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Porphyry-type ore deposits form in hydrothermal fluid circulation systems developed above and around high-level, subvolcanic felsic to intermediate magma chambers and/or cooling plutons. The ore is temporally and genetically related to the intrusions, but did not precipitate directly from the magma. [1]
The ore fluids of MVT deposits are typically low temperature (100 °C–150 °C) and have the composition of basinal brines (10–30 wt.% NaCl equivalent) with pH's of 4.5–5 (buffered by host carbonates). This hydrothermal fluid may or may not carry the required sulfur to form sulfide minerals.
The ore materials are trapped within a fumarole field or a black smoker field when they are expelled into the ocean, cool, and precipitate sulfide minerals as stratiform sulfide ore. [6] Some deposits show evidence of formation via deposition of sulfide via replacement of altered volcanosedimentary rocks and may also form by invasion of sulfur ...
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Even if some deposits clearly indicate a magmatic source, it must be considered that only due to overprinting mineralization with higher gold grades from other sources, these deposits became economic. [23] A hybrid deposit with a combination of a magmatic and a metamorphic (mid- or sub-crustal) source is a much more common scenario. [24]
Classification of hydrothermal ore deposits is also achieved by classifying according to the temperature of formation, which roughly also correlates with particular mineralising fluids, mineral associations and structural styles. [2] This scheme, proposed by Waldemar Lindgren (1933) classified hydrothermal deposits as follows: [2]
The term "polymetallic ore" also includes nodules, principally Manganese nodules, that do not form as terrestrial deposits but as concretions on the ocean floor. [3] [4] Rocks containing polymetallic ores are often altered or formed by hydrothermal processes — chloritization, sericitization and silicification.
Primary deposits contain hydrothermal and igneous processes while secondary deposits are sedimentary and weathering processes. [11] In the case of primary deposits, the minerals and metals are derived from a specific area, where the elements come together to form the deposit. [12] This location is also where the mineral is produced. [12]