The easiest solution: the brazier
Caves need light, and the simplest solution for light is fire. Braziers were regularly used in the ancient world not only for fire, but for cooking. There is an impressive list of things you can cook that would give supplicants in the consecrated chamber all kinds of visions ... especially if the High Priest knows which roots/powders/plants to drop on top of the burning coals. These include:
Acacia • African Dream Root (Silene Capensis) • Amanitas Mushroom • Angel's Trumpet (Brugmansia) • Ayahuasca • Betel Nut • Blue Lily (Sacred Lily of the Nile) • Bolivian Torch Cactus • Brunfelsia • Bullet Ant (go read about this one... Sheesh!) • Bundle Flower (Desmanthus illinoensis) • Caapi • Cacao (raw, in high doses) • Calea Zacatechichi • Cannabis • Capsicum • Capsicum • Cestrum • Chaliponga (Diplopterys cabrerana) • Chacruna • Christmas Vine • Cohoba • Coleus • Colorado River Reed • Colorado River Toad • Coffee • Datura (Jimson Weed or Devil’s Snare... the real one) • Desfontainia • Diplopterys • Ephedra • Ergot • Fly Agaric • Fermented Honey • Guarana • Hallucinogenic fish • Henbane • Iboga • Intoxicating Mint • Jurema • Justicia Pectoralis • Kanna • Kava • Khat • Kratom • Lettuce Opium • Mapacho • Mescal • Mezcal • Mucuna Pruriens • Mandrake • Mimosa • Nutmeg • Ololiuqui • Passionflower • Peyote • Phalaris Grass • Pituri Bush • Poppies • Psychotria Viridis • Salvia Divinorum • Sakae Naa • San Pedro Cactus • Sarpa Salpa • Sinicuichi • Sleepy Grass • Solandra • Saint John’s Wort • Syrian Rue • Thuoc Lao • Tobacco • Virola • Voacanga • Vilca • Wild Lettuce • Wormwood • Yerba Mate • Yopo • Yohimbe Bark • Zacatechichi.
Now, to be fair, some of the items on that list are critters, not plants. But, high priests do get hungry, don't they? Everything on the list is a hallucinogenic. Some also induce convulsions. If you'll forgive me, the list is long enough that I'll let you go to the source to discover specific properties.
An Improbable Solution
Our cave is high on a volcano. A lava river exudes from the base where any one of these plants grow. The plants burn, and a natural chimney takes the smoke up to one of the cave chambers.
However, this is a really capricious solution. A bad year, not enough rain, yadda yadda, I doubt this would be a predictable solution.
A quick Internet search suggests there is no such thing as a hallucinogenic mineral
Which stinks, 'cause that would have been the simplest way to get to where we want to go.
So, maybe this...
What if one of the more feathery or viney of the plants listed above were prolific in bog land many bazzilions of years ago... and over time the volcanic mountain that contains our cave covered the bogs with, well, dirt. This would create a seam of organic, combustable material containing hallucinogenic components. And that seam was exposed in the consecrated chamber.
First discovered by the first high priest who entered the cave on a vision quest looking for answers to life, the universe, and everything (it can't be 42 as that's not evenly divisible by fingers and toes) and, of course, he had a torch to light his way.
He camped in the chamber and enjoyed a nice meal of dried berries and fish, then slept. The torch was conveniently placed by the seam. And sometime in the middle of the night the very first high priest met his god.
The chamber was instantly sacred, which meant moving in braziers, which meant combustion....
Unless you can get naturally occuring Nitrous Oxide
But laughing gas might not be the effect you're looking for!
I find it unlikely any of the following (other than the ethanols/ethelynes) can occur naturally... but maybe. I found the list here, on page 10.
Solvents: Acetone, benzene, difluroethane, ethanol, ethyl acetate, hexane, methanol, methylene chloride, naptha, toluene, trichloroethane, trichlormethane
Fuels: Butane, gasoline, kerosene, propane
Aerosols/Gases: flurocarbons, Freon, helium, xenon
Nitrates: Amyl Nitrate, Butyl Nitrate
But, it has happened before
Concerning the Oracle at Delphi...
The region's underlying rocks turn out to be composed of oily limestone fractured by two hidden faults that cross exactly under the ruined temple, creating a path by which petrochemical fumes could rise to the surface to help induce visions.
In particular, the team found that the oracle probably came under the influence of ethylene -- a sweet-smelling gas once used as an anesthetic. In light doses, it produces feelings of aloof euphoria.