diff --git a/swh/loader/package/loader.py b/swh/loader/package/loader.py
--- a/swh/loader/package/loader.py
+++ b/swh/loader/package/loader.py
@@ -449,6 +449,18 @@
             status_visit = "partial"
             status_load = "failed"
 
+        if snapshot:
+            try:
+                metadata_objects = self.build_extrinsic_snapshot_metadata(snapshot.id)
+                self._load_metadata_objects(metadata_objects)
+            except Exception as e:
+                logger.exception(
+                    "Failed to load extrinsic snapshot metadata for %s", self.url
+                )
+                sentry_sdk.capture_exception(e)
+                status_visit = "partial"
+                status_load = "failed"
+
         try:
             metadata_objects = self.build_extrinsic_origin_metadata()
             self._load_metadata_objects(metadata_objects)
@@ -638,6 +650,42 @@
 
         return metadata_objects
 
+    def get_extrinsic_snapshot_metadata(self) -> List[RawExtrinsicMetadataCore]:
+        """Returns metadata items, used by build_extrinsic_snapshot_metadata."""
+        return []
+
+    def build_extrinsic_snapshot_metadata(
+        self, snapshot_id: Sha1Git
+    ) -> List[RawExtrinsicMetadata]:
+        """Builds a list of full RawExtrinsicMetadata objects, using
+        metadata returned by get_extrinsic_snapshot_metadata."""
+        metadata_items = self.get_extrinsic_snapshot_metadata()
+        if not metadata_items:
+            # If this package loader doesn't write metadata, no need to require
+            # an implementation for get_metadata_authority.
+            return []
+
+        authority = self.get_metadata_authority()
+        fetcher = self.get_metadata_fetcher()
+
+        metadata_objects = []
+
+        for item in metadata_items:
+            metadata_objects.append(
+                RawExtrinsicMetadata(
+                    type=MetadataTargetType.SNAPSHOT,
+                    id=SWHID(object_type="snapshot", object_id=snapshot_id),
+                    discovery_date=item.discovery_date or self.visit_date,
+                    authority=authority,
+                    fetcher=fetcher,
+                    format=item.format,
+                    metadata=item.metadata,
+                    origin=self.url,
+                )
+            )
+
+        return metadata_objects
+
     def build_extrinsic_revision_metadata(
         self, p_info: TPackageInfo, revision_id: Sha1Git
     ) -> List[RawExtrinsicMetadata]:
diff --git a/swh/loader/package/nixguix/loader.py b/swh/loader/package/nixguix/loader.py
--- a/swh/loader/package/nixguix/loader.py
+++ b/swh/loader/package/nixguix/loader.py
@@ -12,6 +12,8 @@
 
 from swh.model import hashutil
 from swh.model.model import (
+    MetadataAuthority,
+    MetadataAuthorityType,
     Revision,
     RevisionType,
     TargetType,
@@ -21,7 +23,11 @@
 )
 
 from swh.loader.package.utils import EMPTY_AUTHOR
-from swh.loader.package.loader import BasePackageInfo, PackageLoader
+from swh.loader.package.loader import (
+    BasePackageInfo,
+    PackageLoader,
+    RawExtrinsicMetadataCore,
+)
 
 
 logger = logging.getLogger(__name__)
@@ -55,8 +61,8 @@
 
     def __init__(self, url):
         super().__init__(url=url)
-        raw = retrieve_sources(url)
-        clean = clean_sources(raw)
+        self.raw_sources = retrieve_sources(url)
+        clean = clean_sources(parse_sources(self.raw_sources))
         self.sources = clean["sources"]
         self.provider_url = url
 
@@ -76,6 +82,20 @@
         """
         return self._integrityByUrl.keys()
 
+    def get_metadata_authority(self):
+        return MetadataAuthority(
+            type=MetadataAuthorityType.FORGE, url=self.url, metadata={},
+        )
+
+    def get_extrinsic_snapshot_metadata(self):
+        return [
+            RawExtrinsicMetadataCore(
+                format="nixguix-sources-json",
+                metadata=self.raw_sources,
+                discovery_date=None,
+            ),
+        ]
+
     # Note: this could be renamed get_artifact_info in the PackageLoader
     # base class.
     def get_package_info(self, url) -> Iterator[Tuple[str, NixGuixPackageInfo]]:
@@ -186,12 +206,16 @@
         )
 
 
-def retrieve_sources(url: str) -> Dict[str, Any]:
+def retrieve_sources(url: str) -> bytes:
     response = requests.get(url, allow_redirects=True)
     if response.status_code != 200:
         raise ValueError("Got %d HTTP code on %s", response.status_code, url)
 
