Stratabound Massive Sulphides

Stratabound Sulphide is a morphological term for base metal sulphide deposits that occur as part of a sequence of volcanic or sedimentary rocks and conform to their host rock’s bedding. That they occur as part of the sequence is strong evidence that they formed along with their host rocks, rather then being emplaced later. They may start with tabular or lens-like shapes, but later deformation can fold them into complex shapes or break them into pieces. The expression massive sulphide has nothing to do with size, rather the mineralized bodies are nearly homogenous and made up almost entirely of sulphides.

Volcanogenic massive sulphides are stratabound deposits in volcanic rocks. Volcanic vent areas and the dykes, sills and stocks that feed them are sources of heat and, consequently, are centers of hydrothermal or exhalative activity. Circulation waters carrying dissolved metals travel through fractures in the volcanic rocks, sometimes depositing sulphide minerals in the fractures themselves. The heat forces the fluids upward to the top of the volcanic sequence, where they are “exhaled” or vented.

The sudden change in temperature as the fluids leave the hot rock makes it impossible for the dissolved species to stay in solution, so they precipitate at the surface of the volcanic pile. It is common for this process to be occurring at several places at once and several deposits may form at a single stratigraphic level or mineralized horizon. If the volcanic pile is under water, the exhaled fluids mix with sea water and sulphides are deposited around the hydrothermal vent, forming layers of massive sulphide material. They may be carried some distance by ocean-floor currents or may slump into ocean-floor depressions.

If the volcanic pile is above sea level, the fluids that were once under great pressure inside the fractures in the rock are suddenly released into the lower pressure of the atmosphere. The fluids respond by rapidly turning into steam. Dissolved minerals precipitate as a sinter around the vent area, much like scale around the mouth of a kettle.

Massive sulphide deposits can contain base metal sulphides like chalcopyrite, sphalerite and galena. The iron sulphides pyrrhotite and pyrite almost always occur along with them. The main ore minerals provide copper, zinc and lead, and metals such as gold, silver, cadmium and tin are common byproduct in these deposits. There are also barren massive sulphide bodies that contain only massive pyrite or pyrrhotite.

It is common for the rocks stratigraphically below the mineralized horizon to have stringer or stock-work mineralization, that is, sulphide minerals in wenlets or disseminated through the rock. This mineralization can contain enough metal to be ore, although the grade is much lower that that of the massive ore in the true stratabound body below it. The surrounding host rocks show very strong hydrothermal alterations and sometimes are converted entirely to chlorite, a gree alteration mineral.

It is useful to note that the stock-work mineralization is younger than its host rocks, or epigenetic, while the stratabound body is syngenetic, even though the two formed at about the same time.

Sedimentary exhalative deposits are similar to the volcanogenic massive sulphides; they are massive sulphide bodies that occur in sedimentary rocks, most often in thick sequence of shale. These deposits also have disseminated or stringer mineralization in the rocks below the mineralized layer and, like their volcanic cousins, may be formed or faulted into complex shapes.

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