As outlined in previously, Kashan, Iran seems to be the initial production site of Blue on White ceramics. This is due to Kashan's proximity to a source of cobalt and other minerals useful in creating glazes. The Kashani cobalt originates from Qamsar aka Ghamsar which is located in the hills south of Kashan. Ghamsar is located proximal the the boundary between the Sanandaj-Sirjan Zone (1) (associated with the subduction of the Neo-Tethys Ocean) and the Urumieh-Dokhtar Arc. The U-D Arc hosts porphyry copper-gold deposits (2) as well as epithermal lead-zinc deposit (3).
Figure1: Geology of Iran - adapted from Geology of the Sari Gunay Epithermal Gold Deposit (4). The blue star marks the location of Kashan and to the south, Ghamsar (aka Qamsar).
The cobalt in Ghamsar is formed as part of a skarn deposit (5) resulting from the intrusion of a microdiorite into the Qom Formation. For non-geologists, the limestone in the Qom Formation is dissolved by the hot hydro-thermal fluids originating from intrusion of diorite, a volcanic rock. The contact between these two rock types metamorphises and recrystalises becoming the skarn deposit. The mineralology of a skarn is determined by the composition of the hydrothermal fluids and the limestone. The Ghamsar deposit appears to be structurally controlled, with the mineralising fluids travelling through faults. The cobalt is spatially associated with magnetite veins in the faults however the cobalt may precede the magnetite ore.
Summary: Due to the unique geological processes and structures involved, cobalt ore is unlikely to form and be accessible at surface in a form useful for glaze production.
We can see why ceramics are produced at Kashan instead of Ghamsar, the source of the ore, by consulting a soils map. Figure 2 indicates that the soils around Ghamsar are calcareous lithosols which are calcium rich, and often contain partly or unweathered rock. They are calcareous as their parent rock is likely the limestones of the Qom Formation. Kashan, on the other hand, has a source of salt marsh soils. Salt marshes are a low energy environment which accumulate sediment over long periods of time. Marsh soils are typically fine grain and well sorted, these appear to be grey and low in organic matter. So while the washed cobalt can be transported a short distance, it's unlikely the clay would be transported.
Figure 2: A map of the soils of Iran indicates which medieval towns would have superior clay. (6)
1: A new tectonic scenario for the Sanandaj–Sirjan Zone (Iran) 2005 A. Ghasemia, C.J. Talbotb Journal of Asian Earth Sciences.
2: Porphyry Copper Deposits of the Urumieh-Dokhtar Magmatic Arc, Iran. 2005. A. Zarasvandi, M. Zentilli, S. Liaghat.
3: The Ay Qalasi deposit: An epithermal Pb-Zn (Ag) mineralisation in the Urumineh-Dokhtar Volcanic Belt of northwestern Iran. DOI: 10.1127/njma/2015/0284
4: Geology of the Sari Gunay Epithermal Gold Deposit 2006 Northwest Iran. by J.P Richards, D. Wilkinson and T. Ulrich. Economic Geology. .
5: Ore Mineralization at Qamsar Cobalt Deposit:Skarn and Metasomatism Evidences . Hadi Mohammaddoost, Majid Ghaderi, Nematollah Rashidnejad-Omran The 1st International Applied Geological Congress
6: Soils of Iran 1961. M.L Dewan, J. Famouri. Food and Agriculture Organisation of the United Nations.