-    return json.loads(response.content.decode("utf-8"))
+    return response.content
+
+
+def parse_sources(raw_sources: bytes) -> Dict[str, Any]:
+    return json.loads(raw_sources.decode("utf-8"))
 
 
 def clean_sources(sources: Dict[str, Any]) -> Dict[str, Any]:
diff --git a/swh/loader/package/nixguix/tests/test_nixguix.py b/swh/loader/package/nixguix/tests/test_nixguix.py
--- a/swh/loader/package/nixguix/tests/test_nixguix.py
+++ b/swh/loader/package/nixguix/tests/test_nixguix.py
@@ -15,11 +15,22 @@
 
 from unittest.mock import patch
 
-from swh.model.model import Snapshot, SnapshotBranch, TargetType
+from swh.model.identifiers import SWHID
+from swh.model.model import (
+    MetadataAuthority,
+    MetadataAuthorityType,
+    MetadataFetcher,
+    MetadataTargetType,
+    RawExtrinsicMetadata,
+    Snapshot,
+    SnapshotBranch,
+    TargetType,
+)
 from swh.loader.package.archive.loader import ArchiveLoader
 from swh.loader.package.nixguix.loader import (
     NixGuixPackageInfo,
     NixGuixLoader,
+    parse_sources,
     retrieve_sources,
     clean_sources,
 )
@@ -27,6 +38,10 @@
 from swh.loader.package.utils import download
 from swh.model.hashutil import hash_to_bytes, hash_to_hex
 from swh.storage.exc import HashCollision
+from swh.storage.algos.origin import origin_get_latest_visit_status
+from swh.storage.interface import PagedResult
+
+from swh.loader.package import __version__
 
 from swh.loader.tests import (
     assert_last_visit_matches,
@@ -38,6 +53,17 @@
 sources_url = "https://nix-community.github.io/nixpkgs-swh/sources.json"
 
 
+@pytest.fixture
+def raw_sources(datadir) -> bytes:
+    with open(
+        os.path.join(
+            datadir, "https_nix-community.github.io", "nixpkgs-swh_sources.json"
+        ),
+        "rb",
+    ) as f:
+        return f.read()
+
+
 SNAPSHOT1 = Snapshot(
     id=hash_to_bytes("0c5881c74283793ebe9a09a105a9381e41380383"),
     branches={
@@ -80,7 +106,7 @@
 
 
 def test_retrieve_sources(swh_config, requests_mock_datadir):
-    j = retrieve_sources(sources_url)
+    j = parse_sources(retrieve_sources(sources_url))
     assert "sources" in j.keys()
     assert len(j["sources"]) == 2
 
@@ -136,7 +162,7 @@
     assert len(clean["sources"]) == 1
 
 
-def test_loader_one_visit(swh_config, requests_mock_datadir):
+def test_loader_one_visit(swh_config, requests_mock_datadir, raw_sources):
     loader = NixGuixLoader(sources_url)
     res = loader.load()
     assert res["status"] == "eventful"
@@ -160,6 +186,34 @@
         loader.storage, sources_url, status="partial", type="nixguix"
     )
 
+    (_, visit_status) = origin_get_latest_visit_status(loader.storage, sources_url)
+    snapshot_swhid = SWHID(
+        object_type="snapshot", object_id=hash_to_hex(visit_status.snapshot)
+    )
+    metadata_authority = MetadataAuthority(
+        type=MetadataAuthorityType.FORGE, url=sources_url,
+    )
+    expected_metadata = [
+        RawExtrinsicMetadata(
+            type=MetadataTargetType.SNAPSHOT,
+            id=snapshot_swhid,
+            authority=metadata_authority,
+            fetcher=MetadataFetcher(
+                name="swh.loader.package.nixguix.loader.NixGuixLoader",
+                version=__version__,
+            ),
+            discovery_date=loader.visit_date,
+            format="nixguix-sources-json",
+            metadata=raw_sources,
+            origin=sources_url,
+        )
+    ]
+    assert loader.storage.raw_extrinsic_metadata_get(
+        type=MetadataTargetType.SNAPSHOT,
+        id=snapshot_swhid,
+        authority=metadata_authority,
+    ) == PagedResult(next_page_token=None, results=expected_metadata,)
+
 
 def test_uncompress_failure(swh_config, requests_mock_datadir):
     """Non tarball files are currently not supported and the uncompress
@@ -305,7 +359,7 @@
     } == stats
 
 
-def test_resolve_revision_from(swh_config, requests_mock_datadir):
+def test_resolve_revision_from(swh_config, requests_mock_datadir, datadir):
     loader = NixGuixLoader(sources_url)
 
     known_artifacts = {
diff --git a/swh/loader/package/nixguix/tests/test_tasks.py b/swh/loader/package/nixguix/tests/test_tasks.py
--- a/swh/loader/package/nixguix/tests/test_tasks.py
+++ b/swh/loader/package/nixguix/tests/test_tasks.py
@@ -3,6 +3,8 @@
 # License: GNU General Public License version 3, or any later version
 # See top-level LICENSE file for more information
 
+import json
+
 
 def test_nixguix_loader(
     mocker, swh_scheduler_celery_app, swh_scheduler_celery_worker, swh_config
@@ -13,11 +15,9 @@
     mock_retrieve_sources = mocker.patch(
         "swh.loader.package.nixguix.loader.retrieve_sources"
     )
-    mock_retrieve_sources.return_value = {
-        "version": 1,
-        "sources": [],
-        "revision": "some-revision",
-    }
+    mock_retrieve_sources.return_value = json.dumps(
+        {"version": 1, "sources": [], "revision": "some-revision",}
+    ).encode()
 
     res = swh_scheduler_celery_app.send_task(
         "swh.loader.package.nixguix.tasks.LoadNixguix", kwargs=dict(url="some-url")
diff --git a/swh/loader/tests/__init__.py b/swh/loader/tests/__init__.py
--- a/swh/loader/tests/__init__.py
+++ b/swh/loader/tests/__init__.py
@@ -59,6 +59,7 @@
             f"Visit_status points to snapshot {visit_status.snapshot.hex()} "
             f"instead of {snapshot.hex()}"
         )
+
     return visit_status