First a mineral brine occurs over millions of years, then you bring your volcano up through it and convert the brine to sulfide ore.
It is known that metals like copper and gold occur in association with volcanoes.
Porphyry deposits represent an important resource of copper; however,
they are also important sources of gold and molybdenum... Porphyry
deposits are formed in arc-related settings and are associated with
subduction zone magmas. Interestingly, porphyry deposits are
clustered in discrete mineral provinces, which implies that there is
some form of geodynamic control or crustal influence affecting the
location of porphyry formation.
These scientists have laid out a model for what those geodynamic controls are.
Generation of porphyry copper deposits by gas–brine reaction in volcanic arcs
Porphyry copper deposits, that is, copper ore associated with
hydrothermal fluids rising from a magma chamber, supply 75% of the
world’s copper. They are typically associated with intrusions of magma
in the crust above subduction zones, indicating a primary role for
magmatism in driving mineralization. However, it is not clear that a
single, copper-rich magmatic fluid could trigger both copper
enrichment and the subsequent precipitation of sulphide ore minerals
within a zone of hydrothermally altered rock. Here we draw on
observations of modern subduction zone volcanism to propose an
alternative process for porphyry copper formation. We suggest that
copper enrichment initially involves metalliferous, magmatic
hyper-saline liquids, or brines, that exsolve from large, magmatic
intrusions assembled in the shallow crust over tens to hundreds of
thousands of years. In a subsequent step, sulphide ore precipitation
is triggered by the interaction of the accumulated brines with
sulphur-rich gases, liberated in short-lived bursts from the
underlying mafic magmas.
In sum: a subsurface brine accumulates dissolved metals over a long period. Mineral brines might be forming all over the world regardless of volcanism. But if there is then an uplift of magma and sulfur rich gases into one of these collections, the minerals precipitate abruptly into the sulfide ores and are carried on up and out by the magma. If your new volcano happens (or is positioned) to be coming up underneath a brine deposit like this, there are your metals.