalative type is the Rammelsberg deposit which is found in the eastern side of Germany.