Die Wurzeln von Ashtnaga

The earliest possibly description of internalized vedic ritual as a process of refinement of the body trough generated Heat (tapas). This is much earlier than Upanishadas!

Here under, is described the process of very first creation. Also as stated in many places in Shatapatha BrAhmana it is
process of getting the elixir of immortality because “gold is
immortality” (6.7.1.2, 7.4.2.17,...). (Which for men is 100 years vishvAyus, sarvAyus = immortality)
Translated by : Julius Eggeling

1. Verily, Pragâpati alone was here in the beginning. He desired, 'May I exist, may I reproduce myself!' He toiled, he practiced austerity (or, became heated). From him, worn
out and heated, the waters were created: from that heated Person the
waters are born.

2. The waters said, 'What is to become of us?'--'Ye shall be heated,' he said. They were heated; they created foam: hence foam is produced in heated water.

3. The foam (m.) said, 'What is to become of me?'--'Thou shalt be heated!' he said. It was heated, and produced clay; for indeed the foam is heated, when it floats on the water,
covering it; and when one beats upon it, it indeed becomes clay.

4. The clay (f.) said, 'What is to become of me?'--'Thou shalt be heated!' he said. It was heated, and produced sand; for this clay becomes indeed heated when they plough it; and if
only they plough very fine then it becomes, as it were, sandy. So
much, then, as to that 'What is to become of me? what is to become of
me?'

5. From the sated he created the pebble: whence sand finally indeed becomes a pebble;--from the pebble the stone: whence the pebble finally indeed becomes a stone;--from the
stone metal ore: whence from stone they smelt ore;--from ore gold:
whence ore much smelted comes, as it were, to have the appearance of
gold.

6.1.3.[1]
prajāpatirvā idamagra āsīt | eka eva so 'kāmayata syām prajāyeyeti so 'śrāmyatsa tapo
'tapyata tasmācrāntāttepānādāpo 'sṛjyanta tasmātpuruṣāttaptādāpo jāyante
6.1.3.[2]
āpo 'bruvan | kva vayam bhavāmeti tapyadhvamityabravīttā atapyanta
tāḥ
phenamasṛjanta tasmādapāṃ taptānām pheno jāyate
6.1.3.[3]
pheno 'bravīt | kvāham bhavānīti tapyasvetyabravītso 'tapyata sa
mṛdamasṛjataitadvai phenastapyate yadapsvāveṣṭamānaḥ plavate sa
yadopahanyate
mṛdeva bhavati
6.1.3.[4]
mṛdabravīt | kvāham bhavānīti tapyasvetyabravītsātapyata sā sikatā
asṛjataitadvai
mṛttapyate yadenāṃ vikṛṣanti tasmādyadyapi sumārtsnaṃ vikṛṣanti
saikatamivaiva
bhavatyetāvannu tadyatkvāham bhavāni kvāha bhavānīti
6.1.3.[5]
sikatābhyaḥ śarkarāmasṛjata | tasmātsikatāḥ śarkaraivāntato bhavati śarkarāyā
aśmānaṃ tasmācarkarāśmaivāntato bhavatyaśmano 'yastasmādaśmano
'yo
dhamantyayaso hiraṇyaṃ tasmādayo bahudhmātaṃ hiraṇyasaṃkāśamivaiva bhavati

Gold as Immortality for example in the 7.4.2.[17]

Here now they say, 'If (the dviyagus) is that same Sacrificer who is that gold man, which then is that (real) form of his?' Well, that (gold man) is his divine body, and
this (brick) is his human one. As to that gold man, that is his
immortal form, his divine form; gold being immortal. And as to this
(brick) being made of clay, it is because this is his human form.

7.4.2.[17]
tadāhuḥ | yadasāveva yajamāno yo 'sau hiraṇmayaḥ puruṣo 'tha katamadasyedaṃ
rūpamiti daivo vā asya sa ātmā manuṣo 'yaṃ tadyatsa hiraṇmayo bhavatyamṛtaṃ vā
asya tadrūpaṃ devarūpamamṛtaṃ hiraṇyamatha yadiyam mṛdaḥ kṛtā bhavati
mānuṣaṃ
hyasyedaṃ rūpam